Experimental observations of early-age drying of Portland cement paste under low-pressure conditions

Plastic shrinkage cracking of concrete surfaces directly affects the long-term durability of structures. It can be attributed to the rapid rate of surface moisture evaporation while the microstructure is undergoing the early phases of hydration. A test method for characterizing evaporation parameters and simulating the sequential formation of shrinkage cracks in two-dimensional cement paste samples under low-pressure condition was developed. Effects of sample size, w/c ratio, initial curing and fiber content on weight loss and evaporation rates were investigated. Results indicate that drying occurs in two stages: a constant drying rate period (stage I) and a falling drying rate period (stage II). Vapor diffusion in stage I and unsaturated flow within porous medium in stage II determine the rate of evaporation. Results of diffusivity analysis shows that moisture diffusivity in stage I is higher than its value in stage II by more than one order of magnitude. Crack surface morphology using image analysis on plain and fiber reinforced cement pastes shows significant effect of fiber on controlling plastic shrinkage cracks.