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This paper provides a background for the 1st Road Dust Management Conference, to be held on November 13 and 14, 2008, in San Antonio, Texas. It will be presented in the opening session to provide a platform for the following presentations, thereby eliminating the need for presenters to provide basic background information at the beginning of each presentation.
1. INTRODUCTION
There are millions of miles of unsealed roads around the world, which are managed by the national road authorities, state or provincial road agencies, local authorities, the forestry and mining industries, agriculture, national park authorities, and tourism, railroad, and utility companies. There are also numerous unproclaimed roads that no authority takes responsibility for, but which serve a need such as access to informal communities in developing countries. Unacceptable levels of dust, poor riding quality, and impassability in wet weather are experienced on much of this global unsealed road network, and although it is acknowledged that these roads are fundamental to the economies of almost every country in the world, many of the management practices followed leave much to be desired, with programs for dust control, chemical stabilization, low-cost upgrading, etc, largely overlooked. There are no comprehensive guidelines for implementing dust control programs.
Chemical dust control on unsealed roads has been researched for decades and there are numerous published papers documenting the establishment and monitoring of experiments. However, much of this has been agency-specific and mostly focused on assessing performance of one additive under a particular set of conditions. There are no specific comprehensive guidelines or specifications available to help practitioners with establishing longer-term dust control programs, identifying which type of additive would be most appropriate for a specific application, undertaking life-cycle analyses, quantifying negative environmental impacts and positive social benefits, designing appropriate treatments, applying the additive, and maintaining the treated road. Similar documentation for sealed roads has long been available and is continuously updated. Additionally, there is no national industry group serving the interests of additive manufacturers and suppliers, similar to the National Asphalt Paving Association (NAPA) and the American Concrete Paving Association (ACPA). There is no “owner” for documentation, procedures and test methods relating to chemical dust control, similar to the American Association of State Highway Officials (AASHTO), nor is there a sustained source of national funding for research to prepare this documentation and develop procedures and test methods.
Increasing concerns with regard to deteriorating air quality, the sustainability of repeatedly replacing gravel on unsealed roads, and the increasing costs of asphalt binders used for sealing roads have placed renewed interest on road dust management. Although upgrading the road to a sealed (asphalt surface treatment, asphalt concrete, or portland cement concrete) standard is always preferable and usually the most economic option in terms of life-cycle costs, the rapidly increasing costs associated with this practice results in less distance being upgraded each year. The application of various additives can provide satisfactory dust control on most road surfaces until such time as sufficient funds become available for a more permanent surfacing. Provided that appropriate construction and maintenance practices are followed, and the additives are rejuvenated at regular intervals, chemically treated surfaces are often structurally adequate to function as a base or subbase in a staged construction of a sealed road.
This paper provides a current status of global road dust management together with some points for consideration that may lead to wider implementation of dust control programs in unsealed road management initiatives. The paper includes discussion on the extent of unsealed road networks, the volume of dust generated, the consequences of dust, categorization of road additives, environmental considerations, and dust control research.
2. UNSEALED ROAD NETWORKS
There is no accurate estimate of the size of the global unsealed road network. Table 1 provides some estimates of the extent of unsealed road networks in the United States1 (1st World, 9,6 million km2), South Africa2 (2nd World, 1,2 million km2), and Tanzania3 (3rd World, 0.9 million km2), indicating the magnitude of global unsealed road management issues.
Table 1: Estimates of unsealed road networks (in kilometers)
|
Owner |
United States |
South Africa |
Tanzania |
|
Land area (km2) Sealed road network (km) Unsealed road network (km) |
9,600,000 3,700,000 2,700,000 |
1,200,000 300,000 600,000 |
880,000 5,000 85,000 |
|
State/county Municipal Forestry Bureau of land management Nature conservation/tourism Agriculture Mine Other* |
850,000 Unknown 620,000 130,000 17,000 Unknown Unknown Unknown |
150,000 200,000 100,000 – 5,000 50,000 5,000 100,000 |
81,000 5,000 Unknown – Unknown Unknown Unknown Unknown |
|
* Includes service roads for railroad, powerlines, military, border patrol, other commercial activities, etc |
|||
3. VOLUME OF DUST GENERATED
Documented studies in the United States indicate that as much as 50 percent of PM10 emissions and 19 percent of PM2.5 emissions are attributed to road dust (Figure 1)3. Road dust is the single biggest source of PM10 emissions and approximately 65 percent of road dust emissions are attributed to unsealed roads. These percentages increase in developing countries that have higher proportions of unsealed roads, and are of particular concern in urban areas with predominantly unsealed infrastructure.
Figure 1: US PM10 and PM2.5 emissions in 2002 by principal source category3
4. CONSEQUENCES OF ROAD DUST
Road dust is often considered only as a nuisance or minor safety hazard by many practitioners. However, using models developed by the United States Environmental Protection Agency5 and calibrated in various countries6, it can be shown that millions of tons of dust are generated on unsealed road networks every year. Although much of this dust falls back onto the road to be regenerated by the next vehicle, studies have shown that at least a third of it is permanently lost in the form of deposits away from the road (Figure 2), with losses increasing under crosswind conditions.
Figure 2: Fines lost from unsealed roads
Apart from the obvious consequences of reduced quality of life and increased safety hazard for road users, pedestrians, and workers, the loss of fines (which perform an integral material- binding function) from the road surface results firstly in accelerated gravel loss, thereby increasing the frequency at which the gravel has to be replaced, and secondly in more rapid deterioration of the riding quality of the road, thereby requiring more frequent grader maintenance6. This has significant economic and environmental implications in terms of regular regravelling programs. Other serious, but often overlooked consequences, include reduced agricultural and forestry yields. These are attributed to retarded plant growth, increased insect activity, crop blemishing, and reduced palatability of pasture and associated reduced yields in terms of dairy production. There are even published reports on accelerated tooth wear of animals grazing in pasture adjacent to unsealed roads7. Environmental consequences in terms of air and water pollution and associated health hazards, primarily those linked to respiratory diseases, are also significant, especially in developing countries where a large proportion of urban road infrastructure is often unsealed. Vehicle operating costs increase significantly in dusty conditions, with numerous publications compiled comparing the cost of operating vehicles in dusty and dust-free environments.
5. DUST CONTROL
Dust control can be achieved either by better selection of base and wearing course materials, mechanical stabilisation using two or more different materials to achieve a better particle size distribution and to increase or reduce the plasticity, or by applying a chemical dust palliative. Only chemical treatments are addressed in this paper.
5.1 Chemical Dust Control Categories
Numerous additives are available for dust palliation, improved compaction, and stabilization of unsealed roads. Most of these bind the fine particles together without any significant chemical reaction occurring in the soil, although certain additives will only perform once a chemical reaction has occurred. A number of additives are material and/or climate-dependent and costs vary significantly. It is therefore important that the bonding nature, limitations and life-cycle costs of these additives be investigated and their performance understood before widespread use is considered.
Most unsealed road additives are proprietary formulations, and information regarding their composition is often not readily available. This knowledge gap can limit the extent of applications if no clear information is available with regard to potential human and environmental impacts and in instances where competitive tendering is required. In order to facilitate research, technology transfer, palliative certification, classification of palliative types for different uses, climates and base material types, selection of appropriate additive type and application rate for particular conditions, and transparent and competitive bidding/tendering procedures, additives need to be categorized based primarily on their function and chemistry. A suggested categorization is provided in Table 26. Similar categorizations are used by the US Forest Service9 and the Environmental Protection Agency. A brief introduction to each category is provided below. Details on the stabilization mechanism and research on laboratory and field testing of each of these categories are discussed elsewhere in the literature.
Most road authorities cannot specify proprietary product names in tender documents. In order to facilitate implementation under these conditions, authorities could consider using category names in tender documentation if a design or experience dictates a specific type of application. Alternatively a performance specification (e.g. dust level reduction) can be used and the contractor can apply an additive of his own choosing, provided that it meets human and environmental safety requirements.
Table 2: Suggested road additive categories
|
Category |
Sub-categories |
Examples |
|
Dust palliatives |
Water and wetting agents |
– |
|
Hygroscopic salts |
Calcium, magnesium or sodium chloride |
|
|
Natural polymers |
Lignosulfonate, molasses, tannin extracts |
|
|
Synthetic polymer emulsions |
Acrylates, acrylics, vinyl acetates |
|
|
Synthetic oils |
Mineral oils, synthetic iso-alkaines |
|
|
Petroleum resins |
Blend of natural polymer and petroleum products |
|
|
Bitumen, asphalt and tar |
– |
|
|
Other |
Industrial wastes |
|
|
Compaction aids and |
Synthetic polymer emulsions |
Acrylates, acrylics, vinyl acetates |
|
stabilizers |
Synthetic oils |
Mineral oils, synthetic iso-alkaines |
|
Sulfonated oils |
– |
|
|
Enzymes and biological agents |
– |
|
|
Bitumen, asphalt and tar |
– |
5.1.1 Dust Palliatives
Dust palliatives can be applied either as a topical application to a prepared road surface, as a mix-in treatment to an existing road, or mixed into the material during construction or regravelling. Mix-in treatments typically provide significantly improved performance compared to topical applications. Standard engineering considerations such as adequate compaction, road shape and drainage should not be overlooked in the application process. If topical applications are used, it should be remembered that applying additives to roads in poor condition will result in some dust reduction, but will not correct ride-related issues. Depending on the degree of compaction on the surface of the road, topical applications are best applied as a series of light applications over a period of time, rather than in a single application, to ensure adequate penetration of the additive.
5.1.2 Compaction Aids and Stabilizers
Compaction aids and stabilizers are typically applied as a mix-in treatment. Little benefit will be gained by applying these additives as a topical application.
6. ENVIRONMENTAL CONSIDERATIONS
There are significant environmental benefits associated with road dust control, including reduced particulate matter and the preservation of scare natural resources. However, care must be taken to ensure that the use of road additives will not have any significant negative environmental impacts. Potential environmental impacts include plant and animal toxicity, contamination of water resources, and corrosion of infrastructure and vehicles.
No internationally recognized laboratory or field procedures have been specifically developed for assessing the environmental impacts associated with the use of road additives10. However, a number of initiatives, mostly voluntary, have been established with a view to assessing potential impacts associated with road dust control (e.g. The Environmental Protection Agency’s Environmental Technology Verification program), while a number of state EPA’s require some form of product assessment before they can be applied. However, the laboratory procedures are based on those developed for other applications, such as assessing leachates from landfills and although in some instances these are practically appropriate, the lack of a single standard complicates the comparison of additives for a given application. The tests often provide a very worst-case scenario that is often not remotely realistic in road applications, resulting in potentially beneficial additives being excluded from use. A number of field trials have been carried out in the United States and elsewhere to assess runoff characteristics, but the findings are typically dependent on a multitude of factors and hence interpretation of the data and extrapolation of the findings to other regions is difficult. There is also no process for deciding whether the benefits of road dust control outweigh the potential negative impacts associated with an application. The problem is exacerbated for those additives that require periodic rejuvenation resulting in residual product build-up over time.
7. DUST CONTROL RESEARCH
The first reported chemical dust control experiments (i.e. those other than water spraying, which probably dates back to Roman times) occurred in the early 1900’s, when chlorides11 (calcium, magnesium, and sodium) and then lignosulfonates12 were applied to road surfaces to reduce dust emissions from passing vehicles. No significant new dust control products appear to have been introduced in the period between the 1930s and 1960s, but in the 1970’s and 1980’s, numerous chemical additives were introduced to the road industry. These included natural and synthetic polymer emulsions, oils and resins, sulfonated oils, enzymes, and various petroleum-based products. Proprietary products, primarily based on these technologies continue to be introduced.
Over the years, varying levels of research have been conducted on the array of dust control and stabilization additives listed above, by additive developers, road owners, and independent researchers. Since the 1920’s, thousands of laboratory studies and full-scale field experiments have been undertaken, and numerous publications prepared on the findings. However, implementation in the form of improved road management practices is almost non-existent world-wide, with no clear indication of why road authorities do not consider chemical improvement a standard practice, despite research continually proving the operational, economic and environmental benefits. For example, the conference proceedings of the 1932 Highway Research Board meeting13 included a paper on the effectiveness of calcium chloride as an unsealed road additive. A literature review of subsequent Highway Research Board and then Transportation Research Board (TRB) publications up to and including the proceedings of the 2006 TRB Low-Volume Roads Conference14 reveals that calcium chloride experiments continued to be established and monitored, and that papers on their performance continue to be published at regular intervals. However, road authorities appear no closer to wide-scale implementation of calcium chloride (or any other additive) than they did in 1932. This appears to be attributed in part to the establishment of experiments to assess performance under a particular given set of circumstances, as opposed to establishing them to identify boundary conditions of performance and develop guideline documentation and specifications. Despite this observation, valuable data on issues such as comparing performance of topical applications with mix-in treatments15, stabilization mechanisms16, and potential environmental impacts17 have also be collected and documented in many of these studies, which if appropriately analyzed, would contribute significantly to the preparation of appropriate documentation.
Conversely, other strategies for low-volume road construction and management such as soil stabilization with cement, lime, and asphalt emulsions, bituminous surface treatments (sand and chip seals), and full-depth recycling (foamed asphalt, asphalt emulsion, and cement and lime), which were all developed long after basic chemical dust control, are widely implemented. Quality design guides and specifications for these strategies have been prepared at state and national levels in many countries; little or no new experimentation is being conducted, and design engineers consider them in their choice of alternatives as a matter or course. The number of TRB publications on topics such as low-volume road cement stabilization and chip seal design were considerable at the time of the research studies, but have since dwindled to papers on specific project implementation or the development of new test methods and design tools.
7.1 Certification of Additives
A number of initiatives have been taken in various countries in an attempt to overcome this lack of implementation. One such initiative is that of fit-for-purpose certification8, which entails reviewing the research conducted on a specific additive and the documentation developed from it to determine whether sufficient information is available for an engineer or manager to make an informed decision on its use as a potential alternative in a road design or for maintenance. Certification systems are also used to ensure that additives comply with certain minimum standards, particularly those related to potential environmental impacts. A series of laboratory control tests are usually carried out as part of the review process. The procedure is based on a relative performance evaluation methodology, which:
The process typically involves the following:
Fit-for-purpose certification is not intended to serve as a formal acceptance or rejection of an additive based on an absolute performance evaluation. It also does not serve as a guarantee of performance, nor does it obviate the need to carry out an engineering investigation, including material testing, for every project where the use of the additive is considered.
8. THE WAY FORWARD
There is no clear way forward to ensure that road dust management initiatives will be implemented on a wider scale than current practice. A number of suggestions are offered for consideration. These are mostly institutional reforms and include:
9. CONCLUSIONS
Road dust control and unsealed road stabilization are significant road management issues. Although considerable experimentation on a variety of chemical additives has been carried out in the last 70 years, very little wide-scale implementation has taken place. There are many reasons for this, including the absence of a national authority, a fragmented industry, and a lack of funding for programs amongst unsealed road authorities and owners.
This conference is aimed at bringing practitioners together to discuss road dust and adjacent area management issues, road dust best management practices, knowledge gaps, research needs, barriers to implementation, and identification of future needs. Participants will attempt to explain why chemical dust control and unsealed road stabilization has not progressed to the point that road authorities can implement wider-scale programs with confidence. Remedies will be sought to initiate the development of nationwide administrative structures, information resources, and consistent experimental and maintenance protocols that, in a manner similar to those already in place for paved/sealed roads, will facilitate the adoption of standards and practices that will improve performance, and reduce both maintenance costs and environmental impacts of unsealed roads. The conference is not intended to be a platform for reporting on another round of experiments, but rather a forum for identifying and overcoming the barriers to wider implementation of the results and recommendations of the past 100 years of research.
A “white paper” documenting the discussion and the recommendations for a way forward will be published after the conference.
10. REFERENCES
Chemical dust control on unsealed roads has been researched for decades and there are numerous published papers documenting the establishment and monitoring of experiments. However, much of this has been agency-specific and mostly focused on assessing performance of one additive under a particular set of conditions. There are no specific comprehensive guidelines or specifications available to help practitioners with establishing longer-term dust control programs, identifying which type of additive would be most appropriate for a specific application, undertaking life-cycle analyses, quantifying negative environmental impacts and positive social benefits, designing appropriate treatments, applying the additive, and maintaining the treated road. Similar documentation for sealed roads has long been available and is continuously updated. Additionally, there is no national industry group serving the interests of additive manufacturers and suppliers, similar to the National Asphalt Paving Association (NAPA) and the American Concrete Paving Association (ACPA). There is no “owner” for documentation, procedures and test methods relating to chemical dust control, similar to the American Association of State Highway Officials (AASHTO), nor is there a sustained source of national funding for research to prepare this documentation and develop procedures and test methods.
Increasing concerns with regard to deteriorating air quality, the sustainability of repeatedly replacing gravel on unsealed roads, and the increasing costs of asphalt binders used for sealing roads have placed renewed interest on road dust management. Although upgrading the road to a sealed (asphalt surface treatment, asphalt concrete, or portland cement concrete) standard is always preferable and usually the most economic option in terms of life-cycle costs, the rapidly increasing costs associated with this practice results in less distance being upgraded each year. The application of various additives can provide satisfactory dust control on most road surfaces until such time as sufficient funds become available for a more permanent surfacing. Provided that appropriate construction and maintenance practices are followed, and the additives are rejuvenated at regular intervals, chemically treated surfaces are often structurally adequate to function as a base or subbase in a staged construction of a sealed road.
This paper provides a current status of global road dust management together with some points for consideration that may lead to wider implementation of dust control programs in unsealed road management initiatives. The paper includes discussion on the extent of unsealed road networks, the volume of dust generated, the consequences of dust, categorization of road additives, environmental considerations, and dust control research.
2. UNSEALED ROAD NETWORKS
There is no accurate estimate of the size of the global unsealed road network. Table 1 provides some estimates of the extent of unsealed road networks in the United States1 (1st World, 9,6 million km2), South Africa2 (2nd World, 1,2 million km2), and Tanzania3 (3rd World, 0.9 million km2), indicating the magnitude of global unsealed road management issues.
Table 1: Estimates of unsealed road networks (in kilometers)
|
Owner |
United States |
South Africa |
Tanzania |
|
Land area (km2) Sealed road network (km) Unsealed road network (km) |
9,600,000 3,700,000 2,700,000 |
1,200,000 300,000 600,000 |
880,000 5,000 85,000 |
|
State/county Municipal Forestry Bureau of land management Nature conservation/tourism Agriculture Mine Other* |
850,000 Unknown 620,000 130,000 17,000 Unknown Unknown Unknown |
150,000 200,000 100,000 – 5,000 50,000 5,000 100,000 |
81,000 5,000 Unknown – Unknown Unknown Unknown Unknown |
|
* Includes service roads for railroad, powerlines, military, border patrol, other commercial activities, etc |
|||
3. VOLUME OF DUST GENERATED
Documented studies in the United States indicate that as much as 50 percent of PM10 emissions and 19 percent of PM2.5 emissions are attributed to road dust (Figure 1)3. Road dust is the single biggest source of PM10 emissions and approximately 65 percent of road dust emissions are attributed to unsealed roads. These percentages increase in developing countries that have higher proportions of unsealed roads, and are of particular concern in urban areas with predominantly unsealed infrastructure.
Figure 1: US PM10 and PM2.5 emissions in 2002 by principal source category3
4. CONSEQUENCES OF ROAD DUST
Road dust is often considered only as a nuisance or minor safety hazard by many practitioners. However, using models developed by the United States Environmental Protection Agency5 and calibrated in various countries6, it can be shown that millions of tons of dust are generated on unsealed road networks every year. Although much of this dust falls back onto the road to be regenerated by the next vehicle, studies have shown that at least a third of it is permanently lost in the form of deposits away from the road (Figure 2), with losses increasing under crosswind conditions.
Figure 2: Fines lost from unsealed roads
Apart from the obvious consequences of reduced quality of life and increased safety hazard for road users, pedestrians, and workers, the loss of fines (which perform an integral material- binding function) from the road surface results firstly in accelerated gravel loss, thereby increasing the frequency at which the gravel has to be replaced, and secondly in more rapid deterioration of the riding quality of the road, thereby requiring more frequent grader maintenance6. This has significant economic and environmental implications in terms of regular regravelling programs. Other serious, but often overlooked consequences, include reduced agricultural and forestry yields. These are attributed to retarded plant growth, increased insect activity, crop blemishing, and reduced palatability of pasture and associated reduced yields in terms of dairy production. There are even published reports on accelerated tooth wear of animals grazing in pasture adjacent to unsealed roads7. Environmental consequences in terms of air and water pollution and associated health hazards, primarily those linked to respiratory diseases, are also significant, especially in developing countries where a large proportion of urban road infrastructure is often unsealed. Vehicle operating costs increase significantly in dusty conditions, with numerous publications compiled comparing the cost of operating vehicles in dusty and dust-free environments.
5. DUST CONTROL
Dust control can be achieved either by better selection of base and wearing course materials, mechanical stabilisation using two or more different materials to achieve a better particle size distribution and to increase or reduce the plasticity, or by applying a chemical dust palliative. Only chemical treatments are addressed in this paper.
5.1 Chemical Dust Control Categories
Numerous additives are available for dust palliation, improved compaction, and stabilization of unsealed roads. Most of these bind the fine particles together without any significant chemical reaction occurring in the soil, although certain additives will only perform once a chemical reaction has occurred. A number of additives are material and/or climate-dependent and costs vary significantly. It is therefore important that the bonding nature, limitations and life-cycle costs of these additives be investigated and their performance understood before widespread use is considered.
Most unsealed road additives are proprietary formulations, and information regarding their composition is often not readily available. This knowledge gap can limit the extent of applications if no clear information is available with regard to potential human and environmental impacts and in instances where competitive tendering is required. In order to facilitate research, technology transfer, palliative certification, classification of palliative types for different uses, climates and base material types, selection of appropriate additive type and application rate for particular conditions, and transparent and competitive bidding/tendering procedures, additives need to be categorized based primarily on their function and chemistry. A suggested categorization is provided in Table 26. Similar categorizations are used by the US Forest Service9 and the Environmental Protection Agency. A brief introduction to each category is provided below. Details on the stabilization mechanism and research on laboratory and field testing of each of these categories are discussed elsewhere in the literature.
Most road authorities cannot specify proprietary product names in tender documents. In order to facilitate implementation under these conditions, authorities could consider using category names in tender documentation if a design or experience dictates a specific type of application. Alternatively a performance specification (e.g. dust level reduction) can be used and the contractor can apply an additive of his own choosing, provided that it meets human and environmental safety requirements.
Table 2: Suggested road additive categories
|
Category |
Sub-categories |
Examples |
|
Dust palliatives |
Water and wetting agents |
– |
|
Hygroscopic salts |
Calcium, magnesium or sodium chloride |
|
|
Natural polymers |
Lignosulfonate, molasses, tannin extracts |
|
|
Synthetic polymer emulsions |
Acrylates, acrylics, vinyl acetates |
|
|
Synthetic oils |
Mineral oils, synthetic iso-alkaines |
|
|
Petroleum resins |
Blend of natural polymer and petroleum products |
|
|
Bitumen, asphalt and tar |
– |
|
|
Other |
Industrial wastes |
|
|
Compaction aids and |
Synthetic polymer emulsions |
Acrylates, acrylics, vinyl acetates |
|
stabilizers |
Synthetic oils |
Mineral oils, synthetic iso-alkaines |
|
Sulfonated oils |
– |
|
|
Enzymes and biological agents |
– |
|
|
Bitumen, asphalt and tar |
– |
5.1.1 Dust Palliatives
Dust palliatives can be applied either as a topical application to a prepared road surface, as a mix-in treatment to an existing road, or mixed into the material during construction or regravelling. Mix-in treatments typically provide significantly improved performance compared to topical applications. Standard engineering considerations such as adequate compaction, road shape and drainage should not be overlooked in the application process. If topical applications are used, it should be remembered that applying additives to roads in poor condition will result in some dust reduction, but will not correct ride-related issues. Depending on the degree of compaction on the surface of the road, topical applications are best applied as a series of light applications over a period of time, rather than in a single application, to ensure adequate penetration of the additive.
5.1.2 Compaction Aids and Stabilizers
Compaction aids and stabilizers are typically applied as a mix-in treatment. Little benefit will be gained by applying these additives as a topical application.
6. ENVIRONMENTAL CONSIDERATIONS
There are significant environmental benefits associated with road dust control, including reduced particulate matter and the preservation of scare natural resources. However, care must be taken to ensure that the use of road additives will not have any significant negative environmental impacts. Potential environmental impacts include plant and animal toxicity, contamination of water resources, and corrosion of infrastructure and vehicles.
No internationally recognized laboratory or field procedures have been specifically developed for assessing the environmental impacts associated with the use of road additives10. However, a number of initiatives, mostly voluntary, have been established with a view to assessing potential impacts associated with road dust control (e.g. The Environmental Protection Agency’s Environmental Technology Verification program), while a number of state EPA’s require some form of product assessment before they can be applied. However, the laboratory procedures are based on those developed for other applications, such as assessing leachates from landfills and although in some instances these are practically appropriate, the lack of a single standard complicates the comparison of additives for a given application. The tests often provide a very worst-case scenario that is often not remotely realistic in road applications, resulting in potentially beneficial additives being excluded from use. A number of field trials have been carried out in the United States and elsewhere to assess runoff characteristics, but the findings are typically dependent on a multitude of factors and hence interpretation of the data and extrapolation of the findings to other regions is difficult. There is also no process for deciding whether the benefits of road dust control outweigh the potential negative impacts associated with an application. The problem is exacerbated for those additives that require periodic rejuvenation resulting in residual product build-up over time.
7. DUST CONTROL RESEARCH
The first reported chemical dust control experiments (i.e. those other than water spraying, which probably dates back to Roman times) occurred in the early 1900’s, when chlorides11 (calcium, magnesium, and sodium) and then lignosulfonates12 were applied to road surfaces to reduce dust emissions from passing vehicles. No significant new dust control products appear to have been introduced in the period between the 1930s and 1960s, but in the 1970’s and 1980’s, numerous chemical additives were introduced to the road industry. These included natural and synthetic polymer emulsions, oils and resins, sulfonated oils, enzymes, and various petroleum-based products. Proprietary products, primarily based on these technologies continue to be introduced.
Over the years, varying levels of research have been conducted on the array of dust control and stabilization additives listed above, by additive developers, road owners, and independent researchers. Since the 1920’s, thousands of laboratory studies and full-scale field experiments have been undertaken, and numerous publications prepared on the findings. However, implementation in the form of improved road management practices is almost non-existent world-wide, with no clear indication of why road authorities do not consider chemical improvement a standard practice, despite research continually proving the operational, economic and environmental benefits. For example, the conference proceedings of the 1932 Highway Research Board meeting13 included a paper on the effectiveness of calcium chloride as an unsealed road additive. A literature review of subsequent Highway Research Board and then Transportation Research Board (TRB) publications up to and including the proceedings of the 2006 TRB Low-Volume Roads Conference14 reveals that calcium chloride experiments continued to be established and monitored, and that papers on their performance continue to be published at regular intervals. However, road authorities appear no closer to wide-scale implementation of calcium chloride (or any other additive) than they did in 1932. This appears to be attributed in part to the establishment of experiments to assess performance under a particular given set of circumstances, as opposed to establishing them to identify boundary conditions of performance and develop guideline documentation and specifications. Despite this observation, valuable data on issues such as comparing performance of topical applications with mix-in treatments15, stabilization mechanisms16, and potential environmental impacts17 have also be collected and documented in many of these studies, which if appropriately analyzed, would contribute significantly to the preparation of appropriate documentation.
Conversely, other strategies for low-volume road construction and management such as soil stabilization with cement, lime, and asphalt emulsions, bituminous surface treatments (sand and chip seals), and full-depth recycling (foamed asphalt, asphalt emulsion, and cement and lime), which were all developed long after basic chemical dust control, are widely implemented. Quality design guides and specifications for these strategies have been prepared at state and national levels in many countries; little or no new experimentation is being conducted, and design engineers consider them in their choice of alternatives as a matter or course. The number of TRB publications on topics such as low-volume road cement stabilization and chip seal design were considerable at the time of the research studies, but have since dwindled to papers on specific project implementation or the development of new test methods and design tools.
7.1 Certification of Additives
A number of initiatives have been taken in various countries in an attempt to overcome this lack of implementation. One such initiative is that of fit-for-purpose certification8, which entails reviewing the research conducted on a specific additive and the documentation developed from it to determine whether sufficient information is available for an engineer or manager to make an informed decision on its use as a potential alternative in a road design or for maintenance. Certification systems are also used to ensure that additives comply with certain minimum standards, particularly those related to potential environmental impacts. A series of laboratory control tests are usually carried out as part of the review process. The procedure is based on a relative performance evaluation methodology, which:
The process typically involves the following:
Fit-for-purpose certification is not intended to serve as a formal acceptance or rejection of an additive based on an absolute performance evaluation. It also does not serve as a guarantee of performance, nor does it obviate the need to carry out an engineering investigation, including material testing, for every project where the use of the additive is considered.
8. THE WAY FORWARD
There is no clear way forward to ensure that road dust management initiatives will be implemented on a wider scale than current practice. A number of suggestions are offered for consideration. These are mostly institutional reforms and include:
9. CONCLUSIONS
Road dust control and unsealed road stabilization are significant road management issues. Although considerable experimentation on a variety of chemical additives has been carried out in the last 70 years, very little wide-scale implementation has taken place. There are many reasons for this, including the absence of a national authority, a fragmented industry, and a lack of funding for programs amongst unsealed road authorities and owners.
This conference is aimed at bringing practitioners together to discuss road dust and adjacent area management issues, road dust best management practices, knowledge gaps, research needs, barriers to implementation, and identification of future needs. Participants will attempt to explain why chemical dust control and unsealed road stabilization has not progressed to the point that road authorities can implement wider-scale programs with confidence. Remedies will be sought to initiate the development of nationwide administrative structures, information resources, and consistent experimental and maintenance protocols that, in a manner similar to those already in place for paved/sealed roads, will facilitate the adoption of standards and practices that will improve performance, and reduce both maintenance costs and environmental impacts of unsealed roads. The conference is not intended to be a platform for reporting on another round of experiments, but rather a forum for identifying and overcoming the barriers to wider implementation of the results and recommendations of the past 100 years of research.
A “white paper” documenting the discussion and the recommendations for a way forward will be published after the conference.
10. REFERENCES
Copyright Soilworks, LLC 2003-. All Rights Reserved. Soilworks®, Soiltac®, Gorilla-Snot®, and Durasoil®are registered trademarks of Soilworks, LCC.
Copyright Soilworks, LLC 2003-. All Rights Reserved. Soilworks®, Soiltac®, Gorilla-Snot®, and Durasoil® are registered trademarks of Soilworks, LCC.
