ANALYTICAL MODELLING OF CORROSION DAMAGE TO R.C. STRUCTURES

Abstract

Due to various factors corrosion occurs in reinforcement in structures. The corrosion changes the physical and strength properties of the structure. Corrosion gives sufficient warning in the form of spalling of cover. But if unchecked, it may lead to failure of structure. It considerably reduces the strength of the R.C. structures. Proper retrofitting is needed to restore the structure. The retrofitting can not be done without a realistic estimate of the residual strength of the members. This study attempts for an analytical procedure to estimate the residual strength of corrosion affected R.C. beams. The factors effecting the strength of R.C. structures due to corrosion are (i) change in bond : as the corrosion proceeds the corrosion products swell in volume. This results in the reaction from the enclosing concrete. This causes initial increase in the bond strength; (ii) loss of bar diameter : due to corrosion the area of the bar gets reduced. Due to this the moment of inertia of the R.C. structure is reduced, this reduces the strength of the R.C. structure. Two models to account for these factors have been developed. The first model is based on R.C. beam formulation and is suitable for hand calculations. It has been described in the following section. The second model is based on non-linear finite element modelling. The first model considers two cases. In the first case, it is assumed that the yield strength of reinforcement is sufficiently high and the failure takes place due to slip of reinforcement due to bond failure. In the second case, it is assumed that the bond is sufficiently strong and failure takes place due to yield of bar. The effect of corrosion is considered in two cases. The strength of the beam is obtained independently in the two cases and superimposed. It can be seen that in beams designed with proper anchorage as specified by the codes, bond strength is sufficient upto about 6% corrosion and the behaviour is not governed by the bond loss. However beyond 6% corrosion the bond loss is expected to govern the behaviour. The second model is based on non-linear finite element modelling. Beam test specimens were analyzed using FEM modelling in ANSYS package. Analytical results were compared with the test results and the propriety of the theory is discussed. The FEM model is of a RC beam with dimensions 150x150x1000. The reinforcement of the beam is model as 2 noded link element (link 1) The concrete is also modelled using the 4 noded solid brick element (Plane 42) The bond between steel and concrete is modelled as 2 noded link element (link 1) The results of theoretical, experimental and F.E.M model were superimposed. All three are compared. It has been seen that F.E.M modeling is more appreciably predicting the strength of structure effected by corrosion. There is considerable loss of strength of beams of series I (i.e.; bond governed beams) and series II beams (i.e.; area controlled beams). Thus it can be seen that the ultimate load of the beams is considerably effect by corrosion 2-D finite element model for modeling the corrosion effect on stiffness and strength of R.C. beams has been developed. The model predicts the behaviour of the beam close to that obtained experimentally. The prediction of the strength by F.E.M model is closer to reality than that obtained from the analytical model. The change in stiffness in the linear range is not appreciable for corrosion less than 10%. However an appreciable change in stiffness in the non-linear range has been observed. Therefore it is difficult to estimate the corrosion in an existing building by the means of non-destructive testing involving dynamic characteristics.