Effect of deflection increment size on apparent performance of fiber reinforced concrete beams and panels

Post-crack performance testing of Fiber Reinforced Concrete (FRC) is commonly undertaken using beam tests, such as ASTM C1609/C1609M, or panel tests, such as ASTM C1550. Both these tests demand the use of sophisticated servo-controlled test machines with data acquisition systems that record several hundreds to thousands of points in a typical test. Existing standard test methods include detailed specifications regarding how load should be applied and how deformation is measured, but they retain simple prescriptions regarding data acquisition and the total number of points required to adequately characterize the performance of a specimen. The present investigation has been undertaken to assess the effect of changes in the size of the deflection increment between recorded points in a load-deflection curve on the apparent outcome of a test. The results indicate that the maximum increment of deflection between successively acquired data points should be 0.020 mm for a ASTM C1550 panel test and 0.0035 mm for a ASTM C1609/C1609M beam test in order to limit potential errors in energy absorption and peak load to within acceptable levels. This is smaller than the current recommendation of 0.05 mm for ASTM C1550 and thus a change to a smaller measurement interval is recommended. For ASTM C1609/C1609M the currently recommendation for rate of data acquisition over the first span/900 deflection is adequate but thereafter the rate of acquisition should be increased by 50%.