Deflection Measurement Considerations in Evaluating FRC Performance Using ASTM C 1018


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Title: Deflection Measurement Considerations in Evaluating FRC Performance Using ASTM C 1018

Author(s): C. D. Johnston

Publication: Special Publication

Volume: 155


Appears on pages(s): 1-22

Keywords: beams (supports); cracking (fracturing); deflection; tests; fiber reinforced concretes; fibers; polypropylene fibers; strength; toughness; Materials Research

Date: 8/1/1995

The issue of how the method of determining midspan deflection in ASTM C 1018 toughness tests influences first-crack strength, first-crack deflection, toughness indices, and residual strength factors is addressed in this paper by comparing results obtained using the method now required in the current standard, which is based on net midspan deflection determined as the nominal midspan deflection minus the average of the deflections measured at the beam supports, with corresponding same specimen results based on nominal midspan deflection only which was not explicitly excluded in earlier versions of the standard. The problem of dealing with the portion of load-deflection relationship immediately after first crack when it is unstable is discussed. The range of test specimens for which comparative data are reported includes a series of third-point-loaded 500 x 150 x 150 mm beams with three different steel fibers ranging in length from 18 mm to 63 mm; a second, smaller series of 350 x 100 x 100 mm beams allowed for assessment of the effects of beam size and fiber alignment. Fiber contents varied from 20 to 75 kg/m 3 (0.25 to 0.94 percent by volume). Also included was a series of 350 x 100 x 100 mm beams with a single type of fibrillated polypropylene fiber of length 38 to 64 mm in amounts of 0.5 to 0.75 percent by volume. The results illustrate the extent to which the ASTM C 1018 parameters I 5, I 10, I 20, R 5,10, and R 10,20 are effective in distinguishing the performance of the various fiber reinforced concretes (FRC) mixtures in terms of fiber type, geometry, and amount. The index I 5 was found to be least effective. A case is made for greater emphasis on use of residual strength factors, especially R 10,20, when employing the test to specify and control the quality of FRC.