Abstract This paper deals with the feasibility and sensitivity of ultrasonic probe waves for characterizing the mechanical damage of reinforced concrete slabs during bending tests either in sound concrete or in concrete affected by alkali-silica reaction (ASR). The results show that the ultrasonic probe waves are capable of distinguishing between the damage phases in the concrete elements (initial structural cracking and bars yielding phases). Three reinforced concrete slabs with dimension of 1.40×0.75×0.3 m3 (made with nonreactive aggregates and ASR-reactive aggregate) were used in this experiment. Both the conventional method of load-deflection measurements and the nondestructive testing based on ultrasonic probe signals were applied in order to evaluate the feasibility of ultrasonic testing for tracking crack growth in the reinforced concrete elements. These slabs were assessed under four-point monotonic bending tests over a span of 1400 mm and were subjected to step-loading until failure. The ultrasonic probe signals were recorded at the each step-load and then energy of received signals were extracted in order to evaluate the energy loss of the signals due to the mechanical cracking. Changes in the energy contents of the signals fairly correlate with the increase of the loads. The results show that the ultrasonic testing is a more robust approach for distinguishing the sound concrete slab from the damaged concrete.