Results of experimental investigation of concrete and rocks under high rate loads of microsecond duration (2.5 µs) are presented. Tested samples are rectangular plates (150/150/15 mm) with a notch (70 mm long and 3 mm thick) containing a pre-crack (about 1 mm long). Dynamic loading was created utilizing electric wire explosion technique. An electric wire was placed between the faces of the notch at a distance of 20 mm from the tip of the pre-crack. After the wire explosion the trajectory of the extended crack and the fracture surface were studied. Measured dependence of the crack extension on the loading pulse energy gives the possibility of determining the threshold (minimal) amplitude of the load required to initiate fracture in the studied samples.