The evaluation of rheological parameters of 3D printable concretes and the effect of accelerating admixture

Direct shear test and rotational rheometry were performed and compared as methods to estimate the rheological parameters of 3D printable concretes. For this, 3 concrete mixes, having a cohesive flow nature were produced with different proportions of accelerating admixtures, in order to evaluate the improvement on the yield strength gain of said mixes. Results have shown that both methods can predict the fresh state evolution of concrete. However, the obtained values for yield strength on the rotational rheometry tests were higher than those collected on the direct shear test. The yield stress for some concretes tends to evolve linearly with increasing resting time, and when using accelerating admixtures, the increase becomes exponential, even during the first few minutes, due to the high ettringite formation caused by the accelerator action. A model for the exponential evolution of the yield stress is proposed and an analytical model, which predicts the buildability capacity of printable concretes, is reviewed and updated.