Plastic shrinkage of 3D printed concrete under different self-weight of upper layers

Abstract

The material properties of 3D printed concrete determine its fragility in preventing plastic shrinkage cracking due to the presence of restraint. Residual friction at the bottom that is positively related to the self-weight of the upper layers is a potential source of constraint that may cause cracking. The present study investigates the effect of self-weight of upper layers on plastic shrinkage of bottom printed concrete at early age. The corresponding surface moisture, bleeding rate, evaporation rate, capillary pressure development, and plastic shrinkage strain of printed concrete specimens under different upper self-weight are determined. It is found that increased self-weight of upper layers can significantly yield more early-age surface moisture and higher bleeding rate of the concrete at the bottom. The increased self-weight of upper layers can induce water to migrate and accumulate on the sides of the printed elements. However, as the self-weight of upper layers reduces the equivalent pore radius, the self-weight of upper layers would still facilitate the development of capillary pressure and further causes larger free plastic shrinkage strain. And when there is friction at the bottom, the horizontal plastic shrinkage is significantly limited by the constraint of the self-weight of upper layers, and the restraint strain increases with the upper self-weight. The maximum principal tensile stress results are calculated by two-dimensional plane strain field on the observation surface, which indicates that the specimen bears a greater risk of plastic shrinkage cracking with larger self-weight of upper layers.