Experimental And Theoretical Investigation On The Anchorage System For CFRP Tendons

Due to it that the ratio of the shear strength to the tensile strength of the Carbon Fiber Reinforced Polymer/Plastics (CFRP) tendons is lower, so the shear strength is the major concern. The wedge anchor for prestressing steel tendons can't be applied to anchor CFRP tendons, otherwise the shear stress causes premature failure of the tendon. Thus, the anchorage system for CFRP tendons must be investigated when used in pretressing concrete or cable structure. The researches on anchorage system for CFRP tendons have arrived considerable accomplishments, but they are still insufficient for CFRP tendons/cables application. So it is of important practical and theoretical significance for the investigation on the anchorage system.An experimental study and corresponding theory analysis are conducted to investigate the anchorage performance of CFRP tendons under the financial support of Natural Science Foundation of China and Excellent Young Teachers Program of Ministry of Education of China. This dissertation involves mainly the following investigations and corresponding results:1. A new bond-type anchor for CFRP tendons is developed, whose bond medium is Reactive Powder Concrete (RPC). Due to the Reactive Powder Concrete with excellent performance, it has been put forward as the bond medium and a series of tests have been carried out to develop a new bond-type of anchorage system for CFRP tendon and cable. Those parameters of surface shape, pretensioning load, bond length, number and space of CFRP tendons, the slope of inner wall of outer steel tube used in the anchorage system, have been studied. The tests results show that the grouted length of 20 diameters of CFRP tendon is enough for the indented surface CFRP tendon of tensile strength less than 3000 MPa. When the pretensioning load arrives 56% of ultimate load of general CFRP bar, the indented surface CFRP tendon with tensile strength is less than 3000 MPa, critical bond length of the pretensioning tendon in RPC of compressive strength 130 MPa is 13 diameters of CFRP bar. The equations developed to determine bond capacity, and the corresponding slip, and critical bond length are applicable. An analytical model of bond stress–slip relationships for CFRP tendon has been proposed.2. The bond stress distribution of CFRP (Carbon Fiber Reinforced Polymer/Plastics) tendon in RPC (Reactive Powder Concrete) along bond length has been studied. Based on the experimental investigation, CFRP tendon stress and slip and displacement funtion along bond length can be presented from the equilibrium equations and the linear elastic relationship between stress and strain and constitutive model. Comparison of experimental and analytical results with CFRP tendon stress and bond stress shows good agreement, so its effectiveness to represent the theory equations has been demonstrated. Bond stresses of CFRP tendon without groove along bond length are obtained and parameter analysis results show that when the grouted length of the smooth surface for CFRP tendon is ranged between 15 bar diameters and 45 bar diameters, the ratios of maximum to minimum bond stress and maximum to average bond stress are about 1.0. When the grouted length of the indented surface CFRP tendon is less than 12.5 diameters of CFRP tendon,bond stress distribution along bond length is comparatively uniform, and the ratios of maximum to minimum bond stress and maximum to average bond stress are about 1.02～1.05. When the grouted length of the indented surface CFRP tendon is more than 12.5 diameters and less than or equal to 20 diameters of CFRP tendon,bond stress distribution along bond length is not uniform, and the ratios of maximum to minimum bond stress and maximum to average bond stress are about 1.06～1.14.3. Base on the investigation on radial elastic modulus of an interface between FRP reinforcement and concrete, bond model and radial elastic modulus of an interface of bond-type anchorage for FRP tendon is defined. Based on this equivalence measure of the strain energies stored in the elastic bodies, closed-form elastic solutions are obtained for axisymmetric hollow or solid cylinders subjected to a radial traction. The overall agreement between the analytical and numerical solutions suggests that the analytical solution is correct. Then radial elastic modulus of an interface for bond-type anchorage is available from the analytical expressions. Those parameters such as constituent properties, outside radius of bond medium, thickness of steel sleeve, radial traction, material constants and so on have been studied. The results show that when bond medium radius is more than 1.0 bar diameters, the periodic length of the bar's surface structure is more than 1.0 bar diameters, the rib's width is more than 0.5 bar diameters, steel sleeve thickness is more than 2.5mm, it is lesser influence to radial elastic modulus of an interface for bond medium radius and the periodic length of the bar's surface structure and the rib's width and steel sleeve thickness. The radial response can be evaluated preferably in Bar-scale model, in which radial elastic modulus is considered.4. Conventional wedge-type anchorage systems for steel tendon must be improved for CFRP tendons due to their material particularity. Wedge-type anchors for CFRP tendons have been developed. The new anchors are assembled from smooth indented wedges, conical barrel, a plastic film, and an aluminum soft metal sleeve. The static tests show, when the anchor involves 3°of slope of conical barrel, 100kN of presetting load level, 1.0 mm of thickness of an aluminum soft metal sleeve, 12.85 mm of space and 0.3 mm of depth of indented wedges, the anchorage system can perform well and undertake 185kN load of the ultimate capability, the related anchorage efficiency is 96.4%. Many factors were demonstrated to affect the anchorage performance and the slip of CFRP tendon, which include length and slope of conical barrel, presetting load level, space and depth and width of indented wedges, and thickness of an aluminum sleeve. Based on the agreement with the analytical and experimental stresses of conical barrel, the present approach is proved. The equations developed for evaluating ultimate load of the anchors are feasible.5. Finite element model of wedge-type anchor for CFRP tendons is established using ANSYS soft with array parameters. The model is axisymmetric with plasticity and contact analysis. Tsai-Wu failure criterion is as terminative condition. Based on the good agreement with the analytical and experimental results, the validity of the present approach is proved. The optimum values of various parameters are obtained. The results show that slope of barrel is 2.5°, angle difference between the wedge and barrel is 0.1°, length of conical barrel is 90 mm, seating distance of the wedge is 10 mm, thickness of an aluminum sleeve is 1 mm, friction coefficient between the wedges and barrel is 0.03, friction coefficient between the tendon and aluminum sleeve is 0.50. So the numerical results are benefit to improve the test results.6. Based on the character of anchorage systems for steel tendon, mechanical gripping-bond type anchorage system is developed for CFRP tendons usage in the stay-cable bridge. Those parameters of barrel length, bond length presetting load level, pretensioning load used in the anchorage system are studied. The static tests show, when the anchor involves 60 mm of barrel length, 100 mm of bond length, 60 kN of presetting load level, the anchorage system can perform well and undertake 208kN load of the ultimate capability, the related anchorage efficiency is 108.2%, which is more than 95%. The equation developed for evaluating ultimate load of the mechanical gripping-bond type anchorage systems is applicable.