ACI 544.8R

Abstract

Scope: This report presents existing methods for estimating characteristic tensile stress-strain or tensile stress crack width response of strain-softening fiber-reinforced concrete (FRC) using flexural beam test data. Methods are proposed for strain-softening FRCs that do not exhibit distributed or parallel microcracking when tested in flexural loading conditions, and strain-softening FRCs that do exhibit distributed or parallel microcracking when tested in flexural loading conditions. A set of definitions for an equivalent stress-strain diagram (Naaman and Reinhardt 2006; Noghabai 1998) are presented first and followed by calculation procedures for obtaining flexural tensile and residual flexural strengths from beam test data. Specific coefficient values for notched beams (RILEM TC 162-TDF 2003) and third-point beams (NBN B 15 238:1992) are validated. Because the coefficient values are not directly reported for beam types tested per ASTM C1609/C1609M and ASTM C1399/C1399M, this report proposes an approach to do so and compares the results with other methods. The relationship is presented in terms of parameter-based stress coefficients that are determined using a step-by-step inverse analysis procedure in Appendix A. The report concludes with the relationship between the parameters that define the stress-strain diagram and the experimental flexural residual strengths. The proposed approach has several drawbacks, as it is for a one-dimensional material behavior model, and yields an effective stress-strain response based on the model assumptions of trilinear tension and bilinear compression. Furthermore, because the results of back-calculation are size- and geometry-dependent, the procedure is presented for test data obtained from ASTM C1609/C1609M specimens. Other geometries and specimen dimensions may need to be correlated with standard-sized specimens. The method can be applied to any size specimen and the results can be used for comparative basis. Different beam sizes may yield different stress-strain values because of the size effect. The subject of the size effect is not addressed in this document because the majority of the flexural test specimens are in the narrow range of 4 x 4 to 6 x 6 in. (100 x 100 to 150 x 150 mm) size. Comprehensive sets of data with different fiber types and specimen sizes have been analyzed using back-calculation procedures, and a comparative evaluation of the results are discussed in recent publications (Bakhshi et al. 2014; Mobasher et al. 2014).