Improving the resistance to moisture damage of cold mix asphalt modified by eco-friendly Microbial Carbonate Precipitation (MCP)

This study investigated the effect of Microbial Carbonate Precipitation (MCP) induced by Sporosarcina pasteurii on the moisture susceptibility of Cold Mix Asphalt (CMA) mixtures. In addition to being eco-friendly, MCP creates a layer of hydrophobic CaCO3 that facilitates great bonding between the bituminous mastic and the surface of aggregates. Two approaches of bio-precipitation treatment were studied. The first approach was implemented by the pre-processing of the aggregates' surface with MCP to be used in manufacturing the CMA mixture. In the second approach, the optimum water content was replaced by a medium culture containing bacteria and Urea-CaCl2 nutrient solution when preparing the CMA mixture. The boiling water test and an image processing technique were utilized to examine the effect of each bio-precipitation treatment approach on the stripping resistance. The Indirect Tensile Strength Ratio (TSR), Fracture Energy Ratio (FER), Percentage of Toughness Loss (PTL), Pulse Velocity Ratio (PVR) specified by the ultrasonic pulse velocity test, and Electrical Resistivity Ratio (ERR) of all the CMA mixtures under wet condition in comparison to dry conditions were studied. The ANOVA test performed on the results at 95% confidence level showed that the mixtures prepared by the first approach had considerably higher TSR and FER, and lower PTL than the control specimens, which reflects significantly improved resistance to moisture damage. For the second approach, these improvements were higher than the control specimens (P-value <0.05), nonetheless, lower than those of first approach. The values of PVR and ERR obtained with the first and second approaches were significantly (P-value <0.05) and slightly (P-value >0.05) higher than the control specimens, respectively. The bitumen coating ratio measured after the boiling water test was higher than that of the control mixtures for each of treatment approaches. The SEM observation confirmed the presence of calcium carbonate precipitates in the modified mixtures. The results showed that while the first approach considerably improves the adhesion within CMA mixtures, the second approach affects their cohesion. (C) 2019 Elsevier Ltd. All rights reserved.