Comparative Experimental Investigation of Reinforced Concrete Beams with Steel and Polypropylene Fiber Reinforcement

Fiber-reinforced concrete is increasingly used to improve the mechanical and structural performance of reinforced concrete elements. This study presents a comparative experimental investigation of reinforced concrete beams incorporating steel and polypropylene fibers. Seven beams were examined, including a reference concrete and fiber-reinforced concrete mixtures containing 1.0%, 1.5%, and 2.0% fiber volume fractions. The experimental program included compressive strength tests on 21 cube specimens, shrinkage measurements on 21 prism specimens, and bending tests on reinforced concrete beams. The properties studied included compressive strength, shrinkage strain, ultimate load capacity, load–deflection response, crack initiation, crack width, and post-cracking behavior. Polypropylene fibers provided the greatest crack-control benefit, reducing shrinkage strain and maximum crack width by up to 50% and 93%, respectively, compared with the reference concrete. Steel-fiber-reinforced beams achieved the highest ultimate load, with an increase of up to 22% relative to the reference beam, and showed higher calculated displacement ductility indices. The results indicate that, under the tested conditions, steel fibers were more effective in improving load-carrying capacity and displacement ductility, whereas polypropylene fibers were more effective in controlling shrinkage and crack development. These findings support fiber selection according to the required balance between load capacity, deformation response, crack control, and serviceability.