Freeze-Thaw Cycles and Concrete Damage

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