Effect of Calcium Carbonate Precipitation by Bacterial Agent on the Bitumen Coating and Stripping of the Cold Mix Asphalt

Finding new solutions to improve the resistance of cold mix asphalt mixture to moisture susceptibility and prevent damages resulting from penetration of water into the mixture is essential and inevitable. In this study, calcium carbonate sedimentation on the cold mix asphalt aggregates by a bacterial agent was employed to create a surface of CaCo3 sediment. This thin layer of CaCo3 makes the surface layer of aggregates hydrophobic and thus improves the bitumen coating and resistance to moisture damage of cold mix asphalt. Biological curing was carried out for 31 days on the aggregates retaining on #50 sieve. X-ray diffraction and Fourier-transform infrared spectroscopy were utilized to determine the formation of calcium carbonate precipitation on the surface of aggregates. The results of these two tests confirmed the formation of calcium carbonate precipitation. Then, two types of cold mix asphalt mixtures were prepared using conventional natural aggregates (CMA) and cured aggregate with the bacterial agent (B-CMA). The modified boiling water test was performed along with the image processing technique on the surface of aggregates to quantitatively investigate the effect of the bacterial agent on the moisture susceptibility. Moreover, the ultrasonic test was done to examine the stiffness and dynamic modulus of the cold asphalt mixtures. Furthermore, statistical t-test was performed at 95% confidence level to evaluate the effect of the bacterial agent sedimentation on the results. Findings revealed that B-CMA mixture has better coatings of aggregates by bitumen, less stripping in the modified boiling test, and higher dynamic modulus than CMA mixture.