Microwave pre-conditioning of granite and concrete and the implications on their geotechnical parameters

Microwave pre-conditioning is considered to be the most promising complementary hard rock cutting method, with a strong potential to improve the advance rate and profitability of future mechanized tunnel boring and mining equipment. This technique is based on selective heating and differential volumetric expansion of minerals causing micro-and macrofracturing in rock. Over 700 samples consisting of granite and three different strength grades of concrete were irradiated and underwent a comprehensive geotechnical testing program. Besides lon-gitudinal wave velocity, porosity, specific heat capacity, density, and temperature difference, additional geotechnical parameters are influenced by material-specific heating intervals: Unconfined compressive strength (UCS), Brazilian tensile strength (BTS), Point load index (PLI), CERCHAR abrasivity index (CAI), the LCPC abrasivity coefficient (LAK) and its breakability coefficient (LBK). Comparisons of thin sections of irradiated and non-irradiated materials show microscopic structural changes. A slight reduction of the UCS was observed after longer irradiation intervals for granite and concrete, while the weakening effect on the BTS and PLI is more pronounced. Rock strength was reduced by up to 48% for granite and 40-48% for concrete, which potentially leads to a substantial increase in penetration rate of a tunnel boring machine and reduced wear. Furthermore, we found a linear correlation between granite strength, sample temperature, and specific energy, resulting in a reduction of the PLI by 0.12% K-1. The tested concrete types showed strength reduction ratios between 0.21 and 0.28% K-1. By calculating efficiencies of the deployed energy, we showed that pre-conditioning with microwaves can be an effective and efficient tool to artificially reduce the strength of rocks in order to increase the advance rate of tunnel boring and mining machines.