Hygro-thermo-mechanical study of concrete elements subject to elevated temperatures: assessment of spalling risk and moisture interactions

摘要

Fire induced spalling of concrete upon exposure to high and rapidly rising temperatures can structurally weaken members and structures through section loss, while leaving reinforcement unrestrained and exposed. Spalling remains a significant challenge in structural design, particularly with the increasing use of higher strength concretes and more durable concretes with denser microstructure. Several decades of experimental and numerical research have demonstrated that fire induced spalling is a complex hygro-thermo-mechanical (HTM) phenomenon; quantitative models for reliably predicting spalling still require significant development.The first part of this dissertation involves development of a comprehensive pseudo one-dimensional coupled hygro-thermo-mechanical model for investigating, and predicting, concrete behaviour at elevated temperatures. Using a novel mechanical hybrid-type fibre model, variations in mechanical boundary conditions and imposed actions, or prestressing, on the fire spalling risk of plate and shell type structures are examined. The model is shown to predict temperature and gas pressure fields well, along with times and extents of first spalling. A simple criterion for explaining fire spalling is given, and the important influence of internal cracking in sections is discussed.Important for high temperature HTM models, together with advanced models for durability, creep and shrinkage of concrete, are good representations of water vapour sorption isotherms (WVSIs). WVSIs describe retained moisture at a prevailing humidity, and are intimately related to the cementitious microstructure. The second part of this dissertation involves development of a new physically-based model for predicting adsorption, desorption and scanning isotherms of hardened Portland cement type materials. After calibration of microstructural parameters from published adsorption data, the model predicts experimental WVSI data sets well. Microstructural predictions are consistent with ¹H nuclear magnetic resonance and mercury intrusion porosimetry data.In the third, and final, part of this dissertation an experimental study of WVSIs and equilibrium kinetics was undertaken. Novel apparatus for accurate determination of cementitious WVSIs at elevated temperatures are developed. Comprehensive data at 23°C–80°C is shown to clarify impacts of temperature on sorption hysteresis; important implications of observed time dependent non-Fickian kinetics are also studied.