No abstract

The long-term evolution of asphalt mixture stiffness in the field represents a complex process in which three factors play an essential role: damage, aging, and densification under traffic. To take the factors into account adequately, not only must the level of each factor be known, but also how each level modifies the modulus of the asphalt mixture for the temperature and frequency range corresponding to field conditions. The research presented is based on experimental data from four flexible sections tested at the test track of the Centro de Estudios y Experimentación de Obras Públicas (CEDEX), in Spain. The stiffness of the asphalt layer was periodically evaluated with falling weight deflectometer back-calculation and with dynamic modulus testing in the laboratory. During a 28-month period, 1,323,600 loads were applied on the pavements. This loading caused a high level of deterioration in all sections and significant asphalt densification. The loading also provided time for some aging effects to develop. From the results of this full-scale test, different conclusions could be deduced concerning the evolution of damage, aging, and densification and how this evolution modifies the stiffness of the asphalt mixture. The effects of changes in the parameters of the original master curve have also been quantified. The model and methodology incorporated in the CalME design procedure were successfully used to reproduce the evolution of the asphalt layer modulus during the test. In particular, the assumptions of this model regarding how the dynamic modulus master curve was modified by each of the three factors were found to be valid for this experiment.

The structural response of a flexible pavement typically is modeled with the multilayer linear elastic theory. This theory is applicable to almost all mechanistic–empirical design procedures but is also used in other fields of pavement engineering, such as in the interpretation of deflection data for assessing a pavement's structural condition. The applicability of this theory, even with some limitations, has been validated for new or undamaged pavements when the hypothesis of continuity is realistic. However, for modeling the structural response of damaged pavements, the presence of discrete cracks is not compatible with continuum mechanics theories. Three flexible sections at the CEDEX Transport Research Center test track were instrumented with sensors. The structural response under moving vehicles was systematically measured for different response variables during a full-scale experiment in which 1.3 million loads were applied. The structural response measured at the beginning of the experiment, when asphalt damage was null, was used as a reference for comparison of the evolution of response during the test. The test showed that the evolution of the response variables could be explained by continuum mechanics, in particular by linear elasticity, as soon as asphalt damage was uniformly distributed in the material. When discrete cracking appeared, the response started to deviate from the response predicted by multilayer linear elastic models. Adoption of a rational approach for determination of the modulus of the asphalt layers is shown to be important.

Asphalt fatigue lives determined in the laboratory must be shifted to predict the performance of in-service pavements. Determination of the shift factor to be applied constitutes one of the main challenges of flexible pavements design. Experimental data from four sections tested for 28 months at the CEDEX test track provided detailed information regarding accumulation of asphalt fatigue damage as a function of loads and temperature. The data were used to draw conclusions about the damage process and to determine shift factors for two cracking levels in the field. An effort was made to quantify the beneficial effect of rest periods in the field through use of a variable shift factor that incorporates the methodology for studying this effect and the concept of reduced rest period that was developed in NCHRP 9-38 and 9-44. Results showed that rest periods were not only the main factor responsible for the need for shift factors but also the primary explanation for why most damage took place at medium to low temperatures and not during the warmest season. The asphalt fatigue model in the CalME design procedure has been used successfully to reproduce actual performance observed in the tested sections in terms of modulus reduction, which was previously determined from falling weight deflectometer backcalculation.