“…This is attributed to the fact that polypropylene fibers are much less rigid than the cement mortar, which, in an isostrain compressive loading mode, should lead to a degradation of the concrete strength level. More rigid fibers, such as steel for example, are expected, as reported by a number of authors 1,[14][15][16][17] , to enhance the concrete compressive strength. Another important factor that should be taken into account refers to the presence of pores, and hence the higher porosity index associated with the incorporation of polypropylene fibers also contributes to lowering the fiber reinforced concrete strength.…”