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Over the years, the road-building community has developed its own technical shorthand, which can be helpful when you speak it fluently but occasionally bewildering when two terms sound almost interchangeable. Prime coat and tack coat are a classic example: most municipal and private-sector road managers have heard both names, yet the exact role each treatment plays can blur together.

At a glance, both products look similar, but their similarities mask two distinct jobs. A prime coat is applied to unbound or granular base material, while a tack coat is sprayed between existing asphalt layers. Because the materials overlap, asking which treatment  goes where can feel counterintuitive, but understanding the difference is essential when you’re reviewing specs from your civil engineer, negotiating contracts for surface treatments, or troubleshooting premature slippage and delamination in the field.

RoadResource exists to make those conversations easier. In this article, we’ll lay out exactly how prime and tack coats are formulated, when they should (and shouldn’t) be used, and the performance payoffs you can expect when each layer is applied correctly. Armed with that knowledge, you can advocate for construction methods that work and protect every dollar of your pavement budget.

What is a Prime Coat?

A prime coat is typically a low-viscosity asphalt mixture that serves as the foundation of a roadway. It can be either a cutback or an emulsion, though emulsions are the more sustainable option due to their lower environmental impact. This mixture is applied over granular aggregate bases, which are often made of crushed stone, gravel, or similar materials.

As the first layer in the pavement structure, the prime coat sits near the bottom of the road, where it acts as the initial foundation. The challenge with granular aggregates is that they are loose and porous, which means they don’t naturally hold together well. This is where the prime coat comes in; it penetrates the top layer of the aggregate and acts as a binding agent, sticking the loose materials together to form a more stable base.

In addition to strengthening the base, the prime coat helps bond the bottom layer to the next layer of asphalt, ensuring a solid connection for long-term durability. Prime coats also seal voids or holes in the granular base to prevent moisture from accumulating and weakening the road structure. Essentially, prime coats act as a waterproof barrier, protecting the granular base from the damaging effects of water and moisture infiltration. Its waterproofing properties can significantly extend the lifespan of the pavement, helping maintain the integrity of the roadway for years to come.

When is Prime Coat Typically Used?

Prime coats are generally applied at the beginning stages of road construction and reclamation projects. They are placed over a loose aggregate base before any hot-mix asphalt (HMA) is applied, providing a stable foundation. Additionally, they are used prior to the application of chip seals, which can also be used to construct low-volume roadways.

Prime coats are especially effective in full-depth reclamation (FDR) projects. In FDR, the existing road material is reclaimed and recycled down to the base layer, which is then rebuilt with new asphalt. The prime coat ensures the new layers bond effectively with the reclaimed base material, resulting in a more durable, long-lasting road structure.

The asphalt mixtures used for prime coats often include MC-30 or MC-70 cutbacks, or AE-P emulsions, each providing the right balance of penetration and bonding for the job at hand. These materials ensure the prime coat can effectively penetrate the granular base and create a solid foundation for the next stages of construction.

What is a Tack Coat?

A tack coat is an asphalt mixture that binds two layers of HMA lifts together. It plays a crucial role in promoting adhesion between old and new asphalt layers, particularly during pavement preservation projects. Applying a tack coat ensures that the layers of asphalt act as a cohesive unit, distributing weight evenly across the road. This reduces the risk of slippage and delamination between layers, ultimately extending the lifespan of the roadway. Because the primary function of a tack coat is to bond layers of asphalt together, it is often referred to as a bond coat.

In addition to improving the bond between asphalt layers, tack coats also provide waterproofing properties. They help prevent moisture from accumulating between the layers of asphalt, which could otherwise lead to road degradation from environmental factors, including the freeze-thaw cycle. By keeping moisture at bay, tack coats protect the road from the damaging effects of water infiltration, further safeguarding its durability over time.

When is Tack Coat Typically Used?

Tack coats are an essential part of nearly every road construction, recycling, and preservation project due to their primary function of binding asphalt layers together. It is commonly applied not only in new construction but also in road maintenance and reclamation efforts.

Tack coats are often used alongside various road treatments such as hot-mix asphalt (HMA) lifts, cape seals, hot-in-place (HIR), and cold-in-place (CIR) recycling methods. They are also vital in full-depth reclamation (FDR) projects. In some cases, especially with very dry or cracked pavement surfaces, micro surfacing treatments may also require a tack coat to ensure proper adhesion. Additionally, because prime coats   serve as a base layer made of asphalt mixtures, tack coat is used to bond this layer to the new asphalt placed above it. This ensures a strong connection between the different layers, enhancing the road’s overall durability.

Tack coats typically consist of asphalt emulsion mixtures, with non-tacking tack (NTT), SS-1, and CSS-1h  being the most common formulations used for these applications.

Key Similarities and Differences Between the Two Treatments

Prime coats and tack coats are similar in many ways, but they each serve distinct roles in road construction and preservation. Both act as binding agents between layers, helping stabilize the structure and reduce issues like slippage and raveling. By allowing road layers to function as a cohesive unit, both treatments help distribute weight evenly across the road surface.

Both prime and tack coats also offer waterproofing properties. They prevent moisture from infiltrating the road layers, which can cause significant damage over time. These layers are often used together, as tack coats are typically applied after the prime coat to bond new asphalt to the prime coat, ensuring the layers stick together effectively. Together, they create a unified system that binds all road layers into one solid structure.

Despite their similarities, there are key differences in their applications. Prime coats are exclusively used to bind loose aggregate bases to the above asphalt layers. It is applied first and is designed to penetrate, bind, and seal the base layer, providing a stable foundation for the subsequent asphalt layers. Prime coats are especially useful for new construction or major preservation treatments such as full-depth reclamation, where the road is essentially rebuilt from its base.  Importantly, prime coats must use slow-setting, stable, and low-viscosity emulsions to ensure proper penetration and bonding, characteristics not required for tack coats.

Tack coats, on the other hand, are primarily used to bond asphalt layers to each other. This includes binding old asphalt to new asphalt or bonding the prime coat to new asphalt layers. Because it acts as an intermediary layer, a tack coat is much more commonly used in road preservation and recycling projects.  Unlike prime coats, tack coats can utilize a broader range of emulsions, including rapid-setting types, depending on the project requirements.

Learn More About Cost-Saving Road Preservation and Recycling Treatments with RoadResource

Both prime and tack coats play essential roles in helping pavement function more efficiently. By preventing moisture build-up and ensuring that all layers of the road work as a cohesive unit, both treatments significantly extend the life of your roadways. While they may have unique functions and applications in different treatments, their ultimate goal is the same: to help you get the most out of your road treatments and maximize the longevity of your infrastructure.

At RoadResource, we’re committed to helping road managers identify the most efficient and cost-effective solutions to increase the performance of existing roads, all while staying within budget. Road preservation and recycling treatments are much more economical than full rebuilds, which are often necessary when regular maintenance is neglected.

To learn more about the various treatment options available and find the best fit for your next project, explore the wealth of information available on our website. With the right knowledge, you can make informed decisions that will save your municipality time and money while enhancing the lifespan of your roads.