Once a region on the pile was sufficiently cleaned, data could be 

 taken with the Schmidt hammer as rapidly as the diver could operate the 

 device. Operating the hammer was very simple for the diver; Vie only 

 had to press the plunger of the hammer firmly against the pile until an 

 impact was felt or heard. The diver then moved the hammer back away 

 from the surface to automatically recock it, then the hammer was simply 

 pressed against the surface again to take another measurement. This 

 sequence was continually repeated and required less than 30 seconds to 

 get 12 readings in any one area of the pile. 



The data were taken in regions near the top and bottom of each pile. 

 These areas were previously cleaned and smoothed with a small carborundum 

 stone by the diver. Twelve readings were taken at each location, then 

 the high and low values were dropped before averaging. Twelve readings 

 were not obtained in two locations for the 1930 piles numbered 93E and 

 93H because the surface was very soft and some of the readings did not 

 register, since they were outside the range of the instrument. This 

 soft surface condition is indicated by the limited data that were 

 obtained. Also, only six readings were collected on pile 88H from the 

 1958 group because some fouling remained on the pile, creating a rough 

 surface. The limited data from pile 88H did indicate a surface condi- 

 tion comparable to the other 1958 piles. 



The average uncorrected rebound number of the data collected from 

 the 1958 pile group was 37.9, which is 23% higher than the average uncor- 

 rected rebound number of the 1930 pile group. This indicates a much 

 softer surface condition on the piles from the 1930 group and would nor- 

 mally indicate a lower strength concrete. This finding of a soft surface 

 condition supports the conclusions . from the previous inspection and 

 indicates that the Schmidt hammer can be used to survey the surface con- 

 dition of concrete underwater. 



In summary, the field test demonstrated that divers can easily use 

 the Schmidt hammer underwater to obtain valid data on concrete surface 

 hardness. The surface must be properly cleaned, however, to obtain 

 consistent data. 



V-Meter . The V-Meter was used to collect ultrasonic data on 

 selected piles from the 1930 and 1958 construction groups, using both 

 direct and indirect transmission methods. For direct transmission, a 

 pair of calipers were used by the diver to measure the transmission path 

 length. The path length for indirect transmission was fixed at 10 inches 

 between the centers of the transmit and receive transducer faces. Sili- 

 cone grease was used on the face of each transducer to improve acoustic 

 coupling. No fixtures were built to hold the transducers for direct 

 measurements, the diver merely pressed them firmly against the concrete 

 surface while the measurement was made. A small guide block was used to 

 maintain the 10-inch spacing for the indirect measurements. 



The received acoustic signal was displayed on an oscilloscope and 

 the pulse transit time was measured from the oscilloscope display to 

 reduce detection threshold errors. If the received signal was a poor 

 quality waveform, which was generally the case for the indirect measure- 

 ments, several signals would be collected and averaged to obtain an 

 improved signal-to-noise ratio. The received signal was also digitized 

 and stored on magnetic tape for later analysis as required. 



17 



