Quality Evaluation of Concrete Exposed to High Temperature by Ultrasonic Spectroscopy

Abstract

In recent years, it became possible to detect feeble electric signals supersensitively owing to the development of electric measuring techniques. Now, an advanced technique using ultrasonic pulses is being applied in the concrete field too. The waveform of ultrasonic pulses through concrete contains various informations related to the internal structure of material. So, it may be possible to clarify the internal structure of material by analyzing the measured waves in detail. Thus, ultrasonic spectroscopy is useful to evaluate the quality of material by measuring the frequency characteristics of ultrasonic pulse through material.
In this study, the ultrasonic spectroscopy was applied to evaluate the quality of concrete exposed to high temperature conditions, as the first step to establish a new non-destructive testing technique for concrete and mortar.
The main results obtained in this study are summarized as follows:
1) Compressive and flexural strengths of concrete decreased according to temperature rise. Especially the flexural strength of mortar was sensitively affected by temperature rise.
2) The ultrasonic pulse velocity, maximum amplitude and energy of measured ultrasonic pulse waves decreased according to temperature rise.
3) The maximum amplitude and energy of frequency transfer function of concrete decreased with increasing temperature, and these decreasing rates were not affected very much by the water-cement ratio and heating duration.
4) The local maximum amplitude and energy of the frequency transfer function in the frequency ranges of 50-100kHz and 200-250kHz were closely related to the change of internal structure of concrete exposed to high temperature.