Which deicers are safest to use on concrete?

One of the most common questions that we get is which deicers are safest to use on concrete or on new concrete. This is a difficult question for me to give you a brief answer to because the effects of deicers on concrete are quite complex and there has been an enormous amount of research that has been done on this question. If I were to share with you all that we know about deicers and concrete, it would take a very long time. As a result, I will provide a brief overview that will hopefully help you better understand the basics of how deicers effect the durability of concrete. 

There are three basic mechanisms by which deicers can attack concrete:

1. They can exacerbate freeze-thaw scaling
2. They can chemically react with the concrete to break it down over time
3. They can cause corrosion of the rebar in steel-reinforced concrete.  

Chemical reactions between deicers and concrete and rebar corrosion are less common outcomes from deicing. When rebar corrodes the iron in the steel is getting converted to iron oxide.  It is absorbing oxygen and as it creates this new compound of iron oxide, it is expanding, and because that rebar is encased in concrete, there is very little room to accommodate that expanding reaction product.  So, as that rust layer grows, it starts to exert pressure on the concrete matrix and it will break it and crack it over time.  The rebar corrosion effects on concrete durability is really an indirect effect of deicers. 

The most common way that deicers attack concrete is by increasing the amount of damage caused by freezing and thawing.  Concrete is a porous material and it generally contains water in the pores at the surface, and when water freezes, it expands.  Its expanding water exerts pressure and stress on the surrounding concrete matrix and it can damage it.  Now it turns out that if the only thing that is present is water, concrete is able to withstand that stress pretty well, but if deicing chemicals are dissolved in the water in the pores, it increases the amount of stress that occurs when that water freezes and it can result in cracking/scaling/spalling of the surface, exposure of the aggregate and ultimately to breaking that concrete down.  This is a process that we call freeze-thaw scaling. 

The good news is that it turns out that properly made concrete is actually very resistant to freeze-thaw damage even in the presence of deicers.  Modern concrete is protected from freeze-thaw damage largely by a process called air entrainment.  Air entrainment introduces tiny bubbles or voids in the concrete and these provide additional space for that water to expand into when it freezes, thereby reducing the pressure and the stress on concrete. 

So we know that properly produced, finished and cured concrete is very resistant to freeze-thaw damage by deicers; however, if the concrete is of marginal quality, if it is not properly air entrained or it has not been given enough time to cure, deicers can cause damage to concrete.