Visualization And Quantification Of Water Transport Evolution In Cementitious Materials

Concrete is the most widely used building material today.However,due to the lack of durability of concrete materials caused by corrosive agents in the environment,the construction structures cannot reach the normal service life.The deterioration of concrete(such as freeze-thaw,steel corrosion,etc.)is often closely related to the existence of water and its transport in the material.As a transport medium,water carrying corrosive ions like Cl~-,intrudes into the material interior and migrates under the action of capillary absorption,causing degradation reactions such as steel corrosion,so as to accelerate the degradation process of the concrete structure.Presently,in order to trace and study the moisture transport process and regularity in cement-based material,most studies have established the functions of water content and absorption depth with exposed time in one-dimension or two-dimensions,while the three-dimensional transport process and distribution of water in material could not be characterized visually.Therefore,X-ray Tomography Technique(i.e.XCT)is applied to visually trace and characterize the water transport process and regularity in cement paste in this paper.The main research contents and results are as follows:(1)To enhance contrast of water in CT images,sodium iodide(NaI)solution instead of pure water was used for water uptake test and XCT test,which could be in-situ and visually characterized the dynamic movement of water in cementitious materials,breaking through the limitation of tracking pure water transport in cement paste directly by XCT.Through several experiments,it was found that 10 wt%NaI solution has a significant contrast in CT images,which could be distinguished the absorption areas and the non-absorbent area clearly in cement paste.(2)Based on relevant image correction theories,the average gray CT values method was used to quantitatively analyze the water uptake height in cement paste.To verify the validity of results tracing water transport in cement paste by XCT technique and average gray value method,two experiments were set up,including measuring the actual absorption depth by splitting and potentiometric titration method for determining the distribution of iodine ion concentration along sample(w/c=0.4)after3 hours of capillary absorption.What’s more,based on XCT method,the 2D and 3D mapping of the evolution process of water transport in four different cement paste specimens(w/c=0.3,0.4,0.5 and 0.6)could be in-situ visually monitored and represented.(3)When studying the regularity of water transport under the capillary suction in cement paste by weighing method,it was found that with the increase of exposed time in water,water uptake process of all samples showed a rapid increase in the amount of water absorption in the early stage and a gradually slower process later.Also,there is a linear relationship between the capillary water absorption per unit area and the square root of time for the four cement pastes,while the slope of the fitted curve is the capillary absorption rate S.The bigger the water-cement ratio,the more water content and the greater moisture absorption rate.(4)When studying the regularity of water uptake height with time under the capillary suction in cement paste by XCT method and splitting measurement,it was found that with the increase of exposed time in water,the evolution process in absorption height of all samples showed a rapid increase of absorption depth in the early stage and a gradual process later.The absorption depths of the four different cement pastes are linear with the square root of time,which is consistent with the Locas-Washburn equation.The slope of the fitted curve is the capillary coefficient k.Also,the larger the water-cement ratio,the deeper the uptake depth,and the greater the capillary coefficient.(5)The water absorption rate shows a good linear relationship with the capillary coefficient under the capillary suction in cement paste,which is consistent with the conclusion obtained by Han?i?et al.using neutron imaging technology.