Investigation of Increase in Roughness Due to Environmental Factors in Flexible Pavements Using Profile Data From Long-Term Pavement Performance Specific Pavement Studies 1 Experiment
CHAPTER 7. SUMMARY AND CONCLUSIONS
Eighteen SPS-1 projects were constructed for the LTPP SPS-1 experiment. The number of projects located on coarse- and fine-grained subgrade were 11 and 7, respectively. The distribution of the projects according to the environmental zones were: DNF—two projects, DF—two projects, WNF—eight projects, and WF—six projects. Data from 201 test sections were analyzed, with 125 of these test sections located on a coarse-grained subgrade and the other 76 sections located on a fine-grained subgrade. Because this was not a balanced experiment, some biases could be present when comparisons are performed between subgrade types and environmental zones.
The median ages of the test sections that were analyzed were 11.8 and 9.3 years for the sections on coarse- and fine-grained subgrade, respectively. The third quartile values for the age of the test sections were 13.7 and 12.2 years for the sections on coarse- and fine-grained subgrade, respectively.
CLIRI
In this project, the increase in roughness that occurred along the center of the lane was assumed to be due to environmental factors. A rate of change of CLIRI was computed for each test section using liner regression using the time-sequence CLIRI data. The following conclusions were drawn from the analysis of the data:
- The median values for the change in CLIRI over the monitored period for the sections on fine- and coarse-grained subgrade were 6.5 and 9.9 inches/mi, respectively. The third quartile values for the change in CLIRI over the monitored period for the sections on fine- and coarse-grained subgrade were 14.1 and 20.6 inches/mi, respectively. As shown by the median value for the change in CLIRI, the change in CLIRI that had occurred over the monitored period for many sections was small.
- The median and the third quartile values of the rate of change of CLIRI when all SPS-1 test sections were considered were 1.05 and 1.19 inches/mi/year, respectively.
- Overall, sections on fine-grained subgrade showed a greater rate of change of CLIRI compared with sections on coarse-grained subgrade. When all SPS-1 sections were considered, the median values for the rate of change of CLIRI for sections on fine- and coarse-grained subgrade were 1.65 and 0.78 inches/mi/year, respectively, while the third quartile values for rate of change of CLIRI for the two subgrade types were 3.49 and 1.45 inches/mi/year, respectively. Sufficient projects were available to compare the performance of the sections on fine- and coarse-grained subgrade in the WF and WNF environmental zones. In the WF zone, the median values for the rate of change of CLIRI for sections on fine- and coarse-grained subgrade were 1.86 and 1.36 inches/mi/year, respectively, while the third quartile values for rate of change of CLIRI for the two subgrade types were 3.56 and 2.18 inches/mi/year, respectively. In the WNF zone, the median values for the rate of change of CLIRI for sections on fine- and coarse-grained subgrade were 0.92 and 0.67 inches/mi/year, respectively, while the third quartile values for rate of change of CLIRI for the two subgrade types were 1.15 and 1.26 inches/mi/year, respectively.
- The provision of drainage through use of a PATB layer reduced the rate of change of CLIRI for sections on fine-grained subgrade. In the WF zone, the median values for the rate of change of CLIRI for sections on fine-grained subgrade with and without drainage were 1.33 and 2.88 inches/mi/year, respectively, while the third quartile values for rate of change of CLIRI for sections with and without drainage were 1.82 and 4.66 inches/mi/year, respectively. In the WNF zone, the median values for the rate of change of CLIRI for sections on fine-grained subgrade with and without drainage were 0.66 and 0.99 inches/mi/year, respectively, while the third quartile values for rate of change of CLIRI for sections with and without drainage were 1.12 and 1.30 inches/mi/year, respectively. This information shows it is very important to provide drainage for pavements built over fine-grained subgrade in the WF zone to reduce the rate of change of IRI.
- The effect of drainage on the rate of change of CLIRI for sections located on coarse-grained subgrade was less than was observed for sections on fine-grained subgrade. For sections on coarse-grained subgrade, when all SPS-1 sections were considered, the median values for the rate of change of CLIRI for sections with and without drainage were 0.66 and 0.85 inches/mi/year, respectively, while the third quartile values for the rate of change of CLIRI for these two cases were 1.36 and 1.46 inches/mi/year, respectively.
- For sections on fine-grained subgrade, the sections where the PATB layer was below the ATB layer (i.e., ATB/PATB) had an overall lower rate of change of CLIRI when compared with sections where the PATB layer was placed over the DGAB layer (i.e., PATB/DGAB). The median values for the rate of change of CLIRI where drainage was provided by ATB/PATB and PATB/DGAB were 1.16 and 1.32 inches/mi/year, respectively, while the third quartile values for these two cases were 1.81 and 2.67 inches/mi/year, respectively. For sections on fine-grained subgrade where no drainage was provided, the median and the third quartile values for the rate of change of CLIRI were 2.40 and 4.58 inches/mi/year, respectively.
- For sections on fine-grained subgrade, a trend of increasing rate of increase of CLIRI with increasing PI values of the subgrade was observed.
- Evaluation of data collected at five sections that were located on coarse-grained subgrade showed no seasonal effects on the IRI values. Two of these sections were in the DNF zone, two in the DF zone, and one in the WNF zone.
MIRI
The profile of the pavement can change due to environmental effects such as frost heave and swelling of the subgrade. Along the wheelpaths, the profile of the pavement can change due to traffic effects. Therefore, the changes in MIRI on a pavement can be due to both environmental and traffic effects. It is possible that the traffic may counteract the upward movements caused in the profile due to environmental effects by smoothing the profile. In addition to evaluating the changes in IRI along the center of the lane, the changes in MIRI were also studied in this project. A rate of change of MIRI was computed using liner regression analysis using the available time-sequence MIRI data at each test section, and the results were used for analysis. The following observations were noted from the analysis of the data:
- The rate of change of MIRI was greater than the rate of change of CLIRI at 62 percent of the projects. Therefore, at 38 percent of the projects, the rate of change of IRI at the center of the lane (which received no traffic) was higher than the rate of change of IRI along the wheelpaths.
- When considering all SPS-1 test sections, the median values of the rate of change of MIRI and CLIRI were 1.56 and 1.05 inches/mi/year, respectively, while the third quartile values for these parameters were 3.22 and 1.19 inches/mi/year, respectively.
- Overall, sections on fine-grained subgrade showed a higher rate of change of MIRI when compared with sections on coarse-grained subgrade. When all SPS-1 sections were considered, the median values for the rate of change of MIRI for sections on fine- and coarse-grained subgrade were 1.87 and 1.43 inches/mi/year, respectively, while the third quartile values for rate of change of MIRI for the two subgrade types were 2.69 and 4.45inches/mi/year, respectively. Sufficient projects were available to compare the performance of the sections on fine- and coarse-grained subgrade in the WF and WNF environmental zones. In the WF zone, the median values for the rate of change of MIRI for sections on fine- and coarse-grained subgrade were 2.80 and 2.26 inches/mi/year, respectively, while the corresponding values for the rate of change of CLIRI were 1.86 and 1.36 inches/mi/year, respectively. The third quartile values for rate of change of MIRI for the two subgrade types in the WF zone were 5.47 and 4.42 inches/mi/year, respectively, while the corresponding values for the rate of change of CLIRI were 3.36 and 2.18 inches/mi/year, respectively. In the WNF zone, the median values for the rate of change of MIRI for sections on fine- and coarse-grained subgrade were 0.67 and 1.09 inches/mi/year, respectively, while the corresponding values for the rate of change of CLIRI were 0.92 and 0.67 inches/mi/year, respectively. In the WNF zone, the third quartile values for the rate of change of MIRI for sections on fine- and coarse-grained subgrade were 0.96 and 1.90 inches/mi/year, respectively, while the corresponding values for the rate of change of CLIRI were 1.15 and 1.26 inches/mi/year, respectively.
- The provision of drainage through a PATB layer had a major effect in reducing the rate of change of MIRI for sections on fine-grained subgrade in the WF zone, while in the WNF zone the provision of drainage also reduced the rate of rate of change of MIRI. In the WF zone, the median values for the rate of change of MIRI for sections on fine-grained subgrade with and without drainage were 1.90 and 3.01 inches/mi/year, respectively, while the corresponding values for the rate of change of CLIRI were 1.33 and 2.88 inches/mi/year, respectively. The third quartile values for rate of change of MIRI for sections on fine-grained subgrade in the WF zone with and without drainage were 3.91 and 6.07 inches/mi/year, respectively, while the corresponding values for the rate of change of CLIRI were 1.82 and 4.66 inches/mi/year, respectively. In the WNF zone, the median values for the rate of change of MIRI for sections on fine-grained subgrade with and without drainage was 0.59 and 0.93 inches/mi/year, respectively, while the corresponding values for the rate of change of CLIRI were 0.66 and 0.99 inches/mi/year, respectively. The third quartile values for rate of change of MIRI for sections on fine-grained subgrade in the WNF zone with and without drainage were 0.67 and 1.03 inches/mi/year, respectively, while the corresponding values for the rate of change of CLIRI were 1.12 and 1.30 inches/mi/year, respectively.
- On coarse-grained subgrades, the provision of drainage also reduced the rate of change of MIRI. A coarse-grained subgrade could have a fines content up to 50 percent, and the effect of drainage for sections on a coarse-grained subgrade could vary depending on the amount of fines content in the subgrade. When all SPS-1 sections were considered, the median value of the rate of change of MIRI for sections on coarse-grained subgrade with and without drainage were 1.40 and 1.55 inches/mi/year, respectively, while the corresponding values for the rate of change of CLIRI were 0.66 and 0.85 inches/mi/year, respectively. The third quartile values for the rate of change of MIRI for sections on coarse-grained subgrade with and without drainage were 2.34 and 3.31 inches/mi/year, respectively, while the corresponding values for the rate of change of CLIRI were 1.36 and 1.46 inches/mi/year, respectively.
- For sections on fine-grained subgrade, the sections where the PATB layer was below the ATB layer (i.e., ATB/PATB) had an overall lower rate of change of MIRI when compared with sections where the PATB layer was placed over the DGAB layer (i.e., PATB/DGAB). The median values for the rate of change of MIRI where drainage was provided by ATB/PATB and PATB/DGAB were 1.12 and 1.24 inches/mi/year, respectively, while the rate of change of CLIRI for these two cases were 1.16 and 1.32 inches/mi/year, respectively. The third quartile values for the rate of change of MIRI where drainage was provided by ATB/PATB and PATB/DGAB were 2.88 and 4.55 inches/mi/year, respectively, while the corresponding values for the rate of change of CLIRI were 1.81 and 2.67 inches/mi/year, respectively. The median and third quartile values for the rate of change of MIRI for sections on fine-grained subgrade where no drainage was provided were 2.72 and 5.92 inches/mi/year, respectively. The median and third quartile values for the rate of change of CLIRI for sections on fine-grained subgrade where no drainage was provided were 1.23 and 4.58 inches/mi/year, respectively.
- For sections on fine-grained subgrade, a trend of increasing rate of increase of MIRI with increasing PI values of the subgrade was observed.
- No relationship between the total length of cracking and the change in MIRI was noted for the analyzed data. Cracks that had a depression associated with the crack had a large impact on the IRI.
Other Observations
- Raveling of the pavement surface contributed to the increase in roughness of the pavement.
- In many cases, construction of patches on the pavement increased the IRI of the pavement significantly. This implies more care should be taken when constructing patches to ensure that the resulting pavement is smooth and adequately compacted.
Benefits of Collecting Profile Data Along the Center of the Lane
State transportation departments can obtain network-level profile data along the two wheelpaths of the travel lane and use MIRI values computed from the collected data to track the roughness of their highway network. Collecting profile data along the center of the lane could provide information on how the profile along the center of the lane, which is mainly influenced by environmental effects, would change over time. This information could be used to modify or improve the agency’s pavement design procedure to minimize large increases in IRI in areas where the combination of environmental and subgrade conditions caused such increases in the center lane. This information could also be used by the agency to build better models for predicting the change in IRI due to environmental conditions. If an agency collects network-level data using the services of a vendor, collection of the center of the lane profile data in addition to the wheelpath data would be expected to increase the cost minimally. If the agency used its own equipment to collect data, there would be a cost associated with upgrading existing equipment with an additional sensor to collect the profile data along the center of the lane. However, if the agency purchased new equipment, it could obtain equipment with an additional sensor that could collect data along the center of the lane at a small increase in cost.