Contribution to the development of colorimetry as a method for the assessment of fire-damaged concrete

Abstract

This study presents a new method of using colorimetry for post-fire assessment of concrete. Changes in concrete colour with temperature are related to chemical changes of cement and aggregates. Two concretes were studied: an ordinary concrete (OC) and a high-performance concrete (HPC). The difference between both concretes was the water–cement ratio. They were made with the same cement and the same riverbed aggregates. Small samples were heated with different target temperatures: 100, 200 ... 1000 °C. After each cooling period, a picture was taken with a flatbed scanner. Two types of colour image decomposition were then used: RGB (red, green and blue) and HSI (hue, saturation and intensity) to analyse the evolution of the colour of the concrete with temperature. The evolution of red, green and blue components with temperature was analysed and two parameters of colorimetric analysis in order to predict the maximum temperature reached by concrete during fire were proposed. As the results have shown, the blue colour histogram gradually widens at its base as the temperature increases. Parameter of colorimetric analysis 1 is the measurement of this width. Parameter of colorimetric analysis 2 is associated with the appearance of very light colours beyond 800 °C. Thus, the number of pixels with intensity of red or green or blue higher than 225 increases very significantly between 800 and 900 C. Parameter of colorimetric analysis 3 is the average hue of the image (in the HSI colour space). It was found that parameter of colorimetric analysis 3 decreases significantly with temperature. This corresponds to the reddening of concrete with temperature due to the oxidation of mineral containing iron. To validate the use of these parameters of colorimetric analysis for the post-fire analysis of concrete, small concrete slabs were heated. They were positioned in place of the oven door, thus providing a thermal gradient through their thickness. Images taken during the cooling process were analysed using the proposed parameters of colorimetric analysis.