Damage Detection of Reinforced Concrete Structures Based on Concrete-steel Interface Element

The interface between concrete and steel in reinforced concrete governs the interaction between the two types of materials under loading. When the interface is seriously damaged, such that a macro-crack is formed, de-bonding takes place or large slip occurs, and the load-transferring capacity of the interface will drop dramatically. In this study, a vibration-based method is presented to detect damage in the reinforced concrete structures using a damage model based on the constitutive law of the lumped model on the concrete-steel interface. Scalar damage parameters characterizing changes in the interface are incorporated into the formulation of a finite element model that is compatible with the vibration-based damage identification procedure. Numerical simulations show that the method is effective to detect failure at the interface between concrete and steel in the reinforced concrete beam. INTRODUCTION The detection of damage in bridge structures using vibration characteristics has been the topic of much research (Salawu, 1997; Doebling et al, 1998). However, most researchers have dealt with homogeneous structural members, where damage is represented by a local reduction in stiffness owing to a crack or cut. Less attention has been given to problems involving non-homogeneous elements, such as reinforced concrete members. Owing to the nature of cracks in reinforced concrete, the length of the damage zone is not small and, depending on other factors, may be many times the height of the beam. Casas and Aparicio (1994) used the equivalent second moment of area of section to evaluate the amount of damage in a cross-section based on finite element analysis. The second moment of area in an element is assumed uniform. Cerri and Vestroni (2000) modelled the damage zone as a beam element with a reduced bending stiffness. Three parameters are used to define the damage: the position, the extent of the damage and the reduction of the elemental flexural stiffness. Frequency change is used to determine the three damage parameters of the reinforced concrete beams (Vestroni and Capecchi, 2000). Wahab et al (1999) also used three parameters to describe the damage zone in reinforced concrete beams which are the length of the damage zone, the magnitude of the damage and the variation of the damage magnitude from the centre to the end of the damage zone. Maeck et al (2000) presented two techniques to calculate the stiffness degradation of the damaged reinforced concrete beam based on this damage model. Law and Zhu (2004) also studied the dynamic behaviour of damaged reinforced concrete bridge structures under moving vehicular loads using this model. The interface failure between the reinforcing bar and the concrete, such as the formation of a macro-crack, is associated with de-bonding or a large slippage at the interface, and the load-transferring capacity of the interface will drop dramatically. Spacone and Limkatanyu (2000) showed the importance of including the bond-slip in the response of reinforced concrete members by displacement-based formulation. Limkatanyu and Spacone (2002) presented the general theoretical framework of the displacement-based, force-based, and two-field mixed formulations of reinforced concrete frame elements with bond slip in the steel bars. Neild et al (2002) presented the non-linear behaviour of reinforced concrete beams under low-amplitude cyclic vibration. He proposed four mechanisms which are responsible for the nonlinear vibration characteristics, i.e. crack closure leading to a bilinear stiffness mechanism, friction across the crack due to matrix-aggregate interaction, slip between the steel and the concrete and the non-linear behaviour of concrete in compression. The most important one is the bonding damage between the reinforcing bar and the concrete. Soh et al (1999) presented a damage model, which included the normal and tangential damage factors, to describe the concrete-steel interface mechanism. A reinforced-concrete element is developed based on this damage model to simulate the bond deterioration in reinforced-concrete structures (Soh et al, 2003). In this study, a vibration-based method is presented to detect damage in the reinforced concrete structures using a damage model based on the constitutive law of the lumped model on the concrete-steel interface. Scalar damage parameters characterizing changes in the interface are incorporated into the formulation of a finite element model that is compatible with the vibration-based damage identification procedure. Numerical simulations show that the method is effective to detect failure at the interface between concrete and steel in the reinforced concrete beam. BEAM ELEMENT WITH DAMAGE AT THE CONCRETE-STEEL INTERFACE Bond Stress Distribution Function Small Tension Dbmax ltx