ACI 544.4R-18 Guide to Design with Fiber-Reinforced Concrete.
3.3—Standard test methods for FRC If crack control under plastic shrinkage is the intended function of the fbers, ASTM C1579 can be used. The efec- tiveness of fbers in controlling the drying shrinkage (under restraint) and reducing the crack widths can be determined following ASTM C1581/C1581M. In these two tests, the efectiveness of fbers in reducing the crack width is deter- mined and expressed in a percentage versus control (plain concrete). If higher levels of crack control and post-crack fexural capacity are expected from fbers, FRC beams or panels should be tested using ASTM C1609/C1609M and ASTM C1550. Equivalent European test methods are BS EN 14651:2005 and BS EN 14488:2006, respectively. Performing a direct tension test (static or fracture) is ideal and desirable for FRC; however, a proper tension test is extremely difcult for cement-based materials because of the potential slippage or crushing of concrete at the grips, or heterogeneous nature of FRC. As an accepted alternative, fexural tests are conducted and the results are used for back- calculating the tensile properties. These fexural tests are designed to obtain the complete pre- and post-crack response of FRC. All the existing design tools for FRC use the test parameters obtained from some type of a bending test. This document describes two test methods that are widely used for measuring the residual strength, the parameter that is implemented in FRC design: ASTM C1609/C1609M and BS EN 14651:2005.
3.3.1 ASTM C1609/C1609M: Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete (Using Beam with Third-Point Loading)—This test measures the complete pre- and post-crack fexural response of FRC beams using accurate defection to control the test (closed- loop control). The test is typically performed on 6 x 6 x 20 in. (150 x 150 x 500 mm) FRC beams with a span of 18 in. (450 mm). Two points of loading (four-point bending) are used until reaching a midpoint defection of 1/150th of the span (L)—that is, 0.12 in. (3 mm). Figure 3.3.1 shows the schematics of a typical ASTM C1609/C1609M test and a beam subjected to fexure (bending). The following param- eters are determined from the test and used in character- izing FRC, which includes the peak strength as well as the residual strength values at L/600 (0.03 in. [0.75 mm]) and L/150 (0.12 in. [3 mm]). Note that b is the width and h is the height of the beam. At least three replicate beams should be tested for determining these parameters, though testing six beams is recommended for achieving a representative average value for residual strength of FRC. a) P P : peak fexural load (maximum load), lbf (kN) b) P D 600 : FRC fexural residual load at a defection of L/600, lbf (kN) c) P D 150 : FRC fexural residual load at a defection of L/150, lbf (kN) d) f P : peak fexural strength, psi (MPa)
3.4—Strain softening and strain hardening Low to moderate dosages of fbers provide enough resis- tance for bridging one main crack in a tension or fexural test and the response is referred to as strain softening. During strain softening, the residual strength gradually declines as the beam defection and crack width increase. Strain soft- ening FRC has a post-cracking tensile stress that is lower than its ultimate tensile strength. ACI 544.4R pdf download.