Experimental Study On Mechanical Properties And Drying Shrinkage Of Concrete Containing Recycled Brick Fine Aggregate

With the steady progress of new urbanization in China,a large number of waste clay red bricks have been produced by the demolition and reconstruction of buildings.After a series of crushing processes,waste clay red bricks can be used as recycled fine aggregate to replace natural river sand in the preparation of concrete,which meets the contemporary trend of green and low-carbon development.In this paper,seven groups of concrete mix proportions with C40 strength grade are designed,and the effects of replacement rate of recycled brick fine aggregate,particle gradation and additional water content on concrete workability,mechanical properties and drying shrinkage are studied.Though the methods of particle shape analyzer,microhardness tester,mercury intrusion meter(MIP),scanning electron microscope and energy spectrum analyzer(SEM-EDS),the mechanism of the influence of fine aggregate characteristics of recycled brick on the mechanical properties and drying shrinkage of concrete was revealed.The results are as follows:(1)By studying the physical characteristics of recycled brick fine aggregate,it is found that the aspect ratio,Waddell sphericity,edge angle and self-hardness of recycled brick fine aggregate are relatively low after secondary crushing.And it has irregular geometry,porous pore structure and high surface roughness,thus decrease the loose packing density and increase both water absorption and crushing value.(2)The influence of recycled brick fine aggregate on the workability and mechanical properties of concrete was studied.The results show that the slump of recycled brick fine aggregate concrete can be similar to that of ordinary concrete by adjusting the amount of water reducer,but its rheological properties are quite different.The static yield stress and plastic viscosity of recycled brick fine aggregate concrete are significantly higher than that of ordinary concrete.Compared with the strength of ordinary concrete,the compressive strength of concrete is improved by adding some recycled brick fine aggregate with additional water,but the compressive strength of concrete is reduced by adding all recycled brick fine aggregate with additional water.In addition,the rough surface and irregular geometric shape of recycled brick fine aggregate help to improve the splitting tensile strength of concrete,but the porous structure of recycled brick fine aggregate reduces the elastic modulus of concrete.(3)The effect of recycled brick fine aggregate on drying shrinkage of concrete was studied.The results show that the addition of recycled brick fine aggregate has obvious adverse effects on the drying shrinkage of concrete.With the increase of substitution rate or the decrease of additional water content,the drying shrinkage of recycled brick fine aggregate concrete gradually increases.Compared with 0.15-5 mm recycled brick fine aggregate,0-5 mm recycled brick fine aggregate can limit the development of concrete drying shrinkage to some extent.(4)By using MIP and SEM-EDS,the differences of pore structure,multi-interface micromorphology and chemical composition between ordinary concrete and recycled brick fine concrete were compared and analyzed.The results show that the addition of recycled brick fine aggregate has obvious influence on the pore characteristics and pore size distribution of concrete.With the increase of replacement rate or additional water,the porosity and total pore volume of recycled brick fine aggregate concrete increase obviously.However,the average pore size decreased and the pore size distribution gradually refined.In addition,SEM-EDS shows that the peak values of Ca,Si and Al elements in recycled brick fine aggregate-cement matrix have changed.It may be due to the pozzolanic reaction between calcium hydroxide provided by cement hydration and aluminosilicate provided by recycled brick fine aggregate,which improves the density of interface transition zone.