Physical Mechanisms

It has long been recognized that chemical deicer applications amplify the physical mechanisms responsible for freeze-thaw deterioration of concrete (ACI 2016). This amplification is thought to be due to multiple factors, including the increased level of saturation that occurs when salt solutions are present, thermal shock that occurs due to the phase change of ice to water, an increase in osmotic pressures due to changes in pore solution chemistry, and the potential for salt crystallization within confined pore spaces (Mindess, Young, and Darwin 2003; ACI 2016; Kosmatka and Wilson 2016). As such, the exposure to chemical deicers constitutes the most severe freeze-thaw exposure condition based on ACI (2016). 

To address this, current recommendations are that the concrete must be air entrained (with a total air content of 6 ± 1.5 percent for typical paving and bridge deck concrete, with some agencies assessing air content after placement) and must have a w/cm below 0.45 (for plain concrete, whereas a value of 0.40 is recommended for reinforced concrete) to prevent damage from the application of deicers (ACI 2016). Until recently, the most common distress associated with deicer applications on concrete flatwork, such as slabs, has been surface scaling, such as shown in figure 3. The strategies described above have historically been effective in minimizing the occurrence of deicer scaling on machine-finished SHA pavements and bridge decks when accompanied with good finishing and curing practices.

However, there has been a recent increase in deterioration observed in the vicinity of concrete joints. As illustrated in figure 1 (presented earlier), this distress differs from scaling in that it initiates and progresses at joint locations. It often first appears as a “darkening” or “shadowing” of the concrete adjacent to the joint, which is attributed to water being trapped in microcracks that have developed near the joints. Over time, this microcracking may progress into significant loss of material (Taylor, Sutter, and Weiss 2012; Weiss and Farnam 2015). Physical mechanisms of this joint deterioration have been linked to the following factors, often working in combination (Taylor, Sutter, and Weiss 2012):

• Locally saturated concrete in the vicinity of the joint.
• Presence of a marginal-to-inadequate entrained air-void system.
• A w/cm greater than 0.40.
• Aggressive use of chemical deicers.
• The presence of coarse aggregates that are slowly susceptible to freeze-thaw damage (i.e., D-cracking).

The deterioration in the study by Taylor, Sutter, and Weiss (2012) was commonly observed in the joint below the pavement surface, being related to a combination of factors including the ponding of water in the bottom of the saw cut, salts collecting in the joints, and increased permeability at the cut faces of the joints. The role of deicers in the occurrence of this deterioration was noted in the report, which states (Taylor, Sutter, and Weiss 2012):

The effects of deicing salts are likely the tipping point, explaining why this is perceived as a relatively new phenomenon. It is likely that this form of distress has been occurring for a long time, but the change in salting practices have made it more common and, therefore, more notable than before.

It is recognized that partially dry concrete will not be damaged from freeze-thaw cycles because the larger pores in the HCP are empty and provide adequate space to accommodate hydraulic and osmotic pressures that develop as ice forms. But if this same concrete undergoes freezing and thawing in a critically saturated state (above approximately 86 percent saturation), damage will occur within even a single freeze-thaw cycle, irrespective of air void volume (Jones et al. 2013).

Critical saturation of concrete is more likely to occur at joint locations for a number of reasons, schematically illustrated in figure 4, which shows a typical sawed and sealed contraction joint in concrete. A properly functioning sealed joint (figure 4a) will significantly reduce the ingress of the water-salt deicer solution so that very little additional saturation of the concrete occurs within the joint. If the joint seal is compromised (figure 4b), the deicer solution will penetrate into the joint, increasing the saturation of the surrounding concrete as illustrated. If the material beneath the activated joint is not free-draining, water entering through the compromised sealant will not be able to drain from the joint. This can lead to critical saturation of the concrete. In this case, a sufficiently low w/cm and an adequate entrained air-void system are essential to protect the concrete against freeze-thaw damage.

Yet there are a number of conditions that may occur that, in combination, may result in the deicer solution remaining in the joint for long periods of time, including (Taylor, Sutter, and Weiss 2012; Jones et al. 2013):

• The joint does not activate and a crack does not form.
• The joint is highly restrained, due to poor tie bar design or misaligned dowels, and thus does not open sufficiently to drain.
• The crack of an activated joint becomes clogged and will not drain.
• The presence of the failed sealant and backer rod delay the evaporation of the solution that is retained within the joint.

Once the water-salt solution pools within the joint, it is absorbed into the exposed concrete and becomes increasingly concentrated over time as additional deicing salts are used (Jones et al. 2013; Weiss and Farnam 2015; Monical et al. 2016). If the joint is not draining, the greatest degree of saturation occurs in a zone at the bottom of the first saw cut, as shown in figure 4b. Due in part to the properties of salt solutions, which increase the degree of saturation, this may result in the concrete within this zone exceeding critical saturation (Jones et al. 2013; Monical et al. 2016). Figure 5 illustrates the type of damage that can occur in this zone of critically high saturation.