Making ultrasonic measurements on concrete underwater was not dif- 

 ficult and required very little diver training. The only real precautions 

 to observe are: (1) make sure the measurement area is thoroughly cleaned, 

 and (2) a good acoustic coupling is obtained. The actual measurement is 

 very straight forward and easily performed by divers. 



In general, the data obtained in the field tests demonstrate the 

 importance of combining Schmidt hammer and ultrasonic testing when 

 inspecting concrete underwater. It is probable the sulfate attack has 

 not penetrated very deep into the piles; consequently the effect on 

 sound velocity through the pile would not be significant. The Schmidt 

 hammer tests, however, did detect a much softer surface condition on the 

 piles from the 1930 group compared to the 1958 piles, which was an 

 indication of the sulfate attack. The extent of the sulfate attack into 

 the piles could be confirmed by taking several core samples and 

 performing a chemical analysis. 



CONCLUSIONS AND RECOMMENDATIONS 



1. A commercially available magnetic rebar locator was successfully 

 modified for underwater use. The R-Meter can be used to determine the 

 location of rebar in concrete, measure the depth of concrete cover, and 

 determine the size of the rebar. Laboratory and field tests of the 

 instrument demonstrated that there was no effect on the output data after 

 modification for underwater use. 



2. A standard Schmidt hammer was successfully modified for under- 

 water use and can be used to rapidly survey concrete surface hardness. 

 The modification, however, introduced an offset in the rebound data of 

 23% compared to the same data obtained with an unmodified Schmidt hammer. 

 Data can be normalized for direct comparison, but the offset does limit 

 the low compressive strength measurements because the lower detection 

 threshold was changed due to the hammer modifications. 



3. Ultrasonics can be used successfully underwater to help evaluate 

 the condition of concrete structures. A commercially available instru- 

 ment was easily modified for underwater use. Laboratory and field tests 

 of the instrument demonstrated there was no effect on the output data 

 after modification. Both direct and indirect transmission methods can 



be used in the field to evaluate the uniformity of concrete and obtain a 

 general condition rating. Cracks in concrete greater than 0.030 inches 

 wide were detected in laboratory tests. 



4. NCEL recommends that a prototype concrete inspection system be 

 developed and evaluated for use by Navy UCT personnel and others to help 

 inspect concrete structures underwater. This system should be comprised 

 of an R-Meter, a Schmidt hammer, ultrasonic test equipment, and a common 

 data acquisition system. The prototype Schmidt hammer should be designed 

 to eliminate any data offset and retain the standard data range of the 

 instrument, thus providing a direct correlation with standard Schmidt 

 hammer test results. The prototype ultrasonic inspection system should 



be designed to minimize acoustic coupling effects and increase the measure- 

 ment reliability. A diver operated mechanical device should be developed 



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