What is Asphalt Made Of?

Blog Home | by on May 3, 2022 in News

The Road to Understanding Asphalt


When you’re driving down the highway or walking along a well-paved road, you might not think much about the material under your feet. But it’s the unsung hero of modern transportation and infrastructure. We’re talking about asphalt, a common building material that plays a huge role in our daily lives. Whether it’s the smooth surface of a well-maintained road, the water-resistant seal on your roof, or the carefully lined airport runway, asphalt is everywhere.

But have you ever wondered, “What is asphalt made of?” It’s not just a simple mixture of rocks and black goo. The composition of asphalt is a complex blend of carefully selected materials, and understanding its makeup can help us appreciate its role even more. In this blog, we’re going to explore the origins of asphalt, its fundamental components, the production process, and its diverse applications. By the time we’re through, you’ll have a clear understanding of what makes asphalt the versatile and essential material that it is.

The Basic Ingredients of Asphalt

Deconstructing the Black Gold

Asphalt is a careful balance of key materials that give it its unique properties. Understanding these components is essential to grasp what asphalt is truly composed of. So, let’s dive into the fundamental elements that make up asphalt.

Bitumen: The Primary Ingredient

Bitumen, often referred to as “asphalt cement” or “asphalt binders,” is the primary component of asphalt. It’s the black, sticky, and highly viscous substance that gives asphalt its distinctive properties. Bitumen is derived from crude oil through a process called fractional distillation. This process separates bitumen from other hydrocarbons, leaving behind a dense, thick liquid that forms the base of asphalt.

Bitumen is crucial in asphalt production as it binds the aggregate materials together, making the final result waterproof and flexible. It allows asphalt to withstand the stresses of traffic and the elements while maintaining its structural integrity.

Aggregates: The Building Blocks

In addition to bitumen, asphalt contains aggregates, which are a blend of crushed rocks, sand, and gravel. Aggregates are carefully selected for their size and properties. The combination of different aggregate sizes and types contributes to the strength and durability of the asphalt mixture.

The aggregates in asphalt do several things. They provide structural support, enhance the load-bearing capacity, and improve skid resistance. The choice of aggregates can vary based on the specific requirements of the asphalt mix, whether it’s for road construction, roofing, or other applications.

Fillers: Enhancing the Mix

Fillers are a lesser-known but equally important part of asphalt. These fine materials, such as limestone dust or hydrated lime, are added to the mixture to improve its properties. Fillers act as mineral reinforcements, strengthening the asphalt and enhancing its resistance to aging and deformation.

By adjusting the number and type of fillers, asphalt engineers can fine-tune the performance of the material to meet the desired specifications. The result is asphalt that can withstand heavy traffic, hot and cold temperature fluctuations, and the wear and tear of everyday use.

Now that you know the basic ingredients of asphalt, let’s take a closer look at each of them. First, let’s talk about bitumen.

Bitumen: The Heart of Asphalt

The Binding Force in Asphalt Mixes

At the core of every asphalt mixture is bitumen, a thick, black, and highly viscous material that serves as the glue holding the various components together. Let’s explore what makes bitumen a unique material and its role in creating durable asphalt surfaces.

Bitumen Composition

As we mentioned before, bitumen is often called “asphalt cement” or “asphalt binder.” It’s derived from crude oil through fractional distillation. During this process, crude oil is heated to separate its various components by weight and boiling point. Bitumen is separated from lighter hydrocarbons, resulting in a dense and thick liquid with extraordinary adhesive properties.

The composition of bitumen primarily consists of hydrocarbons, which are molecules made up of hydrogen and carbon atoms. Bitumen’s unique chemical structure, with long carbon chains and a high carbon-to-hydrogen ratio, gives it its remarkable characteristics.

Key Bitumen Properties

Bitumen possesses several key properties that make it ideal for use in asphalt mixes:

  • Viscosity: Bitumen has high viscosity, which means it is thick and sticky at room temperature. This property is crucial for binding the aggregate materials together and creating a durable asphalt surface.
  • Durability: Bitumen has excellent resistance to environmental factors such as water, sunlight, and temperature fluctuations. It helps protect the aggregate materials from breaking down, improving the longevity of asphalt surfaces.
  • Elasticity: Bitumen’s elasticity allows asphalt surfaces to flex and deform under the weight of traffic, making it an ideal material for roads. It can return to its original shape after deformation, preventing cracks and deformations.
  • Adhesion: Bitumen has a natural adhesive property that enables it to effectively bond with aggregates. This adhesion ensures that the asphalt mixture remains cohesive and provides a strong, durable surface.

Types of Bitumen

Bitumen comes in different grades, each tailored to specific applications. Common bitumen varieties include:

  • Penetration-Grade Bitumen: This type of bitumen is categorized by its penetration depth when a standard needle is applied to it under specific conditions. Penetration-grade bitumen is often used in road construction.
  • Viscosity-Grade Bitumen: Viscosity-grade bitumen is defined by its viscosity at a particular temperature. It’s commonly used for roofing projects.
  • Performance-Grade Bitumen: This classification is based on the desired performance characteristics of bitumen in specific conditions. Performance-grade bitumen is used in high-stress areas like airports and heavy traffic roads.

Bitumen’s versatility and adaptability make it the primary component of asphalt, holding the entire mixture together while providing crucial protective and adhesive qualities. It’s the secret ingredient that transforms loose aggregates into durable roadways and surfaces.

Next, let’s take a look at aggregates and fillers.

Aggregates and Fillers: The Building Blocks

Structural Integrity and Support in Asphalt Mixes

Bitumen is important as a binder, but equally vital are the aggregates and fillers that form the structural backbone of asphalt. Let’s dive into the role of aggregates and fillers, which are the building blocks of asphalt mixtures.

Aggregates: The Foundation of Strength

Aggregates are a diverse group of materials that play a huge role in the composition of asphalt. These materials, which include crushed rocks, sand, and gravel, are carefully picked based on their size, shape, and quality. The choice of aggregates directly impacts the strength and durability of the final asphalt mixture.

  • Aggregate Sizes: Aggregates are categorized into various sizes, from coarse to fine. Coarse aggregates, like crushed stone, provide structural support and help distribute the load across the asphalt pavement. Fine aggregates, such as sand, fill in the spaces between the coarse aggregates, enhancing the overall cohesion of the mixture.
  • Aggregate Types: Different types of aggregates may be used based on the specific requirements of the construction project. Limestone, granite, and basalt are commonly used in asphalt mixes, each offering unique characteristics to meet the desired performance criteria.
  • Skid Resistance: The combination of aggregate sizes influences the skid resistance of the asphalt pavement. Proper gradation of aggregates helps create a surface that provides sufficient traction for vehicles, enhancing safety.

Fillers: Enhancing Asphalt Properties

Fillers are often an unsung hero in asphalt production. These fine materials, which can include limestone dust or hydrated lime, are used to modify the properties of the asphalt mixture. Fillers offer several advantages:

  • Enhanced Mixture Properties: Fillers help improve the workability, cohesion, and durability of the asphalt mix. They play a big role in preventing segregation, where coarse and fine aggregates separate during transportation and placement.
  • Aging Resistance: Fillers help improve the long-term durability of asphalt by enhancing its resistance to aging and deformation. They help maintain the structural integrity of the asphalt pavement over time.
  • Environmental Benefits: In some cases, the use of fillers can lead to environmental benefits by reducing the need for additional bitumen or aggregates. This conserves resources.

The combination of aggregates and fillers is a science in itself. Engineers carefully select and blend these materials to create an asphalt mixture that can withstand the stresses of traffic and environmental factors while maintaining its structural integrity.

The Asphalt Production Process

Mixing the Ingredients: How Asphalt is Created

Now that we’ve explored the key components of asphalt, let’s uncover the process of how these elements are combined to create the impressive material that paves our roads and seals our roofs. So, how exactly is asphalt created?

1. Bitumen Extraction and Refinement

The asphalt production process starts deep beneath the earth’s surface. Bitumen is derived from crude oil through a process called fractional distillation.

Once extracted, bitumen undergoes further refining to remove impurities, making it suitable for asphalt production. This refined bitumen serves as the glue that binds the other components together.

2. Aggregate Selection and Preparation

Aggregates, the structural elements of asphalt, are carefully selected and prepared to meet specific engineering requirements. These aggregates typically include crushed rocks, sand, and gravel of various sizes and types.

The selection of aggregates depends on factors such as the intended use of the asphalt mix, the local climate, and the desired durability. Engineers work with quarries and suppliers to obtain aggregates that meet precise specifications.

3. Mixing and Heating

The heart of asphalt production is the mixing process. Here, the refined bitumen and aggregates are combined in precisely determined proportions. The mixture is heated to high temperatures to allow the bitumen to become fluid and thoroughly coat the aggregates.

The heating process is crucial, as it ensures that the bitumen can effectively bind the aggregates together and provide the necessary structural integrity. Temperature control is a key factor, as the proper temperature range is essential for quality asphalt production.

4. Quality Assurance

To ensure the quality of the asphalt mix, rigorous testing is conducted during and after the production process. This includes assessing the temperature, gradation, and composition of the mix. Quality control measures help guarantee that the final product meets the required specifications for the construction projects the asphalt will be used in.

5. Transportation and Paving

Once the asphalt mix is prepared and quality-tested, it is transported to the construction site. Asphalt paving machines lay the mixture in a uniform layer, which is then compacted to create a smooth and even surface. This is the process you witness when a new road is constructed or an old one is resurfaced.

The production of asphalt is truly a blend of science and engineering. The meticulous selection of materials, precise mixture proportions, and temperature control are essential to ensure the durability, skid resistance, and longevity of the asphalt pavement.

Contact Tolliver & Curl for Your Next Asphalt Project

Asphalt Parking Lots, Pavement, Maintenance & More

We hope you’ve gained a deeper understanding of what asphalt is made of and how its composition can be fine-tuned for different construction projects.

At Tolliver & Curl, we take pride in working with asphalt every day. We’re not just in the business of paving roads; we’re in the business of connecting communities and ensuring the safe and efficient movement of people and goods. Asphalt is at the heart of what we do, and we’re committed to delivering the highest quality asphalt solutions for your construction projects.

Ready to start your next asphalt construction project? Contact us today and discover how we can bring your vision to life with asphalt. Together, we’ll create surfaces that not only stand the test of time but also enrich your community.