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. You may not be a quarry expert at the end, but you will understand the basics for your next concrete or hardscaping project!
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.
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 dont want large stones in ready mix concrete, and you dont 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.
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.
An example of this often occurs when a road is being replaced or resurfaced. 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.
The exact amount of crushed stone recycling is unknown due to a lack of reporting. 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.
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.
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 or heat or pressure.
If you go to a creek or river, you 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.
Pea gravel: Pea gravel is some of the smallest gravel - typically 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 thinking of construction, it is important to know what kinds of rock are ideal for specific applications. After all, if a rock type crumbles easily under pressure, you dont want to use it as a component in ready mix concrete or pavement.
Granite: An igneous rock that is durable and is 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.
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.
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 places that need aesthetic appeal, such as curbing or decorative stone.
Crushed stone: If you hear the generic crushed stone term, it 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.
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.
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.
Stone dust compacts and hardens, especially when it becomes wet. 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.
If you want to learn more about ready mix concrete and preparing for it, read our blog post on the Beginner's guide to concrete and the complete guide to pouring concrete in different types of weather.Get in Touch with Mechanic