Durability of concrete and cement composites

Freezing processes Flow and pressure

The fact that ice has a volume approximately nine per cent greater than the water from which it was formed leads to a forced flow of water when ice crystals begin to form in concrete. An ice body forms locally in a cavity from pure water until the cavity is filled. The excess unfrozen water and air is pushed out into the surrounding network. The ice forms a moving front and pushes water ahead of it. The flow may be modelled by Darcy’s law for transport of fluids through a porous medium. Flow occurs under a pressure head and resistance to flow is proportional to the length of the flow path. Thus, conditions may arise where the flow is impeded due to insufficient pore volume. Resistance to the flow develops a hydraulic pressure. These expansive pressures may reach a level at which the tensile strength of the concrete is exceeded and cracks develop. In addition, the water movement during freeze thaw cycles may result in capillary water moving into microcracks. In subsequent freezing cycles this water may cause propagation of these cracks leading to significant cumulative damage over time (Pigeon and Pleau, 1995).

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