Colour Change in Heated Concrete

This paper presents an analysis of colour change in concrete under the influence of heat. The colour change observed in concrete is primarily a result of the gradual dehydration of the cement paste, but also of transformations occurring within the aggregate. The colour change may be used to reveal the exposure temperature of concrete from which the corresponding fire damage of concrete can be estimated. The paper presents the results of tests carried out on ordinary and high performance concretes (OC and HPC) prepared with natural river-bed aggregates. In addition, mortars and cement pastes prepared with the same components were observed to change colour upon heating. The colour change was investigated using Scion Image v. 4.0.3, an image analysis software package (Scion Corporation ©, USA). In the proposed method the digital image is split into three RGB colour components: red, green and blue, which are then presented as a histogram using counts of pixel intensity. The histogram results show colour distributions in unheated cementitious material and in material heated to temperatures ranging from 100°C to 1000°C. The concrete colour changes as a result of heating are linked to the physical and chemical transformations taking place in the heated material.