Aggregate interlocking allows transferring shear and normal stresses through open cracks, and is considered to significantly contribute to the force transfer in cracked concrete. The complex phenomenon depends on the roughness of cracked surfaces, where material protruding from one side may engage with the opposite one. Two-Phase models were established in the 1980's by Walraven to estimate the force transfer, distinguishing between cement matrix and spherical aggregates. The approach leads to good results but has several shortcomings. In this paper, the fundamental assumptions are reviewed using specific numerical and experimental investigations. Special tests respecting the geometrical assumptions are presented and the results compared with numerically calculated estimates. The model is extended to address some shortcomings and investigate the physical nature of the main parameters. Positive aspects of Two-Phase Models and a number of limitations are highlighted, allowing a consistent step forward in the understanding of aggregate interlocking.
The bond response of deformed bars in structural concrete is a phenomenon governed, to a large extent, by the rib-to-concrete contact and interaction, with contact forces depending on both material properties and the kinematics between the contact surfaces (slip and potential separation between the bar and concrete). This phenomenon, presenting similarities with aggregate interlocking in cracked concrete, is difficult to investigate in an experimental manner by means of conventional test arrangements. In this work, an experimental and theoretical investigation on the phenomenon was conducted by means of an innovative experimental test setup performed with bar off-cuts and allowing tracking of the development of bond and confinement stresses for given contact kinematics. The surface of the rebars was scanned to analyse the contact and roughness properties for various cases. The experimental results were analysed using a mechanical model accounting for the surface properties and rib-to-concrete mechanical engagement. Good agreement in terms of the maximum stresses and load–displacement curves was obtained. On that basis, the practical implications of calculating bond stresses on cracked concrete are discussed.
Aggregate interlock is one of the most significant stress transfer actions in cracked concrete and its understanding is fundamental in order to predict the strength of many concrete structures, particularly members failing in shear. Several test programmes focusing on aggregate interlock have been reported in the literature. These programmes, however, often investigate a limited number of parameters and concrete properties, and do not account for different imposed kinematics of the cracked surface. This paper presents some preliminary results obtained by the authors by using a test setup which allows performing tests on double notched specimens subjected to mode I, mode II or mixed mode displacements with imposed kinematics. A series of mode I tests on small specimens have already been performed and the results are briefly summarized. These tests were categorized depending on the type of the cracked surface (crack through the concrete matrix or at the interface between matrix and aggregates), with results showing a significant dependency on this parameter. A preliminary mixed mode test on a pre-cracked specimen is presented as well and the results are compared to tests from the literature with similar assumed kinematics.
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