Reflective Cracking Study: Backcalculation of HVS Test Section Deflection Measurements

This report is one in a series that describe the results of HVS testing and associated analyses on a full-scale experiment being performed at the Richmond Field Station (RFS) to validate Caltrans overlay strategies for the rehabilitation of cracked asphalt concrete. It describes the analysis of deflection data measured with a Falling Weight Deflectometer (FWD) throughout the study. The test forms part of Partnered Pavement Research Center Strategic Plan Element 4.10: “Development of Improved Rehabilitation Designs for Reflective Cracking.” Findings and observations based on the data collected during this HVS study include: * Variation of material properties were recorded both between sections and within sections, and were mostly attributed to variation in the degree of recementation of recycled concrete particles in the base material. * The asphalt concrete modulus was significantly affected by the pavement temperature, as expected. In general, lower modulus was obtained at high temperatures, and higher modulus at low temperatures. * The modulus of the aggregate base was generally positively correlated with the moduli of the asphalt concrete and subgrade. Correlation between the asphalt concrete modulus and the base modulus was weaker in the untrafficked area and/or in the trafficked area before HVS testing. No significant correlation was found between the asphalt concrete modulus and the subgrade modulus. * The load level of the FWD did not have a significant effect on the values of the backcalculated moduli. * Aging of the asphalt concrete was apparent on five of the six sections. The stiffness of the base increased significantly with time after initial construction, primarily due to recementation of the recycled concrete particles. This increase continued after overlay construction in certain areas of the test road. * Phase 2 HVS testing generally damaged the asphalt concrete layers in the trafficked area of each section. * In the one to three year period after Phase 2 HVS testing, the modulus of the damaged asphalt concrete generally recovered to some extent on all sections. * Seasonal effects on pavement stiffness were not detected from the limited data collected during this study. * The asphalt concrete moduli backcalculated from the overlay sections match reasonably well with the moduli determined during laboratory frequency sweep tests on flexural beam specimens. * There was a difference between the moduli backcalculated from FWD data and from the RSD data. Differences in test conditions and backcalculation assumptions of the two procedures contributed to this difference. No recommendations as to the use of the modified binders in overlay mixes are made at this time. These recommendations will be included in the second-level analysis report, which will be prepared and submitted on completion of all HVS and laboratory testing.