Fatigue bond-slip properties of steel reinforcing bars embedded in UHPFRC: Extraction and development of an accumulated damage law

The occurrence of cyclic loads in RC structures is known to deteriorate the bond between the reinforcing bars and concrete by reducing both the bond strength and stiffness, eventually leading to debonding through large increases in slip. There is much research to quantify this bond deterioration for normal strength concrete but little research has considered UHPFRC, which is the subject of this paper. This research develops a testing approach and analysis procedure to quantify the deterioration in bond as a result of high-cycle fatigue. The procedure has been developed through 18 tests of steel reinforcing bars embedded in UHPFRC with steel micro fibres. A test rig has been developed to directly measure the bond-slip under monotonic and cyclic loads. Procedures are then developed for quantifying the bond stiffness and the incremental set, that is, the increase in slip per cycle, by using the known interaction between the monotonic and cyclic bond-slip already identified by other researchers. It is shown how these procedures can be used to quantify the bond degradation under combinations of fatigue loads and how simply measuring the crack width in a structure can give a very good indication of both the residual fatigue life and bond strength.