Because of these limitations, which are discussed more fully in 

 Reference 2, the estimation of concrete compressive strength obtained 

 with a rebound hammer is accurate only to about ±25%. This applies to 

 concrete specimens cast, cured, and tested under identical conditions as 

 those from which the calibration curves were established. Because of 

 the lack of accurate calibration data correlating compressive strength 

 with rebound numbers, the Schmidt hammer is primarily useful for 

 checking surface hardness and uniformity of concrete. It can also be 

 used to compare one concrete against another if they are assumed to be 

 reasonably similar. 



Modifications for Underwater Use 



To use the Schmidt hammer underwater, it was necessary to place the 

 hammer in a waterproof housing with an O-ring seal on the impact plunger 

 shaft. This required extending the impact plunger approximately 

 4 inches. In order to eliminate the diver recording data manually, an 

 electrical pickup was added to sense the position of the rebound rider. 

 This allowed the diver to take measurements as rapidly as possible. A 

 150-foot-long cable was used to connect the electrical pickup to the 

 data acquisition system on the surface. Figure 11 shows the Schmidt 

 hammer modified for underwater use. 



Laboratory Test Results 



Laboratory tests were performed on the Schmidt hammer to evaluate 

 its basic performance. The modified Schmidt hammer was tested to 

 determine if the modifications had any effect on the output and to 

 evaluate its basic performance underwater. Test results obtained with 

 the modified hammer were compared against the test data obtained with a 

 standard Schmidt hammer. 



The basic Schmidt hammer calibration was checked using a test anvil 

 provided by the manufacturer. The anvil is made of hardened steel and 

 forms a surface upon which a reference reading can be obtained to check 

 the calibration of the rebound hammer. Internal adjustments can be made 

 in the Schmidt hammer to make small variations in the output to match 

 the anvil reference reading. The range of this adjustment is about 

 ±4 points. 



Before modifying the Schmidt hammer, tests were conducted in a dry 

 enviroment using the test anvil to evaluate the performance of the 

 hammer and establish the repeatibility of the measurement. The rebound 

 numbers for three different unmodified Schmidt hammers averaged 60 

 with a standard deviation less than 2.0. This agreed exactly with the 

 reference rebound number on the test anvil. 



After the Schmidt hammer was modified for underwater use, the average 

 rebound number, obtained using the test anvil in a dry enviroment, dropped 

 to 46.5, although the standard deviation remained about the same. This 

 represented a decrease in the standard reference rebound number for the 

 modified hammer of 23%. It was determined that the lower rebound numbers 

 resulted from energy losses associated with the extension of the impact 

 plunger. This reduces the useful operating range of the modified hammer 

 compared to the standard Schmidt hammer. Therefore, low compressive 



