The construction industry uses different types of stone materials depending on the nature of the project. Knowing the differences between each material, including its pros and cons, will help you choose the best materials for your building project. Crushed stone and gravel are popular construction materials for landscaping, hardscaping, and other construction works.
Though commonly confused for each other, gravel and crushed stones are two different materials, produced differently and with varied uses. Let’s explore common facts about these two materials, their sources, the types of crushed stone and gravel, and their different roles in construction projects.
What is crushed stone?
Crushed stone is produced by breaking down rock material using crushing equipment commonly known as crushers. The crushers mine the original stone from its natural deposit and break it into small angular rocks of different standard sizes depending on the project’s needs.
Angular stone is a highly versatile material with many uses in the construction and landscaping industries. It is relatively inexpensive to produce than gravel and other naturally occurring rock materials. Today, many companies mine and sell different grades of angular rocks to clients worldwide.
Mining crushed stone
Like most other construction stones, crushed angular rocks are mined in quarries and broken down into smaller angular stone pieces using man-made crushing machines.
Once the rock deposits are crushed into small pieces, the crushed stones are fed into a screening machine that sorts the angular according to size.
The larger ones are sorted first, followed by medium and small-sized pieces during the screening process. A typical crushing machine will have different sizes of screen traps which allows smaller rocks to sieve through. The size of the screening holes is then altered until all rock sizes have been sorted from largest to smallest.
Stone dust is the finest material of crushed stone and has many uses in the construction industry. However, its primary purpose is as a bed setter or for pavement layering before placing larger rock pieces.
Once sorted, the angular rock piles are stored according to their sizes before transportation to the construction site. Crushed stone is sourced directly from quarry sites, from where it’s shipped to construction sites and wholesale dealers who later sell the material to clients.
In the United States, you can buy crushed stone in many states. The largest crushed stone-producing states include Texas, Missouri, Florida, Pennsylvania, Kentucky, Illinois, and North Carolina. These states account for over half of the United States’ crushed stone production. However, Mexico, The Bahamas, and Canada are other major crushed stone producers in North America.
Interesting facts about crushed stone
Crushed stone became a popular construction material shortly after the Second World War. Before World War II (WW2), there was a lack of equipment capable of efficiently crushing stones. Before the 1940s and 1950s, moving rocks and stone dust from quarries was challenging, given the heavy machinery capable of handling large stones. WW2 ushered in technologies that laid the foundation for most modern-day stone crushers. Today, crushed stone is in high demand for road paving, drainage constructions, and other concrete works.
What affects the cost of crushed stone material?
Compared to other rock materials, angular rocks have one of the lowest costs-to-weight ratios of the finished product.
The production cost of crushed stone is determined by the equipment used, labor, water, and energy costs involved in mining rock deposits. In addition to mining costs, crushed stone companies must comply with environmental safety regulations.
The cost of mining and crushing often varies depending on geographical and geological factors, such as the condition of rock deposits. Distribution is another critical factor and can equal or even exceed production costs depending on the distance between the quarry and the job site. Due to the high cost of transporting crushed stone and its by-products, most construction companies prefer to source their materials locally.
To reduce moving costs, many quarries set up operations near urban centers and populous towns. However, the rapid appreciation of land value in urban centers and the growing environmental concerns are forcing many quarries to move away from urban settings. This trend is part of the rising cost of crushed stone and ballast.
Common sources of crushed stone
Crushed stone or angular rocks come from many rock residues, including limestone, granite, and dolomite.
Limestone is a tough sedimentary rock whose main component is calcium carbonate. Limestone is crucial to the construction industry and is used to make cement, crushed stone, and other building material. Nearly 70% of all crushed stone comes from limestone deposits in the United States alone. Besides being the main ingredient in concrete, limestone forms the base material in railroad and rural road constructions.
Limestone is an ideal source of crushed stone due to its softness and fragility. The rock fragments easily when under steel crushers compared to harder rocks like quartzite. Limestone is preferred over other rocks because it causes less wear and tear on crushing equipment. Its softness and easy-to-chip edges also mean that it causes less damage to truck beds during transportation.
Granite is an igneous rock and comprises large light-colored grains that are visible. Commonly known as trap rock, granite is the most common type of igneous rock and is rich in quart, alkali, and feldspar. This unique combination of minerals gives granite a reddish-gray appearance with some white mineral grains.
Granite forms from the gradual crystallization of lava deposits deep below the earth’s surface. Granite is the third most popular material for making crushed stone and accounts for about 15% of all crushed stone production in the United States.
Dolomite is very similar in appearance to limestone. However, unlike limestone, whose main building block is calcium carbonate, dolomite mainly comprises calcium magnesium carbonate. Dolomite has a higher degree of hardness (Mohs of 4) compared to a Mohs hardness of 3 in limestone.
This slightly higher degree of hardness gives dolomite added durability when used as an aggregate for construction purposes. However, its hard nature also means that dolomite is more taxing on stone crushing machines and leads to more wear and tear than limestone.
Limestone and dolomite deposits frequently occur together and are rarely mined as different products. In the United States alone, nearly 70% of all crushed stone comes from limestone and dolomite deposits.
Crushed stone from marble deposits is highly valuable. This polishable rock is composed mainly of calcium magnesium carbonate. Marble forms from the crystallization of carbonates under heat or geological pressure resulting in large deposits of interlocking crystals. Marble accounts for 5 to 10% of crushed stone products and is used in many construction projects, from railroad beds to highway foundations.
This metamorphic rock is made almost entirely of quartz mineral. Quartzite forms from the crystallization of sandstone under heat or geological pressure. The interlocking nature of quartz crystals is responsible for quartzite’s tough and durable properties. It is primarily white or grey, but the presence of impurities may cause quartzite to appear yellow, green, or orange.
Gravel vs. crushed rock
Unlike crushed rock, gravel deposits occur naturally and are commonly found on beaches or along current rivers or streams. It is mainly formed as a result of weathering and erosion of rocks. Crushed stone and gravel differ in appearance and texture.
Crushed stone often has angular edges as a result of the crushing process. Gravel appears rounded with very smooth edges due to the erosion of natural rock beds by moving water.
Even though gravel is sold in its natural state, it also undergoes a screening process to yield different gravel sizes. Pea gravel and river rock are the most common types of gravel used for construction purposes. Pea gravel is a fine natural rock, typically less than half an inch in size. Pea gravel is ideal for walkways and other high-traffic areas due to its small size.
On the other hand, river rock comes in larger sizes, typically between one and two inches in diameter. Its large size makes it ideal for lining walkway boundaries for aesthetic purposes like garden beds.
Common sources of gravel
Gravel, just like sand, occurs in a few geological zones and is not widespread like other rocks. Gravel forms when rock structures are broken down over time by moving water or melting ice from glaciers. Over time, these rock particles are swept down river channels, accumulating along river beds over the years. Most gravel deposits occur along river deltas where water velocity is at its lowest, which allows the sediments to settle and build up over the years.
Common uses for crushed stone
Below are some common applications for crushed stone in the construction sector.
Walkway and driveway construction
Crushed stone is favored in driveway construction because of its low cost and visual appeal. Walkways can be constructed using angular rocks of different sizes, textures, and colors. Due to its fine size, crushed stone is ideal for comfortable walkways and hiking trails.
The material is available in a broad range of colors, serving as a decorative construction element. Besides the visual appeal of crushed stone, its porous nature boosts the drainage capabilities of walkways.
Commercial and residential landscaping
Because crushed stone comes in various color finishes and textures, it is a great landscaping material. Once installed, crushed stone maintains its value for a long time. It does not decompose like mulch or carpeting.
As base material
Crushed stone is a great base material for constructing concrete or gravel pathways. It also forms the base material in many road projects. It provides an excellent surface for concrete and gravel to bind onto when laying foundations.
Crushed stones add durability and uniformity when used as base material. When used as basement composite, crushed stone helps lower the construction cost due to its affordable cost compared to cement.
Crushed stone basements can last for years and withstand wear and tear better than other materials.
Crushed stone is an excellent material in drainage construction. It is widely used to prevent the degradation of coastal shorelines. In many coastal areas, the crashing ocean waves can be quite destructive because they drag sand and other debris back into the ocean. Over time, the pounding waves cause recession of the shorelines making public beaches unsuitable for human use.
Rock aggregates help reinforce shoreline structures by absorbing most impact from crashing waves.
The angular edges of crushed stone make it an ideal material for railroad construction. The rock aggregates help hold the train tracks in place and prevent the tracks from being compromised. Railroads that use crushed stone can withstand train weight and traffic much better than when railroad ballast is not used.
Crushed stone is usually high in nutrients and minerals, making it an excellent additive in many livestock feeds. Today, many livestock farmers supplement their poultry feed with crushed stone concentrates, which help boost mineral intake and provide a balanced diet for hens.
However, when using crushed stone in chicken feed, it is important to source the supplements from a reputable producer. Dealing with reputable crushed stone producers ensures that the material is safe for poultry consumption and is devoid of broken glass and other contaminants. The rough nature of crushed stone helps grind food particles, thus improving the chicken’s digestion.