Bond in RC structures at high temperature and in fire: lessons from the past and hot issues still open to investigation

High temperatures and fire are definitely among the various exceptional load situations RC structures are required to resist, as demonstrated by the extensive research activity performed so far, from the behavior of cementitious materials to that of single members and entire structures. Reinforcement-concrete bond, however, has become a hot issue at a relatively late stage, with an acceleration in the new millennium. The objective of this paper is to recall some of the major issues treated in the literature since the last conference “Bond in Concrete” (Brescia, Italy, 2012) and still open to investigation, such as: (1) bond micromechanics in fire; (2) bond stress-slip law as a function of the temperature; (3) the role of polypropylene, steel and hybrid fibers; (4) tension stiffening in fire; and (5) bonded fasteners in fire. This reexamination takes advantage of the tests performed in Milan in the last decade on bond-stress distribution along anchored bars in fire, pull-out vs. splitting failures, in-fire capacity of postinstalled fasteners and tension stiffening at high temperature. Only by improving the knowledge – and the modelling - of the basic resisting mechanisms, bond included, today’s refined FE codes will provide rational structural responses based on clearly-recognizable contributions.