Study On The Bonding Properties Of New And Old Concrete With Nano Silica Modified Interface Agent

The repairing and strengthening of concrete overlays involve the interfacial properties between existing and newly cast concrete.The most common repair method is the increasing section method.The survey results show that the interfacial agent and roughness greatly impact the interfacial bonding of new and old concrete.At present,the bonding effect of commonly used interfacial agents such as styrene-butadiene latex,epoxy resin,cement paste（sand）,and so on is not ideal,and the repair effect is weaker than the interfacial roughness factor.The interfacial chiselling treatment may cause disturbance and damage to the existing structure.Therefore,it is of great significance to develop a good interfacial bonding method using only interfacial agents without surface chiselling.At present,the research on the interface of new and old concrete mostly focuses on the short-term mechanical properties,and the research on the reinforce bond of concrete after long-term service,especially after carbonization,is relatively rare.In this paper,nano-silica materials are used in interfacial agents.The effects of nano-silica materials on interfacial adhesions were investigated via mechanical strength tests,endurance test and microscopic observations.The main contents and results are as follows:（1）In the mechanical test without carbonation treatment,oblique shear test and splitting tensile test are carried out for six interfacial agents（water,mortar,nano-Si O₂solution,nano-Si O₂mortar,styrene-butadiene latex and epoxy resin）at different ages（3d,7d,28d）.In the oblique shear test,the water interface agent group was treated with different interface roughness（2mm,4mm,6mm）.In addition,the impermeability tests of different interfacial agents were carried out.The results show that the interface shear effect of the NS mortar group is excellent,and 3.75%nano-Si O₂is the local optimal dosage.It’s 28d oblique shear strength is 27.3MPa is even better than the optimal group（6mm）of roughed specimens,and the anti permeability effect is good（2）Slant shear tests are conducted on carbonated concrete with different carbonization depths（1mm,2mm,3mm）.Five representative interfacial agents（water,mortar,3.75%NS solution,3.75%NS mortar,styrene-butadiene latex）are employed and compared accordingly.The experimental results showed that the hardening effects caused by carbonation could slightly improve the interfacial bond strength.Nevertheless,the nano Si O₂group presented monotonous downward trends with increased carbonation depth.The reductions in CH in the surface regions decrease the excitation effects of non-hydrated（residual）cementitious components on infiltrated nano-Si O₂particles.It is worth noting that the slant shear strength was still close to or even superior to the highest results of the rest groups.（3）The mechanical strength tests of nano Si O2-based matrix with different concentrations are carried out.In addition,the influences of strength gradients within the interfacial transition zones（existing/new concrete and interfacial matrix）on bond performances are tested.The results showed that The similar material property gradient in the interface transition zone is beneficial to improve the properties of the weak interface zone,which is beneficial in improving the coordination of the stress transfer path.（4）TEM showed that the nano-silica particles tended to enrich in high concentration solution.SEM observations demonstrated that the additions of nano-silica reduced the size and quantity of acicular Ettringite（Aft）and generated more network C-S-H gel,which was beneficial in increasing the density and compactness of interfacial regions.