CRACK RESISTANCE OF DOUBLE-HINGED REINFORCED CONCRETE FRAMES WITH ARTIFICIAL STRESS REGULATION

Abstract

Reinforced concrete frames are widely used in the construction of industrial, civil buildings, and special structures (bridges, tunnels, pedestrian crossings, support systems for the lower floors of public buildings, etc.). To study U-shaped reinforced concrete frames with artificial force control, reinforced concrete frame samples were manufactured with the following dimensions: span l = 2000 mm, height h = 1100 mm, frame crossbeam cross-section – 160 × 100 mm, frame upright cross-section 180 × 100 mm.

The width of cracks in frame elements was measured using indicators located in the stretched zone of concrete at the center of gravity of the reinforcement, based on which cracks were formed. The width of cracks was also measured using an MPB-3 microscope. During the first load cycles of the frames, the tensile deformation of the concrete was taken into account, and during the subsequent cycles, the indicators recorded only the width of the cracks in the concrete. In cases where two or more cracks formed at the base of the indicators, their average width was determined by dividing the absolute elongation recorded by the indicators by the number of cracks.

The results of experimental studies of the crack resistance of double-hinged reinforced concrete frames are presented, and it has been established that under repeated loads, with a load level of up to Fcyc = 0.7 of the destructive (operational level), the crack width increases by an average of 30% compared to a single load on the frame. After the fifth cycle, the crack width in the frame girder span began to stabilize. Compared to the first cycle, the crack width increased by 20% in the fifth cycle and by 23.8% in the tenth cycle. After the seventh cycle, the crack width in the frame joints began to stabilize. Compared to the first cycle, the crack width increased by 18.8% in the seventh cycle and by 21.2% in the tenth cycle. A significant increase in the residual crack width was observed in the first three load cycles. Thus, the residual crack width was wres = 0.04 mm in the span and wres = 0.12 mm in the frame joints. Subsequently, the residual crack width practically did not increase, i.e., it stabilized.