Multiple cracking and strain hardening in fiber-reinforced concrete under uniaxial tension

Fiber-reinforced concrete (FRC) showing strain hardening after cracking is commonly defined as High Performance Fiber-Reinforced Cementitious Composite (HPFRCC). In the post-cracking stage, several cracks develop before complete failure, which occurs when tensile strains localize in one of the formed cracks. As is well known, multiple cracking and strain hardening can be achieved in cement-based specimens subjected to uniaxial tension by increasing the volume fraction of steel fibers with hooked ends, or by using plastic fibers with and without steel fibers, or by means of high bond steel fibers (e.g., twisted fibers or cords). To better understand why, in such situations, high mechanical performances are obtained, an analytical model is herein proposed. It is based on a cohesive interface analysis, which has been largely adopted to investigate the mechanical response of FRC or the snubbing effects produced by inclined fibers, but not the condition of multiple cracking and strain hardening of HPFRCC. Through this approach, all the phenomena that affect the post-cracking response of FRC are evidenced, such as the nonlinear fracture mechanics of the matrix, the bond–slip behaviour between fibers and matrix, and the elastic response of both materials. The model, capable of predicting the average distance between cracks as measured in some experimental campaigns, leads to a new design criterion for HPFRCC and can eventually be used to enhance the performances of cement-based composites.