Study On Compressive And Tensile Size Effects Of Polypropylene Fiber Recycled Concrete

In recent years,with the rapid development of social economy,China's urbanization process is accelerated,which promotes the development of the construction industry,and also produces a large number of construction waste.Waste concrete is the main component of construction waste.Recycled concrete is a good way to recycle and reuse the waste concrete.Due to its defects,the mechanical properties of recycled concrete are far from those of natural concrete.In practice,adding fiber is often used to improve the performance of recycled concrete.Polypropylene fiber has the advantages of good crack resistance performance,can effectively improve the ductility of concrete and low cost,so this topic selects polypropylene fiber as the reinforcement material to improve the mechanical properties of recycled concrete.The size effect phenomenon is widely existed in various materials,and the size effect is more obvious for the heterogeneous material of polypropylene fiber recycled concrete.Therefore,it is important to study the size effect of polypropylene fiber recycled concrete.In this paper,the compressive and splitting tensile strength of polypropylene fiber regenerated concrete cubes with different fiber content,different concrete strength grade and different size were tested by combining experimental research and numerical analysis.The main research contents and conclusions are as follows:?1?By conducting compressive and tensile strength tests on 162 cubic test blocks of polypropylene fiber recycled concrete,the influence of fiber content and concrete strength grade on the size effect of compressive and tensile strength of polypropylene fiber recycled concrete and its variation rule were analyzed.The results showed that the size effect exists in the compressive and tensile strength of polypropylene fiber recycled concrete,and the size effect increases with the increase of the specimen size.With the increase of fiber content,the size effect of compressive and tensile strength of polypropylene fiber recycled concrete decreases first and then increases.With the increase of concrete strength grade,the size effect of compressive and tensile strength of polypropylene fiber recycled concrete increases,and the size effect degree of tensile strength of polypropylene fiber recycled concrete is far greater than that of compressive strength.?2?Calculate the size conversion coefficient and size effect degree of compressive and tensile strength of polypropylene fiber recycled concrete under different concrete strength and fiber parameters according to the experimental data,and study the influence of fiber content and strength grade on size effect degree.The results showed that the lower the concrete strength is,the greater the influence of polypropylene fiber content on the size effect of polypropylene fiber recycled concrete on compressive and tensile strength is.That is to say,for low strength recycled concrete,polypropylene fiber can effectively reduce its size effect,while for high strength recycled concrete,the addition of polypropylene fiber has a low impact on its size effect,and the reduction degree is weak.The size conversion coefficient and size effect degree of polypropylene fiber recycled concrete under different fiber content and different concrete strength grade are calculated by the experimental data.?3?By fitting the measured values of compressive and tensile strength experiments with six size effect law formulas corresponding to the three size effect theories of statistical size effect theory,size effect theory based on energy release and Carpinteri multifractal size effect theory,the fitting effect was analyzed.Research results showed that:in the six kinds of law of size effect formula,Bazant size effect formula fit the polypropylene fiber recycled concrete cube compressive and tensile strength of the measured values best,the compressive strength of the fitting correlation coefficient between 0.913-0.982,tensile strength of fitting correlation coefficient between 0.921-0.997,and six kinds of size effect law is obtained by fitting formula under different fiber content of the parameters in the empirical formula and intensity-size fitting curve.?4?Using six size effect law formulas corresponding to the three size effect theories of statistical size effect theory,size effect theory based on energy release and Carpinteri multifractal size effect theory statistics obtained by fitting experimental data,calculated critical strength f_crand critical size D_crof the size effect of polypropylene fiber recycled concrete under different fiber content,and the influence of fiber content and concrete strength level on the critical strength f_cr and critical size D_cr was analyzed.The results showed that with the increase of the fiber content,the critical size of the compressive and tensile strength of the polypropylene fiber recycled concrete cube corresponding to the six size effect law formulas decreases first and then increases,and the critical strength of the cube corresponding to the polypropylene fiber recycled concrete increases first and then decreases.With the increase of concrete strength,the critical size of compressive and tensile strength of polypropylene fiber recycled concrete cube corresponding to the six size effect law formulas increases with the increase of concrete strength,and the critical strength decreases with the increase of concrete strength.