This study was aimed to investigate the effect of steel, polypropylene (PP), and hybrid (steel + PP) fibers on high-temperature mechanical properties of reactive powder concrete (RPC). The mechanical properties considered are cubic compressive strength, axial or prismatic compressive strength, split-tensile strength, flexural strength, elastic modulus, peak strain, and stress-strain behavior. The strength recession due to high temperature was investigated at micro level by scanning electron microscope, energy dispersive X-ray spectroscopy, X-ray diffraction, mercury intrusion porosity, thermogravimetric, and differential scanning calorimetry analyses. The high-temperature tests were carried out at target temperatures of 120, 300, 500, 700, and 900 °C. The hot-state compressive strength of RPC started to decrease at 120 °C; however, a partial recovery at 300 °C and a gradual decrease above 300 °C were observed. The degradation of split-tensile strength, flexural strength, and elastic modulus were gradual with increasing temperature despite the effect of different fibers. Whereas, the peak strain was gradually increasing up to 700 °C. However, after 700 °C, it remained unchanged. Steel fiber reinforced RPC (SRPC) and hybrid fiber reinforced RPC (HRPC) showed a ductile behavior. PP fiber reinforced RPC (PRPC) showed a quite brittle behavior up to 300 °C; however, further heating made the microstructure porous and it became ductile too. Overall the performance of SRPC and HRPC were superior to PRPC because of higher modulus of elasticity, higher strength, and better fire resistance of steel fibers. Fiber reinforced RPC was found to have better fire resistance than traditional types of concrete based on comparative studies with the provisions of design codes and earlier research. The constitutive equations developed can be utilized in computer programs for structural design of RPC structures exposed to fire.

This study provides an experimental investigation on the properties of fiber-reinforced composite concrete consisting of natural fibers (coconut coir) and rounded, straight steel fibers in a comparative manner for both normal-strength concrete (NSC) and high-strength concrete (HSC). This investigation was carried out using several tests, which included workability test, compressive strength test, indirect tensile strength test, and flexural strength test. A total of 10 mix batches of NSC and HSC containing 0%, 0.5%, and 1.0% fiber volume dosage rates on coir and steel fibers were tested to determine the enhancement of mechanical properties of composite concrete. In both NSC and HSC, the workability of concrete significantly reduced as the fiber dosage rate increased. In NSC, the compressive strength test indicated that strength decreases compared to plain concrete for both types of fiber. In HSC, compressive strength decreases compared to plain concrete for coir fiber and also decreases with increasing fiber volume dosage rate and increases for steel fiber with increasing fiber volume dosage rate. In NSC, split tensile test showed that tensile strength decreased for 0.5% coir and increased for 0.5% steel compared to plain concrete. But in HSC, strength increases for 0.5% coir compared to 0.5% steel and plain concrete. In NSC, flexural strength and flexural toughness of composite concrete increased in both types of fiber. But in case of concrete with coir fiber, flexural strength for 0.5% and 1% fiber volume dosages is same and increases for steel fiber. In HSC, increase of flexural strength for 0.5% coir compared to plain concrete was observed. Flexural strength also decreases with increase of fiber volume dosage for both types of fiber. In post-cracking stage, the fibers were fully utilized when it increases the ductility and toughness of both types of concrete. This research has shown that 0.5% and 1.0% coir fibers give improved performance in flexural strength of NSC, and 0.5% coir fiber gives improved performance in flexural and tensile strengths of HSC. Also, addition of coir fibers in both types of concrete increases the ductility and toughness of concrete, whereas reduction of workability of fresh concrete and compressive strength of hardened concrete occurs using coir fibers in both types of composite concrete.