10,500 ft/sec, which indicates low strength concrete. Indirect measure- 

 ments, however, did not indicate the simulated cracks in any of the test 

 specimens during either the dry or wet tests. The calculated equivalent 

 path length, based on the average sound velocity in the test specimen 

 and the measured indirect time of flight, indicated the sound waves 

 reflected off the back surface of the test block. The spacing between 

 the transmit and receive transducers was maintained between 4 and 6 inches 

 for these measurements. The test blocks should have been much larger in 

 size to eliminate the effects of the reflected wave in order to draw 

 conclusions from this series of tests. 



Indirect measurements were also taken on a prestressed octagonal 

 concrete pile that had cracks around its circumference in five different 

 locations along its length, as shown in Figure 19. All of the cracks 

 were clearly visible and appeared to go completely through the pile. 

 One crack near the end of the pile was much wider than the others. This 

 crack was measured to be around 0.025 to 0.030 inch wide at the surface 

 on the face were the measurements were made. The other cracks were 

 estimated to range from 0.001 to 0.010 inch wide on the same surface. 

 The actual width of a crack is very difficult to quantify because of its 

 highly irregular three dimensional shape, and these are very approximate 

 values. 



Indirect transit times were measured as a function of position 

 along the prestressed pile and they are plotted in Figure 19 for a 6- 

 and 8-inch path length. The positions of the transmit and receive 

 transducers are indicated for each measurement, in addition to the 

 location of the cracks. These data were taken with the pile dry and 

 except for the large crack located at position number one, there was no 

 apparent change in measured transit time for either path length due to 

 the cracks. For the measurements taken across crack number one, the 

 transit time for the 8-inch path length increased by 36% and for the 

 6-inch path length, the transit time increased by 68%. The increase in 

 transit time for the sound pulse, due to the increased path length 

 around the crack, can be used as a good indicator of the presence of 

 large cracks in concrete but should not be used to estimate the depth of 

 the crack. The transit time measurements did not change when the cracks 

 were filled with water. 



In summary, ultrasonics can be used to categorize concrete by 

 measuring the sound velocity in the material using direct transmission. 

 Good acoustic coupling will enable accurate time measurements to be made 

 for calculating sound velocity. Direct and indirect transmission can 

 also be used to compare the general condition of the concrete from one 

 location to another on the same structure, assuming the concrete mix is 

 the same. Indirect transmission normally should not be used to obtain 

 sound velocity measurements for categorizing or rating concrete because 

 of the poorly defined sound path length. Indirect transmission appears 

 to detect deep cracks (on the order of 0.030 inch wide) in concrete, but 

 it did not detect other cracks that were much narrower but still clearly 

 visible. 



14 



