Assessment of fire damaged concrete via the hammer-drill pulse transmission technique

Abstract

The point-by-point measurement of the resistance to hammer-drilling is a viable way to scan the steep gradients of residual mechanical properties exhibited by fire damaged concrete members. Despite the interesting pros of this technique (rapidity and immediate availability of the results), previous studies pointed out its insensitivity to low levels of damage and the significant variation that can be attributed to the coarse aggregate. In order to overcome these limitations, a renewed version has been developed, based on the continuous monitoring of two further phenomena during the drilling operation. The first observation is through acquisition of the compressive pulses generated by the hammering mechanism of the drill, which propagate along the bit shank and are partly reflected in the form of tensile stress waves. The amplitude of the reflected waves provides some meaningful information on the local acoustic impedance of the drilled material. The second observation involves measurement of the time of flight of the pulses propagating from the tip of the drill-bit to a fixed ultrasonic receiver on the surface of the member. This translates into an acoustic scan of the concrete volume enabled by the drilling process. Besides the technical challenges associated with the monitoring of these two phenomena, the paper discusses their sensitivity to fire damage and the ability to detect steep gradients in the residual material properties. © RILEM 2013.