Properties Of Recycled Fine Aggregate Self-compacting Concrete

Recycling waste concrete is an effective way to solve the environmental and land occupation issues caused by waste concrete.While significant amount of studies have been conducted on recycled concrete,very limited research is carried out to study the concrete with recycled fine aggregate(RFA).Therefore,most RFA is not reused.This research plans to use RFA to replace nature sand in producing self-consolidating concrete(SCC).RFA could serves as an internal curing agent due to its high water absorption,which could reduce shrinkage of SCC.This paper selects different kind of origin concrete to prepare recycled fine aggregate,and the basic properties of recycled fine aggregate were tested,including the internal pore structure,water absorption/desorption characteristics,density and other properties.Three kinds of recycled fine aggregates were selected to replace natural sand(25%,50%.75%,100%replacement rate)to prepare self-compacting concrete,and the effects of different types,replacement rate and pre-wetting time of recycled fine aggregates on workability,mechanical properties,durability and shrinkage of self-compacting concrete were researched.Workability included slump expansion,T500.J-ring test.L-box test,etc.;mechanical properties included compressive strength,flexural strength,splitting tensile strength,elastic modulus,etc.;durability included chloride ion flux of concrete,freeze-thaw resistance,etc.;shrinkage properties included self-shrinkage,dry shrinkage and restrained shrinkage.Based on the internal curing effect of recycled fine aggregate,the spatial and temporal distribution of the internal humidity field,pore structure and hydration characteristics of cement stone-aggregate interface area of the recycled fine aggregates self-compacting concrete were researched through experiments and theoretical analysis.The relationship between concrete shrinkage and internal relative humidity change was established,and the influence law and mechanism of recycled fine aggregate on the performance of self-compacting concrete was revealed.The following findings and conclusions can be drawn from this study:(1)Recycled fine aggregate mortar has high strength.At the same time,recycled fine aggregate has a certain internal curing effect,which could increase the relative humidity inside the mortar and reduce the drying shrinkage of the mortar.(2)Recycled fine aggregate reduced the workability and strength of self-compacting concrete.However,the workability of recycled fine aggregate self-compacting concrete meets SF2 level.Concrete containing RFA could reach 50MPa at 28 days,which could be used for general ordinary reinforced concrete structures.At 28 days of age,the splitting tensile strength of the recycled fine aggregates self-compacting concrete ranged from 4.27 to 5.07 MPa,the flexural strength ranged from 7.39 to 8.51 MPa,and the elastic modulus ranged from 28.9 to 34.67 GPa.(3)Recycled fine aggresates reduced the durability of concrete.Dry recycled fine aggregate could improve the freeze-thaw resistance of concrete.However,the saturated recycled fine aggregate reduced the durability of concrete,but it could still withstand at least 200 freeze-thaw cycles.At 28 days of age,the recycled fine aggregate self-compacting concrete had a chloride ion permeability between low and medium,with electric flux range of 877 to 2346 C.The electric flux of 56d recycled fine aggregate self-compacting concrete ranged from 412 to 1015C.and the recycled fine aggregate self-compacting concrete had good resistance to chloride ion penetration.(4)The water absorbed by the recycied fine aggregate would be released when the relative humidity inside the concrete reduced,so that the internal relative humidity of the concrete maintained at a high level,and the internal curing effect of recycled fine aggregate effectively reduced the shrinkage of the self-compacting concrete and the risk of constrained cracking.(5)The water released effect of the recycled fine aggregate effectively promoted the hydration of the interface cement stone and make the structure of the interface zone more dense.The mercury intrusion porosimetry test showed that the interface area cement stone of recycled fine aggregate content with pore diameter of 0nm-20nm decreased significantly,the average pore diameter decreased,and The pore structure is refined.The TGA-DTG results indicate that the hydration degree of cement stone in the interface area of recycled fine aggregate is higher than that of ordinary concrete,and the degree of hydration increases with the increase of saturation of recycled fine aggregate.