Winter is once again upon us, and ‘tis the season to shovel and de-ice snowy, slippery stairs and sidewalks.
But before you do, think twice about using salt and de-icing chemicals around historic buildings.
Common rock salt and chloride-based de-icers such as calcium chloride and magnesium chloride are damaging in varying degrees to historic fabric, and especially to concrete and masonry. Salts (chlorides) dissolved in melt water are easily wicked into porous materials such as brick or stone. As the water evaporates, the remaining salts crystallize, exerting powerful expansive pressures within historic masonry and concrete. Over time, these microscopic but destructive expansive forces tear historic fabric apart from within, resulting in spalling concrete, disintegrating sandstone, and often leaving unsightly stains or “efflorescence” (see photograph).
To add insult to injury, salts have a natural affinity for water that enables them to attract and retain moisture within building materials. High moisture levels contribute to freeze-thaw damage during the winter and promote persistent dampness even in otherwise relatively dry summer conditions. Salts also enable the corrosion of steel reinforcement, anchors and ties within concrete and masonry.
(As a side note, these issues aren’t limited to de-icing salts: chemical lawn and garden fertilizers and even naturally occurring salts in the soil can cause similar problems.)
There’s a broad and bewildering variety of other, non-chloride de-icers. Among these are urea-based products and acetates, such as sodium acetate, calcium magnesium acetate and potassium acetate, as well as amide/glycol-based products, which are summarized here. The absence of chlorides, however, is no guarantee that building materials will be unaffected, and there are also potential effects on plants, pets, and the environment to take into consideration, along with costs and overall effectiveness.
From a building conservation perspective, it’s best to avoid or minimize the use of even apparently benign de-icers where possible, for several reasons:
– Any chemical applied to a surface that is subsequently absorbed into building materials becomes a relatively irreversible intervention – not unlike applying a stain to wood. Salts are very difficult to extract once they’ve contaminated historic fabric.
– Encouraging test results from modern concrete structures such as bridges and runways may not apply to historic masonry and concrete where different porosity and other properties may affect durability.
– Keep in mind that even “gentle” de-icing methods can aggravate freeze-thaw damage simply as a result of melting and subsequent saturation of building materials, along with potential shifting of the freezing point into a range where temperature fluctuations may be more frequent (depending on your location and climate) and conducive to frost damage.
The friendliest solution for a historic place is a combination of prompt snow removal to minimize ice buildup along with an application of grit such as sand, kitty litter, or other granular products that are intended to provide traction. (Clumping kitty litter containing bentonite clay may create a bit of a mushy mess once saturated.) Grit itself has no de-icing properties beyond the tendency for less reflective materials to absorb solar radiation and contribute indirectly to melting, although this warming may be significant on some south exposures. Be sure to lay floor mats at entrances to minimize the tracking of grit, snow and meltwater into building interiors.
Written by: Fraser Shaw, Heritage Conservation Adviser.