In recent years, application of high strength concrete (HSC) has attracted increasing interest in the construction industry due to its significant economic, architectural, and structural advantages, compared to the conventional normal strength concrete (NSC). However, under fire condition, which is one of the most common hazards that attack building structures, HSC members may be subjected to explosive spalling. Strength reduction of structural members may occur, leading to severe consequences such as failure of members or even collapse of the whole structure. A newly designed 2layered cylindrical specimen consisting of an HSC core and an NSC outer layer is proposed to improve the fire performance of HSC members under elevated temperature. The NSC layer is designed to act as an outer layer insulation to reduce the thermal gradient and also serve as a lateral confinement to prevent the HSC core from spalling. Compression and thermal tests were performed on the specimens to investigate their strength and behavior under elevated temperature. Test results preliminarily verify the feasibility of 2-layered design and at the same time provide insights for the applicability of 2-layered columns in practical construction projects.

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Assessment of the fire-induced spalling of high strength concrete in concrete structures requires knowledge of the tensile strength of concrete at an elevated temperature. However, previous research is mainly focused on determining residual tensile strength, measured at the post-fire stage after cooling the specimens to ambient temperature. However, such residual tensile strength is only applicable to concrete after fire exposure and not during the fire event, which is usually more critical. In this study, the early residual splitting tensile strength of concrete at high temperature is determined experimentally. The test results indicate that early residual splitting tensile strength decreases with temperature. To understand this phenomenon, temperature distribution is examined. Compared to residual tensile strength, loss of early residual splitting tensile strength is found to be faster due to the elevated temperature effect. Lastly, in order to reproduce early residual splitting tensile strength, a numerical model is developed and empirical expressions are proposed for engineering application.