Development of surface fluorinated polypropylene fibres for use in concrete

D.Ing. (Civil Engineering)Polypropylene (PP) fibre is one of the most widely used fibres for reinforcing concrete. Due to its unique material properties such as economic production cost, relatively high melting point and alkaline resistance, PP fibres in concrete are found to increase the toughness, provide restrained plastic and drying shrinkage cracking resistance, increase impact resistance and flexural loading capacity (especially during the post-crack stage) of concrete structures and components. The wettability of PP fibre surfaces is poor due to the hydrophobic surface nature of polypropylene. The bonding between the PP fibre and the concrete matrix is recognized as poor and currently the PP fibre / concrete interfacial bonding depends upon interlocking and keying (mechanical bonding). As the interfacial bond strength between the PP fibre and the concrete is much smaller than the tensile strength of PP fibre and concrete, in order to fully explore the reinforcing effect of PP fibres in concrete, further improvement of interfacial bonding is necessary. In this investigation, the research work was conducted on the surface oxyfluorinated PP fibre newly developed by the Atomic Energy Corporation of South Africa Limited. Examining the feasibility of using this fibre in concrete, as well as evaluating its superiority over unmodified PP fibres, is the key task of this investigation. This new type of oxyfluorinated PP fibre shows a large increase in interfacial bond strength compared to the unmodified PP fibres. The surface free energy concept and Lewis acid-base interfacial interaction theory were innovatively used to examine the interfacial bonding between the PP fibre and the concrete matrix. The purpose of this study was to develop the background of such oxyfluorinated PP fibres, to establish the mechanism of the increase in interfacial bonding and to investigate the basic properties of the concrete incorporating oxyfluorinated PP fibres. The experimental results on the properties of oxyfluorinated PP fibre reinforced concrete compared with those of unmodified PP fibre reinforced concrete and plain concrete are presented, with some field test results focused on the improvement in shrinkage cracking control capacity. It is concluded in this investigation that the surface oxyfluorinated PP fibres possess significantly increased PP fibre / concrete interfacial bonding due to the chemical bond arising from the acid base interfacial interaction and the intimate interfacial contact arising from the improved fibre surface wettability. When combining oxyfluorinated pp fibre with concrete, a general improvement in the physical and mechanical properties of fibrous concrete compared to that created with unmodified PP fibres, is obtained