strength measurements will be limited by the effect of lower average 

 rebound numbers. However, the lower rebound numbers can be normalized 

 for direct comparison with standard Schmidt hammer data using the following 

 relationship: 



2l<r x Anvil No. _ 2-iV x 60 , . ^ 



n x R n x 46 UJ 



a 



where: R = Corrected rebound number 



r = Measured rebound numbers 



n = Number of measurements 



R = Rebound number obtained on anvil 

 a 



This relationship was used to make comparisons between data obtained 

 with the modified hammer and the standard Schmidt hammer during 

 laboratory tests. 



Laboratory measurements were taken on six different concrete blocks 

 (10x12x24 inches) using the modified Schmidt hammer and two standard 

 Schmidt hammers. The top of each block had a rough wood trowel finish 

 and the remaining sides were smooth, cast surfaces. Each block was a 

 single mix of concrete, except block six, which contained three different 

 concrete mixes and was divided into areas 1, 2, and 3. (The concrete 

 floor in the laboratory was also used as a test block.) 



Dry measurements were taken with the three hammers on the top and 

 sides of each block in the same general area. The average rebound numbers 

 obtained from each block are presented in Table 3. The modified hammer 

 data were normalized, using Equation 1, for direct comparison with the 

 data obtained using the standard Schmidt hammers. Data from Table 4 

 obtained with the standard hammer (No. 1-8140) are compared against data 

 obtained with the standard hammer (No. 2-8155) and the modified hammer 

 (No. 8148) in Figure 12. The mean differences appear to be randomly 

 distributed and are generally within the expected limits of ±20% for the 

 Schmidt hammer. 



Data obtained from the tops of the concrete blocks differed from 

 the side measurements by as much as 44%. The rebound numbers were always 

 much lower on the rough top surface than on the smooth cast-in-place 

 sides. On blocks 3, 4, and 5, no readings were obtained from the top 

 surface with the modified hammer because they were outside its operating 

 range (too low) . These data illustrate the effect of surface roughness 

 on Schmidt hammer data. In actual field use, each measurement site must 

 be thoroughly cleaned and smoothed in order to compare the results from 

 one location with rebound numbers obtained at another point on the struc- 

 ture. 



When the modified Schmidt hammer was initially tested submerged, 

 the average rebound number obtained with the test anvil dropped to 45.2 

 and the standard deviation increased to 4.6. After practicing with the 

 hammer underwater, the standard deviation of the readings dropped below 

 2.0 and the average rebound number increased to 46.1. This test demon- 

 strated that the standard deviation of the readings could vary substan- 

 tially even on the test anvil. Minor things such as keeping the hammer 



