A primer: calcium chloride for stabilizing gravel roads
The 70,000-plus miles of aggregate-surfaced roads in Minnesota are essential for providing all-weather access for people and freight. Unfortunately, these roads can also be dusty and bumpy, and they require periodic replacement of lost gravel and regular blading to smooth the surface.
According to the report Relative Effectiveness of Road Dust Suppressants prepared by the Transportation Information Center at Colorado State University, dust from unpaved roads contributes nearly 34% of the particulate matter in the atmosphere nationwide. Loss of gravel and blading can cost a road agency several thousand dollars per mile per year.
A common tool to stabilize the aggregate surfacing used by counties for many years has been the application of calcium chloride. What is calcium chloride and where does it come from? How does it work? Are there adverse environmental affects? County experiences?
What is calcium chloride and where does it come from?
According to Dr. Steve Clark, a research scientist with EnviroTech, a principal vendor for calcium chloride in our region, calcium chloride results from the happy and stable mating of two chemicals: one calcium atom with a plus 2 electrical charge and two chlorine atoms with a minus 1 charge. It is highly soluble in water. Calcium chloride can be manufactured in several ways, including from limestone and sodium chloride and limestone and hydrochloric acid. However, it is also available as a natural brine that can be filtered and diluted or concentrated as needed for road stabilization or for ice control. A natural brine deposit in Michigan operated by Oxy Chemical is the primary source of the material in this area. The material can be concentrated through evaporation to produce a solid.
Magnesium chloride has very similar dust and ice control characteristics. It is generally a byproduct of potash production. The cost of both materials is similar and relatively low; the logistics of transportation largely determine the difference in delivered cost.
How does calcium chloride work?
The hygroscopic properties of calcium chloride attract moisture from the air. The moisture and electrostatic attraction of the small, minus 200 screen particles create particles heavy enough to stay in place rather than being blown or washed away. Thus, the fine particles, critical for binding the aggregate together, remain in place. The moisture and calcium chloride may also act as a lubricant to facilitate compaction of the aggregate.
Calcium chloride depresses the freezing point of water and is very effective for snow and ice control at very low temperatures. Depending on the solution of calcium chloride solids to water, it can be stored at temperatures much lower than sodium chloride brine.
Counties using calcium chloride for both dust control and deicing have an interesting apparent paradox in storing the material over the winter. To keep the solution from freezing, you add water. Dr. Clark shed some light on this: for dust control, the calcium chloride solids are the effective agent and a high concentration of solids is desirable. For freezing, the proximity of the solids makes it easier for them to attach to each other and solidify into frozen calcium chloride. Diluting the solution moves the solids away from each other, lowering the freezing point (eutectic point of 30% calcium chloride is 60 degrees below zero Fahrenheit).
Calcium chloride is not toxic; in fact, it is used to increase the water hardness in swimming pools and as a food additive. However, there are growing environmental concerns about chloride released by sodium chloride salt and the potential for chloride release from calcium chloride salt. Application in a uniform manner, at an appropriate rate, and on a properly shaped and graded aggregate surface is important in binding the calcium chloride to the road surface.
Olmsted, Dodge, and Blue Earth County practices were reviewed as examples of the widely used product.
Olmsted County policy for many years has been to apply calcium chloride for a 300-foot reach at each residence on its county crushed-limestone-surfaced roads at no charge. One application in the spring is made by contract. It has enjoyed the support of the public and elected officials. The material reduces dust, loss of aggregate, and amount of blading needed substantially. However, by capturing the fines as the limestone crushes under traffic, the fines accumulate, and after a rain the surface may become slimy. This can be corrected by adding clean crushed aggregate with few fines and mixing with a blade. Since the material is hygroscopic, it attracts moisture in the winter and spring, when surface moisture may be a liability. Olmsted County is not using it for ice control.
Dodge County policy for more than 25 years has been to apply primarily magnesium chloride at residences where requested on its county crushed-lime- stone roads. It has found magnesium chloride to be less costly. The resident pays the total cost, currently estimated at $70 for a segment 100 feet long by 24 feet wide. One application is made by a contractor in the spring. The program has enjoyed support from citizens and elected officials. The county emphasizes the need for quality durable aggregate to reduce the amount of traffic-induced aggregate crushing and accumulation of excessive fines. Citizens may object to blading the section of treated road to smooth bumps, since it temporarily reduces dust control. The county has tested calcium chloride for ice control under subzero freezing conditions and found it works well but is expensive.
Blue Earth County policy for more than 30 years has been to apply calcium chloride to gravel roads in the county, including townships and cities as requested by citizens. The citizen is charged the actual cost of the material, currently $125 for a segment 100 feet long by 20 feet wide. Two applications are made for this price, one in the spring and one later in the summer. The program has enjoyed good support over the years. Calcium chloride is also applied for special conditions, such as gravel detour routes or difficult hills and intersections. Accumulation of fines has been less of a problem, perhaps because of the harder natural gravel sources. The county blades the road before application and minimizes blading through the treated section after. The material is purchased by semi-tanker load and stored in a county tank. It is applied by a tank and spray bar unit mounted in a county dump truck. In the fall, additional water is added to the storage tank to obtain the optimum eutectic point for a lowered freezing temperature. During very cold conditions, calcium chloride is sprayed on top of the sand/salt mixture in the dump truck box. This has proven to be very effective, although it is a very corrosive mixture for the trucks.
Is calcium chloride treatment cost-effective?
The Colorado State University report referenced earlier found a 50–70% reduction in fugitive road dust emission when treated with calcium chloride. It also found a 42–61% reduction in aggregate loss. This is highly variable depending on climate, traffic, and aggregate quality. Cost-effectiveness will depend on replacement aggregate cost and cost to treat the road. The study found for an aggregate replacement cost of $15 per ton and a cost to treat with calcium chloride of $2,800 per mile, treatment to reduce aggregate loss would become cost-effective at 105 ADT. Higher replacement aggregate costs will tend to reduce the feasible ADT; higher calcium chloride treatment costs will tend to increase the feasible ADT. This is only one limited study in another region with different climate conditions and aggregate sources from ours, so caution is advised.
—Al Forsberg, P.E., retired Blue Earth County Engineer