Numerical and experimental study on concrete subjected to elevated temperature

Abstract

Fire-induced spalling has caused primary concern in the application of concrete, especially the high-strength and high-performance concrete with more compact microstructure, subjected to elevated temperature. Though decades of experimental and numerical research have been concentrated on the performance of concrete at high temperature, fire-induced spalling remains a challenging topic in the design of building structures. Reliable and practical model is therefore highly desirable and critical to the assessment and prediction of spalling. The first contribution of the thesis involves the development of a coupled thermo-hygro-mechanical (THM) model to study the behaviour of concrete subjected to elevated temperature. Macroscopic governing equations of the numerical model are established based on mass, enthalpy and momentum conservations, supplemented with relevant constitutive laws and state equations. A two-step spalling criterion is developed combining the assessment of mechanical damage and strain energy density. Validated against experiments conducted by various researchers, the model is demonstrated to show good correlations with the experimental data of temperature and pore pressure profiles. An algorithm is proposed to account for the stochastic nature of concrete behaviour with steps of implementation to incorporate the algorithm into the model. The second part of the thesis investigates the early residual splitting tensile strength (ERSTS) of concrete at elevated temperature, which could serve as an important parameter in determining the occurrence of fire-induced spalling. Experimental studies are designed and performed to obtain data of ERSTS while the concrete specimens are still at high temperature, and to compare with the residual splitting tensile strength (RSTS) of concrete measured when specimens have cooled down and resumed to ambient temperature. Differences have been shown to exist between the ERSTS and RSTS of concrete. Numerical analyses are also conducted to simulate the thermo-mechanical process and reproduce the experimental results. The third and final part of the thesis proposes an innovative protective measure to prevent fire-induced spalling. A bilayer column system, consisting of a high strength concrete (HSC) inner core and a normal strength concrete (NSC) outer layer, is introduced to examine the effect of thermal barrier on columns. The behaviour of bilayer column is evaluated by compression and fire tests conducted at ambient and elevated temperatures respectively. Preliminary experimental results show that the bilayer columns properly cast and reinforced are generally effective in preventing spalling. However, further study on thickness of the outer layer and integrity of the whole column is needed to improve the mechanical and thermal performance of bilayer columns.