Impact of Design Features on Pavement Response and Performance in Rehabilitated Flexible and Rigid Pavements

Executive Summary

The main goal of this project was to use Long-Term Pavement Performance (LTPP) Specific Pavement Study (SPS) experiment data to assess the impact of different design, construction, and rehabilitation features on pavement response and performance for specific site conditions. The analysis sought to identify which features could help achieve the best short-term and long-term performance and to evaluate the effectiveness of common maintenance practices used for flexible and rigid pavements.

Preventive Maintenance Treatments

The findings of this study are based on the analysis of 81 SPS-3 flexible pavement sites and 34 SPS-4 rigid pavement sites subjected to different preventive maintenance treatments. Most of the flexible pavement sites were monitored for at least 4 years, and approximately 22 percent of the sites were monitored for 10 years or more. Most of the rigid pavement sites were monitored for at least 4 years.

Of all SPS-3 treatments, thin overlay was the only effective alternative to mitigate and delay the progression of roughness; however, it was effective only for pavements in freeze zones, high traffic, or poor condition. It was found that thin overlays could only perform better relative to roughness compared to other treatments if the International Roughness Index (IRI) level was higher than 7.34 ft/mi (1.39 m/km). For lower IRI levels, the sections performed similarly and independent of the treatment, and there was no advantage of applying thin overlays.

Thin overlays slowed the progression of rutting under all circumstances. Chip seal was more effective than slurry seal in wet freeze zones but was only marginally more effective in dry freeze zones. There were no significant differences among slurry seal, crack seal, and the no treatment scenario with respect to rutting, as expected.

Thin overlays and chips seals were more effective than slurry seal and crack seal treatments in mitigating fatigue cracking. Thin overlays performed better than most other treatments if the pavement was in a freeze zone, in a wet climatic region, initially in poor condition as well as subjected to high traffic. For fatigue cracking, thin overlays and chip seals outperformed the other treatments, as well as the control section, when the initial cracking was lower than 232.13 ft²/mi (13.4 m²/km). For higher levels of cracking, every treatment outperformed the control section. Specifically, chip seals performed best, followed by thin overlays.

The data analysis from SPS-4 sites indicated that the joint/crack sealed sections and undersealed sections performed similarly to the control sections. Also, no meaningful differences were found between the two treatments. The analysis was based on a relatively small number of sites that had 4 years of performance history that included recorded surveys with undersealing treatment. While 34 sites were included in the survey measurements for joint/crack sealed sections, only 10 had data for undersealed sections.

Rehabilitated Flexible Pavements

The findings are based on the analysis of 18 SPS-5 rehabilitated flexible pavement sites, with a total of 162 core test sections. Most of the sections were monitored for at least 9 years.

Rehabilitation strategies with milling prior to overlay provided better performance relative to IRI levels for all site conditions. Moreover, strategies with thick overlays provided smoother pavements for all site conditions. Design alternatives with new or recycled asphalt mixes had equivalent performance when used under wet conditions; however, those with recycled asphalt mixes provided smoother pavements when used in dry conditions. Traffic level and freeze conditions did not affect pavement performance relative to roughness.

With respect to rutting, rehabilitation strategies with thin overlays performed better than thick overlays in the short term. The ranking of best strategies was evenly distributed between the two mix types (virgin and recycled asphalt). In the long term, the ranking of best strategies was more evenly distributed for both thick and thin overlays. Rehabilitation strategies with virgin mixes performed better in most of the sites, with the exception of pavements in fair surface condition prior to rehabilitation and under freeze conditions, which corresponded to 33 percent of all sites. Strategies with milling did not improve rutting performance more than alternatives without milling. Surprisingly, the level of traffic did not affect rutting performance for the selected rehabilitation strategies.

Short-term fatigue cracking performance was not significantly affected by any design feature under any site conditions. This finding was expected because overlays are designed to minimize fatigue cracking in the short term. Rehabilitation strategies with thick overlays provided better performance for fatigue cracking for all site conditions that were evaluated. Strategies with milling prior to overlay performed better to mitigate development and propagation of fatigue cracking in all site conditions. In regions with a dry climate, alternatives without milling performed as well as solutions with milling. Strategies with recycled asphalt mixes were better ranked for sites with low traffic when evaluating fatigue cracking.

When comparing the alternatives evaluated and the overall performance for all types of load-associated distress, overlay thickness was the most influential design feature. As expected, thick overlays consistently performed better. The impact of thickness on performance was more evident in the long term (more than 5 years) for most of the distresses. The exception was rutting, for which no evidence was found, suggesting that either thin or thick overlays provided less rutted pavements. The analysis of milling prior to overlay suggests that replacing the distressed portion of the surface layer improved the performance for the majority of distresses commonly observed in flexible pavements. The majority of sites did not show significant differences in performance between sections overlaid with virgin and recycled asphalt mixes. However, when differences existed, they were mostly in favor of virgin mixes.

For evaluation of structural responses, a maximum falling weight deflectometer (FWD) deflection measured under the center of the load was used as a structural response indicator. Rehabilitation strategies with thick overlays provided the lowest structural response independent of site conditions. Strategies with recycled asphalt mix overlays had the smallest structural deflections in freeze regions, while those with virgin mixes presented smaller deflections under no-freeze conditions. Milling prior to overlay did not further impact the structural response. In fact, in no-freeze zones, strategies without milling presented lower deflections. When comparing wet and dry climates, pavement surface conditions, and traffic levels, none had a significant impact on structural responses associated with each rehabilitation alternative.

As expected, rehabilitation strategies with thick overlays had lower maximum deflection values compared to alternatives with thin overlays. There were no differences in pavement response between strategies with virgin and recycled asphalt mix overlays. Strategies with milling prior to overlay did not affect the structural response more than alternatives without milling.

Rehabilitated Rigid Pavements

Findings for rehabilitated rigid pavements are based on the analysis of 14 SPS-6 rehabilitated rigid pavement sections, 8 jointed plain concrete pavement (JPCP) sections, and 6 jointed reinforced concrete pavement (JRCP) sections. Most of the sections were monitored for at least 6 years. The results from the analysis are described separately for JPCP and JRCP sites.

With respect to JPCP structures, rehabilitation strategies with hot mix asphalt (HMA) overlays provided significantly smoother pavements than treatments without overlays in both the short term and long term. The best alternative to improve roughness performance was crack/break and seat with an 8-inch (203-mm) overlay. This same alternative and minimum restoration with a 4-inch (102-mm) overlay (without crack/break) had statistically equivalent performances and were found to be the best alternatives for most of the scenarios evaluated when both short-term and long-term roughness performance were considered. Crack/break and seat with a 4-inch (102-mm) overlay was among the worst alternatives to improve pavement performance relative to roughness. Saw and seal provided similar performance to other 4-inch (102-mm) overlays.

Rehabilitation strategies without overlays were the best to mitigate cracking development and propagation. HMA overlays over jointed concrete pavements exhibited more surface cracking than alternatives without overlays. Crack/break and seat the JPCP had no significant effect in reducing the amount of cracking because it performed similarly to the 4-inch (102-mm) overlays over noncracked JPCP (with both minimum and maximum restorations). The three alternatives without overlays, the no treatment scenario, minimum restoration, and maximum restoration, were found to be the best choices to mitigate surface cracking for both short-term and long-term performance. Crack/break and seat with 4-inch (102-mm) overlays was the best alternative among those that involved overlays. The sawed and sealed joints did not deteriorate significantly on these sections, and they effectively controlled reflection cracking.

When evaluating the impact of site conditions, different climate regions and surface conditions did not have a significant impact on roughness and total cracking performance for the rehabilitation strategies included in the SPS-6 JPCP experiment.

Similar to the findings for JPCP, JRCP strategies with HMA overlays improved roughness performance, while strategies without overlays were better at improving total cracking development and propagation. Rehabilitation strategies with overlays performed significantly better when compared to treatments without overlays. Minimum and maximum restorations with overlay were the best strategies to improve short-term performance for roughness. For long-term performance, the best alternative was the crack/break and seat and the 8-inch (203-mm) overlay.

Rehabilitation strategies without overlays were the best when considering total cracking. Saw and seal presented the highest surface cracking among all options evaluated; however, the sawing may have had an impact on the monitoring process because this alternative remained in reasonably good condition over time. Crack/break and seat the JRCP had no significant effect on reducing the amount of cracking because it performed similarly to the 4-inch (102-mm) overlay over noncracked JRCP (with minimum and maximum restoration). Sawing and sealing proved to effectively control reflective cracking.

Deflections at the center of the slab and at the transfer joints were evaluated in this study. JPCP and JRCP structures were evaluated independently. Sections that received HMA overlays were monitored like flexible pavements, and deflections at the center of the lane were used in the analysis. There were limitations due to the amount of data available, particularly after the data were grouped by pavement structure type and surface condition.

The only analysis that provided statistically meaningful results was the evaluation of maximum deflection at the center lane of overlaid JRCP structures. The results of that evaluation suggest that crack/break and seat significantly increased the overall deflections measured on the pavement surface. The remaining treatments provided equivalent maximum deflection magnitudes. This was expected since crack/break and seat was an alternative in which the concrete slab was reduced to smaller pieces, resulting in lower stiffness. This process increased the maximum deflection at the center of the slab.

Findings from MEPDG Analyses

The Mechanistic Empirical Pavement Design Guide (MEPDG) analysis was used to compare MEPDG-predicted performance of rehabilitated pavement sections with field measured data to verify current calibration for rehabilitated pavement structures.⁽¹⁾

The roughness models for flexible and rigid pavements provided good estimates for rehabilitated sections with and without HMA overlays, and some bias was identified. The model has a tendency to underpredict roughness for rigid pavement sections with IRI values above 9.50 ft/mi (1.8 m/km). This bias is more characteristic of sections located in dry and freeze regions, and it could be addressed by calibrating the models for local conditions.

The rutting model needs further enhancement to more accurately predict permanent deformation for HMA overlays over flexible and rigid pavements. The model underpredicts rutting of HMA overlays over crack/break and seat restored rigid pavements and overpredicts for HMA overlays with saw and seal and for minimum and maximum restorations prior to overlays.

The cracking models for HMA overlays, particularly the empirical reflection cracking, need further enhancement to provide more accurate estimates for rehabilitated sections. The models for fatigue cracking (new and reflective) and longitudinal cracking were very accurate for estimating consistent and comparable performance with measured values. MEPDG did not predict transverse cracking in any of the SPS-5 or SPS-6 sections, even though some transverse cracking was measured during surveys.