Effect of loss of bond on the behaviour of RC beams under service loading conditions

The deviation of the flexural reinforcement spacing from code specifications appears to underly the causes of loss of bond between concrete and steel which preceded the unexpected collapse of the top-floor balcony of a building under service loading conditions on 15th October 2021 in Athens. Although rare, loss of bond has also been identified as the cause of collapse of buildings under similar conditions in the U.K. and Canada. Yet, Codes of Practice for reinforced concrete design neither make reference to the effect of loss of bond on structural behaviour, nor include loss of bond in the parameters which the formulae currently used for assessing load-carrying capacity are dependent on. In view of the above, the paper provides a description of the effect of loss of bond on the function of reinforced-concrete beams and this description is used as the basis for the derivation of a formula linking loss of bond with load-carrying capacity. The validity of the proposed formula is verified through the use of published experimental information on the subject and applied for the structural assessment of the collapsed balcony taking into account design details, physical state and loading conditions of structural members similar to that which collapsed.