Smooth Anchored Bars in NSC and HPC: a Study on Size Effect

Abstract

The bonding of moderately-long anchorages to ordinary and high-performance silica-fume concrete is studied here with reference to size effect. To this purpose, 24 anchorages (L/db = 10), consisting of a quasi-smooth, micro-roughened bar embedded in a concrete cylinder, were cast and tested up to the pull-out of the bars, which had 4 different diameters (db= 5, 12, 18 and 26 mm). For each of the 8 cases examined here (4 diameters × 2 mixes), 3 nominally-identical specimens were tested. Though the primary objective of this study is to investigate whether a general-type size-effect law applies to bond in high-performance concrete, the modeling of an anchorage by means of a local elastic-fracturing frictional bond-slip law is also carried out, and two approaches are adopted for the description of size dependency, the first based on a stress criterion for the debonding, and the second on an energy criterion. The former approach leads to a size-dependent formulation of the local bond strength, while the second approach makes it possible to evaluate the debonding energy, which is size independent. In both approaches the values of the fundamental parameters of the model are identified by means of a bidimensional or mono-dimensional, least-square regression procedure. The results confirm that size effect in bond should be taken into consideration directly in the codes, and pave the way to the study of size effect in high-bond bars.