INITIAL ROUGHNESS

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.

1. Findings From Specific Pavement Studies
   • Report No. NCHRP 20-50(8/13)
   • Report No. NCHRP 20-50(3/4)
   • Report No. FHWA-RD-00-029
   
   
   
2. Findings From General Pavement Studies
   • Report No. NCHRP 20-50(8/13)
   • Base and Subgrade
   • Climate
   • Miscellaneous Findings

Findings From Specific Pavement Studies

1. 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, and rutting.
     
     
   • 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.
     
     
2. 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.
3. 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.

Findings From General Pavement Studies

1. 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 increase.
     
     
   • 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 values.
     
     
   • 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 tensile strength.
     
     
   • 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.
     
     
2. Base and Subgrade
   Report No. FHWA-RD-O1-166
   
   • 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.
     
     
3. Climate
   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 (90 OF).
     
     
4. Miscellaneous Findings
   Report No. NCHRP 20-50(8/13)
   
   • 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.
     
     
   Report No. FHWA-RD-02-057
   
   • 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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