Enhanced CRCP Backcalculation Procedure and Interface Bond Assessment: Volume 3

Abstract

Falling weight deflectometer (FWD) testing is effective in evaluating the structural response of in-situ concrete pavements through the backcalculated pavement layer parameters. Since Continuously Reinforced Concrete Pavement (CRCP) has closely spaced transverse cracks, the traditional backcalculation assumption of an infinite slab can lead to significant errors in the backcalculated results. In this study, solutions for backcalculated k-value, elastic modulus of concrete (E), and effective thickness (heff) for different crack spacing have been derived from 2-D finite element analysis for both interior- and edge-loaded CRCP. AASHTO sensor configuration (0, 12 in, 24 in, 36 in.) was recommended for CRCP with crack spacing greater than or equal to 6ft, and an alternative solution for crack spacing of 4 and 5ft was proposed with AREA24. Crack load transfer efficiency (LTE) across transverse cracks had limited impact on backcalculated results if the LTE was greater than 80%. According to the sensitivity study, both positive and negative temperature differentials less than 16oF had limited impact on backcalculated parameters. As expected, the backcalulation values were very sensitive to the load plate’s longitudinal position relative to the transverse crack especially for crack spacings smaller than 8ft. For edge-loaded condition, it is important to drop within a 2-inch lateral offset from edge in order to achieve accurate backcalculated results. In addition, the over width of the base provided extra support to CRCP slab resulting in higher backcalculated heff. The proposed backcalculation procedure system was applied to the Tollway field CRCP test section with different crack spacing, reinforcement ratio, and base types. Finally, a procedure for evaluating the slab-base bonding condition using simplified friction model along with backcalculated heff was extended and applied to one of the CRCP test section on Illinois Tollway near Itasca and one on Interstate 57 near Effingham. Based on the analysis of friction coefficient from measured deflection basin, cement treated base (CTB) was shown to provide good support to CRCP slab given the interface bonding condition between slab and base is stable in the long-term (more than 20 years). The combination of warm mix asphalt (WMA) and CTB also has good performance, even though the dynamic modulus of asphalt changes with the temperature causing decreased interface bonding condition in the summer.