When it comes to paving driveways, roads, and parking lots, two primary materials often come to mind: coal tar and asphalt. While they might appear similar at first glance, these substances possess distinct characteristics, applications, and environmental implications that are crucial to understand for any paving project.
Choosing between coal tar and asphalt is not merely a cosmetic decision; it impacts durability, maintenance requirements, cost, and environmental footprint.
This article will delve into the fundamental differences between coal tar and asphalt, exploring their origins, properties, advantages, disadvantages, and typical uses in the world of pavement construction and maintenance.
Understanding the Basics: Coal Tar and Asphalt
At their core, both coal tar and asphalt are black, viscous materials derived from petroleum or coal. However, their origins and chemical compositions set them apart significantly.
What is Coal Tar?
Coal tar is a byproduct of the destructive distillation of coal, a process that heats coal in the absence of air to produce coke and other valuable chemicals. This dark, oily liquid is a complex mixture of hundreds of organic compounds, including aromatic hydrocarbons like benzene, toluene, and xylene, as well as phenols and heterocyclic compounds.
Historically, coal tar was a widely used paving material due to its excellent sealing properties and resistance to water and oil penetration. Its application often involved a sealcoating process, where a thin layer was applied over existing asphalt or concrete surfaces.
However, concerns regarding its environmental impact and potential health hazards have led to a significant decline in its use for general paving applications in many regions.
What is Asphalt?
Asphalt, also known as bitumen, is a naturally occurring viscous substance or a byproduct of petroleum refining. In the context of paving, it is most commonly the refined residue from crude oil distillation, forming the binder that holds aggregates like gravel and sand together in asphalt concrete (often called “blacktop”).
This binder is a thermoplastic material, meaning it softens when heated and hardens when cooled, a property essential for the paving process. Asphalt binders are primarily composed of complex hydrocarbon molecules, including asphaltenes and maltenes, which contribute to their adhesive and cohesive properties.
Asphalt concrete is the most prevalent paving material used globally for roads, highways, driveways, and parking lots due to its versatility, durability, and cost-effectiveness.
Key Differences in Properties and Performance
The divergence in their origins directly translates into distinct properties that influence how each material performs under various conditions.
Durability and Longevity
Asphalt concrete generally offers superior durability and longevity as a structural paving material. When properly installed and maintained, asphalt pavements can withstand heavy traffic loads and extreme weather conditions for many years, often 20 to 30 years or more.
Coal tar sealcoats, on the other hand, are typically applied as a surface treatment. While they provide a protective layer, they are not a structural component of the pavement itself and are more susceptible to wear and tear from traffic and UV exposure. Their lifespan as a protective coating is considerably shorter, usually requiring reapplication every 2 to 5 years.
The inherent flexibility of asphalt binders allows them to accommodate minor ground movements and temperature fluctuations without cracking, contributing to the overall resilience of the pavement structure.
Water and Chemical Resistance
Coal tar exhibits excellent resistance to water penetration and petroleum-based products like gasoline and oil. This property made it a preferred choice for applications where spills were common, such as gas station islands and industrial areas.
Asphalt, while water-resistant, is more susceptible to damage from oil and gasoline spills, which can soften and degrade the binder over time. This is why coal tar sealcoats were often favored for their superior protection against fuel leaks, even when applied over asphalt.
However, modern asphalt formulations and sealants have improved their resistance to petroleum products, mitigating this historical advantage of coal tar.
Temperature Sensitivity
Both materials are temperature-sensitive, but their behavior differs. Asphalt binders become soft and pliable in high temperatures, allowing them to be compacted and shaped during paving. In cold temperatures, they become more brittle.
Coal tar, while also softening with heat, tends to become more brittle at lower temperatures than asphalt and can be more prone to cracking in freeze-thaw cycles when used as a surface sealant.
The ability of asphalt to remain somewhat flexible across a wider temperature range is a significant factor in its widespread use for structural paving.
Adhesion and Binding Properties
Asphalt is a highly effective binder for aggregates, creating a cohesive and durable pavement structure. Its strong adhesive properties ensure that the stones and sand are firmly held together, forming a robust surface.
Coal tar is primarily used as a sealant or rejuvenator, not as a primary binder for structural pavement layers. Its binding capabilities are more suited to sealing existing surfaces and filling small cracks rather than forming the foundation of a road.
The chemical compatibility of asphalt with various aggregate types is a key reason for its success in creating stable road surfaces.
Applications: Where Are They Used?
The distinct properties of coal tar and asphalt dictate their primary applications in the construction and maintenance industry.
Coal Tar Applications
Historically, coal tar was extensively used as a paving material itself, particularly for driveways and low-traffic roads. Its primary modern application is as a sealcoat, applied as a thin layer over existing asphalt or concrete surfaces.
This sealcoating process was valued for its ability to protect the underlying pavement from water damage, UV radiation, and chemical spills, thereby extending its lifespan and improving its appearance. Coal tar sealcoats were particularly popular for their effectiveness in preventing gasoline and oil damage.
However, due to environmental and health concerns, the use of coal tar sealants has been restricted or banned in many areas, leading to a decline in its application for general paving and sealcoating purposes.
Asphalt Applications
Asphalt concrete is the ubiquitous choice for most paving projects today. It forms the structural layer of virtually all modern roads, highways, airport runways, and parking lots.
Its versatility allows it to be used in various mixes, from dense-graded asphalt for high-traffic areas to open-graded asphalt for improved drainage. Asphalt is also used in chip seal applications and as a binder in other construction materials.
The ease of installation, relatively low cost, and ability to be recycled make asphalt a sustainable and practical choice for a vast array of paving needs.
Environmental and Health Considerations
The environmental and health profiles of coal tar and asphalt are significant factors influencing their current usage.
Coal Tar Concerns
Coal tar contains polycyclic aromatic hydrocarbons (PAHs), many of which are known or suspected carcinogens. Exposure to coal tar can occur through skin contact, inhalation of fumes, or ingestion.
Environmental concerns also arise from the potential leaching of PAHs into soil and water, posing risks to ecosystems. Runoff from coal tar-sealed surfaces can carry these contaminants into waterways.
These health and environmental risks have prompted regulatory bodies in many jurisdictions to limit or prohibit the use of coal tar-based products in consumer applications, including sealcoating.
Asphalt Considerations
While asphalt is generally considered safer than coal tar, it is not without its environmental considerations. The extraction and refining of petroleum, the source of most asphalt, have associated environmental impacts.
During the paving process, asphalt fumes can be released, and workers may be exposed to these emissions. However, modern safety protocols and the development of lower-emission asphalt technologies have aimed to mitigate these risks.
A significant advantage of asphalt is its recyclability. Reclaimed asphalt pavement (RAP) can be incorporated into new asphalt mixes, reducing the need for virgin materials and diverting waste from landfills. This makes asphalt a more sustainable option in the long run.
Maintenance and Repair
The maintenance requirements for coal tar and asphalt surfaces differ, reflecting their differing roles in pavement construction.
Maintaining Coal Tar Surfaces
Coal tar sealcoats require regular reapplication, typically every 2 to 5 years, to maintain their protective properties. This involves cleaning the surface and applying a new layer of sealant.
Minor cracks and potholes in coal tar-sealed surfaces can often be repaired with specialized patching compounds, but the sealant itself is more of a surface treatment than a structural repair material.
The effectiveness of coal tar sealcoats diminishes over time due to UV degradation and wear, necessitating periodic rejuvenation.
Maintaining Asphalt Surfaces
Asphalt pavements require a different approach to maintenance. Routine sweeping and cleaning help remove debris that can accelerate wear.
Crack sealing is a crucial maintenance activity for asphalt, preventing water from entering the pavement structure and causing further damage like freeze-thaw cycles. Potholes are typically repaired by patching, which can involve removing the damaged material and replacing it with new asphalt.
Sealcoating of asphalt pavements, often using asphalt-based sealants rather than coal tar, is also a common practice to protect the surface, enhance its appearance, and extend its lifespan.
Cost Comparison
The initial cost and long-term expenses associated with coal tar and asphalt can vary.
Coal Tar Cost
When used as a sealcoat, coal tar application can be relatively inexpensive per square foot for the initial treatment. However, the need for frequent reapplication means that the long-term cost of maintaining a coal tar-sealed surface can escalate over time.
The declining availability and increasing regulatory hurdles in some areas may also influence its cost.
The upfront cost might seem appealing, but the recurring expense of reapplication is a significant factor to consider.
Asphalt Cost
The initial cost of installing an asphalt pavement is generally higher than applying a coal tar sealcoat. This is because asphalt concrete is a structural material that requires proper grading, compaction, and often a base layer.
However, the longevity and reduced maintenance frequency of asphalt pavements often make them more cost-effective over their entire lifespan. The ability to recycle asphalt further contributes to its long-term economic viability.
While the initial investment is greater, the durability and extended service life of asphalt typically result in a lower total cost of ownership.
The Modern Landscape: Trends and Regulations
The paving industry is constantly evolving, driven by technological advancements, environmental awareness, and regulatory changes.
Shifting Away from Coal Tar
Across many parts of the world, there has been a pronounced shift away from the use of coal tar for paving and sealcoating. This is primarily due to the health and environmental risks associated with PAHs.
Many municipalities and states have enacted regulations that ban or restrict the use of coal tar sealants, encouraging the adoption of safer alternatives, such as asphalt-based sealcoats and other proprietary formulations.
This regulatory trend is a strong indicator of the declining relevance of coal tar in the modern paving market for consumer-facing applications.
The Dominance of Asphalt
Asphalt concrete remains the dominant material for pavement construction globally. Ongoing research and development are focused on improving asphalt performance, durability, and sustainability.
Innovations include the use of polymer-modified asphalt binders for enhanced elasticity and resistance to rutting, warm-mix asphalt technologies that reduce emissions and energy consumption during production, and advanced recycling techniques.
The industry’s commitment to sustainability, particularly through increased use of recycled asphalt pavement (RAP), solidifies asphalt’s position as the material of choice for the foreseeable future.
Making the Right Choice for Your Pavement
Deciding between coal tar and asphalt for your paving needs requires a careful consideration of various factors.
For Structural Paving
If you are constructing a new driveway, parking lot, or road, asphalt concrete is almost invariably the material of choice. Its structural integrity, durability, and cost-effectiveness make it the standard for these applications.
The performance and longevity of a well-constructed asphalt pavement far outweigh any perceived benefits of using a coal tar-based structural material, which is largely obsolete for this purpose.
Consulting with experienced paving contractors is essential to ensure the correct asphalt mix design and installation practices are employed for optimal results.
For Surface Protection and Sealing
For existing asphalt or concrete surfaces that require protection and sealing, the choice is typically between asphalt-based sealants and other modern alternatives, rather than coal tar.
Asphalt-based sealcoats offer good protection against water and UV damage, and many are formulated to resist oil and gasoline spills. These are widely available and generally considered environmentally safer than coal tar.
When evaluating sealcoat options, consider the specific needs of your surface, the local climate, and any applicable environmental regulations. An asphalt-based sealcoat provides a protective barrier that can extend the life of your pavement.
Conclusion
In summary, while both coal tar and asphalt are black, viscous materials used in paving, their differences are substantial. Coal tar, a byproduct of coal distillation, was historically used as a sealant and structural paving material but is now largely phased out due to health and environmental concerns. Asphalt, derived from petroleum refining, is the dominant binder in modern pavement construction, prized for its durability, versatility, and recyclability.
For structural paving projects like driveways and roads, asphalt concrete is the clear and current industry standard. For surface sealing and protection, modern asphalt-based sealants and other advanced formulations have replaced coal tar due to safety and environmental considerations.
Understanding these distinctions empowers property owners and project managers to make informed decisions that ensure durable, safe, and environmentally responsible paving solutions.