Ultimate Capacity of Undercut Fasteners Installed in Heat-Damaged Concrete

Abstract

The ultimate behavior and bearing capacity of undercut fasteners installed in heat-damaged concrete are investigated in this paper in which the results of a rather comprehensive research project are presented. Sixty-one fasteners (shank diameter ØN=10 mm) are tested under a pull-out force after being installed in as many preheated and postdrilled slabs made of three different concretes: (1) a low-strength concrete (LSC), fc20=20 MPa; (2) a normal-strength concrete (NSC), fc20=52 MPa; and (3) a high-performance concrete (HPC), fc20=63 MPa. Besides room temperature, five reference temperatures (between 200°C and 450°C at a prefixed distance from the heated surface) are considered to represent as many values of the fire duration prior to the instalment of the fastener. Four values of the embedment depth (45%, 60%, 80%, and 100% of the embedment suggested by the producer=10ØN) are investigated. In all cases, except in the virgin unheated specimens, the failure is caused by the thermally-damaged concrete, with the formation of a conical crack. Two finite-element (FE) models are formulated, based on linear-elastic fracture mechanics (LEFM) and on nonlinear fracture mechanics (NLFM), respectively, and their results are shown to envelop the test results. A third simple model based on linear fracture mechanics is also developed to quantify the extra loss of capacity induced by real fires in which the heating rate is one order of magnitude higher than in the electric furnaces generally used in the labs. Last but not least, to have information on the ultimate load associated with the possible formation of a conical crack, even in those cases in which the collapse due to shank yielding precedes concrete-cone failure (typically in high-grade virgin or undamaged specimens), the well-known concrete capacity method (CC-method) is modified to incorporate the maximum aggregate size, which plays a substantial role in crack mechanics.