This approach cannot be applied to nonhomogeneous materials because 

 echos are generated at the numerous boundaries of the different phases 

 within these materials, resulting in a general scattering of the pulse 

 energy in all directions. Also, the sound velocity through nonhomogeneous 

 materials is not constant and depends on the material composition, density, 

 and elastic properties. However, an average sound velocity can be measured 

 and used to evaluate material composition and uniformity. 



Measuring the average sound velocity in materials such as concrete 

 requires using separate transmit and receive transducers to avoid the 

 energy scattering problem. The sound velocity is calculated by measuring 

 the time required to transmit over a known path length. The measurement 

 of average sound velocity through concrete is recommended as a means to 

 establish the uniformity of the concrete being tested (Ref 2) . It is 

 not recommended that average sound velocity be correlated with concrete 

 compressive strength, but rather that it be used only as an indicator of 

 concrete quality. Table 5 presents some suggested sound velocity ratings 

 for concrete and for comparison includes an average sound velocity for 

 water (Ref 2) . 



The two most important factors that affect the measurement of 

 ultrasonic sound velocity through concrete are listed below and must be 

 considered when making sound velocity measurements. 



Concrete Surface Finish . The smoothness of the surface under test 

 is important for maintaining good acoustical contact between the face of 

 the transducer and the surface of the concrete. Cast surfaces are gener- 

 ally sufficient for routine testing and coupling agents such as silicone 

 grease, water, etc. will help to improve coupling. Good acoustical coup- 

 ling is necessary to obtain accurate sound velocity measurements. 



Presence of Reinforcing Steel . Sound velocity measurements taken 

 near steel reinforcing bars may be high because the sound velocity in 

 steel is 1.2 to 1.9 times the velocity in concrete. When the axis of 

 the rebar is perpendicular to the direction of propagation, the influ- 

 ence on sound velocity is generally small and if the quantity of rein- 

 forcement is small the correction factors are on the order of 1 to 4% 

 depending on the quality of the concrete. If the axes of the reinforcing 

 bars are parallel to the direction of propagation, reliable corrections 

 are difficult to make and it is recommended that sound paths be chosen 

 that avoids the influence of reinforcing steel. The derivation of cor- 

 rection factors to compensate for the effects of reinforcement on sound 

 velocity measurements in concrete are covered in Reference 2 for both 

 the perpendicular and parallel cases. 



Three approaches for measuring sound velocity in concrete are illus- 

 trated in Figure 14. The most common method is direct transmission where 

 the transducers are positioned on opposite sides of the test specimen 

 and the longitudinal waves propagate directly toward the receiver. For 

 indirect transmission, both transducers are placed on the same side of 

 the concrete and energy scattered by discontinuities within the concrete 

 is detected by the receive transducer. The strength of the pulse detec- 

 ted in this case is generally less than 5% of the strength detected for 

 the same path length when direct transmission is used. Semi-direct 

 transmission is not normally used because it is difficult to maintain a 

 consistent or known path length. 



11 



