Aggregates play an important role in the construction sector and can be used for different projects. Their primary use is to strengthen concrete, strengthening its structure and thus minimising instances of cracks. Aggregates are used in the construction of railway and road ballast to distribute the load over a large area. They also create a void structure that forms a path for water to flow.
The Use of Aggregates in Concrete and Construction
Aggregates are an important element in the making of concrete and offer several benefits. The smaller the size of the aggregate, the greater generally the cost of bringing in the binding material. Using excessive amounts of aggregate is not recommended either because it will weaken the binding material. A good compromise is to combine a mixture of fine and coarse aggregates to minimise the two problems mentioned above. It has also been shown that aggregates form a highly valuable product that can fetch a good price in the market.
How to Get the Right Aggregate for Home Improvement Projects?
You must have run into this common issue; which type of aggregate should I get for my home improvement project? Depending on your project, you might want fine aggregates, which are smaller than 5mm in size and can be used for a range of applications. Conversely, you can get coarse aggregates which are larger than 20 mm in size. If you’re not too sure which type to use, please don’t hesitate to call the experts at MP MORAN.
How to Select the Most Appropriate Aggregate for Your Project?
Since aggregates will occupy most of the volume of the concrete, it is important to make sure you choose the right make and model. Here are a few key factors you should consider.
i) Size
Aggregate gradation and size are two of the most important factors you should consider. Aggregates can be small or large, from fine sand to fist-sized rocks. Coarse aggregates are larger than ¼ inch, while anything smaller is termed as fine aggregate. It is not recommended to use aggregates that are greater than 1/3rd in diameter than 1/3rd the depth of the slab.
As a general rule, if the countertop has a thickness of 1 ½ inch, then the aggregate should be no more than ½ inch. It is common to mix fine aggregates with coarser aggregates to fill the void in larger pieces, and it can be used to lock the larger pieces together, known as graded aggregates. This decreases the amount of possible shrinkages and reduces the amount of cement paste required.
ii) Shape
The two most popular shapes are angular particles and rounded particles. Angular, rough aggregates tend to pack more closely together and have a larger surface area, which increases the amount of inter-particle friction than in rounded, smooth particles.
Higher friction is not very conducive to workability. Rounded particles require a bit less cement paste to coat them than angular particles. This is why mixes with rounder particles won’t require as much cementitious content.
iii) Material
Most crushed rocks and natural stones are ideal for use in concrete. Commonly used stones include basalt, quartz, granite, limestone, and marble. If you are using diamond tools to ground concrete countertops, you could end up exposing a large chunk of the aggregate which may not look too pleasing. Problems may arise with reactive, soft, or weak rock or stone.
iv) Gradation
Coarse aggregates are about 10 times larger than finer aggregates, but it is common for their range of sizes to be more diverse. There are three levels of gradations, well-graded, poorly graded, and gap-graded.
Well-graded aggregates have a gradation of particles that evenly span from finest to the coarsest. A cut-section of well-graded aggregate will reveal a packed field of various different sizes of particles. This also means that the particles have virtually no voids between them.
Poorly graded aggregates have virtually no variations in size. This isn’t good because there are large voids between each particle, no matter how densely packed they may be. In order to fill the large voids, poorly graded aggregates are filled with a large amount of cement paste. This is not very economical and doesn’t perform as well. A better option is to use gap graded aggregates, although this isn’t optimal either.
Gap graded aggregates contain a mixture of coarse aggregates and fine aggregate. There is less variation in the size difference between each aggregate, thus there will be a few isolated regions of empty spaces that could reduce the workability of the cement. A cut-section of gap graded concretes will reveal a field fine aggregates mixed with few larger aggregate pieces.
Studies have shown that achieving maximum density yields the highest strength. The best mixtures tend to have a grading curve resembling a parabola. According to experimental data, the surface area of aggregates tends to fluctuate widely without causing any noticeable difference in the concrete strength. The amount of water needed for a certain amount of consistency is dependent on other characteristics.
Mortar Concrete
Concrete made with sand (or fine aggregates) is termed as mortar concrete. Like the mortar used for concrete and brick block construction, which is simply made with sand and mortar cement, mortar concrete has no amount of coarse aggregates in it.
This means that ground finishes have fine grained appearances. The most common use for mortar concrete is in concrete countertop mixes due to the importance of surface finishes.
It is always preferable to use aggregate gradation than an all-sand mix. Size variation in terms of the particles is a good trait because it reduces the inter-particle void volume. The most commonly used sands are bank or bulk-run sands because they already have decent variation in terms of particle size distribution.
To provide workability, one needs a large amount of cement volume to encapsulate as much of the aggregate particles as possible, this should preferably be done while the concrete is still fresh.
4 Main Types of Aggregates
Aggregates are sourced from pits, quarries, and even gravels from the sea and river banks in many countries. There are many types of aggregates, with the four most common ones being described here.
1) Gravel Aggregates
Gravel aggregates are made by crushing natural stone and sifting quarried rock. They don’t’ have much strength compared to granite aggregates but are cheaper to purchase. Their most attractive feature, which ultimately isn’t that useful in the context of construction, is the low radioactive content. Gravel aggregates are used for concretes and foundations.
There are two further sub-categories of gravel aggregates, round gravel and scrabbled stone. The former is sourced from the sea or river gravels.
2) Sand Aggregates
Sand is considered to be the finest aggregate. It does have different gradations that include kiln-dried silver sand, builder’s sand, and sharp sand. The finest of these is kiln-dried silver sand. Finer sands are considered to be a good aggregate for grouts and mortars. Coarser sands are a popular choice as an aggregate for concrete mixes.
3) Granite Aggregates
Experts believe these granites to be among the best aggregates to produce high-quality concrete. This is because granite aggregates come in a variety of shapes, sizes, and even colours, include red, grey, and pink. This allows them to be used as a decorative feature. Granite is composed of mica crystals, quartz, and feldspar. The colour of the stone will ultimately depend on granite’s composition.
4) Limestone Aggregates
These aggregates are created by crushing sedimentary rocks, they are most commonly used in reinforced concrete and road construction, aside from gravel and granite. Limestone aggregates are more jagged and bulkier compared to the relatively smoother gravel. These materials are highly versatile and can be used for hardscaping in the construction industry to create practical or decorative elements in a project.
Aggregates Classified According to Shape
It is not easy to attain the required shape of the aggregate due to the way it is produced in the first place, since the source rocks are blasted and crushed. Only the most ideal parent rock should be used along with the ideal crushing machine to extract aggregates. Aggregates can be classified according to the following types:
i) Rounded Aggregates
Rounded aggregates are found in the form of seashore gravel. They are shaped entirely by attrition. They cause the least amount of voids, hence a higher workability. This also means a lower water to cement ratio is needed. Rounded aggregates are not considered for intensive projects that require exceptional strength due to their weak bond strength and poor interlocking behaviour.
ii) Angular aggregates
As the name suggests, these aggregates consist of finely shaped edges that form at the intersection of planar surfaces. These aggregates are obtained by crushing the rocks. The percentage of voids is between 38 to 45%, which results in reduced workability. They have more compressive strength due to stronger bonds, making them useful in manufacturing high strength concrete.
iii) Irregular aggregates
Irregular aggregates are available in the form of gravel and pit sands. They are partly shaped by attrition. They result in about 35 percent of voids. This results in lesser workability than rounded aggregates. Their strength is slightly higher, but not high enough for high strength concrete.
iv) Elongated Aggregates
When one length of the aggregate is larger than the other two dimensions, the material is called elongated aggregate.
v) Flaky Aggregates
When the aggregate thickness is smaller compared to the other two dimensions, the material is said to be flaky aggregate.
vi) Flaky and Elongated Aggregates
When the width is larger than its thickness and the aggregate length is larger than its width, the material is then called Flaky and Elongated Aggregates.
What is Ballast?
There is no difference between ballasts and aggregates, and they are virtually synonyms of each other. The word ballast has widespread use in the United States, while aggregates are used in the UK.
A Unique Type of Ballast – All-In-Ballast
All in ballast is a mixture of gravel and sand to create a coarse aggregate with a diameter of around 20mm. The result is a very strong coarse grade concrete that is suitable for hard standings, building bases, and foundations. It is not ideal for use in the drainage of water due to the fine content. All-in-ballast can be used for a wide range of domestic and construction applications.
You will typically need around 12 bags of cement or 1700 kg of cement when mixing with all-in-ballast at a 1:6 or 1:4 ratios.
How Are Aggregates Extracted?
Aggregates are extracted from sand-and-gravel pits or natural sand, dredging submerged deposits, underground sediments, or hard rock quarries.
i) Rock Quarries
Explosives are used to shift the rock from the working face. This rock is extracted, crushed, and passed through a series of screens. The crushed rock has varying sizes depending on the specified sizes. The rock is transported by road or rail from the quarries to buyers.
ii) Sand and Gravel Pits
Both working and defunct sand and gravel pits can be found interspersed throughout the UK, particularly towards the East of England. They are typically located in regions where glaciers left behind large reservoirs of stone and sand. The gravel may go deeper than the water table, requiring pumps for extraction. The extraction process leaves behind lakes and ponds in its wake.
iii) Marine Aggregates
Over 20,000 years ago, our sea levels were up to 100m lower, thus, a large percentage of the sea bed was vast swathes of dry land. Once the last ice age was over, the sea levels began to drastically rise, causing the sand and gravel deposits to get submerged into the sea. Today, the action of the sea has left behind well-sorted aggregates.
These require purpose-build dredging vessels to work round the clock to extract the marine aggregate deposits. There are two popular dredging techniques. One involves static dredging where the vessel uses an electronic pump to extract the aggregate. The second involves the use of a pump trailing behind the vessel along the length of the seabed.
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