What is Asphalt Concrete Paving? A Comprehensive Guide

Asphalt concrete, commonly known as asphalt or blacktop, is one of the most widely used materials for paving roads, parking lots, and driveways. Its popularity stems from its cost-effectiveness, durability, and simplicity in construction. This article will discuss the makeup of asphalt concrete, its application processes, and provide examples and resources for further information.

Asphalt Concrete Defined:

A sticky, black, and highly viscous liquid, asphalt is a semi-solid form of petroleum. Asphalt concrete is a composite material consisting of mineral aggregate bound together with asphalt. The mixture is laid in layers and compacted to create a smooth, durable road surface. The composition of asphalt concrete can vary depending on its use. Generally, asphalt concrete includes a mixture of sand, gravel, and crushed rocks, combined with asphalt cement.

Key Components of Asphalt Concrete:

• Aggregate: Sand, gravel, and crushed stone form the foundation of the pavement.
• Asphalt Binder: Derived from crude oil refining, it holds the aggregate together and provides flexibility and waterproof properties.
• Mineral Fillers: Fine sand or limestone dust fills gaps between the larger aggregate particles. This improves the overall performance of the asphalt mix.

How is Asphalt Concrete Paved or Installed?

The process of paving with asphalt concrete typically involves several steps:

1. Preparation of the Subgrade: This is the initial step where the ground is prepared to ensure it can support the weight of the traffic. It involves leveling and compacting the soil, which may be enhanced by adding materials like lime or cement for stabilization.
2. Placement of Base Layers: Before the asphalt mixture is laid, base layers of crushed stone or gravel are placed and compacted. This layer provides a stable foundation and drainage for the asphalt overlay.
3. Mixing and Heating: Asphalt concrete is produced in a plant where the aggregate and binder are mixed and heated to the right temperature. This ensures the asphalt is workable during application.
4. Laying and Compaction: The hot asphalt mix is then transported to the site. It is laid using a paving machine, and compacted with rollers to ensure it is dense and air voids are minimized.
5. Cooling: After compaction, the asphalt needs to cool down to harden and gain strength before it can be opened to traffic.

There are different ways to mix and apply asphalt concrete, depending on the project’s needs and weather conditions. Here are three common types:

Hot Mix Asphalt (HMA)

HMA is the most widely used type of asphalt, heated to high temperatures (300-350 degrees Fahrenheit) before mixing and paving. It includes:

• Dense-Graded Mixes are further subdivided based on the size of the aggregate into fine-graded and coarse-graded. Fine-graded mixes contain a higher percentage of sand and small stones. They are typically used for paving high-traffic areas such as interstates and highways. In contrast, coarse-graded mixes have larger aggregate particles.
• Stone Matrix Asphalt, originally developed in Europe, enhances tire grip on roads and prevents rutting. Stone matrix asphalt made its U.S. debut in 1988. It features a higher concentration of asphalt cement, along with asphalt binders and certain fibers. This makes it more costly than other types of asphalt. Due to its expense, it is usually reserved for large-scale projects.
• Open-Graded Mixes come in two varieties. The first, known as a friction course, is applied as the top layer of pavement. It must have at least 15 percent air voids to function effectively. The second, asphalt treated permeable bases, are designed to drain water away from the pavement’s top layer. This serves as a base for dense-graded asphalt and Portland cement surfaces.

Advantages of Hot Mix Asphalt

1. Durability and Longevity: HMA is known for its ability to withstand heavy traffic and severe weather conditions. This makes it a durable choice for road surfaces. It can last for many years with proper maintenance.
2. Cost-Effectiveness: Compared to other materials like concrete, asphalt is generally cheaper to produce and lay. Its maintenance costs are also lower over time.
3. Speed of Construction: Asphalt paving can be completed quickly, which is crucial for minimizing traffic disruption in busy areas. Once laid, it cools and sets faster than concrete, allowing roads to be opened to traffic sooner.
4. Recyclability: Asphalt is one of the most recycled materials. Old asphalt can be milled and reused in new asphalt mixes. This reduces the need for raw materials and lowering the environmental impact.
5. Smoothness: Asphalt roads provide a smoother ride which is safer and more fuel-efficient for vehicles. This smoothness also reduces wear and tear on vehicles.
6. Noise Reduction: Asphalt surfaces tend to produce less road noise compared to concrete. This is beneficial in urban areas to reduce noise pollution.

Disadvantages of Hot Mix Asphalt

1. Oil Price Vulnerability: Since asphalt is a petroleum product, its price and availability are influenced by fluctuations in oil prices. This can affect the cost-effectiveness of projects.
2. Maintenance Requirements: Although maintenance is generally cheaper, asphalt surfaces require regular upkeep. Sealing cracks and resealing asphalt is needed to maintain integrity and appearance.

Warm Mix Asphalt (WMA)

This is a more environmentally friendly option that uses lower temperatures during the mixing process. However, long-term performance remains uncertain, and it has a higher risk of water damage.

WMA is produced at temperatures between 200 – 250 degrees Fahrenheit. Fewer fossil fuels and resources are consumed during its manufacturing process. By incorporating additional binders and additives like wax, emulsions, and zeolites, it is more easily poured and spread at cooler temperatures. Moreover, WMA is less expensive to produce than hot mix asphalt (HMA).

The lower production and transportation temperatures of WMA prevent it from cooling as quickly as HMA. This allows transportation over longer distances, and extends the paving season beyond traditional months. In addition, WMA enables road, highway, and pathway construction or maintenance during both daytime and nighttime.

Environmentally and occupationally preferable, WMA produces less dust, smoke, and fumes when poured and laid. This enhances safety in confined spaces like tunnels and on days with poor air quality. Under similar conditions, this can reduce delays that typically occur with other asphalt types.

Advantages of Warm Mix Asphalt

1. Reduced Energy Consumption: WMA is produced at temperatures 30 to 120 degrees Fahrenheit lower than traditional HMA. This means less energy is required for heating, leading to lower fuel consumption and energy costs.
2. Lower Emissions: Lower production temperatures reduce the emission of greenhouse gases and other pollutants like volatile organic compounds (VOCs) and sulfur dioxide. This makes WMA more environmentally friendly and improves working conditions by reducing smoke and fumes around production and paving sites.
3. Extended Paving Season: Because it can be compacted at lower temperatures, WMA can be applied in cooler conditions, extending the paving season in colder climates.
4. Improved Workability: The lower temperatures increase the workability of asphalt, allowing for easier placement and compaction, which can result in better quality pavements with longer lifespans.
5. Potential for Recycling: WMA technology improves the incorporation of reclaimed asphalt pavement (RAP) into new mixes, which can further enhance environmental sustainability and reduce costs.
6. Reduced Aging of Asphalt: Lower temperatures during production mean less short-term aging of the asphalt binder, preserving its performance characteristics.

Disadvantages of Warm Mix Asphalt

1. Initial Cost: The technologies used to produce WMA, such as additives and special equipment, can initially be more expensive than traditional HMA production methods.
2. Limited Long-Term Data: Although rapidly gaining popularity, WMA is relatively new, and long-term performance data is less abundant compared to HMA. This can make some road agencies hesitant to adopt the technology widely.
3. Performance Concerns in Heavy Traffic: There are some concerns about the use of WMA in roads with heavy traffic loads, as it may not be as robust as HMA, potentially leading to issues like rutting or premature wear.
4. Complexity in Production: The production of WMA requires precise temperature control and proper mixing to ensure that the additives used function correctly. This can complicate the manufacturing process compared to traditional HMA.

Cold mix asphalt (CMA)

This is a type of asphalt that does not require heating to become pliable and can be used at ambient temperatures. Cold mix asphalt is typically made by mixing bitumen emulsion (a mixture of water and bitumen along with an emulsifying agent) with aggregate (stone, sand, and gravel) at a lower temperature. It is usually used for emergency or small-scale repairs.

Advantages of Cold Mix Asphalt

1. No Heating Required: Since cold mix asphalt does not require heating, it saves energy and reduces emissions associated with the heating process.
2. Easy to Use: It can be prepared and applied without specialized equipment, making it ideal for remote or less accessible locations.
3. Year-Round Application: Unlike hot mix asphalt, which is difficult to work with in colder temperatures, cold mix asphalt can be applied in colder weather conditions. This makes it useful for emergency repairs during winter.
4. Storage and Reusability: Cold mix asphalt can be stockpiled and kept for future use as it remains workable for longer periods.
5. Cost-Effective for Small Repairs: It is cost-effective for small-scale repairs like potholes, utility cuts, and patch works, especially where immediate traffic return is necessary.

Disadvantages of Cold Mix Asphalt

1. Less Durable: Cold mix asphalt is generally considered less durable than hot mix asphalt, making it unsuitable for heavy traffic areas or permanent repairs.
2. Weather Susceptibility: While it can be applied in any weather, the performance can be affected by environmental conditions. Moisture can particularly degrade the material if not properly cured.
3. Longer Curing Time: It takes longer to cure compared to hot mix asphalt. Until it fully cures, the repaired patches may remain more susceptible to damage under traffic.
4. Limited Use: Primarily used for temporary repairs, cold mix asphalt is not suitable for larger or structural paving projects due to its lower performance characteristics.
5. Quality Consistency: The performance can vary depending on the mix design and the quality of the raw materials used.

Cold mix asphalt is a versatile and environmentally friendly option for specific uses, especially for quick, temporary repairs in colder climates, though it is not ideal for all paving needs.

Examples of Asphalt Concrete Applications

Asphalt concrete, commonly referred to as asphalt or blacktop, is a versatile paving material widely used in various applications due to its durability, cost-effectiveness, and ease of construction. Here are some common applications of asphalt concrete:

1. Roadways and Highways: Asphalt is the primary material used in the paving of roadways and highways. It provides a smooth, durable surface that can withstand heavy traffic and diverse weather conditions.
2. Airport Runways: Due to its ability to handle heavy loads, asphalt concrete is often used for airport runways. It can support the weight of large aircraft and resist the stresses of take-offs and landings.
3. Parking Lots: Asphalt is a popular choice for parking lots because it can be quickly laid and easily maintained, providing a cost-effective solution for large- and small-scale parking areas.
4. Driveways: Many residential properties choose asphalt for driveways because it is less expensive than concrete and can be installed relatively quickly.
5. Bicycle Paths: Asphalt provides a smooth surface for bike paths, enhancing safety and comfort for cyclists. It is also adaptable to various landscapes and easy to color for aesthetic purposes.
6. Sports Courts: Asphalt is used in the construction of various sports courts, including basketball, tennis, and other recreational areas, due to its smooth finish and durability.
7. Walking Paths: Similar to bicycle paths, walking and jogging trails often use asphalt because it is gentler on joints compared to harder surfaces and integrates well into natural surroundings.
8. Bridge Decks: Some bridge surfaces are paved with asphalt to provide a waterproofing layer and protect the underlying structural elements.
9. Tunnels: Asphalt is used inside tunnels as it provides a durable, fire-resistant layer that can be essential for safety in enclosed spaces.

Each application benefits from asphalt concrete’s adaptability, resilience, and relatively low maintenance costs, making it a preferred choice for many infrastructure and recreational projects.

Advantages of Asphalt Concrete Paving

Asphalt concrete paving, commonly known simply as asphalt paving, offers several advantages that make it a popular choice for road construction and other paving needs. Here are some of the main benefits:

1. Cost-Effectiveness: Asphalt is generally less expensive than other paving materials like concrete. The initial cost of laying down asphalt is lower, and its maintenance costs can also be more economical due to the ease and cost-effectiveness of repairs.
2. Durability and Longevity: When properly installed and maintained, asphalt paving can last for many years. It is designed to withstand the heavy traffic and varying weather conditions, making it particularly suitable for roads, driveways, and parking lots.
3. Speed of Construction: Asphalt paving projects can be completed relatively quickly. The material itself cools and sets faster than concrete, allowing roads and pathways to be opened to traffic much sooner after installation. This quick turnaround is beneficial in reducing road closure times and minimizing disruption.
4. Recyclability: Asphalt is highly recyclable, which contributes to its sustainability. Old asphalt can be milled and reused, reducing the need for new materials and decreasing the environmental impact associated with the extraction and processing of fresh resources.
5. Noise Reduction: Asphalt roads tend to produce less noise compared to concrete. The material’s texture and composition help to absorb road noise, leading to quieter roadways which is a significant advantage in urban areas.
6. Smoothness: Asphalt provides a smoother surface that enhances safety by offering better contact with vehicle tires. This smoothness also helps in reducing wear and tear on vehicles, potentially leading to lower operating costs for vehicle owners.
7. Flexibility: Asphalt’s flexibility helps it resist cracking under the pressure and slight movements of the ground. It can also adapt to low-temperature conditions without becoming brittle, reducing the risk of cracks and damage during winter months.
8. Water Drainage: Asphalt can be engineered to be porous, which allows water to drain through the surface and reduce standing water and skidding on roads, enhancing safety during rainy conditions.

These advantages make asphalt a highly favorable material in both urban and rural paving projects. Its combination of cost-efficiency, durability, and environmental benefits contribute to its widespread use in infrastructure development.

In conclusion, asphalt concrete paving is a critical component of modern infrastructure. Its versatility and efficiency in construction make it an ideal choice for various paving needs ranging from busy highways to residential driveways. Understanding its composition, installation processes, and benefits can help stakeholders make informed decisions about their paving projects.

To learn more about asphalt concrete paving or to request a quote or consultation, please contact us at 503-999-1235 or complete our contact form. We look forward to serving you.

Further Reading and Resources

For more detailed information about asphalt concrete and its uses, consider exploring the following resources:

• Asphalt Pavement Alliance: http://www.driveasphalt.org/
• Federal Highway Administration: https://highways.dot.gov/
• Asphalt Institute: https://www.asphaltinstitute.org/
• National Asphalt Pavement Association: https://www.asphaltpavement.org/
• Asphalt Magazine: http://asphaltmagazine.com/

These sites provide extensive data on the specifications, research, and technological advancements associated with asphalt paving.