Experimental investigation on mechanical properties of normal strength concrete containing crimped-end hooked steel and modified polymer fibers

An undesirable failure mode of a reinforced concrete beam is shear mode. Low tensile strength of conventional concrete and brittle crushing due to shear failure in reinforced concrete beams can be improved by adding adequate percentage of steel fibers. The combination of high and low elasticity fibers is capable of arresting macro- and micro-cracks. In fact, the bridging action of fibers on crack faces causes a strong limitation on opening of the crack. This phenomenon improves the aggregate interlock on the crack faces which results in increasing the shear strength of the cracked section. In order to accurately study the pull-out characteristics of crimped-steel fibers with end hook and to compare the results with the behavior of hooked steel fibers and crimped steel fibers alone, an experimental study was conducted. Pull-out load versus slip was thoroughly investigated in 25 specimens and parameters such as maximum pull-out force and its associated slip were taken into account for comparison purposes. The results indicated that the crimped-steel fibers with end hook have better performance in pull out test. In fact, the post-peak behavior of this type of fiber shows a slight drop in carried load. This increases the area under the load-displacement curve in comparison with the others. It can be predicted that cementitious composites reinforced with crimped-steel fibers with end hook would be more ductile than those reinforced with other fibers. In addition, the effect of modified polymer fibers along with different amounts of crimped end hook steel fibers on the mechanical properties of conventional concrete such as compressive strength and indirect tensile strength was studied. The modified polymer fibers were added into the mixes for arresting micro-cracks. 45 specimens were made in 5 groups and the volume fraction of polypropylene fiber was kept constant (0.25%). The volume fraction of steel fibers were selected in three ranges of 0.5%, 0.75%, 1.0%. Also a mix was cast without any fibers to be used for comparison purposes. The results of this study showed that by adding 0.25% polypropylene fibers and 1.00% crimped end hook steel fibers, 27.5% and 66.7% increase in compressive strength and indirect tensile strength are observed compared to conventional concrete. In all cases, by adding steel fibers with polypropylene fiber in the mentioned percentages, the fibers can show desirable performance in post-cracking behavior. Finally, the criteria of ACI 318-2011 for using this fiber reinforced concrete (without shear reinforcement) as the minimum shear reinforcement was investigated. The test is based on ASTM C1609 and it is applicable to the sections of a beam when the applied shear is less than the concrete strength from one hand but, on the other hand, it is greater than the half of that. It was found that this requirements is met in all proposed fiber reinforced concretes. It can be concluded that in such sections the cementitious composites studied in this paper can be utilized without accompanying any stirrups. In fact, the ductility required by ACI 318-2011 in this area can be provided with steel fibers, rather than stirrups.