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Key factors affecting distribution and orientation of fibres in steel fibre reinforced concrete and subsequent effects on mechanical properties

Student thesis: Doctoral Thesis

Use of fibres to reinforce brittle materials for better performance in buildings and for construction purposes has been employed since time immemorial. Inclusion of steel fibres in concrete therefore, has always improved the post-cracking strength and concrete ductility to a large extent. Nevertheless, there is no doubt that it has become imperative to have more understanding of the internal workings of steel fibre reinforced concrete to fully exploit its potential in practice.
In this PhD study, investigation of distribution and orientation of steel fibres within steel fibre reinforced concrete, studying how the positioning of steel fibres in SFRC (steel fibre reinforced concrete) matrix affects the post-cracking strength and other properties that enhance concrete ductility is reported. Variables selected for this study were those considered to influence how steel fibres and concrete matrix associate together during mixing. Hooked-end steel fibres with 50 mm and 60 mm length, of varying diameter resulting in different aspect ratio (ratio of length to diameter of fibre) of 45, 65 and 80, and dosages of 0 kg/m³, 25 kg/m³, 40 kg/m³, 50 kg/m³ and 60 kg/m³ were employed with maximum sizes of coarse aggregate of 10 mm and 20 mm. The same mix proportions of concrete were used throughout the investigation.
Workability of the fresh mix was carried out through slump test, flexural performance was assessed through beam and slab tests at 28 day while compressive strength was also measured using cubes. Subsequently, cores were extracted from these panels and X-ray computed tomography was employed for imaging the cores while Insight Toolkit Software was used to analyse the position of fibres in hardened concrete.
The experimental results show that the strength performance of steel fibre reinforced concrete improved drastically when compared to plain concrete without fibres. Remarkable improvements were observed at larger dosages of steel fibres, and with fibres with highest aspect ratio of 80 noted to give the best results which suggests that aspect ratio of fibre is critical to SFRC performance. It was found that fibre effects on compressive strength is slightly pronounced, with optimum compressive strength of 68 MPa noticed at fibre dosage of 50 kg/m³ and with fibre of 80 l/d ratio with 20 mm aggregate mixture which is about increase of 8 MPa when compared with plain concrete. Also, in SFRC beams, there were up to 83% increase in maximum stress reached when compared to unreinforced concrete. Moreover, it was found that the results of X-ray CT image analysis by The Insight Toolkit software correlate well with the outcome of mechanical performance of steel fibre reinforced concrete.
The slab test results show that mixtures containing 10 mm maximum aggregate size sustain higher load than those of 20 mm counterparts. Harmonization of beam and slab results using yield line analysis revealed that the values of theoretical and experimental failure loads are reasonably close for slabs containing 20 mm maximum aggregate size while the analysis does not agree perfectly with slabs containing 10 mm maximum aggregate size. The 3D rendering images of SFRC cores show that steel fibres are generally positioned horizontally in the slabs which can be seen to be more pronounced in 10 mm maximum aggregate mixes resulting in their ability to sustain higher failure loads.
The study has revealed a clear relationship between the geometry of steel fibre and maximum aggregate size, establishing the fibre-aggregate interaction effects on post-cracking capacity of SFRC. Finally, the study has quantitatively measured the distribution and orientation of steel fibre within the concrete matrix while the correlation between the internal mechanism and the mechanical properties of SFRC has been established.
Original languageEnglish
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Award dateJan 2017

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