The bond properties of an interface in composite structures are generally considered as local mechanical characteristics. Through experimental testing and analytical study, it is shown in this work that the interfacial bond characteristics are generally size dependent, which involve both Mode II and Mode III fractures. For fiber-reinforced polymer (FRP) externally bonded (EB) to concrete members, this size effect is called the width effect. Test results in this work show that there is a central region on EB FRP-toconcrete bond face where only Mode II fracture is involved, and hence, the bond properties are size independent. The measured bond test results in the central region are used to determine interfacial bond properties that are size independent. The global test results or overall bond responses are subsequently used to determine the width effect and its model, through rational reasoning and regression of test results. The proposed model includes new factor that has not been considered in the width effect modeling, and consequently shows a better performance.

Introduction

Interfacial bonding connects substrates of different types of materials, by adhesion, friction or mechanical interlocking. The stress and strain fields at the bond interface, involving bonding agent and the local vicinity of connected parts of substrates, are usually much more complicated than those in other parts of a structure. To avoid such complexity and facilitate the application of conventional structural theories to composite structures, a convenient and sensible approach is to lump all additional deformations that cannot be included in conventional structural theory (such as beam theory) into a concentrated displacement at the interface, namely, interfacial slip. The interaction between the connected substrates is subsequently simplified into a resistance to the interfacial slip by a bond-slip relationship which can be identified from bond tests or pull-off tests for a particular type of interfacial joint. Such an ingenious treatment of bond interface converts a highly complicated problem into one within the regime of conventional mechanics and forms the cornerstone of modern composite mechanics.