This report presents the findings from a laboratory experimental program that is designed to develop local calibration factors for warm mix asphalts using the AASHTOWare Pavement ME program. Double Barrel® foamed technology and Evotherm additive were selected for this study because they are the two WMA technologies that are used most commonly in North Carolina. These WMA technologies are applied to four different asphalt mixtures commonly used in North Carolina. Dynamic modulus, fatigue cracking performance, and rutting performance of control (i.e., hot-mix asphalt) and WMA mixtures are determined using axial compression dynamic modulus test (AASHTO PP61), direct tension cyclic test (AASHTO TP107), and triaxial repeated load permanent deformation (TRLPD) test (AASHTO TP79), respectively. Different propensities of WMA mixtures for aging and moisture damage are determined by conducting these laboratory tests on the control and WMA mixtures aged to different levels and with and without moisture conditioning.
The main findings from the laboratory experimental program are: (1) the HMA mixtures showed higher stiffness values, more rutting resistance, and more fatigue resistance than the two WMA mixtures; (2) the aging effect on the rutting resistance of foamed WMA mixture was greater than the effect on WMA Evotherm and the HMA mixtures; (3) in terms of the fatigue cracking properties, foamed WMA mixture was more sensitive to aging than the HMA mixtures; and (4) the S-VECD analysis results suggest that the moisture susceptibility of the WMA Evotherm mixture is the lowest among all the mixtures.
The results from the aging and moisture damage tests were applied to the Pavement ME program to compare the fatigue cracking and rutting performance of the HMA and WMA pavements at the structural level. It was concluded that the local calibration factors developed for the HMA mixtures could be used to analyze WMA pavement performance using the Pavement ME program and that no modification was necessary in terms of the input parameters. With regard to moisture damage, this study found that the moisture susceptibility of the WMA Evotherm mixtures was not statistically different from that of the corresponding HMA mixtures. Therefore, no correction was deemed necessary for the Evotherm mixtures in terms of moisture damage. However, the fatigue cracking performance of the foamed WMA mixtures was significantly affected by moisture conditioning, thereby necessitating modification of the local calibration factors (determined originally for the HMA mixtures). Statistical analysis and theoretical calculations based on moisture diffusion theories were conducted to reflect the different performance levels observed in the WMA Foam mixtures in the presence of moisture in the Pavement ME analysis. Based on the analysis, a 1.088 reduction factor must be incorporated for the fatigue life prediction of foamed WMA mixtures due to the relatively high moisture susceptibility of this WMA technology.
It must be noted that different aging effects on fatigue performance of HMA and WMA mixtures cannot be accurately reflected in the local calibration factors because of the rigid way of GAS model being implemented in the Pavement ME program. Also the Pavement ME program does not account for the effect of moisture damage explicitly through a model; therefore, incorporation of different moisture susceptibility between HMA and WMA mixtures in the local calibration factors is only approximate at best. A more mechanistic pavement design methodology that expresses the effects of aging and moisture damage through performance models is warranted in order to fully capture the difference between HMA and WMA mixtures.
