Mechanical Properties of High and Very-High Strength Steel at Elevated Temperatures and After Cooling Down

High-strength steels (HSS) are produced using special chemical composition or/and manufacturing processes. Both aspects affect their mechanical properties at elevated temperatures and after cooling down, and particularly the residual strength and the ductility of the structural members. As HSS equates the design of lighter structural elements, higher temperatures are developed internally compared to the elements designed with conventional carbon steel. Therefore, the low thickness members, along with the severe effect of high temperature on the mechanical properties of the HSS, constitute to the increased vulnerability of such structures in fire. Moreover, the re-use and reinstatement of these structures are more challenging due to the lower residual mechanical properties of HSS after the cooling down period. This paper presents a review of the available experimental studies of the mechanical properties of HSS at elevated temperatures and after cooling down. The experimental results are collected and compared with the proposed material model (reduction factors) of EN1993–1-2. Based on these comparisons, modified equations describing the effect of elevated temperatures on the mechanical properties of HSS are proposed. Also, the post-fire mechanical properties of HSS are examined. A comprehensive discussion on the effect of influencing parameters, such as manufacturing process, microstructure, loading conditions, maximum temperature, and others is further explored

Rate and temperature dependent relations for CFSTs and CFFTs subject to post-impact fire conditions

Experimental research on the behaviour of conventional construction materials subject to extreme loading condition of post-impact-fire has indicated the effect of rate dependent loading history on the temperature performance of these materials. In order to analyse and design structures to withstand such combined loading, it is desirable to develop models that can reflect the mechanical properties of materials under initial impulsive loading and subsequent elevated temperature. In this study, a rate-pre-damage-temperature dependent empirical expression is proposed to predict the residual strength of partially damaged steel-concrete and fibre rein-forced polymer-concrete composite materials, in the form of concrete-filled steel tubes (CFST) and Concrete filled carbon-fibre reinforced polymer tubes (CFFT), at high temperatures up to 600 °C. The developed model is calibrated and validated on the basis of actual experimental data published by the authors. The proposed expression proves to be capable of successfully reproducing material strengths by considering the extent of high strain rate induced pre-damage together with temperature exposure

Mechanical properties of structural steel under post-impact fire

Accurate prediction of material properties under combined high strain rate and elevated temperature are essential for safe design of structures to withstand post-impact fire situations such as collision by heavy vehicles followed by fire. Numerous material tests performed in recent years do not address the influence of such sequential loading on the mechanical properties of mild steel. An inclusive test program is carried out in the Civil Engineering Lab at Monash University to investigate the post-impact fire properties of Grade 350 structural steel and the results are presented here. Specimens have undergone interrupting high strain rate tensile loading, controlled locally at defined levels of elongation, to account for different deformation states. Different damage levels are introduced for each rate of strain with respect to the displacement corresponding to the ultimate stress (fu). Subsequently, the partly damaged specimens are subjected to static tensile loading to failure at high temperature conditions. Material behaviour of pre-damaged steel is compared to those of each individual loading scenario and to design code expressions. The test results demonstrate that the combined actions are profoundly different from that in which the structure is subjected to either high strain rate or thermal loading and notably vary from those predicted in different codes. Moreover, it is shown that the strength and ductility of mild steel are significantly dependent on the rate of loading, the pre-deformation history and the temperature it is subsequently exposed to. The experimental results can be used by researchers and structural engineers as benchmark data for calibrating current material model constants and/or developing new material models which take into account the coupled effect of high strain rate and temperature for rational fire analysis and design of steel structures

Effect of Post-Fire Curing on the Residual Mechanical Properties of Fire-Damaged Self-Compacting Concrete

Concrete is recognized for being a fire-resistant construction material. At elevated temperatures concrete can, however, undergo considerable damage such as strength degradation, cracking, and explosive spalling. In recent decades, reuse of fire-damaged concrete structures by means of developing techniques to repair the degraded material has gained interest amongst researchers. Autogenic self-healing methods such as re-curing in water has proven to partly restore the strength of concrete. The extent of restoration is dependent upon various parameters such as concrete type, exposure temperature, and post-fire curing conditions for example. The use of selfcompacting/ consolidating concrete (SCC) has become common in the construction industry due to its high workability and low permeability. This paper presents the results of an experimental study aimed at investigating the improved mechanical properties of high temperature exposed SCC concrete by the autogenic self-healing phenomenon resulting from water re-curing. The residual mechanical properties including strength, modulus of elasticity and ultimate strain of the material upon application of different post-fire curing regimes are presented herein with special emphasis on the effect of thermal profile including exposure time, temperature and cooling rate. The experimental results confirm that the recovery of material properties in fire-damaged SCC concrete is contingent on the post-fire water curing conditions.Materials and Environmen

Mechanical properties of partially damaged structural steel induced by high strain rate loading at elevated temperatures - An experimental investigation

© 2014 Elsevier Ltd. All rights reserved. In structural engineering practice, understanding the behaviour of steel under extreme loading conditions is essential for accurate prediction of material response when subjected to a combination of severe load scenarios such as collision by heavy objects and a following fire. Hitherto, the combined effects of high strain loading and subsequent elevated temperature have not been widely investigated on the mechanical properties of structural steel. A comprehensive test program is carried out to investigate the post-impact fire properties of Grade 350 steel under well-defined conditions, the results of which are reported in this paper. Coupon specimens have undergone interrupting high strain rate (HSR) tensile loading at impact level, controlled locally at different levels of elongation, to account for different deformation states. Three different damage levels are introduced with respect to the displacement corresponding to the ultimate stress (fu). Subsequently, the partly damaged specimens are subjected to steady-state quasi-static tensile loading to failure at temperatures ranging from ambient to 600 °C. The overall stress-strain relationship, as well as the mechanical properties of pre-damaged steel, are presented at elevated temperatures and compared to those of each individual loading scenario. The test results demonstrate that the effects of these combined actions are profoundly different from those in which the structure is subjected to either high strain rate or thermal loading individually. It is shown that the strength and ductility of mild steel is significantly dependent on the rate of loading, the pre-deformation history and the temperature to which it is subsequently exposed. This necessitates the development of models which take into account the coupled effect of high strain rate and temperature in rational fire analysis and design of steel structures