the modified probe submerged in water. Figure 6 shows the R-Meter and 

 one of the test blocks. Table 2 presents the test data in terms of 

 measured concrete cover before the modifications were made and after 

 modification with the probe submerged in water. Figure 7 is a plot of 

 data from the same tests with readings from the linearized scale used 

 for comparison. 



The test results showed no significant change in output data after 

 the probe was modified to operate underwater. The maximum deviation was 

 +1/8 inch and indicated increased concrete cover. Since the epoxy coat- 

 ing deposited over the probe tips did raise the probe above the surface 

 of the concrete about 0.05 inches, it was expected that the instrument 

 should indicate increased concrete cover. However, interpretation of 

 the readout to better than 1/16 of an inch is not practical, especially 

 for thicker coverage (>4 inches) where 1/8 of an inch is probably more 

 realistic. 



A qualitative test was performed in the seawater test tank at NCEL 

 to evaluate the performance of divers using the R-Meter. A concrete 

 slab (4x4x0.67 feet) containing different lengths of No. 5 rebar was 

 used as a test specimen. NCEL divers surveyed both sides of the slab 

 with the R-Meter probe and marked the location of the rebar. 



The results of the evaluation showed that the modified R-Meter was 

 very easy for the diver to use. Rebar with less than 4 inches of 

 concrete cover was easily detected and accurately located. Rebar with 

 concrete cover between 4 and 6 inches was more difficult to locate and 

 rebar with concrete cover greater than 6 inches was very difficult to 

 locate because of the small deflection on the remote readout. Also, 

 parallel rebars were not distinguishable from one another when the con- 

 crete cover was 4 inches and the rebar spacing was 6 inches. Measure- 

 ments from the opposite side of the slab did detect the parallel rebars 

 where the concrete cover was only 2 inches. 



In summary, the R-Meter can be used successfully underwater to 

 inspect concrete structures and perform the following functions, within 

 the basic limitations of the instrument: 



1. Determine the location of rebar in concrete structures, both 

 orientation (±10 degrees) and position (±1/2 inch). 



2. Measure the depth of concrete cover over rebar for the range of 

 1/4 to 8 inches thick with an accuracy of about ±10%. 



3. Determine the size of standard rebar (No. 3 to No. 16) with an 

 accuracy of ±10% which is roughly equivalent to one standard 

 rebar size. 



The operation of the modified R-Meter in the laboratory did not reveal 

 any problems with the fundamental operation of the instrument and there 

 was no effect on the output data after the modification. 



