Micromechanism Of Improving Mechanical Property And Durability Of Recycled Aggregate Concrete Based On Material Meso-structures And Micro-structures

The interfacial transition zone (ITZ) is the weakest part in concrete. To enhance its properties, a new mixing method named'W3T4'was proposed to modify the ITZ in recycled aggregate concrete (RAC). It involved three parts of adding water, which include the water in pre-wetting phase, and four stages of mixing procedures during making RAC. The improvement and micromechanism of recycled aggregate concrete (RAC) on mechanical properties, durability by using superfine pozzolanic materials and'W3T4'mixing method was illustrated from different aspects, for example, mechanical properties, resistivity of chloride ions, Scanning Electron Microscope (SEM) images and Energy Dispersive Spectrometry (EDX). The comparison results of strength, stress-strain curve and chloride diffusion coefficient for RAC (with different mixtures) and normal concrete (NC) show that RAC groups have lower strength and larger deformation than NC at 28 days'age, but there was a great promotion for RAC at 90 days due to the pozzolanic activity, and some RAC groups even have greater strengths than NC due to this activity. The apparent chloride diffusion coefficient of RAC was much lower than that of natural concrete even when the replacing level of natural aggregate by recycled aggregate was 50%. Steady state migration test was carried out in this study, and the results showed that there was a great promotion in the properties of mortar in RAC, and the effective chloride diffusion coefficient of mortar containing superfine phosphorous slag (PHS) was only 1/5 of the ordinary mortar. The experiments of RAC under different stress levels showed that the external loads can modify or cause some damages to the microstructures of recycled aggregate concrete (RAC), which affects the ingression process of harmful ions in concrete. The permeability of concrete under flexural load is much more sensitive than that under the same level of compressive load. At low compressive stress levels, the permeability of concrete decreased by closing the pores and micro-cracks within RAC. However, the chloride diffusion coefficient and the chloride content increased rapidly with the increase of compressive stress when it exceeded a threshold stress level of approximate 35% of the ultimate compressive strength in short time. However, under flexural stress, the chloride transport capability increased with the increase of stress level and time. The permeation of new ITZ in RAC was studied systematically through theoretical analysis and experiments, and a related formula was derived to calculate the chloride transport characteristics of ITZ in RAC. The results indicated that the chloride diffusion coefficient of new ITZ in RAC is about 1 to 4 cm2/year. The chloride diffusion coefficient of ITZ in normal RAC is about 10 times greater than that of ordinary mortar. When modified with PHS and superplasticizer, the permeability of chloride in new ITZ is 3 to 7 times greater than that in the ordinary bulk cement paste. PHS can modify the material structure of ITZ and the addition of PHS and superplasticizer not only reduces the chloride diffusion coefficient of mortar but also decreases the thickness and the chloride diffusion coefficient of new ITZ in RAC due to its pozzolanic reaction effect. Energy Dispersive Spectrometry (EDX) and Scanning Electron Microscope (SEM) methods were used to investigate the micromechanism of the improvement of RAC, which results showed that the PHS can modify the structure and the arrangement of pores in concrete by consuming the calcium hydroxide. The generation of C-S-H gel and ettringite enhanced the density and strengths of RAC.