Use of colour image analysis for assessment of fire damaged concrete

The aim of this project was to carry out a fundamental study to assess the potential of colour image analysis for use in investigations of fire damaged concrete. This involved:(a) Quantification (rather than purely visual assessment) of colour change as an indicator of the thermal history of concrete.(b) Quantification of the nature and intensity of crack development as an indication of the thermal history of concrete, supporting and in addition to, colour change observations.(c) Further understanding of changes in the physical and chemical properties of aggregate and mortar matrix after heating.(d) An indication of the relationship between cracking and non-destructive methods of testing e.g. UPV or Schmidt hammer. Results showed that colour image analysis could be used to quantify the colour changes found when concrete is heated. Development of red colour coincided with significant reduction in compressive strength. Such measurements may be used to determine the thermal history of concrete by providing information regarding the temperature distribution that existed at the height of a fire. The actual colours observed depended on the types of cement and aggregate that were used to make the concrete. With some aggregates it may be more appropriate to only analyse the mortar matrix. Petrographic techniques may also be used to determine the nature and density of cracks developing at elevated temperatures and values of crack density correlate well with measurements of residual compressive strength. Small differences in crack density were observed with different cements and aggregates, although good correlations were always found with the residual compressive strength. Taken together these two techniques can provide further useful information for the evaluation of fire damaged concrete. This is especially so since petrographic analysis can also provide information on the quality of the original concrete such as cement content and water / cement ratio. Concretes made with blended cements tended to produce small differences in physical and chemical properties compared to those made with unblended cements. There is some evidence to suggest that a coarsening of pore structure in blended cements may lead to onset of cracking at lower temperatures. The use of DTA/TGA was of little use in assessing the thermal history of concrete made with blended cements. Corner spalling and sloughing off, as observed in columns, was effectively reproduced in tests on small scale specimens and the crack distributions measured. Relationships between compressive strength/cracking and non-destructive methods of testing are discussed and an outline procedure for site investigations of fire damaged concrete is described.

Coupled heat and mass transfer in concrete exposed to fire

The first investigation of this study is concerned with the reasonableness of the assumptions related to diffusion of water vapour in concrete and with the development of a diffusivity equation for heated concrete. It has been demonstrated that diffusion of water vapour does occur in concrete at all temperatures and that the type of diffusion is concrete is Knudsen diffusion. Neglecting diffusion leads to underestimating the pressure. It results in a maximum pore pressure of less than 1 MPa. It has also been shown that the assumption that diffusion in concrete is molecular is unreasonable even when the tortuosity is considered. Molecular diffusivity leads to overestimating the pressure. It results in a maximum pore pressure of 2.7 MPa of which the vapour pressure is 1.5 MPa while the air pressure is 1.2 MPa. Also, the first diffusivity equation, appropriately named 'concrete diffusivity', has been developed specifically for concrete that determines the effective diffusivity of any gas in concrete at any temperature. In thick walls and columns exposed to fire, concrete diffusivity leads to a maximum pore pressures of 1.5 and 2.2 MPa (along diagonals), respectively, that are almost entirely due to water vapour pressure. Also, spalling is exacerbated, and thus higher pressures may occur, in thin heated sections, since there is less of a cool reservoir towards which vapour can migrate. Furthermore, the reduction of the cool reservoir is affected not only by the thickness, but also by the time of exposure to fire and by the type of exposure, i.e. whether the concrete member is exposed to fire from one or more sides. The second investigation is concerned with examining the effects of thickness and exposure time and type. It has been demonstrated that the build up of pore pressure is low in thick members, since there is a substantial cool zone towards which water vapour can migrate. Thus, if surface and/or explosive spalling occur on a thick member, then such spalling must be due to high thermal stresses, but corner spalling is likely to be pore pressure spalling. However, depending on the exposure time and type, the pore pressures can be more than twice those occurring in thick members and thought to be the maximum that can occur so far, and thus the enhanced propensity of pore pressure spalling occurring on thin sections heated on opposite sides has been conclusively demonstrated to be due to the lack of a cool zone towards which moisture can migrate. Expressions were developed for the determination of the maximum pore pressures that can occur in different concrete walls and columns exposed to fire and of the corresponding times of exposure.

Lateral instability of slender reinforced concrete columns in a fire environment

The research concerns the development and application of an analytical computer program, SAFE-ROC, that models material behaviour and structural behaviour of a slender reinforced concrete column that is part of an overall structure and is subjected to elevated temperatures as a result of exposure to fire. The analysis approach used in SAFE-RCC is non-linear. Computer calculations are used that take account of restraint and continuity, and the interaction of the column with the surrounding structure during the fire. Within a given time step an iterative approach is used to find a deformed shape for the column which results in equilibrium between the forces associated with the external loads and internal stresses and degradation. Non-linear geometric effects are taken into account by updating the geometry of the structure during deformation. The structural response program SAFE-ROC includes a total strain model which takes account of the compatibility of strain due to temperature and loading. The total strain model represents a constitutive law that governs the material behaviour for concrete and steel. The material behaviour models employed for concrete and steel take account of the dimensional changes caused by the temperature differentials and changes in the material mechanical properties with changes in temperature. Non-linear stress-strain laws are used that take account of loading to a strain greater than that corresponding to the peak stress of the concrete stress-strain relation, and model the inelastic deformation associated with unloading of the steel stress-strain relation. The cross section temperatures caused by the fire environment are obtained by a preceding non-linear thermal analysis, a computer program FIRES-T.

The transient behaviour of plain concrete at elevated temperatures

This thesis describes an investigation of the effect of elevated temperatures upon the properties of plain concrete containing a siliceous aggregate. A complete stress-strain relationship and creep behaviour are studied. Transient effects (non-steady state) are also examined in order to simulate more realistic conditions. A temperature range of 20-700ºC is used. corresponding to the temperatures generally attained during an actual fire. In order to carry out the requisite tests, a stiff compression testing machine has been designed and built. The overall control of the test rig is provided by a logger/computer system by developing appropriate software, thus enabling the load to be held constant for any period of tlme. Before outlining any details of the development of the testing apparatus which includes an electric furnace and the.associated instrumentation, previous work on properties of both concrete and. steel at elevated temperatures is reviewed. The test programme comprises four series of tests:stress-strain tests (with and without pre-load), transient tests (heating to failure under constant stress) and creep tests (constant stress and constant temperature). Where 3 stress levels are examined: 0.2, 0.4 & 0.6 fc. The experimental results show that the properties of concrete are significantly affected by temperature and the magnitude of the load. The slope of the descending portion branch of the stress-strain curves (strain softening) is found to be temperature dependent. After normalizing the data, the stress-strain curves for different temperatures are represented by a single curve. The creep results are analysed using an approach involving the activation energy which is found to be constant. The analysis shows that the time-dependent deformation is sensibly linear with the applied stress. The total strain concept is shown to hold for the test data within limits.