Optimization Of Recycled Concrete Mix Ratio Based On Fractal Gradatio

In various construction activities such as new construction,reconstruction,and demolition in China,a large amount of waste concrete is generated.The traditional method for handling waste concrete is direct landfill,however,this approach is actually a waste of resources,and the piled up waste concrete occupies a large amount of land resources.Moreover,the dust from waste concrete blown into the air worsens smog,and harmful substances entering the groundwater and soil causes serious environmental pollution.In addition,the production of concrete consumes more than20 billion tons of aggregates every year,and according to current trends,the construction industry’s demand for concrete aggregates will increase year by year.The large-scale mining of sand and gravel has damaged the environment and violated the development concept of"green mountains and clear waters are as good as mountains of gold and silver".Waste concrete is a resource that has been placed in the wrong place".Decomposing waste concrete into recycled aggregates can both alleviate the two difficult problems of the rapid growth of waste concrete and the rapid depletion of natural sand and gravel resources.Recycled aggregates can replace natural aggregates to prepare recycled concrete.However,the old mortar attached to the surface and the micro-cracks inside the recycled aggregates lead to higher water absorption and crushing value of the recycled aggregates,thereby causing defects in the mechanical properties of the recycled concrete compared to ordinary concrete.This article introduces fractal theory and derives fractal gradation formulas,attempting to characterize the gradation curve of recycled concrete aggregates with fractal dimensions,optimizing the mechanical properties of recycled concrete without increasing costs.This article analyzes the effects of different fractal dimensions of fractal gradation(2.17,2.26,2.30,2.33,2.39,2.43,2.47,2.52),different water-cement ratios(0.45,0.35,0.30)and low-concentration Na OH solution(0.3%,0.6%,0.9%,1.2%,1.5%)pre-wetting on the compressive and tensile properties of recycled concrete.Based on fractal gradation,the impact of aggregate replacement on recycled concrete is analyzed through experiments,and a constitutive model of axial compression for recycled concrete based on fractal gradation is proposed.Based on the experimental data,the research results are mainly divided into the following three aspects:(1)The fractal grading with the same fractal dimension has a specific and obvious effect on the compressive strength of recycled concrete.The use of a fractal grading curve with a fractal dimension of 2.39 enables the recycled concrete to obtain a better compressive strength,and the strength difference between the optimal fractal and the worst fractal can span two concrete strength classes.Therefore,in the engineering application of recycled concrete,special attention should be paid to the adjustment of the aggregate gradation of recycled concrete,so that the recycled concrete can play its maximum compressive strength.(2)Through the axial compression test of recycled concrete with different fractal dimensions,it can be concluded that the fractal dimension can characterize the aggregate particle shape,and when the fractal dimension D changes between 2.33-2.39,the influence of aggregate particle shape on the cubic compressive strength,axial compressive strength and tensile strength of recycled concrete is small;when the fractal dimension D<2.30,the cubic compressive strength gradually decreases,while axial compressive strength and tensile strength show a significant decrease.Combining with the preliminary test data,the predicted numerical model of the compressive,axial compressive and tensile strength of recycled concrete under the influence of fractal dimension was derived(R~2=97.8%).(3)The low concentration of Na OH solution with mass fraction of 0.6%-0.9%has a certain optimization effect on the compressive strength of recycled concrete with low water-cement ratio(W/B<0.45).However,this optimization effect has little effect on the early strength of concrete,and the longer the curing time,the higher the compressive strength.The use of low concentration Na OH solutions had little effect on the tensile strength of the concrete.