Test Results 



R-Meter . The modified R-Meter was used to measure the depth of 

 concrete cover over the rebar in five different piles from the ' 1930 

 construction group. The data obtained with the R-Meter are presented in 

 Table 7 and a cross section view of the 1930 piles is shown in Figure 20. 

 The rebar' s configuration in the pile varied depending upon its location 

 in the pile as shown in Figure 20. The amount of concrete cover was 

 measured by positioning the probe of the instrument directly over the 

 No. 6 rebar for measurements near the bottom of the pile and over the 

 No. 7 rebar for the measurements taken near the top of the pile. A 

 water depth reading was obtained at each measurement location. 



A very good peak reading was obtained on the R-Meter when the probe 

 was directly over the No. 6 rebar on the lower portion of the pile. The 

 measured depth of concrete cover over the No. 6 rebar averaged 1.89 inches 

 thick. This number is in error by a small amount due to the effect of 

 adjacent parallel rebar. The spacing between the rebars would have to 

 be about 7.5 inches to eliminate this effect instead of the 5.5 inches 

 indicated in Figure 20. The amount of error is difficult to quantify 

 and the actual depth of concrete cover is thicker than the indicated 

 depth. The actual depth was probably between 2 and 2.5 inches deep. 



When the probe was used near the top of the pile, a narrow peak 

 reading could not be obtained and it was impossible to differentiate 

 between the two adjacent No. 7 rebars. The effect of the parallel rebar 

 was very apparent in these measurements and it strongly influenced the 

 readings. The average depth of the concrete cover measured near the top 

 of the piles was 4.64 inches. The actual depth of cover over the rebar 

 was greater than the measured amount. The data indicate a large differ- 

 ence between the construction plans as shown in Figure 20 and what was 

 actually built. 



When taking measurements of concrete cover over the rebar in con- 

 crete piles, the data usually will be influenced by the effect of 

 closely spaced rebar. In some cases, it will not be possible to obtain 

 narrow peak readings that indicate the actual location of the rebar due 

 to the narrow spacing of the rebar with respect to the depth of concrete 

 cover. The actual depth of concrete cover, however, will always be 

 greater than the measured amount. Reducing the effects of closely 

 spaced parallel rebar would require a major redesign of the instrument 

 to alter the shape of the generated magnetic field. 



The field tests demonstrated that divers were able to use the 

 instrument with very little training. The field tests also demonstrated 

 that the instrument would be more effective if the diver collected the 

 data after orienting the probe rather than depending upon a verbal com- 

 munication link to the surface operator. A reel to handle the instru- 

 mentation cables would also be benefical. 



Schmidt Hammer . The modified Schmidt hammer was used to measure 

 the concrete surface hardness of selected piles from the 1930 and 1958 

 construction groups to evaluate its performance in the field. The data 

 obtained with the modified Schmidt hammer are presented in Table 8 for 

 the two different pile groups and include the anvil calibration data 

 before and after the measurements. 



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