The use of vegetable fibres as a sustainable alternative to non-natural sources of fibres applied for concrete reinforcement has been studied for over three decades. The main issues about plant-based fibres pointed out by other authors are the variability in their properties and concerns about potential high biodegradability in the alkaline pH of the concrete matrix. Aiming to minimise the variability of flax and hemp fibres, this research compares a range of chemical surface treatments, analysing their effects on the behaviour of the fibres and the effects of their addition to concrete. Corroborating what has been found by other authors, the treatment using NaOH 10% for 24 h was able to enhance the properties of hemp fibre-reinforced concrete and reduce the degradability in alkaline solution. For flax fibres, a novel alternative stood out: treatment using 1% of stearic acid in ethanol for 4 h. Treatment using this solution increased the tensile by 101%, causing a minor effect on the elastic modulus. Concrete mixes reinforced with the treated flax fibres presented reduced thermal conductivity and elastic modulus and increased residual tensile strength and fracture energy.
Natural fibre reinforced concrete is been studied for many years as a more sustainable option to current reinforced concrete used in industry. The most common fibre materials currently adopted are steel, glass and synthetic fibres. Apart from the high oxidation and cost, their environmental impact is a serious issue as they are petroleum-based materials. This study assesses the feasibility of replacing polypropylene fibre with hemp and flax fibres. According to the inventory of carbon and energy (ICE) the embodied energy of polypropylene (PP) is 95.4MJ/kg and the embodied carbon is 4.98kgCO2/kg during its lifetime. It represents approximately 3 times more than the estimated values for vegetable fibres. For this, Different concrete mixtures reinforced by 0.5% to 1.0% of hemp, flax and polypropylene fibres were tested, and their post-crack flexural tensile strength, elastic’s modulus, compressive strength and fracture energy were evaluated. The mixtures containing hemp fibres presented properties equivalent to those containing polypropylene under the same proportion. Although both compressive and tensile strength were reduced for the mixes containing flax fibres, the Young’s modulus was 49% smaller and could be an interesting approach for applications that require better elasticity from the concrete, such as industrial floors and structures that may be submitted to impact.
Fibre-reinforced concrete (FRC) is widely employed in the construction industry, with assorted fibre types being used for different applications. Typically, steel fibres give additional tensile strength to the mixture, while flexible fibres may be used in large sections, such as floor slabs, to control crack width and to improve the handling ability of precast sections. For many reasons, including durability concerns, environmental impact, thermal performance, etc, alternatives to the currently available fibres are being sought. This study examines the potential of using basalt fibres, a mineral and natural material, as reinforcement of concrete sections in comparison to steel fibres and plain concrete mix. Mixes were tested containing 0.5% and 1.0% of basalt fibres measuring 25mm length, 0.5% of the same material with 48mm length and steel fibres measuring 50mm by 0.05%, 0.1%, 0.15% and 0.2% of the concrete volume. For the mechanical performance analysis, the 3-point bending test was led and the fracture energy, Young’s modulus and tensile strength in different moments of the tests were calculated. When compared to the control mixtures and the steel-fibre-reinforced concrete, the mixes containing basalt had a reduction in their elastic modulus, representing a decrease in the concrete brittleness. At the same time, the fracture energy of the mixtures was significantly increased with the basalt fibres in both lengths. Finally, the flexural strength was also higher for the natural fibre reinforced concrete than for the plain concrete and comparable to the results obtained with the addition of steel fibres by 0.15%.
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