Influence of fibres on the mechanical behaviour of fibre reinforced concrete matrixes

An experimental analysis focused on the mechanical behaviour of fibre reinforced concrete matrixes (FRCM) is presented using a total of three hundred and twelve specimens. A reference plain mixture was first defined and then three types of fibres were chosen to reinforce it (polypropylene, glass and steel fibres). Within each type of reinforcement, four volumetric proportions were adopted, ranging from 0.5% to 2% in 0.5% increments. The influence of each type of fibre and dosage on the properties of the FRCM, including compressive strength, bending behaviour, cracking and maximum loads and ductility was analysed. In summary, it was observed that the compressive strength generally grows with the reinforcement dosage, and that this growth is greatly affected by the properties of the fibre, namely by its tensile strength. The load-displacement curves are also highly affected by the type of reinforcement. Steel and polypropylene fibres provide the composite material a better capacity to withstand high deformations. Glass fibres have a reduced effect on this regard, due to their brittle behaviour. For each type of fibre, by increasing the fibres percentage, an increase in the load capacity is also observed, with a maximum of 160% for an addition of 2.0% of steel fibres. The cracking loads are consistently lower than that of the reference mixture, due to the loss of homogeneity and increased porosity caused by fibre addition, in spite of the favourable influence associated to the mechanical properties of the fibres. For polypropylene FRCM the cracking loads were approximately 35% lower than that of the reference mixture. For steel and polypropylene fibres the toughness indexes (I5, I10 and I20) were defined, being observed that for 1.5% volume fraction of steel fibres the I5 and I20 are respectively 6.80 and 35.08, whereas for the polypropylene fibres those indexes are respectively of 3.61 and 15.75 for the same fraction.