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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-04-032
Key Findings From LTPP Analysis 2000-2003
Pavement roughness greatly affects ride qualilty, safety, and vehicle operating costs. The following are
key findings from several LTPP studies to enhance understanding of how and why roughness occurs in pavements.
- Findings From Specific Pavement Studies
- Findings From General Pavement Studies
- Report No. NCHRP 20-50(8/13)
- Increase in IRI in the SPS-1 (new AC pavements) projects is
attributed to pavement distresses such as transverse cracking,
longitudinal cracking in the wheel path, fatigue cracking,
- In the SPS-2 (new PCC pavements) projects, no clear relationship
between IRI changes and pavement distress was found.
- Generally, for the SPS-6 (rehabilitation of PCC pavements)
projects, diamond-ground sections that have higher values
of IRI before overlay are showing a higher rate of
increase of IRI.
- Report No. NCHRP 20-50(3/4)
- In the SPS-3 experiment (maintenance of flexible pavements),
the thin (38-mm (1.5-inch)) AC overlay treatment has
a small but significant effect in initial reduction of roughness.
It is the only one of the four treatments studied (thin AC
overlays, chip seals, slurry seals, and crack seals) to have a
significant effect on long-term pavement roughness.
- Based on the SPS-5 (rehabilitation of AC pavements) data
and the GPS-6B (new AC overlays on AC pavements) data,
initial post-treatment asphalt overlay IRI depends on pretreatment
asphalt pavement IRI. Higher pre-treatment
asphalt pavement IRI results in higher post-treatment
asphalt pavement IRI.
- Report No. FHWA-RD-00-029
Based on 8 years of data collected in the SPS-5 experiment
(rehabilitation of AC pavements) in the United States and
Canada, the long-term control of roughness generally can
be attained with thin (51mm (2inch)) and thick (127mm (5
inch)) AC overlays. However, the success of each project
depends on various factors such as surface preparation,
traffic loads, climatic regions, and pavement conditions
before the overlay is placed.
- Report No. NCHRP 20-50 (8/13)
- In the GPS-1 experiment (AC pavements on granular base),
the strongest relationships between the rate of increase of
IRI over time and an evaluated parameter exist for the following
parameters: percentage of base material passing
No. 200 sieve, freezing index, and plasticity index of subgrade.
Higher parameter values induce higher rates of IRI
- In the GPS-2 experiment (AC pavements on stabilized
base), those sections built over asphalt-treated bases
with high AC void ratios (percentage of air voids per unit
AC volume) have a higher rate of increase of IRI. Also,
those sections built over cement-treated bases in
warmer climates have a higher rate of increase of IRI.
- Among the doweled pavements in the GPS-3 experiment
(JPCP), those pavements with dowels have less joint faulting,
which results in lower IRI values than those pavements
without dowels. Higher IRI values are associated with a high
number of wet days.
- In the GPS-4 experiment (JRCP), pavements with low cement
content (less than 300 kg/cubic meter (505 lbm/cubic yard))
or high water-cement ratios (greater than 0.50) have higher IRI
- In the GPS-5 experiment (CRCP), 90 percent of CRCP sections
are located in the wet climatic region (freeze and nofreeze).
In the region, higher levels of roughness are associated with those sections with higher PCC elastic
moduli and higher ratios between PCC elastic moduli and
- In the GPS-6 experiment (AC overlay of AC pavements),
the IRI rate of increase on overlaid pavements is related to
the IRI prior to overlay.
- In the GPS-7 (AC overlay of PCC pavements) experiment,
initial results indicate that high rates of IRI increase were
observed for overlays on PCC sections that have high
PCC elastic modulus.
- Base and Subgrade
Report No. FHWA-RD-O1-16
- HMA pavements with unbound aggregate layers have
slightly more fatigue cracking and higher IRI values than
do those with asphalt-treated base layers.
- Jointed plain concrete pavements constructed on coarsegrained
subgrade soils are smoother than pavements
constructed on fine-grained subgrade soils. This confirms
a similar finding from a previous study.
Report No. NCHRP 20-50(8/13)
- Flexible and rigid pavements in areas that have a high
freezing index or a high number of freeze-thaw cycles
have higher IRI values when other contributors to
roughness are ruled out.
- In hot climates, higher IRI values are noted for AC sections
in areas that have a higher number of days above 32 OC
- Miscellaneous Findings
Report No. NCHRP 20-50(8/13)
Report No. FHWA-RD-02-057
- Placing overlays on pavements (flexible or rigid) that have
an IRI of less than 2.0 m/km (10.6 ft/mile) appear to be an
effective rehabilitation strategy in extending the life of the
pavement. However, the section should have sufficient
structural capacity to carry the anticipated traffic volume.
- Using LTPP profile data, the basis for roughness computations, 54 models were developed to assist highway agencies in transitioning smoothness specification limits from
profile index (PI) (5, 2.5, and 0 mm (0.2, 0.1, 0 inches)) to IRI or to PI0.0. Depending on the current situation and an agency's need, appropriate models can be chosen from the 54 models for transition.
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