A hollow core slab is a precast prestressed concrete member with longitudinal cores that reduce its self-weight. Strengthening of prestressed hollow core slabs is required for several reasons to maintain their structural integrity. The main focus of this research is to understand the flexural behavior of hollow core slabs and failure modes with hybrid strengthening techniques including bonded overlay and near surface mounted (NSM) glass fiber reinforced polymer (GFRP) bars. The test variables include different strengthening techniques such as (i) bonded overlay with and without shear keys, (ii) NSM GFRP rebar and (iii) hybrid strengthening combination which includes a thin bonded overlay in the compression region and NSM GFRP bars in the tension region. A total of seven full scale slabs were cast, strengthened and tested until failure. Test results show that bonded overlay increases the flexural strength by 89% without much compromise on the ductility when compared to control slab, whereas NSM GFRP bar strengthening increases the strength by about 100% but with reduced ductility. Hybrid strengthening led to a highest increase in the ultimate strength (about 200%) and ductility when compared to only NSM and only bonded overlay strengthening. © 2016 Elsevier Ltd.