Experimental Study On Interfacial Bonding Properties Of Concrete Reinforced By Different Types Of Amorphous Nano-SiO₂

The interface bonding of concrete is involved in the connection of assembled concrete components（such as laminated slabs,beam-column joints,plate-beam joints,etc.）,connection of assembled concrete pipe gallery and integrated pipe gallery components,cold joints and construction joints of cast-in-place concrete structures,reinforcement of concrete structures and other engineering fields.At present,the improvement of interface bond performance of concrete is mainly considered from interface treatment,reinforcement of inserts,interface chemical modification and so on.On the other hand,the test method of concrete interface bond strength mainly includes ASTM"C1583/C1583M","CECS 03:207 Technical specification for testing concrete strength by drilling core method"and other methods,but there are still a series of problems in the test program of drilling core disturbance and the need for special tensile machine.At the same time,foreign studies have shown that nanomaterials,especially sol amorphous nano SiO₂,can effectively improve the bonding force of concrete interface due to its good dispersion,high reactivity,large specific surface area and other characteristics,but the nano SiO₂ used in foreign studies has not disclosed its specific composition.Therefore,this paper first designed an experimental device for the bonding strength of concrete interface,and based on this,conducted a comparative study on the bonding performance of two kinds of sol-type nano-SiO₂ sold in China and self-made in the laboratory when applied to concrete interface.The specific work and results are as follows:（1）A direct tension test device was developed to study the interface bond strength of small concrete specimens.The results show that the device can be used to quickly prepare samples and avoid the influence of the disturbance caused by the traditional core method on the interface bond strength test.On the other hand,the test procedure is simplified,and the occurrence and development of concrete interface bond failure can be observed intuitively while the test load-displacement curve is obtained by combining with the ordinary tensile testing machine.（2）The liquid amorphous nano SiO₂ prepared by three different methods was comparatively studied.Namely,the enhancement effect of commercially available nano-SiO₂ prepared by chemical precipitation method（known as HGNS）,commercially available nano-SiO₂ prepared by metal hydrolysis method（known as JHNS）and nano-SiO₂ prepared by sol-gel method（known as RNNS）on the bond strength of concrete interface.The influence of nano-material concentration and interfacial retarder on interfacial bond strength was analyzed.The results showed that the interface tensile strength increased from 2.4mpa to 4.7mpa at 28 days by 95.3%compared with the control group（interface wet water treatment）when the interface was treated with 0.9%RNNS.Compared with complete concrete,the tensile strength of complete concrete can reach76.9%after 28 days.（3）The interface durability of nano-SiO₂ reinforced concrete was explored through freeze-thaw cycling and sulfate erosion resistance tests,and the effects of different types of nano-SiO₂ at the same concentration on the interface durability were studied.The results show that the strength loss and mass loss of the specimen are reduced compared with the control group after the interface strengthening treatment in freeze-thaw cycle and sulfate erosion test.Among them,the effect of RNNS treatment is the best.After 300freeze-thaw cycles,the interface tensile strength decreases by 33%and the mass loss is about 1.1%.During 90 days of sulfate erosion,the interfacial tensile strength decreased by only about 20%and the mass changed by about 3.75%.Compared with the control group,there is a certain improvement.（4）By means of SEM and EDS combined with the phenomenon and results of macroscopic experiments,the mechanism of concrete interface bond performance enhanced by nano-SiO₂ was explored.The results show that,on the one hand,the pore structure of the interfacial zone is improved;on the other hand,the secondary hydration reaction between nano-SiO₂ and Ca（OH）2 crystal in the interfacial zone of concrete promotes the formation of C-S-H gel in the pores,which makes the structure more dense and improves the tensile strength and interfacial durability.