Effect Of Hybrid Fiber And Nano Carbon Black On The Self-sensing Ability Of Crack For Smart Concrete

In the service stage,the concrete structure works with cracks under load.The unfavorable effect of the external loading and the environment will cause the expansion of cracks and the formation of new cracks,reduce the serviceability and durability significantly,and even lead to a sudden concrete structure failure.Therefore,it is of great importance to monitor cracks in concrete structures.Since the shortcomings of the current concrete crack monitoring technology(may not be able to achieve real-time monitoring,or failed in monitoring once crack appears because the deformation coordinated condition assumption is no longer satisfied,or the cost is too much high),a new kind of concrete with a hybrid use of different fibers and nano-carbon black is developed to monitor concrete crack propagation and to enhance the mechanical behavior of structures,based on the carbon fiber reinforced cementitious strain sensor technology.The aim of this work is to explore the feasibility of integration of structural materials and self-sensing functional materials,to provide a new way for structural health monitoring of infrastructures such as bridges and tunnels.The detailed contents and conclusions of this thesis are as follows.(1)The effects of hybrid fiber(steel fiber(SF)and carbon fiber(CF))and nano carbon black(CB)on the fresh concrete property including slump and air content,as well as on the compressive strength of hardened concrete are studied.The results show that:low content of SF(?40kg/m3),CF(?2 kg/m3),CB(?1.5kg/m3)exhibit little influence on the slump and air content of fresh concrete,however,as the conductive material content further increases,the slump of concrete shows a significant decrease trend,while the air content showed a significant increasing trend.Besides,the compressive strength of concrete is not influenced by the monophase,di-phasic and tri-phasic conductive materials at low volume content.(2)The four-electrode method is used to measure the fraction change in resistance during the crack propagation of the concrete beam subjected to bending.The effect of different combinations and content of carbon black,steel fiber,carbon fiber,and different fiber lengths on the crack self-sensing gauge factor(K)of the concrete beam is compared and analyzed.Based on the fractal theory,the effect of different conductive materials on the noise level of the crack self-sensing signal curve(FCR-COD)is studied.The results show that the linear fitting analysis fit well with the FCR-COD curve and the GF can be characterized as the slope of the fitted line.In addition,the K decreases with the increase of steel fiber content.Specimens with the addition of steel fiber and nano carbon black reveal the best crack self-sensing ability among all the specimens,because the steel fibers and nano carbon black can form a continuous conductive network combined with long-range and short-range electron transfer pathway.The K can be improved while the tortuosity of the FCR-COD curve is reduced by the addition of nano carbon black.Specifically,The K values exhibit an increasing tendency till the carbon black increased to a certain content and then GF decreased for even higher content of CB.However,the fractal dimension(D)of the specimens and carbon black content shows an inverse tendency.Furthermore,the optimum dosage of nano carbon black is 1-1.5kg/m3.There exists a negative effect on the K value with the addition of carbon fibers.However,the standard deviation of the fractal dimension(D)of the normalized FCR-COD curves is reduced for the specimens with carbon fibers.The sensitivity can be improved by increasing the proportion of long steel fibers in those specimens hybrid with short and long fibers.(3)With the assistance of the self-designed devices for crack width and crack region monitoring of concrete subjected to bending,two crack location method are proposed,namely continuous-section location method and overlapped-section location method,in order to realize concrete crack location.The mechanism of concrete location and the influence of crack width and crack height on the distribution change of the electric potential and current density of the specimen are analyzed using the finite element software COMSOL.The results show that the development of the crack height has an obvious influence on the electric potential redistribution in the concrete specimens,however the crack width opening has little influence on the electric potential redistribution of the specimens.According to the characteristic of the electric potential and current distribution change before and after cracking,the cracked section can be determined.Therefore,the concepts of electric potential redistribution and current redistribution can explain how these methods can realize the crack location reasonably.(4)The macro steel fiber(SF)and nano carbon black(CB)as conductive materials are added into the concrete to make smart concrete.A new type of concrete beam with double layers combined with the smart concrete and plain concrete are produced.In addition,the effect of the steel fiber content,the depth of the smart concrete layer and the age of concrete on the post-crack toughness and self-monitoring performance of the double-layered beam are studied in this paper.In order to monitor the concrete crack,the four-probe method is used to measure the electrical resistance of the specimens.The results show that the toughness of the concrete beams is increased with the increasing of fiber content and the depth of the smart concrete layer.The crack self-monitoring of the concrete can be realized by analyzing the characteristics of the FCR-Time curve.The value of FCR of the specimen increases rapidly when the crack initiates,after that it increases gradually.Additionally,the self-sensing behavior decreases with the increasing of the steel fiber content and the depth of the smart concrete layer.To increase the age of concrete is beneficial to improve the crack self-sensing sensitivity.The proposed firstorder exponential decay function fits well with the FCR-COD curves.(5)The feasibility of the crack self-monitoring of steel reinforced concrete beams doped with steel fiber and nano-carbon black subjected to bending is investigated.Furthermore,the effect of the concrete cover thickness,the diameter of the steel rebar,the age of the concrete,the longitudinal reinforcement ratio and stirrup ratio on the self-sensing performance are analyzed.The results show that the reinforced smart concrete can realize the crack selfmonitoring by the real-time measurement of the resistance change of the specimen.The crack self-sensing sensitivity of the specimens is increased with the decreasing of concrete cover thickness from 30 mm to 20 mm and the diameter of the steel bar from 6 mm to 4 mm.In addition,the crack sensitivity can be improved by the increase of concrete age,and the increase in concrete age also reduces the noise of the FCR-COD curve.It can be also found that the selfmonitoring ability of reinforced smart concrete beams is weakened with the increase of the reinforcement ratio,but the negative effect will be reduced with the further increase of the reinforcement ratio.Moreover,the crack self-monitoring ability does not change significantly with the change of hoop reinforcement ratio.(6)Splitting tests and direct tensile tests are carried out for the smart concrete hybrid with steel fibers and nano carbon black.The self-sensing performance of the saturated and unsaturated smart concrete are compared.Effect of the fiber content and electrode types on the self-sensing performance of the smart concrete specimens are also studied.Results show that the signal noise of the self-sensing of a saturated specimen is higher than that of an unsaturated specimen.The sensitivity of the crack sensing will be reduced by the increasing of steel fiber content to a certain degree.The FCP(Fractional change in electric potential)value used in the monitoring of the concrete crack is more obvious than FCR value;in addition,smart concrete hybrid with steel fibers and carbon black also has the feasibility of crack self-monitoring ability during direct tensile tests.