Rheology and shrinkage of concrete using polypropylene fiber for 3D concrete printing

This paper proposes a rectangle rheology box for 3D polypropylene fiber reinforced concrete printing, in which dynamic yield stress and plastic viscosity of the concrete mixture are suitable for printing and gaining a low early shrinkage. The rheology in terms of dynamic yield stress and plastic viscosity was measured by ICAR rheometer. A modified standard test following ASTM C1581 was proposed to measure shrinkage strains of the concretes right after 3 h from casting. In this research, fly ash content was used as 60% by weight of cement, whereas silica fume and limestone powder used as 3% and 8% by cement weight respectively. Water to binder ratio (W/B) was of 0.22, 0.24, 0.26, 0.28 and 0.30. Polypropylene (PP) fiber content varied from 1.35 to 5.4 kg/m(3). The experimental results showed that W/B and PP fiber content significantly influenced the plastic viscosity and the dynamic yield stress. The resistance to cracking of the concrete was improved with addition of PP fiber, sand to binder ratio (S/B) increase and with increasing W/B. The shrinkage of the 3D printing concrete was very fast at the first 24 h after casting, it was up to 96% compared with the maximum shrinkage at concrete cracking. The concrete using PP fiber was printable with a low shrinkage when the dynamic yield stress varied from 250 to 500 Pa and the plastic viscosity was various from 22 to 60 Pa s.