Two Important Issues In FRP-Concrete Iterfacial Bond

Fiber reinforced polymer (FRP) composites have been widely used for strengthening reinforced concrete (RC) structures since 1990s. To support the rapid development of this new technology, many issues related to the structural performances of FRP strengthened RC elements have been studied. Among them, the studies on the FRP-concrete interfacial bond-slip (δ-τ) constitutive law and the debonding failure of FRP strengthening reinforce concrete beams have become two important focuces in recent years.Since the FRP-concrete interfacialδ-τconstitutive law plays a key role in the composite performances and the durability of RC structures after being strengthened, the studies on it was regareded to be one of the most fundamental and important issues. Considerable experimental studies have been conducted onδ-τcurve. However, the ultimate slipδ_u, one of the three key control parameters ofδ-τcurve, could not be captured in existing experiments, through several measuring methods have been attempted. By means of finite element method(FEM), it was shown that the key reason for not being able to captureδ_u was the unrational design of the existing popular"simple shear specimens". The unrational design led to a whole debonding failure of the FRP-concrete interface at a very low load level. A new conceptual design named"double pull"specimen with a great potential for measuringδ_uaccurately was proposed in this thesis. The reliability of the proposed double pull specimen was preliminarily proved by comparing the debonding feature of the existed simple shear specimen with that of the double pull specimen through FEM. The calculating results show that, the"double pull"specimen can bear a much higher loading without whole debonding. The"double pull"specimen, therefore, is more suitable for measuringδ-τrelationship than the existed"simple shear"specimen.Based on two exploratory experimental studies, a kind of practical double pull specimen was developed. The existing test mehods for measuringδ-τcurve was also summaried, then a simple and accurate method for measuringδdirectly using displacement transducers was put forward. Consequently, 22double pull specimens were produced, and displacement transducers and strain gauges were coupled for measuring the range ofδ_u. The test results showed thatδ_uwas about 0.4mm under the given experimental conditions.The intermediate FRP-concrete interfacial debonding failure may occur in midspan of strengthened reinforced concrete beams. Such failure decreases the flexural load-carrying capacity and induces abrupt collapse of the strenthened beam. Thus, it must be thoroughly investigated.Several calculating methods for predicting the interfacial shear stress were put forward by a number of researchers. However, existing methods for predicting the shear stresses led to calculation results of great differences. In order to obtain a better understanding on the debonding failure, a new simplified and reliable FEM model named"beam segment model"is proposed and validated. Based on this model, the distributions of interfacial shear stress in FRP strengthened beams under the influence of various parameters were calculated and discussed. Based on the research results, it is consequently concluded that the ultimate interface shear stress is not suitable for judging the debonding failures. It is also suggested that the ultimate slipδ_u may be better to be used as the debonding failure criterion. Twelve beams strengthened by CFPR, GFRP and HFRP respectively were produced to verify the proposed"beam segment model". The test results showed a basic agreement with the calculation results of the"beam segment model". Noticing that to research the FRP-concrete debonding failure by using whole beams was time-consuming and troublesome, a simple conceptual design named"beam segment specimen"was proposed by referring the formerly proposed"beam segment analysis model"and the provided"double pull specimen".Through three times of testing-modifying process, the"beam segment specimen"was produced. The beam segment specimen is easy to produce and has a great potential for investigating the influcences of various parameters on the debonding in detail.Some suggestions for further research were put forward in the last part of this thesis.