Abstract

The objective of this research was to measure and analyze the long-term modulus values of cement-treated base (CTB) layers constructed in Utah using microcracking. Because modulus values of pavement layers are among the most influential inputs affecting mechanistic-empirical pavement design, obtaining reasonable estimates of modulus values is critical. Testing was performed with a portable falling-weight deflectometer, also called a lightweight deflectometer, and modulus values were backcalculated with the computer program BAKFAA. Testing occurred at five asphalt pavement sites in northern Utah, where reconstruction with full-depth reclamation and cement stabilization, in the form of cement slurry, was performed approximately 2 to 14 years previously. Unconfined compressive strength (UCS) data collected for the CTB materials during earlier projects were compiled for all five sites. The correlation between backcalculated CTB modulus values, which ranged from 42 to 433 ksi, and 7-day UCS values, which ranged from 366 to 559 psi, was analyzed, and uniformity and sensitivity analyses were performed. Based on the results of this research, a new correlation is proposed for estimating the long-term modulus values of microcracked CTB layers constructed in a seasonally cold climate, such as northern Utah. For an average 7-day UCS of 450 psi, a CTB modulus value of 114 ksi would be estimated using this correlation, whereas a much higher modulus value of 630 ksi would be estimated from an existing correlation chart that was published in 1972 before microcracking was developed as a CTB construction practice. The results of the uniformity analyses indicate that statistically significant spatial variability in the CTB modulus values exists at each site. In comparison to a proposed maximum threshold coefficient of variation of 40 percent presented in the literature for aspects of CTB construction, the CTB modulus at all of the sites would be characterized as having low uniformity, with values ranging from 42.9 to 90.3 percent. The results of the sensitivity analyses indicate that backcalculated CTB modulus values are sensitive to typical deviations from design values that may occur in pavement layer thicknesses and suggest that CTB modulus estimation errors may range from -22,561 to 62,097 psi, or -3.73 to 10.81 percent, for pavements similar to those studied in this research when the actual asphalt and CTB layer thicknesses are different than the assumed values by up to 0.25 or 0.50 in., respectively.

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