The spalling of concrete in fires

The occurrence of spalling is a major factor in determining the fire resistance of concrete constructions. The apparently random occurrence of spalling has limited the development and application of fire resistance modelling for concrete structures. This Thesis describes an experimental investigation into the spalling of concrete on exposure to elevated temperatures. It has been shown that spalling may be categorised into four distinct types, aggregate spalling, corner spalling, surface spalling and explosive spalling. Aggregate spalling has been found to be a form of shear failure of aggregates local to the heated surface. The susceptibility of any particular concrete to aggregate spalling can be quantified from parameters which include the coefficients of thermal expansion of both the aggregate and the surrounding mortar, the size and thermal diffusivity of the aggregate and the rate of heating. Corner spalling, which is particularly significant for the fire resistance of concrete columns, is a result of concrete losing its tensile strength at elevated temperatures. Surface spalling is the result of excessive pore pressures within heated concrete. An empirical model has been developed to allow quantification of the pore pressures and a material failure model proposed. The dominant parameters are rate of heating, pore saturation and concrete permeability. Surface spalling may be alleviated by limiting pore pressure development and a number of methods to this end have been evaluated. Explosive spalling involves the catastrophic failure of a concrete element and may be caused by either of two distinct mechanisms. In the first instance, excessive pore pressures can cause explosive spalling, although the effect is limited principally to unloaded or relatively small specimens. A second cause of explosive spalling is where the superimposition of thermally induced stresses on applied load stresses exceed the concrete's strength.

The structural permeability of concrete at cryogenic temperatures

The thesis describes a programme of research designed to identify concretes for application at cryogenic temperature, in particular for storage of Liquefield Natural Gas which is maintained at a temperature of -165oC. The programme was undertaken in two stages. Stage 1 involved screening tests on seventeen concrete mixes to investigate the effects of strength grade (and water/cement ratio), air entrainment, aggregate type and cement type. Four mixes were selected on the basis of low temperature strength, residual strength after thermal cycling and permeability at ambient temperature. In Stage 2 the selected mixes were subjected to a comprehensive range of tests to measure those properties which determine the leak tightness of a concrete tank at temperatures down to -165oC. These included gas permeability; tensile strength, strain capacity, thermal expansion coefficient and elastic modulus, which in combination provide a measure of resistance to cracking; and bond to reinforcement, which is one of the determining factors regarding crack size and spacing. The results demonstrated that the properties of concrete were generally enhanced at cryogenic temperature, with reduced permeability, reduced crack proneness and, by virtue of increased bond to reinforcement, better control of cracking should it occur. Of the concretes tested, a lightweight mix containing sintered PFA aggregate exhibited the best performance at ambient and cryogenic temperature, having appreciably lower permeability and higher crack resistance than normal weight concretes of the same strength grade. The lightweight mix was most sensitive to thermal cycling, but there was limited evidence that this behaviour would not be significant if the concrete was prestressed. Relationships between various properties have been identified, the most significant being the reduction in gas permeability with increasing strain capacity. The structural implications of the changing properties of the concrete have also been considered.