The basis for the proposed introduction of calculation of residual flexural strength in the Calculation section of ASTM C1550 is that expression of post-crack performance in terms of residual flexural strength will greatly increase the utility of this test method. The performance of fiber reinforced shotcrete and concrete tested using the ASTM C1550 panel test is currently expressed in terms of energy absorption at 5, 10, 20, and 40 mm central deflection. This is useful for applications such as ground control, where energy absorption capacity has been correlated to satisfactory in service performance, but is difficult to apply in conventional structural engineering. Structural engineers prefer to assess material performance in terms of strength, and for the typical applications of FRS and FRC, residual flexural strength is a very useful performance parameter. Introduction of residual flexural strength to ASTM C1550 has been hampered to date by the lack of suitable thickness correction factors on flexural performance. The thickness of ASTM C1550 panels is difficult to accurately control during production, so limits on acceptable mean thickness are 75 -5/+15 mm. If the mean measured thickness of a panel departs from 75 mm, then the post-crack load resistance must be corrected to that which could be expected if the thickness was 75 mm. If such a correction is carried out in proportion to the thickness squared, it is found that panels thinner than 75 mm produce more favourable apparent performance than panels that are thicker than 75 mm. If such a bias were permitted within ASTM C1550, then parties required to satisfy minimum performance requirements would always produce panels close to 70 mm thick. To preclude such a bias, correction factors on the post-crack load resistance at 5, 10, 20, and 40 mm central deflection have been developed such that no bias in performance arises for thicknesses in the range 70-90 mm. The work undertaken to develop these correction factors is published in: Bernard, E.S., 2020. Estimating Residual Flexural Strength of Fiber Reinforced Concrete Using the ASTM C1550 Panel Test, Advances in Civil Engineering Materials, ASTM, Vol. 9, No. 1, 2020. doi:10.1520/ACEM20190098. Note that the current proposal does not include any change to the test method, only an additional set of calculations within the Calculation section to determine residual flexural strength based on data that is already generated using the existing experimental procedure.
Developed by Subcommittee: C09.42
Staff Manager: W Scott Orthey
Date Initiated: 10-22-2020
Technical Contact: Stefan Bernard
Ballot: C09.42 (21-02)
Status: Will Reballot Item