Chapter 2. Literature Review

Problem of Road Dustiness

Nearly 50 percent of America's roads are unpaved, besides an even larger percentage of private roads and parking lots. Here in Colorado approximately 65 percent of the public roads and over 70 percent of local roads are unpaved. These unpaved roads are located in the agricultural and forest areas and in cities, town and villages. Although they are low-volume and low load bearing roads they are a vital first link in the local economy (Colorado Transportation Information Center. #1, 1989)

A gravel road can be said to consist of a mixture of gravel, sand and fines (silt and clay) in the proportions of 40-80 percent stone or gravel, 20-60 percent sand and 8-15 percent fines (Woods, 1960). The aggregates are blended and compacted into a strong dense surface crust - hard enough to resist breaking down under traffic. A crown is provided at the center of the road and ditches are provided at the shoulders to facilitate drainage.

The problem associated with gravel roads is traffic-generated dust. This involves public economics and environmental quality and is also a nuisance to the public (Hoover J.M. et al., 1981). But even then dust is more than just a nuisance from unpaved roads:

1. By obscuring the vision of drivers, dust clouds are traffic hazards.
2. Dust particulate can be carried several hundreds of feet, penetrating nearby homes and covering crops. Crop growth can be stunted due to the shading effect and clogged plant pores.
3. In human health dust is a common cause of allergies and hay fever and a conveyor of diseases according to a United Nation study (United Nations, 1979).
4. The fine dust particles can be abrasive and therefore greatly increase wear of moving parts of vehicles.
5. The losses of fines (road binder), as dust, represent a significant material and economic loss (Colorado Transportation Information Center. #3, 1989).
6. The fine dust particles are washed off during precipitation and carried into streams, creeks and lakes increasing their respective turbidities.

The severity of a dust problem is determined primarily by the amount of speed of traffic on the unpaved road. The condition can be aggravated by long dry spells, softer road aggregates and initially excessive soil binder in the road surface. Without binder material (enough fines) and adequate moisture, the coarser material will be thrown and washed away from the road surface. When that happens the road begins to ravel, rut and washboard leading to costly repairs.

In 1978, Iowa's 99 county secondary road departments spent $32,267,661 for aggregate replacement, a condition due in large part to traffic abrasion losses from limestone (Hoover, J.M. et al., 1973). Iowa Highway Research Board project HR-151 quantified dust sources and emissions created by traffic on unpaved roads and identified control and surface improvement techniques (Hoover, J.M., 1981). Average dust generation from unpaved roads was found to be one-ton/mile/year vehicle of average daily traffic within 1000 foot wide corridor centered on the roadway (USDA Forest Service 1983).

The problem of road dustiness is a worldwide problem and here in the United States as early as 1909, the "Office of Public Roads", suggested clay-bound stone as the surfacing for roads to alleviate dusting and raveling (U.S. Dept. of Transportation, 1976). Portland cement and asphaltic concrete further stimulated technical solutions to the dust and surface improvement problems. Historically, however, the combination of high initial cost and low traffic volumes on gravel roads did not provide sufficient cost-benefit ratios, a situation that has not significantly changed. Short term relief from dust, raveling and washboarding of gravel roads has thus been accomplished through trial and error technology by a variety of dust suppressants and additives. Such methods have generally been limited in scope and success, with the vast majority of gravel roads receiving only intermittent blading and occasional aggregate replacement.

Mckee, H.C.(1969), showed that evidence exists that suspended particulate have the potential to alter weather patterns and affect human health. A study commissioned by the American Petroleum Institute in 1971 estimated that soil dust from tillage and unpaved roads contributed approximately 10 percent of the annual total particulate emissions on a global scale (Robinson, et al., 1971). Governmental regulations in recent years have accelerated the rate of air quality monitoring and control. For example here in Colorado the Clean Air Act of 1982(Title 25) specifically section III.D.2.a(i.)(B) of Regulation 1 - the general requirements states that, "Any owner or operator responsible for construction or maintenance of any (existing or new) unpaved roadway which has vehicle traffic exceeding 200 vehicles per day in attainment areas or 150 vehicles per day in non-attainment areas (averaged over any consecutive 3-day period) from which fugitive particulate emissions will be emitted shall be required to use all available practical methods which are technologically feasible and economically reasonable in order to minimize emissions resulting from the use of such roadway." All this can have a considerable impact on local government's maintenance and/or construction procedures for unpaved roads.

Literature concerning atmospheric pollution from dust appears to center on two areas of study: 1) atmospheric modeling and prediction, and 2) field measurement and quantification. Several studies have proposed mathematical models for generation and distribution of particulate emissions from various sources (Robinson, E. et al., 1971) and Becker, D.C., 1978). Becker, D.C., (1978) examined a Gaussian plane model applied to dust generated from unpaved roads in Iowa and Kansas. Included were limited field measurements, using a high volumetric sampler and mechanical analysis of collected particulate.

Types of Dust Suppressants

Presently, methods of dust control range from spraying the roads with chemicals, chiefly chloride compounds and resinous adhesives to using geotextiles in the road reconstruction (Colorado Transportation Information Center. Bulletin #3, 1989). The use of these dust suppressants can be justified when:

1. the traffic is low or paving is not feasible for lack of funds,
2. the cost of suppressant and application is low, and
3. when stage construction is planned.

The commonly used dust suppressants are lignin derivatives, calcium chloride, magnesium chloride, sodium chloride, road fabric, resinous adhesives, water and etc. The selection of a particular suppressant depends not only on its performances characteristics, but also on the type and volume of traffic, roadway condition and product cost to achieve the desired level of dust control (Colorado Transportation Information Center. #3, 1989).

Hygroscopic salts such as Magnesium Chloride and Calcium Chloride especially have been used to treat unpaved road surfaces since the last century. Because of their hygroscopic properties they do not only retard evaporation from the road surface during the heat of the day, but draw moisture from their surrounding (environment) to produce a brine that in turn keeps the road surface moist (Larkin Laboratory, 1986). Results from a study comparing Magnesium Chloride with Calcium Chloride shows that although both agents work the same way, there are still some differences between the materials that effect their performance and efficiency. The key criterion for judging these dust control agents is their ability to produce a brine under adverse conditions, such as high ambient and road surface temperatures and low humidity. These conditions generate the worst traffic dust, and it is under these adverse conditions that the properties of Calcium Chloride make it a more suitable alternative than Magnesium Chloride (Larkin Laboratory, 1986).

The solubility of Calcium Chloride compared to Magnesium Chloride increases more rapidly with increasing temperature. This means that Calcium Chloride will produce a brine at low relative humidities and under hot dry conditions that cause the greatest dust control problems. A study done at the Swedish Royal Institute of Technology quantified the difference between the two dust control agents on actual gravel roads and found Calcium Chloride to be 18 percent more effective than Magnesium Chloride (Reyier, J., 1972). Other chlorides are widely used particularly for stabilizing the surfaces of haul roads. Calcium Chloride is relatively expensive so usually a mix of common salt and Calcium Chloride is used to cut cost while controlling dust effectively.

The adhesive and waterproofing characteristics of bituminous materials are well known, but they are very expensive to use. Lignin derivatives are natural cements that bind dust particles. They are aided by associated sugars that act as hygroscopic agents. Various hybrid products are emerging (e.g. a bitumen-lignin dust control agent) which pose opportunities for cooperative test projects.

Effectiveness of Dust Suppressants

Two primary dust suppressant application methods are commonly used. 1) surface or topically sprayed, and 2) mixed-in-place. Surface or topically sprayed involves spraying the suppressant on the road surface after the road surface has been maintained (bladed). The method is simply and fast (Woods, K.B., 1960). Suppressants applied by this method turn out to be effective for short periods of time resulting in the need for repeated applications during a single season. Calcium Chloride, Magnesium Chloride, lignin and cutback asphalts are usually applied using this method.

Mixed-in-place is another method involving the addition of the suppressant to the road surface material in-situ. This is achieved by mixing the suppressant with the aggregate after the road surface has been scarified. The method does not only achieve palliation but provides improved road surface resulting in reduced maintenance cost from continued palliative applications and/or aggregate surfacing replacement (Hoover, J.M. et al., 1973). Furthermore in-depth stabilization may improve the sub-base or base for future higher type pavements.

Laboratory tests on many dust suppressants and surface improvement agents lead to the selection of six agents that were placed in road test sections in Poweshiek, Linn and Clinton Counties (Iowa) for controlled experimentation. The dust control agents used were MC-800 cutback asphalt, a cationic asphalt emulsion, lignin, lignin plus alum, lignin plus lime and a residual waste product of the Chemplex Plastic Co., Clinton Iowa (Hoover, J.M. et al., 1981). The experiments showed that there was approximately one-third to more than 80 percent reduction in dust from the treated roads as compared to the untreated roads, with amount of dust reduction depending on the method of treatment. Aggregate pullout from the road surface was also reduced to as little as one-fourth of that of untreated surface (Hoover, J.M. et al., 1981). Thus, it was shown that dust generation can be controlled, and annual aggregate replacement could also be reduced by a factor of 2 to 4, the latter alone offering a potential annual cost saving of several thousand dollars per mile. Reduction of dust and aggregate loss was also coupled with significant reduction in normal weekly to monthly blading and grading for the palliation sections, and generally no blading operations were required in the mix-in-place applications, a definite maintenance cost saving.

Research was done on six test sections using lignin, lignin plus a herbicide, two cationic emulsions, and a proprietary chemical called Kelpak in a mix-in-place application and the sixth was calcium chloride in a surface application. Several weeks after construction, volumetric dust sampling measurements on the test sections revealed that the lignin and lignin plus herbicide sections produced less than 10 lbs. of dust per million ft³ of air. Both cationic emulsions produced less than 3 lbs/million ft³ of air while the control section for all the above treatments methods produced more than 55 lbs./10⁶ ft³ of air. The proprietary chemical produced 6.5 lbs with its control producing 13 lbs./10⁶ ft³. A year later, the lignin, lignin plus herbicide and both asphalt emulsions still produced significantly less dust as compared to the control while the proprietary chemical was producing dust about equivalent of its control. Initial data on the calcium chloride section and its control were 12 and 27 lbs. of dust/10⁶ ft³ of air respectively. In less then a year the calcium chloride section was producing as much dust as its control (Hoover, J.M., et. al., 1981).

In a cost benefit study on road dust conducted in the Seattle Industrial Valley, Robert, J.W. (1975) reported that dust emissions on 19 miles of gravel road and 110 miles of dusty paved roads contributed an estimated 2700 tons/year of particulate, of which 700 tons were of a particle size of less than 10 microns (0.01 mm). He suggested that paving roads having an average daily traffic (ADT) over 15 were the least costly methods for reducing dust, and it was a good investment when the traffic exceeded 100 ADT. Paving or oiling the road was estimated to produce benefit of nearly $3.9 million yearly in household cleaning, health care, vehicle operation, sewer and road maintenance costs, coupled with increased property values.

The effectiveness of lignin was discovered in a study on three test sections in Taylor county - Iowa, using 0.75, 1.00 and 1.25 percent by soil weight of Bindtite, a lignosulfonate, (Blunck, T.R., 1975). The sections ranged in length from 0.25 to 1.25 miles and before the treatments, they were experiencing complaints of dust, potholes, washboarding, frost heaving and general deterioration of the surface. Pre-construction consisted primarily of blade dressing of the road shoulders and ditch line and reclaiming of aggregate pullout. Finally the surface was scarified, mixed with the lignin and compacted into a 6-inch thick wearing course. The treated sections did not receive a seal coat until way over a year after construction. During this period, all the sections remained nearly free of dust, potholes or washboarding and the surfaces remained relatively dry and solid after each rain.

From various laboratory and field studies done on dust palliative, it can be concluded that for a chemical dust palliative to be effective, it must provide significantly increased physico-and surface-activity in order for the fine particulate to remain in an aggregated form under adverse conditions of both weather and traffic (Hoover, J.M.,et al 1973). For example calcium chloride and lignin improve capillary cohesion while rain does not occur. When mixed-in-place, the lignin and calcium chloride have already been noted as showing greater longevity. For asphalt, the electric potential created by varying the type of emulsifier within an asphalt produces variations in strength also potential effectiveness as a dust stabilizer for a given soil.