Experimental Research On Freeze-thaw Shear Of New And Old Concrete Interfaces

The repair and strengthening of concrete structures has become an important content of project field.The shear resistance of the interface between new and old concrete has been widely existed in the repair and strengthening of concrete structures.In addition,considering the serious freezing damage of concrete structures in the north and other areas,it is of great significance to study the frost-resistant shear properties of the interface between new and old concrete.This paper mainly considers the influence of freeze-thaw on the interfacial bonding performance of new and old concrete.The shear tests are carried out on the new and old concrete interfaces with different rates of bonded rebars,different temperatures and methods of freeze-thaw,different methods of interfacial treatment,different grades of concrete strength and whether to brush interface agent.The main contents are as follows:The frost-resistant shear tests of smooth,chiseling,chiseling and bonded bars,and intact concrete blocks(i.e.no interface)are carried out.The smooth interface is divided into brushing and Non-brushing cement paste;the depth of chipping is in the range of 2-3 mm;the diameter of planting bars is 8 mm,10 mm,12 mm,and the number of planting bars is 1;the center temperature of freeze-thaw in new and old concrete blocks with 8 mm planting bars is-6 ～5℃,-12 ～5℃ and-18 ～5 ℃ respectively.At the same time,the block quality and transverse fundamental frequency of the rapid freeze-thaw test are measured,the failure modes of the interface are observed during the loading process,the rebar-planting block is subjected to secondary loading,some blocks with rough interface are subjected to freeze-thaw in air,and the CT scanning test of the same test block is carried out after different freeze-thaw conditions,and the influence of different factors on the shear strength of the interface between new and old concrete is obtained.Through the test methods,some useful conclusions are drawn in this paper: the mass of test block increases first and then decreases within 25 freeze-thaw cycles,and the loss of relative dynamic elastic modulus is small;the failure of rough interface is brittle,while the interface of bonded bar is ductile,and the smooth interface basically loses its bearing capacity after freeze-thaw damage;within a certain range,the ratio of planting steel,the strength of new concrete are directly proportional to the shear strength of interface,while the absolute value of freeze-thaw negative temperature and the number of freeze-thaw times are inversely proportional to the shear strength of interface;secondary loading test finds that the strength loss of most specimens is less than 50%,and ductility characteristics are obvious;air freeze-thaw(35 times)has little effect on the shear resistance of interface;comparing the CT slices of concrete at interface and non-interface,it is found that there are more pores with wider distribution,fewer aggregates and more mortar at interface.With the increase of freeze-thaw times,the porosity and pore number at interface first increase and then stabilize,and the lower the negative freeze-thaw temperature,the larger the porosity increase.Compared with non-interface,the pores at interface are more and the average pore diameter is smaller.