Performance of fibre reinforced concrete at elevated temperatures

Fibre reinforced concrete (FRC) has gained increasing interest in recent years due to its superior mechanical properties compared to normal concrete. To apply FRC in civilian and strategically important buildings and other infrastructures, its fire resistance is an inevitable matter of concern. The fire resistance of FRC can be influenced by various factors, including fibre type, fibre dosage and cementitious matrix design. At present, a comprehensive and versatile constitutive model that can accurately capture the behaviour of FRC at high temperature is still missing. In particular, 1) the information about the relationship between multiple variables and the material properties of FRC under both ambient and high temperature is still lacking, 2) a temperature FRC constitutive model for a wide range of applications is required, and 3) the simulation of FRC members under transient temperature field has not been reported. In this study, temperature-dependent constitutive models are developed for steel fibre reinforced concrete (SFRC) and hybrid fibre reinforced concrete (HFRC). In the new models, a series of empirical equations are proposed to describe the effect of different factors on FRC performance at high temperature. In addition, the prediction accuracies of the models are improved by introducing new parameters and modifying the governing equations. Besides, a numerical model is proposed to simulate the behaviour of FRC under transient temperature field, and the effects of temperature loading rate, cross-section dimension and the properties of reinforcing fibre (i.e. fibre dosage, fibre aspect ratio and fibre shape) are investigated to provide reliable references and guidance for the design of FRC members. A comprehensive review of the recent research works on the fire resistance of FRC is firstly performed. In particular, the temperature-dependent mechanical properties of SFRC, polypropylene fibre reinforced concrete (PFRC) and HFRC are discussed, including permeability, spalling, compressive strength, tensile strength, elastic modulus, toughness and mass loss. In addition, the currently available predicting equations for FRC residual properties are summarised and compared. A new temperature-dependent constitutive model is then proposed in this study to describe the mechanical behaviour of SFRC at elevated temperatures. In the proposed model, both accumulated shear damage and compaction damage are considered, and a temperature-dependent coefficient is introduced to evaluate the plastic strain at fracture. To predict the mechanical properties of SFRC at elevated temperatures, a set of equations is also proposed based on the collected experimental data. Particularly, three indexes, i.e. water to binder ratio, moisture content and alumina content, are introduced to calibrate the normalised compressive strength of SFRC. Besides, the effects of fibre dosage and fibre shape are taken into account by introducing a new fibre reinforced index. The proposed model is demonstrated to be effective and accurate in predicting the mechanical behaviour of SFRC subjected to elevated temperatures. After that, the proposed constitutive model is further developed for HFRC under high temperature condition. To mimic the behaviour of HFRC, the governing equations of damage function and the strength envelope are modified in the new model. To calibrate the relationship between the mechanical properties of HFRC and temperature, a series of empirical equations are proposed. The effects of various influencing factors are considered, including fibre type, fibre shape, fibre dosage, water-binder ratio, moisture content and chemical composition. The proposed model is validated by comparing the simulation results with the test data from literature. In order to model the response of SFRC members under real fire condition, transient analysis is performed. In this study, a numerical model is established, in which the developed SFRC constitutive model is modified to capture the behaviour of SFRC subjected to transient temperature, and the mechanical properties of SFRC under hot state is calibrated. The new material model is incorporated in the commercial finite element software LS-DYNA as user defined subroutine, and the temperature propagation model is employed to apply transient temperature field. Once the model is validated, parametric study is carried out to investigate the effects of temperature loading rate, member dimension and fibre reinforcement parameter on the performance of SFRC beams under fire condition. The proposed constitutive model and modelling techniques in this study provide a reliable predicting tool for describing the behaviour of FRC under fire conditions.