Where Pipes Freeze

One of the quirks of fate is that frozen water pipes typically burst at locations where freezing last occurs. Thus an insulated portion of an otherwise poorly insulated pipe is likely to be the place where the pipe breaks.

As everyone knows, the freezing of water causes the material to expand. Ice occupies a greater volume than does the water it formed from. While the expansion of water as it freezes is the underlying cause of bursting pipes and bent buckets, the actual deformation or breakage of the water container results from a secondary consequence of the expansion, namely hydraulic pressure in that part of the water last to freeze .

A series of experiments described by John Houk in the Summer 1974 issue of the Northern Engineer illustrates the role of hydraulic pressure rather well. A water-filled metal bucket set on a block of styrofoam ends up with a bulged bottom created by the hydraulic pressure in the last water to freeze, in this case, at the bottom of the bucket. Yet if the styrofoam is placed on the top of the bucket, no bulging occurs because the last water to freeze is near the top surface where there is no metal to be bent.

In other experiments, Mr. Houk wrapped insulation around short sections of otherwise exposed pipe filled with water. The pipe always burst under the insulation, and water leaked into the insulation, proving that liquid water was still present at the time of the breakage.

As the last of the water freezes in a confined space, the increasing hydraulic pressure causes the melting point of the confining ice to decrease. By the time the pressure rises to 8,000 pounds per square inch, the melting point lowers to near -5°C (23°F). If that's not enough pressure to break the pipe, the pressure can build to 28,000 pounds per square inch at -21 °C (-6°F). That ought to be enough pressure to break just about anything.