The Influence Of Recycled Aggregate On The Durability Of Cement Concrete And Corresponding Improvements

The reconstruction of urban roads,the demolition of houses and many other municipal projects generate a large amount of demolished concrete.Simple disposals like landfill or discard could cause environmental and economic problems.After the process of crush,grading,decontamination,demolished concrete can be reused as recycled aggregate in concrete,which avoiding not only economic waste,but also environmental pollution,hence such recycling has broad application prospects.Aimed at the deficiencies of current researches about recycled concrete,this thesis concentrates on the experimental research about the durability of recycled concrete,then on this basis improvement measures are proposed and verified.In this thesis,the method to prepare recycled aggregate is a combination of manual and mechanical crush.The prepared recycled aggregate are handled with four types of surface treatments,then the apparent density,water absorption,moisture content and crush index of the recycled aggregate are measured.Pore size analyzer are used to analyze the structure and dimension of the pore from microcosmic perspective.The experiment results indicate that the basic property of the recycled aggregate is poor.By handling with polyvinyl alcohol,organosilicon based hydrophobic agent and hydrochloric acid,the basic property of the recycled aggregate can be improved,however,due to large water absorption,the recycled aggregate cladded by cement paste is not suitable for concrete.The replacement rate of recycled coarse aggregate is 100%.The recycled concrete with different mixture ratio is devided into several groups according to different water cement ratio,surface treatments and mineral admixtures.The workability and mechanical property of the samples are experimentally studied,after that,the causes for differences among the samples are analysed.The results of the study show that the workability of the recycled concrete is worse than that of ordinary concrete,which can be improved by mineral admixtures and surface treatments;the mechanical property of recycled concrete is poor,recycled aggregate has a greater impact on the fracture resistance of concrete than on the compression resistance of concrete.After handled with polyvinyl alcohol and hydrochloric acid,the recycled aggregate get a better mechanical property,while the treatment using organosilicon based hydrophobic agent is unfavourable for the mechanical property of recycled aggregate.Since coal ash has great influence on the early strength of recycled concrete,high mixing amount of coal ash is not suitable.This thesis experimentally studies the penetration resistance,freezing resistance,sulfate corrosion resistance of prepared recycled concrete,to solve the problem caused by deicing salt,experiment of resistance to concrete sulfate attack is designed.Besides the failure mechanism of durability,the influence of water cement ratio,surface treatment and replacement rate of mineral admixtures on the durability of recycled concrete are studied.The experiment results indicate that the penetration resistance,freezing resistance,sulfate corrosion resistance of recycled concrete are not as well as those of ordinary concrete,but the resistance to concrete sulfate attack of recycled concrete is closed to that of ordinary concrete.The recycled aggregate handled with organosilicon based hydrophobic agent acquires better durability,while the hydrochloric acid-immersed and adhering mortar-removed recycled aggregate acquires worse freezing resistance.The double mixing of coal ash and slag could both ensure the strength and improve the durability of recycled concrete.At last,scanning electron microscope is used to observe the interfacial transition zone and hydrates of recycled concrete,the flaw of recycled concrete is analysed from the microcosmic aspect and the macroscopic conclusions are verified.