This chapter discusses the conclusions that can be drawn following the study. Recommendations for future work are discussed as well.
6.1 Conclusion
The objective of this study was to investigate the mechanical and volumetric properties of SFRC blocks, with regards to its possible use in the LCH sector in South Africa. A particular focus was on developing a matrix with a reduced CH content to improve durability.
In order to evaluate the mechanical performance of the blocks, compression and flexure tests were performed and it was concluded that there was a decrease in both compression and flexural strength with an increase in fibre content. The addition of fibre, however, did lead to a higher ductility. The addition of CSF to matrices did increase both compression and flexural strength, but the extent of the effect was lowered as fibre content increased.
Regarding the volumetric properties of blocks, mixed results were achieved. The addition of fibres decreased the density and increased the porosity, which in turn led to a higher absorption and rate of absorption. Drying shrinkage was increased as well. The dimensional stability of blocks, after repeated wet-dry cycles, was significantly improved with the addition of fibres.
The partial substitution of cement with CSF led to a slight increase in capillary absorption and rate of absorption. Absorption through immersion, however, was reduced somewhat. The density and air void content was reduced, with the biggest reductions at the highest fibre contents. Due to the reduced pore sizes, drying shrinkage and dimensional stability was increased significantly.
From the results the use of sisal fibre as reinforcement in concrete blocks is questionable. All mechanical properties were reduced significantly, apart from the increase in ductility. The volumetric properties were reduced as well, although all still fell within the allowable limits for concrete blocks, where applicable. Nevertheless, sisal fibre does seem to have a use as a means of dimensional stabilisation.
It was also shown that the use of sisal fibre as a cement content reducing material cannot be recommended. The study investigated blocks with fibre contents of 0.5 and 1% by volume and it was found that even at these low volumes the adverse effect on strength was significant. With respects
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to the results obtained in this research, the positive effects of volume that is added to concrete through the addition of fibre is diminished through the negative effect the fibre has on strength. Furthermore, sisal is not as widely available in South Africa as in Brazil and the economic advantages of using it as reinforcing material is difficult to assess.
As a possible new technology for use in LCH, much more research is needed to quantify the durability of SFRC blocks.
6.2 Recommendations for future work
Much research is still needed before natural fibres can be considered as a durable concrete reinforcing material.
The following recommendation can be made with regard to the fibre type used and sustainability:
• Hemp is a fibre similar to sisal and perhaps more suited to cultivation conditions in South Africa. The possibility of the use of hemp fibre in South Africa should be further investigated.
• It is important to quantify the environmental sustainability of sisal (or other natural fibres) in terms of carbon footprint, life cycle assessment, etc. This, however, lies outside the scope of an engineering study.
The following recommendations can be made with regard to the matrix type used:
• The partial substitution of cement with CSF as a means of lowering the CH content and alkalinity does improve the strength and durability of NFRC. The amount of CSF needed for a CH-free matrix requires investigation.
• Since CSF is very expensive in South Africa, other pozzolans with the same CH-reducing effect need to be researched to find a more economically viable alternative.
The following recommendations can be made with regard to the durability and strength:
• Similar tests as were performed in this study could be performed over longer periods (perhaps up to two years) to properly investigate the durability and the effect of CSF over longer periods.
• The effect of fibre length could be studied. Longer fibres might improve the mechanical performance of blocks. However, it should be noted that longer fibre will increase the possibility of fibre balling during mixing and it might not work with hollow blocks, due to its small wall thicknesses.
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• The effects when using finer aggregates need to be researched. The matrices used in this research could have made use of a higher fine aggregate content. This is contrary to suggestions in literature regarding the production of low cost concrete blocks with reasonable strength, but perhaps a less porous matrix could enhance the positive effects of the added natural fibres.
• The fibre-cement interface in a porous matrix requires further investigation in order to better understand how the fibre interacts with cement in such an environment.
• The moisture absorption of NFRC blocks after various wet/dry cycles could make for an interesting research topic. This would shed some light on the effect of wetting/drying on the pore system of a porous matrix.
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