Ultimate capacity of undercut fasteners installed in thermally-damaged high-performance concrete

Medium-capacity metallic undercut fasteners installed in thermally-damaged concrete has been recently the subject of a research project in Milan (0 = 10 mm = net shank diameter). The results of the third phase of this project, concerning a high-performance concrete (HPC, fc = 60-65 MPa) are presented in this paper, but the results obtained in the two previous phases are systematically recalled (low-strength concrete-LSC, fc = 20-25 MPa and normal-strength concrete-NSC, 50-55 MPa). Beside room temperature, five “reference” temperatures (between 200 and 450°C) are considered, to represent as many values of the fire duration prior to the installment of the fastener. The investigation covers four values of the installment depth (h/hN = 0.45, 0.60, 0.80 and 1.00, where hN = nominal installment depth = 100), and includes the mechanical and thermal characterization of the concrete. In all cases the failure was due to the damaged concrete, with the formation of a conical crack. The systematic formation of this crack was instrumental in formulating a relatively simple model based on limit analysis and on the assumption that the conical crack forms in Mode 1. The model allows to quantify the further loss of capacity that occurs in actual fire situations, where the heating rate is one order of magnitude higher that in the electric furnaces generally used in a lab.