shown in Figure 4 (Ref 8). Theoretically, if the separation of the axes 

 of two parallel rebars is at least three times the thickness of the con- 

 crete cover, this effect can be neglected. In routine measurements, if 

 the meter needle drops to a value of one or less on the linear scale 

 when the probe is between the two bars, the effect can be neglected. 



The presence of rebar perpendicular to the axis of the probe has 

 much less effect on the measurement of concrete cover than that of 

 parallel rebar and in most instances it can be ignored. For example, if 

 the probe is not positioned directly above the perpendicular rebar or 

 the perpendicular rebar is located beneath the rebar under test, the 

 effect is negligible. The operations manual provides guidance to reduce 

 these limitations and improve the measurement accuracy. 



Modifications for Underwater Use 



The first attempt at modifying the R-Meter for underwater use con- 

 sisted of placing the entire instrument in a waterproof housing, 12 inches 

 in diameter and 6 inches deep. The probe was located directly below the 

 readout inside the waterproof housing. The readout was visible through 

 a clear acrylic top. In order to operate the modified R-Meter underwater, 

 it was necessary for the diver to use both hands to position the instru- 

 ment. Once the R-Meter was positioned the diver had to manually log the 

 data. This sequence of events was difficult to accomplish and inefficient. 

 Consequently, this approach was dropped in favor of waterproofing only 

 the test probe. The electronics were kept topside, where the data were 

 automatically recorded. 



To evaluate the R-Meter underwater using the second approach, it 

 was necessary to waterproof the test probe, provide a remote indicator 

 to orient the diver while using the instrument, and increase the length 

 of the interconnecting cable between the test probe and the readout 

 that was kept topside. The modified R-Meter is shown in Figure 5, 

 including a closeup of the test probe. 



To waterproof the test probe a thin layer of epoxy was deposited 

 over the exposed metal tips. The remote indicator was a small voltmeter 

 that duplicated the meter movement from the deck unit. The voltmeter 

 was mounted in a PVC housing and attached to the test probe. The diver 

 used this indicator to locate rebar and orient the probe when measuring 

 the depth of concrete cover. The actual measurement of concrete cover 

 was made topside; the diver did not log any readings. The housing also 

 contained a pressure gauge to measure the water depth and a waterproof 

 connector. An underwater electrical cable (180 feet long) connected the 

 probe to the deck unit. 



Laboratory Test Results 



The modified R-Meter was tested in the laboratory to determine if 

 the modifications had any effect on the output and to check its perfor- 

 mance underwater. To evaluate the performance of the modified R-Meter, 

 measurements were taken on four concrete test specimens, each containing 

 a different size of rebar. The size of each test block was 6x6x18 inches 

 and the rebar was located slightly off-center. Measurements were taken 

 before the modifications were done, after they were completed, and with 



