Effect of fire on concrete and concrete structures

Abstract

The behaviour of concrete in fire depends on its mix proportions and constituents and is determined by complex physicochemical transformations during heating. Normal-strength concretes and highperformance concretes microstructurally follow similar trends when heated, but ultra-highperformance concrete behaves differently. A key property unique to concrete amongst structural materials is transient creep. Any structural analysis of heated concrete that ignores transient creep will yield erroneous results, particularly for columns exposed to fire. Failure of structural concrete in fire varies according to the nature of the fire; the loading system and the type of structure. Failure could occur from loss of bending or tensile strength; loss of bond strength; loss of shear or torsional strength; loss of compressive strength; and spalling of the concrete. The structural element should, therefore, be designed to fulfil its separating and/or load-bearing function without failure for the required period of time in a given fire scenario. Design for fire resistance aims to ensure overall dimensions of the section of an element sufficient to keep the heat transfer through this element within acceptable limits, and an average concrete cover to the reinforcement sufficient to keep the temperature of the reinforcement below critical values long enough for the required fire resistance period to be attained. The prediction of spalling hitherto an imprecise empirical exercise`is now becoming possible with the development of thermohydromechanical nonlinear finite element models capable of predicting pore pressures. The risk of explosive spalling in fire increases with decrease in concrete permeability and could be eliminated by the appropriate inclusion of polypropylene fibres in the mix and/or by protecting the exposed concrete surface with a thermal barrier. There are three methods of assessment of fire resistance: (a) fire testing; (b) prescriptive methods, which are rigid; and (c) performancebased methods, which are flexible. Performancebased methods can be classified into three categories of increasing sophistication and complexity: (a) simplified calculations based on limit state analysis; (b) thermomechanical finite element analysis; and (c) comprehensive thermohydromechanical finite element analysis. It is only now that performance-based methods are being accepted in an increasing number of countries.

Effect of elevated temperature on the compressive strength of concrete produced with pulverized steel mill scale

“…The physical changes and chemical decomposition of concrete constituents at elevated temperatures is demonstrated in widely spread cracks, explosive spalling or both. In reinforced concrete members, such changes are reflected by a reduction in the mechanical properties, increase in the permeability of concrete, and weakening of the bond strength between embedded steel and concrete [10][11][12]. Many studies have been carried out to investigate the effect of the elevated temperature on different concrete specimens [13][14][15].…”

Modelling explosive spalling and stress induced thermal strains of HPC exposed to high temperature

“…Numerous experiments [1][2][3][4][5] demonstrated certain aspects of explosive spalling. However, only limited information can be deduced due to the inherent difficulties in such experiments.…”

“…It is associated with violent failure of thin layers of concrete resulting in sudden reduction of load carrying capacity, which may lead to complete collapse. High pore pressures due to low permeability and the thermally induced stresses are considered to be the governing causes of explosive spalling [1,2], however the mechanism has not yet been fully understood.…”

High Temperature Exposure of HPC – Experimental Analysis of Residual Properties and Thermal Response

“…In case of fire, the release of the bound water from the calcium silicate hydrates (CSHs) takes place at about 110-150 °C [10]. Increase of the tensile stresses in the concrete structure is caused by the thermal expansion of the aggregates from 300 °C, where some cracks appearance was noted [11].…”