Surface modified polypropylene fibres for use in concrete

Abstract

The research project reported on is concerned with the effect of modification of the surface of polypropylene fibre by a new chemical treatment process, oxyfluorination, on the properties of polypropylene fibre reinforced concrete. As a world first, the interfacial bond of polypropylene 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–base interaction concept. The contact angle of water on the polypropylene fibre surface as well as fibre surface free energy components were measured. 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 properties and impact resistance of the fibrous concrete, reinforced with different types of oxyfluorinated polypropylene fibres, were determined and compared with those of untreated polypropylene fibre reinforced concrete. The results confirmed that the surface modification largely improves the mechanical performance of the fibrous concretes. Restrained plastic and drying shrinkage cracking tests, using restrained slab specimens and steel ring restrained specimens, indicated that the surface oxyfluorinated fibres possess a higher shrinkage cracking resistance than do unmodified fibres. The effect of surface oxyfluorination on the fibre/concrete matrix interfacial bondings was investigated using a fibre pull-out test. A mechanism for this interfacial bonding improvement 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 application tests were conducted and the good results that were achieved are presented.