Direct transmission of the ultrasonic pulse is the preferred approach 

 for measuring the average sound velocity in concrete because this method 

 provides maximum sensitivity with a well defined path length. Indirect 

 (surface) transmission is used when only one surface of the concrete is 

 accessible, such as a concrete retaining wall. This approach does not 

 have a well defined path length and indicates primarily the quality of 

 the concrete near the surface. 



Equipment Description 



The ultrasonic equipment used for these tests was the Model C-4899 

 V-Meter manufactured by James Instruments, Inc., shown in Figure 15. 

 This instrument is representative of commercially available ultrasonic 

 devices used for laboratory and field testing of concrete. It generates 

 low frequency ultrasonic pulses and measures the time for them to pass 

 from one transducer to the other through the material between them. The 

 V-Meter displays the transit time directly on a digital readout. The 

 overall time measurement range is 0.1 to 9,990 microseconds, in three 

 selectable intervals, with a resolution of 0.1, 1.0, and 10.0 micro- 

 seconds, depending on the selected interval. The accuracy of the time 

 measurement is ±0.1 microseconds. The instrument can be operated from 

 commercial power or a self-contained battery pack that provides 6 hours 

 of continuous use. A detailed description of the Model C-4899 V-Meter 

 and its operation can be found in Reference 11. 



A pair of lead zirconate titanite (PZT-4) piezolectric transducers, 

 operating at a frequency of 54 kHz, were used with the V-Meter. The 

 piezoelectric elements were mounted in rugged stainless steel housings, 

 modified for underwater operation. The coaxial cables connecting the 

 transducers to the V-Meter were about 150 feet in length. A metal 

 calibration bar was provided with the instrument to accurately set the 

 zero time reference to compensate for the effects of cable length. 



Laboratory Evaluation 



The basic purpose of the laboratory tests was to evaluate the oper- 

 ation of the ultrasonic V-Meter for underwater use. Direct transmission 

 data were collected to compare sound velocity measurements in dry concrete 

 with measurements taken underwater. Indirect transmission was examined 

 to evaluate the ability to detect cracks in concrete underwater. In 

 addition, acoustical coupling effects were examined for both modes of 

 transmission. 



Good acoustic coupling is necessary in order to make accurate and 

 repeatable sound velocity measurements. For dry concrete, the surface 

 must be reasonably smooth and a coupling agent, such as silicone grease 

 is placed between the transducer and the concrete surface to make good 

 acoustical contact and transfer maximum energy. If a coupling agent is 

 not used, the transmitted signal is severely attenuated at the interface 

 boundary between the transducer and the concrete surface due to the 

 acoustic impedance mismatch. This results in large errors for the 

 measurement of the transit time of the acoustic signal. Water is a 

 reasonably good coupling agent and provides a significant improvement 

 over air, but it was not good enough to match the dry measurements that 



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