Hammering sound of concrete with defects and spalling risk

Abstract

Inspection is important for preventing concrete spalling and maintaining the soundness of tunnels. Human inspection combining visual inspection and the hammering tests has a proven track record and is considered reliable. However, human inspection is time-consuming and the results vary depending on the inspector. Vibration measurement results obtained for areas with defects in unreinforced concrete sections of railway tunnels reveal that there are many defects that are overestimated the spalling risk. The objective of this study was to elucidate the causes of this overestimation. A concrete specimen with an inclined detachment was prepared, and the change in the hammering sound with the extension of the detachment was investigated. Numerical analyses were conducted to supplement the experimental results. The results reveal that the low-frequency vibrations of the defects are less likely to be transmitted by air as sound pressure. Moreover, low-frequency sounds are relatively hard to hear considering human auditory characteristics. Hence, low-frequency vibration may not affect the hammering sound. Although the presence or absence of defects can be distinguished by the hammering sound, the spalling risk cannot be accurately evaluated from the sound alone, which is one of the main reasons for spalling risk overestimation by human inspectors.