centered on the anvil, cleaning the anvil surface, maintaining a constant 

 hammer position, etc., affect the measurement and the operator must use 

 a consistent technique to increase the repeatability of the readings. 

 It is necessary for each operator to practice with the hammer to develop 

 a consistent technique. 



Measurements were taken underwater with the modified Schmidt hammer 

 on the side of each test block in the same general area where the dry 

 measurements were made. These data are tabulated in Table 4 along with 

 the rebound numbers measured during the dry tests. Figure 13 is a plot 

 of the dry versus wet data obtained with the modified Schmidt hammer. 

 The rebound numbers obtained underwater tend to be higher than the 

 comparable dry data, although they are still within the expected error 

 band of ±20%. The exception was the test results from block No. 4, which 

 were considerably lower. It was determined that the rebound numbers 

 from block No. 4, obtained underwater, were not taken in the same area 

 as the dry measurements. This accounts for the shift in the data since 

 block No. 4 was made with very low strength ready-mix concrete and the 

 uniformity varied significantly. 



In summary, a Schmidt hammer was modified for underwater use and 

 its use demonstrated in laboratory tests. The modification introduced 

 an offset in the rebound data of 23% that limits the low compressive 

 strength measurements compared to the standard hammer. After normaliza- 

 tion, there were no significant differences between rebound numbers 

 obtained with the modified hammer compared to the standard Schmidt 

 hammer. The instrument can be used to rapidly survey concrete surface 

 conditions to look for nonuniformity, provided the surface is adequately 

 cleaned. A major redesign of the hammer will be required to remove the 

 effect of lower rebound numbers that resulted from the initial modifi- 

 cation. 



ULTRASONIC TESTING 



The transit time of high frequency sound waves through concrete can 

 be used to assess its condition. Ultrasonic testing procedures for 

 concrete have been standardized by ASTM Standard C-597 (Ref 10) and test 

 equipment is available from commercial sources. Ultrasonic sound velocity 

 tests were carried out on both laboratory test specimens and completed 

 concrete structures. A detailed description of ultrasonic testing of 

 concrete is presented in Reference 2 for terrestial applications. 



Background 



Ultrasonic testing of nonhomogeneous materials, such as concrete 

 and timber, is significantly different than ultrasonic testing of 

 homogeneous materials (metals). For example, when metals are tested 

 ultrasonically, one objective is to detect internal flaws that send 

 echoes back in the direction of the incident beam. These echoes are 

 detected by a transducer that acts as both the transmitter and receiver. 

 The position of the flaw can be determined from the measurement of the 

 time taken for the pulse to travel from the surface to the flaw and back. 

 This assumes a uniform sound velocity through the material being tested 

 which is the case for metals. The thickness of metals is also measured 

 in the same manner. 



10 



