Effects of Mixing Sequence of Polypropylene Fibers on Spalling Resistance of Normal Strength Concrete

This paper presents an experimental study on the spalling resistance of normal strength concrete with different mixing sequences of polypropylene (PP) fibers subjected to fire. The mixing sequences of PP fibers in concrete mixture investigated are: (1) mixing PP fibers with fresh concrete; (2) mixing PP fibers with coarse aggregates; and (3) mixing PP fibers with fine aggregates. According to the test results, the degree of concrete spalling is reduced for specimens containing PP fibers at 0.2% by volume in the concrete mixture. The beneficial effect on spalling resistance is maximized when the PP fibers are mixed with fresh concrete and when the PP fibers are mixed with fine aggregates. The results are confirmed using scanning electron microscope (SEM) to investigate the dispersion of PP fibers in hardened concrete. In the case of mixing PP fibers with fine aggregates, smaller amount of superplasticizer is needed to obtain the target slump, and it is therefore recommended for practical use.