Experimental Study Of Static And Fatigue Behavior Of Tension Steel Plate Strengthened With Prestressed CFRP Laminates

Bonding prestressed CFRP plates to old steel structures is an innovative and effective reinforcement technique, and it provides a higher static and fatigue strength as well as a non-corroding alternative to other traditional rehabilitative methods. This specialized application combines the benefits of passively bonded CFRP laminates systems with advantages associated with external prestressing, to improve the serviceability and ultimate load of the damaged steel structure. This dissertation focused theoretically and experimentally on the static and fatigue behavior of the pre-crack steel structure strengthened with prestressed CFRP laminates.Firstly the research significance on fatigue damages of steel structure was presented and the existing reinforcement technologies were also investigated. And an attempt was made to analyze the static and fatigue performance, and durability of steel structure strengthened with non-prestressed CFRP laminates. It was noted that un-efficient use of CFRP material limits its wide application, and introduce of prestress is an excellent solution.Secondly the tension steel plate strengthened with pre-stressed CFRP laminates was investigated based on one-dimensional linear elastic theory and a three-dimensional element finite element method (FEM) model. These theoretical results are in a good agreement with the value of the static test. The results show that, the mechanical property of CFRP material is relatively stable with lower creep under high stress state. The prestress eccentricities lead to the bending effect which affects the stress distribution across the steel thickness. By applying a prestress of the laminate, the material is used much more efficiently and the ultimate capacity and stiffness of the reinforced structure increase.Then a modification of Pairs'law, which considers both the global and local reinforcement effect, was used to analysis the fatigue behaviour of central-cracked tension steel plates strengthened with single-prestress-level CFRP laminates in Basseti's test series. Test results show that depending on the ratio of the applied load, reinforcement and prestress level, the application of CFRP laminates results in an increase in the fatigue life by more than 16 times. The application of a high prestress level is a promising method to achieve a significant improvement in term of remaining fatigue life of the damaged structure.The author conducted experimental studies of static and fatigue behaviour of double-edge-notched tension steel plates reinforced by multi-prestress-level CFRP laminates. The test results indicate that the static and fatigue performance depends on stiffness of CFRP laminate and, largely, the prestress level. The relative fatigue life predicted by the modified Paris law is in a reasonable agreement with test results.From the view of the global reinforcement effect, introduce of prestress prolongs significantly the remaining fatigue life of reinforced structure since a greater portion of reduction of cyclic stress and stress ratio is engaged. The fatigue loading, as the driving force for crack propagation, is an important factor for estimation of the fatigue life.From the view of the local reinforcement effect, the prestress influences the local behaviours of crack propagation in the following aspects:1) decrease of the steel plastic zone and the adhesive debonding area in crack front; 2) the crack non-uniform propagation across steel thickness due to bending stress caused by prestress eccentricity; 3) initiation and propagation of the secondary fatigue crack.Two different crack growth measurements, infrared thermography testing and Fractographic approach, were used to detect the crack in the fatigue test. The results show that the crack propagation can be defined as surface-crack propagation and through-crack propagation, and the surface crack propagation consumes a major part (more than 50%) of total fatigue life with short crack length (less than 5mm). Surface-crack propagation depends on the size and shape of the initial crack, and therefore the pretreatment of the initial crack is necessary for the reinforcement, for example, the use of stop holes prior to CFRP application will give a better result.The anchor system and estimation of pre-stress loss in application of prestressed CFRP laminate are highly significant. It was found that mechanical gripping bond-type anchor combines the benefits of Mechanical gripping anchor and bond-type anchor, and it is more reliable and efficient for the anchorage of prestressed CFRP laminates. The prestress losses related to different anchor systems were also investigated. The total prestress loss of CFRP laminate is consist of not only the short-term prestress losses due to elastic deformations and anchorage set, but also the long-term loss in service stage. Fatigue test results shows that the long-term prestress loss under cyclic load is slight and can be neglected. Hence the estimation of the pre-stress losses relies on the short-term prestress losses. The prestress losses of CFRP laminate with bond-type anchor were 30%-50% of the initial prestress because of the large deformation of anchorage set. The slight prestress losses of CFRP laminate with mechanical gripping bond-type anchor are mainly due to elastic deformations. The mechanical gripping and bond-type anchor was proved to be much more efficient than bond-type anchor in the test.Based on the theory of crack analysis, three-dimensional element FEM fracture model was presented to do parameter study on the stress intensity factor, and the effects of the factors such as the stiffness of CFRP laminates, the prestress level and adhesive properties were discussed. The results show that the prestress level becomes the dominated parameter to improve the fatigue behavior of the reinforced structure.In the end the classic beam theories were used to analysis the static flexural behavior of steel girder strengthened with prestressed CFRP laminates, including the flexural stress, deflection and failure modes. The fatigue test results indicate that the application of CFRP laminates results in a significant increase of the fatigue life by more than 10 times. Furthermore, a simplified design and calculation method was presented to estimate the demanding prestress level in reinforcement.