Influence of curing conditions on the shrinkage behavior of three-dimensional printed concrete formwork

The use of three-dimensional (3D) printed concrete as formwork is becoming more widely applied within the industry. However, the technology is still not optimized and there are many reports of preliminary cracking during the curing of cast concrete. This is believed to result from differential shrinkage between the printed and cast concrete. These cracks (in the printed concrete or at the interface between the infill and printed concrete) form a preferential path for aggressive substances and can reduce the durability of the combined concrete element. To ensure the desired service life of the structure, it is important that the differential shrinkage between cast and printed concrete is understood. This study investigated the effect of curing conditions on the differential shrinkage behavior of 3D and cast concrete. The influence of prewetting of the dry-cured 3D printed formwork was also determined. In the experimental program, a vibrated and self-compacting concrete were used as cast material. Linear 3D printed formwork was produced and combined with cast concrete to simulate a concrete structure. Printed formwork was cured for 1, 7, or 28 d exposed to the air (relative humidity: 60% or 95%) or submerged in water. The length change of the combined elements was observed over 56 d after concrete casting and throughout the thickness of the materials. Results show that increasing the curing period in dry conditions of the printed concrete leads to an expansion of the formwork on the first day after casting. The expansion leads to a non-uniform strain evolution throughout the curing period of the combined element. Printed concrete formwork stored in wet conditions does not expand after the casting process but tends to show a decreasing linear deformation within the whole elements.