Low cost colorimetry for assessment of fire damage in rock

Tunnels and caverns excavated for the purpose of mining tend to be designed for short term use and would usually be unlined. Fire hazard within a mining cavern or tunnel poses concerns on the structural integrity of the surrounding rock. Hence, a fast, non-destructive method for assessing the strength of a rock mass after a fire would be useful in deciding whether to salvage a mining operation or to abandon it. Colorimetry as a technique to estimate the heating temperature of concrete has proven useful. However, the use of colorimetry imposes a need to balance ease of use, such as low cost digital cameras and computer scanners, with accuracy achievable with expensive spectrometers and spectrophotometers. This technical note shall investigate the use of a Digital Single Lens Reflex (DSLR) camera and MATLAB as a means of performing non-destructive testing on rocks exposed to temperatures of between 200 °C to 500 °C to estimate the degradation of the rocks' tensile strength. The CIE L*a*b* color space is used. The effects of soot and smoke on the rocks are also simulated using high temperature grease to mimic real world conditions. The Brazilian tensile strength and P-wave velocity are two other properties investigated in this technical note. The results suggest that the a* and b* color values have an almost linear relationship with the heating temperature of the rock, whereas the tensile strength of the rock has a quadratic relationship. Additional tests indicate the robustness of the colorimetry process as the simulated smoke and soot damage did not significantly affect the relationship between heating temperature, a* and b* color values, provided the surface of the rock was cleaned after the fire event. The mechanism of the strength degradation may be attributed to generation of microcracks due to thermal loading, while the color change is attributed to either chemical changes within the rock, or scattering of light due to the generated microcracks. A key finding of this study is that the colorimetry process outlined is not only relatively cheap to use, as compared to expensive spectrometers and spectrophotometers, but also fast, as the photo taking process can be completed within 10 min. This makes it easier for multiple teams to be deployed after a fire incident in an underground rock tunnel or cavern, minimising the amount of time that workers need to spend in potentially hazardous environment.