In simple terms, aggregates are rocks. However, they're more than that. Aggregate is a valuable resource for county road supervisors and private companies in industries like construction, extraction, and agriculture. 

Let’s dig into what aggregate is, what it’s used for, and the different types. 

What Is Aggregate? 

Aggregate is rock grains or fragments that have broken off from a larger “parent” rock. Aggregates can be any color, but they’re usually white, brown, gray, or black. They come in many shapes and sizes (more on that soon).  

However, not all parent rocks make good aggregate for human use. Some rocks are too chemically reactive, breakable, or soft. 

Common parent rocks that produce good aggregate include traprock, dolomite, limestone, granite, sand, basalt, schist, hornfels, quartzite, and flint. In seaside locations, you may see “shell aggregate,” which is broken seashells used for paving and construction.

How Is Aggregate Made? 

Some aggregates form when wind and water erode the parent rock. For example, river rock occurs naturally. 

However, most aggregate in roads and other infrastructure is man-made. Miners extract large chunks of rock from the earth using machines and explosives. Then, they send it to processing plants, where crushers break the rocks into smaller sizes. The crusher type and speed help determine what type of aggregate the plant produces. Conveyor belts transport the crushed rock to storage areas or haul trucks.

Fun fact: Yearly worldwide aggregate production totals over 16.5 billion tons!1

Where Does Aggregate Come From? 

Aggregates come from all over the world. Different regions produce different rocks. For example, Indiana has over 240 limestone quarries.2 New Hampshire has rich granite deposits—hence why it’s called The Granite State.3 

Hauling aggregate long distances is expensive, so most counties and companies buy aggregate from local or regional sources. However, builders sometimes ship aggregate long distances for special projects; the Empire State Building contains limestone from Indiana.4

What Is Aggregate Used for? 

Aggregates help us build most of the world’s infrastructure and make many household products. We use aggregate rock in:

Beyond construction and industry, you’ll even find aggregate in toothpaste, tape, paint, and flour! 

Types of Aggregate

In addition to the type of parent rock, rock sizes determine what type of aggregate you’re dealing with. Aggregates range in size from specks less than one-tenth of a millimeter wide to boulders more than a foot across. Generally, they fall into two categories: fine and coarse aggregate. 

Fine Aggregate

Fine aggregates, or fines for short, are tiny rock grains. The majority of fines in a given sample must measure 4.75mm (0.19 inches) or less in diameter for the sample to qualify as a fine aggregate.5

Sand is a fine aggregate. Many sand grains measure just one millimeter apiece—about the width of a sewing needle. Sand makes good fill material, and it’s easy to compact when wet. However, it’s an unstable building surface because it shifts and settles over time. 

Limestone dust is another fine aggregate. As crushers break limestone into coarse aggregate, some of it crumbles instead, producing this powdery white or gray substance.  

Coarse Aggregate

Coarse aggregates are rocks over 4.75mm (0.19 inches) in size.6 Most aggregates you’ll work with in county road management, mining, construction, and so on are coarse.  

Gravel is a coarse aggregate that ranges in size from about 0.1 inches to around 10 inches across.7 It tends to have rounded edges, so gravel is a good driving surface for unpaved low-cost rural roads.  

Crushed stone is a coarse aggregate that’s more jagged than gravel and ranges from three-eighths of an inch to 1.5 inches across.8 Type II aggregate is crushed stone that’s a popular subbase for paved roads, because it’s strong and compacts well. 

Slag aggregate is a byproduct of iron and steel smelting; removing iron and steel from the parent rocks leaves rock pieces behind. Slag is good for high-performance concrete, especially near water, because it’s not very permeable and resists chlorides. Research suggests slag can also reduce soil acidity and excess phosphate in water.9 

Sizing Rocks with the AASHTO T27

Rock sizes determine aggregate type, which begs the question, “How does one size a lot of rocks in a little time?” Civil engineers use sieve analysis tests like the ASTM C136 and the AASHTO T27. For now, we’ll focus on the AASHTO T27. 

What Is an AASHTO T27 Sieve Analysis? 

AASHTO stands for the American Association of Highway and Transportation Officials. They’re a nationally recognized non-profit that standardizes the different phases of highway construction and development to ensure road building materials in the U.S. will safely hold up to traffic. AASHTO also standardizes tests that civil engineers use to ensure counties and companies comply with those requirements.  

The T27 is a sieve analysis test. It determines the rock sizes in a given aggregate sample to make sure most of the sample is the right size for the job that the project owner wants it to do. This test is essential because aggregate producers can’t make perfectly identical rock sizes. Rather, they create approximate sizes. The T27 sieve analysis determines the distribution—or aggregate gradation—of the different rock sizes in the sample.

The four aggregate gradations are:

    • Dense: With a wide variety of rock sizes, these work best for road bases.
    • Uniform: Most aggregate rocks are close to the same size.
    • Open: With many large rocks and few fines, open graded aggregates have more air gaps.10
    • Gap: Aggregates in the sample are mostly large and small with few mid-size rocks, creating a “gap” in sizes.11

How to Conduct a Sieve Analysis

The AASHTO T27 has highly specific rules, so we’ll just give a brief overview of how to conduct a sieve analysis here.

First, let’s start with sieves. These round metal components look like cake baking pans, and common sizes are 8” to 12” across. For large coarse aggregates, civil engineers use screening trays that are generally at least 12” x 24”. Instead of solid bottoms, sieves and screening trays have screens with different sized holes. A #200 sieve screen has holes that are 74 micrometers (0.003 inches) wide.12 A #4 sieve’s holes are 4.75 millimeters (0.19 inches).13 

Now, on to the test! Here’s how a sieve analysis works: 

  1. Collect and weigh an aggregate sample. 
  2. Dry the sample to prevent false readings from excess moisture. 
  3. Remove rocks that are the size of grapes or larger. 
  4. Grind soil. 
  5. Combine materials and re-weigh the sample.
  6. Prepare the screen tower with the largest screened sieves on top, smallest at the bottom. 
  7. Add the sample to the sieves.
  8. Place the screen tower into the sieve machine for five minutes; it’ll shake the sieves so fines migrate onto lower sieves, while coarse aggregates stay on the upper sieves.
  9. Weigh each screen to determine the aggregate gradation. 

Civil engineers represent that data using a gradation curve, which they compare to standard gradation curves from AASHTO. If their gradation curve matches the ones from AASHTO for the same type of project, they know the aggregate meets safety and durability requirements for their project, too. 

Pros and Cons of Aggregate

Some types of aggregate are nearly as strong as steel. They're hard and durable, so they make an excellent base for paved roads, especially high-traffic ones. Aggregate with coarse shapes helps cement and asphalt bind properly. It also gives traction on unpaved or icy roads. 

However, some aggregate rocks are brittle and easy to break. Aggregate on unpaved roads eventually washes away, migrates to the shoulder, or works itself into the soil. Counties use various soil stabilizers to make gravel roads last longer, like chlorides, concrete, and geotechnical textiles. 

One highly effective option for soil stabilization is Perma-Zyme. Its enzymes chemically react with clay in the soil, causing the clay particles to permanently bind to each other and bind around the aggregate rocks. This creates a strong, concrete-like surface that lasts 10 or more years with little to no maintenance. 


Aggregates are rock grains or fragments that have broken off a larger parent rock like dolomite, limestone, granite, or many others. Some form naturally via erosion, but most aggregates are man-made through mining and crushing. We use aggregates in everything from road construction to household items. 

We classify the types of aggregates as fine or coarse based on rock size. The AASHTO T27 sieve analysis helps civil engineers determine rock sizes and aggregate gradation, so they can make sure that aggregate can hold up to a specific project. 

Aggregates are strong and durable, but they can shift, erode, or wash away over time—especially on unpaved roads. Perma-Zyme is the ideal stabilizer for gravel roads and other areas where aggregates mix with clay-based soils. It’s 100% organic, natural, and non-toxic, so it safely stabilizes the soil and holds gravel in place with no harm to the environment. 



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