Performance evaluation of novel prepacked aggregate concrete reinforced with waste polypropylene fibers at elevated temperatures

The prepacked aggregates fiber-reinforced concrete (PAFRC) is an innovative type of concrete composites that recently has gained popularity and pulled the attention of researchers worldwide. The preparation of PAFRC, which is a novel developed concrete, comprises the placing and packing of coarse aggregates with different sizes and short fibers in a formwork, and the spaces between the aggregates are then filled through the injection of cement grout with high flowability. Fire is one of the most disparaging reasons for the collapse of the concrete structures. Therefore, this study aims to investigate the influence of waste polypropylene (PP) fibers on the mechanical and microstructural properties of PAFRC at elevated temperatures of up to 600 °C. Five mixes comprising fiber volume fractions from 0 to 1.0% with a length of 30 mm were cast by gravity technique. Another five mixtures with the same fiber volume fractions were cast using a pump to inject the grout into the formwork. Additionally, palm oil fuel ash (POFA) was used at the substitution level of 20%. The fire resistance of the PAFRC specimens was then measured in terms of mass loss, ultrasonic pulse velocity, compressive and tensile strengths. The role of fibers was inspected through the analysis of the microstructure in terms of scanning electron microscopy. Besides, the experimental outcomes were statistically analyzed. The findings revealed that the waste PP fiber reinforcement in combination with POFA in PAFRC mixes turned out to deliver high resistance against elevated temperatures by the reduction in spalling and losses in the mass of specimens. The positive interaction between fibers and POFA subsequently led to the higher compressive and tensile strength values of PAFRC specimens at high temperatures. Moreover, the outcomes indicated that PP fibers and POFA are promising materials for the production of PAFRC with satisfactory fire resistance.