Bond and Anchorage of Ribbed Bars in High-Strength Concrete

Abstract: For structural utilisation of high-strength concrete, a good understanding of the bond and anchorage behaviours of ribbed bars in this specific material is essential. The research project aims to increase the knowledge of bond and anchorage behaviours of ribbed bars in high-strength concrete. Furthermore, the validity of the design method in Model Code 1990 aimed for normal strength concrete is investigated.

An extensive experimental study was carried out to examine the bond and anchorage behaviour in high-strength concrete compared with that in normal strength concrete. Tests on four types of specimens were conducted. Detail tests and all major anchorage regions (support regions, cut-off regions and splice regions) are treated. For each high-strength concrete specimen tested, a reference specimen of normal strength concrete with the same detailing was also tested. Both simple numerical modelling based on schematic bond stress-slip relationships and more complex non-linear finite element modelling were used to complement the experiments.

The study showed that the bond and anchorage behaviours were similar in both concrete types, and governed by the same phenomena. The bond strength in well-confined concrete increased linearly with the concrete compressive strength. Furthermore, high-strength concrete gave a much stiffer local bond stress-slip relationship than normal strength concrete. If the yield capacity of the reinforcing steel was reached, the local bond resistance decreased substantially, especially in high-strength concrete. Schematic local relationships between bond stress and slip are proposed for anchorage in confined normal strength and high-strength concrete; these take into account the bond deterioration when the yield capacity of the reinforcement steel is reached or exceeded.

Although the anchorage capacity of a high-strength concrete specimen was higher than that of a comparable normal strength one, the increase was not in proportion to the rise in compressive strength, unlike the local bond strength under well-confined conditions. However, the high-strength concrete showed less capacity for stress redistribution, both along anchorage or lap lengths and between bars anchored in the same region. This can result in premature and brittle failures, unless compensated by a sufficient amount of stirrups. The design method examined usually gave results on the safe side when compared with test results. However, the scatter was large and there was a tendency for the safety to decrease with greater concrete strength.

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