The Research On The Size Effect Of Behavior In Compression Of Recycled Aggregate Concrete Reinforced By Silica Fume

As urbanization accelerates,the application of recycled concrete can solve environmental problems such as construction waste and shortage of gravel resources.However,the recycled aggregates mainly come from old buildings.Due to the old mortar remaining on the surface of recycled aggregate and a large number of micro-cracks inside,the mechanical properties and size effect of recycled concrete and ordinary concrete are different.The mechanical properties of recycled concrete can be effectively improved by surface treatment,but the research on the effect of surface treatment on the size effect of recycled concrete is insufficient.The performance parameters of recycled concrete are usually determined by scale model tests.In order to ensure the applicability of the scale model test results in the actual structure,it is necessary to fully study the size effect of recycled concrete.In addition,due to the limitations of the experimental conditions,when conducting tests on recycled concrete,only small-size specimens can be used.The numerical analysis can be used to study the performance of recycled concrete in a larger size range and also provide another research method for the study.In this paper,experimental research,theoretical analysis and numerical simulation were combined to study the compression behavior and size effect of recycled concrete reinforced by silica fume.The main contents are as follows.(1)The compression tests were carried out on 24 groups cubic specimens with three sizes of 100 mm,150mm and 200 mm.The test results show that the size effect of cubic compressive strength of recycled concrete reinforced by silica fume is obvious.The compressive strength of the specimens with dimension of 200 mm and 150 mm is about 84.9%and 94.3% of the strength with dimension of 100 mm,respectively.And the size effect is more obvious with the increase of the substitution rate.The size effect degrees with the substitution rates of 50%,75% and 100% are increased by 25.6%,33.3% and 35.9%compared with the substitution rate of 25%.The treatment with silica fume can effectively reduce the size effect of the compressive strength.After the treatment with silica fume slurry with water-cement ratios of 0.8,1.0 and 1.2,the size effect degree is reduced by about 10.6%,19.2% and 7.9% compared with the untreated one.When the water-cement ratio of the silica fume slurry is moderate,the size effect degree decreases the most.(2)The compression tests were carried out on 15 groups recycled concrete prism specimens.The test results show that the size effect of axial compressive strength,peak strain and ultimate strain of recycled concrete reinforced by silica fume is obvious.The axial compressive strength,peak strain and ultimate strain of the specimens with dimension of200 mm were about 85.4%,128.4% and 92.5% of the specimens with dimension of 100 mm,respectively.After treatment with silica fume slurries with different water-cement ratios,the size effect of compression performance is reduced.Among them,when the water-cement ratio of the silica fume slurry is 1.0,the size effect of the axial compressive strength,peak strain and ultimate strain decreases the most,which is reduced by about 14.9%,23.0% and19.7% respectively compared with the untreated one.The initial elastic modulus of recycled concrete reinforced by silica fume increases compared with the untreated one,but its size effect phenomenon is so subtle that can it can be ignored.(3)The theories of statistical size effect,the Bazant size effect law of fracture mechanics and Carpinteri size effect law of multifractal size effect theory were analyzed.Based on the measured value of the compressive strength of recycled concrete and references,the values of the parameters in the size effect law were determined.The critical size and critical strength were defined,and the suggested values for different water-cement ratios of silica fume slurries were given.(4)The 3D random aggregate model was establish with MATLAB,and then ANSYS was used to establish a three-phase mesoscopic numerical analysis model of the recycled concrete specimen to determine the mechanical properties of interfacial transition zone(ITZ)under different water-cement ratios of silica fume slurries.(5)The homogeneous elastic brittleness model was used to calculate the cubic compressive strength of the numerical specimens of different sizes under different water-cement ratios of silica fume slurries,and the parameter analysis of its size effect was carried out,and the suggested values of critical size and critical strength of cubic compressive strength of numerical specimens were given.