The Study On Mechanical Properties Of Polypropylene Fiber Recycled Concrete Before And After Freeze-thaw

At present,China is in the period of great urbanization development,a large number of buildings are facing demolition and reconstruction,if the waste concrete is not properly disposed of,it will cause great pressure on resources.Some buildings have been in severe cold areas for a long time,which puts forward higher requirements for their frost resistance.Polypropylene fiber has strong toughness,which can improve the internal structure of recycled concrete and improve its tensile properties and other mechanical properties and frost resistance.At present,the research on recycled concrete is relatively mature in China,but the research on the properties of recycled concrete mixed with polypropylene fiber is relatively scarce.Therefore,it is of great significance for practical engineering to study the mechanical properties of polypropylene fiber recycled concrete before and after freezing and thawing.In this paper,the optimum mixture percentage of polypropylene fiber recycled concrete was determined through experiments,then complete the freeze-thaw cycle test and analyze its antifreeze mechanical performance changes,the freeze-thaw damage criterion is given.The main research contents and conclusions are as follows:(1)Test production of 36 groups of different sizes(100mm×100mm×100mm,150mm×150mm×150mm,150mm×150mm×300mm)of compressive and 12 groups of tensile specimens,to analyze the effects of two factors,namely the replacement percentage of recycled aggregate(0%,30%,60%,100%)and the amount of polypropylene fiber(0%,0.1%,0.2%),on the mechanical properties of concrete and to determine the optimal mix percentage.Make eight groups of optimal mix test blocks(100mm×100mm×100mm),the compressive and tensile tests were carried out after 0,25,50 and 75 freeze-thaw cycles,respectively,to analyze the change of frost-resistant mechanical properties and establish the freeze-thaw damage criterion.(2)Analysis of mechanical properties under normal test results show that 30 %recycled aggregate replacement percentage,fiber content is the same,the test block mechanical properties change is not obvious;When the replacement rate of recycled aggregate exceeds 60%,the compressive and tensile properties of the test block decrease faster,the plasticity of polypropylene fiber recycled concrete increases and the stress decreases.When 0.1% polypropylene fiber content and recycled aggregate replacement percentage are the same,the compressive and tensile properties of the test block are significantly improved.When the fiber content was 0.2%,the compressive and tensile properties of the specimens tended to decrease,and the ductility increased and the residual strength increased due to the increase in fiber doping.It was concluded that the mechanical properties of concrete were optimal at30% recycled aggregate replacement percentage and 0.1% polypropylene fiber admixture.The conversion coefficient of cubic compressive strength of polypropylene fiber recycled concrete is between 0.88 and 0.91.The conversion coefficients of cube and prism compressive strength are between 0.78 and 0.8.(3)Analysis of the freeze-thaw test results shows that the changes in the appearance,mass loss percentage and relative dynamic elastic modulus of the test blocks were not obvious at 25 times of freeze-thaw,and the compressive and tensile strength of the test block decreased by 1.364% and 1.614%,respectively.After freezing and thawing for more than 50 times,the overall damage of the test block was significantly increased,and the growth rate of the compression and tensile strength loss rate became faster.After 75 freeze-thaw cycles,the mass loss percentage of the test block was 0.176%,the relative dynamic elastic modulus was 95.838%,and the compressive and tensile strength decreased by 4.515% and 6.374%,respectively.It was concluded that with the increase of the number of freeze-thaws,the frost resistance of polypropylene fiber recycled concrete weakened,the plasticity of concrete increased,the stress decreased,the strain increased and the residual strength decreased.(4)Based on the experimental results and theoretical analysis,the freeze-thaw damage model is established,the freezing-thawing damage criterion of polypropylene fiber recycled concrete with different freezing-thawing times is given.Establish constitutive model,The concrete values of parameters in the model and the strength failure criterion affected by replacement percentage of recycled aggregate and content of polypropylene fiber are given.