Category: Maintenance

The short version of the article reads as follows: Do not apply chemical deicers to concrete!

As winter approaches, you gaze at the now limp plastic sack which held last year’s supply of deicer. You know that you need to pick a deicer unless you plan on spending the winter in a cast, but with so many options which one do you pick? There are a great many claims about which deicers are harmful to concrete, and how you can keep your walkways safe without damaging them.

In this series of articles I will explore some of the major causes of deicer damage to concrete and how you can select a deicer to best fit your needs and protects your concrete products. But before I get into the nitty-gritty, here are some general tips to keep your concrete looking brand new.

1. Wait 6 months to two years after installing new concrete to apply any chemical deicers. Concrete gains most of its strength in the first 6 months. No matter what you put on your steps waiting at least 6 months will substantially reduce the risk of damage to your concrete.
2. Apply a sealer before using any deicer. Sealers will protect concrete from both chemical and physical damage and are very inexpensive. For best results see a professional on concrete sealing and wait at least 28 days after concrete is poured to seal it.
3. Never use ammonium nitrate.
   The undisputed worst deicer for concrete. It does chemical damage and will even corrodes cured concrete.
4. Avoid Magnesium Chloride. This is the runner up for the most destructive deicer to concrete. Magnesium Chloride causes both chemical and physical damage. Though this product is often advertised as being safe for concrete, these claims are entirely inaccurate.
5. Use sand or kitty litter, if possible. Though these products do not melt ice, they will provide traction and cause virtually no damage to concrete surfaces of any age.

Chemical Damage to Concrete

Despite what is often advertised, most deicers do chemical damage to concrete. All salts, including calcium chloride, sodium chloride, ammonium nitrate, and even magnesium chloride do chemical damage[1,2,3]. Many companies which manufacture deicers will disagree with this claim, and I do not intend to cast stones at the deicer industry without just cause. Fortunately a number of scientific studies have been conducted to tackle this subject. I will review here the mechanics of chemical damage to concrete and how they relate to deicer corrosion.

Concrete Structure

To understand how various chemicals damage concrete, we must first understand how concrete derives its strength. Concrete is made from a combination of stone, sand and cement. Cement is the component of the mix which gives the concrete strength and shape, and is composed of calcium silicates such as (CaO)₂(SiO₂). When cement is mixed with water these calcium silicates combine to form larger molecules which for a hard gel with water and calcium hydroxide. [3]This process occurs according to the following equation:

2(CaO)₂(SiO₂) + 5 H₂0 → (CaO)₃(SiO₂)₂4(H₂O) + Ca(OH)₂

The result is a firm material which holds the sand and aggregate in concrete together. Figure 1 shows the  calcium silicate hydrate crystalline structure [3]. The expansive bond structure which connects all of these molecules together is responsible for the strength of concrete. At the center of each of the molecules is a calcium atom (green in figure 1). Salts and other deicers can damage concrete by removing this atom from these calcium silicate molecules. Without this central molecule, the stability of the crystalline structure breaks down and the concrete loses its strength.

Experimental Support

A study was conducted by the university of Kansas which dealt specifically with the chemical damage that deicers cause to concrete. In the experiment, a number of concrete ingots were cast and exposed to deicer solutions of various concentrations. The strength of the concrete was measured by its change in elasticity after immersion in the salt solutions.

Figure 2 shows the results of this experiment[5]. You can see the 15% drop in elasticity from the solutions of both magnesium chloride and calcium chloride in only the first 10 weeks of exposure. This is because both magnesium chloride and calcium chloride contain ions which can bond to the calcium in the calcium silicate hydrate structure, and remove it [1, 2, 4]. You can see a similar effect with CMA or calcium magnesium acetate, wherein the ions that form from CMA leach calcium from calcium silicate hydrate, although in CMA this process takes much longer.

This is because different elements have varying abilities to ionize and react with calcium and other chemicals. Halides, such as chlorine, tend to ionize easily in salts and be very reactive with other compounds [3]. Thus different salts cause varying amounts of damage to concrete.

Figure 3 shows photos of the concrete samples taken from the University of Kansas Experiment. The photos corroborate the results of the elasticity testing in that magnesium chloride and calcium chloride had the greatest appearance of physical damage, followed by CMA. The experiment also showed that sodium chloride (table salt) is actually far less harmful to concrete than other deicers. Finally, the difference in the damage from the high and low concentration solutions shows that there may be a great difference in the amount of damage done to concrete depending on the amount of deicer used.

In Conclusion:

To hit the main points that I want to stress in this article:

• Calcium chloride, magnesium chloride and all salts do damage to concrete.
• Magnesium chloride and ammonium nitrate are especially bad – do not use these!
• If you have to use salt, use a small amount. It will be better than the alternative.

Good luck dealing with the snow this year. Have a safe and happy winter from Steps Plus!

 

References:

1. Carde, C., R. François, and J.-M. Torrenti. Leaching of Both Calcium Hydroxide and CS-H from Cement Paste: Modeling the Mechanical Behavior. Cement and Concrete Research, Vol. 26, No. 8, 1996, pp. 1257-1268.
2. CODY, R. D. Experimental Deterioration of Highway Concrete by Chloride Deicing Salts: Environmental & Engineering Geoscience, Vol. 2, No. 4, 1996, pp. 575 – 588
3. H. F. W. Taylor, Cement Chemistry, 2nd Ed., Academic Press, London (1997).
4. Darwin, D., J. Browning, L. Gong, and S. R. Hughes. Effects of Deicers on Concrete
   Deterioration. ACI Materials Journal, Vol. 105, No. 6, 2008, pp. 622-627.

It’s March and the sun is out. The crusted snow is thawing, revealing glimpses of your sidewalk beneath the alabaster coating. Only 8 months ago you had a brand new sidewalk installed. It had a perfect broom finish and smooth edges when it went in the ground, but something catches you eye. There small craters all over the surface of your sidewalk. In fact, the smooth finish that you once knew now looks like the surface of the moon. You used a deicer maybe once a week, and the bag said that it was safe for concrete.

What Happened?

The Science of Concrete Degradation

There are two general mechanisms by which salt causes concrete to degrade: by chemical damage and by physical damage. We are going to focus on physical damage first.

Thermal Expansion

One of the most common causes of salt damage to concrete is thermal expansion. Thermal expansion occurs when water makes its way into pores in concrete and then freezes. When the water freezes, its volume increases creating pressure on the concrete which can result in cracks and scaling.

This phenomenon is not caused by salt. However, the damage that thermal expansion does to concrete can be exacerbated by salt application[3].

 

Consider a winter in which the temperature ranges between 0°F and 25°F between December and February (See Figure 1.). You have a slab of concrete, outdoors, exposed to these conditions. Each time the ambient temperature reaches the freezing point the water melts (when the red line crosses above the blue line). When the temperature drops below the freezing point the water freezes, and then expands to cause scaling and cracking of the concrete. This full process is known as a freeze-thaw cycle. The damage done to concrete during a winter is strongly related to the number of freeze thaw cycles experienced that year. The concrete subjected to the conditions in figure 1 experiences no freeze-thaw cycles because the temperature was lower than the freezing point over the entire time interval. This is the ideal condition to maintain the durability of concrete.

Deicers exacerbate the damage caused by thermal expansion by increasing the number of freeze-thaw cycles that concrete is subjected to. Salts and other deicers melt ice by reducing the freezing point of water. When the freezing point is reduced below the ambient temperature, the ice melts. Figure 2 shows how this influences freeze-thaw cycles.

 

The chart to the left shows the same ambient temperature data as figure 1, but the freezing point has been lowered to 5°F. This is the typical freezing point of water after magnesium chloride has been applied to it. We can see that the ambient temperature now drops below the lower freezing point and rises above it again. Though the ambient temperature remains above the freezing point of the MgCl2/water solution, the system now experiences one freeze-thaw cycle instead of none. This is the mechanism by which deicers exacerbate the scaling caused by freeze thaw cycles. However, the actual damage caused by deicers is typically much more severe than the wear caused by a single freeze-thaw cycle[3].

 

Deicers (such as magnesium chloride, calcium chloride, etc) do not consistently alter freezing points to the same temperature. Rather, the freezing point of a solution of water and deicer is dependent upon the concentration of deicer in the solution (see Figure 3). When deicers are applied to concrete they are not applied uniformly, and the distribution of deicer on the concrete can change over time. Thus, various regions will develop which each have distinct concentrations of deicer and, consequently, different freezing points[1].

If we focus on one specific region, we can see the effect that this will have on freeze-thaw cycling. Let us consider a region of the previous concrete slab in which snow falls regularly and deicer is routinely reapplied to melt the new snow fall. As snow falls, water is added and the concentration of deicer in the region is lowered. This consequently raises the freezing point. Adding deicer conversely raises its concentration and lowers the melting point. The result is a region in which both the temperature and the freezing point fluctuate over the time interval (see Figure 4).

 

Here we can see that the freezing point of the deicer/water solution fluctuates between 5ºF and 15°F. From Figure 3 we can see that this might represent a change in concentration on magnesium chloride of about 35%. Figure 4 shows that the ambient temperature now intersects the freezing point 4 times, resulting in 2 freeze-thaw cycles. These additional freeze-thaw cycles are responsible for the physical damage that salt causes to concrete.

Experimental Support

A study performed in 2006, at Iowa State University was performed to show the impact of deicers on freeze-thaw cycles and on damage to concrete. The study measured the effects of sodium chloride, calcium chloride, calcium chloride with corrosion inhibitor, potassium acetate and agricultural deicer on concrete. Concrete samples were exposed to a solution of water and each of the formerly mentioned deicers and then taken through a freeze-thaw cycle. Photos were used to document the results of the experiment[2].

From Figure 5 you can see two photos labeled “a” and “c”. These are photos of blocks which were exposed to deionized water and calcium chloride, respectively. One can easily observe the dramatic difference in corrosion between the water and calcium chloride samples. By following the link in the references page, you can confirm with the experiment’s elasticity testing that this impact is not purely cosmetic.

Conclusion

Deicers are effective at melting ice because they affect the freezing point of the water around them. Unfortunately, this exact quality that makes deicers good for promoting winter safety causes them to damage concrete. There are, however, several things that you can do to keep salt from scaling your concrete.

The first thing that you can do is apply a sealer right away. Since water has to enter the concrete for it to cause any damage when it freezes, the best way to protect your concrete is to keep water from entering the pores on its surface. A sealer will create a coating which keeps water from entering these small pores and freezing to cause concrete scaling.

In my next article I will go into the subject of chemical damage and address that burning question: “Which deicer is actually safe for concrete?”

References:

1.Jozwiak-Niedzwiedzka, Daria & Jain, Jitendra & Olek, Jan & Janusz, Anna. (2012). Effects of Deicing Salt Solutions on Physical Properties of Pavement Concretes. Transportation Research Record Journal of the Transportation Research Board. 2290. 69-75. 10.3141/2290-09.

2.Kejin Wang, Daniel E. Nelsen, Wilfrid A. Nixon. (2006). Damaging Effects of Deicing Chemicals on Concrete Materials. Cement and Concrete Composites. 2802. 173-188. 0958-9465

3.Eric S. Sumsion, W. Specncer Guthrie, Ph.D. (2013). Physical and Chemical Effects of Deicers on Concrete Pavement: Literature Review. Utah Department of Transportation- Research Division. UT-13.09. 5-28