Sensing Properties Of Resin Based Carbon Fiber Reinforced Bar And Its Application In Pre-stressed Concrete Structure

Composite materials are usually used as important structural load-bearing components in large-scale projects,and their health status is the primary factor to ensure the safety,reliability and stability of structures in the life cycle.Traditional structural health monitoring methods embed sensing elements to monitor the safety performance of the structure,which will damage the integrity of the structure or the component materials,thus greatly reducing the mechanical properties of composite materials.While the non-destructive evaluation method(NDE)has complex operation,heavy workload,long test cycle,and it is difficult to realize continuous monitoring in the whole field and full-time domain.Therefore,Composite materials are usually used as important structural load-bearing components in large-scale projects,and their health status is the primary factor to ensure the safety,reliability and stability of structures in the life cycle.Traditional structural health monitoring methods embed sensors to monitor the safety performance of the structure,which will damage the integrity of the structure or component materials,thus greatly reducing the mechanical properties of composite materials.However,the non-destructive testing method has complex operation,heavy workload and long test cycle,and it is difficult to realize continuous monitoring in the whole field and in the whole time domain.Therefore,it is one of the most important requirements in modern engineering structures to explore a structural health detection method which is accurate,reliable,convenient,real-time and runs through the whole life service process by using the sensing characteristics of structural materials.Carbon fiber reinforced polymer composites(CFRPs)not only have high specific strength,corrosion resistance and other load-bearing and durability characteristics,but also have good self-sensing characteristics.It is a potential ideal material to achieve this goal.This research comes from the Natural Science Foundation project,"Research on self-sensing characteristics and application of pre-stressed concrete structure based on carbon fiber materials"(NO.51478209).Firstly,a piezoresistive model of CFRP is established through theoretical derivation,and its accuracy and reliability are verified by literature data and experiments,respectively.Secondly,based on this theoretical model,the influences of preparation process,carbon powder modification and electrode layout on the bearing and sensing characteristics of CFRP bars are explored through multi factor optimization comparative experiments,and CFRP sensing bars with excellent bearing performance and stable sensing characteristics are obtained.Finally,the CFRP sensing bars are used as part of the tensile load-bearing bars of the composite reinforced bending beams and the prestressed tendons of the prestressed concrete beams.The real-time resistance change response process parameters of CFRP bars are measured and analyzed through beam test so as to accurately and real-time monitor the service state of CFRP bars and surrounding structures.The main results are as follows:(1)Establishment and verification of the piezoresistive model.The total strain energy of CFRP reinforcement structure is derived based upon the elastic strain energy theory and the mixing law of composite materials;the total potential energy equation of CFRP Composite in passive electrostatic field is deduced by combining Maxwell velocity distribution and kinematics equation;the stress-impedance effect model based on the relationship between conductivity and strain of the CFRP bar is established according to Thomson's theorem and variational principle.We use the numerical fitting method in Matlab to verify and analyze the model.The fitting results are in good agreement with the literature data and the experimental data of CFRP tendon sensing performance in this paper.The mean value of determinable coefficient reached 0.974,indicating high accuracy and reliability of the stress-impedance effect model.We further derive the piecewise equation of CFRP axial strain and its resistance value through the piezoresistive model,and find that the sensitivity-strain model has three types of curves i.e.concave curve,straight line and convex curve.The accuracy of the model is verified by the test data,and the causes of different curve types are analyzed,which provides a theoretical basis for the sensing performance experiment of CFRP bars in this study.(2)Analysis of strain transfer mechanism between CFRP bars and structures.Based on the interaction mechanism between CFRP and concrete structure,the strain transfer mechanism is deduced.The effective bond zone and strain transfer efficiency are obtained through theoretical deduction and actual calculation,which provides a theoretical basis for the sensing function range and electrode setting of CFRP bars.Referring to the concrete design code and the American Concrete Beam Design standard,according to the principle of "equal strength replacement",the mixed reinforced beams are designed according to the conversion method of equal strength and equal modulus of elasticity,respectively,the results show that the bearing capacity and mid span deflection of the hybrid reinforced beams with CFRP bars instead of some reinforcements have no obvious performance degradation compared with reinforced concrete beams.(3)Fabrication and performance improvement of CFRP sensor bar.CFRP sensing bars were fabricated using three different methods including vacuum process,brushing process and roll extrusion process.The key parameters of mechanical properties and sensing characteristics are obtained through uniaxial tensile test.The test results show that the roll extrusion process has better load-bearing efficiency and sensing stability.By uniformly dispersing the conductive phase of carbon powder with a mass ratio of 6% in the resin,the sensing sensitivity of CFRP bars can be improved effectively and the dispersion of conductivity of CFRP bars can be reduced.The elastic modulus,linearity,repeatability and sensitivity of CFRP bars modified with 6% carbon powder were obtained by static tensile and cyclic loading tests,which further proved the reliability of the CFRP bars as load-bearing components and sensing elements.(4)Calibration and debugging of sensing characteristics of CFRP bars.CFRP-FBG composite sensing bars were constructed by reasonably arranging the optical fiber measuring points,pasting the optical fiber package and setting the copper foil interval electrode to introduce optical fiber testing as standard control.The tensile test results of CFRP-FBG sensing bar show that the CFRP resistance change detected by the electrical signal of the copper foil electrode has a consistent corresponding relationship with the strain measured by the FBG,which can realize the self-sensing and self-monitoring functions of the CFRP sensing bar.The test results also show that the FBG stops monitoring work much earlier than the breaks of CFRP bar,which can still maintain high-precision sensing function after the broken of FBG.Therefore,independent application of CFRP bars can realize high-precision and reliable sensing monitoring under high stress and complex environment conditions.(5)Application Research of CFRP sensing bar.In order to further test the feasibility and accuracy of CFRP sensing bars in practical engineering applications under high stress and complex surroundings.The CFRP bars are used as part of the tensile load-bearing bars of the flexural beams with mixed reinforcement.The results show that the real-time resistance response of each electrode region of CFRP bars in the beam structure can be measured to realize the self sensing of the stress and strain state of the reinforcement,as well as the self monitoring and early warning of the steel yield,load distribution,crack development and reinforcement fracture in the corresponding section of the beam structure.Furthermore,CFRP sensing bars are used as prestressed tendons of prestressed concrete test beams.The high-strength load-bearing characteristics of the CFRP bar are well utilized,and the stress state monitoring and structural health warning of prestressed steel bar and prestressed concrete beam structure are realized.