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Holding the Road with Calcium Chloride Flake


In its initial roadway applications nearly a century ago, the sole purpose of calcium chloride was to provide relief from dusty road conditions. Its use, however, has grown continuously for over 75 years, and calcium chloride is now an integral part of the plans and specifications of contractors and highway departments throughout the nation. Today, calcium chloride is used not only to maintain unpaved roads and streets but also to construct the shoulders and bases of modern superhighways and turnpikes.

Peters Chemical Company has had years of experience using calcium chloride for these highway applications. Our experience, along with the vast research conducted by various organizations, has helped us to gain a wealth of knowledge in this area. This knowledge assures predictable, controlled, economic results.

The following pages examine two current uses of calcium chloride:

  1. Surface stabilization (including dust control and surface consolidation)
  2. Graded aggregate mixes for bases and shoulders.

While these two uses are interrelated, each represents a specific phase of road work, and for this reason they are discussed separately. Although the formulations presented here are based on 83-87% flake calcium chloride, other forms of calcium chloride may be substituted by the use of appropriate adjustments for the product assay.

Surface Stabilization

Regular flake calcium chloride has two properties that make it particularly useful as a surface stabilization material:

  1. It absorbs moisture from the air or surface and retains this moisture for an indefinite period of time. This property is referred to as hygroscopicity – the ability to absorb moisture from the atmosphere.
  2. It dissolves in the moisture it absorbs, forming a clear, colorless liquid, which is extremely resistant to evaporation. This chemical action is known as deliquescence – the ability to become liquid by the absorption of moisture.

When flake calcium chloride is spread on an unpaved surface, it begins to absorb moisture from the air or surface and to dissolve in that moisture. As the resultant calcium chloride solutions penetrate the surface material, they coat the particles of dust and gravel and bind them together. This binding action stabilizes the surface and keeps unpaved roads dense and compacted. Because these same calcium chloride solutions resist evaporation, the surface remains damp, and the particles stay bound together, even on the hottest, driest days.

Dust Control with Calcium Chloride

Billowing clouds of dust are recognized by the general public as a nuisance and sometimes even as a hazard. Yet every highway superintendent knows that the tiny particles, or “fines”, serve a vital function in the roadway surface if they can be held there.

So long as they remain in the surface, the fine particles stabilize the unpaved roadway by acting as a binding agent for the coarser aggregates. But when those fines are lost in the air, the road surface begins to loosen, and traffic starts scattering the expensive roadway aggregate into ditches and culverts. If unchecked, this continuous deterioration of the roadway surface will result in the need for extensive road rebuilding and attendant high costs.

A proper application of calcium chloride helps stop the deterioration before it begins. When applied as a dust control agent, calcium chloride consolidates and stabilizes the roadway to provide a clean, smooth-riding surface.

Benefits of Calcium Chloride

Calcium chloride has a long history of use as a dust control agent for unpaved roadways, parking lots and other unpaved surfaces, and its use in this application is continually growing. The consistent increase in acceptance of calcium chloride can be attributed to the combination of the following benefits:

  1. Retention of Fines:

    The loss of fines in the formation of dust is one of the basic reasons for the deterioration of a riding surface. Calcium chloride helps stop this surface deterioration by enhancing the binding the loss of fines in the formation of dust is one of the basic reasons for the deterioration of a riding surface. Calcium chloride helps stop this surface deterioration by enhancing the binding together of fines and aggregates and thus forming a stabilized surface.

  2. Reduced Material Replacement Costs:

    The replacement cost of lost materials represents an actual loss of road dollars for many highway departments. Often an even more serious factor is the depletion of local deposits of gravel and other surfacing materials. When calcium chloride is applied to an unpaved surface, it coats the fines and aggregates, binding them together to help keep the original surface material in place and thus also to help reduce the need for replacement materials.

  3. Less Blading:

    The amount of blading required to patch and reshape a road is directly related to the extent of the deterioration of the road’s surface. A badly deteriorated road surface will require extensive, heavy blading; a stable, consolidated roadway surface will require a minimal light blading. As calcium chloride binds the fines and aggregates together, the treated road materials form a stable, compacted surface that remains intact and requires less blading.

  4. Elimination of Dust Complaints:

    There is no other phase of highway maintenance that is more appreciated by the driving public and by taxpayers who live and work along unpaved roads than the elimination of dust.

    Soon after an application of calcium chloride for dust control, the telephone calls and letters expressing concern over dust are replaced by thanks for the smooth-riding, dust-free road surface.

  5. Adaptability to Surfaces:

    Almost no two geographical areas have exactly the same composition of material in their road surfaces. However, with proper gradation, calcium chloride can be used on almost all unpaved surfaces (both large and small), including earth, cinders, gravel, sand, bluestone, shale, limestone, clay, graded crushed stone, shell, and other similar local materials.

  6. Increased Safety:

    Dust is not only a nuisance but can also be the cause of accidents on unpaved roads. As billowing clouds of dust reduce visibility, the temporarily “blinded” motorist must confront the dangers of unseen potholes, washboard, and other road hazards.

    Because of its ability to penetrate the road surface and bind fines and aggregates together, calcium chloride greatly reduces the formation of both dust and potholes. The end result is a cleaner, safer, smoother-riding surface.

Why Calcium Chloride

Over the years, a variety of materials including oil, oil emulsions, lignin sulfonate, and water have been used for dust control. A comparative examination of these materials clearly indicates why calcium chloride has proven to be the most effective dust control agent.

Lignin Sulfonate:

From the standpoint of material costs, lignin sulfonate, an impure by-product of sulfite wood pulping operations, may be less expensive than calcium chloride. Yet the use of this material for dust control and stabilization has been minimal because of its lack of availability and its lower effectiveness. To overcome the problem of lower effectiveness, more applications are needed. And more applications can mean only one thing: higher operating costs.


Water is undoubtedly the least expensive material used for dust control. Yet evaporation necessitates multiple applications. In addition, water has no aggregate binding ability. Therefore, soon after the application of water is followed by surface drying, the problems of dusty, deteriorating roads reappear.

Although water is inexpensive, the high operating costs associated with constant watering and patching make water an unattractive alternative for dust control.

Millions of miles of unpaved roads in this country are constructed of excellent surfacing material. A simple treatment with calcium chloride, combined with seasonal shaping, will provide clean, smooth-riding surfaces requiring little, if any, other maintenance. Frequently, the use of calcium chloride for stabilization through dust control more than pays for itself in savings in replacement materials and blading. And roads of this type are often so well accepted that demands for more expensive paving can be held off until the volume of traffic grows to a point where the cost of paving can be fully justified.

Spot Treatment:

Spot treatment with calcium chloride can be employed when local funds or traffic conditions do not permit full-scale dustlaying. Such spot treatment is practical at intersections, corners, detours, markets, in front of houses, on grade crossings, shoulders, breakups in blacktop pavements, and gravel roads adjacent to paved trunk lines.

Many communities handle the expenses of spot dustlaying on a share-the-cost basis whereby taxpayers who live along the road to be treated, or motorists who use the road, pay the local highway department for the calcium chloride plus a nominal charge for labor.

Surface Consolidation

Improvement of Unpaved Road Surfaces and Shoulders

A calcium chloride road is an unpaved road containing a naturally or mechanically balanced mixture of local materials compacted into a smooth, dustless surface with moisture absorbed from the air and maintained by calcium chloride.

The stability of this road results from combining local materials correctly either during the original construction or as part of planned maintenance operations.

In a road improvement program, the calcium chloride road is the intermediate type of improvement between the dusty unimproved road and the more expensive paved surface.

The experienced road man knows the performance of the gravel or stone roads in his system. Certain roads that stand up very well (after they have been shaped) during damp or even wet weather become dusty, ravel, and washboard during dry weather. Other roads are reasonably stable during dry weather but soften, become slippery, break up or pot-hole during periods of wet weather. Some roads may have sections where each of these conditions prevails. If these conditions are not severe, the roads can be converted to calcium chloride roads by shaping – securing of the proper crown – arranging for adequate ditching – and treating the surface with calcium chloride. In those cases, however, where periods of either dry or wet weather severely affect the stability of the road, it will be necessary to adjust materials in order to achieve a balanced mixture. Necessary adjustments can be made either in a single operation or through the addition of the required materials over a period of time as part of a regularly scheduled maintenance program.

Composition of the Calcium Chloride Road

Close examination of the surfacing materials of a typical calcium chloride road reveals that they range in size from coarse aggregate (one inch maximum) to the very finest particles of binder and soil. The smaller-sized particles are found in sufficient quantities to fill in the spaces between each of the larger sizes. Under traffic, these graded particles are compacted into place and interlock to form a dense, smooth-riding surface with sufficient strength to carry all normal wheel loads.

The binder soil, kept moist by treatment with calcium chloride, remains plastic, thereby keeping the voids filled. Thus there is formed a surface that is sufficiently tight and compact to prevent the penetration of water from rain or melting snow.

The surface of this kind of road usually meets the recommendation of the American Association of State Highway and Transportation Officials for surface course gradations. A one-inch top size aggregate is considered maximum for easy blade maintenance. Larger top sizes may be used as long as the other materials are proportionately graded.

Correcting Roads Lacking Proper Gradation

Some roads have unsuitable gradations as a result of either a lack of fines to compact or hold coarser aggregate or a lack of sufficient aggregate to give stability in wet weather. Correcting this deficiency requires either the addition of new material mixed with the present road material or the addition of sufficient graded aggregate to provide a new wearing surface.

Many types of aggregate can be used, including limestone, gravel, and crushed stone. It is essential that this material be graded (it should vary in size from coarse to fine) and not of a uniform size. If local graded material is not readily available, run-of-crusher stone or gravel can be used. In some cases, materials of different sizes can be selected, windrowed, and mixed on the road. This procedure is followed to insure that (1) all the aggregate will be incorporated into the surface, and that (2) no more aggregate is added than is necessary to accomplish the job. Where funds and equipment are available, the road may be corrected in one operation by scarifying the top inch, then adding and mixing the new aggregate with the scarified material.

Corrective Maintenance

If a road is compact and does not rut but becomes slippery in wet weather because of an excess of binder soil, the application of sand, pea gravel, stone chips or similar small-sized material is recommended. This should be applied to the road surface during wet weather in very thin layers, either from trucks or by means of sand spreaders, in sufficient quantities to blot up the mud. And preferably this work should be done in the spring when the road surface is moist and before the calcium chloride has been applied.

Drainage, Shaping and Blading of the Calcium Chloride Road

The calcium chloride road, as any other type of road, produces the best results when it has good drainage and a proper crown. Thus, assuring good drainage and providing a proper crown are the first steps in converting a road to a calcium chloride road and the first items in any spring maintenance program. This work should be carried out as early as practical while there is still moisture in the ground and before the road dries out.


Adequate drainage is vitally important. This requires the maintenance of a proper crown and the cleaning and shaping of ditches. A crown has been well described as the roof structure of the road. A good crown is essential to all calcium chloride roads. Standing water that results from a flat surface or high shoulder is, of course, the main cause of potholes.

Blading and Shaping

To restore the proper crown and shape the shoulders, a road should be bladed in the early spring when there is still moisture in the surface. A straight-line crown of ½” per foot has been found to be most satisfactory (3¼” to 4” on a 14’ to 16’ roadway).

It is important that care be taken not to cut into or otherwise disturb a hard, well-consolidated wearing surface by blading or maintenance when the road is dry. On roads of this type, the spring blading usually consists of pulling in a sufficient amount of loose material from the shoulders to restore the proper crown and cross section.

If, because of insufficient or improper maintenance, the road has developed potholes or poor cross section, heavier blading will be required. It may be necessary to cut into the surface to obtain sufficient material for reshaping. If any frost-boils have developed, it is necessary to remove the poor soil and replace it with granular material.

Summer and Fall Maintenance of Calcium Chloride Roads

During the summer and early fall seasons, the maintenance of calcium chloride roads is usually limited to light blading of the surface and the application of prescribed amounts of calcium chloride. It is recommended that patrolmen take advantage of rainy periods and start their maintenance either during or immediately following a rain. This maintenance consists of lightly blading from the edges toward the center of the road and then reversing the operation, feathering the material to the edges of the road. Care should be taken to limit the length of the sections to be maintained so that each section can be compacted before the road surface becomes too dry to bond under traffic. It is generally not advisable to blade the surface of consolidated roads during dry weather, as this tends to loosen the aggregate and dissipate the calcium chloride.

Application of Regular Flake Calcium Chloride

The initial seasonal application of calcium chloride should be made as soon as possible after roads have received their final shaping. In the spring, calcium chloride should be applied while the road is still moist – before it dries and dust appears. Calcium chloride should be evenly and uniformly applied over the surface of the road within one foot of either side. On roads that have been previously treated, the rate of initial or spring application of calcium chloride depends on the amount of visible carry-over from previous applications, how well the surface is consolidated, and the amount of traffic being carried. This application will range from as low as ½ lb per square yard, on a compact road having carry-over and light traffic, to 1½ lbs per square yard.

On new roads which have not previously been treated, the quantity of calcium chloride to be used usually varies from 1 lb. to 1½ lbs. per square yard, depending on the type and composition of the surface and the traffic count. An additional application of ½ lb to ¾ lb of calcium chloride per square yard should then be made later in the season when the surface shows signs of dusting. Applications of calcium chloride should be made immediately following the blading, when the surface is still moist.

During periods of hot, dry weather, it is recommended that the surfaces of dense, well-compacted roads be sprinkled with water immediately preceding the application of flake calcium chloride. This will speed both the dissolving of the calcium chloride and the absorption of the solution into the surface of the road. This, in turn, prevents the dry flakes from being thrown from the road’s surface by traffic before they dissolve.

Spreading Equipment

Many types and styles of spreaders can be used to apply calcium chloride, including the tailgate, spinner disk, and drill types. In choosing a spreader, important points to consider are the uniformity of the coverage and the ease of adjusting the controls to regulate the rate of application and width of the spread.

Graded Aggregate Bases

Engineered Bases for Modern Highways

Although the axiom “no surface is better than its base” has been familiar to highway engineers for many years, it is only in the comparatively recent past that special design and special construction of bases have been recognized as a practical necessity. It is now known that the quality of the base not only will affect the maintenance costs and riding qualities of rigid and flexible surfaces, but that the base is the factor that determines the life of the pavement itself.

In the past, inadequately drained bases and subgrades have caused millions of dollars worth of damage from heaving and frost boils. No type of pavement or surface was exempt from this trouble if the base was not properly constructed and drained.

Lack of uniformity in bases, uneven settlement, and poor compaction have also been responsible for such difficulties as settlement of slabs, waving, pot-holing, and longitudinal ruts.

It is generally acknowledged that, in order to allow for weakness in base structure, pavements have often been made thicker than would have been required to carry the anticipated traffic load. In designing primary roads, the engineer’s problem is not whether to build a base but what type of base will be most suitable for a given project.

Advantages of the Calcium Chloride Graded Aggregate Base

The rapid increase in acceptance of the calcium chloride graded aggregate base in recent years is the result of the combination of the following advantages:

  1. Dependability. In practical field experience it has proven the most dependable type.
  2. Low cost.
  3. Strength. It has high structural strength to carry the heaviest wheel loads.
  4. Frost and moisture resistance. It is resistant to the detrimental action of excess moisture and frost.
  5. Ease of field control.

    In the calcium chloride graded aggregate base, the interlocking obtained from the gradation of the coarse aggregate provides the structural strength necessary to carry the load. The stability is the result of high densities (up to 150 lbs per cubic foot dry weight) which can be developed. Such densities prevent the main cause of base failures, either through softening or frost action, the existence of detrimental amounts of free water in the base.

Maintenance of Optimum Moisture

To achieve maximum density and stability, the careful control of moisture during the compacting period is essential, and calcium chloride produces the most economical and dependable means of controlling the optimum conditions of moisture accurately, which results in maximum density and stability.

Unless optimum moisture (approximately 7 to 8%) is uniformly maintained during the entire compaction period – despite periods of high temperature, low humidity, and drying winds – It is virtually impossible to achieve a uniform maximum density in all sections of the project. The continuous addition of water is both expensive and difficult to control accurately on the job, but the inclusion of calcium chloride in the mix provides a resistance to usual evaporation, reduces or eliminates the need for additional water, and maintains the necessary optimum moisture content.

The use of calcium chloride frequently adds nothing to the total cost of the completed base because the savings it affects during the construction period offset the cost of the calcium chloride. The addition of calcium chloride to graded aggregate mixes makes it possible to achieve maximum density with only a small fraction of the compactive effort required when plain water is used. In addition, graded aggregate containing calcium chloride reaches a greater density with fewer rollings than when plain water is used.

Preparation of Graded Aggregate Mixes

In the preparation of calcium chloride graded aggregate mixes, it is essential that the gradation and moisture be held to specified limits. This can be done by either of two methods: (1) combining measured amounts of specified materials at a central mixing plant and transporting the prepared mix to the job site; or (2) placing on the road for in-place mixing measured amounts of specified components, as called for in the specifications.

Plant Mix

A typical plant for producing graded aggregate mixes includes stocks of coarse and fine aggregates in bins or stockpiles and equipment for blending the proper proportions of these materials on a conveyor belt. Calcium chloride is usually added directly to the aggregate in the conveyor belt with a vibrator or similar type feeder. The correct amount of water is then added through spray bars located over the mixture.

There are a number of advantages to preparing specification mixes at a central mixing plant:

  1. Maximum control assures a more uniform mix
  2. Moisture content, so vital in attaining density and stability, is easily controlled and maintained.
  3. Inspection is simplified since one worker at the plant can control gradation of material, moisture content, and amount of calcium chloride.
  4. The possibility of segregation is eliminated.
  5. There is accurate control of the amount and even distribution of calcium chloride throughout the mix.

Graded aggregate mixes containing calcium chloride, delivered from central batching and mixing plants are usually spread on the road by mechanical aggregate spreaders, so that rolling can start immediately before there is any moisture lost.

Road Mix

Although the value of plant-mixed materials is generally recognized, it may prove more practical for some projects to construct graded aggregate bases by road mix methods. This method requires more careful inspection and supervision to insure uniformity because the control of materials and moisture must be maintained in the field throughout the entire operation.

The subgrade and subbase must be in a thoroughly compacted condition and must be provided with adequate drainage prior to construction of the base.

To construct a satisfactory base by the road-mix method:

  1. The coarse and fine material should be placed on the prepared surface, with care being taken to insure an even distribution of materials.
  2. Binder-soils should b e in a dry and pulverized condition when placed.
  3. All the material should then be bladed with a patrol grader by standard methods, from side to side or mixed with a travelling mixing machine until it is thoroughly and uniformly mixed.
  4. Generally, lifts should not exceed 5 inches of compacted thickness.

Aggregates should be dry-mixed to some extent before the addition of the calcium chloride. Calcium chloride should then be added by means of a mechanical spreader.

Graded Aggregate Mixes for Wearing Courses and Shoulders

Calcium chloride graded aggregate mixes are also used for construction of wearing courses and for the shoulders of pavements on primary roads. Although the specifications for the mix for surfaces of this type differ slightly from those that are to be used in bases scheduled for immediate surfacing, the materials can be mixed and applied in the same manner. It is the practice of some highway departments to use a base course before paving as a riding surface for periods of up to one year to determine whether any weaknesses develop.

If the base is to be used as a surface for a period of time before placement of the finished pavement, particular care must be taken to allow for quick run-off by insuring that not less than 3/8 inch per foot (and preferably ½ inch) crown is permitted.

Materials and Specifications for Calcium Chloride Graded Aggregate Mixes

Although most of the specifications for graded aggregate mixes are similar, slight variations are made in different sections of the country to suit local weather and traffic conditions and/or the local availability of materials. The following is a typical specification for graded aggregate base material:

For use in the construction of stabilized wearing or base courses, graded aggregate shall consist of coarse aggregate composed of gravel, crushed stone or slag combined with soil mortar or stone fines, or any combination of these materials. The requirements are intended to cover only materials having normal or average specific gravity, absorption, and gradation characteristics. Where other materials are used, appropriate limits suitable to their use must be specified.

The aggregate shall be composed of hard durable particles and shall be free from injurious or deleterious substances.

Detailed Requirements of Gradation
Sieve Designation Wearing Course* Base Course*
1" 100 100
3/4" 85-100 70-100
3/8" 65-100 50-80
#4 55-85 35-65
#10 40-70 25-50
#40 25-45 15-30
#200 10-25 5-15

*The fraction passing the No. 200 sieve shall not be greater than two-thirds of the fraction passing the No. 40 sieve.

The fraction passing the No. 40 sieve shall have a plasticity of not less than 4 or more than 9. The liquid limit of the fraction passing the No. 40 sieve shall not exceed 35. If the wearing course is to be used as a base within a year, the plasticity index shall not exceed 6, and the liquid limit shall not be greater than 25.

**The fraction passing the No. 200 sieve shall not be greater than two-thirds of the fraction passing the No. 40 sieve.

The fraction passing the No. 40 sieve shall have a liquid limit not greater than 25 and a plasticity index not greater than 6.

Moisture Content: The materials herein specified shall contain sufficient moisture to insure compaction to design density. This specification is being followed by several state highway departments and the American Public Works Association.

The Use of Calcium Chloride for Graded Aggregate Mixes for Shoulders and Wearing Corners

Road Mix – Regular flake calcium chloride shall be uniformly spread at a rate of 0.5 pounds per square yard per inch of compacted thickness. The material shall be thoroughly mixed by alternately spreading and windrowing or by multiple blade maintainers, by rotary tillers or other travelling plant mixers, or by other approved methods.

Plant Mix – The finished plant-mixed material shall contain 10 pounds of regular flake calcium chloride per ton of mixture.

Surface Application – Where the course is to serve as a wearing surface, an application of one pound per square yard should be applied. This should be followed by one-half pound per square yard applications as required to keep down dust.

Handling Precautions

CAUTION! Regular and anhydrous flake calcium chloride is harmful if swallowed. Contact with the eyes, mucous membranes, or skin may cause irritation or burns. When using, work gloves should be worn, and dust should be avoided.

If swallowed, drink several glasses of water and then induce vomiting. Call a physician.

In case of contact: For eyes, promptly flush with plenty of water for at least 15 minutes, and call a physician. For skin, flush with plenty of water and wash thoroughly. Contaminated clothing should be removed as soon as possible and washed before being reused.

Regular and anhydrous flake calcium chloride acts by absorbing moisture which in turn liberates heat. This moisture-absorbing property can dry out leather and may damage clothing.

All ice-melting chemicals lower the freezing point of water and increase the number of freeze-thaw cycles. This situation may hasten the occurrence of flaking and scaling of concrete or mortar less than one-year old or of questionable quality. Mortar joints are particularly vulnerable.

Care must be used when applying near lawns or shrubs since damage to them may occur.

Keep out of reach of children.

Store in a cool, dry place.