Relating Ride Quality and Structural Adequacy for Pavement Rehabilitation/Design Decisions

CHAPTER 5. SUMMARY AND CONCLUSIONS

Ride quality and structural adequacy are key pavement performance indicators. The relationship between these two indicators has been a topic of frequent and continuing discussion in the pavement community, but an accepted and widely used relationship has not been identified to date. To address this issue, FHWA sponsored a study to identify and verify the relationship, if any, between ride quality and structural support or ride deterioration and structural adequacy using the LTPP pavement performance data.

The FHWA study was undertaken in an effort to improve the evaluation and use of pavement condition data in pavement rehabilitation and design decisions. More specifically, this project was intended to develop and document a mechanism to include both ride and structural adequacy values within the context of current network-level PMS practices for highway agency implementation. The results of the project were intended for use by PMS engineers to ensure smooth pavements that are also structurally adequate.

A literature search was undertaken to collect, review, and synthesize available information on relating ride quality and structural adequacy for pavement rehabilitation and design decisions. Pertinent information was sought through Web-based searches of State highway agencies, university pavement research centers, TRB, ASCE, industry, and other national and international organizations. A total of 62 references were identified, but only 16 were considered relevant to the study. Moreover, while these 16 references contained valuable information, they did not contribute to the direct or indirect identification of a potential ride quality-structural capacity relationship.

Following completion of the literature search, a review and assessment of selected LTPP pavement performance data were undertaken to see if a ride quality-structural capacity relationship could be identified through the analysis of these data.

A total of 21 LTPP sections were identified and broken down into 5 groups according to pavement type (new AC, AC overlay over AC, and PCC), base type (granular or asphalt treated), changes in IRI over time, and changes in structural capacity over time. Historical pavement smoothness (profile) and structural capacity (FWD deflection) data were extracted from the LTPP database for the 21 sections, and the resulting data were analyzed. IRI was used to characterize ride quality, while SN and D_eff were used to characterize the structural capacity of flexible and rigid pavements, respectively.

Continuous IRI and SN or D_eff plots were generated to see if a viable ride quality-structural capacity relationship could be identified. It was hypothesized that areas where large changes in IRI had occurred should show large decreases in SN or D_eff. However, such relationship could not be observed for the sections and data investigated. In general, no major changes in SN or D_eff associated with localized areas of roughness were observed. In most sections, significant IRI changes were noticed at localized areas. Comparisons of continuous IRI and maximum deflection (i.e., below the FWD load center) or estimated subgrade modulus plots were also performed, but again, a relationship could not be identified.

A number of additional analyses were conducted to validate the study findings and conclusions. The evaluation of maintenance and repair activities showed that only a limited amount of maintenance had been performed on the study sections; with few exceptions, they did not affect the study findings. The review of IRI and FWD time history data confirmed that the use of the first and last test dates at a site to characterize the change in IRI and SN appears reasonable. Finally, the assessment of PCC warping and curling concluded that changes in IRI that occur due to changes in slab shapes have no relationship with the structural adequacy of the pavement.

In view of the findings, actual development of a ride quality-structural adequacy relationship and guidance for implementing the relationship into PMS was not pursued, as it could not be justified on the basis of the approach taken in this study. The lack of correlation found in the study, however, is considered of value to PMS practitioners, as it indicates that good ride quality does not mean good structural support or lower funding requirements to maintain the pavements. This becomes an important consideration for those wishing to base performance measures on ride quality indicators.

It is also important to recognize that these findings are based on the approach taken in the study. It is possible that other researchers will pursue alternate approaches that will yield a reliable relationship. However, while a relationship would be expedient for PMS applications, a fundamental understanding of the differences in factors causing structural deterioration and roughness (as well as the variable cause-effect relationship) makes it unlikely to find a simple relationship, particularly one excluding most other factors.

It is hypothesized that a strong relationship between the two performance indicators in question would require the inclusion of many other variables, potentially undoing its usefulness for the intended purpose in PMS. Clearly, pavement structural and functional performances are not independent of each other, even though they are not related in a one-to-one manner that can provide a PMS shortcut. However, this does not mean that structural parameters are not important for consideration in roughness prediction models, that roughness could not be one factor in the rate of structural deterioration, or that many common factors do not affect both roughness and structural capacity.