H.F.S.G. Pereira et alii, Frattura ed Integrità Strutturale, 31 (2015) 54-66; DOI: 10.3221/IGF-ESIS.31.05 56 conditions. Fig. 3 shows the 3D geometrical model with the representation of the surfaces’ boundary conditions ascribed due to both symmetry conditions and test support conditions. The perpendicular displacements of the finite element nodes at the two symmetry planes were constrained along the yy and zz axis, respectively. Moreover, at the specimen’s front plane (where the protruding end of the rebar arises), the displacement of the nodes perpendicular to the plane were also constrained, i.e. in the xx direction. 100mm 100mm 200mm 200mm 200 mm EMBEDDED DISABLED TOPVIEW FRONTVIEW LENGTH BOND Ø20.5mm Figure 2 : Geometry of the specimens used in the pullout test. Figure 3 : 3D FE mesh and boundary conditions. The pullout specimen was modelled using 3D solid finite elements available from ABAQUS library [26], namely, C3D8 and C3D6 were used to model the concrete bulk and steel rebar, respectively. Additionally, cohesive elements (COH3D8) were considered to model the interface behaviour between the concrete and rebar. (a) (b) Figure 4 : Model with smooth rebar: a) top view, b) front view. (a) (b) Figure 5 : Model with ribbing rebar: a) top view, b) ribbing detail. Local bond-slip law The bond properties of a reinforcing rebar can be analytically described by a local bond stress – slip relationship, = (s), in which is the shear stress acting on the contact surface between rebar and concrete, and s is the corresponding slip, i.e. the relative displacement between steel bar and concrete [31]. Once the relation = (s) is known, using equilibrium and compatibility relations, the second order differential equation governing the slip can be defined as:

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