2. Brief description of the steel-concrete bond

Conceptually, the phenomenon of bonding corresponds to the physical interaction of two different materials, which occurs on an interface zone by allowing the transfer and continuity of efforts and stresses between the two bodies in contact. In the case of reinforced concrete structures, this phenomenon is also known as « tensile stiffness » which develops around a reinforcing element, partially or totally embedded in a volume of concrete. The tensile forces that appear within the reinforcement are transformed into shear stresses on the surface, and are transmitted directly to the concrete in contact which will finally balance them, and vice versa. The overall response will depend on the ability of concrete to deform as much as steel, since steel will tend to slide inside the surrounding concrete. The bonding phenomenon corresponds to this ability of concrete to deform and to degrade locally by creating a type of layer, or envelope, around the reinforcement, whose kinematic and material properties differ from those of the rest of the concrete or reinforcement used.

The phenomenon can be broken down into three well-defined mechanisms:

• an adhesion of chemical origin,

• a friction mechanism between two rough surfaces (steel-concrete or concrete-concrete),

• a mechanical action created by the presence of the ribs of the steel bar on the surrounding concrete.

From this decomposition, it can be clearly deduced that for a smooth bar, the predominant mechanism is the friction between the two materials, while for a ribbed bar (in French commonly called « HA reinforcement: High Adherence »), the dominant mechanism is the mechanical interaction between surfaces. When the reinforcement consists of the wires with steel cables, it is possible to control or combine the various mechanisms since they depend directly on the surface of the cables.

The link will undergo a different degradation depending on the type of load applied, either monotonic or cyclic. Moreover, among the most important parameters that influence the behavior of the link, we can mention:

1. the characteristics of the load,

2. the geometric characteristics of the steel bar,

3. the spacing between active bars,

4. the characteristics of concrete,

5. confinement by passive reinforcement,

6. lateral pressure.

When studying a cylindrical bar embedded in an infinite medium, we can identify the surface of discontinuity where we will place the effects of the bond, which develops in a certain area of cracked and crushed concrete around the steel bar. At some point, this surface will correspond to the cylindrical crack created during the coalescence of shear cracks. Looking at the crack network, we can assume that, under ideal conditions, the crack plane is always perpendicular (normal direction) to the surface of the bar and parallel (tangential direction) to its longitudinal axis (see [Figure 2-a]). This allows us to project the components of the displacement onto the normal and tangential direction of the crack plane, and therefore to obtain the corresponding deformations and stresses.

Figure 2-a: real description of the bonding phenomenon and finite element simplification: coordinates in the local coordinate system of the interface element used as law support JOINT_BA