7. Conclusions and remarks
The benefits of R/ECC, regards to resisting shear, include improved shear resistance and cracking control. Similar to uni-axial tension, multiple cracking of ECC occurs in shear. The first cracking strength was slightly higher for the R/C used in this study; however, the initial crack opening of the R/C was 10 times larger than that in the R/ECC. Crack deformations for the R/ECC were between 3 and 5 times smaller than for R/C at similar load levels. The addition of stirrups in R/C controls cracks widths as more stirrups resulted in reduced initial crack opening and total crack deformations (opening and sliding). Initial crack opening was independent of stirrup spacing in the R/ECC beams. The contributions of ECC on shear behavior of R/ ECC include: Fiber bridging of shear crack, thus increasing the shear capacity; Traditional shear reinforcement is activated at smaller individual crack deformations; Crack deformations are limited by fiber bridging mechanism and by activating traditional shear reinforcement at smaller crack deformations. Based on the shear stress-strain responses and DIC measurements of the specimen deformations, a phenomenological descriptions of the shear crack opening, crack sliding and subsequent failure of R/C and R/ECC are proposed. For the R/C the shear loads over the shear crack can be transferred only by stirrups, aggregate interlock and fiber dowel effect of longitudinal reinforcement. The crack development mechanism for R/ECC is more complex due to the fiber bridging mechanisms, which induces multiple cracking resulting in smaller crack openings at a given shear stress as well as higher peak shear stress. The experimental program demonstrated that the use of R/ECC provides improved shear resistance, better control of crack sizes, and a more ductile shear failure than R/C. However, shear stress transfer over the crack via aggregate interlock is significantly reduced in ECC type materials due to the small particle size in the matrix. Additional increases in shear failure ductility of ECC would be possible by using fibers that can resist higher shear deformations and engineering the composite to fail by fiber pullout rather than rupture.
