2.6 Dust Suppression Measures

The top problem associated with unpaved roads is traffic-generated fugitive dust and the resulting loss of fines. This problem lays the foundation for the eventual degradation of the road surface in the form of ruts, potholes, and corrugations. These conditions translate into high maintenance cost for road departments and as well as higher road user cost in the form of vehicle maintenance.

To maintain unpaved road surfaces at an acceptable standard, the frequency of periodic maintenance may have to be increased to keep the formation of ruts, potholes and washboards under control. The traffic type and volume as well as the base material characteristics, among other factors, influence the frequency of maintenance. At a moderate traffic volume of about 500 ADT, an estimated eight periodic maintenances per year was reported on a two-lane test section in Larimer County, Colo. (Addo and Sanders, 1995). The periodic maintenance involved scarifying, blading and compacting the road surface in the presence of adequate moisture. Once said, "When you see dust coming up from your roads you are seeing dollars thrown to the wind" (Nebraska T2, 1995).

To prevent the loss of road surface fines and prolong the useful driving life of unpaved roads, dust control measures are usually employed before maintenance is required. Commonly used dust control methods include: reduction of vehicular speed, application of water, and the use of dust suppressing chemicals. The use of dust suppressing chemicals have gained wide acceptance because of the many proprietary products available on the market and their effectiveness in controlling dust at a relatively low cost. Chemical suppressants range from organic and inorganic chemical mixes to synthetic fabric used to contain the road material. The primary dust suppressants in use are: water (fresh and sea), chloride compounds, lignin derivatives, and resinous adhesives.

2.6.1 Water (fresh and sea)

Water is probably the oldest of all dust palliatives, it is readily available and generally applied by spraying over the road surface. Its dust suppressing capacity is very temporary because of evaporation. Depending on weather conditions, several light applications should be carried out instead of one heavy application (Compendium 12, 1980). This application technique prevents the excess water from heavy application from turning the dust or fines into soft mud, washing away fines or even penetrating the road to the subbase and causing major road failure. Seawater is generally considered more effective in controlling dust than fresh water because of the presence of salts in it - chiefly magnesium chloride. Under sufficiently humid conditions, the salts will absorb and hold moisture to keep the road surface damp for a longer period of time than if fresh water is used.

2.6.2 Chloride Compounds

Calcium chloride (CaCl₂), and magnesium chloride (MgCl₂) are mainly used. Sodium chloride (NaCl) is also used on a limited basis. Proprietary products using combinations of these salts and other additives are also widely used. The properties of chloride compounds allow them to attract and absorb moisture from the atmosphere and retain it for extended length of time (Compendium 12, 1980). They also have low vapor pressure compared to water in the atmosphere at the same temperature. These properties are closely related to relative humidity and air temperature (Hogentogler, 1938). Relative humidity ranging from 30 to 40 percent has been reported as the cutoff at which CaCl₂ and MgCl₂ will cease to attract and absorb moisture from the atmosphere (Compendium 12, 1980).

When used as dust suppressants these chemicals keep the road surface damp especially in the early morning hours when the relative humidity is higher and temperature is low thus controlling dust generation. At a lower relative humidity and a higher temperature especially in the afternoons they are not so effective in attracting and absorbing moisture from the atmosphere. However, they significantly reduce the evaporation of moisture from the road surface thus making them effective dust suppressants.

Another important characteristic exhibited by chloride compounds is that they have a low freezing point depending on their concentration in aqueous solution. As reported in Woods (1960) a 30 percent CaCl₂ solution freezes at approximately -60F, a 22 percent MgCl₂ solution freezes at approximately -27F, while a 25 percent Na₂Cl solution freezes at -6F. When used as dust suppressants they minimize frost heave and reduce freeze-thaw cycles, which weaken unpaved roads. However, they have the disadvantage of being water soluble and therefore can be washed out during wet weather conditions. They are also corrosive.

2.6.3 Lignin Derivatives

These suppressants include a variety of industrial waste products, animal fats, and vegetable oils. Of these binders, the most widely available and used is ligninsulfonate. This is a waste product from the paper-making industry. Lignin is said to be the natural cement that binds the fiber of wood together in plants. During the paper-making process (pulping) the lignin polymers and wood sugars are released into the processing wastewater. The wastewater is generally called ligninsulfonate because of the sulfite process used for extracting the pulp. When used as dust suppressant, the lignin polymers act as glue binding the soil particles together. The ligninsulfonate is water soluble and therefore easily washed away during wet weather conditions.

2.6.4 Resinous Adhesive

Waste oils, tars, bitumen, and by-products from the plastic industry are in this class of dust suppressants. Of all these products, cutback asphalt and asphalt emulsions are most widely used as dust suppressants (Hoover, et al., 1973). Asphalt is a highly complex material, composed primarily of various hydrocarbon compounds (The Asphalt Handbook, 1995).

Cutback asphalts are formed by adding solvent to asphaltic cement. The type of solvent used determines the type of cutback produced. Highly volatile solvents such as gasoline or napthal produces rapid-curing cutback. Kerosene produces medium-curing cutbacks and light volatile solvents produces slow-curing cutbacks.

Asphalt emulsions are produced by dispersing asphalts as small droplets in water. The dispersion of the asphalt is maintained by adding an emulsifying agent during the formulation of the emulsion. Three classes of emulsions are available for road construction: rapid setting (RS), medium setting (MS), and slow setting (SS). The formulation and properties of emulsions are different from that of cutbacks; for detailed discussions see Bennett, (1968). Specifications on the use of these bituminous materials are set by the Asphalt Institute, American Society of Testing Materials (ASTM), the American Association of State Highway and Transportation Officials (AASHTO) standard testing methods. When used as dust suppressants resinous adhesives provide the most durable dust-free surfacing due to their adhesive properties and insolubility in water.

Common to the success of any dust suppressant is the structure and composition of the road surface material. As Squier (1974) stated "the road should have good mechanical stability in itself, because chlorides are not strong binders." Of lignin derivatives, clay is a very important component of a good road soil. For bitumens and tars, the structure and composition of the aggregate is one of the most important factors that affects adhesion in an aggregate-asphalt system (Mertens, 1959; Day, 1965). For dust-suppressant-treated unpaved roads to perform well under the most adverse conditions, the road surface material must be mechanically stable.

The use of dust suppressants can be an important unpaved road maintenance technique. Their use can be justified when:

1. traffic volume is low
2. paving is not feasible because of budget constraints
3. the cost of suppressant and application is low, and
4. when stage construction is planned.

In selecting a dust suppressant, the performance characteristics to be evaluated include the type and volume of traffic, roadway condition, climate, and product cost to achieve the desired level of dust control. The attributes, limitations, typical application rates, and sources of the primary dust suppressant discussed are noted in Appendix A.

2.7 Effectiveness of Dust Suppressants

Treating an unpaved road with dust suppressants can, in a large measure, improve conditions of the road. To unpaved road users the dramatic difference can be observed in the difference between dust plumes coming off an untreated versus a treated road. Figure 2.4 and 2.5 depict the typical visual dust levels from an untreated and a treated unpaved road.

Although dust suppressants have been used for several decades, research to quantify their relative effectiveness in controlling dust generation and preventing loss of fines and subsequent aggregate pullout are limited. Hoover, et al. (1973), in a highway research project, tested several dust suppressants and road surface improvement agents. Of the 22 tested in laboratory experiments, six were selected and applied on test sections for a controlled experiment. The suppressants tested included a cutback asphalt, a cationic asphalt emulsion, lignosulfonate, lignin/alum mixture, a lignin/lime mixture, and a by-product from Chemplex Plastic Company. Both laboratory and field experiments showed approximately 30 to 80 percent reduction in dust from the treated test sections as compared to the untreated. Aggregate pullout from the treated test section surfaces were found to be approximately 25-27 percent less than that of the untreated test sections.

Sanders, et al. (1997), in a field-based research study, quantified the relative effectiveness of three primary dust suppressants, MgCl₂, CaCl₂, and ligninsulfonate. The quantification was done by measuring the dust amount emitted from each of the four test sections using the Colorado State University Dustometer - a moving dust sampler developed during the research. Total aggregate lost over each test section was estimated. The research indicated that all three dust suppressants studied reduced fugitive dust emissions from the unpaved roadways by 50-70 percent. The treated test sections retained approximately 40-60 percent more aggregate than the untreated control test section. The cost savings of retaining aggregate on the treated test sections more than offset the cost of the dust suppressants, resulting in an estimated cost savings of 28-42 percent over the untreated test section.

The two above-cited projects demonstrated that the use of dust suppressants and road surface improvement agents can control dust generation from unpaved roads while reducing the annual aggregate replacement by a factor of two to four, the latter alone saving thousands of dollars in road maintenance cost.

For many recent years low-volume secondary and tertiary roads have being receiving some attention as exhibited by research studies such as Hoover, et al., (1973, 1981); Lane, et al., (1984); Irwin, et al., (1986); Boresi, and Palmer, (1993); Sanders, et al., (1997) to name a few. Most of the studies have been aimed at understanding and improving unpaved road surface conditions through the use of dust suppressants and other surface improvement agents and various maintenance methodologies. Much more value-add research needs to be done such as:

1) understanding the effect of road surface material characteristics and quality on the effectiveness of different types of dust suppressants, 2) determining optimum suppressant dosages and application rate, and 3) evaluating suppressant application method effect on effectiveness of the different types of suppressants. One of the objectives of this research project is to ascertain if unpaved road surface material characteristics have any effect on the performance of different types of dust suppressants. The following describes the work done.