Q.: We do a lot of concrete work in Canada where frozen ground is common. Most specifications prohibit placing concrete on frozen ground. How do concrete contractors in northern climates meet this specification requirement?

A.: The contractor must usually either prevent the ground from freezing or thaw it before placing concrete. The most common method for preventing freezing is to cover the ground with insulation. Insulating blankets and straw have both been successfully used. Another possible insulating method is to cover the ground with a waterproof plastic sheet and construct a pond of water above the subgrade. The water serves as insulation and as a heat sink when it changes to ice.

The amount of water needed to protect the ground from freezing can be calculated. Make sure no water leaks from the pond into the subgrade below. Another solution is to erect an enclosure and provide heat. The enclosure could be a flat tent propped on blocks or it could be large enough to serve as a construction shelter for use during form setting, concrete placement, and curing.

There aren't many economical options for thawing ground that's already frozen. You can build a heated enclosure and wait for the ground to thaw within it. In Canada, frozen ground has been successfully thawed by drilling holes 4 to 5 feet apart and injecting hot water or steam.

You might be able to safely place concrete on frozen ground if the soil isn't frost susceptible. To successfully place concrete on frozen ground you need to prevent the concrete from freezing until it reaches a compressive strength of 500 psi. Adding an extra inch or two of concrete is one option. This could be viewed as a sacrificial layer that may freeze but maintains the heat generated by the rest of the concrete. An alternative is placing the concrete on an inch of rigid insulation board thus protecting it from the frozen subgrade below. Heating the concrete, using Type III cement, or adding an accelerator reduces the time needed to reach the 500-psi strength level. However, placing concrete on frozen ground isn't recommended if the ground isn't thaw stable. Silts and clayey silts with free access to water aren't thaw stable because they settle after thawing. Also, because of the free water present, these soils are weaker immediately after the thaw. Don't place slabs on grade over frozen soils that aren't thaw stable. Subgrade settlement and decreased bearing capacity may crack and tilt the slabs. Coarse-grained soils such as gravels and sands are typically thaw stable and maintain their strength after freeze-thaw cycles.

If you consider placing concrete on frozen ground, contact a soil testing firm and ask for a thaw-consolidation test on samples of the soil. This test measures the amount of settlement after a soil thaws. Based on the amount of settlement, the soil testing firm can evaluate the advisability of placing concrete over the frozen subgrade.

A simpler test also can be used to help predict thaw settlement. The dry density of frozen and thawed soil is measured. The values are then used in the following equation to predict settlement:

  • Settlement = (1--frozen density/thawed density)
  • x (height of the frozen layer)

This equation states that if the frozen soil contains more moisture than the thawed soil originally contained, settlement will occur as thawing releases the moisture and the soil shrinks to its original volume.