Reinforced concrete cantilever slabs are among structures that are most likely to develop structural integrity problems, as they are statically determinate and often exposed to the outdoor environment. This article presents an experimental study on the strengthening of reinforced concrete cantilever slabs where the anchorage of the top steel reinforcing bars into the adjacent wall was insufficient. The experimental study involved the use of a fibre-reinforced polymer strengthening system and fibre Bragg grating sensors for strain monitoring. The fibre-reinforced polymer strengthening system consisted of glass fibre–reinforced polymer sheets and glass fibre–
reinforced polymer spike anchors which connected the glass fibre–reinforced polymer sheets to the adjacent concrete wall. The test results showed that the fibre-reinforced polymer strengthening system was effective in improving the load-carrying capacity of reinforced concrete cantilever slabs and the fibre Bragg grating sensors worked efficiently and reliably for strain monitoring. The debonding in glass fibre–reinforced polymer sheet/glass fibre–reinforced polymer anchor-to-concrete bonded joints was found to be a progressive process associated with an increasing load. The fibre-reinforced polymer strengthening system examined in this study is thus a potential ductile solution for deficient cantilever slabs.