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-00-166

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

Background

A key 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 Specific

Pavement Study (SPS) 5.

The SPS-5 experiment, “Study of Rehabilitation of Asphalt Concrete Pavements,” has 17 projects. Each project has nine test

sections. The nine test sections consist of one control section

(no rehabilitation applied to the surface) and eight test sections

with different combinations of the following strategies:

• Thin (51-mm) and thick (127-mm) overlays.

• Virgin and recycled mixtures used for the overlay.

• Milled and non-milled surfaces prior to overlay placement.

This TechBrief summarizes the results of a study of the SPS-5

experiment entitled, “Performance of Rehabilitated Asphalt

Concrete Pavements in the LTPP Experiments—Data Collected

Through February 1997.” The study documents performance

trends of the 17 SPS-5 projects using distress data collected

through February 1997. The age of these SPS-5 projects vary

from 0.1 to 6.1 years, but most are less than 4 years old. 

Distresses Considered in the Study

Six distress types or performance indicators were used in the

study to evaluate the performance trends of the LTPP SPS-5 test

sections. These distress types 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 area

or extent of the distress was divided into different categories for

relative comparison. The different levels of distress used in the

study are defined in table 1 on the following page.

Research, Development, and

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Comparison of Rehabilitation 

Strategies for AC Pavements
Table 2 shows that more than

half of the SPS-5 test sections

had no fatigue cracking, longitudinal cracking in the wheelpath, or longitudinal cracking

not in the wheelpath, and only

a small percentage had exceeded the nominal levels of

distress.

Table 3 provides the results

from a comparison of the performance of overlaid test sections with thin and thick overlays, virgin or recycled overlay

mixes, and milled and unmilled test sections. However,

these are preliminary results

and continued monitoring and

materials data are needed to

achieve the objectives of the

experiment. 

The following is a summary of

observations regarding each

treatment type:

Hot-Mix Asphalt (HMA)

Overlay Thickness

• As expected, the nominal

127-mm overlays have less

fatigue cracking, and fewer

transverse cracks and longitudinal cracking outside the

wheelpaths than the nomiDistress 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.

Fatigue cracking 85 10 5 0 

Longitudinal cracking in wheelpaths 58 27 8 7 

Transverse cracking 58 30 12 0 

Longitudinal cracking not in 

wheelpaths 54 19 18 9 

Rutting n/a* 84 16 0 

Roughness n/a 93 7 0

*n/a = not applicable

Distress Type

Levels of Distress

Greater Than Nominal, %

Moderate Excessive

None, % Nominal, %

Table 2. Percentages of SPS-5 test sections with none, nominal, or greater than nominal levels

of distress for AC overlays 2 years of age or older.
nal 52-mm overlays, although there were exceptions. The average area of fatigue cracking that occurred

on those test sections with a

thin overlay is 10.2 m2 and

only 4.1 m2 for those with a

thick overlay.

• The thicker overlays consistently have less longitudinal

cracking not in the wheelpath, as well as a lower incidence of this type of cracking.

• Overlay thickness did not appear to have a strong effect

on the occurrence of longitudinal cracking in the wheelpath and rutting, and there

was no apparent effect on

roughness based on these

early observations.

Milling With Overlays

• In general, it is believed that if

you mill off damaged asphalt concrete (AC) material

and replace it with new AC

material, the overlay will last

longer than if you left the

damaged material in place.

The initial performance

trends from the SPS-5 projects, however, appear to

contradict this hypothesis.

Currently, certain materials

data for these test sections

are not available. Review of

this data, when available,

may provide insight into this

apparent contradiction.

• The test sections that were

milled prior to the overlays

generally performed better

than the unmilled test sections for transverse cracking,

but they seemed to have little or no advantage in resisting both types of longitudinal cracking, fatigue cracking, rutting, or roughness

(over the early life of the

overlay).

• The data consistently show

fewer transverse cracks on

milled surfaces, compared

to unmilled surfaces, prior to

overlay placement. This

seems logical because removal of the top material

from the original AC layer

should reduce the effects of

the cracks in the original

pavement on the overlay,

while replacement of the

milled materials in effect increases the thickness of the

uncracked new material over

the original pavement. This

difference in the number of

transverse cracks, however,

does not represent a significant difference.

• The data show that milling offers no consistent advantage

for resisting longitudinal

cracking not in the wheelpath during the early life of

an overlay. For half of the

projects, the milled test secFatigue cracking Less Less No advantage

over virgin

Longitudinal cracking in wheelpaths No advantage No advantage More

Transverse cracking Less Less No advantage

over virgin

Longitudinal cracking not in No advantage No advantage Less 

wheelpaths

Rutting No advantage No advantage No advantage

over virgin

Roughness No advantage No advantage No advantage

over virgin

Distress Type

Factor

Recycled

Mix

Table 3. Summary of the results from the SPS-5 data.

Overlay Thickness

Increasing

Milling

Surface
tions performed better than

the unmilled test sections,

while the reverse was true

for the other half of the projects. The only conclusion

that can be drawn at this

time with respect to milling

versus non-milling is that

milling apparently had little

effect in the short run.

Overlay Mixture Type—

With and Without Recycled

Asphalt Pavement

• The effect of mix type (virgin

versus recycled) for the

overlays seemed to be important only for the two

types of longitudinal cracking (longitudinal cracking in

the wheelpath and longitudinal cracking not in the

wheelpath). However, the

results contradict one another as shown in table 3. The

test sections with recycled

mixes had more longitudinal cracking in the wheelpath than the test sections

with virgin mixes, while the

test sections with virgin mixes had more longitudinal

cracking not in the wheelpath than the sections with

recycled mixes.

• It is generally believed that

mixtures with recycled asphalt pavement are stiffer

(higher moduli), but are no

more or less susceptible to

repeated load fracture than

those mixtures without recycled asphalt pavement. The

initial performance trends

from the SPS-5 projects are

in line with this hypothesis.

• There is no benefit or advantage derived from using one

mix type over the other (virgin versus recycled mixes) in

reducing the number of

transverse cracks.

• The recycled AC mixes resisted longitudinal cracking not

in the wheelpath substantially better than the virgin mixes for five of the six projects,

with the overall average

amount of longitudinal

cracking not in the wheelpath being only 40 percent of

that for the virgin mixes.

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 will

be 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.

NOVEMBER 2000 FHWA-RD-00-166