A Further Milestone to the Use of Natural Fibres in Concrete – Past Findings, Barriers and Novel Research Avenues

Abstract

Concrete as a building material is much appraised for its good compressive strength; however, its low tensile strength makes it a quasi-brittle material. Experiments have proven that fibres such as steel and some polymeric fibres can reinforce and enhance the mechanical strength of concrete. The relatively high production cost of these fibres coupled with environmental issues for their end of life disposal and decline in mechanical strength beyond a certain fibre fraction have encouraged the use of natural fibres; particularly due to their renewability, low cost and good tensile strength. This paper reviews published literature in the field of natural fibres, their extraction methods as well as their effect on the mechanical properties of concrete. Alkaline fibre treatment to improve strength, wettability and subsequently, fibre-concrete matrix interfacial adhesion has also been discussed. As part of the research, the current authors have found that by just using untreated (raw) fibres as reinforcement in fact leads to a decline between 75 % and 90% in compressive strength tested at 8 days for 2 different fibre lengths and volume fractions, respectively. This decline in strength could be co-related with the phenomenon of fibre agglomeration as seen from microscopic analysis. As such, fibre treatment, to remove different impurities from its surface, constitutes an important step towards the manufacture of natural fibre-reinforced concrete. Furthermore, water adjustment in relation to the total water requirement of the cement, aggregates and water needed to saturate the plant fibres is an important property that requires proper control since water content has a direct impact on the workability of the concrete and can turn into a major constraint. The main challenge of the use of natural fibres in concrete is its degradation with time within the highly alkaline concrete environment. Accelerated ageing experiments for natural fibres in concrete as described in literature have confirmed this deleterious occurrence. Thus, as per findings from the current experimental works and literature, the following recommendations are proposed: natural fibre pre-processing to inhibit agglomeration, adequate water addition to cater for all the constituents of the reinforced concrete and the potential implementation of biomimicry to solve the fibre degradation problem.