Study On Mechanical Propertiesand Media Transport In Natural Fiber Reinforced Cement-based Composites

Plain concrete materials have the characteristics of low tensile strength,easy cracking and brittleness,which seriously limit the durability and service life of concrete structures.According to the composite thought of materials,a series of fiber-reinforced cement-based materials rise in response to the proper time and conditions.In order to increase the toughness of the cement-based material,the incorporation of a certain amount of fiber therein can improve its brittleness.Due to its low cost and low environmental pollution,natural fiber has received more and more attention of domestic and foreign scholars in recent years.Although relevant research has been carried out,there are few studies on the physical properties and durability of natural fiber cement-based composites,and the related theories are still incomplete.Therefore,it is of great theoretical and practical significance to study the mechanical properties and medium transport mechanism of natural fiber cement-based composites.Based on the above-mentioned considerations,this paper carried out the experimental study on the physical properties and medium transport of natural fiber cement-based composites,and analyzed the influence of alkaline environment inside the concrete on the strength and apparent performance of natural fibers.The specific work and main conclusions are summarized as follows:?1?The alkaline environment of concrete was simulated by two solutions of NaOH and Ca?OH?₂?pH=12?.The effects of the alkaline environment on the tensile strength and apparent morphology of Pineapple leaf fiber and ramie fiber were analyzed.It was found that the apparent morphology of natural fibers did not change significantly after soaking in alkali solution at different times,but the tensile strength decreased slightly.When the fiber was immersed in NaOH solution for 60 days,the tensile strength of pineapple leaf fiber and ramie fiber decreased by 13.1%and 14.7%,respectively.When the fiber was immersed in Ca?OH?₂ solution for 60 days,the tensile strength of pineapple leaf fiber and ramie fiber decreased by 15.7%and 19.4%,respectively.?2?The basic mechanical properties tests,e.g.flexural,compressive,tensile and bending properties,of natural fiber cement-based composites were carried out.Based on the stress-strain curves of specimens,the influence of fiber content and type on mechanical properties was analyzed.The results show that with the increase of the volume of pineapple leaf fiber,the flexural strength,fracture energy and ductility index of the specimens increase obviously,but the compressive strength decreases slightly.It shows that the incorporation of natural fibers significantly improves the brittleness of cement-based materials,which shows the characteristics of strain hardening and improves the toughness and load-bearing capacity of the materials.However,pineapple leaf fibers have better toughening effect on cement-based materials than ramie fibers.?3?The capillary water absorption and chloride ion corrosion test of natural fiber cement-based composites were carried out.The effects of different fiber content on capillary absorption coefficient and chloride ion penetration depth were compared and analyzed.The results show that the water absorption of the specimen increases with the increase of the natural fiber content,and the capillary absorption coefficient also increases.However,the water absorption of the ramie fiber specimen is significantly smaller than that of the pineapple leaf fiber specimen.The development trend of the chloride ion content varies with the fiber content and type is consistent with the trend of water absorption of specimens.?4?The drying shrinkage test of specimens with different content of pineapple leaf fiber and ramie fiber was carried out,and it was found that the shrinkage deformation of the specimens mixed with pineapple leaf fibers was significantly smaller than that of the ramie fiber specimens.When the age is 90d,the shrinkage deformation of different pineapple leaf fiber samples is reduced by 9%²4%compared with ordinary specimens,while the shrinkage deformation of specimens with volume of 2%of ramie fiber is reduced by 16%.In general,the incorporation of natural fibers increases the water loss rate of the test piece,but it can effectively restrain the shrinkage deformation of the cement-based material,and the effect is more obvious as the fiber content increases.This is because the natural fiber produces a constrained tensile stress in the cement matrix opposite to the direction of shrinkage,which limits the shrinkage of the test piece.