Shrinkage and Cracking Properties of Cellulose Fiber–Concrete Composites for 3D Printing by Leveraging Internal Curing

Compared with conventional formwork casting materials, 3D printed concrete (3DPC) is characterized by large amounts of cementitious materials, a low aggregate–binder ratio, and a large water evaporation area, which make the printed materials and structures highly prone to plastic shrinkage and cracking. In this study, cellulose fibers were incorporated into concrete to improve its moisture distribution and increase its early-age strength. The effects of both dry and prewet cellulose fibers on properties of 3DPC were experimentally investigated. To ensure consistency in the amounts of dry fibers used, 0.5–2% dry cellulose fibers and 1–4% prewet cellulose fibers were adopted. The effects of the added cellulose fibers on printability, mechanical strength, shrinkage, and cracking performance of the 3DPC were experimentally studied. Particularly, a constraint method was developed to access the cracking behavior of 3DPC. Favorable shrinkage resistance was achieved, and the 120-day shrinkage decreased by 17.9% and 23.3% by addition of 2% dry fibers and 4% prewet fibers, respectively. Cracking was eliminated with addition of 4% prewet fibers, without influencing the printability and mechanical properties.