Compressive strength of concrete at high temperatures: a reassessment
TL;DR: In this paper, a reassessment of the subject is given, which considers material and environmental factors/mechanisms influencing the strength of concrete during the heat cycle and after cooling, not all of which necessarily result in strength loss.
Abstract: Based on experience with siliceous aggregate/OPC paste concrete it is generally believed that the compressive strength of unsealed ‘concrete’ declines sharply above 300°C. This is too pessimistic a view. A reassessment of the subject is given in this Paper, which considers material and environmental factors/mechanisms influencing the strength of concrete during the heat cycle and after cooling, not all of which necessarily result in strength loss. Design of concrete for better performance at high temperatures should aim at minimizing contributions to strength loss, while exploiting the processes responsible for gain in strength. It appears that, in its hydraulic state of binding, a rheological criterion limits the structural usefulness of Portland cement concrete to temperatures of 600°C. Today, many commonly used concretes lose considerable strength at temperatures above about 300°C. There is, therefore, scope for improvement in design within the temperature range 300— 600°C. Raising the ‘working’ temper...
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TL;DR: In this article, the effect of temperature in the mineralogical composition of cement hydration products has been studied using thermogravimetric analysis (TGA) and DTG curves, which can be used to determine fire conditions and the consequent deterioration expected in the cement paste.
709 citations
TL;DR: There are three methods of assessment of fire resistance: (a) fire testing, (b) prescriptive methods, which are rigid; and (c) performance-based methods which are flexible as discussed by the authors.
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 high-performance concretes microstructurally follow similar trends when heated, but ultra-high-performance 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) performance-based methods, which are flexible. Performance-based 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.
569 citations
TL;DR: In this article, an original device was designed in order to make simultaneous measurements of pressure and temperature at various positions in a concrete specimen (30×30×12 cm3) heated on one face up to 800°C.
552 citations
TL;DR: In this paper, the pore structure in HSC and NSC was investigated and it was shown that HSC lost its mechanical strength in a manner similar to that of NSC.
Abstract: Based on normal strength concrete (NSC) and high-strength concrete (HSC), with compressive strengths of 39, 76, and 94 MPa respectively, damage to concrete under high temperatures was identified. After exposure to temperatures up to 1200 °C, compressive strength and tensile splitting strength were determined. The pore structure in HSC and in NSC was also investigated. Results show that HSC lost its mechanical strength in a manner similar to that of NSC. The range between 400 and 800 °C was critical to the strength loss. High temperatures have a coarsening effect on the microstructure of both HSC and NSC. On the whole HSC and NSC suffered damage to almost the same degree, although HSC appeared to suffer a greater worsening of the permeability-related durability.
377 citations
TL;DR: In this article, the authors used an experimental approach to study the influence of curing temperature and combined effect of temperature and CPB components on the main mechanical properties (strength, modulus of elasticity, and stress-strain behaviour) of CPB.
327 citations