Hot mix asphalt, which is also referred to as hot mix, HMA, bitumen, and blacktop, is the most commonly used flexible pavement here in the United States.

You see it everywhere from asphalt driveways to roads and parking lots—but have you ever wondered what it is and how it's made? If you have, then you've come to the right place.

In this post, we're going to cover everything and more about hot asphalt mix, it's variations, uses, and manufacturing. Keep reading to learn more. 

Hot Mix Asphalt 101

Asphalt pavement, in general, is a mixture of sand, cement, and both fine and coarse aggregated stone. It's also petroleum-based. Hot mix asphalt is known to be flexible, water-repelling, and weather-resistant—which is what makes it the most commonly used type of asphalt mixture for roads, highways, and interstates. 

The hot mix asphalt (HMA) is designated by the temperatures at which it is heated and poured. Those temperatures should be anywhere between 300 and 350 degrees Fahrenheit. This type of asphalt is inclined to flash-cooling, which is why it's only used when the outside temperature is at 40 degrees Fahrenheit or above.

There are three different categories in which Hot mix asphalt can be classified:

Dense-Graded Mixes

Dense-graded mixes are the most popular because of their water-repelling capabilities. These mixes are actually categorized by their aggregated-sizes, which are known as fine-graded and coarse-graded mixtures. 

If the final product contains a higher percentage of sand and smaller stones, it will fall into the fine-graded category. Of course, a higher percentage of coarse and larger materials will categorize it as a coarse-graded mixture. 

These types of mixtures are suitable for all weather and traffic conditions because of its performance under construction conditions, friction, as well as the type of road surface it provides and its ease of repairing.

Open-Graded Mixes

This type of mixture is also categorized into two sub-categories. In general, this type of hot mix asphalt is constructed with only crushed stones and minimal grains of sand. The two sub-categories are known as open-graded friction course and asphalt treated permeable-bases.

The open-graded friction course mixture is only used for surface courses thanks to its smoother finish compared to the dense-graded mixes. It requires that air voids are kept at a minimum of fifteen percent, with no true maximum air voids. There is a high cost in producing this mixture, however, placement costs are considerably low. 

Asphalt treated permeable-bases is used as a base layer for drainage. It's usually laid beneath dense-grade mixtures, stone mix, and cement concrete—but never by itself.

Stone Matrix Asphalt

Also referred to as stone mix asphalt, this mixture was created in Europe and designed for durability as well as maximum rutting resistance. It's made up of higher asphalt content, certain types of fibers, and modified asphalt binders.

This stuff was introduced to the United States in 1988 and can be used for a variety of roads as well as driveways. However, due to its design and production process, it's much more expensive than other hot asphalt mixes.

It's typically recommended for interstate and high-volume road use because of the durability and friction it provides for drivers, which increases their safety on the road. It also reduces road noise as well as reflective cracking—and it's long-lasting, which makes it worth the price.

Why Asphalt Temperature and Timing Matter

The type of asphalt mixture isn't the only thing that matters—ambient temperatures, as well as ground temperatures, play a large role in hot mix asphalt applications. And above all, timing is everything.

The Mixing Temperatures

As mentioned earlier, hot mix asphalt has to be mixed and poured at a specific temperature because it has the capacity to cool quickly. When it arrives at the job site, it is already between 275 and 300 degrees Fahrenheit. It should be heated and poured between 300 and 350 degrees Fahrenheit and between 220 and 290 degrees Fahrenheit for the initial rolling.

Upon the final compaction, the mixture cannot fall under 185 degrees Fahrenheit, otherwise, it will be too thick and stubborn to compact and smooth out. The actual length of time in terms of compaction depends on the following: the thickness of the layers poured, the ground temperature, the ambient temperature, and the wind.

The Ambient and Ground Temperatures

Again, the outdoor temperature must be at least 40 degrees Fahrenheit—and rising—while the layers of asphalt are being poured and compacted. On the more windy days, it's preferable to begin with an ambient temperature of 50 degrees Fahrenheit and rising since the wind will cool the hot mix quicker.

The ground temperature is considered the base temperature since it's where the crew will be pouring the asphalt. This temperature needs to be at 50 degrees Fahrenheit and rising to begin pouring the asphalt.

Without the right mixture, ambient, and ground temperatures, the hot mix asphalt cannot be sufficiently compacted—which is also where timing and experienced contractors come in. Even in the right conditions, the crew must compact the poured asphalt within a certain amount of time. Otherwise, the new pavement will not result in the right density or surfacing. 

Schedule an Asphalt Paving Today

When it comes to hot mix asphalt, having a professional and experienced crew for the job is crucial. That's where we come in. Our crew can handle all of your commercial and/or residential asphalt paving and resurfacing projects. Contact us today—and while you're at it, request a free estimate.