Fracture of reinforced concrete: scale effects and snap-back instability

Remarkable size-scale effects are theoretically predicted and experimentally confirmed in low reinforced high strength concrete beams. The brittleness of the system increases by increasing size-scale and/or decreasing steel area. On the other hand, a physically similar behaviour is revealed in the cases where the non-dimensional number: is the same, being the steel yield strength, KIC the concrete fracture toughness, b the beam depth and As/A the steel percentage. The tensile strength and toughness of concrete, usually disregarded, are so high in some cases that the peak bending moment overcomes the bending moment of limit design (hyperstrength). The drop in the loading capacity can hide a virtual softening branch with positive slope (snack-back), which is detected if the loading process is controlled through the crack width.