Keywords: pull-out, finite element, explicit algorithm, size effect.

In this paper numerical analysis of pull-out tests for concrete are discussed. The analysis is carried out by means of finite element models based on an experimental study carried out in the laboratory of the Department of Civil and Environmental Engineering and Architecture (DICAAR) of Cagliari University on properly made concrete samples. The experiments have been performed on 24 cubic 300x300x300 mm specimens, made of six different concrete grades with cubic compressive strength ranging from 15 to 45 MPa. Post-installed expansion anchors have been forced into the drillings and then extracted from the specimens by means of manual equipment. Also a specific correlation between the extraction force and the compressive cubic strength has been provided.

The numerical models have been carried out and calibrated in order to provide the same results as the experimental tests and at the same time a deeper insight into the physics of the phenomenon. First a proper idealization of the tests had to be arranged, discarding all the features considered of minor importance such as prestressing, friction, geometrical details and taking advantage of the axial symmetry of the problem. Several material models have been tested within the framework of plastic and damage theory and the encountered numerical problems have been discussed, such as mesh sensitivity of the results, convergence difficulties of classic implicit algorithms and the lack of results objectivity due to the use of strain softening constitutive laws. Final axisymmetric non linear models have been proposed and solved by means of an explicit dynamic algorithm capable of bypassing the above mentioned convergence problems, though involving other difficulties due to inertial effects, reduced integration finite elements, and mesh sensitivity. The results obtained have been compared with the experimental ones, showing a good accordance between the compressive strengths of the material models and the expected extraction loads. Pros and cons of the chosen approach are finally discussed and possible improvements and future research lines are proposed.

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