Freshly mixed concrete undergoes a variety of chemical events. Though this heat is essential for early strength gain, it can also act as an enemy to the concrete in that is causes expansion. As the concrete continues to harden, a peak temperature is reached. From the point of peak temperature, the concrete begins to cool slowly and thus contract or shrink. This volume change can create stresses in the concrete which may lead to what is referred to as thermal cracking.
Fibers help to bridge across the stressed areas and provide an interlocking matrix of connectivity.
Plastic shrinkage cracking is different than thermal cracking in that it is more related to moisture than internal heat. Before the initial hardened state is reached, weather conditions may dry out the surface of the concrete. Though the interior of the concrete maintains plasticity, the surface may completely dehydrate. Excessive moisture loss at the surface causes a premature stiffening and shrinkage. The similarity between thermal cracking and plastic shrinkage cracking is that both are caused by a volume change of the concrete mass.