Aggregate Supply and Delivery
What is Aggregate?
The dictionary definition of aggregate is “a material or structure formed from a mass of fragments or particles loosely compacted together”. They are a component of composite materials, and are the most mined materials in the world. They serve a number of functions, particularly in construction, and there are several types available.
It primarily serves as a form of reinforcement to add strength to the overall composite material. They are widely used in drainage systems, under railway tracks, and under foundations. They can also act as a cheap extender that binds with costlier cement or asphalt to form concrete.
Types of Aggregate
Here are the types of aggregate available, the mining methods and processing carried out, as well as information on forms of transportation for them. It also looks at the functions each type serves and what they can be used for.
Type 1 MOT Aggregate and Road Construction
Type 1 MOT aggregates can span a wide variety of materials, all of which are listed below. The thing that separates type 1 from anything else, however, is its approval for the construction of roads by the department of transport. Only aggregates that come under this type 1 category and are government approved can be used for road construction. The mining process used to get these aggregates varies according to the material used, and each one is explained in detail below.
As with the mining methods, transportation also varies, although the most commonly used forms are dump truck, train, and front loader – all of which is dependent on the type of material that is being used. There are two variations of type 1 material – slag based and rock based. For this section, we will look at crushed concrete, the most commonly used type 1 aggregate for road construction.
Type 1 MOT Recycled Crushed Concrete.
Type 1 MOT Crushed Stone (mined)
The usual size for crushed concrete is 50mm thick, as road construction requires larger pieces of material. While it has been mentioned that it is heavily used in the construction of roads, highways, and car parks, it can also be used as a high quality filler if there are gaps in an already constructed area.
Crushed stone is a premium aggregate that will cost more, but often this material is a requirement for many highway construction projects.
Crushed concrete is a recycled aggregate (which is further detailed in the final section on this page), and it recognised for its general consistency when it comes to quality. Plus, it is a fantastic way to reduce your carbon footprint as the recycling process has a less damaging effect on the environment. So it is not only good for the world, but can have a positive impact on your company’s image.
However, when purchasing crushed concrete there is a key piece of information to remember – if it’s really cheap then it’s best to stay away. Much of the cheap crushed concrete has an inconsistent mix due to its creation vi temporary crushers that have been placed on demolition sites. This then leads to poor quality roads and construction.
Sand aggregate is a construction aggregate that is typically used for roads, concrete, manufacturing, or sanitation. It is highly absorbent and can become very firm as well as sturdy. Its composition can largely depend on the source, but typically it is composed of fine rock material and mineral particles. It occurs naturally, and varying compositions can be found in different parts of the world.
Sand is classified as a fine aggregate due to its small size, and is often seen as the half way point between silt and gravel. When sand is processed and refined it goes through five main stages before it becomes the finished product. Generally, the plant that carries out this process will be located near to the source of the material (so near a beach or large dune) to cur transportation costs.
The first of the five stages is natural decomposition. During this stage the solid rock is broken down into chunks by natural means, and this process continues over time with the rocks being worn down and shrunk by things like water. Eventually, it becomes sand. The second stage is extraction. This is where a front loader will come and extract large quantities of sand from the area before being dumped onto a truck or train, or even placed on a conveyor directly to the plant.
A Sand Aggregate plant in Phoenix, AZ. Image courtesy of Highslide JS.
It can also be extracted via pipe, which is directly linked to the plant and so is pumped straight in without the need for vehicles. The third stage is sorting. The material is immediately mixed with water before being passed through a perforated screen to filter out any foreign material. It is then passed through several more perforated screens to separate the material according to particle size and they are sent to separate conveyor belts.
The fourth stage is washing. The material that is stuck onto the screen for the coarsest material is washed before further screening takes place. A blade works through the material to remove any remaining clay or soil, and any sand particles are sent to join the rest.
The gravel particles, which are larger, are screened out and collected separately according to size. The material that passed through the finest screen is washed and the silt is separated from it, leaving only the sand. The sand is collected in bins. When the process is complete, the sand is removed from the bins, washed again, and then sent to storage.
The final stage is crushing. Some of the sand is crushed specifically to create a shape that is not naturally available. The sand falls between two fixed cones that rotate, crushing particles between them to create smaller and newly shaped ones.
Generally, sand aggregate delivery is transported between locations via truck or train. It is available in several sizes depending on what your task requires. The transportation of this particular material is very simple and does not tend to require and specialist environments. Sand is a very common material, making the aggregate supply for this quite constant, and its usefulness has it in popular demand.
Gravel aggregate is commonly used in several areas. Most commonly, it is used for roads, the manufacturing of concrete, and sometimes decorative purposes. The deposits are formed through a series of natural weathering and erosion processes, and the material is easy to find. Its composition can vary depending on the location and materials.
Interestingly, gravel goes through the same processing system as sand. They are found in the same or similar environments and tend to be processed together. The stationary plants that are set up on the beaches or dunes will collect the sand via a pump that leads directly to the station, or a front loader that will either dump the contents into a truck or train, or onto a conveyor belt that leads directly to the station.
It goes through the same five stages as sand in order to be formed. Once the sand has been separated through perforated sheets, the larger chunks of material can be collected and processed. These are the gravel pieces and they are separated from the sand towards the end of the process in the washing stage. They are separated by screens according to size before being washed again.
Gravel Aggregate. Image courtesy of Wurtz Brothers LTD.
Once this has been done, the gravel is placed on conveyor belts and sent to stockpile ready for customer needs. Some stations have cement mixing platforms available, ready to create the solution required for construction sites. If this is available on site, some of the gravel will be sent here and added to the mix. This can make it easier for construction teams who are pressed for time or would rather buy the mix ready to go. The aggregate supply and demand for this is both bountiful and high as it is used for a wide range of functions and comes at a relatively low cost.
The aggregate delivery and transportation of this material is generally done by truck or train, although it is most commonly found in the former. No special circumstances tend to be required, much like sand, and usually only a piece or tarpaulin is required over the top to prevent spillages. Here are the two types of gravel aggregate as well as the available fractions.
- Scabbled stone. This type comes in regular, natural, or crushed.
- Gravel. Rounded pebbles that tend to originate from rovers or the sea.
- Factions for gravel are; 3-10mm, 5-20mm, 5-40mm, 20-40mm
Granite aggregate is mined using open pit methods. Once the gravel is extracted a quarry is opened up and ready for use. It is primarily used for decorative purposes as it comes in a wide variety of colours – from the classic grey to shades of pink. The colour is determined by its composition and the types of stone that merge together to create the granite.
When the granite is mined, the top layer (knows as overburden) is removed and transported either to dumps or used as mineral construction raw material, which is often how limestone aggregate is formed. Once this is completed the oversized chunks of stone that were extracted are prepared for further processing. Special prospect holes are drilled into them to plan their separation, and explosives are placed in the holes. The blast is incredibly well calculated and specific so that the rock is separated but the quality is not compromised.
Granite Aggregate with emphasis on the multiple colours that can be found on the stones. Image courtesy of DW Nye.
The stone block loading is carried out by excavation machines and that is how it is transported during this stage. The transportation of the mineral resources to the charging hoppers and the productive stripping is transported via dump truck only.
There are a wide range of sizes for granite aggregate, and it is probably the vastest aggregate. Here are some of the sizes you can expect to see and their applications.
- Up to 0.5mm (granite sand). This is used for road construction, decoration on paths, covering sport grounds and children’s play areas. It can also be used with concrete and for creating non-slip surfaces.
- 2-5mm. Used for paving slabs.
- 3-10mm. The most popular aggregate size for granite. Used for concrete, roads, bridges, fillings, and drainages. It is primarily used for the surfacing of roads.
- 5-20mm. Used in road concrete mixes.
- 20-40mm. Used for the lower layer of road construction with asphalt. Also used as a filler in parking areas and for increased strength concrete.
Limestone aggregate is also known as lime or dolomite aggregate. Next to gravel and granite, it is one of the main types of aggregate that is used in the construction of roads as well as the construction of reinforced concrete objects. It can also be used for soil treatment for agricultural use, changing the pH of the soil.
It is primarily made up of calcium carbonate and is the product of the crushing of sedimentary rocks. It is mined in the same manner as granite, so an open pit excavation which is then developed into a quarry. Or, it is excavated via an underground sediment mine.
Once the raw materials are extracted from the mine/quarry they are separated and taken to different locations for processing. It is transported to and from the next plant via dump truck and no other form of vehicle. A tarpaulin is usually placed over the top to prevent spillages from the truck, protecting the road and anyone who is driving on it.
Limestone Aggregate. Image courtesy of Longwood Quarries.
The limestone starts off as rubble stone before being taken to the plant to be turned into crushed stone. The process can be divided into three main stages. These are the primary crushing stage, the secondary crushing stage, and the screening stage. The first stage is where the rubble is loaded into a charging hopper, and the rock is steadily fed into the primary crusher, which turns the rock into average sized chunks, ready for the next stage.
During the second stage, the now average sized rocks are fed into the secondary crusher. It is fed by conveyor at a steady rate, and the crusher turns the rocks into much smaller chunks. During the third and final stage, the rock is sent through screening. The rock is separated into tradeable factions here and the remote belt transporters then pool each piece of crushed stone separately. After this, they are organised by size and sent either to storage or to those who need it and have purchased it.
Here are limestone aggregate sizes and their uses.
- 14-28mm. Filling and levelling
- 40-50mm. Filling and levelling
- 50-75mm. Filling and levelling
- 10-20mm. Used to mix with cement
Crushed Stone Aggregate
While the process of creating crushed stone has been explained in the previous aggregate topic, crushed stone aggregate can be made from a variety of rock materials. It can be excavated via an open mine/quarry, or even through the mining of underground sediments. It is not limited to limestone and can be created from any type of suitable rock material. It is very distinct from aggregates such as gravel, and tends to have a much rounder shape due to the manufacturing process.
Crushed stone aggregate is most commonly used in the construction of roads as well as the strengthening of concrete along with asphalt. It is also frequently used for landscape design in gardens and public parks, as well as in beds for cacti. It can also be used for pavements and driveways as well as weed suppression. It has a diverse list of potential functions. The aggregate supply for this is quite high due to the bountiful amount of rocks available for extraction.
The process for creating crushed stone can be divided into three stages. These stages are the primary crushing stage, the secondary crushing stage, and the screening stage. The first of these stages is where the rock is fed into the primary crusher. This is done by rock being fed into a charging hopper and onto a slow moving conveyor belt. In this stage, the rock is turned into average sized pieces so that it can be sent to the next stage.
Crushed Stone Aggregate.
In the second stage of processing, the average sized rocks are slowly fed into the secondary crusher by conveyor belt. At this point, the crusher will turn the rocks into much smaller chunks that can be used in the aggregate. It is then sent to the third, and final, stage which is the screening process. Here, the rock is separated by the screens into tradeable factions.
The remote belt transporters will then pool each piece of crushed stone separately. Once this is done, they are organised by size. Afterwards, the aggregate delivery either be sent to consumers or to storage where they will await purchase or shipment. Much like the limestone, the crushed stone is only transported by dump truck with tarpaulin secured over the top to prevent spillages and potential threats to other road drivers.
Here are the sizes and uses of crushed stone aggregate.
- >6mm. Used as a binding agent or for filling voids between other stones.
- 5-20mm. Used for roads, sidewalks, drainage systems.
- 20mm. Used for lawns and other public features
- 50mm. Used for construction.
- 65mm. Erosion control or for decorative features
Slag aggregates are made by the crushing of smelter slag or by a specialised treatment of molten slag aggregates. Generally speaking, slag aggregate is made using steel slag and it has a number of potential uses. It is most commonly used in specific types of concrete that require the use of metallurgical slags as both a binding and component filler. Plus, it offers a concrete that is lower in cost than most due to the use of slag.
Due to its mineral composition, there are some great benefits to using slag aggregates in road development. For one, the mineral composition means that it is harder and takes a much longer period of time to wear down. This means that the roads are able to withstand more and will not need to be replaced as frequently.
It also promotes better adhesion for tyres on the road, so that even in slippery conditions vehicles are likely to experience better grip on the roads. This is due to the micropores it contains, whereas natural rock becomes smoother over time – reducing the levels of grip. Plus, it can hold more on its surface, and so is less likely to become cracked or damaged due to large weights driving over it.
Slag Aggregate. Image courtesy of Gulf Coast Slag.
Slag aggregate is formed in a blast furnace (also known as blast furnace slag) or a steel furnace (also known as steep furnace slag). The blast furnace method often uses the air cooling method, where the slag is dried in the open, or the granulated method, where the molten slag is quenched in water to form sand sized particles.
If the granulated slag comes into contact with free lime during the hydration process, it can become incredibly strong and partly substitute cement in concrete. If the steel slag method is used, then this is typically air cooled each time. Generally, steel slag aggregate is used in road construction or for filler in car parks and other areas as well as asphalt concrete.
Geosynthetic aggregates are typically used for the stabilisation of terrain, and their molecular nature makes them a fantastic solution where high durability in the ground is required. As a result, they are sometimes used in the construction of roads, particularly areas where heavy good vehicles frequent. They are most commonly used on runways and railroads because they are able to withstand the extra weight. They are also used to secure reservoirs, canals, and dams, as well as being used for the lining of landfills.
Geosynthetic aggregates are created by mixing different materials with soil for added strength and reinforcement. It is a technique that has been used since the Roman era to reinforce roads and make them stronger. They were also used to build steep slopes, such as those seen on the Egyptian pyramids. Of course, the main issue with this was the fact that the natural material would degrade over time, and so in the 20th century polymers were introduced to the mix. It is an area that is constantly growing.
Geosynthetic aggregates are primarily used for reinforcement, especially in terms of runways and railroads (as previously mentioned). The soil and the Geosynthetic material are mixed together and fused to create something much stronger and durable. The cells from the Geosynthetic material and the particles from the aggregate lock together to form a mechanically stable aggregate. This increases its stability with regards to loadbearing and other heavy duty tasks. Plus, the production of this particular aggregate has a reduced carbon foot print, making it a more environmentally friendly path to take. Essentially, the aggregate is a way of reinforcing the core strengths of Geosynthetic material.
Recycled aggregate is also known as secondary aggregate, and it comes from crushed construction waste such as bricks, concrete, and asphalt. It comes at a much lower cost than most other aggregates, especially granite, and it is also friendlier in terms of the environment as mining is not required for this particular form – just waste construction materials. In terms of frost protection and strength, it is inferior to aggregate made from natural stone, but it still has a wide range of applications.
It is often used as a large scale filler for concretes as well as in road construction as a filler for roads and ramps, but not those that have wide importance. It is also used for maintenance works on pavements and areas with asphalt concrete. In addition to all of this, it has popular usage in the reinforcement of weak soils, and tends to be used as a reinforcement for the bottom of construction pits.
Recycled Aggregate. Image courtesy of Sand and Gravel Direct.
The process that recycled aggregate goes through is much the same as that which crushed rock goes through. The first stage is where the waste material is fed into the primary crusher by a charging hopper. It moves along on a slow conveyor and into the machine where the material will be changed from large to average sized chunks. At this point it is ready for the next stage.
During the second stage of processing and refinement, the now average sized chunks of material are fed slowly into the secondary crusher by a conveyor belt. It is at this point that the secondary crusher will turn the material into much smaller chunks that can be used to create the aggregate. Following this, it is sent to the third (and final) stage – the screening process.
It is here that the material is separated by screen into tradeable factions. The remote belt transporters pool each piece of, now crushed, material separately before they are organised by size. After this is completed, they will be inspected carefully to make sure that they meet regulation standards, before being sent to stockpile. After this, they are loaded onto trucks and sent out for delivery. This aggregate delivery is transported via truck, usually with some type of tarpaulin or cover over the top to prevent any potential spillages.
Here are the materials that can be recycled to create new aggregates:
- Clay bricks
During the recycling process, not all materials will be washed before they are ready to be used again. Generally speaking, sand will not be washed during the recycling process. This is known as unwashed fill sand and it is generally seen as a high quality, type 1 aggregate. The aggregate supply and demand for this item is quite high, as it is used for bedding cables and backfill for trenches.