Comparative evaluation of the mechanical behavior of synthetic and steel fiber-reinforced concrete

Synthetic macrofibers are commonly used in Brazilian market. However, the characterization of the mechanical behavior of reinforced concretes with these macrofibers still hast to be better evaluated. Among the many methods used to this purpose the tests performed under closed-loop displacement control provide more reliable results. The main advantage of that system is in a more accurate evaluation of these composites at low levels of crack opening. In this study, the performance of these composites was examined according to a recent standard (ASTM C1609 - 2010) and a more traditional one, which is reference in Brazil (JSCE-SF4 - 1984). Both procedures prescribe flexural tests in prismatic specimens for the determination of post-crack residual strength and toughness. Two fibers were analyzed: a polypropylene macrofiber, used in the dosages of 0.22, 0.33, 0.50, 0.66, 0.82 and 1.0 in volume percentage, and a steel fiber, in the dosages of 0.19, 0.32 and 0.45 in volume percentage. A matrix of average compressive strength of 35MPa was used. It was observed that the synthetic macrofiber can provide the same level of residual strength as the steel fiber, as long as the equivalence of the dosages of each fiber is taken into account. Besides that, great efficiency and accuracy were obtained using the net displacement of the specimen as the controlled variable, due to the reduction of post-peak instability. Therefore, the determination of the residual strength of the composite in the initial levels of displacement and cracking of the matrix was better achieved.