Textile-reinforced concrete (TRC) is an innovative product that consists of a combination of fine-grained cement matrix with two-dimensional or three-dimensional textile layers. These materials can be considered as cement composites with strain-hardening behaviour, which are used for strengthening, retrofitting and construction of structural and semi-structural elements, facades and roofs. Two types of fibres including synthetic and natural fibres can be used in manufacturing TRCs. Compared to synthetic fibres such as carbon, glass, natural plant fibres such as Kenaf have many useful properties for the construction industry. These fibres are biodegradable, renewable and recyclable and do not pose any risk to human health, and less energy is used in their production. The purpose of this research was to eva‎luate the mechanical properties and durability of 144 concrete specimens reinforced with kenaf textiles (KTRC) with dimensions of 400 x 60 x 30 mm under the 4-point bending test. For this purpose, parameters such as the first-crack stress (modulus of rupture), Toughness at the moment of cracking (T1), Total Toughness and Toughness Indices (I5, I10 and IF) were investigated in two separate study phases. In the first phase, the effect of the presence of polypropylene fibres in the cement matrix (0.5% and 1% by weight of mortar), the number of reinforcement layers in the thickness of concrete (3 and 5 layers) and the way the layers are arranged (symmetrical and asymmetrical) on the mechanical properties of KTRC have been investigated. In the second phase, which was related to the durability of KTRC samples in an environment containing sulfuric acid (H2SO4), the flexural performance of the samples was eva‎luated after 2, 4 and 6 months. Finally, the performance of the samples was verified using the images prepared from the Scanning Electron Microscope (SEM). The results of the first phase depicted that although the modulus of rupture of KTRC samples has not been affected greatly by the number and the arrangement of kenaf layers, by adding short-cut PP fibres to the cement matrix, the modulus of rupture and T1 has been increased. In addition, the asymmetric arrangement of layers has a more favourable effect on energy absorbtion of the 3-layer samples. In general, with the increase in the number of layers and the percentage of PP fibres, the energy absorption and toughness of the samples increased. The results of the second phase indicated that in the acidic environment after 4 and 6 months, the water in this environment helped to continue the cement hydration reaction in the long term and the positive effect of this reaction prevailed over the negative effect of acidic solution. Moreover, the mechanical properties of the cement matrix, such as the modulus of rupture and the T1, have increased compared to the samples in the laboratory environment. However, the acidic environment created a detrimental condition for the rotting and deterioration of kenaf textiles and in the long term led to a decrease in the toughness, ductility and strain-hardening behaviour of the samples. SEM images indicated the satisfactory durability of kenaf textiles in the alkaline environment of cement paste. Also, these images show that there is no specific chemical or physical connection between the textiles fibers and the concrete matrix.

كياچهر بهفرنيا , مهدي حجازي

فريدون رضائي , محمدرضا افتخار