Study On Mechanics And Durability Of Carbon Fiber Reinforced Nano-metakaolin Recycled Concrete Based On Dispersion Technology

In the process of urban renewal,some old buildings have already reached their service life,and the demolition of old buildings will produce immeasurable construction waste(such as abandoned concrete,etc.).The research shows that the recycled concrete(RAC)technology is one of the most effective means to solve the problem of waste concrete,but the performance of RAC is poor and does not meet the practical engineering application standard,so it needs to be modified.Nanometakaolin(NMK),as a new nano-reinforced material,has excellent performance and low cost,so it has a good application prospect.However,the tensile and bending strength of RAC modified by NMK are still at a low level.In order to overcome the above problems,carbon fiber(CF)reinforced NMK recycled concrete is used in this paper,and its mechanical and durability properties are studied experimentally.However,the mixed CF and NMK need to be evenly dispersed in RAC to better stimulate their role in enhancing RAC performance.Currently commonly used dispersants cannot achieve a good dispersion effect on CF and NMK at the same time.Therefore,this paper intends to explore an ideal dispersant,and use CF to strengthen NMK recycled concrete based on the optimal dispersant,and conduct an experimental study on its mechanical and durability properties.Combined with mercury injection(MIP),nitrogen adsorption and scanning electron microscopy(SEM),the influence rule and mechanism of CF on NMK recycled concrete were analyzed.The main research results are as follows:(1)The dispersion effect of different surfactants on CF and NMK was studied.The surface tension method was used to preliminatively analyze the dispersion effect of surfactant on CF and NMK suspension,and then the compression test was used to further prove the dispersion effect of surfactants.The results show that 1% sodium dodecyl benzene sulfonate(SDBS)has the best dispersion effect on CF and NMK.(2)Based on the optimal dispersant SDBS,orthogonal tests were designed to study the influences of different factors on the compressive strength,splitting tensile strength and tension ratio of RAC,taking the dosage of NMK,the dosage of CF,the substitution rate of reproduced coarse aggregate(RA)and CF length as influencing factors.The results show that the significant factors affecting the compressive strength of RAC are RA substitution rate and NMK content,and the effect of RA substitution rate is more significant than that of NMK content,and the significant factor affecting the splitting tensile strength and tension ratio is CF content.(3)Based on the conclusion of orthogonal test,significant influencing factors NMK and CF content were selected as variables to carry out RAC bending strength test,and their influence rules on RAC bending strength were explored.The results show that the incorporation of CF has a significant effect on the bending strength of RAC,but the optimum dosage of CF varies with different dosage of NMK,indicating that there is a suitable dosage of CF and NMK.Under the same CF content,the bending strength of RAC increases first and then decreases with the increase of NMK content.When the content of NMK is 10% and CF is 0.15%,the bending strength of RAC reaches the maximum,and the strength increases by 59.1% compared with that of ordinary RAC.(4)The chlorine ion penetration resistance and freeze-thaw resistance cycle tests were carried out with NMK and CF content as variables to evaluate their effects on RAC durability.The results showed that appropriate amount of CF and appropriate amount of NMK could enhance the chloride penetration resistance of RAC,and the enhancement effect was better when mixed at the same time.When the content of NMK is 10% and CF is 0.15%,the chloride ion penetration resistance of RAC is the strongest,and the chloride ion migration coefficient of RAC is 29.6% lower than that of common RAC.When CF and NMK were mixed together,the mass loss rate and relative dynamic modulus test results of RAC were better than those of the two materials separately.In this case,the anti-freeze-thaw cycle performance of RAC was the strongest.Therefore,the addition of CF and NMK can enhance the durability of RAC more significantly than that of the two materials separately.(5)Selects the MIP,nitrogen adsorption method and SEM method to analysis the CF to NMK the mechanism of the recycled concrete.The results show that the incorporation of CF leads to the increase of pores in RAC,but the incorporation of NMK at the same time fills part of the increased pores,resulting in a small increase in porosity.The separation and fracture resistance effect of CF refines the pore structure of RAC,and the disorderly distribution of CF makes the fracture propagation path more complicated and difficult to form through-fracture,thus improving the ductility of RAC.CF bridged on both sides of the crack effectively alleviates the stress concentration phenomenon at the crack and plays a role in stress transfer and bearing,resulting in the matrix becoming a whole under common stress.After the incorporation of CF and NMK at the same time,due to the good filling and volcanic ash effect of NMK,the adverse effects caused by the incorporation of CF are eliminated,and the strength of the interfacial transition zone is enhanced,so that the CF and matrix bond more closely and can better play its bearing role.The two materials complement each other and jointly enhance the mechanical and durability of RAC.