±0.002 in. (0.05 mm). The potentiometers were mounted on arms that 

 extended from a center shaft. The center shaft was motor-driven at a 

 rate of one revolution per 90 seconds . 



Radial displacement calibration was accompHshed by mounting 

 aluminum shims of 0.125-inch (3.18-mm) thickness on, the inside wall of 

 the cylinder so that the steel ball passed over the shims to record 

 magnitude and direction of inward displacement. These calibration 

 marks also determined a 360-degree rotation. 



The deflectometer system was insensitive to the axial orientation or 

 lack of straightness of the center shaft. The top and bottom on the 

 center shaft were fixed in location, and the shaft was rotated. The 

 arms were fixed to the center shaft; and, in plan view, the end of each 

 arm scribed a perfect circle. The steel ball at the end of each arm 

 moved in and out to conform to the shape of the concrete cylinder. 

 This radial movement was recorded as changes from a perfect circle. 



In reducing the analog deflectometer data, an analog- to- digital 

 converter was used along with a timing system to control the number of 

 samples taken and the time interval between samples . Over 900 samples 

 of analog data were digitized for each 360-degree rotation. This 

 equates to a radial displacement data point every 0.17 inch (4.3 mm) 

 around the inside circumference of the cylinder. 



Strain Gages . Strain gaging of the specimens proved to be diffi- 

 cult because the concrete was in a wet condition . Various approaches 

 for applying gages to wet concrete were tried, but none were success- 

 ful. The problem was in maintaining the bond throughout the entire 

 test. 



The procedure for strain gaging is described as follows : 



(1) Electrical-resistant strain gages (type FA-06-125, 

 three-element rosettes) were mounted either on brass 

 shim stock 0.002-inch (0.05 mm) thick or on steel shim 



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