A plastic shrinkage cracking risk model for 3D printed concrete exposed to different environments

The absence of formwork leaves early age 3D printed concrete vulnerable to excessive pore water evaporation. This study investigates the fundamental behaviour of the resulting plastic shrinkage cracking by exposing printed specimens to six drying conditions at concrete ages of 0, 1, 2 and 4 h (delay times). The results show that the rate of concrete mass loss is equal to the drying condition's free water evaporation rate for the first few hours, causing significant desiccation and plastic shrinkage. A moderate evaporation rate removed 24% of the total mixing water from the specimen in only 2 h. The minimal surface moisture and low bleeding rate provide limited protection against pore water evaporation. Therefore, increasing the evaporation rate results in an immediate and direct shrinkage response in the concrete. Delaying the evaporation to the initial setting time decreases the shrinkage magnitude by 56%. A model for cracking risk is proposed based on the evaporation rate, surface to volume ratio, delay time and stiffening rate. Experimental results are used as validation.