Development of design procedures for bonded whitetopping overlays has been based on the assumption that failure mechanisms are a function of overlay thickness; namely, thin whitetopping results in longitudinal cracking and ultrathin whitetopping results in corner cracking. However, field data from whitetopping sections indicate that failure modes are dictated by slab size rather than overlay thickness. The revised procedure presented here for thin whitetopping and ultrathin whitetopping offers four primary enhancements to the Portland Cement Association and Colorado Department of Transportation procedures that traditionally have been used: (a) the failure mode is dictated by the joint spacing and not the overlay thickness, (b) the stress adjustments factors have been calibrated with an extensive data set, (c) the equivalent gradients to be used as the design input are defined according to the pavement structure and geographical location of the project and, (d) the effect of temperature change on hot-mix asphalt stiffness is considered. Comparisons of the predicted performance for the revised procedure with the actual performance for four separate projects showed that the predicted thicknesses are reasonable. It was also found that the predicted thickness obtained with the revised procedure was sensitive to the thickness of hot-mix asphalt, the level of traffic, and the modulus of rupture of the portland cement concrete, as expected.
In 1990, the Arizona Department of Transportation (ADOT) constructed a Specific Pavement Studies (SPS)-6 experiment on eastbound Interstate 40 (I-40) east of Flagstaff, AZ, as part of the Long-Term Pavement Performance Program. The SPS-6 experiment was designed to evaluate treatment and rehabilitation options for an existing concrete pavement. Nineteen sections were constructed, one of which was a 10 in. thick unbonded concrete overlay (UBCOL). Additionally, ADOT planned to construct an 11 in. unbonded portland cement concrete (PCC) overlay on the adjacent I-40 westbound to investigate benefits of additional concrete thickness. The eastbound unbonded PCC overlay has significantly outperformed all other pavement sections and remains in good condition after 27 years of service. However, the westbound PCC section is severely distressed and in need of immediate rehabilitation. An investigation was conducted to evaluate why two similar unbonded PCC overlays performed with vastly different results. It was found that the westbound “unbonded PCC overlay” was not an overlay after all, but instead a 12 to 13 in.thick jointed plain concrete pavement (JPCP). The high level of distress in the westbound JPCP is likely related to the presence of a weak subgrade that infiltrated the aggregate base. Additionally, during coring, water filled the core holes suggesting that the subgrade in the westbound direction was saturated. Further, petrographic analysis indicated deicing chemicals contributed to corrosion of the dowel bars and deterioration of the concrete joints. This investigation demonstrates some advantages for using unbonded overlays as rehabilitation alternatives for existing concrete pavements.
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