In winter conditions, or in cold climates, when the temperature falls below , the crystallization of ice in the pores of concrete is also a physical mechanism (change of state) responsible for the volumetric expansion of a substance exerting a high tensile strength inside the concrete matrix. When the tensile strength of concrete is exceeded, cracks appear. Adding an air entrainment agent during the mixing of fresh concrete induces the formation of tiny air bubbles in the fresh concrete slurry. This creates numerous small air-filled micro-cavities in the hardened concrete serving as empty volume reserve to accommodate the volumetric expansion of ice and delays the moment tensile stress will develop. Air entrainment makes concrete more workable during placement, and increases its durability when hardened, particularly in climates subject to freeze-thaw cycles.