Experimental Study On Salt Frost Resistance And Mechanical Properties Of Basalt Fiber Reinforced Concrete

Aiming at the problems of concrete cracking and durability reduction in the cold and dry irrigation area of Northwest China,Jingdian Irrigation Area of Gansu Province is selected as the research object to conduct field investigation to determine the causes of damage and the main erosion media.Basalt fiber with excellent mechanical properties and environmental protection is selected as the concrete strength enhancing material.Indoor rapid freeze-thaw test is used to simulate the field environment to study the effect of salt freezing on concrete strength The influence of the same basalt fiber content（volume content 0.05%,0.1%,0.15%,0.2%）on the working performance,mechanical properties and durability index of concrete was studied.The internal damage of concrete under different freeze-thaw conditions（clear water,3%NaCl,5%Na₂SO₄）,different freeze-thaw times and different fiber content was detected by ultrasonic nondestructive testing method,and the internal damage cloud of concrete specimen was drawn The damage rule is consistent with the change rule of durability index.The mechanical attenuation model is established,which is related to the relative dynamic modulus of elasticity and the compressive strength and flexural strength.The model is verified by the test data,and the correlation coefficient R² is high.The main conclusions are as follows:（1）The addition of fiber has a great influence on the workability of concrete.The larger the content of fiber is,the smaller the slump of concrete is,and the worse the working performance is,which makes the viscosity effect of fiber obvious and is not conducive to the mixing and pouring of concrete.It is suggested that the volume addition of fiber should not exceed 0.2%in practical engineering application.The addition of basalt fiber can improve the initial compressive strength of concrete,but not obviously.The compressive strength can be increased by about 8.4%at most,and the flexural strength of concrete can be increased by about 26.5%at most.The addition of basalt fiber can make the concrete specimen more dense to some extent.The overall performance of concrete is the best when the content of fiber is0.1%～0.15%.（2）The damage degree of salt freeze to concrete is much greater than that of water freeze-thaw.The addition of basalt fiber can effectively slow down the rate of mechanical property decline of concrete under the action of salt freeze,but it can not prevent the erosion and damage of concrete.The more the fiber content is,the better it is.If the fiber content is too small,the concrete effect will not be improved obviously.If the fiber content is too large,it will easily cause the mechanical property of concrete With the decrease of energy,the concrete with fiber content of 0.15%～0.2%has the best anti freeze-thaw effect.After 150 times of freeze-thaw,the compressive strength of the three solutions decreased by 21%,14.52%and 8.93%respectively.The reduction of flexural strength is slightly higher than that of PC,but the residual flexural strength is still significantly higher than that of PC.Under the freeze-thaw environment,the descending rate of mechanical properties of concrete specimens in three media is 3%NaCl>5%Na₂SO₄>water.（3）Under the same freeze-thaw condition,the mass loss and relative dynamic modulus of elasticity of concrete mixed with fiber are lower than that of PC.the incorporation of fiber slows down the mass loss and relative dynamic modulus of elasticity of concrete,which has the effect of toughening and crack resistance.After the freeze-thaw of PC,brittle failure occurs directly.After the freeze-thaw of fiber concrete,the failure process is relatively slow,and the specimen is completely damaged After that,the integrity of the whole structure can be maintained.The hydraulic concrete structure in the cold and dry areas can be added with basalt fiber to prevent the concrete from cracking.（4）The damage rule of concrete defect nephogram by ultrasonic nondestructive testing is consistent with the change of concrete appearance and mechanical properties.The damage order of three freeze-thaw media to concrete is 3%NaCl>5%Na₂SO₄>water.When the fiber content is 0.15%,the performance of concrete is the best,the ultrasonic wave velocity decreases the slowest,the internal damage of the specimen is the lightest,and the second is bf0.2%.Under the action of salt freezing,the order of frost resistance of basalt fiber concrete specimens with different content is BF0.15>BF0.2>BF0.1>BF0.05>PC.（5）The mechanical attenuation model related to compressive strength,flexural strength and relative dynamic modulus of elasticity is established,and the model is fitted and verified by test data.The fitting correlation coefficient R² is high,which verifies the rationality and reliability of the model.Similar environmental concrete structures can evaluate the compressive strength and flexural strength of concrete structures in service period by detecting relative dynamic modulus of elasticity.Relevant research can provide basis for design and maintenance of concrete structure in cold and dry irrigation area.At the same time,it can provide reference for durability research of fiber concrete（such as PVA fiber and polypropylene fiber）.