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• Alkali leaching on dense-graded asphalt paved on a concrete bridge deck. • Investigated the static water transport inside asphalt mixtures for various modes. • Examined the distinction between the alkali and non-alkali leaching cores by Computed Tomography (CT). • Revealed the major driving mechanism for alkali leaching on as-built concrete bridge deck pavements. 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 CO 2 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. [ABSTRACT FROM AUTHOR]

• Efflorescence of dense-graded asphalt paved on a cement concrete bridge deck. • Calcium Carbonate was found to be the main efflorescence material. • Cement concrete bridge deck provide the alkaline source for efflorescence materials. • Carbonation is the result of Ca(OH) 2 transformation in the presence of H 2 O and CO 2. • Migration model of the efflorescence material was established. A new distress type, namely alkali leaching, was recently observed to have developed on an as-built dense-graded asphalt concrete (DGAC) paved on a cement concrete bridge deck in China. Not only does alkali leaching affect the aesthetic appearance of the cement concrete bridge deck pavement (CCBDP) due to the distinctive color contrast, it also creates a potential threat to the service life of the bridge. In this paper, an alkali leaching distressed section of a CCBDP in Shaanxi Province (China) was studied. The study results indicated that whitish spots cannot be removed by natural water. In situ pH measurements indicated highly alkalinine materials appear on the surface of pavement layers. Based on these findings, the formation mechanism of alkali leaching on CCBDP was analyzed. The corresponding results showed that the main mineral of alkali leaching was Calcium Carbonate (CaCO 3). Under wet conditions, rainwater infiltrates into the cement bridge deck from the sides of the pavement. Continuous exposure to moisture or humid environments facilitates the subsequent hydration to generate Ca(OH) 2 , which is the alkaline source for alkali leaching. The generated Ca(OH) 2 migrates with rainwater to the pavement surface through capillary action. With the repeated circles of wet and dry conditions, the in-situ carbonation products lead to formation of alkali leaching on the pavement surface. [ABSTRACT FROM AUTHOR]