Thermal effect of high temperatures on the physical and mechanical properties of a granite used in the UNESCO World Heritage of the north of Portugal

Abstract

This work discusses the results from a set of physical and mechanical test conducted on 46 cubic samples of a granite from the north of Portugal widely used in local granite-built UNESCO World Heritage, heated in a furnace at temperatures ranging from 105 to 700º C and cooled under different conditions. Then, the obtained results are compared with an extensive database of published case studies, showing that porosity, unit weight, ultrasonic waves velocities, elastic modulus and uniaxial compressive stress variations are mostly related to the induced thermal-stress caused by the anisotropic thermal expansion of the mineral grains. Only mineralogy and specific gravity do not exhibit changes for the studied range of temperatures. For specimens heated to above 500ºC, degradation processes predominate due to alpha-beta phase transition in quartz grains, considerably increasing the linear crack density, length and width and reducing the rock properties. Regarding the influence of the cooling method, thermal effects are amplified when cooling is performed by water immersion because of the inhomogeneous shrinkage and the subsequent tension stress between adjacent particles due to temperature decrease that develop at different pace within the samples. Scanning electron microscopy confirms the increase of damage and the negative effect on the granite properties caused by thermal treatment from 500ºC and by water cooling. For temperatures lower than 500ºC a hardening effect slightly improving elastic modulus and uniaxial compressive strength is observed in slowly and quickly cooled samples. The derived conclusions considerably improve the knowledge about the behaviour against temperature of this type of granite, with the purpose of evaluating the integrity level of granite-built UNESCO cultural heritage of the north of Portugal.