Study On Durability Of Ultra-high Performance Concrete Under Sulfate-wet And Dry Cycle Coupling

China has a vast territory,a large number of salt lakes and saline-alkali land,the soil and water in these areas contain a large number of,coupled with the influence of wet and dry cycle makes concrete structure by sulfate-wet and dry cycle for a long time to retire in advance,the study of high performance,high strength and high durability concrete is imminent.Super high performance concrete is a new cement-based composite material with high strength,durability,ductility,toughness and good volume stability.At the same time,steel fiber contained in ultra-high performance concrete effcctively inhibits the initiation and development of cracks in concrete members,changes the distribution of internal stress in concrete,and improves the toughness of concrete to a certain extent.The advent of ultra-high performance concrete provides more excellent material properties for the construction of large concrete projects such as Bridges,roads and tunnels,and improves the service life of components However,the structure is often affected by the harsh and complex natural environment in the process of use.At present,ultra-high performance concrete materials are generally used in large-span,ultra-high rise and large-volume structures.Structural damage in the service process will cause serious social harm.Therefore,it is particularly important to analyze the damage performance of ultra-high performance concrete buildings under the coupling effect of various factors and monitor the deterioration trend in the whole life cycle.In this paper,a series of durability tests on ultra-high performance concrete in the aspect of sulfate-wet and dry cycle have been done.The research contents and results are as follows:(1)Four sulfate solutions of different concentrations were used:Group A(0%Na2SO4 solution),Group B(5%Na2SO4 solution),group C(10%Na2SO4 solution),group D(15%Na2SO4 solution),carried out ultra-high performance sulphate-wet and dry cycle coupling test.During the test,the data of sample mass,compressive strength and dynamic modulus were analyzed.The mass change rate,compressive strength change rate and relative dynamic modulus of specimens in different concentrations of solution were compared.The results show that the mass,compressive strength and dynamic elastic modulus of ultra-high performance concrete increase first and then decrease.In the solution with higher concentration,the change trend was also obvious.The mass loss rate and compressive strength loss rate of the pilot block in group D(15%Na2SO4 solution)were 0.83%and 3.65%,respectively,after 540d dry and wet cycling.The mass and compressive strength of the pilot block were reduced by 13.8g and 4.1MPa compared with that before cycling.The mass loss rate and compressive strength loss rate of group A(0%Na2SO4 solution)were 0.36%and 0.32%,respectively,and the mass and compressive strength decreased by only 8.7g and 1.0MPa.However,the overall mechanical properties of super high performance concrete vary gently in the whole cycle.(2)The ultrasonic wave method is used to analyze the sound time,sound velocity and ranging of ultra-high performance concrete.By observing the influence of four sulfate solutions with different concentrations on ultrasonic parameters of ultra-high performance concrete under the mechanism of dry and wet circulation,the variation rule of damage layer thickness of ultra-high performance concrete is analyzed.The results show that:In the four sulfate solutions with different concentrations,the thickness of the damage layer of the prismatic specimens increased gradually with the increase of the cycle period.The thickness of the damage layer of the specimens in group D(15%Na2SO4 solution)with the highest solution concentration reached 7.26mm.Group A(0%Na2SO4 solution),group B(5%Na2SO4 solution)and group C(10%Na2SO4 solution)were 5.97mm,6.03mm and 6.44mm,respectively.(3)Through paste acoustic emission test,the internal characteristics of ultrahigh performance concrete material damage were explained,the whole process of damage was monitored in real time,and the damage process of ultrahigh performance concrete was studied by comparing the cumulative ringing technology,cumulative energy and amplitude characteristic parameters with the load development curve.The results are as follows:The concentration of sulfate solution in the specimen is represented by the AE characteristic parameter curve.The higher the concentration of sulfate solution is,the longer the fluctuation time of AE characteristic parameter curve is,and the earlier the peak appears.The AE signal response of the ultra-high performance prismatic test block after being eroded by the high concentration of sulfate solution is relatively severe.Steel fiber plays an obvious role in preventing crack development during concrete damage.(4)The microstructure of ultrahigh performance concrete under the coupling action of sulphate-wet and dry cycles was analyzed by SEM test,and the influence of the change of microscopic morphology on the macro damage of concrete was studied.The results showed that:The ultrahigh performance concrete matrix itself is very dense,and the steel fiber is closely bonded to the concrete.With the increase of the dry-wet cycle,hydration products and crystals gradually fill the internal pores of the concrete until the excessive expansion stress leads to the appearance of tiny cracks.The passage of SO42-into the interior of concrete continues to expand,eventually causing concrete damage.Analysis at the micro level verifies the failure law of ultra-high performance concrete at the macro level.