Damage assessment of FRP-encased concrete using ultrasonic pulse velocity

Since concrete members with fiber-reinforced polymer (FRP) reinforcement ar e not as ductile as conventional structures, and since concrete in retrofit ted members is often covered and not readily available for visual inspectio n, development of nonintrusive and nondestructive damage assessment tools i s essential. Ultrasonic pulse velocity (UPV) can provide such an assessment tool, and help quantify the extent and progression of damage in concrete. Continuous UPV monitoring of a number of concrete cylinders and concrete-fi lled FRP tubes showed the concrete-filled tubes to have higher sensitivity at lower stress ratios as compared to plain concrete. When compared with th e axial strain energy, the UPV damage index has a much better resolution fo r stress ratios up to about 50% of ultimate strength. This ratio depends on the thickness of the jacket. It was also shown that the UPV has a strong c orrelation with volumetric strains after confinement is activated. Therefor e, at high levels of axial loads when the confining pressure of the jacket results in volumetric compaction, the UPV increases by about 6% of the init ial pulse velocity. Repeated loading generally induces the same rate of cha nge of UPV damage index as that of the initial loading on the virgin specim en. This phenomenon occurs irrespective of the stress ratio at which the sp ecimen is unloaded or reloaded. A comparison of the UPV damage index with t he normalized acoustic emission counts reveals that the two methods have di fferent sensitivities at different stages of loading and could potentially complement each other as a hybrid damage assessment tool.