The degradation of reinforced concrete structures due to chloride penetration has a significant influence on structural durability. In this study, an experiment is conducted to investigate the effects of the aggregate type, mixing method, and admixture type on the chloride penetration resistance of concrete. Based on the experimental results, it is found that, since more pores and microcracks are included in recycled aggregates, the chloride penetration resistance of recycled aggregate concrete is larger than that of natural aggregate concrete. Slag is more efficient than fly ash in increasing the chloride penetration resistance of recycled aggregate concrete. It is also found that the mixing method has a significant influence on the chloride penetration resistance of recycled aggregate concrete. These conclusions can provide theoretical evidence for the design optimization of recycled aggregate concrete properties.

A novel triple mixing method (TM) was developed to realize surface-coating of aggregate with pozzalanics materials for further improving microstructure of interfacial transition zone (ITZ) and properties of recycled aggregate concrete (RAC). Effects of mixing method and coating by various admixtures on compressive strength and chloride ions penetration resistance of RAC were investigated. Through preparing the new–old–new concrete sandwich specimen and SEM observation of the old concrete surface after fracture, effect of surface-coating of the recycled aggregate (RA) in RAC with various admixtures on ITZ microstructure was also studied. The results showed that properties of RAC can be further enhanced by using TM, as compared to that of using the double-mixing method (DM). Through SEM observation, it is revealed that the coated pozzalanic particles can consume CH accumulated in the pores and on the surface of the attached mortar to form new hydration products, which can not only in situ strengthen the RA, but further improve the microstructure of the ITZ, thus the strength and durability of the RAC was further enhanced.

研究了采用掺合料浆液包覆处理老混凝土表面对新老混凝土界面微观结构与性能的影响,结果表明,与未经界面处理相比,采用质量浓度为20%的粉煤灰浆液处理的新老混凝土界面粘结试样3 d和28 d界面剪切强度可提高45.1%和18.2%,经20%矿渣浆液界面处理试样各龄期界面剪切强度分别提高达74.2%和69.2%.扫描电镜分析表明,未经界面处理试样老混凝土表面孔隙和新老混凝土界面处存在CH富集现象,而经粉煤灰和矿渣掺合料浆液包覆处理后,在老混凝土表面及孔隙中吸附的微细掺合料粒子,可吸收在新老混凝土界面区和老混凝土表面孔隙中富集的CH,在老混凝土表面孔隙中和新老混凝土界面处形成对界面粘结强度有贡献的水化产物,从而提高界面粘结强度.

A novel mixing method was applied to realize surface coating of aggregate by admixture for further improving microstructure of interfacial transition zone(ITZ) and properties of recycled aggregate concrete(RAC).Effects of mixing methods,slump of the fresh concrete and coating by various admixtures on compressive strength and chloride ions penetration resistance of RAC were investigated.The results show that compressive strength and chloride ions penetration resistance of the concrete can be further enhanced through surface coating of admixture around the aggregates during mixing,as compared to that by the surface-coating-by-cement mixing method,especially at early ages.Increasing slump of the fresh concrete helps enhance compressive strength but weakly harm the chloride ions penetration resistance of the hardened RAC for both mixing methods.However,the negative effects of increasing slump on properties of RAC can be reduced by using the surface-coating-by-admixture mixing method.Using slag as admixture helps improving properties of RAC as compared by using fly ash.