Studies On Electric Properties Of Cementitious Composite Containing Carbon Fibers And Their Applications

Because of influencing factors such as natural environment and cumulative damage, concrete structure will suffer from degeneration of materials and debasement of application function, which will curtail its longevity and reduce the security, durability and applicability. Detection for new and old concrete structures is effective measure to monitor the structure's reliability. Cementitious composite containing carbon fibers is one kind of smart materials which can monitor itself. Cementitious composite containing carbon fibers is new detective materials for concrete structures which will lead to low cost and extensive application. In last decades, this materials allured attention of many researchers whose applied field is extending fleetly. Based on the project of the National Natural Science Foundation of China (No. 50438010), influencing factors which affect the electric properties of cementitious composites containing carbon fibers and how to apply this smart materials to monitor concrete structure were investigated in this dissertation. The major contents are mainly summarized as follow:1. According to the effect of the non-linear elastic behaviour of concrete, the normal stress at fiber end and the effect of thermal stress caused by temperature change on shear stress and normal stress in fiber, the traditional shear-lag theory of stress transfer in short fiber reinforced concrete-matrix composites was improved. The paper analyzes the effect of concrete strain, stress coefficients at fiber end and temperature difference on carbon fiber reinforced concrete (CFRC) and concludes that the impact of strain on shear stress and normal stress in fiber is more dominant in plastic stage than in elastic stage, the effect of stress coefficients at fiber end on shear stress and normal stress in fiber is linear, shear stress and temperature change with the same tendency, but temperature could not affect normal stress in fiber.2. Based on actuality how to detect resistance of cementitious composites containing car--bon fibers and requirement of structural experimentation and engineering, a resistance measurement device for cementitious composites was developed in this paper, which surmount not only complexity and high cost of existent device with electrode, but also characteristics of noncontacting electrical resistivity measurement device like CCR-1 which is disturbed by surroundings, only suitable for material experimentation and leaks magnetism. The results suggest that this electrical resistivity measurement device can be applied into material experimentation, three-point bending beam experimentation and mode â…¡ fracture experim--entation, which can acquire satisfactory relationship between resistance and parameters of materials and mechanics.3. Relationships between DC resistance of CFRM and CFRC and curing age were researched using the two-probe method. To simulate crack inside material under load, insulators were embedded into specimens. Then the effects of insulative location andinsulative quantity on DC resistance of CFRM and CFRC were investigated at different curing age. For CFRM, the results suggest that DC resistance inceases greatly with its curing age, which illustrates relationship like Sigmoidal curve. The quantity of insulation has few effect on DC resistance, but the shorter the distant between electrode and insulating trip is, the stronger its conductibility is. For CFRC, the results suggest that DC resistance increases greatly with its curing age, which illustrates relationship like Gaussian curve. Insulative location in optimal position can only result in optimal conductibility. In same curing age, section and insulative area, more quantity of insulation is, stronger its conductibility is. From grey relational analysis, influencing factors for CFRM abate in following order: curing age, insulative location, insulative quantity and insulative area. Influencing factors for CFRC abate in following order: curing age, insulative quantity, insulative location and insulative area. Curing age is decisive factor for electric properties of CFRM and CFRC.4. The electric properties of CFRC was utilized to attain initial load of three-point bending beams. Double-K criterion was imported to compute k% and &ï¿¡ ? Relationships betweenfractional resistance increase and load, deflection and CMOD were analyzed, which are accord with Lorentz fit. The curves of fractional resistance increase before maximum load can illustrate three stages such as elastic deformation-, stable fracture and unstable fracture.5. The electric properties of carbon fiber reinforced concrete was applied to investigate mode II fracture of double edge notched specimen. The results suggest that resistance increases with increase of load and displacement. Electric properties of carbon fiber reinforced concrete can be used to ascertain k nc values of double edge notched specimen. The curves of fractional resistance increase before maximum load can illustrate three stages such as elastic deformation^ stable fracture and unstable fracture.6. To avoid the disadvantages of high cost and complex operation of actual methods whic--h were applied to monitor the structure's reliability, the electric properties of carbon fiber rei--nforced concrete was imported to investigate the reliability of three-point bending beam and analyze the relationship between the reliability index of load bearing and damage and fraction--al change in resistance. Results indicate two kinds of reliability indices of three-point bending beam decrease with the increase of fractional resistance increase whose relationship accor--ds with Lorentz curve, whose parameters are stable under the effect of different ratio between variances and means of load and deflection. All conclusions illuminate that the smart properti--es of carbon fiber reinforced concrete can to monitor the process of damage of three-point be--nding beam and reflect the rule of reliability index well. Based on bilinear model, damage model based upon deflection was pointed out to analyze the damage of three-point bending beam. Then grey forecasting theory was utilized to predict tendency of damage. The results indicate that grey forecasting theory can be utilized in damage analysis and prediction well, whose error is small and reliability is excellent.