HomeMy WebLinkAbout11/01/2015 - Report: Stree Maintenance Audit - City Council - Audit CommitteeStreets Audit 1
Street Maintenance Audit
November 2015
City Internal Auditor’s Office
City of College Station
File#: 15-02
2 Streets Audit
Audit Executive Summary:
Street Maintenance Program
What We Found
Process Effectiveness. The Street Maintenance
Program experiences difficulties in certain areas of
operations. Turnover rates in the Streets Division are
higher than other comparable city departments and the
private construction industry. Employees are not as skilled
as required for efficient job task completion, which is
influenced by high turnover. High levels of equipment
maintenance impact the ability to complete work in a
timely manner.
Policy Considerations. Citizen complaints have resulted
in a shift away from the preventative maintenance
techniques of chip and crack sealing. This shift has led to
increased costs for the Streets Division, both in time and
money. These complaints are primarily driven by the
aesthetic look of streets when chip sealing or crack sealing
methods are utilized.
Streets maintenance is irrevocably linked to street
construction standards. The City utilizes the Bryan-College
Station Unified Guidelines and Specifications in its capital
improvement projects and requires the same
specifications be used by developers. However, there are
opportunities in the City’s system to construct streets
below current standards. The City should consider taking
on the construction of streets classified as minor collector
or higher in order to ensure better quality roads. In
addition, while many city streets are constructed in
asphalt, concrete roads may be more effective and less
costly in the long-term.
Flexibility is key in operating street maintenance programs
effectively. Given the uncertainty that comes with weather
and high turnover, the funding source should reflect this
flexibility. Some cities institute transportation fees
allocated to a street maintenance fund for this purpose.
Why We Did This Audit
The City’s largest capital asset is its streets
infrastructure. In fiscal year 2015, $2,330,000
was allotted for street maintenance projects
and $20,435,000 was appropriated for street
related capital improvement projects. Actions
taken regarding street maintenance and
construction will have long-term impact on
streets’ useful life and quality. Street
maintenance programs were also the most
audited area across local government audit
shops in the nations during fiscal year2014 due
to the deterioration of roads nationwide.
What We Recommended
The Streets Division should reinstitute
skill based pay for heavy equipment
operators (recommendation 1).
Consider only having an engineering firm
conduct a Pavement Condition Analysis
every three years and in-house in years
in between. Investigate alternative data
collection methods (recommendation 3).
Consider allowing contractors to take
over the majority of millings and overlays
so that the City can reallocate more
resources toward preventative
maintenance; if not, consider a dedicated
mechanic for streets heavy equipment
(recommendations 2, 4, and 5).
Consider raising construction standards
for local roads and moving toward more
concrete construction for city streets
(recommendations 6 and 7).
Institute a transportation fee and
allocate the revenue into a dedicated
street maintenance fund.
(recommendation 8).
Streets Audit 3
Street Maintenance Audit
Table of Contents
Introduction ................................................................................................................. 4
Audit Objectives ................................................................................................. 4
Scope and Methodology ..................................................................................... 4
Streets Maintenance Background ...................................................................... 6
Several Factors Impact the Useful Life of Streets....................................................... 7
The City Utilizes Different Types of Maintenance Techniques ...................................... 9
Municipal Maintenance Strategies Vary ................................................................... 11
Findings and Analysis .................................................................................................. 15
Obstacles to Optimal Maintenance of City Streets Exist .................................. 15
Several Factors Impact Street Maintenance Crews’ Productivity ................................ 15
Turnover has contributed to Operator Inexperience ................................................. 16
Several Factors Contribute to High Turnover Rates .................................................. 17
Skill-based Pay Should Be Implemented for Heavy Equipment Operators .................. 19
Street Maintenance Equipment Is Prone to Problems ............................................... 20
City Streets Could Be Impacted by Policy Considerations ............................... 22
Effectiveness of Pavement Condition Analysis Reports is Uncertain ........................... 22
The City Should Consider Modifying its Use of Contractors ....................................... 23
Being Responsive to Citizen Complaints Comes at a Cost ......................................... 24
Strengthening Development Standards Would Lead to Longer Useful Lives................ 27
Concrete Roads Can Reduce Maintenance Costs and Increase Useful Life .................. 29
A Dedicated Maintenance Fund Would Grant Greater Flexibility ................................ 30
Summary of Audit Recommendations .............................................................. 32
Appendix A: Skill-based Pay Methodology ...................................................... 35
Appendix B: Management’s Responses to the Audit Recommendations ......... 38
Appendix C: Flexible and Rigid Pavements Report ......................................... 40
4 Streets Audit
Introduction
The Office of the City Internal Auditor conducted this performance audit of the street
maintenance program pursuant to Article III Section 30 of the College
Station City Charter, which outlines the City Internal Auditor’s primary duties.
A performance audit is an objective, systematic examination of evidence to assess
independently the performance of an organization, program, activity, or function. The
purpose of a performance audit is to provide information to improve public
accountability and facilitate decision-making. Performance audits encompass a wide
variety of objectives, including those related to assessing program effectiveness and
results; economy and efficiency; internal control; compliance with legal or other
requirements; and objectives related to providing prospective analyses, guidance, or
summary information. A performance audit of street maintenance was included in the
fiscal year 2015 audit plan based on direction given by the Audit Committee.
Audit Objectives
This audit addresses the effectiveness of the City to maintain its most valuable municipal
asset—city roadways. This report answers the following questions:
Is Public Works utilizing appropriate techniques, staffing levels, and equipment to
best maintain the City’s street inventory?
Are proper policies being implemented and standards being upheld?
Are the methods used to construct and maintain city streets successful at
ensuring the highest level of street infrastructure at the lowest possible cost?
Scope and Methodology
This audit was conducted in accordance with government auditing standards (except for
the completion of an external peer review),1 which are promulgated by the Comptroller
General of the United States. Audit fieldwork was conducted from June 2015 through
September 2015. The scope of review varied depending on the analysis being
performed. The methodology used to complete the audit objectives included:
1 Government auditing standards require audit organizations to undergo an external peer review every three years.
Streets Audit 5
Reviewing the work of auditors in other jurisdictions and researching professional
literature to identify (1) street maintenance best practices, (2) current challenges
facing street infrastructure, and (3) industry trends.
Reviewing applicable policies and procedures and relevant state and federal laws
and regulations.
Examining applicable financial and performance reports and data.
Observing street maintenance operations to examine how maintenance actions
are carried out in the field and how the work environment affects productivity.
Interviewing various Public Works and Planning and Development employees to
identify (1) potential improvements to overall street performance and (2)
possible problems that can impact street performance.
Examining the effectiveness of the street maintenance program and its alignment
to the Streets Division’s goals through (1) observing employees in the field, (2)
interviewing management, (3) reviewing records of applications and work orders,
and (4) comparing Streets Division employment statistics with similar positions in
other city departments.
Analyzing the maintenance performed by the Streets Division and identifying
patterns between (1) amount of maintenance and road type, (2) types of
maintenance and their frequency, (3) costs per maintenance type, and (4)
maintenance costs per street type.
Analyzing reports regarding Streets Division equipment maintenance and
equipment use in the field.
Identifying the Pavement Condition Analysis report (the street condition data
collected and analyzed by an engineering firm) functionality and appropriateness
by (1) reviewing report variations, (2) interviewing the report reviewer (City of
College Station employee), (3) interviewing Streets management, and (4)
observing whether the recommendations issued were appropriate through
observations of randomly selected streets.
Identifying CityWorks’ (the work order catalog information system) functionality
and reporting capabilities by (1) reviewing the CityWorks software, (2)
interviewing the system administrator (City of College Station employee), and (3)
observing whether work orders reported in the system were accurate through
observations of randomly selected streets.
6 Streets Audit
Streets Maintenance Background
Streets, Drainage, & Landscape is a division within the Department of Public Works. The
division consists of 36 full-time employees maintaining approximately 310 miles of paved
streets, 90 miles of storm water lines, 21 miles of concrete valley gutters, and 150 miles
of natural creek line. They also provide heavy equipment support for other departments
and traffic management support for major community events. In addition, the division
provides 24 hour, seven day a week response to street maintenance problems, drainage
issues, and incident management support for public safety agencies.
Although there are three unique business units within the division, employees are
crossed trained. Therefore, occasions arise when crews from one unit will aid others in
another unit, especially when weather prohibits Street Maintenance Work. Figure 1,
below provides the flowchart of the division.
Figure 1: Streets, Drainage & Landscape Organization Chart
Streets
Foreman
Drainage
Foreman
Streets, Drainage &
Landscape Supervisor
Landscape
Supervisor
Streets Crew
Leader
Drainage Crew
Leader
Landscape Crew
Leader
Irrigation Equip.
Operator
Equipment
Operator (8)
Equipment
Operator (13)
Light Equipment
Operator (5)
Light Equipment
Operator
For the purpose of this report, the streets business unit of the Streets, Drainage, &
Landscape Division will be referred to as the “Streets Maintenance Division” or “Streets
Division.” In fiscal year 2015, the total number of authorized Streets Division employees
was seventeen. This includes thirteen Equipment Operators, a Light Equipment
Operator, a Crew Leader, and a Foreman.
Streets Audit 7
The Streets Maintenance Division of the Public Works Department strives to ensure that
the street system within the City of College Station is properly maintained. This is done
through a number of programs, including a street maintenance program that addresses
street repair before more expensive reconstruction measures are needed.
The effectiveness of this service is currently measured by the average pavement rating
of the City’s streets conducted by an outside consultant. The citywide average pavement
rating for 2014 was 88 out of 100, which indicates that city streets are in good condition
overall, a slight improvement over 2013 which had a score of 86. A decision matrix is
used to determine the maintenance strategy for a particular roadway and is based upon
the type of distresses present, the density of the distresses, and the roadway
classification. The Street Maintenance Division is funded out of the General Fund (see
Figure 2 below).
Figure 2: Street Maintenance Division Funding
Several Factors Impact the Useful Life of Streets
Although many factors can play a significant role in impacting the useful life of a street,
the following are likely the greatest contributors to reducing the longevity of city streets:
Soil Conditions: College Station is inundated with clay soils that have a high plasticity
and shrink-swell capability. This can be especially dangerous for concrete roads as the
high level of soil expansion and contraction can interfere with the uniformity of the soil
under the pavement. Having uniformity of soil under concrete pavement is perhaps the
most important aspect of construction. Concrete distributes the weight of loads traveling
over its surface more evenly across the entirety of the slab. Asphalt on the other hand
channels the weight deeper into the subgrade under the localized area the weight is
traveling. Careful attention to the design and construction of subgrades and subbases is
0
500,000
1,000,000
1,500,000
2,000,000
2,500,000
3,000,000
3,500,000
4,000,000
4,500,000
FY11 Actual FY12 Acutal FY13 Actual FY14 Actual FY15 Budget
8 Streets Audit
essential to ensure the structural capacity and ride quality of all types of bases.
Therefore, having a thorough soil testing process, high quality subgrade materials, and
proper subgrade depth are important for ensuring a full life for the street.
Traffic Load: The traffic load can have a great impact on the useful life of a street.
Streets are built to certain standards based on the expected volume of traffic they will
carry during their lifetime, resulting in the residential, collector, and arterial
classifications. Streets of a certain classification are built assuming that the street will
only carry the traffic load determined by its specifications. When a road carries more
than its specifications were designed for, the road experiences more stress. This results
in a larger number of distresses faster, shortening the useful life of the street. It is
important to ensure that streets are being built for the proper amount of traffic to
mitigate this problem. Research concerning traffic loads throughout the City are key for
proper street specification designations.
Material Quality: In addition to soil conditions and traffic, the quality and strength of the
paving materials used are important for determining the useful life of a street. Asphalt is
composed of bitumen cement and aggregates of different sizes that are then heated to
create a viscous material. Having the correct amount of aggregate, emulsion, and other
additives is necessary for ensuring the pavement is able to function properly over the
course of its life. Materials’ strength and quality is also affected by the process used to
create it, which requires specific temperatures and storage strategies before application
of the materials begin. It is necessary to ensure that materials are up to standards
through an inspection process, as well as to determine whether material standards are
high enough to reach the full useful life of a road.
Weather: Lastly, weather can have an adverse impact on road conditions. Rain and
freezing temperatures exacerbate cracks and potholes when water seeps into the
pavement. Water erodes the subgrade and pavement while cold temperatures cause the
trapped water to expand and shrink, resulting in more distresses in the pavement. This
is true for both concrete and asphalt. Long periods of rain also mean that asphalt and
concrete cannot be laid, as the water would then be trapped under the pavement.
Water works its way into cracks in the roads and causes further damage as seasons
change. Wet winters and springs are likely to lead to more distresses in the roads, which
may not have occurred as quickly if the weather was dry. Extreme fluctuations in
temperature over the course of the year can also result in distresses developing more
quickly. Weather is an uncontrollable factor for street maintenance and therefore the
best strategy for handling unexpected weather events is to maintain flexibility in
scheduling and operating capabilities in case disruption occurs.
Streets Audit 9
The City Utilizes Different Types of Maintenance Techniques
The City of College Station utilizes multiple techniques to maintain and repair city
streets. They can be broken into three categories: (1) preventative, (2) corrective, and
(3) reconstruction. Each type of maintenance techniques plays a role during the life
cycle of a street.
Figure 3: Pavement Life-Cycle
When pavement is young, it deteriorates at a slower rate. At a certain point in the
young road’s life, the timing becomes optimal for preventative maintenance techniques
to be used, keeping the road in good condition. As time goes on, preventative
techniques lose effectiveness due to the aging process of the street. As potholes and
other more severe distresses begin to appear, corrective maintenance techniques are
used to patch and repair those areas of a street that require more help than
preventative maintenance can give. Eventually a street will reach such an advanced age
that reconstruction of the road becomes necessary.
As Figure 3 illustrates, preventative maintenance extends the life of the road and helps
ensure that pavement can undergo the full life cycle of a street rather than waiting until
reconstruction becomes necessary. Roads that generally are in good condition do not
register a major change in condition rating after a preventative treatment is applied—the
rating continues as good. What is important, however, is the condition rating several
years later—roads that receive preventative maintenance are in better condition than
those left without. For the purposes of this audit, we define the techniques used by the
City in the following ways and in the following categories.
10 Streets Audit
Preventative: These maintenance techniques are used to prevent more serious road
deterioration and generally are conducted in the early life of the street. The following
are common examples of preventative maintenance techniques that are used
nationwide.
Crack sealing: Sealing and filling asphalt or concrete pavement cracks is a common
road maintenance activity. Specialized materials are placed into or above cracks to
prevent the intrusion of water and incompressible material into the cracks and to
reinforce the adjacent pavement.
Seal coating/Chip sealing: Seal coats have been used for decades to preserve riding
surfaces. The most common type of seal coat the City has used in the past is chip
sealing. A chip seal is a surface treatment in which the pavement is sprayed with
emulsion and then immediately covered with aggregate and rolled. Chip seals are used
primarily to seal a pavement with non-load-associated cracks, and to improve surface
friction. A chip seal’s main purpose is to seal the fine cracks in a pavement’s surface and
prevent water intrusion into the base and subgrade. To achieve the pavement
preservation benefits of a chip seal, an agency must apply it on roadway surfaces when
the level of pavement distress is low. This technique is used primarily for asphalt roads
though it can be used for concrete as well.
Joint sealing: Resealing concrete pavement joints is a common pavement maintenance
activity. Joint sealants reduce the amount of water entering the pavement structure and
prevent incompressible materials from filling the joints. Joint sealing is used for concrete
roads and areas where concrete curbs and aprons connect to asphalt pavement.
Corrective: These maintenance techniques are used when a road is experiencing more
severe distresses or failures and generally are conducted in the later life of the street.
Corrective maintenance occurs when preventative maintenance is no longer effective or
cannot be used given the severity of the distresses. Examples of common corrective
techniques used by the City of College Station are listed below.
Level Ups: This maintenance technique is utilized when a road begins to recede below
the curb along the side of the street. Level ups require milling out the asphalt along the
curb and then laying new asphalt, making the street flush with the curb again. This
usually becomes necessary as a road ages. It is an asphalt maintenance technique.
Patching: A patch can be done in a few ways based on which type of distress is being
repaired. Potholes consist of small bowl-shaped depressions in the pavement surface
typically having sharp edges and vertical sides near the top of the hole. They are
localized failures and are repaired by filling the pothole with asphalt and compressing it
down into the pavement. Failures usually require milling out a larger portion of asphalt
to ensure the subbase is still functioning and then laying new asphalt over the milled
Streets Audit 11
area and compressing the material. Patching is mostly used for asphalt roads but may
also be used as a measure for concrete surface distresses.
Overlays: For severely distressed surfaces, overlays will provide a new surface, prolong
pavement structure life, and make a pavement stronger. Overlays generally require
milling up old asphalt and laying between 0.5 and 2 inches of new asphalt.
Reconstruction: Reconstruction refers to removing all or a significant portion of the
pavement material and replacing it with new or recycled materials. This may include full-
depth reclamation, where the pavement surface is demolished in place and new
pavement surface is applied. It also may be partial reconstruction if the road does not
have enough severe distresses to necessitate the replacement of the entire road.
Asphalt: Partial reconstruction consists of localized base repairs along with a milling of
the existing hot mix asphalt concrete pavement and replacing it with a new asphalt
surface. Full reconstruction occurs when severe levels of distress in most or all areas
necessitate a full replacement of the roadway. The old asphalt will be removed, the
subbase repaired, and then new asphalt will be laid.
Concrete: Once a concrete slab has been injured beyond the point of joint and crack
sealing, the entirety of the slab must be removed to replace. If the problem has to do
with joint failure, there is a strong possibility that a neighboring slab, if within six feet of
the problem, must be removed as well. For some types of concrete pavement, any
cracking, breaking, or spalling of slab edges on either side of a transverse joint need to
be repaired. This can be full depth repair or partial depth repair.
Municipal Maintenance Strategies Vary
Comparing the City of College Station to peer cities found that there is a wide variety of
staffing and maintenance strategies used amongst surveyed Texas cities. Although there
is no city that exactly matches College Station, we attempted to identify cities that best
resembled College Station’s soil, weather and traffic. As a result, we surveyed the Public
Work’s departments of Bryan, Mansfield, Sugar Land, San Marcos, and Waco.
College Station is well within the normal range for staffing of the Streets Division,
especially given comparable number of lane miles2 and work done in-house3. Figure 4 on
the next page provides a comparison of College Station Streets Division staffing with
those of comparable Texas cities.
2 College Station maintains the second most lane miles of streets compared to cities we surveyed (see Figure 6, p. 13).
3 Like Mansfield and Waco, College Station conducts most street maintenance in-house (see Figure 5, p. 11).
12 Streets Audit
Figure 4: Streets Division Staffing Comparison amongst Texas Cities
While there are different opinions about how much and for which projects contractors
should be used, there is a consensus among peer cities that contractors are a necessary
aspect of street maintenance. Generally speaking, city staff conducts most maintenance
in peer cities with the exceptions of Sugar Land and Bryan which both use contractors at
higher rates.4
Figure 5: Percentage City Staff vs. Contractor
Peer cities also differ in allocation of their employees versus contractors. See Table 1
below.
4 Bryan is not included in the Figure 3 due to a lack of numerical data, however, per our conversation with staff, Bryan
contracts out most its maintenance work.
0
5
10
15
20
25
30
Waco College
Station
Average San Marcos Bryan Mansfield Sugar Land
0
20
40
60
80
100
College Station Mansfield Sugar Land San Marcos Waco
City Staff Contractors
Streets Audit 13
Table 1: Current Division of Labor between Contractors and City Staff
City Overlays Potholes Base
Failures
Crack
Sealing
Concrete
Reconstruction
College Station City Staff/
Contractors City Staff City Staff Contractors Contractors
Mansfield City Staff/
Contractors City Staff City Staff City Staff Contractors
Bryan Contractors City Staff City Staff Contractors Contractors
Sugar Land Contractors City Staff/
Contractors Contractors Contractors Contractors
San Marcos City Staff/
Contractors City Staff City Staff City Staff Contractors
Waco
Contractors City Staff City Staff City Staff City Staff
City staff repair potholes and base failures in every peer city except Sugar Land, likely
due to the high amount of concrete in that particular city. Contractors are used for
major street rehabilitation projects, as well as concrete reconstruction in every city
excluding Waco. There is a division of labor between city staff and contractors for crack
sealing and overlays, but it is important to note that the two other cities that contract
out crack sealing (Bryan and Sugar Land) contract out most of their maintenance as
shown in Table 1.
With regards to asphalt and concrete, there are two schools of thought: (1) concrete
has a longer useful life and is more aesthetically pleasing and (2) asphalt is cheaper and
easier to repair. This is reflected in the percentage of asphalt and concrete streets that
peer cities have, some of which are chiefly concrete and some that are predominantly
asphalt.5 See Figure 6 on the next page.
Hiring requirements were largely the same throughout each of the peer cities. Each city
also relied on city employees doing visual inspections of the roads to determine how
maintenance scheduling should be done regardless if they had a plan put together by an
outside engineering firm. In fact, with the exception of Bryan (who does in-house data
collection and analysis), College Station has the most frequent Pavement Condition
Analysis. Each division supervisor we interviewed also stated that street maintenance
depends on multiple factors and therefore requires a high level of flexibility.
5 While the City of Bryan does not appear in the Figure 6 due to a lack of numerical data, it has significantly more asphalt
than concrete per our conversation with staff.
14 Streets Audit
Figure 6: Miles of Asphalt and Concrete Comparison6
6 These numbers include all of the miles of street in each City, ownership notwithstanding.
0
100
200
300
400
500
600
Waco College Station Sugar Land San Marcos Mansfield
Asphalt Miles Concrete Miles
Streets Audit 15
Findings and Analysis
Obstacles to Optimal Maintenance of City Streets Exist
The stated mission of College Station with regards to transportation is to ensure that the
City has a “safe, efficient and well-connected multimodal transportation system that
contributes to a high quality of life and is sensitive to surrounding uses.” This suggests
that city streets, sidewalks, bike lanes, and multi-modal paths should be well-maintained
and free of hazards. To accomplish this, the City provides streets that accommodate
vehicles, bicyclists and pedestrians, plans for infrastructure that meets projected growth
and development, and maintains and rehabilitates the system to avoid costly
replacement.
Several Factors Impact Street Maintenance Crews’ Productivity
Maintaining and rehabilitating city streets requires that street maintenance be
accomplished efficiently and effectively. Therefore, we examined multiple aspects of the
College Station street maintenance program to determine if street maintenance services
are meeting the stated goals of the City of College Station Strategic Plan.
During the course of the audit, we observed the process of a street overlay, which
encompasses removing old asphalt and laying new asphalt on a stretch of road.
Overlays are the most labor intensive function the Streets Division performs, and can
take multiple days to complete depending on the size and length of the road. The
overlay process is illustrated in Figure 7. We also observed the pothole repair process.
Figure 7: Steps Used to Overlay a Street
Flagging (0-4) Milling (2-4)
Edging (2-4)
Dumping (2-4)
Remove Old Asphalt
Sweeper (1)
Street Sweeping
Driver (1)
Sprayer (1)
Lay Down Tack
Flagging (0-4) Paving (2-3)
Laborers (2-4)
Truckers (3-4)
Lay New Asphalt
1 2 3 4
There are bottlenecks in the process of overlaying a street. This results in the number of
crew members allocated for a certain job exceeding the number of members necessary
to accomplish the task. For example, when laying down asphalt for an overlay, the crew
must wait for the asphalt to be delivered to the site. This means that if there is a
situation at the asphalt plant which delays the delivery of asphalt, then the crew may be
16 Streets Audit
unable to conduct meaningful work on the project until the delivery is completed. Trucks
drive to the asphalt plant to get more asphalt continually throughout the day. Tonnage
of asphalt required for a stretch of road is estimated by the crew leader, who is
generally conservative in his estimates to ensure that funds are not wasted on excess
materials.
Turnover has contributed to Operator Inexperience
In our observations, we noted that many of the crew members were not skilled in their
work, which negatively affects the efficiency and effectiveness of the street maintenance
crews. This can be partially attributed to the large turnover rate that exists in the Streets
Division. New workers are going to be less skilled than those who have been on the job
for a number of years, but crew members generally do not stay for very long in
comparison to similar jobs in other city departments (illustrated in Table 2 below). This
means that employees do not gain those skills that would make them more effective in
doing street maintenance, especially in the operation of maintenance equipment.
Table 2: Comparison of ‘Blue Collar’ Employee Turnover Rates
FY10
Actual
FY11
Actual
FY12
Actual
FY13
Actual
FY14
Actual
FY15
Estimate7
Streets 22.6 31.5 48.6 33.4 24.5 16.6
Public Works Turnover 13.6 12.0 21.7 16.7 32.0 11.1
Parks & Rec 5.1 14.7 8.8 13.2 15.9 9.8
Water Services 0 0 0 0 0 5.3
Waste Water 2.9 0 0 2.9 0 0
Electric Utility 3.1 6.3 6.3 3.1 15.6 0
Construction Industry 6.3 6.3 5.7 5.3 4.7 5.0
Turnover is more prevalent amongst street maintenance employees than comparable
‘blue collar’ employees in other city departments.8 This is true for every year except
fiscal year 2014, in which the Public Works Facilities, Drainage, and Traffic Signs and
Markings divisions had a 32 percent turnover rate amongst their ‘blue collar’ staff and
Streets had 24.5 percent turnover rate. While a number of these terminations occur
during the probationary period, the division still averages over 26 percent turnover
across the five years analyzed. Regardless, Streets is losing nearly a third of its
workforce on an annual basis. Not only is this significantly higher than other comparable
positions throughout the City, it is also much higher than published9 national
construction industry turnover rates.
7 October 1, 2014 to June 30, 2015.
8 We only included positions that most resembled heavy or light equipment operators and their supervisors in our analysis.
9 Data taken from the US Bureau of Labor Statistics
Streets Audit 17
Additionally, the heavy equipment operator position seems to take more time to fill and
has a higher turnover rate than other positions within the Streets Division. This high
turnover can reduce efficiency as new members must be hired and trained for their new
job. It also can hinder work if employees resign during a busy period.
Several Factors Contribute to High Turnover Rates
The causes of turnover can be varied, but widely accepted reasons are as follows:
inadequate compensation, lack of opportunities for advancement, perception of unfair
treatment, feeling undervalued, and job stress. The strength of these causes for
turnover in the Streets Division are addressed below.
Compensation: There is a perception in the Streets Division that pay is not adequate for
the work conducted. Recently, the City hired a consultant to conduct a salary survey,
which found that the current average salary of light equipment operators in the City of
College Station is 98 percent the average of comparable cities.10 The current average
salary of heavy equipment operators in the City of College Station is 96 percent of
comparable cities. Though no survey could be completely accurate, the salary survey
results are an indicator that pay is unlikely to be the primary cause of turnover. Other
factors are likely to have an equal or greater effect on why employees leave the Streets
Division.
The primary goal of an effective compensation strategy should be to attract and retain
top performers not to eliminate turnover. Assuming that streets employees are
compensated at or close to market, the average employee is less likely to leave solely
based on pay, though it is an important factor. High performers, however, are more
likely to seek work elsewhere because of the combination of pay and few opportunities
for financial acknowledgement of their performance level.
Lack of Advancement: There are few opportunities for advancement in the Streets
Division which is likely due to multiple factors. There are only nineteen allotted positions
in the Streets Division for fiscal year 2015. The only opportunities to advance are for
light equipment operators to become heavy equipment operators, heavy equipment
operators to become crew leaders, and crew leaders to become foremen. However, this
is only possible when those positions have a vacancy. During the time period between
fiscal year 2010 and fiscal year 2015, there were three crew leader position postings for
the Streets Division. All of those positions were filled by a current city employee at the
time. The foreman position was also filled by a current Streets employee when a
vacancy opened. So while there are not many opportunities for advancement due to the
small size of the division and the limited vacancies that open for higher level positions,
those vacancies that have opened in the past were filled by internal hires.
10 No public survey data sources were available.
18 Streets Audit
Personality Conflicts: Another cause of turnover can be conflicting personalities among
employees and supervisors. Conflicting personalities may lead to a tense and inefficient
workplace and disrupt trust between the different levels of employees in the division,
which can lead to higher rates of turnover. Personality conflicts also lead to
communication failures and the perception that an employee is undervalued or is treated
unfairly in comparison to other coworkers. Much of this problem can be traced to a
difference in values. A possible solution to this problem is to hire employees based more
on whether their values fit with the values of the Streets Division rather than solely
whether the applicant is qualified to run certain equipment or has a certain amount of
experience in similar work. It also means that current employees should be informed of
the division’s foundational values and a culture of dedication to those values should be
developed within the division. This requires identifying the overarching values that drive
the Streets Division and formulating a hiring strategy to determine whether an applicant
shares those fundamental values.
Feeling Undervalued: In addition, feeling undervalued by the employer can be another
reason for higher turnover rates. Feelings of this nature were noted during our field
observations. Employees noted that people performing more difficult tasks than others
receive the same pay without regard to skill differentials. Some employees expressed
that they felt that those who had more skill were undervalued considering the
differences in work are not reflected in salaries.
This perceived lack of appreciation for the skill needed to do the work could be a
contributing factor to the turnover rate in the Streets Division. While some turnover is
good for an organization, the goal is to retain high performers which is difficult with the
current incentive and reward system. Acknowledgement of individual and group success
in multiple ways could be helpful in creating a feeling of appreciation among employees.
Job Stress: One of the other causes for turnover is the nature of the work and stress
that comes with the job. During our observations, many crew members commented on
how difficult it was to work in the heat during the summer, especially when laying
asphalt. Some employees stated that they believed one of the reasons for the high
turnover in the Streets Division compared to other ‘blue collar’ divisions in the City was
due to the heat that comes from working with asphalt in the summer. We found that 46
percent of all service separations occur between June and September, indicating that
heat is a factor in turnover. Regardless, there is not much that can be done to lower the
physical demands of the job beyond making sure that basic needs like water are easily
accessible. Other causes resulting in turnover are more easily mitigated.
Applicant Qualification: While turnover is significant, we found that there seem to be
enough qualified applicants for open positions based solely on stated qualifications.
However, Streets Division management noted that while there are qualified applicants in
Streets Audit 19
terms of physical ability, the interview process is where they judge whether the
applicant is motivated to do the work and willing to rotate between different equipment
as needed. The interview process is where it becomes clearer that some applicants are
not appropriate. In this case, we believe that Streets is receiving enough applications to
fill the positions for the light equipment operator, foreman, and crew leader positions.
Though there are less qualified applicants for the heavy equipment operator position
there are still enough to adequately fill the position based solely on capabilities stated in
applications.
Table 3: Qualified Applicants
Position Title Percent of Qualified
Applicants
Number of
Applicants
Qualified Total
Heavy Equipment Operator 14.5% 43 296
Light Equipment Operator 23.1% 57 247
Crew Leader 35.3% 6 17
Foreman 18.8% 3 16
Skill-based Pay Should Be Implemented for Heavy Equipment Operators
While turnover is a problem that can affect the skill level of the overall crew and should
be addressed, there should also be a system put in place to give crew members
incentives to gain skills and implement those skills effectively. The current low skill level
affects how efficient street crews are when conducting maintenance work. During our
observations, we noted that there was little pro-active behavior on the part of the crew.
This may be a result of a new crew leader and crew members—as most Streets Division
employees were hired less than a year ago. The lack of senior leadership that can train
new employees how to perform the job may be an issue. For instance, we noted that
the crew leader often had to halt different essential tasks in order to ensure that other
tasks were performed up to standards. This lack of confidence may be due to the skill
deficiency demonstrated by some crew members, which led to forced inactivity as the
crew leader switched between tasks.
There is currently no incentive to become skilled on the machinery required for the job
which limits the effectiveness of crews. Skill-based pay could provide incentive for
workers to gain the skills necessary to perform the job as quickly as possible, which in
turn would increase overall crew effectiveness. It would also address some of the causes
of turnover, particularly employees feeling undervalued and the lack of advancement.
Skill-based pay objectively acknowledges the value of the employee and results in
greater competency in work. Therefore more responsibility can be given to employees in
the field because of their greater skill level.
While we believe that skill-based pay should be reinstituted, we find that skill-based pay
should only be instituted for the position of heavy equipment operator. The reasons for
20 Streets Audit
this are twofold. First, rather than making skill-based pay a part of the development
track that naturally occurs from light equipment operator to heavy equipment operator,
limiting it to heavy equipment operator requires that a light equipment operator already
have demonstrated their ability to perform at the higher level required for the position.
Therefore, the promotion to heavy equipment operator would open the door to further
skill-based raises that the individual would be further motivated to obtain beyond their
already high performance. Secondly, the possibility for abuse is present within a skill-
based pay system. Limiting skill-based pay to a position requiring those higher skills
makes it less likely that system abuse will occur, unwittingly or otherwise.
Recommendation: Reinstitute skill-based pay for heavy equipment operators.
Street Maintenance Equipment Is Prone to Problems
The City currently owns over 50 separate pieces of equipment that are used in
conducting street maintenance work. We were able to observe some of the equipment
being used in the field and gain access to maintenance reports for 20 of the different
pieces (summarized in Table 4).
In the field, there were clear instances in which the equipment was not functioning as it
should. At one point, it was observed that there was difficulty with the Milling Machine.
Because the milling machine is an essential piece of equipment to perform a street
overlay, when this occurs work on an overlay project would need to be halted.
Table 4: Unscheduled Maintenance of Streets Equipment
Equipment Name First Maint.
Date
Last Maint.
Date
Avg. Hrs
in Shop
Days in
Shop
Avg. time
between visits
Milling Machine 8/05/13 7/24/15 6.61 33 15.50
2011 Multi-Terrain Loader 9/03/13 5/28/15 3.35 10 48.20
Truck Tractor w/Winch 12/04/13 7/01/15 3.75 5 131.67
Autocar w/Schwarze Patcher 6/20/14 5/08/15 1.67 6 44.20
Ford F250 Crewcab 9/26/13 5/23/15 0.93 6 83.00
Entyre Lowboy 8/12/13 5/12/15 5.61 8 62.57
F750 Diesel Dump Truck 7/28/14 - 0.25 1 -
9 Wheel Pneumatic Roller 8/09/13 6/23/15 2.33 23 20.82
Freightliner Dump Truck 8/27/13 7/21/15 2.10 38 12.86
Tool Trailer 5/27/14 6/05/15 7.46 6 51.60
Tandem Drum Roller 8/14/13 6/08/15 3.30 24 19.74
2009 Peterbilt CC w/Patcher 10/07/13 7/22/15 2.99 55 8.44
Highway Sweeper 4/28/15 - 1.00 1 -
2014 Carlson Paver CP100 5/16/14 3/23/15 3.80 4 70.33
Peterbilt Dump Truck 9/10/13 7/21/15 1.55 15 31.07
Tilt Trailer 1/28/14 9/26/14 1.50 3 84.50
250G Tack Oil Trailer 1/07/14 7/11/14 0.83 3 64.50
4x2 Super Cab SRW 10/15/13 2/03/15 1.13 4 132.33
Flatbed Haul Trailer 8/27/13 6/26/15 1.16 10 51.00
2014 Cat Excavator 11/18/14 - 0.50 1 -
Streets Audit 21
Multiple pieces of equipment experience significant amounts of time in the shop
undergoing maintenance. In particular, the Milling Machine, Peterbilt Pothole Patcher
Truck, and Freightliner Dump Truck are in the shop at least every 20 days on average.
They have also been in the shop the most within the two year time frame for which we
had maintenance records. Of these pieces of equipment, the milling machine spends the
most time in the shop for each visit, averaging 6.67 hours per visit. In cross referencing
the maintenance reports with work orders, we identified an instance in which work on a
street (a level up) was halted for a week while the milling machine underwent
maintenance.11 In this case, maintenance resulted in an inability to finish an assigned
task within the expected time frame. According to Public Works management, when this
occurs, street crews will be reassigned to other maintenance activities.
It seems the most significant delays in work occur when multiple pieces of equipment
are undergoing maintenance at the same time. For instance, between April 23, 2014 and
May 13, 2014, the following heavy equipment vehicles were all in and out of the shop
for unscheduled repair: (1) Milling Machine, (2) Entyre Lowboy, (3) Peterbilt Pothole
Patcher Truck, (5) Freightliner Dump Truck, and (6) Peterbilt Dump Truck. That time
also did not see significant rainfall or other weather problems that could have caused
the lack of street maintenance work. Again, between June 23, 2014 and July 14, 2014,
seven different pieces of equipment underwent unscheduled repair and there was not
significant weather that could explain the long period with no work orders written up,
especially since the average time between work orders is three days.
The key concern regarding equipment is how necessary it is to effectively and efficiently
complete maintenance tasks. Currently, more resources have been allocated toward
corrective maintenance techniques. This means that heavy equipment is needed more
frequently to accomplish overlays, level ups, and other large distresses. For instance,
though the milling machine is prone to needing repair, it is used for overlays, level ups,
and base failures. Without a milling machine, many of the distresses would be much
more difficult to fix. While rentals have been used in emergency situations if a piece of
equipment cannot function, it would ultimately be less efficient with the flexibility
needed for street maintenance. Sometimes equipment is not needed for a particular
maintenance activity, but when needed it should be available. Given the frequent
necessity of heavy equipment with the current maintenance strategy, a more effective
option would be to have a dedicated mechanic for heavy equipment or greater
compensation for mechanics with more skill and expertise to ensure that equipment is
being repaired quickly and thoroughly.
Recommendation: If the City chooses to retain its current maintenance strategy, it
should consider increasing funding and instituting skill-based pay for mechanics.
11 The two different work orders (12672 and 12757) were identified as covering the same stretch of road and having
the same maintenance work being done.
22 Streets Audit
City Streets Could Be Impacted by Policy Considerations
During our audit we identified several street infrastructure policy related issues that
could have a significant impact on the long-term effectiveness of the street maintenance
program. These issues include (1) the reliability of the pavement condition analysis
report for scheduling purposes, (2) a shift in maintenance techniques, (3) street
construction standards, and (4) constraints in current funding for street maintenance.
Effectiveness of Pavement Condition Analysis Reports is Uncertain
Each year, the City contracts an engineering firm to conduct data collection and analysis
on the conditions of city streets – the Pavement Condition Analysis report. The firm
assigns each segment of road a score based on the number of distresses present which
in turn is input into an algorithm to determine what type of maintenance should be
carried out and when. The firm then recommends a three-year plan for maintenance
and rehabilitation, based on estimated deterioration rates from current distresses.
In an effort to ensure an unbiased report, the City hired an outside engineering firm to
conduct the pavement condition analysis. Prior to 2012, the City used in-house
personnel to annually evaluate street conditions and recommend appropriate
maintenance for the year. The shift to using an independent engineering firm for
analysis was motivated by a desire for impartiality regarding street maintenance
performance and funding estimates. The first engineering firm to conduct the pavement
condition analysis only did so in 2012 before the City decided to utilize a different firm
for the 2013 and 2014 reports due to questions regarding the quality of the report.
The Pavement Condition Analysis report is used only as an aide, not as a comprehensive
pavement management plan that can contribute to operational scheduling. This aligns
with the Pavement Condition Analysis report data, which only gives the year in which it
believes the maintenance should be conducted, not an actual timetable within those
years. Management stated that it usually sends a Streets employee to look over the
streets to check if the Pavement Condition Analysis report gave an accurate
recommendation for the road before the division begins maintenance work. When we
assessed street conditions on site, we determined that sometimes recommendations
were not appropriate for given street segments, supporting the necessity of street
condition review by the division. The actual scheduling of street maintenance is done by
management and is decided based on which job seems most pressing. The report does
not seem to offer a logical long term pavement management plan that could be
appropriately scheduled given the shifting yearly estimates that are stated in the report
or the amount of revision necessary to meet the City’s desired level of quality. This year,
data collection and report finalization occurred between October 2014 and May 2015,
indicating that an annual report may be unnecessary. It was noted that the report did
Streets Audit 23
not offer consistency in strategy and consistency in year for adjacent street segments on
a year by year basis.
However, the Pavement Condition Analysis report is useful for giving a general idea
about expected deterioration and current street conditions during the three-year
estimation period, which can contribute to scheduling and development of the annual
work plan. Hiring an outside engineering firm to develop a roadway condition analysis is
helpful in three major ways: (1) as a method of independently evaluating street crew’s
performance in maintaining city streets, (2) as a means to assess overall street
conditions, and (3) estimating funding needed to maintain the City’s roadways. For
these reasons, we recommend that an engineering firm still be hired to provide a
roadway condition analysis to mainly be used as a strategic document to assess street
maintenance performance and funding needs for upper level management and policy
makers. Contracting with an outside firm to conduct this work on an annual basis,
however, may not be warranted.
Research into an alternative possibility for creating an unbiased record of pavement
conditions is currently being done. There is the possibility of using a DTS mobile asset
collection vehicle for in-house data collection and analysis. This may address concerns
for both the scheduling component of the report and the necessity for an impartial
assessment of street conditions.
Recommendation: Consider only having an engineering firm conduct a Pavement
Condition Analysis every three years and in-house in years in between. As an option to
maintain the highest level of objectivity, the City should continue to investigate using a
DTS mobile asset vehicle for in-house data collection and analysis.
The City Should Consider Modifying its Use of Contractors
Contractors are used by the City to complete major projects, assist on overlays, and
conduct preventative maintenance like crack sealing. However, one of the large benefits
of contractors is related to the effect of weather on street maintenance work and
increased schedule flexibility.
Street maintenance is unique because the weather has a large impact on how much
work can be completed depending on seasonal changes and unexpected weather
events. For instance, the large amounts of rain in June of 2015 meant that maintenance
involving asphalt could not be conducted until the roads had dried sufficiently. Rain
stronger than a slight drizzle means that the asphalt plant closes and therefore repairs
requiring asphalt cannot be performed. In addition, when water is present in the road
while asphalt is being laid, asphalt will not properly adhere to the surface and the road
is more likely to experience major flaws and distresses more quickly. Asphalt also should
24 Streets Audit
not be lain when the temperature are less than 40-50 degrees so temperature
fluctuations must be taken into account.
The use of contractors gives the Streets Division more flexibility, allows for more
variability in workload, and assists when there are unforeseen circumstances that can
affect scheduled maintenance. Weather, vacant positions, broken-down equipment, and
injured personnel can all disrupt maintenance. Generally, street crews repair any failures
or distresses in preparation for overlays and then contractors conduct the actual overlay.
Crew members perform overlays but generally only for shorter street segments. The
Streets Division does not have enough resources to complete all desired street
maintenance in-house. Therefore, contractors are essential for completing scheduled
maintenance tasks. The City should consider letting contractors take over millings and
overlays completely due to the technical and expense costs that come with using and
maintaining the necessary equipment, plus the amount of time and cost that overlays
require. This system has been implemented by peer cities like Bryan and Sugar Land,
though it should be noted that Sugar Land’s roadways are primarily concrete.
However, we acknowledge that flexibility is an important and necessary part of street
maintenance and that contractors, while important contributors, are limited by the
amount of work required of them by multiple customers. While there are certain years
when contractors may be able to carry the workload associated with the amount of
overlays the City requires, there may be times when they cannot, limiting their
usefulness as an asset to schedule flexibility. Therefore, there may be circumstances
where city staff performing overlays is justified. This requires the use of equipment that
is necessary for those tasks, such as the milling machine and paver. The Streets Division
should retain the capability to do overlays in these circumstances, but overlays should
not be the primary maintenance technique. Instead, preventative techniques should be
the heart of a long-term street maintenance program.
Recommendation: The City may want to consider allowing contractors to take over a
larger percentage of milling and overlay projects. Doing so would allow the Streets
Division to reallocate in-house activities to do more preventative maintenance.
Being Responsive to Citizen Complaints Comes at a Cost
Preventive maintenance is usually the first priority when implementing a planned
pavement maintenance program and provides the highest return on investment for
pavements. In a 2008 report the City contracted to analyze whether changing pavement
thickness in College Station would be a feasible choice, emphasis was put on the
importance of a preventative maintenance plan for the City. Preventive maintenance
consists of both localized maintenance (e.g. crack and joint sealing and patching) and
global maintenance (e.g. surface sealing). The 2013 Pavement Condition Analysis Report
also focused on preventative maintenance, stating that preventative maintenance was
Streets Audit 25
treated as the highest priority in decision tree calculations, with all crack seal, patching,
and chip seal assigned first in order to prevent further degradation of pavement as
much as possible.
Chip sealing and crack sealing are two cost-effective techniques that the City has
historically used to extend the useful life of its roadways. Prior to 2010, chip seal was a
useful method used to seal cracked roadways. However, chip sealing in non-rural roads
was discontinued over five years ago due to citizens’ complaints concerning texture and
appearance. This led to a focus on crack sealing as the next best cost-effective
preventative maintenance measure. But recently, the amount of crack sealing has
decreased significantly in reaction to further citizen complaints over the aesthetic look of
crack sealed streets. This has led to an increase in overlays and level ups. Meanwhile,
Public Works has been exploring alternative preventative maintenance methods in order
to find the most cost-effective approach that will satisfy citizens’ desires for an
aesthetically pleasing street.
Between March 2013 and July 2015 some crack sealing was performed by outside
contractors, but at levels below previous crack sealing or chip sealing efforts We
examined work order data from the 29 months after this shift and compared it to the
previous 38 months (Jan 2010 – Feb 2013) when crack sealing was performed in-house.
We found that average maintenance costs and labor hours were greater while the
number of work orders completed decreased during the months when crack sealing
wasn’t performed in-house (see Table 5 below).
Table 5: Comparison of Before and After Crack Sealing in-house Suspension
Crack sealing
Suspension
Avg. Cost
per Month
Avg. Hrs.
per Month
Avg. Number of
WO Complete per
Month
Before $83,407.42 1,407.4 37
After $87,355.59 1,471.6 31
Crack sealing
Suspension
Avg. Cost
per Year
Avg. Hrs.
per Year
Avg. Number of
WO Complete per
Year
Before $1,000,889.14 16,888.5 447
After $1,048,267.09 17,659.7 372
Overlays and other corrective techniques are more costly and labor intensive. After
analyzing work order data from the past five years, we found crack sealing to be the
least expensive maintenance method and overlays to be the most expensive. A
breakdown of the cost of each maintenance type is described in Table 6 on the next
page.
26 Streets Audit
Table 6: Costs per Maintenance Type (2010 – 2015)
Work Type Avg. Hrs. per WO Avg. Cost per Hr. Avg. Materials
Cost per Hr.
Crackseal 23.08 $12.47 $1.96
Sealcoat 24.00 $15.83 $3.53
Potholes 19.20 $20.98 $8.76
Failures 59.01 $55.55 $45.13
Level Up 54.59 $81.30 $68.70
Overlay 83.77 $96.24 $83.09
Crack sealing and seal coating are preventative maintenance techniques while failure
repair, pothole repair, overlays and level ups are all considered restructuring or repaving
techniques, or corrective maintenance. Given the above costs, it was determined that
the overall costs based on average cost per hour and average materials cost per hour
for preventative techniques versus corrective techniques are as follows:
Table 7: Costs per Maintenance Technique
Maintenance Technique Weighted Avg.
Cost per Hr.
Weighted Avg.
Materials Cost per
Hr.
Preventative (Crackseal and Seal Coat)
$12.50 $1.98
Corrective (Failures, Level Up,
Potholes, and Overlays) $161.06 $49.65
Overall, the corrective techniques that the City uses are over thirteen times more
expensive in total cost and almost twenty-five times more expensive in materials cost
than preventative techniques.
In addition to the considerations of cost, preventative maintenance seems to ensure that
streets remain in better condition for a longer amount of time, lengthening the useful
life of the road before more expensive corrective measures need to be taken. Street
segments that have experienced crack sealing in the last five years had on average 91.5
pavement score out of 100, while assets that had not been crack sealed had an 87.9
pavement score. The average for all assets was about 88.3.
In summary, the Streets Division implemented a fiscally prudent strategy of chip sealing
or crack sealing roads as pavements begin to exhibit signs of distress12 in order to
extend street useful life prior to the need of more expensive corrective measures. In a
desire to be more responsive to citizen complaints, street maintenance resources were
shifted from the preventative maintenance methods of chip sealing and crack sealing to
12 See Figure 3 on page 8 of this report.
Streets Audit 27
corrective maintenance techniques such as mills and overlays. Consequently, being
responsive to citizen complaints over the aesthetic or texture of streets has resulted in a
less cost-effective approach to street maintenance.
The decisions to halt chip (for non-rural roads) and reduce crack sealing due to citizen
complaints, especially when those complaints are driven by aesthetic reasons, should be
reevaluated moving forward. If there is investigation into new techniques for possible
future use, it should occur parallel to ongoing preventative maintenance strategies. The
possibility of a concrete alternative for streets will be addressed in the next sections.
Recommendation: Reallocate more resources for preventative maintenance techniques,
such as crack sealing, chip sealing (or other comparable techniques) for asphalt roads.
Strengthening Development Standards Would Lead to Longer Useful Lives
In addition to city-built streets are those that are constructed by developers, primarily in
residential areas. The City has a growing number of development projects due to the
continuous influx of new residents.
Due to the nature of the development business, developers do not have strong
incentives to build streets beyond the lowest possible standards. Developers are driven
by profit first and foremost. Therefore, the development community is likely to contest
increased costs of production or added building requirements set forth by the City. Given
that developers currently build most local streets and minor collectors, the standards
they must reach are important for ensuring that streets can meet the full range of their
useful life.
There are two enforcement mechanisms in place to try and ensure that developers
adhere to the standards set forth by the Bryan-College Station Unified Guidelines. The
first of these mechanisms is the
warranty, which lasts for one year from
the completion and approval of the
road. After that period of time, the
Streets Division takes over maintenance
of the road. However, a year-long
warranty is a short amount of time to
recognize flaws in a street and any
underlying faults that can cause major
problems shortly after that warranty
period will then be handled by the City.
An example of a developer-built
residential road that was not built to
standards is Baker Meadow Loop in the
Figure 8: Baker Meadow Alligator Cracking
28 Streets Audit
Creek Meadows subdivision, which is a year old and has already developed severe
alligator cracking and other obvious distresses.
Possible reasons why this may have occurred are poor quality construction (subgrade
not being installed properly), poor quality material (asphalt with shingles and wrap
rather than pure emulsion and aggregates), and poor workmanship. This road in
particular demonstrates that the warranty system may not be an effective deterrent for
subpar street construction, especially as in this case the developer will be repairing the
road after construction is completed.
The second enforcement mechanism is the city inspection process. This process, while
thorough, is not without the possibility of fraud or deceit. There are only four inspectors
for a large number of projects occurring within the City. Inspectors do not take samples
or test materials themselves. Instead this process is contracted out to labs, who, then
send the results to the inspectors with the data showing whether the developers have
passed or failed the required tests. Therefore, inspectors are not required to go out to
sites, although we were told and observed that most inspectors choose to do so in order
to verify the numbers and processes that the lab technicians are conducting on site.
There have also been instances in which inspectors said that a street did not meet
requirements but were instructed to pass the street anyway, despite it not meeting
standards. This constitutes a flaw in the effectiveness and objectiveness of the
inspection process. These weaknesses in the inspection mechanism can exacerbate the
possibility of poor construction and existing flaws in development standards.
In addition, when developers build roads that are residential, the City provides them
with specifications for those types of roads without considering traffic load. During the
first few years of the road’s life it experiences a traffic load that is much greater than a
normal residential street because of continuing development in the subdivision. Concrete
trucks, dump trucks, and other heavy equipment put increased pressure on the road and
cause shifting underneath the pavement, leading to abnormal rates of distress. Baker
Meadow Loop’s condition could be exacerbated by this problem, as we watched a heavy
construction truck coming onto the street during our fieldwork. Recently a consultant
evaluated how asphalt and concrete support different traffic loads in College Station and
what the ideal thickness of road building materials would be to ensure that streets
perform as long as they should and as well as they should. This Flexible and Rigid
Pavements report, located in Appendix C, noted that traffic loads are key in
understanding how long a street may last and how well it is likely to perform,
mentioning in particular the construction trucks that travel down residential roads during
development. The current specifications then do not seem appropriate for the type of
loads that the streets are bearing, especially in development areas.
Streets Audit 29
Increasing pavement thickness and requiring more virgin materials in construction could
increase the useful life of the road in residential and collector areas.13 Given the recent
report submitted to the City, there are clear changes that have been suggested and
could function as a starting point for creating new standards. However, updating the
Bryan-College Station Unified Guidelines is done with collaboration from community
developers and the City of Bryan, which may make raising standards difficult for the City
of College Station.
Recommendation: Consider raising standards of street construction for residential
streets.
Concrete Roads Can Reduce Maintenance Costs and Increase Useful Life
The City of College Station currently maintains 310 miles of roads, of which 87 percent
are paved in asphalt and 12.5 percent are concrete. The different classifications of
streets from lightest traffic load to heaviest are local/residential streets, minor collectors,
major collectors, minor arterials, and major arterials. The different types of pavement
and street classifications require different maintenance and development standards.
Therefore, the way in which the City decides to build and maintain its streets has a
direct impact on whether a certain street type is more or less effective. The City almost
exclusively uses concrete in capital improvement projects unless there are extenuating
circumstances that require the use of asphalt. Conversely, developers generally use
asphalt for road construction in residential areas and for collectors as part of
development projects.
While initial costs of concrete street construction are higher than comparable asphalt
street construction, in the long-run total concrete maintenance costs may be less than
asphalt. An analysis conducted by the Public Works Department found that, in capital
improvement project bids, average asphalt costs were $36.00 per square yard and
average concrete costs were $42.00 per square yard—approximately 15 to 25 percent
higher than asphalt. However, the maintenance required for concrete streets is much
less extensive. The Flexible and Rigid Pavements report found that concrete streets can
support more heavy traffic loads for a longer amount of time and that the building
material requirements for streets to meet their potential is less for concrete than for
asphalt. This supports analysis conducted by the City that has found that vehicles with
heavier loads like buses and dump trucks have a larger impact on street conditions and
cause more distress. Where concrete becomes more expensive than asphalt is when
reconstruction must be done. Once a concrete slab has been injured beyond the point of
joint and crack sealing, the entirety of the slab must be removed to replace. However,
the useful life of a concrete street (40 years) is much longer than an asphalt street (20 –
13 Current standards can be found the Bryan-College Station Unified Guidelines.
30 Streets Audit
25 years) so it may be more cost effective in the long run to build concrete streets
rather than asphalt.
Collectors and arterials see the most traffic loads and therefore constructing them in
concrete would be the most cost effective option so as to require less maintenance than
asphalt would need. However, developers currently build many of the minor collectors in
College Station. Given that concrete reconstruction costs are very extensive and
expensive, and given that developers do not have incentive to construct streets to the
highest standard, the City should consider building streets that are classified as minor
collector or higher. The City has greater incentive to ensure that concrete streets are
built up to standards, which means that the street should be less likely to fail and have a
greater useful life. There are three funding options for implementing this strategy: (1)
the City pays for all of the project, (2) a portion of the project is paid for by developers
and a portion paid by the City, or (3) the developers pay for the whole project through a
fee.
In addition, concrete offers a more uniform appearance and is more pleasing to the eye.
This reason was cited in the majority of cities that we interviewed for peer comparisons.
Citizen complaints about aesthetics also appear to be a driving force in policy decisions
concerning street maintenance. However, such a decision to convert to concrete would
be a long-term venture rather than one exclusively dealing with street maintenance.
Such a decision will also have implications for the overall purpose of the Streets Division
and street maintenance program.
Recommendation: Consider having the City build arterials and collectors in concrete.
Conduct further research into a long-term shift to concrete streets.
A Dedicated Maintenance Fund Would Grant Greater Flexibility
Given the seasonal and unpredictable nature of street maintenance work, a separate
fund with rollover capability should be created for the Streets Division. Currently the
Streets Division is under the General Fund and therefore must use all their money by
October 1st because it does not roll over into the next fiscal year. However, weather
patterns during the year can derail the street maintenance schedule and therefore
impact the ability of the Streets Division to use the allotted funds despite the need to
still complete scheduled maintenance. Because zero-based budget system creates
incentives for money to be spent in its entirety prior to year-end regardless if it makes
sense to do so, a better option could be to create a fund that is dedicated to street
maintenance. Doing so would allow the money for maintenance and repair to roll over
each fiscal year as needed for Streets projects.
One possible option for funding is to institute a transportation fee in College Station
similar to a drainage/utility fee. The fee for transportation could be used for
Streets Audit 31
maintenance every year and put into a Streets Maintenance Fund rather than relying
only on the General Fund. The City of Bryan has instituted a fee for street maintenance
called a Transportation fee. The fund is used to help prolong the life of city
infrastructure and assets. By managing and maintaining public right of way
infrastructure, the City of College Station would be able to save taxpayer money by
intervening before full reconstruction is needed. Full street reconstruction can be costly
and time-consuming, therefore maintenance financed by the fee could help reduce these
costs.
Recommendation: Create a dedicated street maintenance fund
32 Streets Audit
Summary of Audit Recommendations
The City of College Station is growing. The population has dramatically increased over
the past two and a half decades, growing by almost three percent each year, and
growth will likely continue.
In the future there will be more streets that will need substantial maintenance. The past
five years has seen a consistent increase in centerline miles in the City, more of which
are then being turned over to the City for maintenance and upkeep. Fifty centerline
miles in five years is substantial.
Maintaining the current street infrastructure and ensuring it does not deteriorate is
important for the City, especially given the expanding population. The City is at a critical
juncture regarding the creation of a long-term strategy for handling the inevitable wear
and tear of city roads. The following recommendations are to help the City’s street
maintenance program function more effectively and efficiently and to address policy
issues that affect city streets’ useful life going forward.
2010
2011
2012
2013
2014 2015
440
450
460
470
480
490
500
510
920 940 960 980 1,000 1,020 1,040 1,060
Ce
n
t
e
r
l
i
n
e
M
i
l
e
s
Population (in hundreds)
Figure 9: Relationship between Centerline Miles and Population (2010-2015)
Streets Audit 33
(1) Reinstitute skill-based pay for heavy equipment operators. Skill-based pay
at the heavy equipment operator level will motivate workers who begin in the light
equipment operator level to learn the skills necessary to advance as quickly as
possible. This would increase efficiency as more workers would have more skills,
and low performers could be more easily distinguished from high performers. Skill-
based pay would also address some of the issues causing higher turnover, such as
feeling undervalued and lack of advancement.
Though we recommend skill-based pay, we also limit this recommendation to the
heavy equipment operator position, not the light equipment operator position. This
is because (1) it creates extra motivation for light equipment operators to gain the
skills necessary to become heavy equipment operators, and (2) limiting skill-based
pay to heavy equipment operators reduces the likelihood of system abuse.
(2) If the City chooses to maintain its current maintenance program strategy,
it should consider increasing funding and reinstituting skill-based pay for
mechanics. Equipment is a key factor in maintaining City streets. If equipment is in
the shop for significant amounts of time it can have a negative impact on street
maintenance productivity. Therefore, having mechanics that are skilled at repairing
the heavy equipment the Streets Division requires will be a necessity. Skill-based
pay may be a way to do this. In the long-term, if the City maintains its current
strategy, it may want to consider hiring on a full-time mechanic to handle heavy
equipment for the Streets Division.
(3) Consider only having an engineering firm conduct a Pavement Condition
Analysis every three years and in-house in years in between. As an option
to maintain the highest level of objectivity, the City should continue to investigate
using a DTS mobile asset vehicle for in-house data collection and analysis. A
pavement condition analysis conducted by a contractor allows for outside expertise
and has a certain amount of value. However, we believe that this analysis need not
be conducted by a contractor every year, as it is more useful as a strategic
document (assessing the above conditions) than a planning document (directing the
Streets Division maintenance schedule day-to-day). In-house data collection and
analysis in the intervening years should be sufficient for understanding street
conditions.
(4) The City may want to consider allowing contractors to take over a larger
percentage of milling and overlay projects. Doing so would allow the
Streets Division to reallocate in-house activities to do more preventative
maintenance. A redirection towards preventative maintenance will help the Streets
Maintenance Division save money in the long-run through prolonging streets’ useful
life and materials cost. The money that is saved could be reallocated to help pay for
the increase in contractor work. However, this redirection does not warrant a
34 Streets Audit
downsize in staffing levels or equipment as the streets crews may still need to do
occasional overlays and much of the equipment needed for overlaying is also
needed for other maintenance techniques.
(5) Reallocate more resources for preventative maintenance techniques, such
as crack sealing, chip sealing (or other comparable techniques) for
asphalt roads. While there have been citizen complaints that led to the current
lack of preventative techniques, the City should not let public pressure result in poor
street maintenance strategies. Preventative techniques also extend the useful life of
streets and keep them in a better condition for a longer amount of time. If there is
investigation into new techniques for possible future use, it should occur parallel to
ongoing preventative maintenance strategies.
(6) Consider raising standards of street construction for residential streets.
Because of continuing development, many developer-built streets in residential
areas experience unusually high strain for a number of years after being
constructed. There are also opportunities for developers to cut corners in building
streets with few consequences. Raising the standards for these streets to adjust to
actual traffic usage could help to solve this problem and save money in the long run
because these streets would need less maintenance. However, a raise in standards
might upset the development community whose focus is on keeping costs low.
(7) Conduct further research into a long-term switch to concrete streets.
Though this would cost more initially, the reduction in necessary maintenance would
likely make up the cost over the useful life of the street. However, it is important
that concrete streets are constructed correctly, otherwise maintenance costs could
be much larger than if the street had been asphalt. Formulating a long-term plan
could include the City taking on the majority of minor collectors rather than having
developers build them and constructing more streets in concrete on a citywide level.
(8) Create a dedicated fund for street maintenance. A dedicated fund for street
maintenance would help to save money from year to year as it could roll over and
would allow the streets management greater flexibility on an annual basis. The
inherent fluctuations that come with weather, contractor supply, and unforeseen
events requires a more flexible method of funding. Creation of this fund would also
be helped by instituting a transportation fee that is assessed to residents like the
drainage/utility fee. This fee would help pay for street maintenance, which could
also help balance the City’s budget.
Streets Audit 35
Appendix A: Skill-based Pay Methodology
Skill based pay is one of the most widely-implemented, poorly understood, and under-
researched organizational practices. In implementing skill-based pay for specific types of
positions in the City of College Station, a system must be put in place to ensure that positions
with skill-based pay meet certain criteria. Ultimately, skill-based pay must assist in furthering
productivity and efficiency of the department in pursuit of the department’s goals. For this
reason, some basic requirements must be met.
Positions Eligible for Skill-based Pay:
1. Entry-level positions
2. Job requires a wide depth/breadth of skills to be performed effectively (flexibility in tasks
done for positions)
3. Skills increase self-management capability
4. Non-clerical/non-exempt positions
5. Hourly positions
The goals of a skill-based pay system are to reward an appropriate balance between employee
flexibility through skill breadth (the ability to do different jobs in the organization); skill depth;
and self‐management skills, all of which are critical in system with supervisors. The jobs that fall
into this category typically require higher skills, flexibility to do different jobs, the ability to work
without close supervision, and a high level of training. Employees must understand the overall
production or service delivery process and respond quickly when problems arise. None of this is
possible if employees know only one job and therefore one small part of the overall process. To
implement skill-based pay (SBP), the following steps must be taken:
1. Identify Positions: Identify potential SBP jobs; that is, a job in which development of
skill depth and/or breadth is possible and desirable.
2. Define Skills: For each job level, identify the specific skills (both depth and breadth)
sought. It is encouraged that employees be included in the skill identification process as
well as which skills are more difficult to master. Each skill on the plan must be directly
applicable to job performance.
3. Determine Benefits: Evaluate the potential costs and benefits of the SBP plan (these
are discussed below); proceed with further consideration of the SPB plan only if the
likely benefits outweigh the costs for the organization.
4. Develop Assessment: Develop the appropriate techniques that will be used to assess
the new skills, knowledge and competencies gained and/or developed.
5. Establish Standards: Establish certification standards and processes for employees to
demonstrate their successful skill acquisition. This can be manifested as different skill
bands.
6. Specify Payment System: Determine the dollar amount or pay percentage of SBP for
the acquired skills, such as indicating the payout for each skill block.
One city department previously utilized skill-based pay for a specific set of its employees. Its
policy is given below. Per the opinion of the audit department, the above guidelines,
36 Streets Audit
indicated in bold, are met through the procedures below. While this exact policy may
not be appropriate for every department, it is an adequate example of a skill-based pay system.
Example – Water Utilities Department
1. Purpose of Skill Band Pay (SBP) system (Determine Benefits, Identify Positions14)
a. Incentivize training in job skills to maximize the value of our Employees
b. Determines skills and experience for employees rate of pay
c. Communicates professional development and advancement roadmap
d. Pay raises up to Midpoint based on achieving an established set of
skills/competencies
e. Possible for employee to reach Midpoint in 3 years
i. Can be less, only if they start above Entry level pay
f. Applies to certain specifically designated Non-Exempt, Non-Clerical positions
g. Applies to new hires or employees promoted/transferred into a Skill Band
position
2. Structure of Skill Band system (Define Skills, Establish Standards)
a. List of Skill Band positions, and any future changes, will be approved by DCM
b. Skill sets required to achieve each Skill Band level will be approved by
Department Director
i. Each Skill on the plan must be directly applicable to job performance
ii. Any substitutions or changes must have written approval of Dept Director
iii. Achievement of each Skill must be signed off by Crew Leader or
Supervisor
iv. Completion of a Skill Band requires Supervisor through Director approval
c. Two acceptable breakdowns of Skill Levels:
i. 6 Skill Levels total; can earn max of 2 per year
ii. 3 Skill Levels total; can earn max of 1 per year
iii. Year is defined to start at hire date (or promotion date)
3. Administration of Skill Band pay (Specify Payment System)
a. Newly hired (or promoted/transferred) employee starts at Entry level pay
i. Existing skills or experience may justify starting at a certain Skill Band
Level
b. Eligibility starts when Probationary period is complete
c. Upon completing all Skill Bands, at Midpoint, employee shifts to Merit Pay system
d. Department Directors will manage their budget for pay increases to be certain
that Skill Band pay increases are funded.
e. If Budget dictates no funding for any pay increases, the Skill Band raises earned
will be deferred until funds are available, but not paid retroactively.
f. Employees shall be eligible for additional City-wide pay increases regardless the
date of any SBP increase
g. Employees who complete all their Skill Bands and transition to Merit Pay shall be
eligible for Merit Pay increases regardless of the date of SBP increases.
14 Guidelines the Audit Department feel are met by this section.
Streets Audit 37
To be consistent throughout the Water Services Department the following process is being
implemented to establish a standard for an employee to ensure timely review and/or approval
of their skill bands. (Develop Assessment, Establish Standards)
1. The employee will email their supervisor requesting the supervisor review the required
skills documentation that was given to them and schedule a date and time within 5
working days from the date the email was sent to review and demonstrate the skill if
needed.
2. If the supervisor does not schedule a review, and demonstration if required, the
employee will resend the email to their supervisor and copy the supervisor’s
superintendent or manager.
3. The supervisor must schedule a review, and demonstration if required, within 3 working
days from the date the email was resent.
4. If the supervisor does not schedule a review, and demonstration of the skill if required,
within 3 days the employee will resend the email and copy the supervisor’s
superintendent or manager and the assistant director. The superintendent or manager
shall schedule a review, and demonstration if required, within 2 days of the date the
email was resent.
The above four steps ensure that an employee’s request, that will potentially impact their rate
of pay is handled in ten working days from the initial request.
If the superintendent or manager has to be copied because of inaction by the supervisor a copy
of the emails shall be placed in the supervisor’s file and be considered during their next
evaluation.
If the assistant director has to be copied because of inaction by the supervisor and
superintendent or manager a copy of the emails shall be placed in all of their files and be
considered during their next evaluation and a verbal warning be given to each.
38 Streets Audit
Appendix B: Management’s Responses to the Audit Recommendations
The following is the Public Works Department’s response to the recommendations made in the City
Internal Auditor’s Office Street Maintenance Audit. Each of the eight recommendations includes a
response describing how the recommendation will be addressed by the Public Works Department.
1. Audit Recommendation
Reinstitute skill-based pay for heavy equipment operators.
Management Response
Management concurs with this recommendation and will work with the City Manager and the Human
Resources Director to reinstitute skill-based pay for heavy equipment operators.
2. Audit Recommendation
If the City chooses to maintain its current maintenance program strategy, it should consider increasing
funding and reinstituting skill-based pay for mechanics.
Management Response
Management concurs with this recommendation and will work with the City Manager and the Human
Resources Director to reinstitute skill-based pay for mechanics.
3. Audit Recommendation
Consider only having an engineering firm conduct a Pavement Condition Analysis every three years and
in-house in years in between.
Management Response
Management concurs with this recommendation. For clarification, in-house analysis would not preclude
data collection and reporting from a contractor in lieu of utilizing Public Works personnel.
4. Audit Recommendation
The City may want to consider allowing contractors to take over a larger percentage of milling and
overlay projects. Doing so would allow the Streets Division to reallocate in-house activities to do more
preventative maintenance.
Management Response
Management concurs with this recommendation and will increase the percentage of contracted milling
and overlay projects.
Streets Audit 39
5. Audit Recommendation
Reallocate more resources for preventative maintenance techniques, such as crack sealing, chip sealing
(or other comparable techniques) for asphalt roads.
Management Response
Management concurs with this recommendation and will work with the City Manager to increase
preventative maintenance techniques that will extend the useful service life of the roadway and meet
citizen expectations.
6. Audit Recommendation
Consider raising standards of street construction for residential streets.
Management Response
Management concurs with this recommendation and will work with the City Manager and the Planning
and Development Services Director to study increasing the design standards for residential streets.
7. Audit Recommendation
Conduct further research into a long-term switch to concrete streets.
Management Response
Management concurs with this recommendation and will work with the City Manager and the Planning
and Development Services Director to study requiring concrete streets.
8. Audit Recommendation
Create a dedicated fund for street maintenance.
Management Response
Management concurs with this recommendation and will work with the City Manager to create a
dedicated street maintenance fund that would roll over unexpended funds from year to year.
40 Streets Audit
Appendix C: Flexible and Rigid Pavements Report
320 Graham Road Office (979) 690-3600
College Station, TX 77845 Facsimile (979) 690-3668
September 24, 2015
City of College Station P.O. Box 9960
College Station, Texas 77842
Attention: Mr. James Smith, P.E., City of College Station Public Works Department
Re: Report of Limited Analysis of Flexible and Rigid Pavements City of College Station, Brazos County, Texas CME Project Number: 55143
Texas Board of Professional Engineers Firm Registration Number: F-1068
Dear Mr. Smith:
CME Testing and Engineering, Inc. (CME) is pleased to submit to the City of College Station (City) this
report of a limited analysis of flexible and rigid pavements.
This letter transmits one (1) electronic copy in pdf format of the report that is entitled “Report of Limited
Analysis of Flexible and Rigid Pavements; City of College Station, Brazos County, Texas.”
The limited analysis was performed in accordance with our proposal to the City that was dated August 12,
2015, and was transmitted to Mr. James Smith, P.E., of the City, on the same date. The proposal was
accepted following the issuance of a purchase order that was dated September 2, 2015, and was received
by CME via the United States Postal Service on September 11, 2015.
The accompanying report focuses on two (2) primary objectives which can be described as follows: (1) the
evaluation of existing minimum flexible and rigid pavement sections outlined in the 2012 Bryan/College
Station Unified Design Guidelines for Streets and Alleys (BCSUDGSA) for residential roadways and minor
collector based on variable subgrade stiffnesses or strengths and (2) performing a limited analysis of various
flexible and rigid pavement sections based on the following factors: (a) variable traffic frequencies that are
typical of the roadway classifications outlined in Table V – Street Classification Definitions of the
BCSUDGSA, i.e. residential roadways, minor collectors, major collectors, minor arterials, and major
arterials; (b) variable percentages of truck traffic that could be considered typical for each roadway
classification, i.e. 2 percent, 4 percent, and 6 percent; (c) variable design periods or analysis periods, i.e. 10
years, 20 years, 30 years, and 40 years; and (d) a single subgrade soil stiffness or strength that is considered
representative of the predominately cohesive subgrade soils, i.e. clays, typically encountered in the College
Station area. The limited analysis was performed utilizing the design methods outlined in the 1993
American Association of State Highway and Transportation Officials (AASHTO) for flexible and rigid
pavement sections.
Mr. James Smith, P.E., City of College Station Public Works Department
Report of Limited Analysis of Flexible and Rigid Pavements
City of College Station, Brazos County, Texas Page 2
C M E T E S T I N G A N D E N G I N E E R I N G , I N C .
The limited analysis presented in the accompanying report focuses predominately on the following types
of pavement materials (with the assumption that the pavement sections are constructed on a subgrade that
can be characterized as having an effective California Bearing Ratio value of 5): (1) flexible pavement
sections consisting of a surface course of hot-mix asphalt concrete, a flexible base course of crushed
limestone, and a subgrade that is chemically stabilized and (2) rigid pavement sections consisting of a
surface course of Portland cement concrete (PCC) and a chemically stabilized subgrade. However,
computations performed as part of the evaluation of existing minimum flexible pavement sections outlined
in the BCSUDGSA for residential roadways and minor collectors also included flexible pavement sections
with a cement stabilized base course. In addition, the evaluation of existing minimum flexible and rigid
pavement sections also considered the pavement sections being founded on a very weak subgrade soil that
exhibited an effective CBR of 3. The results of this limited analysis are presented in the accompanying
report for your review and consideration.
CME sincerely appreciate the opportunity to have performed this work for the City of College Station and
look forward to continuing our working relationship in the future. Please do not hesitate to contact us at
(979) 690-3600 if you have any questions or need additional information concerning this matter.
Sincerely,
CME TESTING AND ENGINEERING, INC.
M. Frederick Conlin, Jr. P.E.
Senior Engineer
GTS:MFC:ts
REPORT OF LIMITED ANALYSIS OF FLEXIBLE AND RIGID PAVEMENTS CITY OF COLLEGE STATION, BRAZOS COUNTY, TEXAS
Prepared for The City of College Station
P.O. Box 9960 College Station, Texas 78842
Prepared by
CME Testing and Engineering, Inc.
320 Graham Road
College Station, Texas 77845 Texas Board of Professional Engineers Registration Number: F-1068
CME Project No. 55139
September 24, 2015
M. Frederick Conlin, Jr., P.E. Taylor Stinson, E.I.T., M.S. Senior Engineer Project Engineer
C M E T E S T I N G A N D E N G I N E E R , I N C .
Limited Analysis of Flexible and Rigid Pavements The City of College Station, TX
ii
TABLE OF CONTENTS
Page
1.0 INTRODUCTION ............................................................................................................................ 1
1.1 PROJECT DESCRIPTION ...................................................................................................... 1
1.1.1 Sources of Project Information ................................................................................... 1
1.1.2 Understanding of Limited Analysis ............................................................................ 1
1.2 OBJECTIVES AND SCOPE OF THE LIMITED ANALYSIS .............................................. 2 1.3 OUTLINE OF REPORT .......................................................................................................... 3
2.0 PAVEMENT DESIGN OUTLINE ................................................................................................... 5
2.1 GENERAL SUMMARY OF PAVEMENT SYSTEMS .......................................................... 5
2.1.1 Flexible Pavement Systems ........................................................................................ 5
2.1.2 Rigid Pavement Systems ............................................................................................. 5
2.1.3 Key Differences between Flexible and Rigid Pavement Systems .............................. 6
2.2 SUMMARY OF METHODOLOGIES TYPICALLY USED FOR PAVEMENT DESIGN .. 7
2.3 PAVEMENT DESIGN METHODOLOGY SELECTED FOR LIMITED ANALYSIS......... 7
2.3.1 DESIGN INPUTS OF SIGNIFICANT IMPORTANCE IN THE 1993 AASHTO DESIGN GUIDE......................................................................................................... 8
2.3.1.1 Design Inputs of Significant Importance for Flexible and Rigid Pavement Design .......................................................................................................... 8 2.3.1.2 Analysis Period............................................................................................ 9
2.3.1.3 Traffic .......................................................................................................... 9 2.3.1.4 Reliability and Overall Standard Deviation .............................................. 10
2.3.1.5 Serviceability ............................................................................................. 10
2.3.1.6 Subgrade Stiffness or Strength .................................................................. 11
2.3.2 Other Design Inputs Pertinent to Flexible Pavement Design .................................... 12 2.3.3 Other Design Inputs Pertinent to Rigid Pavement Design ........................................ 13
3.0 EVALUATION OF MINIMUM PAVEMENT SECTIONS OUTLINED IN THE BCSUDGSA .. 14
3.1 GENERAL ............................................................................................................................. 14
3.2 EVALUATION OF FLEXIBLE PAVEMENT SECTIONS OUTLINED IN THE
BCSUDGSA .......................................................................................................................... 15
3.2.1 Assumptions Incorporated in Minimum Flexible Pavement Section Evaluation ..... 15
3.2.1 Results of Minimum Flexible Pavement Evaluation ................................................ 15
3.3 EVALUATION OF RIGID PAVEMENT SECTIONS OUTLINED IN THE BCSUDGSA 17
3.3.1 Assumptions Incorporated in Minimum Rigid Pavement Section Evaluation .......... 17
3.3.2 Results of Minimum Rigid Pavement Section Evaluation ........................................ 18
3.4 EVALUATION OF “EQUIVALENCY” OF MINIMUM PAVEMENT SECTIONS OUTLINED IN THE BCSUDGSA ....................................................................................... 19
4.0 LIMITED ANALYSIS OF PAVEMENTS BASED ON VARIABLE TRAFFIC FREQUENCIES, MAGNITUDES OF LOADING, AND DESIGN PERIODS ........................................................... 20
4.1 GENERAL ............................................................................................................................. 20
4.2 ASSUMPTIONS MADE PERFORMING LIMITED ANALYSIS OF FLEXIBLE AND RIGID PAVEMENT SECTIONS .......................................................................................... 21
C M E T E S T I N G A N D E N G I N E E R , I N C .
Limited Analysis of Flexible and Rigid Pavements The City of College Station, TX
iii
4.2.1 Similar Assumptions Pertinent to both Flexible and Rigid Pavement Analysis ....... 21
4.2.2 Assumptions Pertinent to Flexible Pavement Section Analysis................................ 22 4.2.3 Assumptions Pertinent to Rigid Pavement Section Analysis .................................... 22
4.3 RESULTS OF LIMITED FLEXIBLE PAVEMENT ANALYSIS ........................................ 22 4.4 RESULTS OF LIMITED RIGID PAVEMENT ANALYSIS ............................................... 28
5.0 CONCLUSIONS AND RECOMMENDATIONS ........................................................................... 35
5.1 OBJECTIVE OF LIMITED ANALYSIS REVISITED ......................................................... 35 5.2 CONCLUSIONS .................................................................................................................... 35 5.3 RECOMMENDATIONS FOR FUTURE STUDIES AND PRACTICES ............................ 36
LIST OF TABLES
Page
Table 3.1.1 Minimum Pavement Thickness Criteria (Adapted from Table VIII of the BCSUDGSA) . 14
Table 3.2.1 Design Inputs Selected for Limited Analysis of Minimum Flexible Pavement Systems
Outlined in the BCSUDGSA .............................................................................................. 15
Table 3.2.2 Maximum Allowable Design ESALs for Minimum Flexible Pavement Sections Outlined in the BCSUDGSA (Based on Assumptions Outlined in Section 3.2.1 and Table 3.2.1) .. 16
Table 3.3.1 Design Inputs Selected for Limited Analysis of Minimum Rigid Pavement Systems Outlined in the BCSUDGSA .............................................................................................. 18
Table 3.3.2 Variations in Computed Design ESALs based on Variable Subgrade Stiffness ................. 19
Table 4.3.1 Limited Flexible Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of Loading, and Design Periods (ADT = 2,500 VPD) ........................................................ 23
Table 4.3.2 Limited Flexible Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes
of Loading, and Design Periods (ADT = 5,000 VPD) ........................................................ 24
Table 4.3.3 Limited Flexible Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes
of Loading, and Design Periods (ADT = 10,000 VPD) ...................................................... 26
Table 4.3.4 Limited Flexible Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of Loading, and Design Periods (ADT = 20,000 VPD) ...................................................... 27
Table 4.3.5 Limited Flexible Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes
of Loading, and Design Periods (ADT = 40,000 VPD) ...................................................... 28
Table 4.4.1 Limited Rigid Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of
Loading, and Design Periods (ADT = 2,500 VPD) ............................................................ 29
Table 4.4.2 Limited Rigid Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of Loading, and Design Periods (ADT = 5,000 VPD) ............................................................ 30
Table 4.4.3 Limited Rigid Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of
Loading, and Design Periods (ADT = 10,000 VPD) .......................................................... 31
Table 4.4.4 Limited Rigid Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of
Loading, and Design Periods (ADT = 20,000 VPD) .......................................................... 32
C M E T E S T I N G A N D E N G I N E E R , I N C .
Limited Analysis of Flexible and Rigid Pavements The City of College Station, TX
iv
Table 4.4.5 Limited Rigid Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of
Loading, and Design Periods (ADT = 40,000 VPD) .......................................................... 33
C M E T E S T I N G A N D E N G I N E E R , I N C .
Limited Analysis of Flexible and Rigid Pavements The City of College Station, TX
1
1.0 INTRODUCTION
This report was prepared by CME Testing and Engineering, Inc. (CME) for the City of College
Station (City) to document the results of a limited analysis of flexible and rigid pavements.
The limited analysis was performed in accordance with our proposal to the City that was dated
August 12, 2015, and was transmitted to Mr. James Smith, P.E., of the City, on the same date. The proposal
was accepted following the issuance of a purchase order that was dated September 2, 2015, and was
received by CME via the United States Postal Service on September 11, 2015.
The work effort associated with the limited analysis was initiated shortly after CME received the
previously referenced purchase order and was completed on September 24, 2015.
A summary the pavement design methodologies utilized to perform the limited analysis, key
assumptions made during the limited analysis, the results and conclusions of the limited analysis, and
recommendations for future studies and practices that should be considered by the City are presented in this
report for your review and consideration.
1.1 PROJECT DESCRIPTION
1.1.1 Sources of Project Information
Information concerning the limited analysis was provided during two (2) meetings that occurred at
the City’s Department of Public Works offices. The first meeting occurred on June 3, 2015, and was
attended by Messrs. Donald Harmon, P.E. and Alan Gibbs, P.E., of the City, and Messrs. Rick Conlin, P.E.
and Taylor Stinson, E.I.T., M.S., of CME. The second meeting was conducted on August 3, 2015, and was
attended by Messrs. Harmon, Troy Rother, P.E., and James Smith, P.E. of the City, and Messrs. Conlin and
Stinson of CME. Additional information pertaining to the limited analysis was obtained in an e-mail
communication from Mr. Smith that was dated August 6, 2015.
1.1.2 Understanding of Limited Analysis
Based on the conversations of the previously referenced meeting conducted on June 3, 2015, CME
understands that the City is experiencing issues and conflicts related to the existing minimum pavement
design thicknesses for flexible and rigid pavement sections which are outlined in the 2012 Bryan/College
Station Unified Design Guidelines for Streets and Alleys (BCSUDGSA) for residential roadways and minor
C M E T E S T I N G A N D E N G I N E E R , I N C .
Limited Analysis of Flexible and Rigid Pavements The City of College Station, TX
2
collectors. More specifically, some of the flexible pavement sections designed utilizing the minimum
pavement sections outline in the BCSUDGSA are approaching terminal serviceability, i.e. the lowest
acceptable serviceability level of a roadway before resurfacing or reconstruction becomes necessary, shortly
after construction of the minimum flexible pavement section, and as a result are requiring excessive
maintenance. In addition, the City feels there is a discrepancy between the minimum pavement sections
outlined for flexible and rigid pavement sections in the BCSUDGSA, e.g. the structural integrity of the
flexible pavement section is not comparable to the rigid pavement section. Following the initial
conversations of the meeting, CME and the City developed an approach to evaluate design sections of
flexible and rigid pavements based on the following factors: (1) traffic frequencies that are typical for
various roadway classifications outlined in Table V of the BCSUDGSA, i.e. minor collectors, major
collectors, minor arterials, and major arterials; (2) variable percentages of truck traffic that could be
considered typical for each roadway classification, i.e. 2 percent, 4 percent, and 6 percent; and (3) variable
design periods or analysis periods, i.e. 10 years, 20 years, 30 years, and 40 years.
The second meeting was conducted so that additional employees of the City, i.e. Messrs. Smith and
Rother, could voice their options regarding the limited analysis proposed by CME. Based on the
conversations of the second meeting and the previously referenced e-mail communication from Mr. Smith,
the City requested that an additional traffic frequency be added to the limited analysis so that a typical
residential roadway would also be reflected in the analysis. In addition, the City requested for CME to
evaluate the existing minimum flexible and rigid pavement sections outlined in the BCSUDGSA for
residential roadways and minor collectors.
1.2 OBJECTIVES AND SCOPE OF THE LIMITED ANALYSIS
The specific objectives of the limited analysis were to perform the following:
Analyze the existing minimum flexible and rigid pavement sections outlined in Table VIII – Minimum Pavement Thickness Criteria of the BCSUDGSA for “equivalency”. Two effective
subgrade stiffnesses or strengths were considered during this evaluation.
Perform a limited analysis of flexible and rigid pavement sections by evaluating three variable conditions which will include: (1) variations in average daily traffic (ADT); (2) variations in
the percentage of trucks that contribute to the ADT; and (3) variations in the analysis period or
design period. Stiffness or strength of the subgrade or foundation soils were not varied in this portion of the analysis.
Evaluate the findings of the limited analysis to determine if any conclusions can be made.
Based on the finding of the limited analysis and CME’s experiences with pavement design in the College Station area, provide recommendations for future studies and/or practices that
should be considered by the City.
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It should be recognized that the results of the limited analysis are meant to provide examples of
variations in design pavement sections based on numerous assumptions which are outlined in subsequent
sections of this report. The results of the limited analysis are not meant to be utilized for the design of a
specific roadway in the College Station area due to variable soil conditions. Prior to the selection of a final
pavement section for a given roadway, numerous variables should be evaluated by a Professional Engineer
to ensure that the pavement section is adequate for the roadway under consideration. Some of the influential
variables that should be evaluated include the following, among others: (1) anticipated traffic volumes or
frequency over the course of the pavements design life or design period; (2) anticipated magnitudes of
loading that will occur over the pavements design life, e.g., volume of heavy trucks; (3) the pavement
materials utilized to support the anticipated traffic frequency and magnitudes of loading and the reliability
of the assumed properties of such materials; (4) drainage conditions along the roadway alignment; and (5)
the underlying subgrade soils that will support the pavement section and the anticipated traffic frequency
and magnitudes of loading.
1.3 OUTLINE OF REPORT
Section 2.0 of the report presents an overview of flexible and rigid pavement sections and the design
methodologies utilized to perform the limited analysis. In addition, Section 2.0 discusses some of the
variables that are considered to significantly impact the required thickness of a flexible or rigid pavement
section based on the design guidelines selected for the limited analysis of flexible and rigid pavements.
Section 3.0 presents the results of the evaluation of the existing minimum pavement sections
outlined in the BCSUDGSA for residential roadways and minor collectors. Assumptions made for the
evaluation are outlined and the results are present in terms of the maximum allowable traffic “capacity” or
maximum allowable equivalent single-axle loads (ESALs) of each type of pavement section. In addition,
an extended evaluation is performed to estimate the theoretical design life of the minimum pavement
sections based on CME’s various design assumptions. The minimum pavement sections are evaluated based
on two effective subgrade stiffnesses or strength that could be considered typical of the cohesive soils, i.e.
clays, prominent in the College Station area.
Section 4.0 presents the assumptions that were developed by CME, with assistance from the City,
to perform the limited analysis of flexible and rigid pavements based on the following factors: (1) variable
traffic frequencies; (2) variable percentages of truck traffic; (3) variable design periods or analysis periods;
(4) a single subgrade soil stiffness or strength. In addition, the section presents the results of the limited
analysis in tabular form based on the outlined assumptions.
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Section 5.0 provides a discussion of the findings of the limited analysis and attempts to formulate
conclusions from the generated data. In addition, the section also provides recommendations for future
studies and practices that may be considered by the City.
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2.0 PAVEMENT DESIGN OUTLINE
2.1 GENERAL SUMMARY OF PAVEMENT SYSTEMS
Pavements with asphalt and concrete surface layers have been used in the United States since the
late 1880s (FHWA, 1996). In general, a pavement system with an asphalt surface course is referred to as
flexible pavement system and a pavement system with a concrete surface course is referred to as a rigid
pavement system; however, flexible and rigid pavement sections cannot be characterized purely by the
surface course utilized to construct the pavement system. The following subsections provide a more detailed
description of the components that contribute to a flexible and rigid pavement system. In addition, the
subsections present a summary of the theory associated with each pavement system. Key differences
between the two pavement systems are also addressed.
2.1.1 Flexible Pavement Systems
Flexible pavements in general consist of an asphalt-bound surface course or layer on top of unbound
base and subbase granular layers over the subgrade soil. In some cases, the subbase and/or base layers may
be absent, e.g. full-depth asphalt pavements, while in others the base and/or subbase layers may be stabilized
or bound using cementitious or bituminous admixtures. A flexible pavement section may also incorporate
a chemical stabilization agent such as lime or cement to increase the strength of the surficial subgrade soils.
The design of a flexible pavement system is based on load distributing characteristics of the
component layers used to develop the pavement. More specifically, the flexible pavement layers transmit
the vertical or compressive stresses imposed by vehicles through grain transfer across the contact points of
the materials comprising the pavement layers. As one might expect, the vertical compressive stresses are
largest on the pavement surface directly under the wheel loads and is equal to the contact pressure under
the wheels. Due to the flexible nature of the pavement system, the stresses are distributed in the shape of a
truncated cone and the stresses decrease as the stresses travel through the layers comprising the pavement
system. Due to the nature of stress distribution in a flexible pavement system, materials with larger strengths
are typically used at or near the surface of the pavement section; whereas materials with lower strength are
typically utilized in the lower layers of the pavement section.
2.1.2 Rigid Pavement Systems
Rigid pavement in general consist of Portland cement concrete (PCC) slabs constructed on a
granular base layer over the subgrade soil. The base layer serves to increase the effective stiffness of the
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slab foundation; however, the base course may not always be utilized to construct a rigid pavement system.
The base course is typically omitted when a rigid pavement surface course is constructed over a coarse-
grained, i.e. granular or sand, subgrade or when a roadway will be subjected to low traffic volumes, e.g.
typically less than 1 million equivalent single axle loads (ESALs). A rigid pavement system is typically
designed as one of the following types of pavement systems: (1) continuously reinforced concrete pavement
(CRCP); (2) jointed reinforced concrete pavement (JRCP); or (3) jointed plain concrete pavement (JPCP).
Another method of rigid pavement design is known as post-tensioned concrete pavement design; however,
this method of rigid pavement design is typically only incorporated for airport pavements and not for
roadways.
The design of a rigid pavement system is based on providing a structural PCC slab of sufficient
strength to resist the loads of traffic. The rigid characteristics of the pavement system are associated with
the rigidity or flexural strength and high modulus of elasticity of the PCC slab which distributes traffic
loads over a relatively wide area.
2.1.3 Key Differences between Flexible and Rigid Pavement Systems
The primary difference between a flexible and rigid pavement system is associated with the manner
in which each type of pavement distributes traffic loads over the subgrade. As previously discussed, a rigid
pavement system is designed with a very high stiffness that allows for the distribution of loads over a
relatively wide area of subgrade. Due to the high stiffness of a rigid pavement system, a major portion of
the pavements structural capacity is derived from the PCC slab itself. Alternatively, the load carrying
capacity of a true flexible pavement system is derived from the load distribution characteristics of a layered
system. Due to the manner in which a flexible pavement system distributes loads, minor variations in
subgrade strength have a significant influence on the structural capacity of the flexible pavement system.
On the other hand, minor variations in subgrade strength have less influence on the structural capacity of a
rigid pavement system due to the distribution of loads over a wide area. Additional key differences between
flexible and rigid pavement systems can be summarized as follows:
Flexible pavement systems will reflect the deformations of pavement layers and subgrade layers on the surface; whereas rigid pavement systems are typically able to bridge over
localized failures and isolated areas of inadequate support.
Temperature variations in flexible pavement systems do not produce stresses in flexible pavement; whereas temperature changes in rigid pavements can induce heavy stresses in rigid
pavements.
Flexible pavements are capable of recovering from larger stresses to some extent; whereas excessive deformations in rigid pavement systems are not recoverable, i.e. settlements are
permanent.
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Flexible pavement systems will often times require significant maintenance costs during the
design life of the pavement system; whereas properly designed and constructed rigid pavement systems will require much less maintenance.
2.2 SUMMARY OF METHODOLOGIES TYPICALLY USED FOR PAVEMENT DESIGN
There are generally three approaches that can be employed to design flexible and rigid pavements:
(1) an empirical approach; (2) a mechanistic approach; or (3) a mechanistic-empirical approach. An
empirical approach is one that is based solely on the results of experiments or experience. Observations
made during an experiment or over a given period of time are used to establish correlations between inputs
and the outcomes of a process, e.g. pavement design and pavement performance. Empirical pavement
design procedures generally do not have a scientific basis; however, the procedures and design inputs
implemented in the design process are typically considered reasonable and have been confirmed based on
previous experiments or experience. On the other hand, a mechanistic approach is based on the theories of
mechanics and relates a pavements behavior and performance to traffic loadings and environmental
influences. As the name suggest, a mechanistic-empirical approach to pavement design combines features
from both the mechanistic and empirical approaches.
In the United States, there are various flexible and rigid pavement design procedures currently
available for the design of a specific paved roadway. One of the most well-known design guides was
developed by the American Association of State Highway and Transportation Officials (AASHTO). The
AASHTO design guide utilizes empirical methods that are based on field performance data measured
during a series of experiments which were conducted between 1958 and 1960 in Ottawa, Illinois. Several
design methodologies based on mechanistic-empirical concepts include the Asphalt Institute design
procedure for flexible pavements (Shook et al., 1982), the Portland Cement Association (PCA) design
procedure for rigid pavements (PCA, 1984), and the National Cooperative Highway Research Program
(NCHRP) 1-37A design procedure (NCHRP, 2002) for both flexible and rigid pavements. Although several
attempts have been made to develop purely mechanistic design procedures for pavement, a fully
mechanistic design approach does not exist at this point in time due to the overall complexity of flexible
and rigid pavement systems, e.g. modeling environmental impacts, forecasting future traffic, modeling
bound and unbound materials as a system, etc.
2.3 PAVEMENT DESIGN METHODOLOGY SELECTED FOR LIMITED ANALYSIS
As previously discussed in Section 2.2, the AASHTO design guide is one of the most well-known
design guideline for both flexible and rigid pavements. The design procedures outlined in the most recent
1993 AASHTO design guide, while complex in theory, are much simpler than those outlined by the NCHRP
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1-37A design procedure which is currently considered one of the more sophisticated design approaches for
flexible and rigid pavements. Due to the relative simplicity of the 1993 AASHTO design procedures
compared to other design procedures, the 1993 AASTO design guide was selected to perform the
computations required for the current limited analysis. It is worth noting that the AASHTO design guide is
the primary document used to design new and rehabilitated highway pavements. Approximately 80 percent
of all states use the AASHTO pavement design procedures, with a majority using the 1993 version (NCAT,
2014).
2.3.1 DESIGN INPUTS OF SIGNIFICANT IMPORTANCE IN THE 1993 AASHTO DESIGN
GUIDE
As previously discussed, the 1993 AASHTO design guide incorporates an empirical approach that
can be utilized to design flexible and rigid pavements. The empirical approach outlined in the design guide
focuses predominately on relating traffic, pavement structure, and pavement performance. The overall
approach of the 1993 AASHTO design guide for flexible and rigid pavements is to design for a specified
serviceability loss at the end of the design life of the pavement. The serviceability loss that occurs over the
design life of a pavement system is considered to be affected by traffic and environmental effects, such as
may be associated with frost heave and/or swelling of subgrade soils. The following subsections of this
report discuss the design inputs that are considered of significant importance when utilizing the 1993
AASHTO design procedures to design both flexible and rigid pavements. In addition, inputs used
exclusively for a given type of pavement design procedure are discussed.
2.3.1.1 Design Inputs of Significant Importance for Flexible and Rigid Pavement Design
The empirical expressions outlined in the 1993 AASHTO design guide for both flexible and rigid
pavements rely heavily on five (5) input variables that are outlined below, along with a reference to the
subsequent subsections of this report in which the design inputs are discussed:
Analysis period (see Section 2.4.1.1);
Traffic (see Section 2.4.1.2);
Reliability and Overall standard deviation (see Section 2.4.1.3);
Serviceability (see Section 2.4.1.4); and
Subgrade soil stiffness or strength (see Section 2.4.1.5).
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2.3.1.2 Analysis Period
The analysis period or design period refers to the time that a pavement is intended to last before it
needs to be rehabilitated or reconstructed. The term analysis period is synonymous to the overall duration
that the design strategy must last. The analysis period may be identical to the performance period; however,
performance limitations may require planned rehabilitation within the desired analysis period, e.g. the
analysis period may encompass multiple performance periods. The 1993 AASHTO design guide provides
recommended ranges for flexible and rigid pavement analysis periods based on the following roadway
classifications and anticipated traffic volumes: (1) high-volume urban roadways – 30 to 50 year analysis
period; (2) high-volume rural roadways – 20 to 50 year analysis period; (3) low-volume paved roadway –
15 to 25 year analysis period; and (4) low-volume aggregate surface roadway – 10 to 20 year analysis
period. As one might expect, multiple performance periods are typically assumed during the analysis period
for high-volume roadways; whereas the analysis period may be identical to the performance period for low-
volume roadways.
It’s worth noting that low-volume roadways constructed with a rigid pavement section are typically
evaluated with longer analysis periods due to the costs associated with constructing or rehabilitating a rigid
pavement system. The analysis period for any rigid pavement system will typically range from 30 to 50
years regardless of the roadway classification and anticipated traffic volume.
2.3.1.3 Traffic
Traffic is one of the most important design inputs in pavement design. In order to accurately model
traffic for a given roadway, the initial traffic volume, traffic growth, directional distribution, lane
distribution, and traffic type (vehicle characterization) must be established. The 1993 AASHTO design
guide is based on cumulative 18 kip equivalent single-axle loads (ESALs) that are anticipated to travel
across the design lane of a roadway during a certain analysis period or design period. ESALs provide a
means of expressing traffic loading from numerous types of vehicles with various axle configurations
and loadings in terms of unit 18 kips single-axle loads. Thus, every vehicle, no matter what the axle
loading, can be expressed as a number of 18 kips equivalent single-axle load units. For example,
passenger cars with single-axle loads of 1 kip can have an ESAL of 0.00018, whereas a large truck
with a single-axle loading of 20 kips can have an ESAL of 1.51. Therefore, the type of truck traffic
and percentage of truck traffic expected to travel on a given roadway will have a significant impact
on the ESALs calculated, whereas the volume of passenger cars expected to travel on a given roadway
will have a significantly smaller impact on the ESALs calculated.
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It is worth noting that the ESALs calculated for a rigid pavement will differ slightly from those
calculated for flexible pavement subjected to the same frequency of traffic and magnitudes of loading over
a given period of time. ESALs are dependent upon the pavement type (flexible or rigid) and the pavement
structure (structural number for flexible pavements and slab depth for rigid pavements). This is primarily
due to the way loads are distributed by the two pavement systems (see Sections 2.1.1 and 2.1.2 for a
summary of load distributions in flexible and rigid pavement systems). As a rule-of-thumb, the
1993 AASHTO design guide, recommends the use of a multiplier of 1.5 to convert flexible ESALs to rigid
ESALs (or a multiplier of 0.67 to convert rigid ESALs to flexible ESALs), although the conversion
multiplier is not constant and can vary with different factors.
2.3.1.4 Reliability and Overall Standard Deviation
Design reliability, R (%), is defined in the 1993 AASHTO design guide as “the probability that the
design will perform satisfactorily over the analysis period.” The reliability must account for uncertainties
in traffic loading, environmental conditions, and construction materials. The 1993 AASHTO design
procedure accounts for the uncertainties by incorporating a reliability level to provide a factor of safety into
the pavement design and thereby increases the probability that the pavement will perform as intended over
its design life. The 1993 AASHTO design guide recommends ranges in reliability based on a roadways
functional classification as follows: (1) local roadways – 50 to 80 percent; (2) collectors – 75 to 95 percent;
(3) principal arterials – 75 to 99 percent; and (4) interstates and other freeways – 80 to 99.9 percent. The
selection in reliability will also depend on whether the roadway is located in an urban or rural area. As one
might expect, higher reliabilities are typically utilized for roadways subject to higher frequencies of traffic
and larger magnitudes of loading in an urban setting, whereas lower reliabilities are selected for roadways
with lower anticipated traffic frequencies in a rural environment.
In addition to reliability of pavement performance, the 1993 AASHTO design guide also
incorporates an overall standard deviation, So, which is used to describe how well the design inputs for a
given roadway fit the 1993 AASHTO design equations. For flexible pavements, values of overall standard
deviation typically range between 0.40 and 0.50. For rigid pavements, values of overall standard deviation
typically range between 0.30 and 0.40. The lower the overall standard deviation, the better the equation
models the data for a given roadway.
2.3.1.5 Serviceability
The serviceability of a pavement system can be simplistically compared to smoothness of the
pavement riding surface and is quantified by the “Present Serviceability Index” (PSI). PSI values
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theoretically range from 5 to 0 with the value of 5 corresponding to a roadway with the best “rideability”
characteristics and the value of 0 corresponding to a roadway with the worst “rideability” characteristics.
However, the actual range of PSI values for pavements is between about 4.5 and 1.5. The initial
serviceability, Pi, of a roadway corresponds to road conditions immediately after construction and the
terminal serviceability, Pt, of a roadway corresponds to the lowest acceptable PSI before resurfacing or
reconstruction become necessary. The initial serviceability of a rigid pavement is typically considered
higher than the initial serviceability of a flexible pavement. Typical values of initial serviceability range
from 4.5 (for rigid pavements) to 4.2 (for flexible pavements). The terminal serviceability of a pavement is
typically selected based on a roadway functionality classification and can be summarized as follows: (1)
2.0 for secondary roads and local residential streets; (2) 2.25 for minor collectors and commercial streets;
and (3) 2.50 for major collectors and arterials. Therefore, a typical allowable serviceability loss due to
traffic for a local residential street that is constructed with a flexible pavement section is 2.2 (4.2 – 2.0 =
2.2). It worth noting that the serviceability loss of a pavement system can also arise from environmental
conditions such as frost heave and/or soil expansion. The 1993 AASHTO design guide provides methods
to determine serviceability loss due to these environmental impacts; however, these methods are considered
crude by many agencies and design firms, and as a result, are often ignored during pavement design
calculations.
2.3.1.6 Subgrade Stiffness or Strength
The stiffness or strength of the subgrade soils supporting a given pavement section are considered
important in the design of any pavement. Subgrade soils are typically characterized in pavement design by
their response to deformation under load, which can be either a measure of the stiffness or strength. Two
common stiffness properties used to characterize the stiffness of subgrade soils are: (1) the resilient
modulus, MR, which is the measurement of the elastic property of a soil (recognizing certain nonlinear
characteristics); and (2) the modulus of subgrade reaction, k, which depends upon the soil’s stiffness and a
slab’s size and stiffness. The 1993 AASHTO design procedure for flexible pavements utilizes the direct
input of the effective modulus of rupture of a subgrade. The effective modulus of rupture is determined
based on the variations in the modulus of rupture of a subgrade as a result of environmental changes, e.g.
increase/decrease in moisture content, changes in temperature, etc. On the other hand, 1993 AASHTO
design procedure for rigid pavements utilizes the direct input of a subgrade’s effective modulus of subgrade
reaction, k, which is determined by considering similar environmental changes.
The resilient modulus is a basic material property that can be measured directly in the laboratory,
evaluated in-situ for nondestructive tests, or estimated using various empirical relations such as soil
classification. The resilient modulus of a subgrade soil is typically estimated based on correlations between
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the resilient modulus and the California Bear Ratio (CBR) values for a subgrade soil. On the other hand,
the modulus of subgrade reaction of the foundation soils (natural soil and embankment) can be measured
by plate bearing tests but is usually estimated from correlations with soil type, soil strength measures such
as the CBR, or by back calculation from deflection testing on existing pavements. The stiffness or strength
of cohesive soils, i.e. clays, is typically much lower than the stiffness or strength of granular soils, i.e. sands.
As previously discussed, the stiffness or strength of a subgrade soil is typically estimated based
results of CBR tests. Typical CBR values for cohesive soils range from 3 to 20; whereas CBR values for
granular soils typically range from 15 to 40 (USACE, 1953). Therefore, the resilient modulus and modulus
of subgrade reaction for a subgrade soil will be much lower for a cohesive soil than for a granular soil.
2.3.2 Other Design Inputs Pertinent to Flexible Pavement Design
There are two other design inputs that are considered for the 1993 AASHTO design procedures
outlined for flexible pavement design. The following paragraphs summarize these inputs and discuss the
importance of each input.
As one might expect, the design of a flexible pavement system will rely heavily on the types of
materials incorporated in the layered pavement system. The 1993 AASHTO design procedure for flexible
pavements requires material properties for each layer which are typically referred to as structure layer
coefficients or layer coefficients. The structural layer coefficient for a material will depend on the stiffness
or strength of the material under consideration and the location of the material within the layered flexible
pavement system. Based on the 1993 AASHTO design guide, the following ranges in structural layer
coefficients were determined for various materials based upon design charts that utilize correlations
between structural layer coefficients and various stiffness and strength parameters such as the CBR, the
elastic modulus of a material, unconfined compressive strength, etc.: (1) hot mix asphalt concrete (HMAC)
surface course with an elastic modulus greater than approximately 350,000 pounds per square inch (psi) –
0.40 to 0.45; (2) cement treated granular base with an unconfined compressive strength ranging from
approximately 200 psi to 650 psi – 0.12 to 0.20; (3) flexible base with a CBR ranging from 40 to 100 – 0.12
to 0.14; and (4) chemically stabilized and compacted subgrade with a CBR ranging from 6 to 100 – 0.06 to
0.14.
Another design input for flexible pavements is the drainage coefficient for unbound materials such
as a flexible base. Drainage coefficients of unbound layers are meant to be selected based on the percentage
of time a material approached a saturated condition and also `based on the quality of drainage in the
pavement layer. It is worth noting that a drainage coefficient basically makes a specific layer thicker if a
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fundamental drainage problem is suspected. A thicker layer may only be of marginal benefit if a drainage
problem is suspected. A better solution to address a potential drainage concern is to utilize very dense layers
in the pavement system or to design a drainage system.
2.3.3 Other Design Inputs Pertinent to Rigid Pavement Design
There are four other design inputs that are considered for the 1993 AASHTO design procedures
outlined for rigid pavement design. The following paragraphs summarize these inputs and also discuss the
importance of each input.
Similar to flexible pavement design, the material properties of a rigid PCC slab are of significant
importance when selecting a design slab thickness. There are two material properties that are considered of
significant importance in the design of a rigid pavement section and these properties include: (1) the 28-
day modulus of rupture, MR, i.e. flexural strength, of the PCC slab and (2) the modulus of elasticity, Ec, of
the PCC slab. The 2011 TxDOT – Pavement Design Manual recommends the following default values be
utilized for rigid pavement design: (1) a 28-day modulus of rupture of 620 pounds per square inch (psi) and
(2) a modulus of elasticity of 5,000,000 psi. It’s worth noting that based on the current correlations that
exist between the 28-day compressive strength of concrete, f 'c, and the two required inputs, the default
values recommended by TxDOT appear to be significantly over-estimated. For example, an elastic modulus
of 5,000,000 psi correlates to a 28-day concrete compressive strength of approximately 7,700 psi based on
the correlation outlined in the American Concrete Institute publication 318 (ACI-318). In addition, the
default value recommended by TxDOT for a 28-day modulus of rupture correspond to a 28-day
compressive strength of approximately 4,750 psi. Based on the correlations outlined in ACI-318, a modulus
of rupture of approximately 570 psi and an elastic modulus of approximately 3,600,000 psi appear to
correlate to cast-in-place concrete with a 28-day compressive strength of 4,000 psi.
Additional required properties for rigid pavement design include a joint transfer coefficient, J,
which depends on various design conditions such as the shoulders constructed with the pavement system
and the type of rigid pavement system design, i.e. JPCP, JRCP, or CRCP, and a drainage coefficient, Cd,
which is similar to the drainage coefficients previously discussed for flexible pavement design.
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3.0 EVALUATION OF MINIMUM PAVEMENT SECTIONS
OUTLINED IN THE BCSUDGSA
3.1 GENERAL
As previously discussed, the City requested CME to evaluate the existing minimum flexible and
rigid pavement sections outlined in the BCSUDHSA for residential roadways and minor collectors. More
specifically, the standard pavement sections that the City requested to be analyzed for “equivalency” are
outline in Table VIII – Minimum Pavement Thickness Criteria found in the BCSUDGSA. In addition, the
standard pavement sections are also outlined on BCS Detail Nos. ST4-04 and ST4-05. The minimum design
sections outlined for flexible and rigid pavements in the BCSUDGSA are summarized below in Table 3.1.1
– Minimum Pavement Thickness Criteria (Adapted from Table VIII of the BCSUDGSA).
Table 3.1.1 Minimum Pavement Thickness Criteria (Adapted from Table VIII of the BCSUDGSA)
FLEXIBLE PAVEMENTS
STREET
CLASSIFICATION
SUBGRADE
TREATMENT BASE MATERIAL SURFACE
TREATMENT
RESIDENTAL 6-in. Lime-Stab. 6-in. Limestone, 6-in. Cement Stabilized Base 2-in. HMAC
MINOR COLLECTOR 6-in. Lime-Stab. 8-in. Limestone, 8-in. Cement Stabilized Base 2-in. HMAC
RIGID PAVEMENTS
STREET CLASSIFICATION SUBGRADE TREATMENT CONCRETE PAVEMENT
RESIDENTAL
(includes alleys) 6-in. Lime-Stab. 6-in.
COLLECTORS AND
PRIVATE LOCAL STREETS 6-in. Lime-Stab. 8-in.
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3.2 EVALUATION OF FLEXIBLE PAVEMENT SECTIONS OUTLINED IN THE
BCSUDGSA
3.2.1 Assumptions Incorporated in Minimum Flexible Pavement Section Evaluation
There were several assumptions that had to be made in order to evaluate the minimum flexible
pavement sections previously outlined in Table 3.1.1 for residential roadways and minor collectors. The
inputs of particular interest included the effective stiffness or strength of subgrade soils supporting the
flexible pavement, the reliability and overall standard deviation that was selected to evaluate the minimum
flexible pavement sections, and the structural layer coefficients that were utilized to develop the design
thickness of each layer comprising the flexible pavement sections. Table 3.2.1 – Design Inputs Selected for
Limited Analysis of Minimum Flexible Pavement Systems Outline in the BCSUDGSA summarizes the design
inputs selected to evaluate the minimum flexible pavement sections. In addition to the inputs outlined in
Table 3.2.1, the following structural layer coefficients were selected for the pavement materials being
evaluated: (1) HMAC – 0.44; (2) Cement stabilized base – 0.16; (3) Flexible base or crushed limestone
base (flex base) – 0.13; and (4) chemically stabilized subgrade – 0.08.
Table 3.2.1 Design Inputs Selected for Limited Analysis of Minimum Flexible Pavement Systems
Outlined in the BCSUDGSA
Street
Classification
R (%)
(Reliability)
So (Overall
Standard Deviation)
EFFECTIVE CBR VALUES OF SUBGRADE
SOIL CONSIDERED
Note 1
Pi (Initial Serviceability)
Pt
(Terminal Serviceability)
RESIDENTIAL 80% 0.5 3 and 5 4.2 2.0
MINOR COLLECTOR 90% 0.5 3 and 5 4.2 2.25
Note: 1. The CBR values considered in the evaluation correspond to a lower bound and upper bound for clay of high plasticity or fat clays (3 to 5)
(FHWA, 2006).
3.2.1 Results of Minimum Flexible Pavement Evaluation
Based on the assumptions previously outlined in Section 3.2.1 and Table 3.2.1, the minimum
flexible pavement sections outline in the BCSUDGSA for residential roadways and minor collectors were
evaluated and the maximum allowable design ESALs for each minimum flexible pavement section was
computed. The results of this evaluation are presented in Table 3.2.2 – Maximum Allowable Design ESALs
C M E T E S T I N G A N D E N G I N E E R , I N C .
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for Minimum Flexible Pavement Sections Outlined in the BCSUDGSA (Based on Assumptions Outlined in
Section 3.2.1 and Table 3.2.1).
Based on the results presented in Table 3.2.2, one can infer that the maximum allowable traffic
“capacity” or maximum allowable ESALs for each type of flexible pavement section will rely heavily on
the effective subgrade stiffness or strength. In addition, it can be deduced that a flexible pavement system
utilizing a cement stabilized base course will out-perform a flexible pavement system utilizing a crushed
limestone base course of the same thickness. However, consideration of reflective cracking of the flexible
surface course may have to be examined with the use of a relatively rigid cement stabilized base course.
Table 3.2.2 Maximum Allowable Design ESALs for Minimum Flexible Pavement Sections Outlined
in the BCSUDGSA (Based on Assumptions Outlined in Section 3.2.1 and Table 3.2.1)
Street Classification Pavement Layers Maximum Allowable ESALs
CBR = 3 CBR = 5
RESIDENTIAL
2-in. HMAC,
6-in. limestone,
and 6-in. lime-stab.
35,000 74,000
2-in. HMAC,
6-in. cement stabilized base,
and 6-in. lime-stab.
57,000 123,000
MINOR
COLLECTOR
2-in. HMAC,
8-in. limestone,
and 6-in. lime-stab.
41,000 87,000
2-in. HMAC,
8-in. cement stabilized base,
and 6-in. lime-stab.
73,000 157,000
Note: 1. Maximum allowable ESALs rounded to the nearest 1,000 ESALs.
Based on the results presented in Table 3.2.2, an extended analysis was performed to evaluate the
design life of a given flexible pavement section based on variations in the subgrades effective stiffness or
strength. The following assumptions were made to perform the extended analysis: (1) a residential and
minor collector with a crushed limestone base course were the only pavement sections considered; (2) the
ADT of the residential roadway was assumed to be 2,500 VPD and the ADT of the minor collector was
assumed to be 5,000 VPD; (3) 2 percent of the ADT for each street classification was assumed to be
associated with truck traffic; (4) the 2 percent truck traffic was assumed to be due to medium-weight trucks
only, i.e. Federal Highway Administration (FHWA) vehicle classifications such as Classes 4, 5, 6, and 7;
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and (5) effective CBR values of 3 and 5 were considered. Based on these assumptions, it was determined
that the design life of the minimum flexible pavement sections outlined in the BCSUDGSA would vary
with the two (2) considered strengths of the supporting soils by as much 10 years. More specifically, the
minimum flexible pavement section outlined for residential roadways (with a crushed limestone base
course) founded on a subgrade soil with an effective CBR of 3 would theoretically have a design life of 10
years; whereas a residential roadway and minor collector founded on a subgrade soil with an effective CBR
of 5 would theoretically have a design life of 20 years. On the other hand, the minimum flexible pavement
section outlined for minor collectors (with a crushed limestone base course) founded on a subgrade soil
with an effective CBR of 3 would theoretically have a design life of 6 years; whereas a residential roadway
and minor collector founded on a subgrade soil with an effective CBR of 5 would theoretically have a
design life of 12 year. As one might expect, the design life computed for each minimum flexible pavement
section considered in this extended evaluation would decrease even more if larger percentages and
magnitude of truck traffic, i.e., heavier trucks with FHWA vehicle classifications of greater than Type 7,
traveled a given residential roadway or minor collector. As a result, it should be recognized that the design
life and performance of the minimum flexible pavement sections outlined in the BCSUDGSA for residential
roadways and minor collectors will depend heavily on the stiffness or strength of the subgrade supporting
the flexible pavement section and on the frequency and types of traffic utilizing a given roadway.
3.3 EVALUATION OF RIGID PAVEMENT SECTIONS OUTLINED IN THE BCSUDGSA
3.3.1 Assumptions Incorporated in Minimum Rigid Pavement Section Evaluation
There were several assumptions that had to be made in order to evaluate the minimum rigid
pavement sections previously outlined in Table 3.3.1 for residential roadways and minor collectors. The
inputs of particular interest included the effective stiffness or strength of subgrade soils supporting the rigid
pavement, the reliability and overall standard deviation that was initially selected to develop the minimum
rigid pavement sections, the 28-day modulus of rupture and elastic modulus of the concrete slab, and the
load transfer coefficient selected for the minimum pavement section evaluation. Table 3.2.2 – Design Inputs
Selected for Limited Analysis of Minimum Rigid Pavement Systems Outline in the BCSUDGSA summarizes
the design inputs selected to evaluate the minimum pavement sections. In addition to these assumed design
inputs, a load transfer coefficient of 3.6 was selected which corresponds to a JRCP section with curb and
gutter edge treatment. Values utilized for the modulus of rupture and elastic modulus of the PCC slab were
570 psi and 3,600,000 psi, respectively, and correlate to a 28-day concrete compressive strength of 4,000
psi based on correlations outline in ACI-318.
C M E T E S T I N G A N D E N G I N E E R , I N C .
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Table 3.3.1 Design Inputs Selected for Limited Analysis of Minimum Rigid Pavement Systems
Outlined in the BCSUDGSA
Street
Classification
R (%)
(Reliability)
So (Overall
Standard Deviation)
EFFECTIVE CBR VALUES OF SUBGRADE
SOIL CONSIDERED
Note 1
Pi (Initial Serviceability)
Pt
(Terminal Serviceability)
RESIDENTIAL 80% 0.4 3 and 5 4.5 2.0
MINOR COLLECTOR 90% 0.4 3 and 5 4.5 2.25
Note: 1. The CBR values considered in the evaluation correspond to a lower bound and upper bound for clays of high plasticity or fat clays (3 to 5)
(FHWA, 2006).
3.3.2 Results of Minimum Rigid Pavement Section Evaluation
Based on the assumptions previously outlined in Section 3.3.1 and Table 3.3.1, the minimum rigid
pavement sections outline in the BCSUDGSA for residential roadways and minor collectors were evaluated
and the maximum allowable design ESALs for each minimum rigid pavement section was computed. The
results of this evaluation are presented in Table 3.3.2 – Maximum Allowable Design ESALs for Minimum
Rigid Pavement Sections Outlined in the BCSUDGSA (Based on Assumptions Outlined in Section 3.3.1 and
Table 3.3.1).
Based on the results presented in Table 3.3.2, one can infer that the maximum allowable traffic
“capacity” or maximum allowable ESALs for each pavement section will vary depending on the effective
subgrade stiffness or strength. In addition, it can be deduced that an 8-inch PCC slab will have a much
larger traffic “capacity” than a 6-inch PCC slab as one would expect.
Similar to the extended analysis previously discussed for flexible pavement sections outlined in the
BCSUDGSA (see Section 3.2.2), an extended analysis of the results presented in Table 3.3.2 was also
performed to evaluate variations in the design period of rigid pavement sections based upon variable
subgrade stiffness or strength. Similar to the extended evaluation performed in Section 3.2.2, the following
assumptions were made: (1) the ADT of the residential roadway was assumed to be 2,500 VPD and the
ADT of the minor collect was assumed to be 5,000 VPD; (3) 2 percent of the ADT for each street
classification was assumed to be associated with truck traffic; (4) the 2 percent truck traffic was assumed
to be due to medium-weight trucks only, i.e. FHWA vehicle classifications such as Classes 4, 5, 6, and 7;
and (5) effective CBR values of 3 and 5 were considered. The theoretical design periods computed for each
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minimum rigid pavement (based on the variable effective subgrade stiffness or strength) were quite large
and ranged from 35 to 75 years in length. This is primarily due to the low frequencies and magnitudes of
traffic that were assumed for the extended evaluation.
Table 3.3.2 Variations in Computed Design ESALs based on Variable Subgrade Stiffness
Street Classification Pavement Layers Maximum Allowable ESALs
CBR = 3 CBR = 5
RESIDENTIAL 6-in. PCC slab and
6-in. lime-stab. 190,000 260,000
MINOR COLLECTOR
8-in. PCC slab and
6-in. lime-stab. 670,000 830,000
Note: 1. Maximum allowable ESALs rounded to the nearest 1,000 ESALs.
3.4 EVALUATION OF “EQUIVALENCY” OF MINIMUM PAVEMENT SECTIONS OUTLINED IN THE BCSUDGSA
As previously discussed, the purpose of this portion of the limited analysis was to evaluate the
“equivalency” of the existing minimum flexible and rigid pavement sections outlined in the BCSUDGSA.
Based on the results presented in Sections 3.2.2 and 3.3.2, it can be concluded that the minimum rigid
pavement sections outlined for residential roadways and minor collectors are capable of supporting larger
traffic “capacities” or ESALs than a flexible pavement section founded on the same type of subgrade. More
specifically, a rigid pavement section for a residential roadway (founded on a subgrade with an effective
CBR of 5) can support approximately 3.5 times as many ESALs (260,000/74,000 ≈ 3.5) as a flexible
pavement section (constructed with a flexible base course) and approximately 2.1 times as many ESALs as
a flexible pavement section (constructed with a cement stabilized base course) (260,000/123,000 ≈ 2.1).
Alternatively, a rigid pavement section for a minor collector (founded on a subgrade with an effective CBR
of 5) can support approximately 9.5 times as many ESALs as a flexible pavement section (constructed with
a flexible base course) (830,000/87,000 ≈ 9.5) and approximately 5.3 times as many ESALs as a flexible
pavement section (constructed with a cement stabilized base course) (830,000/157,000 ≈ 5.3) for a minor
collector founded on the same type of subgrade. Due to these significant variations, the minimum rigid
pavement sections outline in the BCSUDGSA will theoretically exhibit much longer design lives than the
“equivalent” flexible pavement sections outline in the BCSUDGSA.
C M E T E S T I N G A N D E N G I N E E R , I N C .
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4.0 LIMITED ANALYSIS OF PAVEMENTS BASED ON VARIABLE
TRAFFIC FREQUENCIES, MAGNITUDES OF LOADING, AND
DESIGN PERIODS
4.1 GENERAL
As previously discussed, the second portion of this limited analysis consists of evaluating flexible
and rigid pavements based on variable traffic frequencies, magnitudes of loading, and design periods. In
order to simplify this portion of the limited analysis, a single subgrade stiffness or strength was assumed.
The following paragraphs summarizes the scope of this portion of the limited analysis. The scope was
developed by CME with the assistance of the City during the previously referenced meetings conducted at
the City’s Department of Public Works offices of June 3 and August 3, 2015
Variations in ADT utilize five (5) different ADT conditions which include: (1) 2,500 vehicles per
day (VPD); (2) 5,000 VPD; (3) 10,000 VPD; (4) 20,000 VPD; (5) 40,000 VPD. The ADTs selected are
meant to be representative of the various roadways common to the College Station area such as: residential
roadways, minor collectors, major collectors, minor arterials, and major arterials. The two (2) lower
volumes of traffic, i.e. 2,500 and 5,000 VPD, will assume one (1) design lane in each direction with 100
percent of the traffic loads being transmitted to the design lane; whereas the three (3) higher volumes of
traffic, i.e. 10,000, 20,000, and 40,000 VPD, will assume two (2) design lanes in each direction with
approximately 80 percent of the traffic loads being transmitted to the design lane. For each ADT condition,
three (3) alternative truck traffic conditions are assumed and include: (1) 2 percent truck traffic; (2) 4
percent truck traffic; and (3) 6 percent truck traffic. The truck traffic was evaluated based on the percentage
of medium-weight trucks, i.e. FHWA vehicle classifications such as Classes 4, 5, 6, and 7, and heavy-
weight trucks, i.e. FHWA vehicle classifications such as Classes 8, 9, and 10, that travel a given roadway.
For an ADT of 2,500 VPD, 90 percent of the truck traffic will be assumed to be medium-weight trucks and
the remaining 10 percent will be assumed to be heavy-weight trucks. For an ADT of 5,000 VPD, 80 percent
of the truck traffic will be assumed to be medium-weight trucks and the remaining 20 percent will be
assumed to be heavy-weight trucks. For ADTs of 10,000, 20,000, and 40,000 VPD, 70 percent of the truck
traffic will be assumed to be medium-weight trucks and the remaining 30 percent will be assumed to be
heavy-weight trucks. Finally, the design periods considered during the analysis include: (1) 10 years; (2)
20 years; (3) 30 years; and (4) 40 years.
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4.2 ASSUMPTIONS MADE PERFORMING LIMITED ANALYSIS OF FLEXIBLE AND
RIGID PAVEMENT SECTIONS
As one might expect, there were several assumptions that had to be made in order to perform the
limited analysis for flexible and rigid pavements in addition to the assumptions previously outlined in
Section 4.1. These assumptions were generally focused towards design inputs previously discussed in
Section 2.0; however, other assumptions were also required in order to simplify the limited analysis. Some
of the assumptions were similar for both the flexible and rigid pavement section analysis and other
assumptions were pertinent to just a flexible or rigid pavement system. The following subsections describe
the various assumptions made to perform this portion of the limited analysis.
4.2.1 Similar Assumptions Pertinent to both Flexible and Rigid Pavement Analysis
Similar assumptions made during the limited analysis for both flexible and rigid pavement sections
are summarized below as follow:
The flexible and rigid pavement sections are founded on a subgrade that exhibits a single
stiffness or strength that is considered representative of the predominately cohesive subgrade soils, i.e. clays, typically encountered in the College Station area. Based on previous project
experience, a clay exhibiting an effective CBR value of 5 was selected to perform the
computations required for the limited analysis.
The following reliability levels were selected for the analysis based on roadway functionality
classification: (1) 80 percent for residential roadways (ADT = 2,500 VPD); (2) 90 percent for minor collectors (ADT = 5,000 VPD) and major collectors (ADT = 10,000 VPD); (3) 95 percent for minor arterials (ADT = 20,000 VPD); and (4) 99 percent for major arterials (ADT
= 40,000 VPD).
Terminal serviceability of a pavement was varied based on roadway functionality class and can be summarized as follows: (1) 2.0 for residential roadways (ADT = 2,500 VPD); (2) 2.25 for
minor collectors (ADT = 5,000 VPD); and (3) 2.5 for all other roadway functionality classes analyzed.
All pavement section were assumed to have a 6-inch chemically stabilized and compacted
subgrade layer.
Drainage coefficients were neglected for the analysis.
The computed design ESALs for flexible and rigid pavement sections were held constant even
though ESALs calculated for a rigid pavement typically differ from those calculated for flexible pavement subjected to the same frequency of traffic and magnitudes of loading over a given
period of time (see Section 2.4.1.2).
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4.2.2 Assumptions Pertinent to Flexible Pavement Section Analysis
As previously discussed, there are variable inputs required by the 1993 AASHTO design
procedures for flexible pavement design that deviate from those required for rigid pavement design. The
assumed inputs pertinent for the analysis of a flexible pavement section are summarized below as follows:
The pavement materials considered for the analysis of flexible pavement sections were a surface course of HMAC, a flexible base course of crushed limestone, and a subgrade that is
chemically stabilized.
The structural layer coefficients assumed for flexible pavement materials were the same as those assumed in Section 3.0 and were as follows: (1) HMAC – 0.44; (2) Flexible base or
crushed limestone base – 0.13; and (3) chemically stabilized subgrade – 0.08.
An overall standard deviation of 0.5 was selected for the analysis of flexible pavement sections.
An initial serviceability of 4.2 was selected for the analysis of flexible pavement sections.
The HMAC surface course thickness selected for each flexible pavement section was based on the ESALs anticipated to travel over a given roadway for a given design period. The minimum thicknesses were determined based on minimum asphalt thickness surface course thicknesses
outline in the 1993 AASHTO design guide and in Figure 14 - Suggested Minimum Thickness of Asphalt Concrete Pavement in TEX-117-E, September 1995.
4.2.3 Assumptions Pertinent to Rigid Pavement Section Analysis
The assumed inputs pertinent for the rigid pavement analysis are summarized below as follows:
The pavement materials considered for the analysis of rigid pavement sections were a surface course of Portland PCC and a chemically stabilized subgrade.
The stiffness properties of a PCC slab remained the same as those utilized in Section 3.0 and
consisted of a modulus of rupture of 570 psi and an elastic modulus of 3,600,000 psi.
An overall standard deviation of 0.4 was selected for the analysis of rigid pavement sections.
An initial serviceability of 4.5 was selected for the analysis of rigid pavement sections.
A load transfer coefficient of 3.6 was assumed for rigid pavement computations.
4.3 RESULTS OF LIMITED FLEXIBLE PAVEMENT ANALYSIS
The results of the limited flexible pavement analysis are presented on the subsequent pages of this
report in Tables 4.3.1 thorough 4.3.5. Each table is meant to represent one of the five roadway functionality
classes previously discussed, i.e. residential roadways, minor collectors, major collectors, minor arterials,
and major arterials.
The results presented for flexible pavement sections did not consider overlays or rehabilitation
techniques being incorporated during the analysis period. It’s worth noting that the 1993 AASHTO design
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procedure for flexible pavements does not directly consider rutting or thermal cracking of a flexible
pavement section, which will likely occur over the design life of any flexible pavement section. In addition,
environmental impacts due to swelling soils were not considered in the computations. As a result, the
theoretical pavement sections presented in Tables 4.3.1 through 4.3.5 would likely require rehabilitation or
overlays during the design life of each section, especially for any flexible pavement sections with a design
life exceeding 10 to 20 years in duration.
The results presented in Tables 4.3.1 through 4.3.5 for various flexible pavement sections are
significant and are primarily attributable to the number of variables that were evaluated for this portion of
the limited analysis. Nevertheless, a number of conclusions can be made based on the results obtained.
Some of the most notable conclusion can be generalized as follows:
The total design thickness of a flexible pavement section will increase with an increase in traffic
frequency, magnitudes of loading, and design period.
For a given design period, the total required thickness of a flexible pavement section (for a
given roadway functionality class) will vary by as much as 4 inches (typically 2 to 3 inches)
based on the variable percentages in truck traffic considered for each roadway functionality class.
Based on the variable ESALs calculated for a residential roadway, a minimum HMAC surface
course thickness of 2.5 inches appears most appropriate for any residential roadway subjected to significant truck traffic over the course of its design period.
Based on the variable design ESALs calculated for minor and minor collectors, a minimum HMAC surface course thickness of 2.5 to 3.0 inches appears most appropriate for a 20 year design period. This minimum thickness may be greater if large percentages of truck traffic are
expected to travel on the minor collector or if a smaller thickness of crushed limestone base is desired.
For minor and major arterials, the required thickness of crushed limestone base course begins
to become excessive. This could be avoided by either using a larger thickness of HMAC or by replacing the crushed limestone with a stiffer material such as cement stabilized base. As previously discussed, the replacement of the crushed limestone base with a cement stabilized
base would require the consideration of reflective cracking of the flexible surface course.
Table 4.3.1 Limited Flexible Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes
of Loading, and Design Periods (ADT = 2,500 VPD)
ADT = 2,500 vehicles per day (Residential Roadways)
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Pavement Layers Design Period
10 years 20 years 30 years 40 years
Design ESALs for 2 Percent Truck Traffic 42,000 84,000 126,000 168,000
HMAC Surface Course (inches) 2.0 2.0 2.0 2.5
Crushed Limestone Base Course (inches) 5.0 6.5 8.0 7.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 13.0 14.5 16.0 15.5
Design ESALs for 4 Percent Truck Traffic 84,000 168,000 252,000 336,000
HMAC Surface Course (inches) 2.0 2.5 2.5 2.5
Crushed Limestone Base Course (inches) 6.5 7.0 8.0 9.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 14.5 15.5 16.5 17.5
Design ESALs for 6 Percent Truck Traffic 126,000 252,000 378,000 504,000
HMAC Surface Course (inches) 2.0 2.5 2.5 3.0
Crushed Limestone Base Course (inches) 8.0 8.0 9.5 8.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 16.0 16.5 18.0 17.5
Notes:
1. Minimum thicknesses of hot mix asphalt concrete were determined utilizing minimum thickness tables outline in the 1993 AASHTO design guide and in Figure 14 - Suggested Minimum Thickness of Asphalt Concrete Pavement in TEX-117-E, September 1995. 2. The stabilized subgrade was assumed to be 6 inches for all conditions. 3. Structural layer coefficients used in calculations were 0.44 for HMAC, 0.13 for flexible base, and 0.08 for stabilized subgrade.
4. The existing subgrade soils below the flexible pavement sections were assumed to have an effective CBR value of 5. 5. Summary of other design inputs: So = 0.5, R = 80%, Pi = 4.2, and Pi = 2.0.
Table 4.3.2 Limited Flexible Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of Loading, and Design Periods (ADT = 5,000 VPD)
C M E T E S T I N G A N D E N G I N E E R , I N C .
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ADT = 5,000 vehicles per day (Minor Collector)
Pavement Layers Design Period
10 years 20 years 30 years 40 years
Design ESALs for 2 Percent Truck Traffic 95,000 190,000 285,000 380,000
HMAC Surface Course (inches) 2.0 2.5 2.5 2.5
Crushed Limestone Base Course (inches) 8.5 9.0 10.5 11.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 16.5 17.5 19.0 20.0
Design ESALs for 4 Percent Truck Traffic 190,000 380,000 570,000 760,000
HMAC Surface Course (inches) 2.5 2.5 3.0 3.0
Crushed Limestone Base Course (inches) 9.0 11.5 11.5 12.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 17.5 20.0 20.5 21.5
Design ESALs for 6 Percent Truck Traffic 285,000 570,000 855,000 1,139,000
HMAC Surface Course (inches) 2.5 3.0 3.0 3.5
Crushed Limestone Base Course (inches) 10.5 11.5 13.0 12.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 19.0 20.5 22.0 22.0
Notes:
1. Minimum thicknesses of hot mix asphalt concrete were determined utilizing minimum thickness tables outline in the 1993 AASHTO design guide and in Figure 14 - Suggested Minimum Thickness of Asphalt Concrete Pavement in TEX-117-E, September 1995. 2. The stabilized subgrade was assumed to be 6 inches for all conditions. 3. Structural layer coefficients used in calculations were 0.44 for HMAC, 0.13 for flexible base, and 0.08 for stabilized subgrade. 4. The existing subgrade soils below the flexible pavement sections were assumed to have an effective CBR value of 5. 5. Summary of other design inputs: So = 0.5, R = 90%, Pi = 4.2, and Pi = 2.25.
C M E T E S T I N G A N D E N G I N E E R , I N C .
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Table 4.3.3 Limited Flexible Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes
of Loading, and Design Periods (ADT = 10,000 VPD)
ADT = 10,000 vehicles per day (Major Collector)
Pavement Layers Design Period
10 years 20 years 30 years 40 years
Design ESALs for 2 Percent Truck Traffic 170,000 339,000 509,000 678,000
HMAC Surface Course (inches) 2.5 2.5 3.0 3.0
Crushed Limestone Base Course (inches) 9.0 11.5 11.5 12.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 17.5 20.0 20.5 21.5
Design ESALs for 4 Percent Truck Traffic 339,000 678,000 1,017,000 1,355,000
HMAC Surface Course (inches) 2.5 3.0 3.5 3.5
Crushed Limestone Base Course (inches) 11.5 12.5 12.5 14.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 20.0 21.5 22.0 23.5
Design ESALs for 6 Percent Truck Traffic 509,000 1,017,000 1,525,000 2,033,000
HMAC Surface Course (inches) 3.0 3.5 4.0 4.0
Crushed Limestone Base Course (inches) 11.5 12.5 13.0 14.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 20.5 22.0 23.0 24.5
Notes:
1. Minimum thicknesses of hot mix asphalt concrete were determined utilizing minimum thickness tables outline in the 1993 AASHTO design guide and in Figure 14 - Suggested Minimum Thickness of Asphalt Concrete Pavement in TEX-117-E, September 1995.
2. The stabilized subgrade was assumed to be 6 inches for all conditions. 3. Structural layer coefficients used in calculations were 0.44 for HMAC, 0.13 for flexible base, and 0.08 for stabilized subgrade. 4. The existing subgrade soils below the flexible pavement sections were assumed to have an effective CBR value of 5. 5. Summary of other design inputs: So = 0.5, R = 90%, Pi = 4.2, and Pi = 2.5.
C M E T E S T I N G A N D E N G I N E E R , I N C .
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Table 4.3.4 Limited Flexible Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes
of Loading, and Design Periods (ADT = 20,000 VPD)
ADT = 20,000 vehicles per day (Minor Arterial)
Pavement Layers Design Period
10 years 20 years 30 years 40 years
Design ESALs for 2 Percent Truck Traffic 339,000 678,000 1,017,000 1,355,000
HMAC Surface Course (inches) 2.5 3.0 3.5 3.5
Crushed Limestone Base Course (inches) 13.0 14.5 14.5 16.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 21.5 23.5 24.0 25.5
Design ESALs for 4 Percent Truck Traffic 678,000 1,355,000 2,033,000 2,710,000
HMAC Surface Course (inches) 3.0 3.5 4.0 4.5
Crushed Limestone Base Course (inches) 14.5 16.0 16.5 16.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 23.5 25.5 26.5 26.5
Design ESALs for 6 Percent Truck Traffic 1,017,000 2,033,000 3,049,000 4,065,000
HMAC Surface Course (inches) 3.5 4.0 4.5 5.0
Crushed Limestone Base Course (inches) 16.0 16.5 17.0 16.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 25.5 26.5 27.0 27.5
Notes:
1. Minimum thicknesses of hot mix asphalt concrete were determined utilizing minimum thickness tables outline in the 1993 AASHTO design guide and in Figure 14 - Suggested Minimum Thickness of Asphalt Concrete Pavement in TEX-117-E, September 1995.
2. The stabilized subgrade was assumed to be 6 inches for all conditions. 3. Structural layer coefficients used in calculations were 0.44 for HMAC, 0.13 for flexible base, and 0.08 for stabilized subgrade. 4. The existing subgrade soils below the flexible pavement sections were assumed to have an effective CBR value of 5. 5. Summary of other design inputs: So = 0.5, R = 95%, Pi = 4.2, and Pi = 2.5.
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Table 4.3.5 Limited Flexible Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes
of Loading, and Design Periods (ADT = 40,000 VPD)
ADT = 40,000 vehicles per day (Major Arterial)
Pavement Layers Design Period
10 years 20 years 30 years 40 years
Design ESALs for 2 Percent Truck Traffic 678,000 1,355,000 2,033,000 2,710,000
HMAC Surface Course (inches) 3.0 3.5 4.0 4.5
Crushed Limestone Base Course (inches) 18.0 20.0 20.5 20.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 27.0 29.5 30.5 31.0
Design ESALs for 4 Percent Truck Traffic 1,355,000 2,710,000 4,065,000 5,420,000
HMAC Surface Course (inches) 3.5 4.5 5.0 5.0
Crushed Limestone Base Course (inches) 20.0 20.5 21.0 22.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 29.5 31.0 32.0 33.5
Design ESALs for 6 Percent Truck Traffic 2,033,000 4,065,000 6,100,000 8,130,000
HMAC Surface Course (inches) 4.0 5.0 5.0 5.5
Crushed Limestone Base Course (inches) 20.5 21.0 23.5 23.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 30.5 32.0 34.5 35.0
Notes:
1. Minimum thicknesses of hot mix asphalt concrete were determined utilizing minimum thickness tables outline in the 1993 AASHTO design guide and in Figure 14 - Suggested Minimum Thickness of Asphalt Concrete Pavement in TEX-117-E, September 1995.
2. The stabilized subgrade was assumed to be 6 inches for all conditions. 3. Structural layer coefficients used in calculations were 0.44 for HMAC, 0.13 for flexible base, and 0.08 for stabilized subgrade. 4. The existing subgrade soils below the flexible pavement sections were assumed to have an effective CBR value of 5. 5. Summary of other design inputs: So = 0.5, R = 99%, Pi = 4.2, and Pi = 2.5.
4.4 RESULTS OF LIMITED RIGID PAVEMENT ANALYSIS
The results of the limited rigid pavement analysis are presented on the subsequent pages of this
report in Tables 4.4.1 thorough 4.4.5. Similar to the results presented in Sections 4.3 for flexible pavement
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sections, each table is meant to represent one of the five roadway functionality classes previously discussed,
i.e. residential roadways, minor collectors, major collectors, minor arterials, and major arterials.
It’s worth noting that the 1993 AASHTO design procedure for rigid pavements does not directly
consider faulting or thermal cracking of a rigid pavement section, which will likely occur over the design
life of any rigid pavement section. In addition, environmental impacts due to swelling soils were not
considered in the computations. Significant distress in rigid pavement systems due to these conditions
generally warrants total reconstruction of the rigid pavement.
Similar to the results previously presented in Section 4.3 for the flexible pavement analyses, the
variations of the results presented in Tables 4.4.1 through 4.4.5 for various rigid pavement sections are
considerable and are primarily based on the number of variables that were evaluated for this portion of the
limited analysis. Nevertheless, a number of conclusions can be made based on the results obtained. Some
of the most notable conclusion can be generalized as follows:
The total design thickness of a rigid pavement section will increase with an increase in traffic
frequency, magnitudes of loading, and design period.
For a given design period, the total required thickness of a rigid pavement section (for a given
roadway functionality class) will vary by as much as 2 inches (typically 1 to 1.5 inches) based
on variable percentages in truck traffic.
For a given roadway functionality class, doubling the amount of ESALs expected to travel on
a given roadway will result in approximately a 1-inch increase in the required PPC surface
course.
Table 4.4.1 Limited Rigid Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of Loading, and Design Periods (ADT = 2,500 VPD)
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ADT = 2,500 vehicles per day (Residential Roadways)
Pavement Layers Design Period
10 years 20 years 30 years 40 years
Design ESALs for 2 Percent Truck Traffic 42,000 84,000 126,000 168,000
PCC Surface Course (inches) 4.5 5.0 5.5 6.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 10.5 11.0 11.5 12.0
Design ESALs for 4 Percent Truck Traffic 84,000 168,000 252,000 336,000
PCC Surface Course (inches) 5.0 6.0 6.0 6.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 11.0 12.0 12.0 12.0
Design ESALs for 6 Percent Truck Traffic 126,000 252,000 378,000 504,000
PCC Surface Course (inches) 6.0 6.0 6.5 7.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 12.0 12.0 12.5 13.0
Notes:
1. The stabilized subgrade was assumed to be 6 inches for all conditions.
2. The PCC slab was assumed to have a 28-day modulus of rupture of 570 psi and an elastic modulus of 3,600,000 psi. 3. The existing subgrade soils below the flexible pavement sections were assumed to have an effective CBR value of 5. 4. A load transfer coefficient of 3.6 was assumed. 5. Summary of other design inputs: So = 0.4, R = 80%, Pi = 4.5, and Pi = 2.0.
Table 4.4.2 Limited Rigid Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of
Loading, and Design Periods (ADT = 5,000 VPD)
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ADT = 5,000 vehicles per day (Minor Collector)
Pavement Layers Design Period
10 years 20 years 30 years 40 years
Design ESALs for 2 Percent Truck Traffic 95,000 190,000 285,000 380,000
PCC Surface Course (inches) 5.5 6.5 7.0 7.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 11.5 12.5 13.0 13.0
Design ESALs for 4 Percent Truck Traffic 190,000 380,000 570,000 760,000
PCC Surface Course (inches) 6.5 7.0 8.0 8.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 12.5 13.0 14.0 14.0
Design ESALs for 6 Percent Truck Traffic 285,000 570,000 855,000 1,139,000
PCC Surface Course (inches) 7.0 8.0 8.5 8.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 13.0 14.0 14.5 14.5
Notes:
1. The stabilized subgrade was assumed to be 6 inches for all conditions.
2. The PCC slab was assumed to have a 28-day modulus of rupture of 570 psi and an elastic modulus of 3,600,000 psi. 3. The existing subgrade soils below the flexible pavement sections were assumed to have an effective CBR value of 5. 4. A load transfer coefficient of 3.6 was assumed. 5. Summary of other design inputs: So = 0.4, R = 90%, Pi = 4.5, and Pi = 2.25.
Table 4.4.3 Limited Rigid Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of Loading, and Design Periods (ADT = 10,000 VPD)
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ADT = 10,000 vehicles per day (Major Collector)
Pavement Layers Design Period
10 years 20 years 30 years 40 years
Design ESALs for 2 Percent Truck Traffic 170,000 339,000 509,000 678,000
PCC Surface Course (inches) 6.5 7.0 7.5 8.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 12.5 13.0 13.5 14.0
Design ESALs for 4 Percent Truck Traffic 339,000 678,000 1,017,000 1,355,000
PCC Surface Course (inches) 7.0 8.0 8.5 9.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 13.0 14.0 14.5 15.0
Design ESALs for 6 Percent Truck Traffic 509,000 1,017,000 1,525,000 2,033,000
PCC Surface Course (inches) 7.5 8.5 9.0 9.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 13.5 14.5 15.0 15.5
Notes:
1. The stabilized subgrade was assumed to be 6 inches for all conditions.
2. The PCC slab was assumed to have a 28-day modulus of rupture of 570 psi and an elastic modulus of 3,600,000 psi. 3. The existing subgrade soils below the flexible pavement sections were assumed to have an effective CBR value of 5. 4. A load transfer coefficient of 3.6 was assumed. 5. Summary of other design inputs: So = 0.4, R = 90%, Pi = 4.5, and Pi = 2.5.
Table 4.4.4 Limited Rigid Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of Loading, and Design Periods (ADT = 20,000 VPD)
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ADT = 20,000 vehicles per day (Minor Arterial)
Pavement Layers Design Period
10 years 20 years 30 years 40 years
Design ESALs for 2 Percent Truck Traffic 339,000 678,000 1,017,000 1,355,000
PCC Surface Course (inches) 7.5 8.5 9.0 9.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 13.5 14.5 15.0 15.5
Design ESALs for 4 Percent Truck Traffic 678,000 1,355,000 2,033,000 2,710,000
PCC Surface Course (inches) 8.5 9.5 10.0 10.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 14.5 15.5 16.0 16.5
Design ESALs for 6 Percent Truck Traffic 1,017,000 2,033,000 3,049,000 4,065,000
PCC Surface Course (inches) 9.0 10.0 10.5 11.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 15.0 16.0 16.5 17.0
Notes:
1. The stabilized subgrade was assumed to be 6 inches for all conditions.
2. The PCC slab was assumed to have a 28-day modulus of rupture of 570 psi and an elastic modulus of 3,600,000 psi. 3. The existing subgrade soils below the flexible pavement sections were assumed to have an effective CBR value of 5. 4. A load transfer coefficient of 3.6 was assumed. 5. Summary of other design inputs: So = 0.4, R = 95%, Pi = 4.5, and Pi = 2.5.
Table 4.4.5 Limited Rigid Pavement Analysis Based on Variable Traffic Frequencies, Magnitudes of Loading, and Design Periods (ADT = 40,000 VPD)
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ADT = 40,000 vehicles per day (Residential Roadways)
Pavement Layers Design Period
10 years 20 years 30 years 40 years
Design ESALs for 2 Percent Truck Traffic 678,000 1,355,000 2,033,000 2,710,000
PCC Surface Course (inches) 9.5 10.0 11.0 11.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 15.5 16.0 17.0 17.0
Design ESALs for 4 Percent Truck Traffic 1,355,000 2,710,000 4,065,000 5,420,000
PCC Surface Course (inches) 10.0 11.0 12.0 12.5
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 16.0 17.0 18.0 18.5
Design ESALs for 6 Percent Truck Traffic 2,033,000 4,065,000 6,100,000 8,130,000
PCC Surface Course (inches) 11.0 12.0 12.5 13.0
Chemically Stabilized Subgrade (inches) 6.0 6.0 6.0 6.0
Total Thickness of Pavement Section 17.0 18.0 18.5 19.0
Notes:
1. The stabilized subgrade was assumed to be 6 inches for all conditions.
2. The PCC slab was assumed to have a 28-day modulus of rupture of 580 psi and an elastic modulus of 3,600,000 psi. 3. The existing subgrade soils below the flexible pavement sections were assumed to have an effective CBR value of 5. 4. A load transfer coefficient of 3.6 was assumed. 5. Summary of other design inputs: So = 0.4, R = 99%, Pi = 4.5, and Pi = 2.5.
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5.0 CONCLUSIONS AND RECOMMENDATIONS
This section of the report provides a summary of the limited analysis along with conclusions that
can be drawn from the results of the limited analysis. In addition, the section also provides recommendations
for future studies and practices that may be considered by the City.
5.1 OBJECTIVE OF LIMITED ANALYSIS REVISITED
As previously discussed in Section 1.0, there were two primary objectives associated with the
limited analysis of flexible and rigid pavements. The two primary objectives can be summarized as follows:
(1) evaluation of the existing minimum flexible and rigid pavement sections outlined in the BCSUDGSA
for residential roadways and minor collector based on variable subgrade stiffness or strength; and (2)
performance of a limited analysis of various flexible and rigid pavement sections based on the following
factors: (a) variable traffic frequencies that are typical of the roadway classifications outlined in Table V –
Street Classification Definitions of the BCSUDGSA, i.e. residential roadways, minor collectors, major
collectors, minor arterials, and major arterials; (b) variable percentages of truck traffic that could be
considered typical for each roadway classification, i.e. 2 percent, 4 percent, and 6 percent; (c) variable
design periods or analysis periods, i.e. 10 years, 20 years, 30 years, and 40 years; and (d) a single subgrade
soil stiffness or strength that is considered representative of the predominately cohesive subgrade soils, i.e.
clays, typically encountered in the College Station area. The results of the evaluation performed for the
existing minimum flexible and rigid pavement sections outlined in the BCSUDGSA are presented in
Section 3.0 along with a summary of the results. The results of the limited analysis of flexible and rigid
pavement sections was presented in Section 4.0 along with a summary of some of the conclusions developed
from the results. The various assumptions required to perform both portions of the analysis were outline in
each respective section.
5.2 CONCLUSIONS
Based on the assumptions previously outlined in Sections 3.0 and 4.0 and the results of the limited
analysis, the following conclusions can be made:
The performance of the minimum flexible a rigid pavement sections outlined in Table VIII of
the BCSUDGSA for residential roadways and minor collectors will rely heavily on the
effective stiffness of strength of the subgrade soils supporting the pavement section and the frequency and magnitudes of traffic utilizing the pavement section. More specifically, the
maximum allowable ESALs on a residential roadway and minor collector will be
approximately twice as much for a roadway founded on a subgrade with an effective CBR of
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5 compared to a roadway founded on a subgrade with an effective CBR or 3. It’s worth noting
that larger effective subgrade stiffnesses or strengths than those considered in this limited analysis will allow for an increase in the maximum allowable ESALs that the minimum flexible
pavement sections can support over a given design period.
Based on the extended evaluation performed for the minimum flexible pavement sections outlined in the BCSUDGSA, the theoretical design life computed for flexible pavement
sections (with a flexible base course of crushed limestone) founded on subgrade soils with an effective CBR of 3 ranged from approximately 6 to 10 years. This agrees with the City’s previously referenced observations which concluded that some residential roadways and minor
collectors constructed using the current minimum design pavement sections are requiring significant maintenance or rehabilitation shortly after construction.
The current minimum pavement sections outlined in the BCSUDGSA for residential roadways
and minor collectors are not “equivalent”. More specifically, the structural capacities of the minimum rigid pavement sections are much larger than the structural capacities of the minimum flexible pavement sections.
The required design thickness of any flexible and rigid pavement section will rely heavily on the percentage of truck traffic and magnitudes of truck traffic that travel either type of pavement system over a given period of time. As a result, it is imperative that accurate traffic information
be obtained prior to the selection and design of a pavement section for a given roadway functionality class.
5.3 RECOMMENDATIONS FOR FUTURE STUDIES AND PRACTICES
Based on the results of the limited analysis and previous experience regarding the design of flexible
and rigid pavement systems, CME has developed various recommendations for future studies and practices
that may be considered by the City. The recommendations for future studies and practices that appear most
notable for the current problems being experienced by the City can be summarized as follows:
Consider implementing in-situ field CBR testing or dynamic cone penetration testing on subgrade soils prior to the construction or reconstruction of a roadway in order to verify existing
subgrade stiffnesses or strength.
Consider requiring that a subgrade soils stiffness or strength is verified prior to utilizing the minimum flexible pavement sections outlined in the BCSUDGSA. This would require
establishing a minimum subgrade stiffness or strength for residential roadways and minor collectors. If the minimum subgrade stiffness or strength is not met, the minimum flexible pavement sections should not be utilized for a given roadway.
Evaluate the physical properties of existing flexible and rigid pavements in the College Station area for variable roadway functionality classes based on falling weight deflectometer (FWD) testing. This evaluation should be performed on distressed and un-distressed roadways so that
a determination can be made as to what is the primary source of failure or success of the pavement systems evaluated.
Consider revising the current minimum flexible and rigid pavement sections outlined in the
BCSUDGSA based on the numerous variables addressed in this report.
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Traffic studies should be performed for roadways currently in use to develop more extensive
data concerning the volume and characteristics of the traffic utilizing that various classifications of roadway. In particular, CME believes that construction traffic on residential
roadways and minor collectors may be contributing to the rapid failure in the minimum flexible
pavement sections outlined in the BCSUDGSA. One segment of the suggested traffic studies should focus on documenting construction traffic. Another segment of the study should also
focus on the characterization traffic associated with the production and operation of oil and gas wells within the City’s jurisdiction...
In addition, extended traffic studies should also be undertaken for recently constructed
roadways to document initial serviceability and to track the loss in serviceability over time.
Based on previous project experience, CME believes that traffic studies are often performed inadequately if performed at all. As previously discussed, the frequency and magnitude of
traffic a roadway is subject to will have a significant impact on the total design thickness of a given roadway. As a result, CME recommends that detailed traffic studies be performed for any roadway that is desired to perform successfully over the selected design period.
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REFERENCES
AASHTO (1993). AASHTO Guide for Design of Pavement Structures, American Association of State
Highway and Transportation Officials. Washington, D.C.
FHWA NHI Course 132040 reference manual (2006). Geotechnical Aspects of Pavements, Participants
Reference Manual, prepared by Ryan R. Berg & Associates, Inc., Federal Highway Administration.
Washington, D.C.
NCHRP (2004). Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures, NCHRP
Project 1-37A, National Cooperative Highway Research Program, National Research Council,
Washington, D.C.
NCAT Report 14-04 (2014). Flexible Pavement Design – State of the Practice, National Center for
Asphalt Technology, University of Auburn.
PCA (1984). Thickness Design for Concrete Highway and Street Pavements, Portland Cement
Association, Skokie, IL.
Snook, J. F., F. N. Finn, M. W. Witczak, and C.L. Monismith (1982). “Thickness Design of Asphalt
Pavements – The Asphalt Institute Method.” Proceedings, 5th International Conference on the Structural
Design of Asphalt Pavements, Vol. 1. Pp. 17-44.
TEX-117-E (1995). Triaxial Compression for Disturbed Soils and Base Materials, Texas Department of
Transportation, Austin, TX
TxDOT (2011). Pavement Design Guide. Texas Department of Transportation, Austin, TX
U.S. Army Corps of Engineers (1953). “The Unified Soil Classification System,” Technical
Memorandum 3-357, Waterways Experiment Station, Vicksburg, MS.