CHAPTER 9

Summary and Suggested Research

Summary

Asphalt and concrete pavement performance is influenced by temporal and diurnal variations in temperature and moisture conditions. For asphalt layers, warmer temperatures result in increased deflections and lower layer moduli and colder temperatures result in decreased deflections and higher layer moduli. Both conditions impact the response to pavement loading and performance. For JPCP, changes in the temperature and moisture gradients can impact slab curvature and result in curl and warp, which impacts rideability and can increase distress (e.g., transverse cracking, corner breaks). Unbound aggregate layers and subgrade soils are subject to moduli changes due to changes in moisture (e.g., saturated versus unsaturated) and temperature (e.g., frozen versus thawed) conditions.

The SMP sections were specially included in LTPP to assess the impact of climate conditions (e.g., arid, moist, freeze, no freeze) on material properties and pavement performance. Data from all SMP sections was extracted from the LTPP database and used to develop approaches for adjusting FWD and longitudinal profile measurements to account for temporal and diurnal changes in temperature and moisture. The data contained within the LTPP SMP database includes, for example, pavement structure, material properties, and pavement performance (e.g., cracking, rutting, longitudinal profile, FWD measurements).

The research identified several significant factors that influence FWD-measured deflections of asphalt pavements. These factors included climatic zone (WF, DF, WNF, DNF), asphalt layer thickness (thick or thin), pavement condition (excellent, good, fair, or poor), amount of rainfall just before FWD testing, and subgrade moisture conditions. The developed approach includes conducting FWD testing on a “limited” one-time series of pavement segments. From the collected data, a regression analysis is conducted to determine temperature and moisture adjustment factors that consider the significant factors. The developed adjustment factors are applied to subsequent project-level FWD measurements. While the research approach includes adjustment for moisture conditions, this may require pavement instrumentation to directly measure the subgrade moisture content. Therefore, the approach includes separate adjustment efforts for temperature and moisture. In the event an agency decides to account for the interaction of temperature and moisture, the developed approach can be used; however, additional evaluation segments may be required.

The approach for assessing changes in moisture and temperature conditions on JPCP uses longitudinal profile measurements and the 2GCI method. The approach includes conducting longitudinal profile measurements (at least five passes) multiple times in 12 hours to capture the maximum temperature gradient across the JPCP slab depth. The longitudinal profile data is analyzed to identify transverse joint locations and each slab evaluated is “fit” to an assumed shape (curvature). A relationship between the PSG value and the measured IRI is determined. This effort is applied to other site visits to identify and remove the influence of curling and warping from the IRI-measured data, thereby allowing for the determination of other factors (e.g., distress) causing increased roughness.

The approaches developed as part of this research establish the foundation for assessing temperature and moisture impacts on asphalt pavement and JPCP. While the LTPP SMP data provided the needed information to establish the proposed approaches, additional FWD and longitudinal profile measurements are recommended to further the implementation of the approaches to state-specific conditions.

Suggested Future Research

The following provides suggested future research:

• FWD deflection measurements:
  • Subsurface temperature measurements obtained from the LTPP SMP sections were limited and inconsistent. Research is needed to quantify the significance of moisture impacts on FWD measurements and material properties.
  • Research to develop a framework for automating, as much as possible, the proposed approach for adjusting deflection measurements based on temperature and moisture at the time of FWD testing.
  • As summarized in this research report, the extent of the SMP data is insufficient (e.g., minimal number of sites, few sections with sufficient repeat runs, consistency issues with thermistor probes) to fully characterize regional and/or statewide climatic adjustment factors for FWD measurements. Conduct an implementation project on the recommended approach by obtaining additional FWD and temporal and diurnal temperature and moisture measurements.
• Longitudinal profile measurements:
  • Research to develop a framework for automating, as much as possible, the proposed approach for assessing longitudinal profile measurements on JPCP curl and warp.
  • As summarized in this research report, the extent of the SMP data is insufficient (e.g., minimal number of sites, insufficient temporal and diurnal runs) to fully characterize the effect of JPCP curling and warping. Conduct an implementation project on the recommended approach by obtaining additional longitudinal profile measurements.
  • Quantify and assess the impact of built-in curl during JPCP construction. JPCP slabs constructed with built-in curl may not experience the typical curling noted in this research. However, if the right conditions can be determined on when to capture and obtain the longitudinal profile measurements an assessment can be made using the approach described in this research.