Brick-Concrete Recycled Coarse Aggregate Self-Compacting Concrete Performance Study

The use of brick-concrete construction wastes to prepare brick-concrete recycled coarse aggregates is an effective way to reuse these solid wastes.The recycled coarse aggregate prepared from mechanically crushed brick concrete waste has good particle gradation and shape,which provides a possibility for the preparation of self-compacting concrete with medium and low strength grade.However,brick-concrete recycled coarse aggregate has more open pores,large porosity,low apparent density and low cylinder compressive strength,which will adversely affect the workability and durability of concrete.In the actual production process of concrete,the use of brick-concrete recycled coarse aggregate is mainly based on pre-wetting treatment and adding additional mixing water,but at present,there is little research on the influence of different pre-wetting degree of brickconcrete recycle coarse aggregate on the performance of concrete and its mechanism.In this paper,the properties of brick-concrete recycled coarse aggregate were investigated,selfcompacting concrete was designed and prepared,and the effects of different pre-wetting degrees of brick-concrete recycled coarse aggregates on the workability,physical and mechanical properties and durability concrete were studied.The main results are as follows:(1)The brick-concrete recycled coarse aggregates with particle size of 5～10 mm and10～20 mm are mixed by mass ratio of 5.5:1,which can meet the requirements of 5～20 mm continuous gradation.The sphericity of brick-concrete recycled coarse aggregate is approximately the same as that of natural gravel,but the water absorption rate is higher,up to 13%,which is much higher than that of natural aggregate and ceramsite light aggregate,and the water absorption rate increases rapidly within 10 min.The range of water absorption of brick-concrete recycled coarse aggregate concrete with different pre-moisture degrees is6.8 %～7.8 %,and the range of softening coefficient is 0.77～0.87.(2)Compared with natural aggregate concrete,the use of dry and air-drying bricks to mix recycled coarse aggregates and add additional mixing water,or the use of saturated surface dry coarse aggregates can improve the initial slump of concrete,but the slump loss of concrete when using dry bricks-concrete recycled coarse aggregate is larger.Compared with the dry and saturated surface dry state,the static yield stress and dynamic yield stress of the air-dry state of brick-concrete recycled coarse aggregate increase the least within 60 min,and change steadily.At the same time,the 0min,30 min and 60 min plastic viscosity of the air-dried state brick-concrete recycled coarse aggregate is the minimum.(3)Compared with natural coarse aggregate,the compressive strength and splitting tensile strength of concrete prepared with recycled brick-concrete coarse aggregate decreased.Compared to the dry and saturated surface dry state,the adverse effect of air-dried state brickconcrete recycled coarse aggregate on the 7d,28 d and 60 d compressive and splitting tensile strengths of concrete were less.The linear correlation between splitting tensile strength and compressive strength of brick-concrete recycled coarse aggregate concrete is poor.(4)The autogenous shrinkage of concrete can be reduced and the structure of ITZ can be improved by using saturated water or additional water technology for the recycled coarse aggregate of brick-concrete.Compared with dry and saturated surface dry state,the autogenous shrinkage of brick-concrete aggregate concrete in aired-dried state is the smallest within 28 d.The moisture absorbed inside the recycled coarse aggregate of brick concrete has a certain internal curing effect on the concrete in the later stage,which can reduce the shrinkage of the concrete.(5)Compared with natural coarse aggregate,the freeze-thaw resistance and chloride ion penetration resistance of brick-mixed recycled coarse aggregate concrete are poor.Compared with the dry state and the saturated surface dry state,the air-dry state brickconcrete recycled coarse aggregate has the least harmful pore size(> 50 nm)and pore volume,the 28 d electric flux value is the smallest,and the resistance to chloride ion penetration is the best.