The driving mechanism of cement hydrated products leaching on as-built cement concrete bridge deck pavements

Abstract

Recently, a new distress type in as-built dense-graded asphalt concrete (DGAC) paved on concrete bridge decks, namely alkali leaching, has been detected in China. The water carries dissolved alkali ions from the cement concrete deck to the pavement surface and reacts with CO2 in the air to form whitish carbonate deposits. The “bottom-up” water transportation process in the presence of the natural environment without vehicle loading effect is a complicated problem. Thereby, scrutiny of the driving force of the “bottom-up” water transportation process in cement concrete bridge deck pavements (CCBDPs) plays a vital role in realizing the mechanism of alkali leaching. In the present study, three potential driving mechanisms that may cause this process are proposed and simulated in the laboratory, including water vapor diffusion under action of multiple temperatures and relative humidity (RH), capillary rise, and leaching water caused by variation of atmospheric pressure. The microstructure of alkali leaching samples cored from distress pavement is also carefully examined by X-ray CT. The obtained results show that the diffusion mass of water vapor demonstrates a rapid lessening with the reduction of the relative humidity differences. A highly linear correlation between the RH difference and the diffusion flux of the water vapor for different temperatures is also identified, indicating that the DGAC with various air voids follows Fick’s first law of diffusion. Further, capillary rise indeed occurs in the DGAC but does not contribute to the alkali leaching process. The pressure difference could continuously drive the leaching water to the surface of DGAC samples, and the critical pressure difference (0.3–0.7 kPa) would be generally less than the daily variation of the atmospheric pressure (approximately 0.8 kPa in weather alternate days). The quantitative analysis of the internal structure for alkali leaching cores also reveals that the distressed cores present relatively higher void ratios, especially for small volume and narrow pores. The entrapped air within unsaturated pores provides the precondition for water transport subjected to the atmospheric pressure differences. These findings contribute to the understanding of static moisture transportation in the dense-graded asphalt mixtures and clarify the main driving mechanism of moisture-induced alkali leaching on CCBDPs.