Study On The Frost Resistance Of Multi-Scale Polypropylene Fiber Concrete

Frost resistance is an important index of concrete materials.Especially in cold regions,the reinforced concrete road pavement,piers,hydraulic structures and so on,will crack and flake after a period of operation.Ordinary concrete has the disadvantages of poor durability and low strength.Adding fibers into cement matrix is one of the effective ways to improve the mechanical properties and durability of concrete.Freeze-thaw tests of fine polypropylene fibers in two sizes and crude polypropylene fibers in one size,single-doped and mix-doped with concrete by the same mixing technique and in the same dose ratio,are completed.Through freeze-thaw cycle test,tensile and compressive mechanical properties test,NMR test and SEM test,the frost resistance,mechanical properties of tension and compression,freeze-thaw failure mechanism and damage prediction model of multi-scale polypropylene fiber concrete are studied.The main research contents and achievements are as follows:(1)The freeze-thaw cycle tests of polypropylene fiber concrete are carried out.The results show that single-doped crude polypropylene fiber has a negative effect on the spalling resistance of concrete,and single-doped fine polypropylene fiber or mixed fiber can improve the spalling resistance of concrete.And under the condition o f the same total amount of fiber,the more proportion of fine fiber,the better the spalling resistance.Among all the groups,multi-scale polypropylene fiber concrete specimens have the best spalling resistance.By testing the mass loss rate and the relative dynamic modulus of elasticity of concrete under different freeze-thaw cycles,the frost resistance of multi-scale polypropylene fiber concrete is optimized.Secondly,the frost resistance of concrete with mixed fiber is better than that of single-doped polypropylene fiber concrete,and the concrete with the worst frost resistance is plain concrete.(2)The tensile and compressive mechanical properties of specimens of multi-scale polypropylene fiber concrete and specimens after 200 times of freeze-thaw cycles are tested respectively.The results show that: the loss rate of compressive strength and splitting strength of concrete specimens mixed with fiber are lower than those of plain concrete specimens.The loss rate of compressive strength and splitting str ength of single-doped polypropylene fiber concrete specimens is only slightly lower than that of plain concrete.Under the condition of the same total fiber content,the strength loss rate of the mix-doped fiber concrete specimens is lower than that of plain concrete and single-doped fiber concrete specimens.Also,among them,the loss rate of compressive and splitting tensile strength of multi-scale polypropylene fiber concrete specimens is much lower than that of concrete specimens mixed with two kinds of fibers,and the greater the proportion of fine fibers,the lower the strength loss.(3)The pore structure of multi-scale polypropylene fiber concrete is studied by nuclear magnetic resonance test.The results show that most probable pore size of concrete can be greatly reduced when the fiber is added.Secondly,after adding fiber,the proportion of harmless holes in the concrete matrix will be improved,the proportion of the less harmful pore and harmful pore will decrease,and the pore structure of the concrete matrix will be improved.Moreover,under the condition of the same fiber content,the larger the proportion of fine fiber,the smaller the fiber diameter,the more obvious the improvement of pore structure.(4)Based on the test results of frost resistance and the results of nuclear magnetic resonance test and scanning electron microscopy,the mechanism of freeze-thaw damage of multi-scale polypropylene fiber concrete is discussed.The results show that fiber plays two roles in the matrix.Firstly,the pore structure of concrete matrix can be effectively improved by adding fiber.Secondly,it can also play a role in crack reduction and toughening: in the early stage of concrete pouring,the early cracking of concrete is prevented,and during the freezing and thawing cycles,as the harmful holes gradually increase and form a micro fracture,the fibers will slow down the formation and coalescence of micro-cracks,thus decreasing the dynamic modulus of elasticity(5)Based on dynamic modulus of elasticity,two parameters Weibull probability distribution,freeze-thaw damage evolution equation,three element function type and double exponential type concrete freeze-thaw damage model are established respectively.The results show that the freezing thawing damage model of double exponential type concrete has the highest fitting accuracy and can well describe the freeze-thaw damage degree of concrete under different fiber content.The maximum freezing thawing times of fiber concrete are predicted by double exponential model.Among them,the plain concrete specimen A0 can resist only 280 freeze-thaw cycles,and the frost resistance is lowest.Multi-scale polypropylene fiber reinforced concrete specimen A8 has the best frost resistance,and the maximum freeze-thaw cycles are 380 times.