Effect of joints on microwave fracturing of the Bukit Timah granite using an open-ended antenna

Microwave fracturing of rocks before mechanical breakage could improve the performance of mechanical excavators and reduce environmental impacts.Previous research focused on the microwave fracturing of intact rock blocks.By using an open-ended antenna,this paper investigates the effect of pre-existing joints on the microwave fracturing of the Singapore Bukit Timah granite blocks.The results show that the specimens are weakened in the manners of cracking,spalling,melting,or a combination of them.The crack number and the total crack length produced by microwave treatment of jointed rock blocks are slightly smaller than those in the intact rock blocks.The interaction between joints and microwave-induced cracks can be summarized into the following four patterns:(1)microwave-induced cracks become arrested so that the crack propagation is terminated;(2)microwave-induced cracks penetrate the joints and continue to propagate;(3)microwave-induced cracks become deflected along the joints;and(4)microwave-induced cracks propagate forward following the joints.The smaller the approach angle between the microwave-induced crack and the preexisting joint is,the more microwave-induced cracks tend to be arrested at the joint.Increasing the approach angle between the microwave-induced crack and the joint can increase the chance of microwave-induced crack penetrating the joint.The results also show that the smaller the distance is between the microwave radiation point and the joint,the easier it is for microwave-induced cracks to penetrate the joints;otherwise,the microwave-induced crack is more likely to be arrested at the pre-existing joint.