USE OF FOAMED POLYPROPYLENE FIBERS TO IMPROVE FIBER/MATRIX BOND FOR CEMENTITIOUS COMPOSITES

Fibers are used in cementitious composites to improve some mechanical properties, i.e. toughness and tensile strength, and/or to avoid shrinkage cracking. For these purposes, fibers of different nature and shape are generally used: synthetic, natural, steel, smooth, crimped, hooked. In composite materials the importance of fiber/matrix adhesion is widely recognized. To this extent, polyolefin fibers are chemical inert, relating to a cementitious matrix, and their surface is smooth thus leading to a low adhesion. This work is aimed to improve fiber/matrix bond using polypropylene (PP) fibers with porous structure and improved surface roughness produced by a laboratory scale melt extrusion process using two different amounts (1 and 2 wt%) of a chemical foaming agent. Density of fibers considerably decreases (from 0.91 g/cm(3) of neat PP to 0.57 g/cm(3) of foamed fibers). As shown by SEM images, the surface morphology of fibers, changes from smooth to highly rough at increasing the foaming agent content. To evaluate fiber/matrix bond a cementitious mortar (one part of cement, three parts of sand and w/c of 0.55) has been prepared. Fiber/matrix adhesion was investigated by single fiber pull-out tests and SEM images which confirmed the increased mechanical bond of foamed fibers. Usefulness of fibers against early age shrinkage cracking has been investigated using restrained mortar slabs. Increasing fibers volume fraction a reduction of cracks is observed.