Polymer-modified asphalt (PMA) mixtures are routinely used today in flexible pavement structures or overlays carrying high volumes of traffic. Although there have been numerous laboratory and field studies comparing the performance of PMA and conventional hot-mix asphalt (HMA) mixtures, for example, Superpave® and Marshall, there has not been a concerted effort to quantify the benefits of using PMA mixtures or to develop guidance on when the use of PMA mixtures is cost-effective. An investigation of nearly three dozen real-world pavement sections in North America was conducted to quantify the benefits of using PMA mixtures. The test sections used in performance comparisons included both roadway and accelerated pavement test sections. Performance data for the test sections were derived from published literature or other public sources such as the Long-Term Pavement Performance or the National Center for Asphalt Technology databases. On the basis of the performance comparisons made between PMA and conventional sections, it was found that PMA mixtures significantly enhance not only the rutting performance of flexible pavements but also their fatigue and fracture performance. The examples used in this study show an extended service life for deep-strength HMA pavements of 5 to 10 years through the use of PMA mixtures, on the basis of the performance observations from the companion test sections, which were constructed mostly with older Marshall or Hveem mixtures. A definite bias exists between the predicted and measured distress values for the sections with PMA mixtures when using current mechanistic-empirical distress prediction models. This finding suggests a need for different calibration factors in PMA mixtures for use in rutting and fatigue cracking prediction equations.
The coefficient of thermal expansion (CTE) is a fundamental property of concrete. It has long been known to have an effect on joint opening and closing in jointed plain concrete pavement, crack formation and opening and closing in continuously reinforced concrete pavement, and curling stresses and thermal deformations in both types of pavements. However, it has not been included as a variable either in materials specifications or in the structural design of concrete pavements. Hundreds of cores were taken from Long-Term Pavement Performance sections throughout the United States and were tested by FHWA's Turner–Fairbank Highway Research Center laboratory, using the AASHTO TP 60 test procedure. The CTE values were then assimilated into groups on the basis of aggregate types, and the mean and range of CTE were calculated. These results were then used in the new mechanistic–empirical pavement design guide to determine the significance of the measured range of CTE on concrete pavement performance. The CTE of the concrete was found to vary widely, depending on the predominant aggregate type used in the concrete. Sensitivity analysis showed CTE to have a significant effect on slab cracking and, to a lesser degree, on joint faulting. Its overall effect on smoothness was also significant. Given that CTE has not been used before in routine pavement structural design, the conclusion is that this design input is too sensitive to be ignored and must be fully considered in specifications and in the design process to reduce the risk of excessive cracking, faulting, and loss of smoothness.
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