If you're interested in home improvement projects, you may be intimidated by the more complex-seeming projects that involve concrete, stone, or other construction materials.
But there's no need to be!
Here at Gra-rock, we provide supplies for construction projects, and we'd love to help you familiarize yourself with one of the most important construction materials: crushed stone.
We'll take a deep dive into the different 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!
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 into smaller pieces.
Most crushed stone is produced in quarries. Instead of being shaped or formed naturally, such as in a riverbed or canyon, crushed stone is produced through man-made processes.
So what does this stone-crushing process look like?
It begins with a rock crusher in a place where there are plenty of large rocks, such as a quarry. 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.
The crushed stone then goes 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 most contractors need 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.
You can even purchase bags of stone, whether it is for construction or decoration, from Lowes and Home Depot.
Has crushed stone always been used widely for construction?
Actually, no.
Crushed stone did not become a staple in construction until after World War 2 when inventors created the equipment needed to crush and move stone efficiently.
Because large stones and quarries are hard on tires, crushed stone was challenging to make and transport until heavy machinery with tracks arrived on the scene. WW2 expedited the development of this type of machinery, and crushed stone began to be used widely 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 suddenly all required large quantities of crushed stone.
However, while crushed stone is handy, some people are concerned about its widespread use.
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, construction companies often use recycled construction products as a replacement for crushed stone.
For instance, when a road needs to be resurfaced or replaced, many road construction companies grind and crush the existing road as they remove it, and the ground blacktop is then used as the base for the new road. This process saves time and money and has less environmental impact.
Due to a lack of reporting, we don't know the exact amount of crushed stone recycling. 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 these two materials don't occur in the same way. While crushed stone is a product of human manufacturing, gravel occurs through natural processes.
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 because of 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 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 simply becomes crushed stone.
It is important to note that some contractors refer to any stone within 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 its 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 layered 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 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 among 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 explore 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 rock types 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 because you want the rock to bond 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. It's also used for masonry projects.
Granite: An durable, easily polished igneous rock. Because of its color, grain, and ability to be polished, it's often used inside homes for countertops or on the outside of monumental or civic buildings. However, it can also be used on bridge piers and river walls.
Limestone: A sedimentary rock and the most commonly used rock type for 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 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, statuary, 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?
After all, this is important because you'll need to know what type of stone to order for your project.
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 classified as clean, it has been screened to remove the majority of the stone dust, 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, especially with bare feet, than jagged crushed stone.
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 and shiny.
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 you don't put fabric 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.
Crushed stone is commonly used as aggregate 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, such as a driveway, for instance.
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.
Conclusion
Although we will gradually use less crushed stone in favor of recycled materials, crushed stone and gravel will continue to be staples in construction, decoration, and industry for years to come.
And now that you know the basics of crushed stones, such as what they look like and how they are used in construction, you'll be well-informed the next time you visit a quarry or a canyon or see bags of stones at the local hardware store.
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