The use of thermal analysis in assessing the effect of temperature on a cement paste

Effect of elevated temperatures on geopolymer paste, mortar and concrete

Effects of elevated temperatures on properties of concrete

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.

Effect of fire on concrete and concrete structures

Spalling and pore pressure in HPC at high temperatures

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...

Compressive strength of concrete at high temperatures: a reassessment

Synopsis This is the first of three papers presenting the results of an investigation into the effect of material and environmental factors upon the transient thermal strain behaviour of concrete during the first heat cycle under load to 600°C. A literature review of transient thermal creep of concrete is presented. It reveals inadequate understanding of this subject for temperatures above 100°C and a lack of data for conditions pertinent to the analysis of concrete structures during first-time heating. This paper also presents results of preliminary work forming background information for the analysis of the transient thermal strain behaviour of unsealed concrete specimens. The complex temperature, moisture and thermal stress conditions developing during thermal transients in concrete test specimens have, therefore, been investigated experimentally and/or theoretically. The ‘structural’ effects and modification of material behaviour caused by these conditions have consequently been minimized by appropria...

Transient thermal strain of concrete: literature review, conditions within specimen and behaviour of individual constituents

This paper presents the basic principles and details of the physical, mathematical and numerical models forming the main structure of the numerical analysis of the thermal, hydrol and mechanical behaviour of normal, high-performance and ultra-high performance concrete structures subjected to heating. A fully coupled non-linear formulation is designed to predict the behaviour and potential for spalling, of heated concrete structures for fire and nuclear reactor applications. Concrete is considered as a multiphase material consisting of a solid phase, two gas phases and three water phases. The physical model is described with emphasis being placed upon the real processes occurring in concrete during heated based on tests carried out in several major laboratories around Europe as part of the wider HITECO research programme. Examples of preliminary outputs are presented in the form of isoline diagrams compared with those obtained from experiment.

Modelling of heated concrete

Temperatures of significance are identified in the range 20 to 700 C (68 to 1292 F) for changes in strength and elastic modulus both static and dynamic) of unsealed hardened cement paste measured at the various temperaturues as well as after cooling. These changes are correlated with chemical microstructural changes reported in the literature. The beneficial effects of thermal drying and loading, within limits, upon these mechanical properties are observed. It is concluded that these properties are dependent primarily on the maximum temperature of exposure as opposed to the temperature at testing.

G. A. Khoury

Papers

Effect of fire on concrete and concrete structures

Compressive strength of concrete at high temperatures: a reassessment

Transient thermal strain of concrete: literature review, conditions within specimen and behaviour of individual constituents

Modelling of heated concrete

Mechanical Properties of Hardened Cement Paste Exposed to Temperatures up to 700 C (1292 F)