Concrete undergoes three stages of cracking or fracture, which are important to understand in terms of the mode of crack initiation and propagation. The formation of small fissures or micro-cracks in concrete occurs due to the strain and stress concentrations resulting from the incompatibility of the elastic moduli of the aggregate and paste components.
In Stage I, intrinsic volume changes in concrete due to shrinkage or thermal movements can cause strain concentrations at the aggregate–paste interface even before loading. Localized cracks are initiated at the microscopic level at isolated points throughout the specimen where the tensile strain concentration is the largest. These cracks are stable, and at this load stage, do not propagate.
As the applied load is increased beyond Stage I, Stage II begins, where initially stable cracks start to propagate. There is no clear distinction between Stages I and II, as stable crack initiation is likely to overlap with crack propagation, resulting in a gradual transition from one stage to another. During Stage II, the crack system multiplies and propagates, but in a slow and stable manner. If loading is stopped and the stress level remains constant, propagation ceases.
The eventual macroscopic crack pattern at the engineering level is a result of the propagation of cracks during Stage II, which can lead to failure in Stage III. Understanding the stages of concrete cracking is crucial for designing structures that can withstand the stresses and strains they will experience during their lifetime.
The length of the stable crack propagation phase depends on the applied state of stress. It is brief for brittle fractures under primarily tensile stress states, while it is more prolonged for plastic fractures under primarily compressive states of stress.
The third stage of the fracture process happens when the crack system has advanced to an extent where it becomes unstable under load, and the release of strain energy is enough to make the cracks propagate until complete disruption and failure occurs. Even if the stress is increased, failure will occur once Stage III is reached. This stage typically begins at about 70-90% of ultimate stress and is accompanied by an overall expansion of the structure, as indicated by a reversal in the volume change behavior.
It is worth noting that the load stage at which Stage III occurs roughly corresponds to the long-term strength of concrete.