The transport process of moisture in concrete is a physical phenomenon and a medium for the transmission of many harmful substances, having a far-reaching impact on the durability and structural safety of concrete. Moisture flows through pores and cracks in concrete that can provide penetration channels for harmful substances such as chloride ions, sulfate ions and carbon dioxide. Especially in humid environments, moisture promotes the migration of these harmful substances and accelerates the chemical processes such as chloride corrosion caused by chloride ions, thus significantly reducing the durability of concrete. The quantitative study of moisture transport can reveal the migration patterns of moisture in concrete, and identify/quantify various durability problems caused by moisture, providing a theoretical support for the maintenance, repair and life prediction of concrete structures.

In this study, the quantitative principle of moisture transport in mortar was derived based on Beer's law theory, and a triangular prism moisture layer sample mold was designed and conceptualized. Water could be directly infused into the prism, and in the process of X-ray transmission, a superposition model of dry mortar and water layer was formed. The relationship between known water content and X-ray transmittance was obtained by this calibration model, and then the calibration was performed through the change of X-ray transmittance of mortar sample before and after water absorption.

According to the standard GB 175—2007, China, mortar samples with the dimensions of 40 mm×20 mm×60 mm (h×b×d) were prepared with water, sand, and P·O·42.5 ordinary cement. The samples were the samples with different water-cement ratios of 0.4, 0.5, and 0.6 (W/C) (at a cement-to-sand ratio of 1:1), the mortar samples with different cement-sand ratios of 1:1, 1:2, and 1:3 (C/S)(at a water-cement ratio of 0.5), and the mortar samples with different sand particle sizes of 0.5–1.5 mm, 1.5–3.0 mm, and 3–5 mm (at a water-cement ratio of 0.5 and cement-sand ratio of 1:1), totaling 8 groups. Prior to the capillary water absorption test, the calibration model for each sample was measured by X-ray projection, and the X-ray radiography error was corrected for field inhomogeneity. A water calibration curve was then obtained, showing a relationship between X-ray transmittance (–ln(I_w/I_w0)) and moisture thickness, ranging from 0 to the maximum thickness d. Subsequently, a capillary water absorption experiment was conducted. After the absorption experiment, X-ray radiography and weighing were performed to obtain the X-ray transmittance (–ln(I_w/I_w0)) at different water absorption stages, with field inhomogeneity correction and normalization applied. Finally, the mass after water absorption was calibrated via the calibration curve and verified by the weighing results.

The relationship curve between water thickness and X-ray transmittance in mortar samples is obtained, having the fitting results (R²>0.99). During water absorption, the bottom of the sample begins to absorb water, forming a distinct water peak, indicating a one-dimensional water absorption process. As water absorption progresses, the water front gradually moves upward. However, the water distribution during capillary water absorption varies significantly due to the differences in composition and porosity among the mortar samples. The height of the water absorption front increases with time, though the rate of increase gradually slows. The water absorption front disappears when capillary water absorption approaches a saturation after a prolonged absorption period. For the total water absorption of each mortar, the maximum relative error between the calibration and weighing results is 4.8%.

A quantitative method for cement mortar was proposed. This method could allow for the quantification of moisture content in samples with different water-cement ratios, cement-sand ratios, and sand particle sizes through a single calibration process. The method could reduce both cost and time via eliminating a need to prepare different calibration samples. The method could accurately track a moisture transmission in mortar in real-time, capture the distribution and evolution of moisture in various mortars, and enable local quantification of moisture content in different regions of the samples. The experimental results showed that the smaller the C/S and sand particle size was, the better the water absorption uniformity would be. The seepage velocity and height of water surface peak increased with the increase of W/C and C/S. This method could obtain a reliable calibration for different mortar samples. Compared to the weighing method, the maximum relative error of all the samples during the water absorption process by this method could be less than 4.8%.