Pore structure, internal relative humidity, and fiber orientation of 3D printed concrete with polypropylene fiber and their relation with shrinkage

The shrinkage properties and its mitigation strategies for 3D printed concrete (3DPC) are of great significance, especially for structural applications in construction. This study aims to understand the effect of polypropylene fiber (PP fiber) on the total shrinkage of 3DPC under drying. The rheological properties, mechanical properties, internal relative humidity (RH), pore size distribution and fiber orientation of 3DPC were investigated and compared with cast concrete. Without PP fiber, the total shrinkage of the printed specimens is similar to the cast ones. The addition of PP fiber reduces the total shrinkage for cast specimens more significantly than for printed specimens. This phenomenon is explained from both pore structure and fiber orientation perspectives. A lower total shrinkage is also observed on specimens cut along the printing direction when compared with those cut perpendicularly to the printing direction due to higher quantity of fiber oriented along the printing direction. The total shrinkage of 3DPC is found inversely proportional to the fiber orientation factor.