Experimental Study On The Bond Properties Between Square Steel Tube And Recycled Coarse Aggregate Concrete With Machine-made Sand

Using recycled coarse aggregate and machine-made sand as coarse and fine aggregate instead of natural aggregate to prepare concrete can not only alleviate the contradiction between the depletion of natural sand and gravel resources and the increasing demand of engineering construction to a certain extent,solve the environmental problems caused by overexploitation of natural river sand,but also properly handle waste concrete,avoid occupying land and polluting the environment,and realize the recycling of construction waste resources at the same time.The recycling of material resources conforms to the concept of green environmental protection and sustainable development.Concrete-filled steel tubular structure has many advantages,such as high bearing capacity,good seismic resistance,good fire resistance,simple construction and good economic benefits.Its application advantages are particularly prominent in structural engineering.At present,recycled coarse aggregate concrete with machine-made sand is mostly used for non-bearing components,but seldom used in structural engineering.In this paper,the machine-made sand recycled coarse aggregate concrete is poured into square steel tube to form a new composite structure-machine-made sand recycled coarse aggregate concrete filled square steel tube.It opens up a new way for expanding the application of recycled coarse aggregate concrete with machine-made sand in structural engineering and has a wide application prospect.There are many experimental studies on the bond properties of concrete filled steel tubular at home and abroad,but most of them are carried out around ordinary concrete filled steel tube,while there is no report on the bond properties between square steel tube and recycled coarse aggregate concrete with machine-made sand.In this paper,192 standard cubic compressive specimens of recycled coarse aggregate concrete with machine-made sand and 12 standard cubic compressive specimens of recycled coarse aggregate concrete with natural sand were made.Water cement ratio,fine aggregate type and stone powder content were taken as factors to study the influence of various factors on the compressive strength of recycled coarse aggregate concrete with machine-made sand,and the relationship model between compressive strength of recycled coarse aggregate concrete with machine-made sand and water cement ratio and stone powder content was established.At the same time,32 push-out specimens of machine-made sand recycled coarse aggregate concrete filled square steel tube and 16 push-out specimens of natural sand recycled coarse aggregate concrete filled square steel tube were manufactured.The influence of various factors on the interface bond strength of push-out specimens was studied,taking concrete strength,width-thickness ratio,raw material form of machine-made sand(limestone/pebble)and stone powder content as factors.The main research contents are as follows:(1)The load-slip curves(P-S curves)of the specimens are obtained through push-out tests,and the shape characteristics of P-S curves are analyzed and summarized.(2)The strain of square steel tube and the relative slip between square steel tube and concrete are measured by the longitudinal uniform strain gauge and displacement sensor.The distribution law of strain along the height direction of square steel tube is analyzed.According to the differential relationship between strain and bond stress,the longitudinal distribution law of bond stress is obtained.(3)The influence of concrete strength and width-thickness ratio on the interfacial bond strength of natural sand recycled coarse aggregate concrete filled square steel tube is studied.(4)The influence of concrete strength,width-thickness ratio,raw material form of machine-made sand and stone powder content on interfacial bond strength of machine-made sand recycled coarse aggregate concrete filled square steel tube is studied.(5)The average bond stress-slip curves at the free end of machine-made sand recycled coarse aggregate concrete filled steel tube and natural sand recycled coarse aggregate concrete filled steel tube are analyzed,and their bond-slip constitutive models are proposed and validated.At the same time,the position function F(x)related to slip and the position function G(x)related to bond stress are introduced to establish the bond slip constitutive relation considering the change of position.