Concrete Tips for Cold Weather Climates

Although concrete can be frozen in its early stages of setting, it is a risky business and one is better off taking steps to prevent it.  Ideally, the pour should be kept warm until a seven MPa strength is attained before allowing the concrete to cool and freeze.  While frozen, concrete will continue to slowly gain strength and once thawed reach its design strength.  Using chemical accelerators in winter will speed up the initial set, reducing both the time and expense needed to keep the concrete heated.  The thicker a slab or wall, the more internal heat it will generate.  Consider ordering a richer mix that has more cement powder.  It will generate more heat and set quicker.  Insulating blankets placed over the slab can play a double role in both keeping the surface from freezing and helping in the curing of the slab.  Give your forms a thorough inspection before starting a pour, to insure no snow or ice is present.  When it melts after the concrete is set, holes or voids will remain.  There should be no ice on the rebar.  Never pour on a frozen sub-grade.  Not only is there the possibility of settlement when the frost thaws, but concrete poured directly against frozen ground will likely freeze before reaching the minimum 3.5 MPa required to prevent frost failure.*  Placing insulation over frozen ground for a few days can cause it to thaw.  Where the frost is deeper, the ground may have to be heated.  Again, insulated thermal blankets can be used to cover concrete work and keep in the heat produced by setting concrete.  Be sure to check corners and edges, as these places are more susceptible to cold air creeping in and freezing the concrete work.

Covering the area with plastic and heating the interior, commonly called hoarding, will allow the concrete to set in warmer conditions.  Avoid venting space heaters inside a heated enclosure, as the carbon dioxide produced will react with the setting concrete resulting in a powdery surface.

Consider using ICF's, or Insulated Concrete Forms for walls.  These will prevent freezing and reduce or even eliminate the need for a heated enclosure.  Covering the top of these forms is essential to retain all the heat of hydration. These forms take advantage of all the heat produced and cool gradually preventing thermal shock.

Why risk a solid investment?  Follow the recommendations listed above to worry-free winter concrete work.

*This is a minimum, generally concrete kept above the freezing point will attain 3.5 MPa within twenty-four hours.

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