Study On Interfacial Bonding Property And Freeze-Thawing Damage Model Of The Concrete Repaired With Alkali Activated Slag/Fly Ash

As the weakest part of new-to-old concrete,adhesive interface is an important factor influencing the functionality and durability of new-to-old concrete structure.In order to make full use of industrial solid waste and considering the strong bonding effect of slag and fly ash mixed cementitious material under the action of alkali activator,the alkali activated slag/fly ash was applied to repair old concrete.In this thesis,the modification mechanism of new-to-old concrete adhesive interface under different slag and fly ash mass ratio was studied.The bond property and durability of the interface between old and new concrete were studied by splitting tensile test,uniaxial compression test,shear test,ultrasonic nondestructive test,freeze-thawing cycle test,chlorides penetration test and high temperature resistance test.X-ray diffraction（XRD）,thermogravimetric analysis（TG-DTG）,scanning electron microscopy（SEM）and mercury intrusion method（MIP）were used to study the hydration degree,interface microstructure and pore evolution of the adhesive interface systematically.In freeze-thawing and high temperature environment,the deterioration of strength of new-to-old concrete with different repairing materials was analyzed,and the damage of adhesive surface in mesoscale was evaluated combined with ultrasonic nondestructive test results.Through comprehensive analysis of strength and durability,the repair performance of various materials was determined,and the most appropriate slag and fly ash content was given.Through the comprehensive comparison of the repaired interface microscopic morphology under initial state,freeze-thawing cycles and high temperature environment,the deterioration characteristics of the adhesive interface under different freeze-thawing cycles and different temperatures were summarized.Fractal theory was applied to describe the pore evolution quantitatively under different environments and the XRD and TG-DTG tests were combined to prove the advantage of alkali activated slag/fly ash as repair material in terms of pore distribution,phase composition and hydration product development.Based on the S-N equation proposed by Aas-Jakobsen considering the influence of constraint stress on the fatigue strength of concrete,combined with the constitutive stress-strain relationship of damaged concrete,the damage variable was expressed by the time of freeze-thawing cycles and the limit time of freeze-thawing cycles,and the phenomenological model of freeze-thawing damage of new-to-old concrete was established preliminarily.Since the most direct factor affecting the deterioration of new-to-old concrete was the porosity,the porosity was further taken as an internal factor.According to The Damage theory of Kachanov-Rabotnov and the effective bearing area of pores,a prediction model of adhesive interface degradation based on pore fractal was established.The main work and achievements are as follows:（1）The splitting tensile strength of concrete increased linearly with the increase of surface roughness（or fractal dimension）under same old concrete age.When the age of old concrete reached more than 60 days,the change of old concrete age had little influence on the splitting tensile strength.With the old concrete age getting shorter,the splitting tensile strength of new-to-old concrete increased.When alkali activated slag/fly ash was used as repairing material,the bonding property of new-to-old concrete was improved obviously.When the repairing material was 50%slag and 50%fly ash（GPC50）,its bonding property with old concrete was the highest,and the splitting tensile strength was73.8%of the complete concrete.Compared with ordinary Portland concrete repair,the splitting tensile strength was increased by 62%.（2）With the increase of the adhesive interface roughness,the fractal dimension of the adhesive interface increased,and the frost resistance of the new-to-old concrete was improved.Under the same freeze-thawing times,the new-to-old concrete repaired with GPC50 had the least defect,the least harmful hole content,the largest compactness and the strongest freeze-thawing resistance.In terms of chlorides penetration resistance,proper amount of fly ash in the slag-fly ash binary system could reduce the harmful pores at the bonding interface and provide additional bonding at the interface effectively,and the addition of slag was the guarantee of the overall strength of the new-to-old concrete structures.The new-to-old concrete repaired with GPC50 had the best chlorides penetration resistance,and the existence of the adhesive interface led to the decrease of chlorides penetration resistance of new-to-old concrete structures obviously.（3）In the high temperature environment,the bonding property of concrete repaired by alkali activated slag/fly ash before 200℃was higher than that of room temperature.The bonding property of concrete repaired by alkali activated slag/fly ash after 200℃was lower than room temperature,and then decreased with the increase of temperature obviously.The main reason was that the high temperature before 200℃promoted the further reaction between alkali activated slag/fly ash and Ca（OH）₂ in old concrete at the adhesive interface,forming a large number of C-S-H,C-A-S-H and N-A-S-H gelations,which was the key to improve the bonding performance of alkali activated slag fly ash repaired concrete.At the same time,the harmful holes at the repairing interface were inhibited under 200℃,and the volume content of the corresponding transition holes increased significantly.The volume of the transition holes at the repairing interface at100℃was increased by 8.70%compared with room temperature.（4）The fractal dimension of pore surface was closely related to the time of freeze-thawing cylces,which could be used to evaluate the bonding strength of the adhesive interface.According to the functional relationship between(D_s,FT-D_s,min)/(D_s,0-D_s,min)and freeze-thawing cycle times,the fractal dimension was transformed into freeze-thawing cycle times through linear change.The prediction model of adhesive interface deterioration based on pore fractall reflected the changes of bonding properties and defects of new-to-old concrete under freeze-thawing environment.The established model reflected the macroscopic mechanical properties from the evolution characteristics of micro pores and predicted the change of adhesive interface freeze-thawing deterioration with limited measuring points.