Flax Frp Composites And Strengthening The Shear Capacity Of Reinforced Concrete Beam

Flax fiber have some advantages,such as high specific strength,sho rt growth cycle,wide variety of resources and low cost,etc.As a kind of reinforced fiber used in the civil engineering structure,the flax fiber could reduce the consumption of non-renewable resources and damage to the eco-environment.There has some important significance to solve the global warming,develop the low-carbon buildings and cities,and promote the civil engineering with sustainable development.However,compared with the carbon fiber and glass fiber,the mechanical properties of flax fiber is relatively low,e.g.the tensile strength is only 1/3 to 1/4 of the glass fiber.Meanwhile,a large amount of hydroxyl groups on the surface of flax fibers lead to a high water absorption and poor moisture durability of flax FRP composites.Therefore,in this dissertation,the multi-walled carbon nanotubes?MWCNTs?and nano-TiO₂ were applied to modify the epoxy resin and flax fiber,respectively.The aim of modification is to improve the mechanical and interfacial properties of the relevant flax FRP composites.Then,the flax fiber sheets were used to strengthen the shear capacity of the reinforced concrete?RC?beams.It will explore the applications of flax fiber as structural materials in the civil engineering.The research achievements of this dissertation will lay the theoretical and design foundations of the natural FRP composites in the civil engineering.There have some important scientific significance and practical values to develop the green and sustainable structures.The main contents and achievements in this dissertation are as follows:Firstly,the epoxy resin was modified by the MWCNTs.The effect of MWCNTs` mixture on the mechanical and thermal properties were studied.Based on the molecular dynamics?MD?method,the interactions with MWCNTs and the cured epoxy resin were investigated.The results of this chapter show that,the hydroxylated MWCNTs could improve the mechanical and thermal properties of epoxy resin,and the optimal mixture of MWCNTs is about 1.0 wt.%.The MD model could predict the glass transition temperature?Tg?of modified epoxy resin accurately.A large amount of hydroxyl groups on the MWCNTs generate effective bond with the polymer matrix,so that the MWCNTs will exert the reinforcement completely.Secondly,the preparation of flax fiber grafted by nano-TiO<sub>2</sub> was put forward and studied.The effects of nano-TiO<sub>2</sub>`s grafting content on the mechanical properties and interfacial bonding properties with epoxy resin was investigated.With the ultrasonic,nano-TiO₂ particles were grafted onto the flax fiber surface through the silane coupling agent.After that,the covalent bond of-Ti-O-Si-and-Si-O-C-were generated on its surface.The results of this chapter show that,the grafting modification by nano-TiO₂ could effectively improve the mechanical properties & interfacial bonding properties of single flax fibers.The optimal grafting content of nano-TiO₂ is about 2.34 wt.%.The nano-TiO₂ grafting could reduce the defect degree of flax fiber`s surface,at the same time,it could generate a nanocomposites layer between the flax fiber and epoxy resin.Thirdly,based on the modification results of epoxy resin and flax fiber,the flax FRP composites with epoxy resin modified by MWCNTs and flax fiber sheet grafted by nano-TiO<sub>2</sub> were prepared to improve the mechanical and interfacial properties of the relevant flax FRP composites.After that,the effects of different modification and preparation parameters on the mechanical and interfacial properties were investigated.The results of this chapter show that,two different modification could both improve the mechanical and interfacial properties of the flax FRP composites in varying degrees.It can be seen that the properties of flax FRP composites have close relationship with the modification and preparation parameters,such as the optimal MWCNTs` mixture is about 1.0 wt.%,the optimal nano-TiO₂`s grafting content is about 2.34 wt.%,and the optimal MWCNT`s grafting time is about 6 minutes.The effective method for the high-performance flax FRP composites is the flax fiber grafted by nanoparticles?e.g.nano-TiO₂ grafting?.Fourthly,the flax fiber sheets were used to strengthen the shear capacity of the RC beams.The bearing characteristics and failure behavior of the strengthened RC beams were investigated.After that,the shear capacity formula for the RC beams strengthened by the flax fiber sheets was put forward.The results of this chapter show that,the shear capacity and central deflection of the RC beams are increased significantly after strengthened by the flax fiber sheets.The improved effect of flax fiber sheets grafted by nano-TiO₂ is greater than the general sheets.After shear strengthening,the shear capacity of RC beam has a good linear relationship with the ‘stirrup ratio'?Eftf?of flax fiber sheets,and it could predict the shear capacity of the RC beams strengthened by the flax fiber sheets accurately.