TECHBRIEF

The Long Term Pavement Performance

(LTPP) program is a 20-year study of inservice pavements across North America. Its goal is to extend the life of highway pavements through various designs of new and rehabilitated pavement structures, using different materials and under different loads, environments, subgrade soil, and maintenance practices. LTPP was established

under the Strategic Highway Research

Program, and is now managed by the

Federal Highway Administration.

Publication No. FHWA-RD-00-165

FHWA Contact: Cheryl Richter, HRDI-13, (202) 493-3148

Background

A primary objective of the Long Term Pavement Performance (LTPP) program is to develop improved design methodologies and strategies for the

rehabilitation of existing pavements. One of the experiments designed to

address this objective is General Pavement Study (GPS) 6. 

The GPS-6 experiment, “AC Overlay of AC Pavements,” involves pavement test sections where an asphalt concrete (AC) overlay was placed on

an existing AC pavement. The GPS-6 experiment is further divided into

two parts—GPS-6A and GPS-6B test sections. The GPS-6A part of the experiment includes those test sections for which a detailed condition survey of the existing surface was not performed prior to overlay placement.

Conversely, the GPS-6B part includes those sections for which detailed

distress surveys were performed prior to overlay. There are 60 GPS-6A

test sections and 65 GPS-6B test sections in the LTPP program.

This TechBrief summarizes the results of a study of the GPS-6 experiment,

entitled “Performance of Rehabilitated Asphalt Concrete Pavements in the

LTPP Experiments—Data Collected Through February 1997.” The study

documents performance trends of the 125 GPS-6 test sections using distress data collected through February 1997. The test sections represent a

diverse range of conditions. The age of the overlays range from 0.1 to 26.4

years (with an overall mean age of 7.3 years), while the traffic levels range

from 10 to 1,900 thousand equivalent single-axle loads (KESALs) per year

(with an overall mean of 300 KESALs per year).

Distresses Considered in the Study

Six distress types or performance indicators were used to evaluate the performance trends or characteristics of the LTPP GPS-6 test sections. They include fatigue cracking, longitudinal cracking in the wheelpath, longitudinal

cracking not in the wheelpath, transverse cracking, rutting, and roughness

(as measured by the International Roughness Index [IRI]). The extent of

these distresses was divided into different categories for relative comparisons. The different levels of distress used in the study are defined in table

1 on the following page. Table 2, also on the following page, shows the percentages of the GPS-6 test sections having nominal and greater than nominal levels of distress, respectively. As table 2 shows, more than half of the

GPS-6 test sections have no fatigue cracking, longitudinal cracking in the

wheelpath, or longitudinal cracking not in the wheelpath.

Research, Development, and

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Performance Trends of 

Rehabilitated AC Pavements
Fatigue and Longitudinal

Cracking in Wheelpaths

Only 15 percent of the GPS test

sections have more than a

nominal level of fatigue cracking and only 10 percent have

longitudinal cracking in the

wheelpath greater than the

nominal level. Most of the GPS

test sections have performed

well past 10 years of age with

little fatigue cracking. The following is a summary of observations regarding the occurrence of fatigue cracking and

longitudinal cracking in the

wheelpath:

• The GPS-6A data show that

overlay designs that provide

pavement structure consistent with traffic expectations

can be expected to perform

well for more than 10 years.

• The study concludes that fatigue cracking and longitudinal cracking in the wheelpath are related. Specifically,

the longitudinal cracking in

the wheelpath will eventually propagate or evolve into

fatigue cracking with continued traffic loading.

Transverse Cracking

Thirty-five percent of the GPS-6

test sections have more than a

nominal amount of transverse

cracking. Although transverse

cracking was found to increase

with age, some overlays have

survived with limited or no

transverse cracking for long periods of time. The following is a

list of general observations regarding the occurrence of

transverse cracking:

• As the thickness of the overlay increases, the incidence

of transverse cracking decreases.

• Where transverse cracking has

occurred, the data show that

the amount of cracking is dependent on the condition of

the original pavement prior to

overlay placement. The overlays placed on pavements

that were classified as being

in good condition exhibit

about a 50-percent increase in

time to the same level of

transverse cracking as those

overlays placed over pavements that were classified as

being in poor condition.

• The age of the AC overlay

was found to have an effect

on the occurrence of transverse cracks for thin overlays (less than 60 mm), but

no measurable effect for the

thicker overlays. As the

thickness of the AC layer increases, the binder and mixture properties become

much more important and

the age of the mix becomes

less important. Although the

LTPP data do not conclusively support those findings,

they do not contradict them.

• While it is a widely accepted

belief that transverse cracking is, to some degree, a result of low temperatures, only a moderate amount of

transverse cracking occurs

in the Canadian test sections, where low temperatures are common.

Distress Type Nominal Moderate Excessive

Fatigue cracking, m2 1 - 10 11 - 60 > 60

Longitudinal cracking in the wheelpath, m 1 - 50 51 - 160 > 160

Longitudinal cracking not in the wheelpath, m 1 - 50 51 - 160 > 160

Transverse cracks, no. 1 - 10 11 - 50 > 50

Rutting, mm < 7 7 - 20 > 20

Roughness (IRI), m/km < 1.6 1.6 - 2.4 > 2.4

Table 1. Magnitude of distress for each category.
• The data show that traffic levels are not particularly important to the occurrence of

transverse cracks in AC overlays.

Longitudinal Cracking Not

in Wheelpath

Twenty-one percent of the

GPS-6 test sections have exhibited greater than nominal

longitudinal cracking not in the

wheelpath. The following is a

list of general observations regarding the occurrence of longitudinal cracking not in the

wheelpath:

• Thicker overlays consistently

have less longitudinal cracking not in the wheelpath, as

well as a lower incidence of

cracking.

• Overlay age and condition of

the pavement prior to the

overlay appear to have little

or no impact on the performance of the overlay in resisting longitudinal cracking

not in the wheelpath. However, 45 percent of the overlays over pavements known

to have longitudinal cracking

not in the wheelpath prior to

the overlay have successfully resisted reflection of these

cracks through to the surface

during their early years. 

Rutting

Thirty-three percent of the

GPS-6 test sections have rut

depths that are greater than

the nominal level of rutting selected for this study. The following is a list of general observations regarding the occurrence of rutting:

• Thick overlays are not superior to thin overlays in resisting rutting.

• Traffic levels are important in

predicting rutting, but other

factors (such as material

properties and construction

techniques/quality control)

are probably more important.

• Although rutting is a problem

nationwide, it is not a significant problem in the LTPP

test sections. Indeed, as far

as rutting as a pavement distress is concerned, the LTPP

test sections represent pavements that tend not to rut.

Hence, LTPP can provide information on how to design

pavements that do not rut

versus data explaining why

pavements are rutting.

• For the majority of AC overlays in the LTPP program, rutting becomes sufficient to require rehabilitation only after

15 years or more (with traffic

levels ranging from 10 to

1,877 KESALs per year). In

fact, excessive rut depths

Fatigue cracking 76 9 8 7 

Longitudinal cracking in wheelpaths 61 30 10 0 

Transverse cracking 40 25 27 8 

Longitudinal cracking not in 

wheelpaths 52 27 17 4 

Rutting n/a* 67 33 0 

Roughness n/a 79 17 4

*n/a = not applicable

Distress Type

Levels of Distress

Greater Than Nominal, %

Moderate Excessive

None, % Nominal, %

Table 2. Percentages of GPS-6 test sections with respective levels of distress.
have been measured on only

a limited number of the LTPP

test sections through 1997. 

Roughness

Twenty-one percent of the

GPS-6 test sections have IRI

values that are greater than

the nominal level of roughness

selected for use in this study.

The following is a list of

general observations regarding roughness:

• Substantial reductions in

pavement roughness can be

obtained by the placement

of an AC overlay, even for

pavements that were not especially rough when constructed.

• The long-term control of

roughness can be attained

with thin and thick overlays.

In other words, the increase

in roughness for both thin

and thick overlays is quite

nominal for many years after

an overlay is placed, even

when placed on pavements

that have moderate to excessive levels of roughness.

• The condition of the original

pavement for the LTPP test

sections appears to have little to do with the initial

roughness of the overlay or

in the long-term growth of

roughness in the overlay.

• The amount of traffic (or

ESALs) on an overlay clearly affects the growth of

roughness or increasing IRI

values, but it is quite possible to construct AC overlays

for heavy traffic that will remain smooth for 15 or more

years.

General Summary and

Conclusions

Clearly, the majority of the AC

overlays included in the LTPP

database have served for 15

years or more before the loadand non-load-related distresses became sufficient to require

rehabilitation. More importantly, there are a number of

test sections where the overlays have less than only nominal levels of distress for more

than 20 years of service. Additional monitoring on these

sites will be extremely valuable as the materials, traffic,

and climate data become

available.

From this and other studies, it

appears that rutting and

roughness distress are somewhat easier to deal with than

cracking. The data show that

the long-term control of rutting and roughness is gained

or lost during construction. If

the AC mix will resist rutting

adequately and is placed at a

reasonable density, the early

permanent deformation will

be limited and the future rutting rate will be nominal. Similarly, if the overlay material is

not subjected to excessive permanent deformation and is

placed relatively smooth (approximate IRI of 0.8 m/km), it

probably will not become very

rough over its service life.

Researcher—This study was performed by Fugro-BRE, 8240 MoPac Expressway, Suite 220, Austin, TX

78759, telephone: (512) 346-0870, fax: (512) 346-8750. Contract No. DTFH61-96-C-00003.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution

is being made to the Resource Centers and Divisions.

Availability—This TechBrief is based on Performance of Rehabilitated Asphalt Concrete Pavements in

the LTPP Experiments – Data Collected Through February 1997 (Report No. FHWA-RD-00-029). Copies are

available from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161. A

limited number of copies will be available from the R&T Report Center, HRD-11, FHWA, 9701 Philadelphia

Court, Unit Q, Lanham, MD 20706, telephone: (301) 577-0818, fax: (301) 577-1421.

Key Words—Asphalt concrete pavements, rehabilitation, and asphalt concrete overlays.

Notice—This TechBrief is disseminated under the sponsorship of the Department of Transportation in

the interest of information exchange. The TechBrief provides a synopsis of the study’s final publication.

The TechBrief does not establish policies or regulations, nor does it imply FHWA endorsement of the conclusions or recommendations. The U.S. Government assumes no liability for the contents or their use.

OCTOBER 2000 FHWA-RD-00-165