Fractal Study On Crack Propagation Path Of Recycled Concrete Bending Beams

Green sustainability of materials is a hot topic of current research,and recycled concrete just meets the current needs.It is a kind of green concrete made from waste concrete such as building demolition,which is crushed,screened and cleaned,and replaces natural aggregate in a certain proportion,so as to realize garbage removal and resource reuse.There are some defects such as micro-pore and micro-crack in concrete structure itself.When the concrete is under constant load,the crack extends,expands and penetrates continuously,which eventually leads to structural damage.The evolution characteristics of cracks reflect the mechanical properties of concrete and the characteristics of concrete itself to a certain extent.Through exploring the surface cracks of components,it is very important to evaluate concrete components for analysis.Safety has practical application value.Concrete is a kind of multi-phase composite material with irregular and non-linear characteristics,which leads to random crack propagation of concrete.Traditional damage mechanics knowledge can not properly solve this problem.The development of cracks has self-similarity.Fractal theory can better analyze the crack evolution characteristics of concrete under different conditions,and then analyze the stress characteristics of structural components.Based on the water cement ratio and recycled aggregate substitution rate as test variables,12 test beams were fabricated for four-point bending test.The fractal dimension of single crack,pure bending section,bending-shear section and lateral crack distribution of recycled concrete under different water cement ratio were analyzed,and the relationship between the deflection,maximum crack width and fractal dimension of recycled concrete in bending process was also analyzed.The analysis shows that the crack development of recycled and ordinary concrete has obvious fractal characteristics.When the replacement rate of recycled coarse aggregate is 30%,the water cement ratio has less influence on the crack distribution and fractal dimension of concrete surface.When the water cement ratio is the same,the fractal dimension decreases with the increase of substitution ratio in the earlystage of concrete bending.With the increase of load,the fractal dimension decreases first and then increases with the increase of substitution ratio of recycled aggregate under the same load,which indicates that the replacement of 30% recycled coarse aggregate is relatively better.By calculating the fractal dimension of cracks in pure bending and shearing sections of test beams,the fractal dimension of bending and shearing sections is larger than that of pure bending sections,which indicates that the distribution of cracks in bending and shearing sections is more dense than that in pure bending sections during the failure process of recycled concrete beams.However,for the fractal dimension of a single fracture,the average fractal dimension of a single fracture in a pure bending section is larger than that in a bending shear section.The fractal dimension of test beam is correlated with midspan deflection and maximum crack width,and a good correlation is found.