The Effect of Recovery Fibers and the Sand/Gravel Ratio on the Direct Tensile Behavior of Concrete

The purpose of this research is the experimental analysis of the mechanical behavior of steel chip reinforced concrete. The fibers used are obtained from metal chips recovered in steel machining workshops in Algeria (SNVI), where they are obtained in the form of cylindrical rolls. Once stretched, these rolls take the form of a coil, which is then cut to produce fibers of small lengths. This helical configuration ensures better anchoring in the concrete and prevents the formation of cylindrical voids that could weaken the matrix. The fibers were subjected to tests to assess their mechanical strength and pull-out resistance capacity. Direct tensile tests were conducted on gypsum samples of square section [100x100] mm², requiring the design and fabrication of a special fixing device on the tensile machine. Four percentages of fibers were selected for this study (W=0.3%, W=0.5%, W=0.8%, and W=1%), where W represents the volumetric fraction of added fibers. In order to determine the optimal composition of fiber-reinforced concrete in terms of workability, three concrete mixes with different sand/gravel ratios (S/G=0.642, S/G=0.8, and S/G=1) were made, all with a fixed water/cement ratio E/C = 0.543 for each fiber percentage. Compression tests were carried out on cylinders with a diameter of Ø16 cm and a height of H32 cm, using a computer-controlled hydraulic press. The analysis of the results reveals that the behavior of fiber-reinforced concrete is divided into two distinct phases: an initial linear phase, corresponding to elasticity, followed by a post-cracking phase where the fibers continue to provide resistance. The fibers provide significant ductility to the material after concrete cracking (post-rupture), while limiting cracks and improving strength and stiffness for certain percentages of fibers and specific sand/gravel ratios (S/G).