Study On Properties Of Multi-Scale Polypropylene Fiber Concrete

Polypropylene fiber is a kind of synthetic fiber which is used to improve the performance of concrete earlier.It has the characteristics of excellent dispersion performance,acid and alkali resistance,fatigue resistance,low production price,wide range of raw materials,etc.In order to meet the complex requirements of the construction industry for concrete performance,the concrete performance can be further improved by changing the fiber size.In this paper,the polypropylene fiber concrete was taken as the research object,and the multi-scale polypropylene fiber concrete was tested by single mixing and mixing,and the compression,splitting,bending resistance and working performance of concrete were taken as reference indexes to determine the optimal scheme of single mixing and mixing test of multi-scale fiber concrete.Under the optimum conditions,the following tests were carried out:(1)the durability tests of concrete for impermeability,frost resistance and sulphate resistance in dry and wet cycles;(2)the performance tests of concrete after high temperature at 25?,200?,400?,600? and 800? and the performance tests under thermal mechanical coupling;(3)the test of internal pore structure of concrete under different temperature(25?,400?,800?).The effect of multi-scale polypropylene fiber on the performance of concrete was studied from the macroscopic and microscopic point of view.The following conclusions are drawn:(1)The addition of polypropylene fiber can improve the compressive,splitting and flexural properties of concrete.In the single mixing test of concrete,the polypropylene fine fiber concrete(PPF)with mass content of 0.9 kg/m3,diameter of 31 ?m and length of 12 mm has the best performance,while the polypropylene coarse fiber concrete(PPM)with mass content of 6 kg/m3,diameter of 450 ?m and length of 45 mm has the best performance;In the mixing test,the polypropylene coarse fiber with mass content of 6 kg/m3,length of 45 mm,diameter of 450 ?m and the polypropylene fine fiber mixed concrete with mass content of 0.9 kg/m3,length of 12 mm and diameter of 31 ?m have the best performance.(2)Polypropylene fiber can improve the durability of concrete.The addition of fiber can reduce the content of harmful holes and multiple harmful holes in concrete,refine the internal hole diameter of concrete,increase the micro hole diameter obviously,and improve the content of less harmful holes and harmless holes.Comprehensive comparison of durability test results and pore structure characteristics can be obtained:the improvement effect of polypropylene fine fiber on concrete durability is superior to that of polypropylene coarse fiber,but the effect is worse than that mixing method of polypropylene coarse fiber and fine fiber.(3)With the increase of heating time,the thermal strain of concrete increases first and then decreases,while the thermal damage of concrete increases gradually.The relationship between the maximum value of thermal strain and the maximum value of thermal damage of four kinds of concrete is:JZ>PPM>PPF>PPC.Compared with JZ specimens,the maximum thermal strain of three types of fiber reinforced concrete specimens(PPM,PPF and PPC)are 86.8%,80.8%and 72.5%of the maximum thermal strain of JZ specimens respectively;the maximum thermal damage of three types of fiber reinforced concrete specimens(PPM,PPF and PPC)are 90.7%,72.9%and 68.0%of the maximum thermal damage of JZ specimens respectively.It shows that the addition of polypropylene fiber can effectively improve the high temperature performance of concrete under thermal mechanical coupling,and mixing method is the best,followed by the polypropylene fine fiber and the polypropylene coarse fiber.(4)After high temperature concrete test,with the increase of temperature,the compressive strength and residual rate of compressive strength of the four kinds of concrete show a decreasing trend,and the compressive strength and residual rate of compressive strength of fiber reinforced concrete specimens after high temperature are greater than that of JZ specimens,which shows that the incorporation of fiber can alleviate the high temperature damage of concrete.(5)According to the attenuation of concrete strength after high temperature action,the constitutive relation of concrete under normal temperature is modified,and the constitutive relation of concrete after high temperature is established:?=KE?k(1-D).The test curve of concrete under the action of 600? temperature is selected to verify it.The coincidence between the test curve and the theoretical curve is high,and the theoretical curve can better express the mechanical properties of concrete after high temperature.