Effect Of Mixing Approaches And Recycled Aggregate Types On Properties Of Self-compacting Concrete

Self-compacting concrete(SCC)refers to a special concrete that can flow under its own weight and fill formwork easily even in the presence of a high density of reinforcement bars,without mechanical vibration or undergoing any excessive segregation and bleeding.Unlike natural aggregate,the surface of recycled concrete aggregate(RCA)contains old cement mortar resulted in higher porosity and water absorption and low mechanical strength.SCC containing recycled aggregates have poorer workability,hardened properties and durability.In order to promote the utilization of recycled aggregates and self-compacting recycled concrete(SCRC),it is important to study the properties of SCC with recycled aggregate and find an economical and practical way to improve them.This research focuses the following three aspects: First,the influence of RCA content on the workability,compressive and flexural strengths,drying shrinkage,water absorption and permeable voids of SCC;second,the effects of slurry-wrapped aggregate approach,two-stage mixing approach and pre-carbonation method on the workability,compression and flexural strengths,drying shrinkage,water absorption and permeable voids and Interface transition zone of SCC prepared with RCA;third,the effect of using different types recycled aggregates(recycled red brick aggregate,recycled block aggregate,recycled concrete aggregate)as 100% nature aggregate replacement on the workability,compression and flexural strengths,drying shrinkage,water absorption and permeable voids,as well as high temperature performance.The results of first part show that workability of the SCC prepared with RCA can meet the requirements by the use of superplasticizer and the fluidity of the SCC decreases when the replacement rate of RCA increases.The compressive strength and flexural strengths decrease with the use of RCA.And the maximum reduction of 28 d compressive and flexural strengths is within 15%.The dry shrinkage,water absorption and permeable voids of SCC increase as the replacement ratio of RCA increases.Compared with normal SCC,the 90 d dry shrinkage of SCRC with 100% substitution increased 36.7% and the water absorption increased 30%.The results of second part show that compared with normal mixing approach,the slurry-wrapped aggregate approach and two-stage mixing approach can improve the workability,compression and flexural strengths and reduced the drying shrinkage,water absorption and permeable voids.The interfacial transition zone of SCRC formed by these two approaches has a larger microhardness value and it is more dense.The pre-carbonation method can also improve the compressive and flexural strengths,dry shrinkage,water absorption,permeable voids and interface transition zone.The results of third part show that the workability of SCC prepared with RCA,red brick aggregate or block aggregate at 100% replacement ratio can meet the requirements by the use of superplasticizer.Regardless of recycled aggregate type,the compressive strength and flexural strength of SCRC were lower than that of natural aggregate SCC.And the 28 d compressive strength of SCC with recycled block aggregate reduced less than 10%,the reduction of recycled red brick aggregate group is as high as 30.93%.The dry shrinkage,water absorption and permeable voids of SCRC were higher than natural aggregate SCC.Among the aggregate type,recycled red brick aggregate group caused the largest dry shrinkage,water absorption and permeable voids.After exposure to high temperature,the compressive strength loss rate of SCRC is smaller than that of natural aggregate SCC.SCC with recycled red brick aggregate has the best high temperature resistance.