Effects of different sonic access tube materials on the signal strength of ultrasonic waves in the cross-hole sonic logging technique

摘要

The Cross-hole Sonic Logging (CSL) technique is one of the most effective quality assurance methods for testing foundations in terms of assessing the homogeneity and integrity of in-situ concrete foundation elements. This paper aims to study the effects of sonic access tube materials (such as polyvinyl chloride (PVC) and steel) on the signal strength of ultrasonic waves when carrying out a CSL test. This aim was achieved by the use of an in-house developed multi-channel ultrasonic data acquisition prototype, referred to as the 'Multiple Referencing System' (MRS). The prototype was developed based on National Instruments (NI) hardware in a LabVIEW (Laboratory Virtual Instrument Engineering Workbench) environment. It was designed for pre-programmed data acquisition, processing and visualising. With the development of the in-house developed prototype MRS, the transmission velocity and signal strength of the acquired waveforms were determined by the use of different sonic access tubes. According to the experimental results, PVC tubes produce a lower attenuation than steel tubes due to a smaller difference in acoustic impedance between PVC and the testing medium. For each material, the signal strength increases with increasing sonic access tube diameter due to the effect of a curved surface of the walls of sonic access tubes. Sonic profiles with unclear and fuzzy successions of white and black lines may be obtained by the use of a larger-diameter (ie 150 mm diameter) sonic access tube, even though no defect is presented inside foundation concrete elements, as the probes can move around inside the tube. Inside sonic access tubes with a diameter more than twice the diameter of the probes, the probes should befitted with centralisers to prevent undue movement during measurement.