Study On Compressive Behaviors And Microstructure Of Recycled Blend Concrete At Early Age

With the rapid development of society and economy, a large number of buildings have been built and demolished which has made much waste concrete, causing great damage to the environment. In order to protect the natural environment and realize the sustainable development of the construction industry, it is imperative to recycle the waste concrete to reduce the production of new concrete. Recycled blend concrete (RBC) is a new way of recycling waste concrete and the RBC is directly mixed with the new concrete by large size blocks or section of waste concrete. The use of RBC which is more simple, efficient and energy saving simplifies the process of recycling waste concrete. Based on the previous research,this paper carried out the age and the demolished concrete block replacement ratio influence on compressive behaviors of RBC, using scanning electron microscopy on the microstructure of mixed recycled concrete which was observed and analyzed, mainly including the following work:Total 60 standard cubic specimens and 60 standard cylinder specimens were prepared for uniaxial compression test. Meanwhile the effect of different replacement rates of demolished concrete blocks, different age and other factors on RBC physical properties were conducted which including compressive strength, elastic modulus and peak strain. Based on the experimental results, the relationship between the compressive strength, the elastic modulus and the peak strain of the RBC is obtained. The results show that: (1) If the compressive strength of demolished concrete blocks is higher than that of the new concrete, the compressive strength of RBC increases with the increasement of replacement rate. With the increasement of replacement rate, the elastic modulus increases first and decreases lately while the peak strain behaves in the contrast way; (2) Both the compressive strength and modulus of RBC increase with age, while the peak strain decreases with age; (3) The effect of demolished concrete blocks replacement rate on cylinder specimen is lower than that on the cube specimen, and the compressive strength conversion factor of RBC is less than the compressive strength conversion factor of normal concrete; (4) All of the empirical formulas are in good agreement with experimental data.Scanning Electron Microscopy (SEM) was used in this study to observe the microstructure of RBC at different ages. Comparative analysis of the morphology of the new and old cement matrix and its hydration products were conducted. Comparative analysis was also performed at new - old cement matrix interface transition zone, new cement - new aggregate interfacial transition zone and the old cement - old aggregate interfacial transition zone. The quantitative analysis has been taken to investigate the hydration products of new and old cement matrix and the new - old cement matrix interface transition zone. Based on the results of compression test,the micro mechanism for affecting the compressive behaviors of RBC was analyzed. The test results show that: (1) New concrete could combine with demolished concrete blocks well. The new-old cement matrix is dense and the interfacial transition zone between the old cement matrix and the old aggregate is the weakest region of the RBC. (2) With the increasement in age, the hydration products, C-S-H gel, Ca(OH)2 and AFt crystals, growing and overlapping and filling up the crack make the matrix structure much denser also enhance the compressive behaviors of RBC.