ACI 544.8R-16 Report on Indirect Method to Obtain Stress- Strain Response of Fiber- Reinforced Concrete (FRC).
It is noted that many of the characteristics of strain- softening and strain-hardening responses in tension testing are also observed when the specimen is tested in fexure. The responses in these conditions are referred to as defec- tion softening and defection hardening; however, there is no one-to-one correlation between the two modes. Defec- tion hardening occurs for samples that have a suffciently high level of fbers to exhibit strain-softening behavior in tension, but hardening behavior under fexural testing. The post-crack response in fexure exhibits ascending load with increasing defection after the frst crack point. For example, if the volume of the fbers is increased in excess of a level designated as critical fber content, then the entire stress- strain response is also affected because the manner of load and strain distribution change in the sample. This is because, as the plain matrix cracks, there are suffcient fbers to carry the load that is being released by the matrix. Because the fexural design of FRC elements is normally governed by the tensile stress-strain response, the post-cracking behavior has a direct effect on the results (Soranakom and Mobasher 2009b). Proper characteriza- tion of the load-deformation response and computation of valid material behavior data from these tests is essential to successful design applications (Banthia and Trottier 1995a,b; Mobasher 2011; Cunha 2010). Many structural systems with strain-softening materials, such as structural foors or indeterminate structures, can exhibit defection-hardening behavior and an increase in strength values in proportion to the residual strength.
3.4—Equivalent tensile stress-strain responses As an alternative to direct measurement of the stress-strain by uniaxial tension test, a back-calculation approach can be applied to fexural tests. This is attractive, as fexural tests are routinely conducted as quality control tests and, therefore, the data collected can be used to verify the material properties and further used in the design of the next sections. Back-calcu- lation of equivalent tension results from fexural tests, also defned as inverse analysis, is a useful tool because it provides equivalent tension response needed in member design. Approaches have been developed by RILEM TC 162-TDF (2003) and DIN EN 14561:2006-08. The equivalent tensile response can then be used in the design of fexural members for applications such as slabs, panels, and other applications where fber contribution is introduced using equivalent soft- ening parameters. Different approaches do not always yield unique results, as assumptions regarding the shape of the stress-strain response inherently affect the back-calculated parameters and, therefore, several test methods for calcu- lation of the equivalent tensile properties are summarized. An overview of the existing inverse methods for obtaining uniaxial tensile stress-strain curves from beam test data follows in 3.5.ACI 544.8R pdf download.