Investigations on fracture in reinforced concrete beams in 3-point bending using continuous micro-CT scanning

Abstract

This study explores a fracture process in rectangular reinforced concrete (RC) beams subjected to quasi-static three-point bending. RC beams were short and long with included longitudinal reinforcement in the form of a steel or basalt bar. The ratio of the shear span to the effective depth was 1.5 and 0.75. The focus was on the load–deflection diagram and crack formation. Three-dimensional (3D) analyses of the size and distribution of pores and cracks were carried out with an X-ray micro-computed tomography system SkyScan 1173 of high resolution that is a very valuable non-destructive tool for studying a 3D material interior. The tomography system was connected with a quasi-static loading machine ISTRON 5569 to continuously follow fracture changes without loading breaks. The beams failed in shear due to a diagonal shear crack that was steeper with basalt reinforcement. The shear strength and flexural strength of RC beams with steel reinforcement were higher by about 10% than of RC beams with basalt reinforcement. The deflection corresponding to the maximum load of RC beams was higher by about 20–25% in RC beams with basalt reinforcement due to its lower basalt modulus of elasticity. The final volume of cracks in beams reinforced with basalt bars was higher by about 9–20% than in concrete beams reinforced with steel bars due to a higher beam deflection whereas the maximum crack width in concrete beams reinforced with basalt bars was higher by about 20–40% than in concrete beams reinforced with steel bars. The critical shear crack in RC beams with basalt reinforcement was wider by about 20–40% and steeper by about 10–45% as compared to concrete beams with steel reinforcement. The relationship between the crack volume and beam deflection was bi-linear. Both, aggregate breakage and crack branching occurred during beam bending.