In the paving world, your final product is only as good as what you put into it. For asphalt, this means that the quality of a pavement can be correlated to the quality of the asphalt cement and aggregate it is composed of. Previously, we’ve taken a look at the variations in asphalt binder and how these dictate its use. In this week’s RoadReady Newsletter, we’re going to explore the other side of the coin and discuss how different aggregate properties affect the characteristics of a finished asphalt pavement.
In a typical asphalt, aggregates can represent up to around 96% of the total mass, so aggregate quality has a very direct impact on the final quality of the pavement. While asphalt cements are manufactured from crude oil, and can be modified to meet specifications, aggregates are largely dependent on their natural properties and present a different kind of problem. With careful planning, we can select the right kind of rocks to keep us rollin’.
An aggregate quarry
Gradation and Size
One of the most important characteristics to look for when selecting aggregate is the size of individual particles. Because individual particles vary in size, we more commonly refer to aggregate gradation, or the distribution of particle size within a collection of aggregate. If the largest size particles in a mix are too small, the mix can be unstable. However, large aggregate will make the mix difficult to work with. In addition to the maximum size of aggregate particles important gradation properties include the amount and size of the finest particles, and the proportion represented by each size category.
The density of an aggregate is a direct result of its gradation. The distribution of particle size determines how particles will fit together, and what air voids will exist. For instance, you can imagine an aggregate that consists of only large rocks will contain a high volume of space between the rocks. As smaller and smaller particles are added to the mix, these spaces are filled and overall density of the mix increases. Typically, we want to achieve a high density in our asphalt pavements designed for higher volumes. This increases the surface contact between particles, which creates a stronger more stable pavement and allows less water to infiltrate the pavements which can cause damage. With this in mind however, a certain amount of air voids are required to leave space for the asphalt binder. While dense-graded aggregate is desirable for many paving jobs, specific types of asphalt may warrant use of other gradations. For instance, permeable pavements are created using aggregates with higher percentages of air voids. This is achieved by reducing the amount of fine aggregate in the blend.
For the above reasons, aggregate gradation and size is closely controlled in asphalt mixes. Specific size ranges of aggregates are commonly separated and stockpiled, allowing blends to be created with specific gradations. Aggregates can also be crushed to reduce the maximum size of larger particles. Crushing has additional benefits which we will discuss later.
Aggregate faces can be classified as angular or non-angular
In addition to size, aggregate shape influences its utility as a component of asphalt pavement. Because aggregate represents such a large portion of the overall pavement structure, the interaction between individual particles is a large factor in pavement strength. In general, angular particles create a stronger interlocked matrix when compared to rounded particles. While this makes placement and compaction of the asphalt more difficult, asphalt containing angular aggregate is less susceptible to shoving and rutting and is generally more desirable. Like gradation and size, particle shape can also be modified and improved for use in asphalt by crushing, which creates more fractured and angular surfaces in the aggregate. Apart from angularity, other important shape considerations include the ratio of width to length of particles in the aggregate. Long skinny particles are likely to break during compaction, reducing the strength of the finished pavement.
Aggregates are kept in stockpiles based on their size
When it comes to asphalt, soft rock just doesn’t cut it. Aggregates in a roadway need to stand up to the construction process, heavy vehicle loads, and freezing and thawing. Before they’re even exposed to traffic loads, aggregate particles see some pretty gnarly action in the form of:
- Construction Processes– Aggregates are stored in large piles that create massive loads at the bottom, and have to withstand some heavy metal in the form of mixing drums at the plant, and compaction devices at the jobsite.
- Freeze-Thaw Cycles-Once the roadway is paved, water can become trapped in the aggregate pores. In colder months, this water can freeze and expand, causing particles to crack and split. Resistance to freeze thaw cycles is known as aggregate durability.
- Polishing – Exposed aggregate on the roadway surface can be polished by traffic. This causes the pavement surface to lose traction and become dangerous. Resistance to polishing is known as toughness.
Finally, aggregate moisture impacts the amount of preparation that aggregate needs before use. The more water that is present in an aggregate sample, the more time and money must be used to dry it at the asphalt plant. Because water weakens the asphalt-aggregate bond, it is essential that it is removed before use in pavement. Moisture is primarily contained in the air voids of aggregate particles. In extreme cases however, water can coat the particle surface as well.
While this is far from an exhaustive list of important aggregate properties, this gives a general idea of the main factors road builders look for and control in their aggregates. Ensuring that your aggregate has all the right characteristics requires performing a number of tests. Click here for a summary of common aggregate tests.
No amount of proper construction practices can make up for a sub-par aggregate on an asphalt project. Make sure you’ve got the right rock to make every roadway a smash-hit.