Research On Mechanical Properties And Durability Of Hybrid Fiber (Bundle) High-strength Concrete

High-strength concrete structures can save energy and materials,but have problems such as low tensile strength,high brittleness,easy cracking and so on.When high-strength concrete structures are in service for a long time in harsh environment,the existence of cracks gives harmful substances a chance to enter the concrete,which reduces its durability and causes premature failure.Adding fibers with excellent crack resistance,strengthening and toughening effects into concrete evenly can make up for the lack of concrete performance.Chopped basalt fibers(CBF)have superior properties and are green and environmentally friendly.The research on CBF reinforced concrete properties has been relatively mature.When the dosage is too large,it is easy to agglomerate and affect the strengthening and toughening effect.Basalt macro-fibers(BMF)are prepared by a twisting process,which not only retains the excellent properties of chopped basalt fibers,but also has better dispersion.In this thesis,BMF was considered as a reinforcing material to study the effect of its content and length on the performance of high-strength concrete.BMF and CBF were mixed into highstrength concrete to study the influence of their mixing proportion and content on the performance of concrete.Calcium sulfate whisker(CSW)is a sub-nano-sized inorganic salt whisker,which is formed from industrial by-product gypsum through a special process,green and with low environmental protection costs.In view of the multi-scale failure process of concrete,considering the addition of CSW on the basis of obtaining the dominant mixing ratios of BMF and CBF,the influence of the change of calcium sulfate whisker content on the mechanical properties and durability of high-strength concrete was studied.The main conclusions are as follows:(1)Basic mechanical properties test of hybrid fiber(bundle)high-strength concrete.The maximum increase rates of concrete compressive strength,split tensile strength and flexural strength were 6.1%,23.4%,and 20.7%respectively when singlemixed with basalt macro-fibers;when double-doped basalt macro-fibers and chopped basalt fibers,the corresponding maximum increase rates were 13.5%,37.0%,and 24.1%respectively;when the three-doped basalt macro-fibers,chopped basalt fibers and calcium sulfate whiskers were used,the corresponding maximum increase were 9.2%,45.4%,and 29.8%,respectively.(2)Test of resistance to chloride ion penetration of hybrid fiber(bundle)highstrength concrete.The chloride diffusion coefficient(DRCM)of the hybrid fiber(bundle)high-strength concrete was determined by the unsteady chloride migration coefficient method(also known as the RCM method).The results show that the chloride ion permeation resistance of high-strength concrete is reduced when the basalt macrofibers was added alone,and the DRCM increases with the increase of fiber content and length;the appropriate amount of BMF and CBF can improve the chloride ion permeation resistance of high-strength concrete,and the maximum increase rate is 6.7%;when three admixtures are used,the resistance to chloride ion penetration of high-strength concrete was improved,and the maximum increase rate is 9.1%.(3)Frost resistance test of hybrid fiber(bundle)high-strength concrete.By evaluating the apparent damage morphology,mass loss rate,relative dynamic elastic modulus and compressive strength loss rate of different types of high-strength concrete after freeze-thaw cycles,their frost resistance performance was studied.The results show that the fiber greatly reduces the appearance damage of high-strength concrete caused by freeze-thaw;the mass loss rate of different types of high-strength concrete after freeze-thaw is negative,because the appearance denudation degree of highstrength concrete by freeze-thaw is relatively light,which is always smaller than the water absorption quality of concrete.The quality of surface slurry spalling;from the perspective of relative dynamic elastic modulus and compressive strength loss rate,plain high-strength concrete has the worst frost resistance,and its compressive strength loss rate is 30.9%after 150 freeze-thaw cycles.High-strength concrete has the best frost resistance,and the strength loss rate of CSW3CBF0.15BMF0.3 specimen after 150 freeze-thaw cycles is only 2.5%.Finally,a damage model of hybrid fiber(bundle)high-strength concrete after freezing and thawing was established,and combined with the model and site temperature,the service life of concrete in natural freezing and thawing environment was predicted based on the definition of equivalent damage.(4)Combined with mercury intrusion test(MIP)and scanning electron microscope test(SEM),the microstructure of different types of high-strength concrete was analyzed,and the fiber reinforcement mechanism was revealed.The results show that the porosity,total pore volume and total pore area of high-strength concrete are increased when single and double fibers are added,but the most probable pore size and average pore size are decreased,indicating that the addition of fibers refines the pore structure.When BMF,CBF and CSW are mixed,all pore structure characteristic parameters such as porosity,most probable pore diameter,total pore volume,total pore area and average pore diameter are reduced,and the pore structure is refined;the reinforcement mechanism of chopped basalt fiber is fiber Tensile failure;basalt macrofibers are cemented into a bundle as a stress unit;compared with single-doped basalt fiber bundles,the cement slurry increases the encapsulation rate of basalt macro-fibers when double-doped;The whiskers fill the pores between the fibers and the matrix,and strengthen the connection between the fibers and the matrix.(5)In the research scope of this thesis,comprehensively considering the compressive strength,splitting tensile strength,flexural strength,chloride ion permeability resistance and freezing resistance,when the basalt macro-fibers is singledoped,the optimal dosage is 0.3%,and the optimal length is 30mm.When basalt fiber and basalt macro-fiber are doped,the optimal total volume dosage is 0.45%,and the mixing ratio is 1:2.The overall effect is the best when the content of basalt macrofibers is 0.3%,the content of chopped basalt fibers is 0.15%,and the content of calcium sulfate whiskers is 2%of the cement mass.The research shows that by twisting and mixing different sizes of BF,the content of basalt fiber in concrete can be increased,and its reinforcing effect can be exerted to improve the durability of high-strength concrete finally.After adding an appropriate amount of sub-nanometer CSW,the mixing effect even better,it benefits from the filling effect of calcium sulfate whiskers and participation in hydration reaction to generate ettringite.