Bond Stiffness at High Temperature and in Fire: an Open Issue

Bond stiffness at high temperature has been rarely treated in the very numerous papers on bond behavior at high temperature and in fire, since (a) bond as such is mostly associated with structural safety (equilibrium at the Ultimate Limit State) than with structural serviceability (where bond stiffness comes into play); (b) the generally good – and even excellent – behavior of bond in ribbed bars makes bond-related resisting mechanisms – tension stiffening included - fairly heat-tolerant; and (c) tension stiffening in cracked concrete – where bond stiffness is a major actor – has a rather indirect and not easily quantifiable role in structural behavior, all the more in fire. To improve the knowledge of bond stiffness at high temperature, the bond stress-bar slip curves resulting from the pull-out tests of four selected ex-perimental campaigns spanning a forty-year period (from 1981) are re-examined in this paper. Bond stiffness (= initial slope of the loading branch of the curves) is derived from the bond stress-bar slip curves in either hot or residual conditions. Comparisons are also made with the authors’ results coming from the pull-pull tests carried out in a fifth experimental campaign on tension stiffening. The prelimi-nary conclusions of this study are that (a) below 150-300°C, bond stiffness is from little to moderately af-fected by the temperature; (b) at higher temperatures, bond stiffness decreases with the temperature, even very sharply (300-500°C); (c) medium- and high-grade concretes (fc  45 MPa) seem to be more affected by high temperature than low-grade concretes (fc  30 MPa); and (d) the values of the stiffness obtained indirectly from pull-pull tests (tension-stiffening tests) confirm the marked decrease of bond stiffness above 500°C.