Assessment of Thixotropy of Flowable and Self-Consolidating Concrete

An experimental investigation was carried out to determine the influence of thixotropy on the development of formwork lateral pressure. Five self-consolidating concrete (SCC) mixtures prepared with different combinations of cementitious materials and two flowable mixtures of different stability levels were evaluated. This article seeks to quantify the thixotropy of fresh concrete by evaluating the variations in yield stress and the evolution of the structural breakdown curves. Changes to the impeller of a modified Tattersall concrete rheometer are proposed for the protocol used to assess thixotropy. Instead of the H-shaped impeller that rotates in a planetary motion, a four-bladed vane impeller rotating coaxially around the main shaft was used. This resulted in less slip flow of fresh concrete and an increase in the sheared surface during rotation. Test results show that thixotropy is not an inherent property of SCC of a given slump flow consistency of 650 ± 10 mm. The concrete exhibits a high degree of thixotropy when proportioned with ternary cement containing 6% silica fume and 22% fly ash compared with similar concrete made with 4% silica fume and no fly ash. The incorporation of set-accelerating and set-retarding agents resulted in greater and lower degrees of thixotropy, respectively. In the case of flowable concrete with 200 mm slump, the addition of a viscosity-modifying admixture is shown to significantly increase the thixotropy compared with similar concrete made without any viscosity-modifying admixture.