SURFACE-MODIFIED POLYPROPYLENE FIBERS FOR USE IN CONCRETE

The research project reported on is concerned with the effect of modif ication of the surface of polypropylene fibre by a new chemical treatm ent process, oxyfluorination, on the properties of polypropylene fibre reinforced concrete. As a world first, the interfacial bond of polypr opylene fibres with the cementitious matrix is improved by increasing the surface free energy of the fibre surface. The reasons for the poor bonding between untreated polypropylene fibre and cementitious matrix are discussed, using the fibre surface free energy and Lewis acid-bas e interaction concept. The contact angle of water on the polypropylene fibre surface as well as fibre surface free energy components were me asured. This showed reduced contact angles, as well as increased acid- base components of the surface free energy because of oxyfluorination. Mechanical properties such as compressive strength, flexural properti es and impact resistance of the fibrous concrete, reinforced with diff erent types of oxyfluorinated polypropylene fibres, were determined an d compared with those of untreated polypropylene fibre reinforced conc rete. The results confirmed that the surface modification largely impr oves the mechanical performance of the fibrous concretes. Restrained p lastic and drying shrinkage cracking tests, using restrained slab spec imens and steel ring restrained specimens, indicated that the surface oxyfluorinated fibres possess a higher shrinkage cracking resistance t han do unmodified fibres. The effect of surface oxyfluorination on the fibre/concrete matrix interfacial bondings was investigated using a f ibre pull-out test. A mechanism for this interfacial bonding improveme nt is proposed. Oxyfluorinated polypropylene fibre surfaces and their interfaces with concrete matrix compared with that of unmodified fibre , were observed using scanning electronic microscopy. Some field appli cation tests were conducted and the good results that were achieved ar e presented.