Influence of Chemical Admixtures on Expansive Behavior and Mechanical Properties of K-Type Self-Stressing Concrete in CFST Systems

Concrete-filled steel tubes (CFSTs) have attracted increasing interest in research during the last decade as a construction system that increases its performance due to the use of self-stressing concrete. The efficiency and confining pressure of this construction solution mainly depends on the expansion of the concrete mixture introduced into the steel tube. This study shows the importance of the correct design of the expansive mixes and the correct selection of components. The effect of three types of chemical admixtures (air-entrained, setting retarder, and hardening accelerator) commonly used in the ready-mixed concrete industry when added to a self-stressing concrete made with a K-type calcium sulfoaluminate-based expansion agent has been analyzed. This study showed that certain chemical admixtures, added in a fixed proportion, conditioned both the compressive strength and the magnitude and kinetics of the expansion in a restricted regime of the reference self-stressing concrete mix. The efficiency of the CFST and self-stressing concrete construction solution represented by the bond failure strength and the sliding load of the concrete core against the steel tube was influenced not by the compressive strength of the expansive concrete but by the degree of restrained expansion (confinement effect) achieved. In this research, an alternative test method for measuring expansion was proposed, which allows continuous recording of positive strains (expansions), eliminating the dependence on setting time for demolding; this method even allows the exact identification of the expansion beginning. Finally, it was proposed that the ettringite detectable by the X-ray diffraction (XRD) technique was not representative of the magnitude of expansion generated from K-type calcium sulfoaluminate expansive agents because the proportion of ettringite was not related to the expansion determined by the different test methods proposed.