Experimental Study On The Effect Of Original Concrete And Mixing Method On Properties Of Recycled Concrete

Every year, there are lots of architectures were demolished in China because ofthe speeding up of urban construction, and at the same time, many cities have beendamaged by the earthquake and other disasters, and all of these have produced alarge number of waste concrete. Therefore, it has become one of one of the hotspotsthat how to reuse the waste concrete and the sustainable development of build ingmaterials. From a long-term point of view, it has important theoretical significanceand engineering application value about the development and application of thetechnology of recycled aggregate concrete.Compared with natural aggregate concrete, recycled aggregate concrete ismore complex, whose properties are influenced b y many factors. The source ofrecycled aggregates, namely the original concrete and the mixing method of mixtureare both the important aspects, which are rarely studied. Therefore, this topic aimsto study on the effect of original concrete and mixing method on properties ofrecycled aggregate concrete.Three kinds of original concrete with different design strength grade and agroup of air-entrained original concrete were produced, which were all crushed atthe age of60days to obtain seven different kinds of recycled coarse aggregates.And the relationship between recycled aggregates and the original concrete wereexplored by testing the basic properties of recycled aggregates.Then groups ofrecycled concrete were produced using these different recycled coarse aggregatesthat replaced natural coarse aggregate by100%, and in the meantime three kinds ofmixing methods were used (normal mixing method and two double mixingmethods). The mechanical properties tests were carried out after28days. The testfor resistance of these recycled concrete to rapid freezing and thawing was alsostarted at the same time, and four kinds of evaluation index, i.e. weight loss, relativedynamic modulus of elasticity, ultrasonic relative dynamic modulus of elasticity andstrength loss rate wre used to evaluate the frost resistance of recycled concrete.A microscopic investigation was carried out on some specimens of recycledconcrete, which were sampled by cutting slices when different freeze-thaw cycles.Through the micro statistics to each slice, the cracking degree of every componentin recycled concrete were quantitatively evaluated by two microstructural damageparameters：mortar cracking ratio and interface cracking ratio. The results reflectedthe freeze-thaw failure mechanis m of recycled concrete, and the influencemechanism on frost resistance of recycled concrete when original concrete wasair-entrained. The results showed the quality of recycled aggregate was effected by theproperties of original concrete, resulting in a significant influence on the strengthand frost resistance of recycled aggregate concrete. The quality of recycledaggregate that came from origina l concrete with higher strength was better, leadingto the higher strength and better frost resistance o f recycled aggregate concrete.When the original concrete was air-entrained, on one hand, the strength of recycledaggregate became lower, which led to the lower strength of recycled aggregateconcrete, and on the other hand, the pore structure of the adhe red mortar of recycledaggregate was more reasonable, and the cracking of the adhered mortar in theprocess of freezing and thawing is less, which significantly improve the frostresistance of recycled aggregate concrete. The two double mixing methods couldimprove the strength of the recycled aggregate concrete, but there was littleinfluence on its frost resistance.This topic is a sub-project of―Experimental study and design method for thecomposite mechanical properties of recycled concrete and structural mechanicalproperties of recycled concrete under the freeze-thaw environment‖(Projectapproval No.51278151) funded by National Natural Science Foundation.