Dust Suppression

How big is the need for dust suppression in road construction? To answer this question we need to look no further than the size of the road network in South Africa. The South African road network is the 10th largest in the world, covering a distance of 750,000km. Of this distance, about 140,000km are tarred roads, with the remaining being gravel roads.

The dust from these gravel roads presents road safety as well as a health risk where the necessary caution is not applied. We approached our colleagues from SANRAL to gain some insights into the use of dust suppression materials and techniques on our roads.

Dust Suppressants (Fines Retention)

A number of available non-traditional soil additives are marketed with the specific objective of treating the upper layers of unsealed roads to reduce dust and often have the additional benefit of reducing gravel loss. They can also be useful for the reduction of dust on construction sites and/or deviations.

An example of a road that has a dust suppressant applied is shown below.

Road with Dust Palliative

Dust palliatives include primarily lignosulphonates and chloride-based products:

Lignosulphonates are produced as a residue from the wood pulping industry and are effective binders of fine particles. When applied to unsealed roads with a relatively wide variation in properties, they can be highly effective in reducing dustiness as well as giving the added benefit of reducing gravel loss and maintenance requirements through what is essentially a glueing action. They are, however, highly water-soluble and are thus rapidly leached from the road surface during the wet season. Fortunately, they are most effective, and needed, during the dry season, when road dust is at its worst.

Chloride-based products are primarily calcium and magnesium, although sodium is sometimes included. The effectiveness of these products is a function of their hygroscopic or deliquescent properties, i.e., their ability to absorb moisture from the atmosphere and retain the road surface in a slightly moist condition. This suppresses dust generation under traffic. They are also highly soluble in water, and thus leached out of the road with time.

These products are only likely to be successful when the minimum relative humidity in the area is more than 50%, for the majority of the time.

Repeated application of both generic products is necessary.

There are currently no standards for these materials, with each supplier having their own internal “recipes” and standards. Their use should thus be accompanied by a Product Performance Guarantee (PPG) from the Supplier. The PPG should include a definition of the degree of dust reduction, the effective time over which this will occur and possibly any maintenance benefits that will accrue.

The PPG must be agreed upon between the parties involved. The effectiveness of these products on individual materials is best assessed through localised trial sections.

Certain physical and chemical information regarding the products is usually provided on the Material Data Safety Sheets, which can be used as a simple guide to the quality of the product provided. The supplier should be requested to provide the data sheets.

The use of any dust palliative should be in accordance with a strict maintenance programme, involving the correct timing of maintenance and rejuvenation activities. Any conventional routine maintenance activities, e.g., grader blading or spot gravelling should be cleared with the supplier before being carried out.

The use of dust palliatives should be considered when:

It is evidently uneconomical to apply a bituminous surfacing, even after life cycle cost comparisons.
There are definite traffic safety issues due to dusty conditions.
It is established that dust damages the environment.

Soil Stabilization

The use of proprietary chemical products as soil stabilizers is less understood than their use as dust palliatives.

There is a much wider range of products, with little scientifically supported marketing, and little information on the reaction mechanisms of many of the products. Many of these proprietary soil additives promise significant increases in the strength of the soil materials after treatment.

The products cover a wide range of raw materials, including sulfonated oils, enzymes and polymer emulsions. The ingredients and processes used in their manufacture are wide and varied. Handling and storage conditions should be ascertained from the suppliers.

Many of the products have restricted shelf lives. The quality of any product that has been in storage for more than 3 months should be referred to the supplier, for confirmation of its continued acceptability for use.

As in the case of dust suppressants, there are no standards for these proprietary chemicals. Each manufacturer has their own “recipe” which appears to change on a frequent basis, as do the names of many of the products.

Those materials accredited by a body such as the Agrément Board require that the manufacture of the product conforms to an in-house quality control system and that all products sold shall be consistent and within the standards of the company. There is, however, little that the end-user can do to confirm the quality other than to assess the information provided by the manufacturer on the delivery of the additives. Reference to the Material Safety Data Sheet may also provide some limits for specific properties of the material.

There is, however, definite local and international evidence that some of the chemicals can be effective. However, little is known about the life cycle costs, and thus, the overall cost-effectiveness of the majority of the products. It is recommended that some form of Product Performance Guarantee system is agreed upon between the product supplier and the end user.

Types of Stabilizer

The proprietary soil stabilizer types can be generically grouped, as shown in Table 62. This table also shows the type of improvement in soil properties that can be expected, as well as the types of material for which soil compactibility may be improved.

Actions/Reactions of Stabilizers

There is considerable debate regarding the actions of many of these stabilizers. The following reflects general consensus on the action/reactions of stabilizers.

Ionic stabilizers, which include sulfonated petroleum products, are generally accepted as undergoing an ionic exchange reaction with suitable clay minerals and/or attaching to the adsorbed water as a result of the hydrophilic nature of the ionic portion of the chemical. It is well known that clay minerals have a surface charge, typically negative, which attracts the “double layer” water that affects the properties of the clay minerals.

The actions of the ionic soil stabilizers result in a decrease in the double-layer water and an improvement in the properties of the treated material. This has been observed in the field and proven in the laboratory.

Enzymes apparently act as catalysts only and accelerate a bonding reaction between organic and inorganic soil components, according to a number of suppliers. The construction techniques and application rates, however, are similar to those for ionic stabilizers, and it is considered that base materials may contain similar active components.

Petroleum resins are not widely used in South Africa, but appear to be derived from natural petroleum resin, a tacky aromatic hydrocarbon derived from crude oil, emulsified with water. The mechanism of these products is to glue the soil particles together.

Polymers are gaining acceptance as stabilizers but can be quite costly. These are essentially plastic materials derived from acrylic polymers, commonly used as household glue. The effect of these is to bond the soil particles together, forming a compacted and bonded water-resistant material. Various suppliers of these products are operating in South Africa, each with specific formulations to develop their own products. As for petroleum resins, the bonding or glueing action of these products is used to improve the strength of the treated materials.

Other products cover a wide range of materials primarily based on cement, bitumen emulsions or other standard components, such as lime or granulated blast furnace slag. The mechanism of action of these products depends on their primary component. Cement-based products hydrate and bond the particles together, as is the case with normal cement stabilization.

Many of the cement-based products, however, have special additives or other components, e.g., polypropylene fibres, to make the treated material more flexible. A similar situation applies to bitumen-based products, where additional chemicals, e.g., urea formaldehyde, are added to enhance the bonding and or strengthening properties of the product. These products rely on a glueing action for their performance.

The use of non-traditional soil stabilizers should be considered when:

Cost comparisons show them to be economically viable compared to traditional stabilizing agents, such as lime or cement.
Testing shows that the non-traditional stabilizers are effective in achieving the desired properties of the material to be stabilized. Chapter 3: 6 covers this aspect in more detail.
The product is accredited by a body, such as Agrément.