The Assault On Concrete by Deicing Chemicals

Exterior concrete slabs and pavements are bombarded every winter with deicing chemicals to remove snow and ice more quickly. Even though these deicing chemicals make concrete surfaces—like roads, sidewalks and parking lots—safer for pedestrian and vehicular traffic, they can be detrimental to the concrete itself.

Winter moisture already does a number on concrete by itself, as water—snow melt, sleet, runoff—expands 9 percent its original size when frozen. That means when concrete freezes, any moisture in the concrete can create pressure in the pores of the concrete. When this pressure exceeds the concrete’s tensile capacity, it causes cracking and spalling of the concrete surface.

If large aggregate in the concrete is porous or absorptive, the moisture in the aggregate can freeze and crack and split. If these aggregates are close to the surface, they can create pop outs that look like cone shaped indentions on the surface of the concrete. This cracking can also create D-cracking, which is closely spaced cracks that parallel joints. These cracks will continue to multiply over time and show up further and further from the joint.

Deicing chemicals can make this process worse in different ways.

First, the deicing chemicals reduce the freezing temperature of the offending moisture. So rather than the moisture in the concrete freezing at 32 degrees and staying frozen until the concrete temperature exceeds 32 degrees, it thaws at a lower temperature and refreezes at a lower temperature. Therefore, the concrete goes thru more freeze/thaw cycles than it would have if deicing chemicals were not used. Each time the concrete freezes, it repeats the expansion pressure that can create more cracking and spalling.

Most deicing chemicals contain chlorides. These chlorides are like sponges that attract and hold moisture. Therefore, concrete that is more saturated with chlorides is probably more saturated with moisture and thus more susceptible to damage when the concrete freezes.

Even worse, certain deicing chemicals can actually chemically attack concrete. Sodium chloride, calcium chloride and urea are the most used deicers. Sodium chloride has little to no chemical effect on concrete, but can damage plants and is highly corrosive. Calcium chloride in weak solutions has little effect on concrete and is less offensive to plants but is also highly corrosive, but calcium chloride in concentrated solutions can chemically attack concrete. Urea has little effect on concrete, plants or metal. Deicers containing ammonium nitrate and ammonium sulfate aggressively attack and disintegrate concrete and should not be used.

But it’s not just the concrete these chemicals harm. Chlorides in deicing chemicals can aggressively attack metal and transform metal into rust. When metal rusts, the rust can become four times larger than the original metal. Therefore, rusting reinforcing steel embedded in concrete can expand and crack and spall the concrete covering it. Once this process starts, both the concrete and steel deteriorate even faster than before.

Deicing chemicals can certainly have a negative effect on concrete. In January, we will discuss ways we can try to protect our concrete from these effects.