Study On The Recycled Concrete Multi-interface Structure And The Damage Mechanism Of Performance

The surface of the recycled coarse aggregate prepared from waste concrete is attached by large numbers of low strength,high water absorption and weakly bound hardened cement pastes.Therefore,the multiple interfacial structures are produced in recycled concrete,which in turn leads to obvious deterioration of mechanical properties and durability of recycled aggregate concrete.Currently,the study on the performance of interfacial transition zone(ITZ)in recycled concrete is rarely reported,and these reports are simply for qualitative analysis of hydration products and microstructure in different ITZ of recycled concrete.However,it has not yet established the relationship between micro and macro properties of ITZ,so it is necessary to study characteristics of all kinds of interfacial structure in recycled concrete,to create scientific and reasonable research methods on the performance of microstructure,to explore the mechanism of performance degradation,and to find out the corresponding improvement process.Firstly,This thesis,supported by the National Natural Science Foundation of China(51578297),studies the content of attached mortar on the surface of recycled coarse aggregate with different quality,quantitatively analyzes the relationship between internal defects and basic performance,and then reveals the influence laws of strength difference of recycled concrete and quality of recycled coarse aggregate on the performance of recycled aggregate concrete.Sequentially,combined with the performance and characteristics of recycled coarse aggregate and recycled concrete,the reconstruction model of recycled concrete multi-interface structure is proposed,which has accurately distinguished and located the old interface,new interface of aggregate and new interface of mortar.Based on the reconstruction model of regenerated concrete multi-interface structure,the microcosmic research method of regenerated concrete multi-interface structure is proposed by using micro hardness tester,optical scanning and other testing techniques.Finally,the effects of carbonation,sulfate and chloride erosion on micro hardness,microstructure and hydration products of recycled concrete were systematically studied,the mechanism of performance degradation was quantitatively analyzed,and the improvement technology of recycled concrete was explored.(1)The effect of physical strengthening on the properties of recycled coarse aggregates are contrastively analyzed,and the content of mortar attached to recycled coarse aggregates with different quality are quantitatively analyzed by means of calcinations-grinding,and the calcinations temperature of recycled coarse aggregates are finally determined as 400°C,the grinding time is 20 min,and the content of mortars attached to Class I,II and III recycled coarse aggregates is 7.5%,22.8% and 39.7% in turn.Through good linearity between the attached mortar content and water absorption of recycled coarse aggregate,apparent density and crush index properties,the adhesive mortar content on the surface of the recycled coarse aggregate can be calculated accurately and the internal defects of recycled coarse aggregate are reflected intuitively.(2)By studying the influence law of recycled coarse aggregate quality and replacement rate on the performance of recycled concrete,the improvement technology and application method of recycled coarse aggregate quality were put forward.The replacement ratio and quality of recycled coarse aggregate have little effect on the performance of C30 concrete,but have a greater impact on the performance of C50 concrete.Compared with ordinary concrete,the 28 d compressive strength of C50 concrete made by low quality and high quality of recycled coarse aggregate mixture was reduced by 30.1% and 5.4% respectively when the replace rate was 100%.And the durable performance of recycled coarse aggregate concrete with high quality is relatively close to the natural aggregate concrete,it is superior to the recycled coarse aggregate concrete with low quality too.(3)Based on the performance and characteristics of recycled coarse aggregate,the reconstruction model of recycled concrete multi-interface structure is innovatively proposed,which can accurately distinguish and locate the old interface of recycled concrete,the new interface of aggregate and the new interface of mortar.Using waste concrete core sample instead of the recycled coarse aggregate with random distribution,casting the new mortar with different strength grade around the cylindrical core samples,a sample of regenerated concrete with multiple interfacial structure reconstruction is prepared.And the width and performance of the interfacial transition zone are quantitatively analyzed by combining the testing techniques of core section,microcrack observation,microhardness and scanning electron microscopy.(4)Using the reconstruction model of recycled concrete multi-interfacial structure,the mechanical property and durability of recycled concrete multi-interface structure with different strength grades were studied experimentally.The damage mechanism of regenerated concrete performance degradation was studied by using microhardness testing technology to quantify the performance of interfacial transition zone.Through the study of the micro-morphology and hydration products of the interfacial transition zone,the passage of corrosive medium into and destroy recycled concrete is clarified,and the invasion process,interface damage and durability degradation mechanism of different corrosive medium are revealed.In this present study,the weak link of the internal defects in recycled aggregate and multi-interface structure in recycled concrete are chiefly studied.The main factors affecting the performance of recycled concrete are grasped,the technical approaches to improve the interfacial structure of recycled concrete is explored.The mechanism of performance degradation of recycled concrete is revealed,the basic research theory of recycled concrete is further completed.It is hereby beneficial to the production and application of recycled concrete and of great significance to promote the development strategy of comprehensive conservation and recycling of resources.