The Complete Guide to Crushed Stone and Gravel
Updated: Aug 24
Most people don't understand the ins and outs of construction materials. In fact, it's easy to assume that only the experts know anything about it.
This is especially true when it comes to one of the most basic construction materials: stone.
We all know a stone when we see one, and we know they are important in construction, but what specific roles do they play in different construction projects?
In this article, we are going to take a deep dive into the types of crushed stone and gravel, how they are made, and their basic applications. By the end, you will understand the basics for your next concrete or hardscaping project.
Are you ready to learn more about gravel and crushed stone? Let’s get started!
What is crushed stone?
By its simplest definition, “crushed stone” is as basic as it sounds: Stone that has been crushed.
Most crushed stone is produced in quarries and is crushed when machinery breaks up and crushes larger rocks. Instead of being shaped or formed naturally, such as in a riverbed or canyon, crushed stone is produced with man-made machinery and processes.
So what does this stone-crushing process look like?
It begins with using a rock crusher in a quarry or site with plenty of large rocks. There are many types of crushers, but their main job is the same: Crush larger rocks into smaller pieces to be used for construction material.
Crushed stone is then passed through different screeners to be organized and stored in different piles according to their size. The screening process starts by removing larger stones, then medium stones, and eventually goes all the way down to the stone dust.
This screening is important because contractors need very specific types of crushed stone to complete different types of projects. For example, you don’t want large stones in ready-mix concrete, and you don’t want stone dust in drainage systems.
After being sorted into different piles depending on the size of the stone, the stone is ready to be shipped from the quarry. Quarries deliver directly to job sites, to concrete plants, or to wholesale distributors who sell the stone through retail to customers.
So has crushed stone always been used widely for construction?
The simple answer is "no."
Crushed stone did not become a staple in construction until after WW2 for a simple reason: The equipment did not exist to crush or move stone efficiently.
Because large stones and quarries are hard on tires and require heavy metal, crushed stone was hard to make and transport until heavy machinery with tracks was developed. WW2 expedited the development of this machinery, and crushed stone began to be widely used in construction projects in the 1940s and 1950s.
Large-scale building projects, particularly in infrastructure like the Eisenhower Interstate System, helped usher in an era where crushed stone was used in almost every part of construction. Foundations, concrete, drainage systems, and roads were all needing large quantities of crushed stone.
What are the concerns with the widespread use of crushed stone?
Because crushed stone mostly comes from quarries, there has been growing geological and environmental concern over the large number of quarries operating and their long-term effects.
In response to this, construction companies are beginning to use recycled construction products to replace crushed stone.
When a road is being replaced or resurfaced, for instance, many road construction companies are beginning to grind and crush the existing road as they remove it. This crushed road, which is essentially crushed stone, then becomes the base for the new road.
Due to a lack of reporting, the exact amount of crushed stone recycling is unknown. Much of the crushed stone is also recycled right on the construction site, especially with road construction, and this makes it difficult to measure.
The most common use for recycled crushed stone is as a base for roadways, especially when the old road can be torn up, crushed, and reused. Concrete blocks and bricks can also be crushed and recycled as a base.
What is gravel?
Gravel is similar to crushed stone because it is a type of rock, but gravel is produced naturally.
A geological definition of gravel is “a natural material that consists of water-transported materials and usually has a rounded shape as a result of the water transport.”
One of the key differences in the look and feel of crushed stone and gravel is the edges of the stone.
Crushed stone often has an angular and jagged edge that occurs during the crushing process. Gravel, on the other hand, typically has a very smooth texture and surface because of the natural weathering and wear of being exposed to the effects of running water.
Unlike crushed stone, gravel is usually sold and used in its natural state. If it is crushed, gravel loses its unique smooth rounded texture and becomes crushed stone.
It is important to note that some contractors refer to any stone in certain size ranges as “gravel,” even if it is crushed stone.
For example, depending on the region of the United States, crushed stone in the size of a few mm to 2 inches is referred to as “gravel,” even if it has been crushed and is no longer smooth.
To provide clarity and avoid confusion, we will stay with the definition that gravel cannot refer to crushed stone and must remain in a natural state.
There are three types of natural rock and stone: igneous, sedimentary, and metamorphic.
Igneous: Igneous rocks are formed after molten rock or lava solidifies.
Sedimentary: Sedimentary rocks are formed over time by the accumulation of small particles cementing together, often producing layers in the rock.
Metamorphic: Metamorphic rocks become changed through intense heat or pressure. Similar to clay hardening in an oven, metamorphic rocks become very hard and crystallized by intense heat or pressure.
Common types of gravel
If you go to a creek or river, you'll see all types of rocks, both large and small. These larger rocks can be used for foundations or other building projects, but typically gravel is screened and only the smaller pieces are used.
What are the most common types of gravel?
Pea gravel: Pea gravel is some of the smallest gravel—typically ½” or smaller in size. Pea gravel is often used in places like fish tanks, walkways, swimming pools, or other places where foot traffic occurs or small gravel is needed.
When using pea gravel, it is important to have set edges and boundaries for the gravel. Because of its small size, it quickly spreads out unless it has a clear edge.
River rock: River rock is larger than pea gravel and typically used for aesthetic purposes. It is typically 1”-2” in diameter.
Even though river rock usually has rounded edges, it's still larger than pea gravel, which makes it more painful to walk on.
River rock is often used as edging, in decorative landscaping like flower beds, or to line a walkway or driveway.
There are other types of gravel, but pea gravel and river rock are the most common. You can also purchase gravel based on color, but this tends to be more expensive since it takes more work to screen for color than size.
Common types of rock used in construction
There are many types of natural stone and rock all around us. If you dig into the ground, go to a river, or look into a canyon you'll see a wide variety of colors, sizes, and styles.
However, like trees or dirt, not all natural rock is created equally. Some rocks have a softer texture, while some are harder. Different rocks react differently to pressure.
When planning your construction project, it's important to know what kinds of rocks are ideal for specific applications. After all, if a rock type crumbles easily under pressure, you don’t want to use it as a component in ready-mix concrete or pavement.
Different types of rock also bond differently. This is important in ready-mix concrete in which it is imperative the rock bonds correctly with the sand, water, and other components.
So what are some of the most common rocks used in construction, and what are they used for?
Basalt: An igneous rock often used for road pavement or concrete aggregates. They are also used for masonry projects
Granite: An igneous rock that is durable and easily polished. Because of the color, grain, and polishing ability, they are often used inside homes for countertops or on the outside of monumental or civic buildings. However, they can also be used on bridge piers and river walls.
Limestone: A sedimentary rock that is the most commonly used to make crushed stone in the United States. One of the most versatile rocks for construction, limestone is able to be crushed easily making it a primary rock used in ready-mix concrete, road construction, and railroads. It is widely available in quarries across the country.
Sandstone: A sedimentary rock used primarily for concrete and masonry work. It is unsuitable for use as a building stone because of its sediment composition.
Slate: A metamorphic rock typically found in layers. Because it is easily mined and cut in these natural layers, it works well in applications requiring thin rock layers. Common examples are roofing tiles, certain types of chalkboards, gravestones, and some pavement applications.
Laterite: A metamorphic rock with a highly porous and sponge structure. It is easily quarried in block form and used as a building stone. However, it is important to plaster the surface to eliminate the pores.
Marble: A metamorphic rock. Like granite, it can be polished well and is often used for decorative purposes. Common uses are columns, flooring, or steps in monumental buildings.
Gneiss: A metamorphic rock. Due to the harmful components of the rock, it is rarely used in construction, although hard varieties are sometimes used in building construction.
Quartzite: A metamorphic rock that is used in building blocks and slabs. It is also used as an aggregate in ready mix concrete.
As you can see, there are multiple kinds of rocks used for construction!
Many of them have a specific purpose for their use in construction, and if you know what to look for, you'll start seeing these rocks in their different applications.
Labeling crushed stone used in construction
As we already discussed, crushed stone is simply natural rock that is crushed by machinery and then sorted and screened into different groups depending on size and components.
This crushing and screening process is typically done at quarries. Depending on the rock available at a construction site, crushers are hauled directly to the site for use.
So how is crushed stone categorized?
There are several specific terms to know when working with crushed stone. If you know what these terms mean, you can quickly know what kind of stone you are working with or ordering.
Stone dust: this is the very fine dust, similar to sand, that is created as the stone is crushed. Stone dust is useful when tamping or packing stone, but it causes problems for applications where water needs to drain, such as behind a retaining wall.
Clean stone: If crushed stone is clean, it has been screened so the majority of the stone dust has been removed, but some dust is still mixed in. This is useful for the top layer of a stone driveway or other places where some minor compaction is not harmful.
Washed clean stone: This is stone that has been screened like clean stone, but then also washed to ensure there is no stone dust on the finished product. This is often used for drainage purposes, for ready mix concrete, or for places that need aesthetic appeal, such as curbing or decorative stone.
Crushed stone: The term “crushed stone” usually refers to stone that has a mixture of stone dust in it. This type of stone is best used for a base when heavy compaction is needed. As a result, it is typically used for the base of concrete and paving projects, foundations of structures, and driveway bases.
Size: Stones are often referred to in size. For example, 2” stone means the stones used have been screened to be roughly 2” in diameter.
Now, let’s put these terms together!
If we are doing a drain system, we want larger rocks with no stone dust. We would call the quarry and order “4” washed clean stone.”
Or, if we were putting the base down for a patio, we want stone that compacts well and makes a strong base. Therefore, we want our stone to have stone dust, so we would call the quarry and order “2” crushed stone.”
Best applications for gravel
Natural gravel is often used in walkways, driveways, and decorative hardscaping for several reasons.
First, because gravel typically has a smooth rounded edge from water erosion, it is an easier surface to walk on with shoes and even bare feet.
Second, gravel often has aesthetic appeal due to its color and texture. Smooth gravel is nicer to look at than jagged crushed stone, and gravel is usually more colorful.
Gravel, especially river rock or similar types, is also popular in flower beds when mulch or cover plants are not a good option.
When putting down gravel in a flowerbed, make sure you start by laying down a quality landscape fabric, securely stake the fabric in place, and then layer the gravel on top of the fabric, usually 2”-3” thick.
If the fabric is not put down first, grass and weeds quickly grow up among the gravel resulting in a lot of maintenance and a weedy flower bed.
Like crushed stone, gravel can be used as an aggregate for pavement, ready-mix concrete, or other construction applications.
Best applications for crushed stone
We have already referenced many places where crushed stone is used, but the most common are construction projects that need concrete, solid bases, or drainage systems.
Parts of crushed stone are commonly used as aggregates for ready mix concrete. Concrete plants keep large quantities of different parts of crushed stone on hand to make their concrete batches.
Crushed stone with stone dust in it is highly compactable and is therefore commonly used when making any kind of base for construction.
Whether it is a road, driveway, building foundation, patio base, retaining wall base, or other projects needing a solid foundation, you typically find crushed stone at the bottom.
Crushed stone that has been cleaned and washed is typical for drainage systems.
Why can’t regular crushed stone with stone dust be used for drains?
When stone dust becomes wet it compacts and hardens. Since drains need to always be open, it is important to keep stone dust out of drains. Therefore, construction projects needing drainage systems make sure they use only stone that has been cleaned and washed.
Crushed stone and gravel will continue to be a staple in construction, decoration, and industry for years to come. As recycling picks up, mining and quarries may slow down, but we will always need crushed stone in general construction and industry.
You don’t need to be a geological or quarry expert to know some of the basics of these products, such as what they look like or how they are used in construction.
So next time you visit a quarry, canyon, or see bags of stones at the local hardware store, see if you know what stone it is and how it is being used!
If you want to learn more about ready mix concrete and preparing for it, read our blog post on the Beginner's Guide to Ready-Mixed Concrete, Everyone's Guide to pouring Concrete in Any Weather, and Understanding Concrete Strength.
Contact us today for more information on how we can help you with your construction project!