DEVELOPMENT OF PAVEMENT DESIGN CONCEPTS for Rational Pavement Design

DEVELOPMENT OF

PAVEMENT DESIGN CONCEPTS

Prepared for:

METROPOLITAN GOVERNMENT PAVEMENT ENGINEERS COUNCIL

Job No. 24,442 February 5, 1998

MGPEC VOL. II FEBRUARY 5, 1998 CTL/T 24,442

TABLE OF CONTENTS PURPOSE AND SCOPE.................................................................................................1

Acknowledgments ...........................................................................................................1

BACKGROUND ..............................................................................................................2

PRELIMINARY STUDIES................................................................................................3

DESIGN TRAFFIC ..........................................................................................................3

VEHICLE TYPES............................................................................................................4

Automobiles ....................................................................................................................4

Trucks and Buses ...........................................................................................................4

Construction Traffic .........................................................................................................6

DESIGN TRAFFIC EQUATIONS.....................................................................................8

Residential Streets ..........................................................................................................9

Commercial Streets.......................................................................................................10

Industrial Streets ...........................................................................................................11

Arterials.........................................................................................................................12

AXLE LOADS................................................................................................................14

TIRE-RELATED STRESSES ........................................................................................15

Summary.......................................................................................................................19

MATERIALS..................................................................................................................19

CONVENTIONAL TESTING..........................................................................................20

Resilient Modulus..........................................................................................................21

Moisture Content Effects ...............................................................................................23

Resilient Modulus Correlation for Subgrade Soils..........................................................24

Stabilized Subgrade ......................................................................................................29

Aggregate Base ............................................................................................................30

Asphalt Cement Concrete Pavement ............................................................................31

SWELLING SOILS ........................................................................................................32

Characterization ............................................................................................................33

Swell Mitigation Recommendations...............................................................................36

DRAINAGE ...................................................................................................................37

PAVEMENT DESIGN EQUATIONS..............................................................................39


Flexible Design Sensitivity Analysis...............................................................................39

Asphalt Pavement Parametric Studies ..........................................................................41

Flexible Pavement Design.............................................................................................45

Rigid Design Sensitivity Analysis...................................................................................47

Rigid Pavement Design.................................................................................................49

Design of Arterial Roadway...........................................................................................51

PAVEMENT MAINTENANCE........................................................................................52

Pavement Life Curves ...................................................................................................53

Pavement Maintenance Strategies................................................................................54

LIFE-CYCLE COST ANALYSIS ....................................................................................56

Discount Rate................................................................................................................58

Cost Definition...............................................................................................................58

Analysis Period .............................................................................................................59

Salvage Value ...............................................................................................................59

Life of Maintenance Treatments ....................................................................................59

Sensitivity Analyses.......................................................................................................61

CONCLUSION ..............................................................................................................61

REFERENCES

APPENDIX A - SURVEY RESULTS

APPENDIX B - GLOSSARY OF TERMS

APPENDIX C - ANNOTATED LITERATURE REVIEW

APPENDIX D - WARRANTIES AND QUALITY ASSURANCE / QUALITY CONTROL

APPENDIX E - MATERIAL TEST RESULTS


1

PURPOSE AND SCOPE

The Steering Committee of the Metropolitan Government Pavement Engineers

Council (MGPEC) desired to secure engineering and technical support to develop

standard pavement design and construction methodology for its member agencies. The

purpose of this document is to provide technical support for the companion Pavement

Design and Construction Manual and does not serve as a design document. Technical,

performance, and analytical data are presented in the form of charts, graphs, and

discussions. This report discusses traffic, materials, analysis procedures and results,

design equations, and life-cycle cost. Included as appendices are survey results

conducted to determine the state-of-practice, a glossary of terms, annotated literature

review, a discussion of QA/QC and warranty issues, and material test results. The

primary tasks focused on the characterization of pavement subgrade materials using

Resilient Modulus (Mr) testing, development of a pavement thickness design method for

new pavements, and creation of a uniform set of pavement construction specifications

for the Denver metropolitan area, exclusive from urban streets. It is believed that when

the design process and quality measurements recommended in the Pavement Design

and Construction Manual are implemented, future projects will yield better-performing

and longer-lasting roadways at an overall reduced life-cycle cost.

The scope of the study was based upon a contract between CTL/Thompson,

Inc., and the Colorado Department of Transportation (CDOT). The contract was

administered by CDOT for local agencies represented by a steering committee

composed of members of the Metropolitan Government Pavement Engineers Council

(MGPEC). The project was funded in cooperation by the Colorado Transportation

Institute (CTI), and Federal Highway Administration (FHWA) through Denver Regional

Council of Governments (DRCOG).

Acknowledgments

We would like to acknowledge and thank members of the Metropolitan

Government Pavement Engineers Council Steering Committee and the Review Panel for


2

their participation and assistance. These individuals are: Kevin Curry, Adams County;

Bob Martin, City of Edgewater; Don Petersen, City of Aurora; Ray Porter, City of

Westminster; Dave Potter, City and County of Denver; Randy Schnicker, City and

County of Denver; John Suess, Jefferson County; Stan Szabelak, City of Federal

Heights; Bryan Weimer, Arapahoe County; and Stephany Westhusin, City of Boulder;

Bill Attwooll, Terracon Consultants Western, Inc.; Roger Johnson, City and County of

Denver; Larry Lukens, Lukens and Associates; and Scott Shuler, CAPA/Western Mobile.

BACKGROUND

Higher wheel loads and tire pressures, variable material quality, expansive soil

subgrades, and the obvious increase in traffic volume have resulted in a significant

number of pavement failures in the Denver metropolitan area in recent years. This has

created an adverse economic impact on agencies trying to maintain a degrading

infrastructure with relatively small and limited budgets.

The characterization of subgrade properties and their associated support

strength has been an issue of debate and disagreement. Agencies in the Denver metro

area have historically used either the California Bearing Ratio (CBR) or Hveem

Stabilometer (R-value) test to measure subgrade support. Both tests have been suspect

as to their ability to predict accurate subgrade support and provide test repeatability.

The American Association of State Highway and Transportation Officials (AASHTO)

outlined, in their 1986 Guide for Design of Pavement Structures1, the use of Resilient

Modulus as the recommended test for subgrade support. Lacking the capability to test

for Resilient Modulus, local agencies desiring to use the AASHTO design equations

have been forced to use generalized correlations to equate CBR, R-value, or Group

Index to Resilient Modulus.

The design of a pavement system has traditionally been based upon research

and data collected from the American Association of State Highway Officials (AASHO)2

road tests, using geographically limited materials and limited load characteristics. Since


3

the mid-1970s, changes in traffic loadings, tire pressures, and quality of materials have

made it necessary to modify the design process.

Once a roadway is constructed, maintenance becomes the critical factor in the

long-term serviceability of a pavement system. A properly planned and implemented

maintenance program is necessary to extend the design life of any pavement system.

Included in this document are a life-cycle cost analysis and recommendations for

maintaining asphalt and concrete pavements.

PRELIMINARY STUDIES

A state-of-practice survey (Appendix A) was performed to assist in determining

the direction of the study. Survey data suggest base failures, thermal and fatigue

cracking, and rutting are the most common maintenance problems in the Denver metro

area. All of the agencies require a form of the 1986 AASHTO design procedure for

design of new pavements and also have specifications for asphalt mixes. Forty percent

have lime-stabilization specifications. Only 50 percent have a Pavement Management

System (PMS).

As part of the research effort, a Glossary of Terms was developed to standardize

the technical language and reduce confusion. The Glossary is presented in Appendix B.

Appendix C contains an annotated review of the literature conducted as part of this

effort. Appendix D contains a discussion of QA/QC and warranty issues pertaining to

testing requirements and penalties.

DESIGN TRAFFIC

Traffic loads are the basis for determining the structural requirements of any

rational pavement design. The original AASHO and subsequent AASHTO3 design

methodologies use the concept of pavement serviceability related to “Equivalent” 18 kip

Single Axle Loads over a twenty-year service period (ESAL20). The critical factors in

design are traffic volume, wheel load, and subgrade support. As wheel loads and tire

pressures increase, damage to pavement increases exponentially. Traffic volume has a


4

similar degrading effect on pavement life and serviceability. As part of this traffic study,

a significant level of effort was expended to quantify traffic loads and to develop rational

pavement loading based upon anticipated traffic for each pavement roadway land use

classification (i.e., residential, commercial, industrial, and arterial).

VEHICLE TYPES

The loads transmitted to the pavement surface are dependent upon tire types,

axle weight, gross vehicle weight, tire pressure, and axle configuration. Loads

transmitted to the pavement are dependent on the type of tire, i.e. radial or bias-ply. In

the process of categorizing traffic load, typical vehicle configurations were analyzed.

Axle and tire configurations and gross vehicle weights were used to model each

roadway pavement, based upon roadway land use classification, in the development of

the design traffic equations.

Automobiles

The most prevalent volume of traffic on any pavement surface is from

automobiles. Owing to the relatively light weight and low tire pressure, a single

automobile causes insignificant measurable damage to the pavement structure. This is

reflected by the load equivalent factors used by AASHTO,3 equating an auto load to

approximately 0.0002 of a single 18,000-pound axle load. This value indicates

approximately 5,000 automobiles are required to equate to one ESAL20. Automobile

loads were considered in the design traffic calculations for this study, even though their

effect is insignificant.

Trucks and Buses

Heavier loads, such as delivery trucks, garbage trucks, buses, and tractor-trailer

trucks, are the most significant loads on most pavement surfaces. The Regional

Transportation District (RTD) operates multiple bus units on daily routes throughout the

Denver metro area. Based upon AASHTO Load Equivalent Factors, a single RTD bus


5

can load a pavement structure equal to the load from 19,240 automobiles, whereas the

load from one heavy H-20 truck can be equal to 64,235 automobiles.

CTL/Thompson published a report in 19934 which discussed various axle loads,

axle configurations, tire pressures, and gross vehicle weights of RTD buses and was

used to develop load-equivalent factors for flexible design. In addition to this report,

Load Equivalent Factors were developed for typical truck and RTD bus loadings (Figure

1) for this traffic study, assuming a serviceability of 2.5 and a Structural Number of 3.0.

Figure 1 - Standard AASHTO Truck and Bus Load Equivalency Factors for Flexible Pavements

Single Axle Tandem Axle Bus

32

1818

8

8

1616

6 24

4 17 17

1710

14 24

18 19

12 22

16 16 16 16

Type 3 - heavy

Type H-10

Type H-15

Type H-20

Type HS-20

Type 3S2

School

RTD

0.004 0.613 = 0.617

0.017 3.49 = 3.507

Load Equivalent Factors




0.047 12.8 = 12.847

0.047 1.0 1.0 = 2.047

0.613 1.08 = 1.693

0.102

0.198

1.0

0.358

0.843 0.843

2.38

1.73

3.49

= 1.788

= 2.578

= 2.73

= 3.848

Type 3 - light

6 12 12

0.017 0.273 = 0.29

Commercial Carrier


6

Trash trucks are the most common heavy-load vehicles and are likely the

heaviest and most damaging loads to residential pavement structures. Trash trucks are

typically either two- or three-axle, single-unit trucks. Most trash trucks are either 30,000-

pound trucks with a 24,000-pound single-axle load (Type H-15) or 50,000-pound trucks

with two rear axles at 18,000 pounds each (Type 3-Heavy). For design purposes, the

Type 3-heavy trash truck should be used, because it is the most common trash truck.

Construction Traffic

Construction traffic loads are not considered in the design of new pavement

systems, particularly for residential pavements. Construction traffic includes numerous

heavily loaded trucks for concrete, drywall, brick, framing, and sod delivery. For

residential streets, the construction period represents the highest concentration of loads

imposed on the pavement during its service life and must be considered as a design

factor.

Experience and analysis of residential construction materials and equipment

deliveries indicate that a single moderate sized residence will require as many as 80

Equivalent Single Axle Loads (ESAL20) to deliver materials and construct the building

(Table A). This number was based on an analysis of construction practices for a typical

single family residence.

TABLE A

RESIDENTIAL CONSTRUCTION TRAFFIC1

No. Truck Axle Load

Type Load Factors

ESAL20

2 Front-End Loader 32k 3S2 1.788 3.576

1 Drill Rig 24k H 15 3.507 3.507

18 Concrete 34k 3-Heavy 1.693 30.474

1 Steel 34k 3-Heavy 1.693 1.693

7 Framing 34k 3-Heavy 1.693 11.85

3 Drywall 34k 3-Heavy 1.693 5.079


7

2 Brick 34k 3-Heavy 1.693 3.386

3 Floor Finishing 24k 3-Light 0.290 0.870

2 Roofing 34k 3-Heavy 1.693 3.386

2 Sod Truck 32k 3S2 1.788 3.576

1 Rock / Gravel 34k 3-Heavy 1.693 1.693

1 Landscaping 34k 3-Heavy 1.693 1.693

20 Moving / Delivery 24k 3-Light 0.290 5.800

15 Miscellaneous 24k 3-Light 0.290 4.350

80.9 ESAL20

1 Based on conversations with Residential Building Contractors Pavement damage experienced during the construction period is often not detected nor

properly repaired, resulting in high future maintenance costs and decreased pavement

life. Construction of residential and other streets should not be phased by leaving the

top pavement lift off until housing construction is complete. Thin pavement sections

loaded by heavy construction traffic results in early fatigue failures and should not be

allowed. The practice of measuring deflections under an 18-kip axle load after

construction is completed, but before acceptance, will normally detect weak pavements

and allow them to be corrected before acceptance by the agency. Deflection testing

procedures for low traffic load pavements require more research to develop a proper

acceptance procedure. In the meantime, use of Benkelman Beam, Dynaflect, or Falling

Weight Deflectometer should be encouraged to assess the structural capacity of

pavements prior to agency acceptance. Due to the limited scope for this project,

procedures and levels of acceptance have not been researched or reviewed for proper

use of deflection testing procedures. It is recommended that MGPEC consider

substantial research and development of acceptance testing criteria.

DESIGN TRAFFIC EQUATIONS

Standard design traffic equations were developed assuming streets will receive

all traffic loading conditions. Default equations were written using the load equivalent

factors for design trucks, which are transformed into a 20-year ESAL20 value by the daily


8

traffic volume and the constant value of 7,300 (assuming a 20-year design life and 365

days in one year). No growth factor is considered since the traffic numbers are

estimated for total “build-out” volume. The pavement design procedure is dependent

upon the accuracy of traffic studies and load equivalency predictions. Tire pressures

and stresses should be considered in the development of load equivalent factors to

provide the most reliable pavement design possible. The trucks used in these equations

were chosen as typical vehicles expected in the corresponding roadway land use

classifications.

The roadway land use classifications are divided into four basic service

descriptions: residential, commercial, industrial, and arterial. These classifications are

defined by the zoning of the land accessed by the street. The Pavement Design and

Construction Manual contains a flow chart which aids the designer in selecting the

proper equation for design.

ROADWAY LAND USE CLASSIFICATION FLOWCHART

Is more than 79% of the property serviced by the street zoned as residential?

Is any of the property servicedby the street zoned as Industrial?

Residential Industrial CommercialEquation 1 or 2 Equation 5 or 6 Equation 3 or 4

Arterial Is the calculatedEquation 7 ESAL > 1,400,000?

Use preceeding classification

Yes

No

No

YesNo

Yes


9

Residential Streets

The majority of the traffic on residential streets consists of light automobiles.

Residential areas rarely receive heavy loads other than trash trucks, construction traffic,

and school buses. Residential streets are defined as those having less than 20 percent

of the area served zoned as commercial property.

These values are formatted into design Equation 1, as shown below.

Equation 1: ESAL20 = [(a)(2.578) + (b)(1.693) + (c)(0.0002)] 7300 + 80 (R) where: a = number of school buses per day b = number of Type 3-Heavy trash trucks per day c = number of automobiles per day R = number of residential density units serviced by the street The default equation for residential streets was developed using the applicable

traffic loads. A typical residential street is expected to receive:

< two school buses per day (a)

< two Type 3-Heavy trash trucks per day (b)

< five-hundred automobiles per day (c)

Using the estimated traffic volumes for residential streets, the default Equation 2 was

reduced as:

Equation 2; default Residential: ESAL20 = 62,000 + 80 (R) where: R = number of residential density units serviced by the street Commercial Streets

Commercial streets provide access to retail stores, businesses, offices, and other

commercial areas. The definition of a commercial street is one where more than 20

percent of the land served is zoned as non residential. These streets typically receive a

large mix of residential traffic along with trash services and delivery trucks. The number


10

of garbage and delivery trucks generally determines the structural requirements of these

streets. The standard equation for commercial streets is developed as:

Equation 3: ESAL20 = 62,000 + 80 (R) + [(a)(2.578) + (b)(1.693) + (c)(0.0002) + (d)(0.617) + (e)(3.848) + (f)(1.788) + (g)(2.047)] 7300 where: a = number of school buses per day b = number of Type 3-Heavy trash trucks per day c = number of automobiles per day d = number of Type H-10, light two-axle trucks per day e = number of RTD buses per day f = number of Type 3S2, tractor-trailer trucks per day g = number of Type HS-20, three-axle trucks per day R = number of residential density units serviced by the street Typical traffic volume was estimated for a one-acre commercial fill-in property

surrounded by developments, similar to a strip shopping center, with a 20,000 square-

foot facility on the property. It was estimated a commercial site will include:

< two school buses per day (a) < two Type 3-Heavy trash trucks per day (b) < one-thousand automobiles per day (c) < twelve Type H-10, light two-axle trucks per day (d) < two RTD buses per day (e) < two Type 3S2, tractor-trailer trucks per day (f) < four Type HS-20, three-axle trucks per day (g)

In addition, it was assumed the residential traffic was added to account for the residential

areas accessed by the street. The construction traffic loads for a commercial property

were considered, yet the loads did not exceed the expected daily traffic volume for a non

residential street. Therefore, the construction period is considered typical service not

warranting special consideration.

Using the estimated traffic volumes for one acre of commercial property, the

default commercial Equation 4 was reduced as:

Equation 4; default Commercial: ESAL20 = 62,000 + 80 (R) + 260,000 (CA)


11

where: R = number of residential density units serviced by the street CA = acres of commercial property serviced by the street Industrial Streets

Industrial streets are defined as streets having property zoned for industrial use

(manufacturing, distribution, warehousing, etc.). These streets will also receive some

commercial traffic. Industrial streets demand higher structural requirements primarily

due to the large number of multiple-unit trucks used for deliveries. The frequency of

trash trucks and buses also increase in industrial areas. Experience indicates

Equivalent Daily 18 kip axle loads can vary from less than 50 to over 1500 depending on

the use of the property. It is strongly recommended to require a full use traffic study for

each facility and require improvements accordingly.

Industrial streets are very similar to commercial streets yet are subject to an

increase in Type 3S2, tractor-trailer trucks. Industrial streets are often not associated

with residential areas, therefore the residential construction factor is not used in the

derivation of the default equation. Since industrial areas are often connected to

commercial areas, the equation should consider the commercial default traffic. The

following Equation 5 is developed for fill-in development of industrial streets:

Equation 5: ESAL20 = 260,000 (CA) + [(b)(1.693) + (c)(0.0002) + (e)(3.848) + (f)(1.788) + (g)(2.047)] 7300 where: CA = acres of commercial property services by the street b = number of Type 3-Heavy trash trucks per day c = number of automobiles per day e = number of RTD buses per day f = number of Type 3S2, tractor-trailer trucks per day g = number of Type HS-20, three trucks per day It was estimated that industrial lots would include:

< three Type 3-heavy trash trucks per day (b) < one-thousand automobiles per day (c) < two RTD buses per day (e)


12

< twenty Type 3S2, tractor-trailer trucks per day (f) < three Type HS-20, three-axle trucks per day (g)

Equation 6 was reduced using the estimated traffic volumes for one acre of

commercial and one acre of industrial property:

Equation 6; default Industrial: ESAL20 = 260,000 (CA) + 400,000 (IA) Where: CA = Acres of commercial property serviced by the street IA = Acres of industrial property serviced by the street Arterials

Arterials are roadways that serve as primary routes across the city, linking major

population, commercial, and industrial areas. They do not fall into one of the previously

discussed categories, or at a minimum, are found to be four-lane roads that service large

subdivisions, and/or commercial and industrial properties. Therefore, a detailed traffic

study should be performed for arterials.

The traffic study should address traffic volume, the distribution of truck types, and

the variations in traffic loading by lane. As a minimum, the traffic study should detail the

estimated number of automobiles, the number of residential units, and areas of

commercial and industrial areas served by the roadway. The study should estimate the

number of trucks of each type and the lane distribution. Buses should be considered a

definite probability on any major arterial, even if the current RTD plans do not include the

roadway. The design traffic should be calculated for lane-by-lane ESALs.

Because of the various functions of arterial streets (i.e., serving as major routes

to airports, trash or recycling centers, or residential parkways), their design ESAL20

should be calculated using Load Equivalency Factors and detailed 20-year traffic

projections. A typical range of ESALs20 is 300,000 to 3,000,000 or more and the design

should account for variations in lane loading in the traffic analysis.


13

For arterial streets, or in situations where the designer must develop a detailed

traffic study for a miscellaneous roadway use classification, Equation 7 should be used

to develop the design ESAL20. Equation 7 presents the design traffic equation with each

possible loading condition.

Equation 7: ESAL20 = [(a)(2.578) + (b)(1.693) + (c)(0.0002) + (d)(0.617) + (e)(3.848) + (f)(1.788) + (g)(2.047) + (h)(12.847) + (i)(3.507) + (j)(0.290) + (k)(2.73)]7300 + 80(R) where: a = number of school buses per day b = number of Type 3-Heavy trash trucks per day c = number of automobiles per day d = number of Type H-10, light two-axle delivery trucks per day e = number of RTD buses per day f = number of Type 3S2, tractor-dual trailer trucks per day g = number of Type HS-20, three-axle trucks per day h = number of Type H-20 trucks per day i = number of Type H-15 trucks per day j = number of Type 3-Light trucks per day k = number of Commercial Carrier buses per day R = number of residential lots serviced by the street

AXLE LOADS

A parametric study on axle loads and tire pressures was performed to determine

the effect of traffic loads on the fatigue life of a pavement system. These factors were

not considered as input variables into the AASHTO design equation. This evaluation

was not intended to change the design method, but was to provide a discussion of other

traffic loading variables. The study emphasizes the importance of load calculations and

shows internal pavement stresses are much higher than normally assumed. The

thickness design is not normally influenced by the tire pressures and loads, but more

accurate load equivalent factors and improved material properties can help counter

these stresses.

The DAMA program, by the Asphalt Institute,5 was used to perform the analysis.

The program analyzes multilayered elastic pavement structures by cumulative damage


14

techniques for single and dual wheel load systems. The program was chosen based on

results of research by Chen et al.6 that evaluated available computer programs for

pavement structural analysis. DAMA

is one of the best programs because it

analyzes most correctly the maximum

surface deflection, tensile strain at the

bottom of the asphalt layer, and

compressive strain at the top of the

subgrade. In addition, DAMA satisfies

the natural boundary conditions in

which the vertical stresses equal the

imposed contact pressure, and it

allows a single or dual wheel

configuration to be considered.

An axle load parametric

analysis was performed to determine

the significant effect of an axle load. Elastic layered analysis predicts tensile strain in the

asphalt cement concrete layer, which is used to calculate the ESAL20 to cause 40

percent fatigue cracking in the pavement system. The load cycles, referred to as ESALs

to Failure, are measured by tensile strain at the bottom of the asphalt cement concrete

pavement.7 Figure 2 shows that for a 7-inch-thick asphalt cement concrete pavement

section over prepared subgrade, axle loads heavier than 18,000 pounds greatly reduce

the fatigue life of the pavement. An increase from 18,000 to 24,000 pounds can reduce

the fatigue life by up to 50 percent.

The 18,000-pound axle load is the standard design load used by AASHTO;

24,000-pound axle loads are typical on over-the-road transport trucks. As shown (Figure

2), axle loads in the 30-kip range, typical of some overloaded vehicles and buses, can

severely reduce pavement life and may cause failure in a marginal pavement after a few

load applications. These heavy loads must be accounted for in design, specifically in

Figure 2 - Estimated Fatigue Life 7" AC @ Mr = 250,000 psi, Tire Pressure = 130 psi

0 10,000 20,000 30,000 40,0000

200

400

600

800

1,000

Axle Loads, lbs.

ESALs to Failure (x 1000)

Subgrade = 13,000 psi


18 Kips

24 Kips


15

bus lanes of arterial streets. These types

of loads are accounted for by use of the

Load Equivalency Factors shown on

Figure 1.

TIRE-RELATED STRESSES

Over the past 30 years, truck tire

pressures have increased from 90 psi to

130 psi8 and will likely continue to

increase in the future. Increased

pressure combined with the change from

bias ply to radial tires has significantly

increased the stress applied to the

pavement. To determine the effect of tire stresses on pavement life and serviceability, a

series of elastic layer and finite element analyses were performed. The results

presented below suggest tire stresses may explain one of the causes of higher

occurrences of rutting and shoving observed in the past few years.

A tire pressure parametric analysis was performed to determine the relationship

between higher tire pressures and fatigue failure in pavement structures. The elastic

layer method was used to calculate fatigue failure. The data indicates tire pressure

could decrease the life of a pavement system by 40 percent or more (Figure 3).

Pavement design has traditionally been based on the assumption that a tire’s

footprint is circular and exerts uniform pressure on the pavement surface. In 1989 the

Figure 3 - Effect of Tire Pressures 7" AC @ Mr = 250,000 psi, with an Axle Load of 18,000 lbs.

40 60 80 100 120 140 1600

200

400

600

800

1,000

1,200

Tire Pressure, psi




16

Texas Transportation Institute (TTI)

conducted a study on tire pressure

distributions for the Air Force

Engineering and Services Center.9

The TTI study looked at pavement

pressure distributions produced by a

variety of radial and bias-ply tires used

by the Air Force aircraft. The wheel

loads and tire pressures used for this

study were similar to conventional

truck tires.

The TTI study reported tire radii

and the distribution of tire pressures.

The results from the TTI study were

normalized (dividing measured tire

stresses by the maximum tire pressure

for various tire sizes) to formulate a

typical distribution for heavily loaded

tires (Figure 4). The stresses at the

centerline are about one-half the tire

pressure, whereas at the tire’s edge, the stresses are over three times

the tire pressure. For an inflation

pressure of 130 psi, the stresses at the

edge of the tire could be over 390 psi.

The TTI tire pressure

distribution was modeled to calculate

the internal stresses in the asphalt

cement concrete pavement and the

amount of stress transmitted to the

subgrade owing to the variable tire

Figure 4 - Tire Pressure Distribution

Figure 5 - Tangential Shear Stress

Figure 6 - Asphalt Stress Bulb

CL

0 0.2 0.4 0.6 0.8 10

0.5

1

1.5

2

2.5

3

3.5

Tire Footprint Radius from Mid-tire Centerline

Contact Pressure/MaximumTire Pressure

80 90 100 110 120 130 140150

160

170

180

190

200

210

Rated Tire Pressure, psi

Maximum Shear Stress, psi

8.0" Asphalt Concrete on SubgradeTire Pressure = 105 psi, Wheel Load = 4500 lbsContour Interval = 20 psi


17

pressures. The analyses were

conducted using SIGMA/W, a two-

dimensional finite element stress

analysis program.10 Tire pressures

were modeled according to the

pressure distribution shown in Figure

4. The resulting tangential shear

stresses (Figure 5) in the pavement

layers were found to concentrate

along the outside tire wall of a dual tire

configuration. Holding the pavement

section properties constant, the tire

stresses were varied to illustrate the

effect on the maximum shear stress

with the pavement layers. The maximum shear stress of 210 psi (130 psi tire pressure)

was significantly higher than the stress used for design, 162 psi (90 psi tire pressure).

The modeled tire pressures were analyzed on typical tire footprints supported by

equivalent pavement sections developed by AASHTO equations. The analysis

compared 8 inches of asphalt cement concrete pavement (Figure 6) with 5 inches of

asphalt cement concrete pavement over 10 inches of aggregate base (Figure 7), and 4

inches of asphalt cement concrete pavement over 12 inches of lime-stabilized subgrade

(Figure 8). Many agencies and consultants are using chemical stabilizers, such as lime,

to produce a firm pavement subgrade after moisture treating

5.0" Asphalt Concrete on 10.0" Aggregate BaseTire Pressure = 105 psi, Wheel Load = 4500 lbsContour Interval = 20 psi


18

an expansive subgrade. Lime was chosen for this analysis due to its present popularity

in the Denver metro area and the experience with material characteristics. Chemical

stabilizers, if used correctly, will provide a long-term, durable pavement platform.

The results indicated slight differences in the maximum tangential shear stresses

measured in the asphalt cement concrete pavement sections. Since the lime-stabilized

subgrade carries up to 60-80 psi, the maximum shear stress in the asphalt cement

concrete pavement is lowest in this section. This indicates the lime-stabilized subgrade

is capable of withstanding higher stresses and distributes the stresses over a larger

area. This reduces the stresses in the asphalt cement concrete pavement, which results

in an increase in the pavement life. The aggregate base carries up to 50 psi with a

higher maximum shear stress in the asphalt cement concrete pavement than the lime-

stabilized subgrade section.

Summary

Tire stresses were not considered in the recommended design methodology

since complete analysis of the data

and design implications were beyond

the scope. The high shear stresses

calculated due to radial tires could

help explain the increased rutting and

shoving observed in many pavements

subjected to bus and truck traffic. At

intersections and starting and stopping

points such as bus stops and truck

loading zones, the shear stresses in

the pavement section are even higher

than shown herein.

For the design of heavily

Figure 7 - Asphalt over Aggregate Base

Stress Bulb

Figure 8 - Asphalt over Stabilized Subgrade

Stress Bulb

4.0" Asphalt Concrete on 12.0" Lime StabilizedTire Pressure = 105 psi, Wheel Load = 4500 lbsContour Interval = 20 psi


19

loaded commercial, industrial, and arterial pavements, it is believed the results

presented indicate the tire pressure and loading effects on pavement deserve further

study and consideration. Where the subgrade exhibits low expansion potential, use of

Portland cement concrete pavements should be seriously considered to counter the

effects of high tire stresses. In areas of high swell potential subgrades, use of stabilized

subgrade can reduce the stresses in asphalt cement concrete pavements and is

recommended. Asphalt cement concrete mix designs should consider use of special

high performance mixes including Superpave mixes developed from the Strategic

Highway Research Program (SHRP) and splitt mastix ashphalts. Asphalt cements

should be significantly stiffer and consider both high temperature properties as well as

low temperature properties. For SHRP Superpave mixes, the asphalt cement should be

at least two grades stiffer based on pavement temperature ranges (high end

temperature only) than would normally be required11 for heavily loaded commercial,

industrial, and arterial pavements.

MATERIALS

Proper measurement of the support characteristics of the underlying subgrade is

critical to the success of a pavement system. The AASHTO and most other design

methodologies use the Resilient Modulus test (MR) to characterize pavement materials

including subgrade, stabilized subgrade, aggregate base, and asphalt. Portland cement

concrete pavement is treated differently by using the subgrade Modulus of Reaction (k).


20

A primary task in the scope of work was to test subgrade, stabilized subgrade,

aggregate base and asphalt cement concrete pavement to measure typical values for

local materials.

The Resilient Modulus test is very difficult to perform, requiring 4 to 8 hours to

manufacture and test a sample. The cost of the equipment to perform the test can be on

the order of $100,000 or more. Given these costs and time requirements, it is neither

realistic nor practical to require measured MR values for most pavement design projects.

Typically, conventional testing results are used to correlate with MR values used in the

design. The existing AASHTO and CDOT correlations were not based on testing of local

Denver metro soils and are, at best, a rough “rule of thumb.” Given the unique nature of

local soils, such as high swell potential and moisture susceptibility, it was deemed

appropriate to spend significant time and funds to attempt to provide a better correlation.

CONVENTIONAL TESTING

Forty samples of typical soils and bases were collected from the Denver metro

area. Each sample was thoroughly mixed to provide a large uniform sample and

subjected to standard classification testing and Proctor compaction. Sandy soils were

compacted to modified Proctor (ASTM D1557) and clayey soils to standard Proctor

(ASTM D698) in accordance with local practice. Samples were also subjected to swell

testing under an applied pressure of 200 psf, California Bearing Ratio, and R-value tests.

The CBR test was conducted using a surcharge weight of 15 pounds, typical of a

majority of the pavements at 2 percent above optimum moisture content. R-values were

determined at 250 and 300 psi exudation pressures at approximately 2 percent above

optimum moisture content. A 300 psi pressure is typical for the metro area, yet the 250

psi pressure provides data at a more saturated condition. Both CBR and R-value tests

were also conducted on soil samples prepared at 5 percent above optimum moisture

content; a moisture content that is thought by area engineers to represent subgrade

failure. All tests were conducted in accordance with the applicable AASHTO or ASTM

testing standards. Results of all testing performed are presented in Appendix E.


21

Resilient Modulus

The Resilient Modulus test is used by AASHTO to measure the strength of

pavement materials under simulated highway loading. The Resilient Modulus test is

conducted in a device similar to a triaxial chamber and subjects samples to repeated

loading under constant confining pressures. A total of 2,500 cycles of repeated axial

loads are applied as haversine stress pulses representing the shape and duration of a

truck load on the pavement.

Figure 9 shows a graphical

representation of the Resilient

Modulus test using a

conventional stress-strain

relationship. The Resilient

Modulus for subgrade soils is

calculated as:

Equation 8:

Where: Mr = Resilient Modulus Φd = average deviator stress over the last 5 cycles ,r = average resilient axial strain over the last 5 cycles A Resilient Modulus of 3,000 psi was measured in the AASHO laboratory during

the 1961 road test.12 The 1993 AASHTO design guide uses the AASHO road test

subgrade modulus of 3,000 psi as a parameter for the development of the design

Figure 9 - Resilient Modulus Stress-Strain Relationship

Volumetric Compression

Plastic Strain

Resilient Strain

Deviator Stress

Confining Pressure

Strain

Stre

ss 1 cycle

M SUB r= sigma SUB d OVER varepsilon SUB r


22

equation. The methods used to measure subgrade modulus have changed significantly

since the AASHO road test. Measured Resilient Modulus values must be adjusted to

provide consistency with the values used in the development of the 1993 design

equation. Thompson and Robnett13 studied the behavior of the AASHO soils and

concluded that a deviator stress of 6 psi and a confining pressure of 0 psi approximated

conditions of the road test, which were used for the 1993 AASHTO design guide. In this

material property study, the subgrade Resilient Modulus was determined using these

stresses on the same forty samples tested in the conventional testing study.

Typical specifications for subgrade compaction require moisture contents at

optimum moisture to 2 percent above optimum. Since no major changes are foreseen in

local specifications, Resilient Modulus testing was conducted at 2 percent above the

optimum moisture content.

Laboratory Resilient Modulus tests were performed using the Colorado

Department of Transportation’s equipment, manufactured by Structural Behavior

Engineering Laboratories and following AASHTO T 294-94 (Strategic Highway Research

Program Protocol P46). Laboratory procedures prevented preparing samples with exact

target moisture contents. Measured Resilient Modulus values were adjusted for

moisture content variations using Equation 9 presented by Li and Selig14 to obtain a 2

percent over optimum moisture content.


23

Equation 9: % = 0.96 - 0.18(w - w1) + 0.0067 (w - w1)2 Mr = Mr1 / % Where: w = desired moisture content w1 = measured moisture content % = moisture content adjustment factor Mr = Resilient Modulus at desired moisture content (w) Mr1 = measured Resilient Modulus at measured moisture content (w1)

Moisture Content Effects

Moisture variations occur during the life of a pavement system and cause

changes in subgrade support. These variations are dependent on the surface condition

of the pavement system, drainage, water table, season, and type of subgrade. The

moisture content of the subgrade reaches equilibrium generally within 3 to 10 years after

construction. Local experience and applicable CDOT research indicates clay subgrades

are typically constructed at optimum moisture content to 2 percent wet of optimum and

fail near 5 percent over optimum. For design, the industry traditionally has assumed the

subgrade support is a constant regardless of moisture content and is represented by the

design CBR or R-value. A portion of the material property study evaluated the effects of

moisture content change on the subgrade support value. Resilient Modulus also varies

on a seasonal basis depending upon moisture content and temperature.


24

To ascertain Resilient

Modulus support loss due to a

moisture content increase, three of

the originally tested soil samples

representing typical Denver soil

types (a sand and two clays) were

compacted at various moisture

contents to a targeted density of 93

percent of maximum Standard

Proctor dry density (AASHTO T-

99). This density level was used to

allow the addition of moisture

without changing the density of the

tested sample. Results, shown in

Figure 10, indicate a 2 percent

increase in moisture content

generally reduces Mr values by 25 percent, regardless of material type. The use of 25

percent reduction factor is a direct way of accounting for the AASHTO drainage factor

(mi) since there is no direct experience with these factors in the Denver metro area.

Support decreases by approximately 50 percent when moisture content increases from

optimum moisture to 5 percent above optimum moisture content.

Resilient Modulus Correlation for Subgrade Soils

Comparisons were made between Mr and CBR and R-value test results.

AASHTO shows a relationship for clay soils with CBR values of less than 10 as:

Equation 10: Mr = 1,500 (CBR)

Figure 10 - Effect of Moisture Content

-4 -2 0 2 4 60

5

10

15

20

25

30

Moisture Content from Optimum, %

Resilient Modulus (x1000), psi

A-7-6 A-6 A-2-6


25

The AASHTO constants

ranged from 750 to 3,000 due to a

wide variation in test results. As

shown in Figure 11, the best fit

linear relationship obtained from

this data has a similar relationship

(Mr = 1140 CBR). However,

neither set of data provides a

reliable Mr estimate, since both

have correlation coefficients (R2)1

less than 0.1. Even when soils

are separated by soil type, all

have very low correlations.

The AASHTO guide provides similar relationships between R-value and Mr.

However, CDOT suggests Modulus values for soils can be calculated using:

Equation 11:

where: S = [(R-value -5) / 11.29] + 3

The Denver metro data, shown in Figure 12, does not correlate to the CDOT equation

(R2 <0.1).

1 The existence of a relationship between variables can be described by the following “rule of thumb” guidelines.

R2: <0.04 slight correlation 0.49 to 0.81 high correlation

.04 to 0.16 low correlation >0.81 very high correlation

0.16 to 0.49 moderate correlation

Figure 11 - Mr with CBR

0 2 4 6 8 100

5

10

15

20

25

California Bearing Ratio (CBR)

Resilient Modulus (x 1000), psi

Denver Metro DataMr = 1140 (CBR)

AASHTO EquationMr = 1500 (CBR)

M r = 10 { { S+18.72 } / { 6.24 } }


26

Thompson and Robnett13 reported that the Resilient Modulus for subgrade soils

can be predicted from percent saturation. When data from this study are plotted against

saturation, there is no apparent correlation (Figure 13) (R2 = .301). Analysis of the

Thompson and Robnett data indicated a correlation coefficient R2 value of 0.301.

Correlation at this low level would not reliably predict Resilient Modulus for use in

pavement design.

Published data from Thompson and Robnett13 and Lee et. al.15 suggest some

correlation between unconfined compressive strength tests and Resilient Modulus.

Samples were compacted for unconfined compressive strength (qu) using the same

method used in the Resilient Modulus test (AASHTO T-294). The tests were performed

using a strain-controlled application of load and separated samples into different material

types for analysis.

Figures 14 through 17 show the linear relationships between the unconfined

compressive strength and Resilient Modulus for four types of soils. Regression analysis

yielded correlation coefficients (R2) of 0.61 to 0.80. These relationships provide a

reasonably good correlation between the unconfined compressive strength testing and

measured Resilient Modulus. The

relationships applicable to the soils in

this material property study are as

follows:

Figure 13 - Mr Correlation with Saturation

60 70 80 90 1000

5

10

15

20

Saturation, %



27

Equation 12: Mr = 2.15(qu) for A-6 soils (R2 = 0.80)

Equation 13: Mr = 1.68(qu) for claystone bedrock (R2 = 0.70)

Equation 14: Mr = 3.13(qu) for A-7-6 soils (R2 = 0.61)

Equation 15: Mr = 2.23(qu) for A-2-4 and A-2-6 (R2 = 0.79)

Where qu = unconfined compressive strength, psf (remolded) - AASHTO T208

(compacted at 2% above optimum moisture content in accordance with AASHTO T294).


28

Figure 15 - Correlation for Claystone Figure 14 - Correlation for A-6 Soils

Figure 16 - Correlation for A-7-6 Soils Figure 17 - Correlation for A-2-4

and A-2-6 Soils

1,000 2,000 3,000 4,000 5,0000

5

10

15

20

Unconfined Compressive Strength, psf


ClaystoneMr = 1.68 (q u)

1,000 2,000 3,000 4,000 5,0000

5

10

15

20



A-6Mr = 2.15 (q u)

Sample 11 data was not used

0 1,000 2,000 3,000 4,000 5,0000

5

10

15

20



A-7-6Mr = 3.13 (q u)

Sample 30 data was not used

0 1,000 2,000 3,000 4,000 5,0000

5

10

15

20



A-2-4 & A-2-6Mr = 2.23 (q u)


29

When projects encounter comparatively clean sands, unconfined strength testing

will not be possible. Given the rare occasions when this will occur in Denver, we believe

conventional R-value testing will provide satisfactory estimates of subgrade support.

These R-values should be converted to MR values using Equation 11.

Stabilized Subgrade

Two types of subgrade soils were tested (A-7-6 and A-6) to indicate the expected

strength of Denver metro subgrades when stabilized. Three samples of A-7-6 type soils

were stabilized with 6 percent quicklime and one sample of A-6 type soil with 3 percent

quicklime and 3 percent fly ash. The stabilizers were chosen due to their present

popularity in the Denver metro area.

The modulus values of the soils were

improved by 500 percent with lime

stabilization and by 1,400 percent with

lime/fly ash, over the natural conditions.

These values are considered typical of

most chemical stabilizers.

Dr. Dallas N. Little at Texas

A&M University has established a

correlation between Resilient Modulus

and unconfined compressive strength

for stabilized soils.17 The equation is as

follows:

Equation 16: Mr = 10,000 + 124qu

where: qu = compressive strength, psi

Figure 18 - Correlation for Chemical

Stabilized Subgrades

0 50 100 150 200 2500

10

20

30

40

50

60

Compressive Strength, psi


Denver Metro DataDallas Little Equation


30

The correlation from Equation 16

appears applicable to local data as shown

in Figure 18 and is recommended for use

in design for all chemical stabilizers.

Denver metro data presented in the figure

are duplicate sets of three samples of the

A-7-6 soils.

Aggregate Base

The AASHTO method T-292-91

was used to test the granular aggregate

base materials. Samples were collected

from Denver area quarries and alluvial

pits; an additional three samples were obtained from suppliers of recycled Portland

cement concrete pavement. The Resilient Modulus values for base courses, as

illustrated in Figure 19, indicate significantly lower values than typically (30,000 psi) used

by AASHTO, CDOT, and local agencies (strength coefficient = 0.12) to represent this

type of material.

The low Resilient Modulus values suggest granular base provides less support

than typically assumed in design. Granular base material has little cohesion, making the

material an effective strain absorbing layer over potentially expansive soils. However,

the more significant concern is the variance in strength of the bases in the Denver metro

area. The test results presented on Figure 19 were measured with a confining pressure

of 17 psi, as determined by a FEM analysis of a typical pavement section, and at near

optimum moisture content. The base courses sampled could not be confined in a mold

for any strength tests at 5 percent above optimum moisture content. Agencies should

also understand that aggregate base provides a reservoir for moisture to accumulate

and drive moisture deeper into the subgrade, which will accelerate and increase

heaving.

Figure 19 - Resilient Modulus of Local

Aggregate Base Courses

0 5 10 15 200

10

20

30

40

50

60

70

80

Moisture Content, %


Quarried ABC Gravel ABC Recycled PCC


31

Based on the values presented in Figure 19, a minimum Resilient Modulus value

of 20,000 psi was chosen for design purposes. A Resilient Moduli less than 20,000 psi

becomes relatively equivalent as a support layer to typical Denver metro subgrades,

resulting in little benefit of use of a base layer. Typical Denver metro clay subgrades

produce Mr values of 3,000 to 5,000 psi while Denver metro sands provide Mr values of

10,000 to 15,000 psi. This minimum Mr of 20,000 psi was used in the design procedure

to represent typically produced aggregate base course. It is recommended that

aggregate base only be used in residential streets due to the low support value and

potential for loss of support of the aggregate base. This recommendation is based upon

the integrity of the base course regardless of the subgrade type.

Experience has shown aggregate base tends to migrate into a clay subgrade as

the clay also pumps into the aggregate base. In an effort to reduce the migration of clay

fines and base material, a woven, high strength fabric should be utilized as a separation

layer.18 In areas of clay soils, a fabric placed on top of the subgrade, beneath the

aggregate base will help confine the base and reduce the loss of support experienced

with migration.

Asphalt Cement Concrete Pavement

We obtained asphalt cement concrete pavement cores from locations in the

metropolitan area to measure Resilient Modulus values to be used with the design

methodology. Asphalt cement concrete varied in design method and the required

compaction level. All pavements were placed during the 1996 construction season.

Resilient Modulus and Indirect Tensile strength testing were performed by the

Central Federal Lands Highway Division located in Denver, Colorado. Testing was

performed at 25ΕC (77ΕF) and results are presented in Table B.


32

TABLE B

ASPHALT CEMENT CONCRETE PAVEMENT TEST RESULTS

Design Compaction

Method

Mixture Grading

Voids, %

Resilient Modulus,

psi

Tensile Strength,

psi Marshall 50-blow

C 12.8 9.7 9.6

122,000 132,000 158,000

15.7 19.6 21.4

Marshall 50-blow

C 5.1 5.4 5.5

162,000 196,000 160,000

32.3 26.1 33.8

Texas Gyratory EP = 100 psi

C 4.7 4.7 7.7

212,000 180,000 155,000

32.9 26.1 33.8

Texas Gyratory EP = 75 psi

C 8.6 8.4 8.8

409,000 471,000 471,000

65.9 72.7 69.8

Superpave 3/4" Nominal

5.5 2.6 4.1

281,000 241,000 344,000

68.3 73.7 73.2

The limited data suggest that Marshall designed mixes have considerably lower

Mr values than the normally assumed values for pavement design. A typically used

strength coefficient of 0.40 correlates to a Resilient Modulus of 370,000 psi according to

the AASHTO correlations. The lower Resilient Modulus values, measured in the cores,

indicate the use of lower strength coefficients for design of asphalt cement concrete

pavements should be considered. It is believed the low values obtained for the majority

of tests are indicative of a problem in the mix design methodologies in use and deserves

consideration and future studies.

SWELLING SOILS

Swelling soils have been the subject of increasing interest in the Denver metro

area. Aspects of swelling soils were studied because of the serious nature and expense

of the damage due to swelling soils, sometimes requiring complete reconstruction.


33

Characterization

Classification testing of the soils included measurements of Atterberg Limits and

percent passing the No. 200 sieve (-200). We also measured swell of samples

compacted to about 95 percent of maximum (ASTM D698) dry density at a moisture

content about 2 percent above optimum moisture. Test specimens were confined under

an applied pressure of 200 pounds per square foot (psf) prior to wetting. The measured

swell was compared with sample liquid limit (LL), plasticity index (PI), and optimum

moisture content. The results showed that measured swell increased with increasing LL,

PI, and optimum moisture content.

Atterberg limits are performed

on the fraction of samples smaller than

the No. 40 sieve. Swell is most likely

influenced by the fines contained within

the sample (the silts and clays), and

most specifically by the clays. To

further judge the influence of the clay

and silt fraction on measured swell, we

plotted measured swell versus the

quantity (PI x -200); this comparison

showed high correlation with an R2 of

0.63 (Figure 20).

Samples compacted at about 2

percent over optimum moisture which contained less than 60 percent silts and clays

exhibited low comparative swell, ranging from 0.7 percent compression to 2.6 percent

swell under an applied pressure of 200 psf. Samples exhibiting a PI less than or equal

to 20 percent did not swell. Samples with a PI of 20 to 30 percent swelled 0.4 percent to

2.3 percent. Seven of these eight samples swelled less than 2 percent. Samples with a

Figure 20 - Correlation of Swell to Plasticity

Index and Clay Material

0 5 10 15 20 25 30 35 400

1

2

3

4

5

6

Plastic Index x -200 Material, %

Remolded Percent Swell, @ 200 psf.

PI = 20-30 PI = 30-40 PI > 40


34

PI between 30 and 40 percent swelled 1.6 to 4 percent. Two samples with a PI greater

than 40 percent swelled 3.2 percent.

The data also indicate it is not possible to achieve “zero swell” with moisture

treatment achievable with common construction techniques. Rather, the results

demonstrate that the goal of moisture treatment should be to reduce swell to control

potential differential heave by creating a more uniform material below the pavement and,

to some degree, reduce total heave.

A parametric study was performed using numerical simulation to evaluate the

magnitude of differential surface deformations resulting from heave of soils below a

moisture treated layer. The calculations were also used to simulate the effect of varying

the depth of moisture treated soil, which to some degree reduces heave and provides

some strain absorption. The analyses were conducted using SIGMA/W, a two-

dimensional finite element stress analysis program.10

A 100-foot-long cross section was formulated for this numerical modeling study.

The moisture treated zone was treated as non-expansive, although this is not practically

achievable using compacted swelling materials. Differential displacements at the surface

of the expansive material below the moisture treated zone were calculated based on the

swell of the underlying subgrade soil.

The effect of the moisture treatment in reducing differential heave was calculated

using six different moisture treated thicknesses (2, 4, 6, 8, 10, and 12 feet). The

potential swell of the soils below the moisture treated layer ranged from 2 percent to 12

percent, assuming a depth of wetting of 11 feet. The depth of wetting was based upon

measurements taken at Front Range Airport at the time of construction and again after

more than 15 years of service.16 The swell of the native materials was reduced with

depth to account for the effects of increasing overburden pressure.

With each variation in depth of moisture treatment and swell of the underlying

soils, a maximum deflection was calculated at the surface of the modeled asphalt


35

cement concrete pavement layer.

A heave feature in the street was

produced by the model to

represent a bump, perpendicular to

the direction of travel, felt by a

driver. Since the perception of a

bump for a driver is directly related

to the slope of the bump, the slope

of each heave feature was plotted

versus the depth of moisture

treatment (Figure 21).

A driver’s perception of

pavement roughness is related to

the vehicle speed. For purposes of

design, the criteria was separated

at a speed of 35 mph. Higher

speed streets were evaluated

based on one percent slope of the

heave feature. Residential streets

with speeds less than 35 mph were

evaluated for 2 percent change in

slope. These slopes were used to

represent the maximum allowable

movement before causing

discomfort to the driving public,

based on our experience. The

resulting depths of moisture

treatment based upon the design

speed assumptions and swell of

the native materials are presented on Figure 22. Based upon this information, 4 percent

is the level at which moisture treatment should be implemented due to possible damage

from swelling soils.

Figure 21 - Effective Depth of Moisture Treatment

Figure 22 - Recommended Depth of

Moisture Treatment

0 2 4 6 8 10 120

2

4

6

8

10

12

Depth of Moisture Treatment, feet

Slope of Heave Feature, percent

12% S

well

10% Sw

ell

8% Swell

6% Swell4% Swell2% Swell

0 2 4 6 8 10 12 140

2

4

6

8

10

12

Percent Swell, 200 psf

Depth of Moisture Treatment, feet

1% sl

ope f

or S

peed

s > 35

mph

2% slope for Speeds < 35 mph

12 inches of Moisture Treatment at optimum to 2% above


36

Swell Mitigation Recommendations

The results of the analyses performed as part of this investigation indicate a

reduction in heave and differential heave of pavements can be achieved through

moisture treatment of the soils below the pavement. It is not possible to eliminate

heave. The experience of the practitioners involved in this study indicates total heave is

generally not a significant cause of pavement failure and distress. Rather, it is

differential heave which usually results in a rough pavement surface, leading to a shorter

pavement life.

In most cases, a combination of moisture treatment of expansive soils below a

pavement in accordance with Figure 22 and stabilization immediately below the

pavement will significantly enhance the performance of pavements. Extensive sub-

excavation and replacement with low expansion materials has been used in the metro

area. This alternative is generally impractical, expensive, and often with little success.

Without proper drainage, sub-excavation and replacement with non-expansive

permeable soils creates a “bathtub” effect which will trap moisture, forcing the swell

deeper. The subexcavation and replacement techniques should not be used in

residential areas where proper drainage can not be provided.

Moisture treatment of clays should be designed to reduce the swell of materials

within the treated zone. For clays, moisture contents over optimum moisture content will

be required. The data from this study indicate where high plasticity clays and claystone

are present, moisture contents averaging 2 percent above optimum may result in about

2 to 4 percent potential swell of the moisture treated materials under ideal laboratory

conditions. There will be variations in swell of the treated zone. Moisture treatment will

likely produce a yielding subgrade. To provide a stable working platform and add

significant structural capacity to the pavement system, it is believed chemical

stabilization of at least 8 inches of the subgrade is appropriate for sites underlain by

moderate to highly plastic clay subgrade materials. Lime is the preferred material for

stabilization of expansive clays and claystone.17 As a minimum, the percentage of lime


37

used to stabilize subgrade should produce a soil/lime mixture having the following

properties: 1) pH > 12.3 after mellowing, 2) an unconfined compressive strength of 160

psi or greater, 3) swell less than 1 percent under an applied pressure of 200 psf, and 4)

a plasticity index of less than 10.19 More documentation concerning the strength and

durability of lime stabilization is available. Other chemical stabilizers may be suitable, if

the strength and swell reduction criteria are met and approved by the Agency.

DRAINAGE

The moisture content of the subgrade significantly affects the performance of the

pavement system. Moisture contents of 5 percent over optimum moisture content as

typically found below failed pavements, significantly decrease the modulus of the

subgrade. In the past, the AASHTO design procedures have accounted for drainage

through a drainage coefficient applied to the structural thickness equation. However, the

drainage factor has been on a scale of 0 to 1, unrelated to performance characteristics

and chosen at the discretion of the design engineer. Poor drainage was accounted for

by indirectly reducing the support strength and increasing the pavement thickness

through a factor that did not relate to any measurable loss due to poor drainage. Since

most subgrades are placed at optimum to 2 percent above optimum moisture content,

additional amounts of moisture introduced into the subgrade will reduce the support

strength. The designer should provide a method to control the amount of moisture

entering and draining from the pavement system, or design for the loss of support due to

an increase in moisture.

A method of controlling moisture entering the subgrade is the construction of

interceptor or subsurface drains behind the curb and gutter during street construction.

This method requires routine maintenance to assure effective operation of the drain.

The cost to provide the subsurface drainage system and its maintenance may not justify

its use as a solution.

Another approach is to design for the loss of support resulting from the increase

of moisture content in the subgrade. This research indicated an increase in soil moisture


38

to 5 percent above optimum, from a

constructed moisture content 2

percent above optimum, will reduce

a material’s Resilient Modulus by

approximately 25 percent (Figure

23). Similar conclusions

concerning loss of strength in the

subgrade, due to an increase in

moisture content, have been

documented through other

research.20 In design, the

subgrade Modulus should be

reduced to account for deficiencies

in drainage and the increase in soil

moisture which will occur over time,

especially when irrigation is nearby.

subgrade as the strength is decreased during the pavement service life. Rural

pavements constructed with good surface drainage where moisture is allowed to escape

the subgrade, and no adjacent landscape irrigation will be present, should not have

Resilient Modulus values reduced.

Proper drainage of the pavement system should include the consideration of

drainage characteristics. In rural areas, AASHTO directs the design engineer to design

the road cross-section so that the pavement system can drain into adjacent drainage

ditches. In urban settings, the drainage ditch concept is not possible, reducing the

drainage of the pavement system. In addition, many urban designs include landscaped

medians that must be irrigated, thus adding more moisture into the subgrade. Irrigation

lines within irrigated median can break, causing more damage. During design and

construction of such median features, the amount of irrigation required, drainage of the

median, and the impact of drainage on the pavement system should be considered in

the determination of subgrade support. The medians should be designed to drain into a

Figure 23 - Loss of Subgrade

Resilient Modulus

0 2 50

5

10

15

20

25

10.8

7.8

4.9

11.9

8.6

5.4

17.1

13

8.8

Moisture Content from Optimum, %


A-7-6 A-6 A-2-6


39

controlled drainage system to allow for proper plant growth and to protect the pavement

subgrade from loss of support due to an increase in moisture content.

For design, loss of support should be accounted for by providing a drain system

or reduction in the design modulus for any cohesive soils. This restriction does not apply

for free-draining sand soils. Rural pavements often have good surface drainage and

borrow ditches, lower than the pavement surface, where free draining moisture can

escape from the subgrade. Where surface drainage is good and there is no adjacent

landscaping, the modulus reduction is not applicable.

PAVEMENT DESIGN EQUATIONS

The pavement design state-of-practice in the Denver area is the AASHTO design

nomographs as published in 1993. Our literature review indicates that this methodology

is the most common “empirical” method available. “Mechanistic” analysis methods, such

as finite element or elastic layer, require sophisticated computer programs. Even with

these “mechanistic” methods, material property assumptions are required. It is

recommended that agencies should consider “mechanistic” methods in the design of

arterial level streets and other high traffic industrial roadways to improve the accuracy

and reliability of pavement designs.

Flexible Design Sensitivity Analysis

The 1993 AASHTO design equations for both rigid and flexible pavements

serve as the basis of the recommended MGPEC pavement design equations. The

development of the equations included a parametric study of the AASHTO design

variables to assess the sensitivity of the various parameters within the equation. The

AASHTO design method uses five variables in the calculation of a structural number.

These variables include Reliability, Standard Deviation, ESAL20, subgrade Resilient

Modulus, and Loss of Present Serviceability Index (PSI).

A typical range of values was used for each design variable.


40

< Reliability: 50 to 99% < Standard Deviation: 0.4 to 0.5 < ESAL: 36,000 to 1,460,000 < Mr: 3,000 to 14,000 psi < Serviceability

Loss:

1.8 to 2.5

Each variable was divided by the

maximum value of that variable to

obtain a normalized value. A

resulting pavement thickness was calculated for each range of variables. Figure 24

summarizes the results of the analyses. The figure shows that only three of the five

variables (Reliability, ESAL20, and Resilient Modulus) significantly influence the design.

The Standard Deviation has a negligible effect on pavement thickness. Considering that

most pavement thicknesses are rounded to the nearest one-half inch, the Loss of PSI

becomes insignificant.

Of the three variables found to influence the design thickness, the Resilient

Modulus of the subgrade material has the greatest influence. The Equivalent Single-

Axle Load (ESAL20) is the second most influential variable. These variables are

dependent variables, determined from the site conditions and predicted traffic loading,

respectively. Reliability is an independent variable determined by the designer.

Reliability is defined by AASHTO as a factor representing the probability that 1) the

Figure 24 - Flexible Sensitivity Analysis

4 6 8 10 120

0.2

0.4

0.6

0.8

1

Asphalt Concrete Thickness, inches

Normalized Variable

Reliability Standard Deviation ESAL Mr Loss of PSI


41

serviceability will be maintained, 2) the load applications used for design are correct, and

3) the pavement will perform its intended function over the design life.3

Asphalt Pavement Parametric Studies

AASHTO recommends the use of structural layer coefficients to represent

various pavement materials. The layer coefficient is a measure of the relative ability of

the material to function as a structural component of the pavement system. Construction

materials were tested in the original AASHO road tests in Illinois21 to determine Resilient

Modulus values. Charts for various materials are published in AASHTO’s Guide for

Design of Pavement Structures3 correlating the structural coefficients to Resilient

Modulus values, R-value, CBR, and unconfined compressive strengths. Data from this

and other studies suggests the correlations are, at best, only rough estimates.

“Equivalent” structural

sections were calculated from the

AASHTO equation based on the

same calculated structural

number. The purpose of this

analysis was to illustrate the

differences in current Denver

metro practices of calculating layer

thicknesses. The various layer

thicknesses were calculated

based on typical local AASHTO

layer coefficients. The layer

coefficient used were: asphalt

cement concrete pavement

Figure 25 - AASHTO Equivalent Sections

Loaded by a 9000 lb Dual Wheel Load with a 130 psi Tire PressureSubgrade Mr = 5,000 psi

100 200 300 6000

50100150200250300350400450500550600650700

Design ESAL (x1000)

ESALs to Failure (x1000)

AC+LSS AC AC+ABC


42

(ACCP) = 0.40 (Mr = 360,000), aggregate base course (ABC) = 0.12 (Mr = 30,000), and

lime stabilized subgrade (LSS) = 0.14 (Mr = 40,000). Local practice and experience

were the reasons for selecting these values for the analysis.22 Three typical sections

were developed including Asphalt Cement Concrete Pavement, Asphalt Cement

Concrete over Lime-Stabilized Subgrade, and Asphalt Cement Concrete over Aggregate

Base Course. Each section was modeled using elastic layered analysis (DAMA) to

calculate the number of cycles required to cause 40 percent fatigue cracking in the

pavement. Elastic layered analysis

predicts tensile strain in the asphalt

cement concrete layer, which is

used to calculate the ESAL20 to

cause fatigue failure in the

pavement system. The design

ESAL section was compared to

resulting fatigue life and the results

are shown in Figure 25.

It is apparent that the

calculated equivalent sections from

AASHTO layer coefficients are not

equal in terms of ESALs to failure,

or service life. Since the structural

numbers are the same for each alternative, the reason the alternatives are not expected

to perform the same is that the layer coefficients do not properly represent the fatigue

performance of the material. The performance difference is exaggerated as the required

structural number is increased.

The demonstrated inequality of the equations and structural coefficients is not

acceptable for design purposes. To increase the accuracy of the design methodology,

structural coefficients were eliminated in favor of ESALs to failure. The design

philosophy is to make Asphalt Cement Concrete Pavement (ACCP) and composite

sections structurally equal under the same loading conditions. This was accomplished

Figure 26 - Comparative Fatigue


0 5 10 150

1,000

2,000

3,000

4,000

5,000

Asphalt Concrete Thickness, inches


AC

ove

r 12"

LS

S

AC

ove

r sub

grad

e

ResidentialCommercial

Industrial

ArterialA

C o

ver 1

0" A

BC


43

by plotting ESALs versus ACCP thickness. The results were obtained from elastic-

layered analysis for various material thicknesses and strengths. A set of curves was

developed for one soil type (clay subgrade, Mr = 5,000), illustrating the relationship of AC

thickness to the number of ESALs to cause fatigue failure (Figure 26).

Each curve was developed by measuring the strain level for various ACCP

thicknesses modeled in DAMA and calculating the resulting ESALs to cause fatigue

failure. To determine equivalent structural sections, a horizontal line is drawn at a

constant ESAL to determine the intersection at the corresponding ACCP thickness for

each alternative base or subgrade section.

The results of the pavement thickness analyses indicate a minimum ACCP

thickness of 6.0 inches for residential pavements, assuming Denver metro area

subgrades of Mr = 5,000 psi (Figure 26). The fatigue equivalent sections consist of 3.0

inches of asphalt over 12.0 inches of stabilized subgrade or 4.5 inches of asphalt over

10.0 inches of aggregate base course with a separation fabric.

Considering constructability and future maintenance, the minimum ACCP

thickness for composite sections was selected to be 4.0 inches. Using 4.0 inches allows

the mat to be constructed in two lifts and provides for a 2-inch rotomill and overlay at 12

to 15 years. A 3-inch mat would require complete reconstruction of the ACCP at the end

of 15 years. When the top 2.0 inches is removed by a rotomill, the bottom one inch will

either be seriously distressed or will also be removed.


44

The final Design Equation

(Equation 17) developed from this

work was used to develop

minimum required ACCP sections.

Figure 27 illustrates one of the

curves developed, which vary with

subgrade strength. For residential

pavement, a minimum ACCP of 6.0

inches is recommended for

subgrades with Mr less than 10,000

psi. A minimum ACCP thickness of

5.0 inches can be used for

subgrades with Mr > 10,000 psi.

Commercial pavements require at

least 8.0 inches of ACCP; Industrial

streets should have at least 10.0

inches of ACCP.

Figure 28 presents a

graphical relationship between the

design ESAL and the required

asphalt cement concrete pavement

Resilient Modulus, to show a

minimum ACCP modulus to satisfy

the existing 1993 AASHTO

equation criteria. The three curves

were developed from relationships

between the design ESAL and the

Resilient Modulus of ACCP for

each thickness, using fatigue analysis. The 1993 AASHTO design equation was used

with a Reliability of 85 percent to determine the design ESAL corresponding with various

ACCP thicknesses. The horizontal lines were drawn from the design ESAL to the

Figure 27 - Minimum Thickness

Figure 28 - Required Asphalt Resilient Modulus


0 2 4 6 8 10 120

100200300400500600700800900

1,0001,1001,2001,3001,400

AC Thickness, inches


Residential

Commercial

Industrial

MG

PE

C


100 150 200 250 300 350 400 450 5000

50

100

150

200

250

300

350

400

450

500

AC Modulus (x 1000), psi

Design ESAL (x1000)

7" A

C

6" AC

8" A

C


45

resulting ACCP thickness according to the AASHTO layer coefficient of 0.4 for ACCP.

At the intersection of the fatigue curves for each thickness, a vertical line is drawn to

determine the ACCP Mr required to provide adequate structural integrity for each

thickness. The data illustrated in Figure 28 shows a range of 200,000 to 280,000 psi for

ACCP Resilient Modulus, required to meet design criteria.

A Resilient Modulus of 250,000 psi has been chosen as the minimum,

acceptable value for design, reinforced by field core testing performed earlier in this

report (Table B). The AASHTO layer coefficient of 0.40 assumes the asphalt cement

concrete will provide Mr values of 370,000 psi, which are rarely achieved in typical

Denver metro paving projects. This assumption results in under designed pavements

that do not provide adequate fatigue. It is recommended that a minimum Mr value of

250,000 psi be used to model the asphalt cement concrete layer during design

procedures. If using the AASHTO design equations, this would correspond to a

structural layer coefficient of 0.33, according to AASHTO charts. This value also

represents what the Resilient Modulus of the asphalt concrete is expected to average

over the design life, if constructed with a higher modulus.

Flexible Pavement Design

The 1993 AASHTO equation for flexible pavements was based upon analyses of

the performance of the flexible pavement test sections in the AASHO road test. The

initial MGPEC modification of the AASHTO flexible pavement Design Equation was to

set several variables constant, in an effort to reduce the complexity of the equation

(Flexible Design Sensitivity Analysis). Three variables were held constant in the

equation: 0.44 for standard deviation, 95 percent Reliability, and 2.0 for loss of present

serviceability index.

The 0.44 value for standard deviation is widely accepted as the statistical

constant determined by AASHO during calculations of the road test data. The 95

percent Reliability was selected as an acceptable level of confidence for design.

AASHTO recommends high reliabilities be used when the designer desires confidence

that serviceability will be maintained, the load applications used for the design are


46

correct, and the pavement will perform its intended function over the design life.3

Accurate design traffic calculations and traffic studies, as addressed in this report, allow

the designer to be more confident in the requirements from future traffic. In addition,

new correlations for material strengths and methods for reducing subgrade failure also

improve the designer’s confidence. The higher reliability improves the effectiveness of

capital funds used to build public roads, by designing for a higher probability of providing

serviceability over the design life. A loss of support of 2.0 was chosen since the

difference between 2.0 and 2.5 was relatively insignificant.

The AASHTO design equation was adjusted to obtain equivalent ACCP

alternative sections based upon fatigue (service life). The adjustments were

accomplished using an Alpha factor (∀). The alpha factor is simply a variable used to

adjust the resulting thickness to meet fatigue requirements.

The alpha factor varies in accordance with Resilient Modulus and ESALs: TABLE C ALPHA FACTOR EQUATIONS

Subgrade Mr ∀

# 2,000 ∀ = -2.5852 + 0.176 x Ρn (ESAL20\1000)

2,000 to 4,000 ∀ = -1.62816 + 0.18832 x Ρn (ESAL20\1000)

4,000 to 6,000 ∀ = -1.59763 + 0.25959 x Ρn (ESAL20\1000)

6,000 to 8,000 ∀ = -1.48333 + 0.28088 x Ρn (ESAL20\1000)

8,000 to 10,000 ∀ = -1.57457 + 0.32263 x Ρn (ESAL20\1000)

10,000 to 12,000 ∀ = -1.58545 + 0.34045 x Ρn (ESAL20\1000)

> 12,000 ∀ = -1.53700 + 0.34120 x Ρn (ESAL20\1000)

Accounting for the factors discussed, the AASHTO flexible Design Equation can

be simplified. The resulting reduced Equation is as follows:

Equation 17:


47

where: ESAL20 = the number of equivalent 18-kip axle loads allowed for the design period

∀ = alpha value adjustment factor to equate sections based on cycles to failure or fatigue

ta = asphalt cement concrete pavement thickness Mr = subgrade Resilient Modulus in psi (reduced by 25% if

drainage is not provided) The equation is indeterminate since there are two unknowns and only one equation and

requires an iterative solution.

When compared to results of the 1993 AASHTO equation, the MGPEC Design

Equation yields slightly thinner pavements at low traffic volumes and thicker sections as

traffic volume increases (Figure 29). Both equations use a Reliability of 95 percent.

Figure 29 indicates that the AASHTO design equation produces thinner ACCP sections

for higher volume streets than the MGPEC equation. According to structural fatigue, the

AASHTO equation under-designs these higher volume streets.

Figure 29 - Difference in AASHTO and

MGPEC Equations

log SUB 10 ESAL SUB 20 = -0.7238 ∀ + 9.36 log SUB 10 ((t SUB a x 0.33)+1) +{ { -0.176 } OVER { 0.4 + { 1094 OVER {{(( t SUB a x 0.33)+1) } SUP { 5.19 }} } } } + 2.32 log SUB 10 { M SUB r } - 8.07


0 2 4 6 8 10 120

100200300400500600700800900

1,0001,1001,2001,3001,400

AC Thickness, inches


Residential

Commercial

Industrial

MG

PE

C

AA

SH

TO


48

After determining the ACCP thickness, it is recommended that the fatigue criteria

be used as a means of making composite sections “equivalent.” This correlates more

closely to the original intent of the AASHO Committee on Design, by comparing sections

directly by strain levels in the asphalt cement concrete pavement rather than choosing

structural layer values that may not correlate to any measurable physical characteristic

of Denver metro soils. These relationships are illustrated by fatigue curves shown in

Figure 26.

Rigid Design Sensitivity Analysis

The AASHTO design method for rigid pavements, often referred to as Portland

Cement Concrete Pavements (PCCP), uses nine variables for the design of a pavement

section. These variables include concrete Elastic Modulus, concrete Modulus of

Rupture, Load Transfer, Drainage Factor, Reliability, Standard Deviation, ESAL20,

Subgrade Reaction (k), and Loss of Present Serviceability Index. The original AASHO

road test provided the basis for the rigid pavement design equation. Numerous

modifications have been made to create the existing 1993 AASHTO equation.

A rigid design sensitivity analysis was performed using the AASHTO design

variables for rigid pavements, to evaluate their importance and influence on the resulting

pavement thickness and to determine which variables should be used in the design

equation.

< Serviceability Loss: 1.5 to 3.0 < Subgrade Reaction: 40 to 320 pci < Reliability: 60 to

99% < Standard Deviation: 0.2 to 0.45 < Elastic Modulus: 2.5 x 106 to 7

x 106 psi < Modulus of Rupture: 550 to 800 psi < Load Transfer: 2.0 to 5.0 < Drainage: 0.7 to

1.2 < ESAL: 36,500 to

3,650,000


49

Figure 30 illustrates the results. The figure shows that four of the nine variables have a

significant influence on the thickness design; two have some influence; and three have

very little influence. The significant variables include ESAL20, Load Transfer, Subgrade

Reaction (k), and Reliability. Of the four variables that were found to significantly

influence the design thickness, the ESAL20 has the greatest influence. Load Transfer is

a factor that is dependent upon the types of reinforcement and dowels used at the joints.

The variables having some influence include Drainage Factor and concrete Modulus of

Rupture. As expected, the Standard Deviation, Serviceability Loss, and concrete Elastic

Modulus have very little effect on the results. These last three variables do not affect the

pavement thickness by more than 0.25 inches and default values will be considered as

the design variables.

Rigid Pavement Design

The AASHTO equation serves as the basis for modification to provide a

simplified design procedure that

produces structurally adequate

sections. The modified design

equation includes only the most

significant variables. The

remaining design variables are as

follows:

Concrete Elastic Modulus = 3,400,000 psi

Concrete Modulus Rupture = 650 psi

Drainage Factor = 1.0

Standard Deviation = 0.34

Loss of Serviceability Index = 2.0

Figure 30 - Sensitivity Analysis

4 5 6 7 80

0.2

0.4

0.6

0.8

1

Portland Cement Concrete Thickness, inches

Normalized Variable

Serviceability Loss Subgrade Reaction Reliability Standard Deviation Elastic Modulus

Modulus of Rupture Load Transfer Drainage ESAL


50

Concrete strength values are commonly specified levels that are typically held

constant in the current design procedures. A Drainage Factor of 1.0 is typical. Drainage

is accounted for in the calculation of the Resilient Modulus and the resulting Modulus of

Subgrade Reaction. The Standard Deviation and Loss of Serviceability Index are typical

design variables that have very little influence on the design thickness.

The design ESALs for use in the rigid pavement design are based upon the

calculations presented earlier in this report using flexible Load Equivalency Factors.

AASHTO provided different Load Equivalency Factors for flexible and rigid pavements to

account for different performance of the two pavement types. The alpha factor used in

the ACCP design equation, serves to distinguish between expected performance of the

ACCP and PCCP design sections. Furthermore, most Denver metro jurisdictions

currently use the same ESAL for both pavement types. It is recommended that the

same ESAL be used for rigid pavements as calculated for flexible pavement sections.

The resulting equation is as follows:

Equation 18:

Where: ESAL20 = the number of equivalent 18-kip axle loads allowed for the design

period tp = Portland cement concrete pavement

thickness required for the pavement system J = load transfer coefficient k = modulus of subgrade reaction. where: k = 0.8932 (Mr / 19.4)(0.827)

log SUB 10 ESAL SUB 20 = -0.5624 + 7.35 log SUB 10 (t SUB p+1) - 0.06 + { { -0.176 } OVER { 1+ { { 1.6248*{10 SUP { 7 } } } } OVER { (t SUB p+1) SUP { 8.46 } } } }+ 3.42 log SUB { 10 } { {650 { ({ t SUP {0.75 } } - 1.132 )} } OVER { 215.63 x J [ { {( t SUB p SUP { 0.75 } ) } - { { 18.42 } OVER { { ({ 3,400,000 } OVER k )} SUP { 0.25 } } } } ]} }


51

The equation must be solved through an iterative solution to calculate the PCCP

thickness (tp). Portland cement concrete pavements are recommended on sand

subgrades classified as A-1, A-2-4, A-2-5, A-3, A-4, or A-5 by AASHTO classification

procedures. AASHTO provides equations for determining modulus of subgrade reaction

based on Resilient Modulus, as shown above.

In the Denver metro area, the majority of performance problems related to PCCP

are a direct result of swelling soils causing differential movement. In areas where PCCP

has been placed on non-swelling soils, the pavements have performed very well. PCCP

provides a very durable platform with relatively low maintenance requirements when

compared to ACCP. Areas of heavy truck traffic also perform much better when paved

with PCCP, which do not experience the shoving and rutting often experienced with ACC

pavements.

The main concern with the application of a PCCP is the preparation of the

subgrade. PCCP are not recommended for use in areas of greater than 4 percent swell.

If they are used, the subgrade should be stabilized to reduce the effect of heave

damage. Even with stabilization, damage often occurs because of improper drainage of

the subgrade. Asphalt Cement Concrete Pavements are flexible and tend to allow small

amounts of swell without experiencing cracking. If swelling does occur in the subgrade,

ACCP often provide a smoother ride than PCCP and are easier and cheaper to repave

in isolated areas.

Design of Arterial Roadways

The variability associated with arterial roadways does not lend itself to simplistic

design methods. The recommended design process begins with the traffic study and the

lane-by-lane breakdown of volumes.


52

Pavement alternatives from the MGPEC design equations should consider

thickness ratios for stabilized subgrade/asphalt cement concrete pavement thickness, in

the range of 2 to 3 to properly distribute stresses. Use of aggregate bases beneath

arterial pavements is not recommended as part of the structural section. Rigid (PCCP)

pavements should not be used in areas where treated or untreated subgrade soil swell

potential is greater than 4.0 percent, according to swell potential damage levels

discussed in SWELLING SOILS.

Once preliminary designs have been established and rational thicknesses

obtained, at least three design alternatives, generally consisting of asphalt cement

concrete pavement, asphalt cement concrete pavement over stabilized subgrade, and

Portland cement concrete pavement should be evaluated. The MGPEC equations may

provide sections that could be deficient in thickness according to fatigue life for traffic

volumes over 1,500,000 ESALs. The evaluation should consist of analysis using finite

element or elastic layer techniques to verify ESALs to failure for each of the alternatives

on a lane-by-lane basis. The DAMA program is recommended as one of the best

programs because it analyzes most correctly the maximum surface deflection, tensile

strain at the bottom of the asphalt layer, and compressive strain at the top of the

subgrade.6 Those sections which fail to exceed the design ESALs should either be

adjusted or removed from consideration. After preliminary design, each alternative

should be subjected to life-cycle cost analysis to determine the best present worth

section for each travel lane to be paved.

The pavement design for arterial streets should also include recommendations

for pavement maintenance that consider the user cost associated with lane closures and

delays. The surface course, whether asphalt cement concrete pavement or Portland

cement concrete pavement, should be designed to minimize maintenance through use

of treatments that will ensure the highest practical level of service and the least user

cost.

PAVEMENT MAINTENANCE


53

Proper pavement

maintenance is the single most

important aspect, after construction,

in achieving the design life and

serviceability of a pavement system.

Consistent and proper maintenance

can also extend the pavement life

past its design life. Changes in traffic

loading, environmental stress, poor

construction or construction materials

will affect the life; however, improper

or deferred maintenance can have an

even more significant effect on

pavement life. The investment in a

pavement infrastructure is the largest single investment for most agencies. It is good

public policy and fiscally sound to protect and maintain the investment at the highest

possible level. To achieve this goal, it is important that each agency implement and use

a Pavement Management System (PMS). The savings associated with properly

scheduled maintenance make a PMS a sound public investment. The ability to

document pavement conditions and model the effectiveness of pavement maintenance

dollars to the public further enhance the value of a PMS. In addition, after several years

of experience, the maintenance treatment program can be fine-tuned to optimize

maintenance treatments and maintenance dollars.

Pavement Life Curves

Numerous rating systems are available to measure pavement performance and

to predict future deterioration. Each system has its advantages and complexities. One

of the most common and likely easiest to use is the Pavement Condition Index (PCI)

system, as developed by the U.S. Army Corps of Engineers.23 The PCI rating system

measures the amount and severity of surface distress. Prediction of pavement

performance uses PCI deterioration curves developed from the performance of existing

Figure 31 - Pavement Deterioration Curve

0 5 10 15 20 25 300

20

40

60

80

100

Service Life, Years

PCI Value

Terminal Life


54

pavements.24 The shape of the deterioration curves is based on a sigmoidal curve fitted

to actual field data.

Normal deterioration of a flexible pavement without maintenance treatments is

shown in Figure 31. Maintenance increases the PCI value and extends the serviceable

life of the pavement beyond the design life, providing a “benefit” to the user when

applied at proper times. The effect of typical maintenance for a residential street is

shown in Figure 32.

Figure 32 - Typical Maintenance on

Residential Streets

0 5 10 15 20 25 300

20

40

60

80

100

Service Life, Years

PCI Value

Terminal Life

Applied Maintenance


55

Pavement Maintenance Strategies

Tables D and E provide recommended guidelines for pavement maintenance

strategies for ACCP and PCCP, as used in life-cycle cost models. Each treatment

serves a specific purpose and reduces distress in the pavement. These guidelines are

generic and do not consider all the traffic, environmental conditions, and other factors

that can either accelerate or decelerate degradation. The effectiveness of the

recommended treatments is dependent upon having the proper engineering and

construction control. A sound pavement management system would use the strategies

like those outlined. These guidelines were prepared based on local experience and are

for residential, commercial, and industrial streets. Annual maintenance procedures,

including tasks such as pothole patching and the emergency maintenance procedures

that are performed by the maintenance department, are not scheduled repairs. A thirty-

year analysis was used to account for one rehabilitation treatment cycle in each

alternative. After thirty years, the same maintenance treatments should be used, but at

an increased frequency or consideration may need to be given to fully reconstruct the

roadway.

The wide variations in traffic loading make development of a typical maintenance

strategy for arterial streets impossible. Arterials, because of their importance and high-

service levels, should be evaluated on a case-by-case basis and specific maintenance

plans developed considering traffic volumes, user and agency costs to close the road,

allowable roughness, type and severity of distress, and other site specific characteristics.


56

TABLE D GUIDELINES FOR

FLEXIBLE PAVEMENT MAINTENANCE TREATMENTS FOR RESIDENTIAL, COMMERCIAL & INDUSTRIAL ROADWAYS1

Year Residential Commercial and Industrial

1 Fog Seal Fog Seal 3 --- Crack Sealing 5 Crack Sealing, Fog Seal --- 6 --- Crack Sealing, Fog Seal

10 Crack Sealing, Slurry/Chip Seal 2-inch Overlay 13 --- Crack Sealing 15 2-inch Overlay with Milling at Edges --- 16 --- Crack Sealing, Fog Seal 20 Crack Sealing, Fog Seal 4-inch Planing, 3-inch Overlay 23 --- Crack Sealing 25 Crack Sealing, Slurry/Chip Seal --- 26 --- Crack Sealing, Fog Seal 30 3.5-inch Planing, 2.5-inch Overlay 2-inch Overlay

1-30 Annual Maintenance Annual Maintenance 1 CDOT and Local Experience

TABLE E GUIDELINES FOR

RIGID PAVEMENT MAINTENANCE TREATMENTS FOR RESIDENTIAL, COMMERCIAL & INDUSTRIAL ROADWAYS1

Year Residential Commercial and Industrial

5 --- Clean & Seal Cracks and Joints 7 Clean & Seal Cracks and Joints ---

10 --- Clean & Seal Cracks and Joints 14 Clean & Seal Cracks and Joints --- 15 --- Clean & Seal Cracks and Joints 20 Grind ½ inch as necessary (25%) Grind ½ inch as necessary (25%) 25 --- Clean & Seal Cracks and Joints 27 Clean & Seal Cracks and Joints ---

30

---

Clean & Seal Cracks and Joints, Grind ½ inch as necessary (25%)

1-30 Annual Maintenance Annual Maintenance 1 CDOT and Local Experience


57

Fog seals and crack seals minimize the effects of oxidative embrittlement and

reduces the amount of air and water entering the pavement surface. Pavements

experiencing weathering and raveling benefit from a fog seal application. Fog seals can

range from 0.10 gallons per square yard to 0.20 gallons per square yard, depending

upon the condition of the pavement surface and sealant material. Each pavement

should be evaluated, and the fog seal material and application rate should be

engineered to fit the condition. Crack sealing does not eliminate the crack, however it

can change the severity level and reduce further deterioration of the pavement layers.

Slurry or chip seals are used to waterproof the surface, improve the ride, and to

provide skid resistance where aggregates may have been polished by traffic and the

pavement has become smooth. The improvement in condition level provided by these

seals depend on the amount of deterioration present in the pavement section before

application of the seal. On higher distressed pavement, these seals provide little to no

benefit due to structural deficiencies. If seals are placed prior to cracking, the pavement

will deteriorate at a slower rate. When seals are applied after cracking has occurred, the

seals tend to ravel as the cracks of the underlying material reflect through the surface

seal.

Overlays are considered rehabilitation and usually improve the PCI value of the

pavement to the maximum value of 100. Designed overlays should be done on a project

level basis and must consider the surface distress and the structural capacity of the

pavement. Urban streets require different maintenance treatment because of curb

height constraints, whereas rural roads do not have the height constraints that require

rotomilling.

LIFE-CYCLE COST ANALYSIS

Paving public streets necessitates the expenditure of capital funds. The

decisions made in the design process not only affect the capital but also affect the


58

maintenance budget. These decisions consequently require that an economic analysis

be performed during the design process. Life-cycle cost analysis determines the relative

financial effectiveness of a set of alternatives over a fixed time period. This analysis

provides a means to select the preferred alternative based on estimated costs of initial

construction and future maintenance costs.

For the analysis to be valid, the analysis period of each pavement section must

be equal. Costs assumed for initial construction and maintenance operations must also

be valid and based on best estimates. Construction costs are highly erratic and time

dependent. Predictions of cost thus represent an indicator, rather than a true estimate.

Interest rates used in the analysis are often debatable and politically sensitive. High-

interest rates promote decisions based on first costs and a short-term approach;

whereas low-interest rates encourage consideration of long-term costs. The

effectiveness and service life of maintenance operations and rehabilitation operations

significantly influence cost and can create bias. However, life-cycle cost analysis is a

tool to be used by design engineers along with sound engineering practice to judge the

best design alternative based on capital and maintenance costs over the analysis period,

not just the construction costs.

The three generally accepted methods for calculating life cycle costs are the

present worth method, equivalent annual worth method, and the future worth method.

All methods calculate the cost of a project over its life. The difference is whether the

cost is measured in today’s dollars (at present worth), as a uniform series of annual

dollars (at present worth), or the cost at the end of the project life (future). The present

worth method is used by most engineers and agencies and was selected for this work.

The present worth method discounts all costs to present-day based on the

assumed discount rate (interest). The present worth of a project is equivalent to the

amount of money that would have to be invested to cover the costs of construction,

maintenance, and rehabilitation over the analysis period. Variables that must be

established to calculate present worth include discount rate (interest rate), cost definition

(cost of construction and maintenance), analysis period, salvage value, and life of


59

maintenance and rehabilitation alternatives. Using these values and the pavement

maintenance recommendations, the present worth can be calculated as:

Equation 19:

Where: Pw = present worth (dollars) i = discount rate (annual) I = initial construction cost (dollars) A = annualized future maintenance costs (dollars) n = number of years for analysis (number) S = salvage value (dollars) The calculation of life-cycle costs can be time consuming and subject to

significant bias. As a standard for MGPEC, it is suggested the Colorado Department of

Transportation model and computer code be used. It serves as the basis for the

analyses performed in this study.

Discount Rate

The discount rate is the effective interest rate per interest period. The discount

rate is determined by subtracting the inflation rate from the market interest rate. The

discount rate of capital has been between 3.7 and 4.4 percent since 1966 (Epps and

Wootan, 1981,25 and Corps of Engineers, 198726). The January 1997 rate is

approximately 3.8 percent, based on a 30-year bond. Based on this information, a

discount rate of 4 percent per year was used for analysis purposes in this document.

Cost Definition

The cost definition is the actual dollar value assigned to each item and procedure

defined during construction and maintenance operations for the project. The costs are

listed as present worth values for construction and predicted inflated prices for

maintenance procedures. These prices are often very susceptible to bias costing.

P SUB w = I + { A[(1+i) SUP n - 1] } OVER { i(1+i) SUP n } -S OVER { (1+i) SUP n }


60

A yearly cost data book, available from the Colorado Department of

Transportation (CDOT), lists the quantity of material used for CDOT projects and

associated unit prices. CDOT data for the 1995 construction season were used in this

report. Time of construction and location affect the unit prices. The costs for each

project should be determined based on projects of similar size and location. Where

CDOT cost data are not available, quotes from contractors or material suppliers can be

used. It should be noted that often these costs can be biased, and competitive quotes

should be obtained.

Analysis Period

The analysis period is the selected time period, in years, over which the

alternatives are compared. A single analysis period allows all alternatives to be

compared on a common basis. The analysis period should be chosen to allow for a

minimum of one major rehabilitation for each alternative, to account for unequal

performance of the various pavement alternatives (PCCP and ACCP). A 30-year

analysis period is recommended to allow for a minimum of one major rehabilitation for

each alternative for a twenty year design life.

Salvage Value

Salvage value is the future value of the pavement materials at the end of the

analysis period. Salvage values are assumed to be equal at the end of the analysis

period for this report, therefore canceling each other. Furthermore, it was assumed that

the ACCP and PCCP will be recycled.

Life of Maintenance Treatments

In predicting the performance of pavement sections, the design engineer should

bear in mind that maintenance and rehabilitation alternatives have significant impacts on

the life cycle cost analysis. The design engineer must be aware of the appropriateness


61

of the assumptions made. Pavement maintenance records and the engineer’s

experience with local materials can provide models to predict typical deterioration of

pavement and the effect of maintenance in their jurisdiction.

The service cycles of pavement maintenance techniques are a function of traffic

loads, environment, materials, and thickness of structural sections. The service life of a

pavement or treatment refers to the number of years before that particular pavement or

treatment will no longer be serviceable and will require total reconstruction. The values

used are the actual years the particular pavement is expected to perform if no

maintenance is applied. Guideline assumptions for average service lives are provided in

Table F.

TABLE F AVERAGE SERVICE LIFE (YEARS)

Residential Commercial,

Industrial Crack Sealing (flexible) 3 to 5 2 to 3 Crack Sealing (rigid) 5 to 7 3 to 5 Joint Sealing (rigid) 5 to 7 3 to 5 Fog Seal (flexible) 3 to 5 3 to 5 Chip or Slurry Seal (flexible) 3 to 5 --- Grinding (rigid) 10 to 14 8 to 10 Overlay (flexible) 10 to 15 8 to 10


62

Sensitivity Analyses

Discount rate, analysis period,

cost definition, and maintenance

service life were varied individually in

the typical flexible life cycle cost model

to determine the sensitivity of each

variable. The percent change of the

variable was plotted against the

resulting present worth in dollars as

shown in Figure 33. Variation of the

discount rate and cost definition does

not significantly affect the present

worth value of the model. Analysis life

beyond 30 years does not appear to be beneficial. Life-cycle costs are most sensitive to

the assumed life of the maintenance treatments selected. This sensitivity analysis

reinforces the importance of properly designed maintenance treatments and how their

schedule will reflect the expected performance and cost.

CONCLUSION

The goal of the project was to provide members of the Metropolitan Government

Pavement Engineers Council with data to support development of a design

methodology. As in most research projects, there are still questions remaining to be

answered. Revisions to this document and to the Pavement Design and Construction

Manual should be anticipated and encouraged as additional research is performed and

experienced gained. The documents should be reviewed and revised to reflect new

knowledge and experience.

As the study progressed, the team was required to make assumptions regarding

unknowns that materially affect the design. It is suggested the Council consider

undertaking research to:

Figure 33 - Life Cycle Cost

Sensitivity Analysis

200,000 400,000 600,000 800,0000.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Dollars, $

Percent Change

Discount Rate (4.0%) Cost Value (1997)

Analysis Life (30) Maintenance Life

20

30

40

4%

3%

5%

0%4%

8%

6 yr. Seal

3 yr. Seal

5 yr. Seal


63

1. Determine actual weights and local distribution of trash trucks. 2. Reexamine asphalt cement concrete pavement mix design philosophies

in light of high-contact stresses and comparatively poor performance of some mixes tested in this study.

3. Expand the database of material properties to better evaluate and

characterize subgrade and paving materials. 4. Accumulate and standardize cost and performance data for initial

construction and maintenance techniques.


64

REFERENCES 1. American Association of State Highway and Transportation Officials, AASHTO Guide

for Design of Pavement Structures, 1986. 2. AASHO Operating Subcommittee on Roadway Design, AASHO Interim Guide for

Design of Pavement Structures 1972, AASHO, Washington, D.C. 3. American Association of State Highway and Transportation Officials, AASHTO Guide

for Design of Pavement Structures, 1993. 4. V. J. Peters and G. Scot Gordon, Determination of Bus Equivalent Factors,

CTL/Thompson, Inc., Report for City and County of Denver, Colorado, 1993. 5. Asphalt Institute, A Computer Program for the Analysis (Including Seasonal

Variations) of Highway Pavements with Dual Wheel Loadings - DAMA, 1993 Edition.

6. Dar-Hao Chen, Masharraf Zaman, Joakin Laguros, and Alan Soltani, “Assessment of

Computer Programs for Analysis of Flexible Pavement Structure,” Transportation Research Record No. 1482, 1995.

7. H. L. Von Quintus, J. A. Scherocman, C. S. Hughes, and T. W. Kennedy, “Asphalt-

Aggregate Mixture Analysis System,” National Cooperative Highway Research Program Report 338, TRB, March 1991.

8. Reynaldo Rogue, Conversation with Professor at the University of Florida during

Presentation at Annual Association of Asphalt Pavement Technologists (AAPT) Conference, 1997.

9. John T. Tielking, “Aircraft Tire/Pavement Pressure Distributions,” Prepared for Air

force Engineering and Service Center, Texas Transportation Institute, MM 7111-89-1, January 1989.

10. Geo-Slope International, Calgary, Alberta, Canada, SIGMA/W, for Finite Element

Stress/Deformation Analysis, Version 3.06, September 1995. 11. G. King, H. King, O. Harders, P. Chaverot, and J. P. Planche, Influence of Asphalt

Grade and Polymer Concentration on the High Temperature Performance of Polymer Modified Asphalt Proceedings, AAPT, Vol. 61, 1992.

12. E. L. Skok, Jr., and F. N. Finn. Theoretical Concepts Applied to Asphalt Pavement

Design. Proc., International Conference on the Structural Design of Asphalt Pavements, Ann Arbor, Michigan, 1962, p. 421.


13. M. R. Thompson and Q. L. Robnett. Resilient Properties of Subgrade Soils, Final Report--Data Summary. Transportation Engineering Series No. 14. Illinois Cooperative Highway Research and Transportation Program Series No. 160. University of Illinois at Urbana-Champaign, 1976.

14. Dingqing Li and Ernest T. Selig. Resilient Modulus for Fine-Grained Subgrade Soils. Journal of Geotechnical Engineering, Vol. 120, No. 6, June 1994.

15. W. Lee, N. C. Bohra, A. G. Altschaeffl, and T. D. White, Resilient Modulus of Cohesive Soils, Journal of Geotechnical and Geoenvironmental Engineering, February 1997, pp. 131-136.

16. D. V. Holmquist, “Subgrade Investigation and Design of Airside Pavements at Denver International Airport,” Presentation to National Airport Conference on Pavement, 1993, Denver, Colorado.

17. Dallas N. Little, “Evaluation of the Structural Properties of Stabilized Pavement Layers,” Interim Report to the Texas Department of Transportation, Research Project 1287, 1993.

18. J. L. Cowley, P.E., “Geotextile Technology: Substitution Factors Between Mirafi Woven Stabilization Fabrics and Aggregate Base Thickness in Asphalt Pavements, Law Engineering, December 1981.

19. Dallas N. Little, “Stabilization of Pavement Subgrades and Base Courses With Lime,” National Lime Association, Kendall/Hunt Publishing Company, 1995.

20. Eric C. Drumm, Jason S. Reeves, Mark R. Madgett, and William D. Trolinger, “Subgrade Resilient Modulus Correction For Saturation Effects,” Journal of Geotechnical and Geoenvironmental Engineering, American Society of Civil Engineers, July 1997, Vol. 123, No.7.

21. E. J. Yoder and M. W. Witczak, Principles of Pavement Design, Second Edition, John Wiley & Sons, Inc., 1975.

22. G. Scot Gordon and Victoria Peters, Evaluation of Lime Stabilized Subgrade Structural Layer Coefficient, CTL/Thompson, Inc., Report for Jefferson County, Colorado, 1995.

23. M. Y. Shahin and J. A. Walter, Pavement Maintenance Management for Roads and Streets Using the PAVER System, USACERL Technical Report M-90/05, July 1990.

24. R. E. Smith, M. I. Darter, S. H. Carpenter, M. Y. Shahin, Adjusting Performance Curves for the Influence of Maintenance and Rehabilitation, ERES Consultants, Inc. for Metropolitan Transportation Commission, Oakland, California.


25. J. A. Epps and C. V. Wootan, Economic Analysis of Airport Rehabilitation Alternatives – An Engineering Manual, Report DOT/FAA/FD-81/78, Federal Aviation Administration, 1981.

26. U. S. Army Corps of Engineers, Life Cycle Costs for Pavements, Report F84-63, 1987.


APPENDIX A

SURVEY RESULTS


APPENDIX B

GLOSSARY OF TERMS


B-1

GLOSSARY OF TERMS

Adhesive Failure: Loss of bond between the joint sealant and the joint, or between the aggregate and the binder.

Aggregate Base (base course): Crushed stone or gravel, immediately under the surfacing material.

Aggregate Interlock: Interaction of aggregate particles across cracks and joints to transfer load.

Alligator Cracks: Interconnected cracks forming a series of small blocks resembling an alligator’s skin or chicken wire.

Analysis Period: The period of time for which the economic analysis is to be made; ordinarily will include at least one rehabilitation activity.

Asphalt Cement Concrete Pavement (ACCP): High-quality, thoroughly-controlled hot mixture of asphalt cement and well-graded, high quality aggregate, thoroughly compacted into a uniform dense mass.

Asphalt Emulsion Slurry Seal: A mixture of emulsified asphalt, fine aggregate and mineral filler, with water added to produce slurry consistency.

Asphalt Leveling Course: A course (asphalt-aggregate mixture) of variable thickness used to eliminate irregularities in the contour of an existing surface prior to superimposed treatment or construction.

Asphalt Overlay: One or more courses of asphalt construction on an existing pavement. The overlay generally includes a leveling course, to correct the contour of the old pavement, followed by uniform course or courses to provide needed thickness.

Asphalt Cement Concrete Pavements: Pavements consisting of a surface course of mineral aggregate coated and cemented together with asphalt cement on supporting courses such as asphalt bases; crushed stone, slag or gravel; or on Portland cement concrete pavement, brick, or block pavement.

Asphalt Cement Concrete Pavement Structure: A pavement structure, placed above the natural subgrade, with courses consisting of asphalt-aggregate mixtures, untreated aggregate courses, improved or stabilized subgrade, drainage layers, etc. which act in a structural capacity.


B-2

Asphalt Tack Coat: A light application of emulsified asphalt applied to an existing asphalt or Portland cement concrete pavement surface. It is used to ensure a bond between the surface being paved and the overlying course. Typically 0.10 gals/yd2 of CSS1h.

Binder: Asphalt Cement used to hold stones together for paving.

Binder Course: The layer of asphalt cement concrete pavement underlying the surface course.

Bituminous: Like or from asphalt.

Bleeding or Flushing: The upward movement of asphalt in an asphalt pavement resulting in the formation of a film on the pavement surface. It creates a shiny, glass-like, reflective surface that may be tacky to the touch in warm weather.

Block Cracking: The occurrence of cracks that divide the asphalt surface into approximately rectangular pieces, typically one square foot or more in size.

California Bearing Ratio Test (CBR): An empirical measure of bearing capacity used for evaluating bases, subbases, and subgrades for pavement thickness design.

Centerline: The painted line separating opposing traffic lanes.

Channels: See Rutting.

Chipping: Breaking or cutting off small pieces from the surface.

Chip Seal: A thin layer of emulsified asphalt cement in which aggregate is embedded. The seal is placed to improve the texture of the pavement surface to increase skid resistance and decrease permeability of the surface.

City Street: A street whose traffic is predominantly local in character.

Cohesive Failure: The loss of a material’s ability to bond to itself or its substrate. Results in the material splitting or tearing apart from itself or its substrate (i.e. joint sealant splitting).

Composite Pavement: A pavement structure composed of an asphalt cement concrete pavement wearing surface and Portland cement concrete pavement slab; an asphalt cement concrete pavement overlay on a PCC slab is also referred to as a composite pavement.

Corrugations (Washboarding): A form of plastic movement typified by ripples across the pavement surface. Most common in aggregate surficial pavements but occurs in asphalt cement concrete pavements as well.

Crack: Approximately vertical random cleavage of the pavement due to thermal or load action.


B-3

Deflection: The amount of downward vertical movement of a surface due to the application of a load to the surface.

--Rebound Deflection: The amount of vertical rebound of a surface that occurs when a load is removed from the surface.

--Representative Rebound Deflection: The mean value of measured rebound deflections in a test section plus two standard deviations, adjusted for temperature and most critical period of the year for pavement performance.

--Residual Deflection: The difference between original and final elevations of a surface resulting from the application to, and removal of one or more loads from, the surface.

Design ESAL: The total number of equivalent 80kN (18,000 lb) single-axle load applications expected during the Design Period.

Design Lane: The lane on which the greatest number of equivalent 80kN (18,000 lb) single-axle loads is expected. Normally this will be either lane of a two-lane roadway or the outside lane of a multi-lane highway.

Design Period: The number of years from initial construction or rehabilitation until terminal service life. This term should not be confused with pavement life or Analysis Period. By adding asphalt overlays as required, pavement life may be extended indefinitely, or until geometric considerations or other factors make the pavement obsolete.

Disintegration: The breaking up of a pavement into small, loose fragments due to traffic or weathering.

Distortion: Any change of a pavement surface from its original shape.

Drainage Coefficients: Factors used to modify layer coefficients in flexible pavements or stresses in rigid pavements as a function of how well the pavement structure can handle the adverse effect of water infiltration.

Edge Cracking: Fracture and materials loss in pavements without paved shoulders which occurs along the pavement perimeter. Caused by soil movement beneath the pavement.

Effective Thickness: The thickness that a pavement would be if it could be converted to Full-Depth asphalt cement concrete pavement.

Embankment (embankment soil): The prepared or natural soil underlying the pavement structure.

Embrittlement: Premature (surficial) cracking of an asphalt concrete pavement due to oxidative aging of the asphalt cement.


B-4

End Result Specifications: Specifications that require the contractor to take the entire responsibility for supplying a product or an item of construction. The highway agency’s responsibility is to either accept or reject the final product or apply a price adjustment that compensates for the degree of compliance with the specifications. (End result specifications have the advantage of affording the contractor flexibility in exercising options for using new materials, techniques, and procedures to improve the quality and/or economy of the end product.)

ESAL to Failure: The number of design 18 kip axle load cycles required to produce approximately 40 percent fatigue cracking as calculated using AAMAS equations based on asphalt cement concrete pavement Resilient Modulus and tensile strain at the bottom of the ACCP layer.

Equivalent 80kN (18,000 lb) Single-Axle Load (ESAL): The effect on pavement performance of any combination of axle loads of varying magnitude equated to the number of 80kN (18,000 lb) single-axle loads required to produce an equivalent effect.

Fatigue Cracking: A series of small, jagged, interconnecting cracks caused by failure of the asphalt cement concrete pavement surface under repeated traffic loading (also called alligator cracking.)

Fault: Difference in elevation between opposing sides of a joint or crack.

Flexible Pavement: see Pavement.

Free Edge: Pavement border that is able to move freely.

Full-Depth Asphalt Pavement: The term FULL-DEPTH (registered by the Asphalt Institute with the U.S. Patent Office) certifies that the pavement is one in which asphalt mixtures are employed for all courses above the subgrade or improved subgrade. A Full-Depth asphalt pavement is laid directly on the prepared subgrade.

Functional Classification: A method of separating and classifying streets according to their purpose or function in the network of streets, i.e. residential collectors, commercial collectors, residential locals.

Grade Depressions: Localized low areas of limited size which may or may not be accompanied by cracking.

Hairline Crack: A fracture that is very narrow in width, less than 3mm (0.12 in.).

Heavy Trucks: Two axle, six-tire trucks or larger. Pickup, panel and light four-tire trucks are not included. Trucks with heavy-duty, wide base tires are included.

Hydroplaning: The dangerous action of a vehicle being driven on a pavement over which a film of rain or other water has formed; on reaching a certain speed, the vehicle’s tires tend to ride upon the water surface rather than the pavement, drastically reducing the driver’s control of the vehicle.


B-5

Incentive/Disincentive Provision (for quality): A pay adjustment schedule which functions to motivate the contractor to provide a high level of quality. (A pay adjustment schedule, even one which provides for pay increases, is not necessarily an incentive/disincentive provision, as individual pay increases/decreases may not be of sufficient magnitude to motivate the contractor toward high quality.)

Instability: The lack of resistance to forces tending to cause movement or distortion of a pavement structure.

Lane Line: Boundary between travel lanes, usually a painted stripe.

Lane-to-Shoulder Drop-off: The difference in elevation between the traffic lane and shoulder.

Lane-to-Shoulder Separation: Widening of the joint between the traffic lane and the shoulder.

Layer Coefficient: The empirical relationship between structural number (SN) and layer thickness which expresses the relative ability of a material to function as a structural component of the pavement.

Lime Stabilized Subgrade: A prepared and mechanically compacted mixture or lime, water and soil below the pavement system.

Lime-Fly Ash Base: A blend of mineral aggregate, lime, fly ash and water, combined in proper proportions which, when compacted, produces a dense mass.

Load Equivalency Factor (LF): A factor used to convert applications of axle loads of any magnitude to an equivalent number of 80kN (18,000 lb) single axle loads.

Longitudinal: Parallel to the centerline of the pavement.

Longitudinal Crack: A crack that follows a course approximately parallel to the center line.

Maintenance: The preservation of the entire roadway, including surface, shoulders, roadsides, structures, and such traffic control devices as are necessary for its safe and efficient utilization.

Materials/Methods Specifications: Specifications that direct the contractor to use specified materials in definite proportions and specific types of equipment and methods to place the material.

Method Specifications: see Materials/Methods Specifications.

Parametric Analysis: A study of a set of physical properties whose values determine the characteristics or behavior of something. Used to isolate the significance of individual variables.


B-6

Patch: An area where the existing pavement has been removed and replaced with a new material.

Patch Deterioration: Distress occurring within a previously repaired area.

Pavement Structure (pavement): A combination of subbase, base course, and surface course placed on a subgrade to support the traffic load and distribute it to the roadbed.

Pavement Condition Indicator (PCI): A measure of the condition of an existing pavement section at a particular point in time, such as cracking measured in feet per mile, or faulting measured in inches of wheelpath faulting per mile. When considered collectively, pavement condition indicators provide an estimate of the overall adequacy of a particular roadway.

Pavement Design (design, structure design): The specifications for materials and thicknesses of the pavement components .

Pavement Distress Indicator: see Pavement Condition Indicator.

Pavement, Flexible: A pavement structure generally consisting of asphalt cement concrete pavement surfacing, base and/or subbase.

Pavement Performance: The trend of serviceability with load applications.

Pavement Rehabilitation: Work undertaken to extend the service life of an existing facility. This includes placement of additional surfacing material and/or other work necessary to return an existing roadway, including shoulders, to a condition of structural or functional adequacy. This could include the complete removal and replacement of the pavement structure.

Pavement, Rigid: A pavement structure consisting of Portland cement concrete pavement surfacing, with or without subbase.

Performance Period: see Design Period.

Performance Specifications: Specifications that describe how the finished product should perform over time. For highways, performance is typically described in terms of changes in physical condition of the surface and its response to load, or in terms of the cumulative traffic required to bring the pavement to a condition defined as “failure”. Specifications containing warranty/guarantee clauses are a form of performance specifications. (Other than the warranty/guarantee type, performance specifications have not been used for major highway pavement components (subgrades, bases, riding surfaces) because there have not been appropriate nondestructive tests to measure long-term performance immediately after construction. They have been used for some products (e.g., highway lighting, electrical components, and joint sealant materials) for which there are test of performance that can be rapidly conducted.)

Performance-Based Specifications: Specifications that describe the desired levels of fundamental engineering properties (e.g., Resilient Modulus, creep properties, and


B-7

fatigue properties) that are predictors of performance and appear in primary prediction relationships (i.e., models that can be used to predict pavement stress, distress, or performance from combinations of predictors that represent traffic, environmental roadbed, and structural conditions.) [Because most fundamental engineering properties associated with pavements are currently not amenable to timely acceptance testing, performance-based specifications have not found application in highway construction].

Performance-Related Specifications: Specifications that describe the desired levels of key materials and construction quality characteristics that have been found to correlate with fundamental engineering properties that predict performance. These characteristics (for example, air voids in asphaltic pavements, and strength of concrete cores) are amenable to acceptance testing at the time of construction. True performance-related specifications not only describe the desired levels of these quality characteristics, but also employ the quantified relationships containing the characteristics to predict subsequent pavement performance. They thus provide the basis for rational acceptance and/or price adjustment decisions.

Planned Stage Construction: The construction of roads and streets by applying successive layers of asphalt cement concrete pavement according to design and a predetermined time schedule.

Plant-Mix Base: A foundation course, produced in an asphalt mixing plant, which consists of a mineral aggregate uniformly coated with asphalt cement or emulsified asphalt.

Pothole: A bowl-shaped depression of varying sizes in the pavement surface, resulting from localized disintegration.

Prepared Roadbed: In-place roadbed soils compacted or stabilized according to provisions of applicable specifications.

Prescriptive Specifications: see Materials/Methods Specifications.

Present Serviceability: The ability of a specific section of pavement to serve, for the use intended, mixed traffic on the day of rating.

Present Serviceability Index (PSI): A mathematical combination of values, obtained from certain physical measurements of a large number of pavements, so formulated as to predict, within prescribed limits, the Present Serviceability Rating (PSR) for those pavements.

Present Serviceability Rating (PSR): The mean of the individual ratings made by the members of a specific panel selected for the purpose.

QA/QC Specifications: see Quality Assurance Specifications.

QC/QA Specifications: see Quality Assurance Specifications.


B-8

Quality Assurance: All those planned and systematic actions necessary to provide confidence that a product or facility will perform satisfactorily in service. Quality assurance addresses the overall problem of obtaining the quality of service, product, or facility in the most efficient, economical, and satisfactory manner possible. Within this broad context, quality assurance involves continued evaluation of the activities of planning, design, development of plans and specifications, advertising and awarding of contracts, construction, and maintenance, and the interactions of these activities.

Quality Assurance Specifications: A combination of end result specifications and materials and methods specifications. The contractor is responsible for quality control (process control), and the highway agency is responsible for acceptance of the product. (Quality assurance specifications typically are statistically based specifications that use methods such as random sampling and lot-by-lot testing, which let the contractor know if his operations are producing an acceptable product.)

Quality Control: Those quality assurance actions and considerations necessary to assess production and construction processes so as to control the level of quality being produced in the end product. This concept of quality control includes sampling and testing to monitor the process but usually does not include acceptance sampling and testing.

Raveling: The wearing away of the pavement surface caused by the dislodging of aggregate particles.

Recipe Specifications: see Materials/Methods Specifications.

Reflection Cracking: Cracks in asphalt overlays that reflect the crack pattern in the pavement structure underneath.

Resilient Modulus Test: A measure of the modulus of elasticity of roadbed soil or other pavement material.

Resistance Value (R-value): A test for evaluating bases, subbases, and subgrades for pavement thickness design.

Roadbed: The graded portion of a highway between top and side slopes, prepared as a foundation for the pavement structure and shoulder.

Roadbed Material: The material below the subgrade in cuts and embankments and in embankment foundations, extending to such depth as affects the support of the pavement structure.

Roadway: All facilities on which motor vehicles are intended to travel such as secondary roads, interstate highways, streets and parking lots.

Roadway Land Use: A classification based on the use of land adjacent or serviced by the street. The classification is used to separate streets for different volume assumptions.


B-9

Roughometer: A single-wheeled trailer instrumented to measure the roughness of a pavement surface in accumulated millimeters (inches) per mile.

Rubberized Asphalt Cement: Blend of asphalt cement and pre-vulcanized rubber.

Rutting: Longitudinal surface depressions in the wheelpaths.

Selected Material: A suitable native material obtained from a specified source such as a particular roadway cut or borrow area, of a suitable material having specified characteristics to be used for a specific purpose.

Serviceability: The ability at time of observation of a pavement to serve traffic (autos and trucks) which use the facility.

Shoving: Permanent, longitudinal displacement of a localized area of the pavement surface caused by traffic pushing against the pavement.

Single Axle Load: The total load transmitted by all wheels of a single axle extending the full width of the vehicle.

Skid Hazard: Any condition that might contribute to making a pavement slippery when wet.

Slippage Cracks: Cracks, sometimes crescent-shaped, that point in the direction of the thrust of wheels on the pavement surface.

SMA (Stone-Matrix Asphalt, Split-Mastic Asphalt): An asphalt mix design composed of large stones creating a stone to stone matrix, often containing large percentages of asphalt cement and fillers.

Soil Cement Base: A hardened material formed by curing a mechanically compacted intimate mixture of pulverized soil, Portland cement and water, used as a layer in a pavement system to reinforce and protect the subgrade or subbase.

Stabilized Subgrade: A subgrade soil that has been altered by a chemical agent to make suitable for subgrade construction and pavement support.

Standard Deviation: The root-mean-square of the deviations about the arithmetic mean of a set of values.

Statistically Based Specifications: Specifications based on random sampling, and in which properties of the desired product or construction are described by appropriate statistical parameters.

Structural Number (SN): An index number derived from an analysis of traffic, roadbed soil conditions, and environment which may be converted to thickness of flexible pavement layers through the use of suitable layer coefficients related to the type of material being used in each layer of the pavement structure.


B-10

Subbase: The layer or layers of specified or selected material of designed thickness placed on a subgrade to support a base course.

Subbase (subbase course): The layer of graded sand-gravel or stabilized subgrade material between the surface of the embankment soil and the base course (or surfacing course when there is no base course).

Subgrade: The soil prepared to support a structure of a pavement system. It is the foundation for the pavement structure. The subgrade soil sometimes is called “basement soil” or “foundation soil”.

Subgrade, Improved: Any course or courses of select or improved material between the subgrade soil and the pavement structure.

Subgrade Resilient Modulus: The modulus of the subgrade determined by repeated load triaxial compression tests on soil samples. It is the ratio of the amplitude of the accepted axial stress to the amplitude of the resultant recoverable axial strain.

Surface (Surface Course): One or more layers of a pavement structure designed to accommodate the traffic load, the top layer of which resists skidding, traffic abrasion, and the disintegrating effects of climate. The top layer of flexible pavements is sometimes called the “wearing course”.

Surface Thickness (surfacing thickness, surface, slab thickness (rigid)): The thickness of surfacing materia, usually expressed in inches.

Tandem Axle Load: The total load transmitted to the road by two consecutive axles extending across the full width of the vehicle.

Thermal Cracking: Cracking occurring in pavement material introduced within the material resulting from a change in temperature.

Traffic Equivalence Factor: A numerical factor that expresses the relationship of a given axle load to another axle load in terms of their effect on the serviceability of a pavement structure.

Transverse Crack: A crack that follows a course approximately at right angles to the centerline.

Triple (Tridem) Axle Load: The total load transmitted to the road by three consecutive axles extending across the full width of the vehicle.

Truck Factor: The number of equivalent 80kN (18,000 lb) single-axle load applications contributed by one usage of a vehicle. Truck Factors can apply to vehicles of a single type or class or to a group of vehicles of different types.

Twenty-Year ESAL: (ESAL20) The Equivalent Single Axle Load application for a twenty-year design. The value is the product of the Load Equivalency factor for each vehicle


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type, the number of each particular vehicles per day, 365 days per year, and a twenty-year period.

Upheaval: The localized upward displacement of a pavement due to swelling of the subgrade or some portion of the pavement structure.

Washboarding: see Corrugations.

Water Bleeding: Seepage of water from joints or cracks.

Weathering: The wearing away of the pavement surface caused by the loss of asphalt binder.


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APPENDIX C

ANNOTATED LITERATURE REVIEW


APPENDIX D

WARRANTIES AND QUALITY ASSURANCE / QUALITY CONTROL


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QUALITY CONTROL / QUALITY ASSURANCE / WARRANTIES

Any discussion of quality must consider the Engineer’s, Contractor’s and Owner’s

perspectives. Owners must first decide the level of control they wish to exert over the

Contractor’s operations. There are three levels of control; 1) complete testing and

inspection, 2) oversight testing, or 3) warranty control. Each of these levels requires a

different type of specification from the Engineer and a different level of effort by the

Contractor.

The most difficult specification to develop is the procedural specification. This

specification dictates to the Contractor what to build, what to build it from and how to

build it. It is the easiest type of specification for the Owner to test and inspect. It is

prescriptive in nature and easily lends itself to a “cookbook” procedure. The Owner can

impose penalties or price reduction if the Contractor does not follow the recipe.

Unfortunately, most penalties are only token in nature and seldom reflect the actual cost

to the public. For instance, analyses have shown that asphalt cement concrete

pavement compacted to 90 percent rather than 93 percent will lose 20 to 50 percent of

its service life, while penalties are less than 10 percent. Warranties for this type of

specification are typically one to 3 years in length and only cover workmanship and

materials. The procedural specification can be tempered to some extent by providing

performance criteria rather than procedural. For instance, the specification may require

compaction to 95 percent rather than “a minimum of six passes with a 20-ton steel wheel

roller at 3 mph.”

This modified procedure/performance specification is the most common in the

industry. It serves to specify certain procedures at times and performance criteria for

other items. It is easily tested and inspected and provides the Owner with control over

the project and some recourse to penalties, price reduction and warranties.

The Quality Control / Quality Assurance or oversight testing specification is normally

applicable only to large projects. In this type of specification, the Contractor is

responsible for his own quality both in terms of materials and procedures. The

Contractor is required to develop a QC plan and provide testing and inspection of his

own work. The Owner’s testing and inspection is largely oversight in nature and usually


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involves 10 to 15 percent of the Contractor’s QC effort. Like the procedural

specification, the QA/QC specification can have both procedural and performance

components as well as penalties and price reductions. Warranties are likewise limited to

workmanship and materials. CDOT has been using the QA/QC specification on a limited

basis. Both Denver International Airport and E-470 use or used QA/QC specifications.

Experience to date has been mixed. Some contracts have worked well while other have

been nightmares leading to litigation.

The last type of specification is warranties. This is a relatively new form of contract

specification in the U. S.; but has been successfully used in Europe for years. Warranty

periods are typically 5 years in the U. S. and more often 10 years in Europe. This

specification will have limited testing and inspection on the Owner’s part and must not be

restrictive. An Owner cannot require specific procedures, nor materials in the design.

For instance, the Owners cannot specify an asphalt mix then expect the Contractor to

warrant the mix against rutting or raveling. Furthermore, the Owner cannot ask the

Contractor to warrant things beyond his control. The development of a warranty

specification must be a joint effort between the Owner, Engineer and Contractor. The

warranty specification must address measurement of performance. Asphalt pavements

will not be in perfect condition after 5 years of service. The questions then become; How

much distress and of what type is allowed? Other issued in warranties include:

1) Who is responsible for maintenance and at which intervals?

2) What if the traffic load changes?

3) Are there penalties and price reductions imposed during construction? If so, what basis?

4) What are the bonding requirements?

5) Should Contractors be pre-qualified?

6) How does the Contractor share design responsibility?

It is believed that warranty specifications will become more popular in the future.

The projects built to date appear to be successful and success will promulgate similar


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programs. Major projects lend themselves to warranties, particularly design/build. At

this time, warranty projects offer little to the municipality or county. The time and effort

involved with specification development would tax the abilities of most agencies.

The QA/QC specification is attractive because the costs associated with testing and

inspection can be rolled into construction budgets. The experience base with the

contracting community is very limited and the previous projects have not been totally

successful. For these reasons, it is recommended to not implement QA/QA

specifications. Given time and direction by CDOT, Contractors will become more

comfortable and proficient. Likewise, warranty specifications are too new to develop any

experience base and should be reviewed at a later date. Exceptions to these

recommendations include a large project, particularly design/build where sufficient time

and cooperation between the parties are likely.

At this time, the more common procedural/performance specification is most

appropriate for municipalities and counties. The agencies should keep control through

testing and inspection. Penalties and price reduction should be carefully evaluated and

should reflect life and serviceability reduction. Specifications should also consider

remedial measures to correct or replace defective work and/or materials.


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APPENDIX E

MATERIAL TEST RESULTS

Documents

DEVELOPMENT OF PAVEMENT DESIGN CONCEPTS for Rational Pavement Design