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SP-180 During the ACI 1997 Spring Convention, ACI Committee 408, Bond and Development of Reinforcement, organized four sessions intending to assess the state of the art in bond research, practical applications, and code development. The sessions were organized into a symposium honoring Dr. Peter Gergely, a longtime member of Committee 408, who had recently passed away.

This paper reports on research in progress conducted at the American University of Beirut to evaluate the effect of silica fume on bond and anchorage of reinforcement in high performance concrete (HPC) structures. The program includes testing the effect of a wide range of variables on the bond strength of beam bar splices and bars anchored in pullout specimens. Results of the first phase of the research program have been analyzed. Ten beam specimens were tested. Each beam was designed to include two bars in tension, spliced at the center of the span. The splice length was selected so that bars would fail in bond, splitting the concrete cover in the splice region, before reaching the yield point. The beams were loaded in positive bending with the splice in a constant moment region. The variables used were the percentage replacement of cement by silica fume and the casting position. Test results indicated that replacement of 5 to 20 percent of the cement by an equal weight of silica fume resulted in an average 8 percent reduction in bond strength regardless of casting position.

The Comite Euro-International du Beton (CEB) has set up under its Commission 2 “Material and Behavior Modelling” a Task Group TG 2/5 “Bond Models” with terms of reference to write a State-of-the-Art report. Chapter 8 in the report will deal with bond of non-metallic reinforcement, FRP. The work has been started using the Japanese State-of-the-Art knowledge presented in [5]. Now research experience is added. The bond concept elaborated for steel rebars is used to interpret the action of the FRP rods/bars. The different bond influencing factors are discussed also for FRP rods/bars. Comparison is performed with steel rebars. The bond of FRP rod/bar depend on more parameters than bond of steel. Variables of interest are form of rod/bar section, type of surface deformations and treatment, modulus of elasticity, Poissons ratio etc. Therefore it is appropriate to use the known bond action of deformed steel rebars in its different stages as a reference, when investigating the bond performance of FRP rods/bars.

The increasing significance of performance-based criteria in modem structural design has motivated new considerations in bond design of conventional reinforcing steels, relating to more reliable assessment of both the demand and the supply sides of the anchorage/development design problem. Accurate identification of the required anchorage lengths needed to ensure strain compatibility, by proper consideration of the conditions affecting bond, is necessary to limit slippage of the steel relative to the concrete. While minimum development lengths calculated by designers imply that the bar is fully anchored, it is well established by experimental observation that in practice there is always some bar slip. Recent research results from around the world provide the basis for improved understanding of the effects on bond performance of critical parameters such as confinement, spacing, and material properties. Much of this work has been empirical in nature and the applicability of empirical design expressions in calculations is limited. Nonlinear finite element calculations and other sophisticated analysis re-quires more information as to how the bond failure proceeds than simply an upper limit. This paper will summarize the available information that exists both within North America through AC1 and within the CEB as to the viable approaches and philosophies that can be applied to the bond problem. The range of application of the various techniques will be identified as will limitations and needs for more re-search.

Splitting does always occur in some way prior to bond failure, in the form of either partial splitting (quite often undetected) or full splitting, the latter being the subject of several recent papers, owing to the importance of cover splitting in R/C elements. Starting from the test results on fully-split specimens (like those by the authors on special specimens having a fabricated crack) it is possible to formulate suitable bond stress-confinement stress relationships. These models can be introduced into the limit-analysis models developed lately for the description of partial splitting up to the onset of full splitting and bar pull-out in short anchorages. In this way, a linkage between the bar-concrete pressure (studied here through a limit-analysis elastic-cohesive model) and the bond stress is established, in order to evaluate the ultimate bond capacity and to investigate the transition from a splitting-type failure to a pull-out failure. At the same time, such important topics as concrete tensile strength and fracture energy, crack cohesion and localization, concrete cover and bar diameter, fiber content and external pressure can be incorporated into the model. A set of diagrams showing the bond capacity and crack number/opening/penetration versus concrete cover is presented, and the design implications of both the theoretical and experimental results are discussed.