Rock salt, also known as halite, is a commonly used deicing agent during winter months. It is highly effective at melting snow and ice on roads, sidewalks, and driveways. However, there are concerns about whether the use of rock salt can damage concrete surfaces over time. In this article, we will examine the effects of rock salt on concrete and discuss whether it is considered a safe option for deicing concrete.
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What is Rock Salt?
Rock salt refers to the mineral halite or sodium chloride (NaCl). It forms as large, chunky crystals and is typically sourced from salt mines or underground salt deposits. Rock salt contains at least 98% sodium chloride and may also contain small amounts of calcium, magnesium, and potassium salts.
The main uses of rock salt are for deicing, food seasoning, water conditioning in swimming pools, manufacturing, and agriculture. As a deicer, rock salt works by lowering the freezing point of water through a process called freezing point depression. This effect prevents ice from bonding and accumulating on surfaces.
For deicing purposes, rock salt is applied to roads, driveways, sidewalks, and steps in advance of or during wintry weather. The salt dissolves into the water and penetrates pores in the concrete or pavement. As vehicles drive over the treated surface, the salty water splashes and spreads, further distributing the salt.
How Does Rock Salt Affect Concrete?
While rock salt is highly effective for melting snow and ice, it can potentially damage concrete surfaces. Here are some of the ways rock salt impacts concrete:
Spalling and Pitting Damage
One of the most common effects of rock salt on concrete is increased spalling and pitting. Spalling refers to chips, flakes, or fragments of concrete that break loose from the surface. Pitting describes the formation ofsmall indentations and holes in the concrete surface. This physical deterioration is caused by the freezing and thawing cycles that occur when salt penetrates the concrete. The salt lowers the freezing point of the water inside the pores and cracks of the concrete. During freeze/thaw cycles, the expanding ice puts internal pressure on the concrete, causing pieces of the surface to break off.
Surface Erosion
The dissolved salt also causes abrasion damage each time vehicles drive over it. The friction from tires moving over the gritty, salty concrete gradually wears down and erodes the surface. Concrete can become markedly rough and uneven over time from this abrasion effect.
Corrosion of Reinforcing Steel
Salt speeds up the oxidation or corrosion of steel rebar inside reinforced concrete. While concrete normally provides alkaline protection for the steel, salt reacts with the alkaline compounds and destroys this passive layer. Exposed rebar will eventually rust and expand, leading to cracks, structural issues, and reduced strength.
Deterioration of Joint Materials
The salt solution also negatively impacts joint sealants, inducing shrinkage, embrittlement, and cracking. Loose or compromised joint sealants allow for increased water penetration.
Scaling
Scaling occurs when the concrete surface develops a thin, flaky, crust-like texture and pieces detach. This effect is also due to the freeze/thaw cycling of salty water inside the concrete. The salt crystals exert outward pressure, causing scaling fragments to pop off the surface.
White Hazing
A white haze may develop on concrete surfaces that have been treated with rock salt. This efflorescence is caused when excess salt crystallizes on the surface as the water evaporates. The white residue can detract from the appearance of decorative concrete.
Factors That Influence Damage
The extent of damage caused by rock salt depends on several factors:
Concrete Properties
The quality and characteristics of the concrete itself directly affect deterioration rates. Concrete that is porous, permeable, or poorly consolidated is much more vulnerable to salt damage than high-density, low permeability concrete. Weaker concretes with lower compressive strengths typically have higher porosities. The proportioning of the concrete mixture is also important. Insufficient cement paste volume and inadequate curing conditions lead to increased porosity and permeability. Any cracks or flaws in the concrete provide entry points for salt solutions to penetrate into the matrix.
Environmental Conditions
Areas with many recurring freeze/thaw cycles tend to exhibit worse salt deterioration. The rate of freezing and thawing contributes too. A rapid drop to extreme subzero temperatures can cause severe, sudden damage. Marine environments where concrete is exposed to salty coastal air and water causes enhanced deterioration.
Salt Concentration
Higher concentrations of dissolved road salts create greater potential for damage. Using excessive amounts of salt on concrete surfaces intensifies the negative effects. Salts also penetrate deeper into concrete when they are more concentrated.
Drainage
Proper drainage plays a key role. Concrete surfaces that do not drain well allow salty water to pool and remain in contact with the concrete for prolonged periods. The longer the wet contact time, the greater the overall salt exposure. Positive drainage minimizes contact time and leaching depth.
Maintenance
Regular cleaning and preventative maintenance helps limit buildup of excess salts. Techniques like pressure washing remove accumulated salts from surfaces. Timely repairs seal any cracks that allow water intrusion. Concrete coatings and sealants provide a protective barrier against chlorides.
Is Rock Salt Considered Safe for Concrete?
There is an ongoing debate about whether or not rock salt should be considered a safe option for deicing concrete. Here are some key considerations:
Effective Ice Removal
Rock salt is highly soluble and very effective for melting snow and ice on concrete surfaces. It works rapidly at relatively low temperatures down to about 5°F (-15°C). No other deicer works as fast or is as cost-effective as sodium chloride rock salt. It is easy to apply manually or through mechanized spreader equipment. These advantages make rock salt an excellent choice for ensuring winter motorist safety and mobility.
Widespread Usage
Rock salt represents about 90% of the deicing salts used on U.S. pavements and surfaces. It has been widely utilized as a concrete deicer for decades with generally acceptable results when proper precautions and application guidelines are followed. Billions of tons of rock salt are produced in the U.S. alone showing just how ubiquitous it has become.
Manageable Concrete Damage
Research indicates that concrete damage from rock salt is noticeable but often minimal or acceptable if preventative measures are implemented. The abrasion effects are typically superficial. Corrosion of rebar only occurs at very high chloride concentrations that are preventable. Most concrete deterioration is manageable provided the concrete quality is sufficient and regular maintenance is performed.
Environmental Effects
A major downside to rock salt is its environmental impacts. Runoff into soils and groundwater raises sodium levels, damaging plants and contaminating drinking water. Chloride runoff into lakes, streams, and wetlands negatively affects aquatic ecosystems. Strategies like reducing application rates, alternative deicers, and collection of runoff help mitigate ecological effects.
Need for Balanced Approach
Given its essential safety benefits but potential detriments, most experts recommend a balanced approach for using rock salt on concrete. This involves:
– Only applying minimum effective amounts for ice/snow removal
– Sweeping up excess once ice has melted
– Proper concrete specifications and construction
– Utilizing alternative deicers when practical
– Using concrete sealants to protect surfaces
– Implementing drained collection systems for runoff
Guidelines for Using Rock Salt on Concrete
When rock salt is utilized for deicing concrete, following proper guidelines can help minimize damage:
– Only apply salt at amounts needed to melt current ice. Use sparingly. Over-salting should be avoided.
– Select high-quality concrete mixes with low permeability and sufficient strength. Incorporate air-entrainment for freeze/thaw resistance.
– Ensure adequate curing of concrete for maximum density and strength.
– Use proper construction methods like insulating forms to prevent frozen concrete placements.
– Consider admixtures like corrosion inhibitors to increase protection.
– Apply protective surface treatments like quality sealers to limit salt penetration.
– Design concrete joints and drainage systems to avoid standing water.
– Inspect concrete regularly for early damage and make repairs quickly.
– Implement preventative maintenance like frequent sweeping and washing of salts.
– Divert runoff away from vegetation and mitigate environmental impacts.
Alternative Deicers
While rock salt remains the primary concrete deicer, various alternatives exist:
Calcium Chloride
Calcium chloride pellets work at lower temperatures than salt, down to -25°F (-30°C). However, calcium chloride draws moisture from the air and can leave concrete surfaces wet. Acetate-based additives help minimize this effect.
Magnesium Chloride
Magnesium chloride is effective to about 5°F (-15°C). It is less likely to cause surface erosion but salt residue can still damage concrete over time.
Potassium Acetate
Potassium acetate performs well in extremely cold climates since it remains effective down to -75°F (-60°C). It causes less corrosion than salt but is more costly.
Calcium Magnesium Acetate
CMA does not contribute chloride ions but has limited ice melting capacity. It is bio-degradable and less harmful to the environment.
Urea
Urea is a moderately effective organic deicer. It does not cause scaling or pitting but may damage vegetation. requiring higher application rates.
Sand and Ash
Abrasives like sand or wood ash provide traction but do not melt ice. Use on top of alternative deicers to improve pedestrian safety.
Conclusion
Rock salt is an economical, readily available deicer that melts ice extremely effectively. However, it also poses risks of surface erosion, scaling, pitting, cracking, and other forms of deterioration to concrete surfaces and steel reinforcement over time. Managing these effects requires restricting salt usage, utilizing alternative deicers when possible, implementing preventative maintenance, and following best practices in concrete mixture proportioning, construction, joints, and drainage. While a highly useful winter maintenance tool, rock salt requires cautious and sparing application to minimize its long-term impacts on concrete. With responsible use and mitigation strategies, rock salt can be an acceptable option for keeping concrete infrastructure safe through wintry conditions.