Keywords: pinching, plasticity, bond deterioration, anchor slip, plastic hinge, continuum damage and fracture mechanics.

This paper shows the influence of bond deterioration between concrete and reinforcement in the behavior of RC frames. Wide beam-column connections and short elements, two cases where this phenomenon is particularly important are investigated. In this work, a model that simulates bond deterioration under cyclic loading is presented. The model combines the plastic hinge concept with continuum damage and fracture mechanics. The model uses a parameter named "slip moment". The calculation of this parameter is based on the bond strength of RC sections.

Bond deterioration between concrete and steel in RC frames under reverse loading is a very important topic because the energy dissipation capacity of the frame may be affected by this phenomenon. Bond deterioration affects the load vs. displacement response. Bond deterioration occurs more dramatically in wide beam– column interior connections. A similar phenomenon may occur in short elements. In this paper, the implementation of a model that can simulate the deterioration of bond between concrete and steel in RC elements under hysteretic loads is presented. This model combines the plastic hinges concept with fracture and continuum damage mechanics. This model has been developed within the lumped damage framework.

This lumped damage model is based in previous theoretical and experimental studies [1,2,3]. The model is able to represent the nonlinear behavior of RC elements due to concrete cracking, steel yielding, low cycle fatigue, influence of varying axial load in the flexural response, and bond deterioration between concrete and steel. Bond deterioration was implemented in the model by incorporating a new evolution law, which assumes that this phenomenon can be represented as the slipping of a rigid body (i.e., a corrugated steel bar) on a rough surface (i.e., the surrounding concrete) [3]. Slippage between concrete and steel occurs when the maximum bond strength is reached.

The transfer of stresses between a reinforcing bar and the surrounding concrete is a very complex problem. The pull-out of a rough or deformed bar involves failure in the surrounding concrete. The stresses between reinforcement and concrete are called bond stresses. The slip moment of a RC section depends on the maximum bond stress that develops between the concrete and the reinforcing bars.

The pinching effect in the hysteretic curves can also be simulated. This pinching is due mainly to bond deterioration. Bond deterioration is incorporated into the model by a new behavior law that depends on the maximum bond strength that can be reached in a RC section. Its formulation is based on the anchor plastic theory and is represented in the model by a new slipping function that incorporates a new parameter. This parameter is called the "slip parameter". The different simulations obtained using the model were compared with several experimental test. The results were accepted. All model coefficients are computed from mechanical and geometrical properties of the RC members. The model combines the concept of plastic hinge with continuum damage and fracture mechanics. The model can be incorporated into any commercial finite element code. The frame with wide beams has a behavior stable, although it permits high lateral displacements.

1

Flórez-López, J. "Simplified model of unilateral damage for RC frames". Journal Structurral Engineering ASCE 121(12) Dec. 1995 pp. 1765-1772. doi:10.1061/(ASCE)0733-9445(1995)121:12(1765)

2

Picón, R. and Flórez-López, J. "Evolución de la degradación de rigidez e pórticos de concreto armado". Jornadas Sudamericanas de Ingeniería Estructural. Uruguay 1999.

3

Quintero-Febres, C. and Wight, J. "Experimental study of Reinforced Concrete Interior wide beam-column connection subjected to lateral loading". ACI Structural Journal, V98, No4 July-August 2001, pp. 572-582.

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