Effect Of Polypropylene Macro-fiber On Properties Of High Strength Concrete After Being Exposed To High Temperature

The fire resistance of high strength concrete (HSC) structures has attractedextraordinary concern from the field of civil engineering. Studies have shown that thepressure relief channels formed by the melting of polypropylene (PP) micro-fiber can beused to ease the burst of concrete and increase the fire resistance of structures. Comparedwith PP micro-fiber, the widely used PP macro-fiber has the same material, melting point,and the relatively larger diameter. After the melting of PP macro-fibers, different pressurerelief channels will be formed, and they may have different effects on the properties ofHSC after being exposed to high temperature. Therefore, it is necessary to study deeply onthe properties of PP macro-fiber reinforced HSC after being exposed to high temperature.By changing the dosage of PP fibers, five groups of HSC specimens (size of40mm×40mm×160mm) with different mix proportion were prepared. Using of the box-typeresistance furnace for high temperature testing (the target temperature was200℃,400℃,600℃and800℃). Through the studies on the change law of compressive strength andflexural strength of different fiber dosage HSC after being exposed to high temperature,the effects of PP macro-fiber on the mechanical properties of HSC after being exposed tohigh temperature were analyzed. The results show that adding PP macro-fiber is beneficialto the improving of HSC residual compressive strength, but has disadvantage effects onthe flexural strength, while the synergistic reaction of the hybrid fibers (PP micro-fiberand PP macro-fiber) can make the effects to minimize. The matrix mass loss rate andwater absorption rate were measured and the effects of PP macro-fiber on the porestructure of HSC were explored. Compared with the PP micro-fiber, the pressure reliefchannels which formed by the melting of PP macro-fiber were more coarsening,meanwhile the matrix mass loss rate and water absorption rate of HSC mixed with PPmacro-fiber are higher. But if the dosage of PP macro-fiber is too high, the pressure reliefchannels will be too coarse, and the total porosity of HSC will be increased significantly.Also the internal damages of specimens were analyzed by the ultrasonic non-destructivetesting technology, and the mathematical model of damage degree varying with exposed temperatures was established. According to this model, the exposed temperature of highstrength concrete can be calculated in the projects.This subject is helpful for the fire resistance designing of HSC structures and it hascertain guiding significance for the correct damage assessment of HSC after beingexposed to high temperature.