Experimental evaluation of flat slabs strengthened with post-installed shear bolts with pre-loading

Based on the possibility of brittle failure of flat slabs, there are relevant requirements for their strengthening in cases of deficient design or increasing loads. One of the essential factors to consider is, that an existing structure requiring strengthening always has some history of loading. An experimental study was performed in order to verify the effect of strengthening with post-installed shear reinforcement on flat slabs with pre-loading. Four full-size square flat slabs with dimensions of 2500 x 2500 mm, strengthened with shear bolts, were tested, as well as a reference slab without any shear reinforcement. The shear reinforcement was installed before loading in two specimens. For this purpose, bolts consisting of threaded rods, nuts and washers were used. Two types of threaded rods were used, i.e. rods of a diameter of 12 mm and a steel grade of 8.8 and rods of a diameter of 10 mm with 4.8 steel grade, which predestined the failure mode of the flat slab. Two other specimens were loaded up to 50% and 75% of their initial shear resistance without shear reinforcement - obtained as the resistance of the reference slab. Subsequently, they were unloaded or loaded on 50% of their resistance and strengthened with shear bolts. The pre-loading of flat slabs before strengthening demonstrated a decreasing effect on the achieved resistance of flat slabs strengthened by post-installed shear reinforcement (resistance 1715 kN for slab without pre-loading, 1647 kN for slab pre-loaded to 50% of initial resistance, and 1563 kN for slab pre-loaded to 75% of initial resistance). The measured decrease in the resistance compared to the slab strengthened before loading was 5% to 10%, depending on the pre-loading level.