Assessment of fire damaged concrete structures usually starts with visual observation of colour change, cracking and spalling. On heating, a change in colour from normal to a pink/red is often observed and this is useful since it coincides with the onset of significant loss of concrete strength. Optical microscopy combined with colour image analysis has been used to quantify changes in colour for concrete subjected to elevated temperatures. Samples were examined in reflected light and measurements of hue, saturation and intensity for colour definition, were taken. The measurement of hue was particularly useful and enabled the quantification of colour at different temperatures. This technique is superior to the subjective visual assessment currently used. The full development of the pink/red colour is coincident with substantial reduction in compressive strength and the method may be used to define the distance from a heated surface where strength degradation has occurred.
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