Microcracking in concrete refers to the tiny cracks that can form within the material even before any external load is applied. These microcracks typically occur at the interface between the coarse aggregate and the hydrated cement paste, often as a result of differential volume changes prompted by variations in stress-strain behavior, as well as thermal and moisture movement. Initially, these microcracks remain stable and do not grow substantially until the concrete is stressed to about 30 percent of its ultimate strength.

As the concrete approaches 70 to 90 percent of its maximum load-bearing capacity, cracks propagate through the mortar matrix—composed of cement paste and fine aggregate—thereby connecting bond cracks and forming a continuous cracking pattern. This phenomenon, termed fast propagation of cracks, could lead to failure over time if the load persists, a condition known as static fatigue.

If the applied load is further increased, rapid failure might occur at the peak strength level. The formation and evolution of microcracks can also be observed through non-destructive testing methods like ultrasonic and acoustic emission tests. As the concrete undergoes further stress, the crack propagation rate accelerates, leading to structural failure. Understanding microcracking is crucial for predicting concrete's behavior under load and its eventual need for repair or reinforcement.