Influence Mechanism Of RCA Quality On Mechanical Properties Of Concrete

As a low-carbon green building material,the application of recycled aggregate concrete in engineering has direct economic benefits,and at the same time solves social and environmental problems such as the accumulation of construction waste caused by the replacement of new and old buildings.On the other hand,it also effectively alleviates the current tension between supply and demand of high-quality natural aggregates.It has broad application prospects.However,due to the different sources of recycled coarse aggregate and the fact that the production process is not strictly uniform,the quality of recycled coarse aggregate is often quite different,which brings security risks to its application in practical engineering.In view of this,this paper,funded by the National Natural Science Foundation of China(51908244),studied the morphological characteristics of recycled coarse aggregates from different sources,quantitatively evaluated the quality of recycled coarse aggregates,and discussed its influence on the mechanical properties of recycled concrete.It is expected to reveal the strength formation mechanism of recycled coarse aggregate concrete and provide a scientific basis for the structural design and safety assessment of recycled concrete.The main research contents and results are as follows:(1)The differences in shape and physical properties of 4 types of recycled coarse aggregates and natural aggregates were investigated.The 3D models of natural coarse aggregate and 4 types of recycled coarse aggregate(Non-desizing road recycled coarse aggregate,desized road recycled coarse aggregate,urban construction recycled coarse aggregate and civil construction recycled coarse aggregate)were obtained by 3D laser scanning technology.Then,six shape index parameters that can quantitatively describe the apparent shape of coarse aggregates are determined,namely axial coefficient,flake coefficient,sphericity,uniformity,specific surface area and roughness.The physical property parameters of the coarse aggregate and the residual mortar content of the recycled coarse aggregate were obtained through experiments.The results show:The apparent shape of the recycled coarse aggregate is affected by the characteristics of the base material and the production process;Mechanical grinding and desizing can significantly improve the quality of the recycled coarse aggregate,both S_P and _RP of the road recycled coarse aggregate after desizing are increased by 6.3%and 13.5%respectively.The water absorption rate decreased by about 52%compared with that before desizing;The residual mortar content has an adverse effect on the physical properties of the recycled coarse aggregate,but due to the difference in the base material,the water absorption,apparent density and crushing index of the aggregate with a higher residual mortar content can also be better than those of the aggregate with a higher residual mortar content aggregate.(2)The influence of the shape and physical properties of recycled coarse aggregates on the basic mechanical properties of concrete was studied.The compressive and splitting tensile tests of 4 types of recycled concrete cubes at different ages were carried out.The 28d strength of recycled concrete with higher water absorption,larger crushing index and lower density of coarse aggregate can be increased by 5.5%and 5.0%respectively compared with ordinary concrete;With the increase of the sphericity and uniformity of the coarse aggregate,the strength of the concrete increases,the sheet coefficient is positively correlated with the compressive strength,and the axial coefficient is negatively correlated with the splitting tensile strength of the concrete.The smaller the water absorption,crushing index and residual mortar content,the higher the concrete strength.(3)The microstructural characteristics of four types of recycled coarse aggregate concrete were studied.Four types of recycled concrete microstructure tests at different ages were carried out,and the hydration products,pore structure,interface transition zone size and mechanical properties of recycled concrete were tested.The results show that:The types of hydration products of the four types of recycled concrete are the same,but the compactness of the microstructure before the age of 14d is worse than that of ordinary concrete.At the age of 14d to 28d,the developing hydration products appear again in the recycled concrete.During this period,the microstructure of the concrete becomes denser;The concrete with the highest residual mortar content is non-desizing road recycled concrete,which has the largest porosity and pore size among the 4 types of recycled concrete after standard curing for 28 days,and its pore distribution is the most densest.Larger porosity and pore size are detrimental to concrete strength.The elastic modulus of the new ITZ and new mortar of desized road recycled concrete and civil construction recycled concrete is higher than that of ordinary concrete,and the thickness of the ITZ of the 4 kinds of recycled concrete are also larger than that of ordinary concrete;The Ca/Si near the new ITZ of recycled concrete is significantly lower than that of ordinary concrete,and the lower the ratio of Ca/Si,the higher the strength of recycled concrete.