Development of strength and elastic modulus of concrete sealed in steel tube under sustained load at early age

Concrete-Filled Steel Tubular Structures (CFSTSs) have become popular among the structural engineering community due to significantly higher load carrying capacity compared to conventional reinforced-concrete structures. Much research has been conducted on understanding the behavior of CFSTSs under various loading conditions and design theories have been established to predict the load carrying capacities of such structures. However, existing models do not consider the effects of sustained early loads on concrete strength and elastic modulus development of CFSTSs. With the need for rapid construction, CFSTSs may be subjected to loading at an early stage before concrete is fully cured. Such early loading may incur negative effects on strength and elastic modulus development of concrete within the confined environment. This paper propose theoretical models based on the compressive packing model (CPM) to simulate strength and elastic modulus development of early-age concrete under sustained stress. Development of concrete properties at early age is described using Hydration kinetics, and maximum paste thickness in the CPM model is modified using energy conservation to simulate sustained loads. Early concrete strength and the elastic modulus development rules were investigated experimentally for sustained loads. Predictions from the proposed models are compared with conventional models from CEB-FIP Model Code. Results showed that when loaded at a very early stage, a relatively high stress to strength ratio will result in causing damage in concrete. Such damage significantly affects the strength and elastic modulus development. Compared with concrete loaded at 28 days, concrete loaded at early stages showed significant reduction in concrete strength and elastic modulus.