Influence of Steel Fibers and Casting Direction on the Bond Between Concrete and Reinforcement Bars

This research aimed to analyze the stress transfer mechanism in specimens under direct tensile stress, through the cracking process on two types of self-compacting concrete mixtures, without fibers and using steel fibers (hybrid reinforcement), which were cast horizontally and vertically. To achieve the purpose, prismatic specimens were produced and their mechanical properties evaluated through of the stiffness, load and displacement of the cracking process characterization. The specimens used dimensions of 150 x 150 x 750 mm and were cast in the vertical and horizontal position with 20 mm steel bars longitudinally, centralized in the cross section. The composition of self-compacting concrete using fibers has a volumetric fraction of 1.5%. The results of experimental tests indicated different crack patterns and fracture mechanism. To mixture without fibers, the vertical direction casting group shown a cracking process synchronously on each specimen faces, demonstrating homogeneity of the mixture and less presence of voids in steel-concrete interface. Moreover, both groups without fibers, vertical and horizontal, performed a brief cracking level, quickly reaching the steel bar stiffness. On the other hand, the steel fibers presence stiffened the prismatic rods, which resulted in gradual levels of stress transfers between steel and concrete. The first crack opening in specimens of horizontal direction casting group showed increase in load and stiffening modulus, when compared to vertical group, and a steady stiffening modulus after. The vertical group performed a gradual decrease stiffening modulus in post-first crack stage. The specimens face of vertical group in the post-cracking stage showed regular and simultaneous spacing between the cracks, while the horizontal group performed an asymmetric crack pattern. Thus, the direction of horizontal casting influenced the inside distribution of fibers, resulting in a favorable mechanical orientation caused during the casting of the fresh mixture.