Infrared thermography as a tool to detect increasing cracking in granitic stones exposed to high temperatures

A fire affecting the rocks of the built heritage has a devastating effect both aesthetically and structurally. After it, a damage assessment is necessary and must be carried out with non-destructive methods, although in extreme cases of degradation, the use of non-contact techniques is the preferred option. Infrared thermography allows the measurement from a distance to the building and can be used to assess the decay. The effect of fire-related temperatures on granitic stones was evaluated by means of non-destructive testing, as porosity by water immersion, capillary water uptake and p-wave velocities (Vp). The samples were heated up to 400 °C, 600 °C and 800 °C and the evaluation techniques were applied before and after each temperature change. Results showed an increase in porosity, capillary water uptake and a decrease in Vp, progressive up to 600 °C and abrupt at 800 °C. As thermal properties of stones are directly related to their porosity, they can be used as an estimator of decay. The kinetic of heat exchange with the environment is a widely used parameter to estimate the physical properties of the materials. Thus, stone cooling measured by infrared thermography was assessed as a tool to predict the decay of cracked crystalline stones. Two parameters characterizing the cooling kinetics (CRI10) and (CRITOTAL), were calculated to compare the variations of granitic stones and minerals between them and to the rest of physical properties. The CRI10 parameter, related to thermal properties, presented a homogeneous and progressive decrease through the whole heating test. The cooling rate was lower with an increase in porosity, due to the low diffusivity of the air from the cracks. However, the trend was not linearly correlated to the porosity increase, and a more detailed interpretation is needed. Two main hypotheses working together arose to explain this behavior found in all the granites. The first is that when the environment is at lower temperature than a fractured rock, the cracks appear as hot areas in the thermal image captured by the infrared camera, and creates a retarding in the cooling. The second one is the influence of the mineral proportion, since each mineral has different thermal properties that may influence the stone behavior stronger than the cracking. In all cases, during the first 30 s, the cooling was fast enough to influence the individual mineral results and this must be studied together with the rest of properties.