Study On Bonding Surface Characteristics Between Recycled Concrete And Ordinary Concrete

With the continuous deepening of China’s urbanization process,the construction of projects,demolition of old buildings,and urban renovation in recent years have produced a large amount of construction waste.Due to the difficulty of transporting and degrading construction waste,resource utilization is superior to conventional on-site landfilling.For example,crushing construction waste to produce recycled aggregate for the production of recycled concrete not only facilitates the treatment of construction waste but also reduces the exploitation of natural resources,while meeting the requirements of economic benefits and social benefits.Given the complex sources and varying qualities of recycled aggregate,the use of prefabricated production methods in the production of prefabricated components or products using recycled concrete ensures higher quality.Therefore,the application of recycled concrete in prefabricated components has become a new research direction.In the construction process of prefabricated buildings,a bonding interface is formed between recycled concrete prefabricated components and cast-in-place concrete.Due to the difference in properties between recycled concrete and ordinary concrete,as well as the influence of pouring sequence,the water-cement ratio near the bonding interface differs from that of the concrete matrix.In addition,the effect of water absorption and desorption of recycled concrete prefabricated components results in a higher porosity and some initial micro-cracks in the bonding interface,making it a weak link in the structure and threatening the overall integrity,load-bearing capacity,and durability of the structure.Currently,there is relatively little research on the bonding interface properties of prefabricated concrete in construction.This paper studies the bonding interface properties between prefabricated recycled concrete and cast-in-place ordinary concrete,including the early-age water migration characteristics,mechanical properties,and resistance to chloride ion erosion of the bonding interface,and provides a theoretical basis for the application of recycled concrete technology in prefabricated buildings.The main research contents are summarized as follows:(1)The water migration process at the bonding interface between prefabricated recycled concrete and cast-in-place ordinary concrete within 24 hours was studied.The surface of the recycled concrete was exposed with recycled aggregate after rough treatment,resulting in a higher porosity,which quickly absorbed water when it came into contact with cement slurry.After the water potential reached a balance between the recycled concrete and the cement slurry during the water absorption period,water absorption stopped.As the cement slurry hydrated,the negative pressure in the capillary of the slurry began to rise,and the recycled concrete entered the desorption period.As the relative humidity difference between the recycled concrete and the cement slurry decreased rapidly,the water potential between the recycled concrete and the cement slurry reached a balance during the desorption period,and the desorption ended.The water migration characteristics of the bonding interface between recycled concrete and ordinary concrete were affected by the pre-wetting degree of recycled concrete and the water-cement ratio of ordinary concrete.(2)Conducted splitting tensile and flexural tests on the bond specimens of precast recycled concrete and cast-in-place ordinary concrete.It was found that increasing the strength of recycled concrete reduced the water-cement ratio at the bond interface,reduced the particle size of crystal such as calcium hydroxide at the bond interface,improved the pore structure at the bond interface,and thus improved the bonding performance.The random distribution of steel fibers at the bond interface increased the roughness of the bond interface,reduced the shrinkage difference between recycled concrete and ordinary concrete,and greatly increased the mechanical interlocking force between the two types of concrete.Increasing the roughness of the bond interface also increased the contact area between recycled concrete and ordinary concrete,and cement slurry infiltrated into the old mortar and pores of recycled aggregate on the surface of recycled concrete and hydrated,providing additional mechanical interlocking force to the bond interface.The interface agent can adsorb on the exposed aggregate surface of recycled concrete,forming chemical bonds and physical adsorption layers,improving the adhesion and interaction forces between cement slurry and recycled concrete aggregate in ordinary concrete,and thus improving the bonding performance.(3)Through scanning electron microscopy(SEM)analysis of the bond interface between precast recycled concrete and cast-in-place ordinary concrete,it was found that there is an obvious transition zone in the bond interface,which is the weak area of the bonded structure.This transition zone is divided into three parts: the permeable layer,the strong reaction layer,and the gradient layer,with the strong reaction layer considered to have the most profound impact on the bond performance.Combined with macroscopic mechanical tests of bond specimens and microscopic analysis of bond interface structures,the bond mechanism between recycled concrete and ordinary concrete and the factors influencing bond behavior are summarized.(4)The chloride salt wet-dry cycle test was carried out on the bond surface between prefabricated recycled concrete and cast-in-place ordinary concrete,and it was found that the pores on the bond surface were the main pathway for chloride ion erosion.At the same depth of erosion,the chloride ion concentration on the bond surface was the highest,and after a distance of more than 35 mm from the bond surface,the chloride ion concentration no longer changed significantly.At the same distance from the bond surface,the chloride ion concentration decreased with the increase of erosion depth.The addition of steel fibers can significantly reduce the chloride ion concentration in the bond surface and the interior of the recycled concrete.Increasing the strength of the recycled concrete has a limited effect on reducing the chloride ion concentration on the bond surface.When the roughness of the bond surface increases from 1.5mm to 4.5mm,the chloride ion concentration on the bond surface decreases,but the chloride ion concentration on the left and right sides of the concrete increases.