Investigation On Static And Fatigue Behavior Of UHPC Beams Prestressed With External CFRP Tendons

The application of high performance materials is an important driving force for the innovation and development of civil engineering.Owing to their excellent mechanical properties,research on advanced composite materials,fiber-reinforced polymers(FRP)and ultra-high-performance concrete(UHPC),have significantly progressed throughout the world in recent years.The combination of these composite materials may have the potential to overcome the corrosion problem of traditional steel prestressing strands,improve the ductility of FRP-reinforced structures,reduce the structural dead weight,and enhance the span length.It is expected to develop a novel FRP externally prestressed UHPC beam with remarkable structural behavior as well as durability.Nevertheless,to the best of the author’s knowledge,few if any studies have reported the application of external FRP tendons in UHPC beams.Thus,this dissertation presented experimental researches and theoretical analyses on the flexural behavior,shear behavior,and fatigue behavior of UHPC beams prestressed with external carbon FRP(CFRP)tendons.These investigations were sponsored by the National Key Research and Development Program of China(“Key Technology Integration and Demonstration for the New Structural Application of Fiber Reinforced Composites”,Grant No.: 2017YFC0703008),and the National Natural Science Foundation of China(“Theories and methodologies for structural design and wind-induced catastrophe of super-long span cable-supported bridges with high performance materials”,Grant No.: 51938012).It aims to provide experimental data and application basis of UHPC beams prestressed with external CFRP tendons.The main results and conclusions are as follows:(1)Eight T-shaped specimens were designed and tested to study the flexural behavior of UHPC beams prestressed with external CFRP tendons.The effects of the design variables,including the effective prestressing stress,partial prestressing ratio,deviated angle,and loading condition on the flexural behavior were investigated.The load-deflection relationship,the stress variation in CFRP tendons,the strain variations in UHPC and non-prestressed steel bars,and the crack distribution of each specimen were measured.The differences of failure modes,ultimate flexural capacity,and ultimate deformation between the fully prestressed beam and the partially prestressed beam were discussed in detail.(2)A prediction method for calculating the ultimate stress of the external CFRP tendons was proposed based on the plastic hinge theory,and thus the simplified equations for predicting the cracking load and ultimate load of UHPC beams prestressed with external CFRP tendons were presented.Moreover,based on computational software MATLAB,a nonlinear analysis program of UHPC beams prestressed with external CFRP tendons was developed.Hence,the differences between the novel beam and the externally prestressed beam with traditional engineering materials,and the impacts of main design variables were analyzed in-depth using the analysis program.(3)Seven T-shaped specimens were designed and tested to study the shear behavior of UHPC beams prestressed with external CFRP tendons.The effects of the design variables,including the shear span ratio and effective prestressing stress in CFRP tendons on the shear behavior were investigated.The load-deflection relationship,the stress variation in CFRP tendons,the strain variations in UHPC and internal glass FRP(GFRP)bars,and the crack distribution of each specimen were obtained.The influences of the GFRP stirrups and steel fibers in UHPC on the shear behavior were analyzed in detail.(4)The applicability of prediction equations in the current FRP structure and UHPC structure design specifications were evaluated.Based on the results of shear tests,a prediction method of the shear capacity was proposed,which takes into accounted the effects of the steel fibers and shear span to depth ratio.This method could appropriately predict the shear capacity of fully FRP reinforced UHPC beams both in this study and literature.(5)Five T-shaped specimens were designed and tested to study the fatigue behavior of UHPC beams prestressed with external CFRP tendons.The effects of the design variables,including the effective prestressing stress,partial prestressing ratio,and fatigue load level on the fatigue behavior were investigated.The stiffness degradation,propagation of cracks,stress variation in CFRP tendons,and strain variations in UHPC and non-prestressed steel bars of each specimen were discussed.Besides,The fatigue deflection of the specimens was analyzed,and a cumulative damage model was proposed to evaluate the fatigue damage of UHPC beams prestressed with external CFRP tendons.(6)The fatigue damage evolution model and fatigue failure criterion of the related materials of UHPC beams prestressed with external CFRP tendons were presented in detail.Based on the local stress-strain analysis of the non-prestressed steel bars,a prediction model of the fatigue life of UHPC beams prestressed with external CFRP tendons was proposed.