Aggregates are the most basic component used in construction. Aggregates refer to a group of raw materials that are used in construction, like sand, gravel, stone etc. which are combined with mediums like water, clay, bitumen etc., to form building compounds like Portland cement, plaster cement, and bituminous concrete. Generally, aggregates consist of 92-96% bituminous concrete and 70-80% of Portland cement.
Aggregates are also used for building sidewalks and pavements, including their use to make the sub-base and base courses that are used to make both rigid and flexible pavements. Aggregates can be naturally occurring or manufactured in factories as per the use case required.
Natural aggregates are usually excavated through a quarry from larger rock formations. Then, the extracted rock is broken down into usable-sized rocks and then mechanically crushed. Manufactured or artificially occurring aggregate is usually made as a byproduct of other manufacturing and industrial processes.
See also: How flexural strength test helps evaluate concrete
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known about the soundness of cement
Properties of aggregates
- Strength: Strength is an important factor when it comes to choosing the right materials for your aggregate, especially when making the topmost layers for roads and pavements to withstand stress action due to traffic wheel load, wear and tear and crushing. The aggregate should consist of higher resistance to crushing and withstand higher amounts of stress in order to build high-quality pavements.
- Hardness: The aggregates used to make surface layers are exposed to constant abrasion and rubbing due to traffic stress. Ideally, the hardness of the aggregate should be enough to withstand the damage caused by moving traffic, especially when it comes to steel tyred vehicles.
- Toughness: When we say toughness, we refer to the ability of the aggregate surface to withstand impact forces. Ideally, aggregates used in pavements should be able to resist the impact steel tyred heavyweight vehicles have on them on different levels.
- Durability: Aggregates also have to withstand the cruel forces of nature. The property of an aggregate’s ability to withstand adverse action of weather is called soundness.
- Adhesion: Adhesion to bitumen is a very essential property to be taken into account when picking the right aggregate
known about fine aggregate
Test of aggregates
Aggregates are a very essential part when it comes to surficial strength and durability, and are chosen based on the needs of a project. There are various ways to test an aggregate for its properties and how viable of an option it is regarding a project.
Crushing test of aggregate
A pavement is bound to fail under compression of load it cannot handle, which may lead to public disaster and can definitely prove to be hazardous.
Therefore, an aggregate crushing test is conducted to ascertain the aggregate’s structural soundness and crushing strength. The aggregate crushing value gives a comparative indication of the resistance to crushing under a crushing load that is applied gradually. The recommended value of a crushing test should ideally be 30% for roads and pavements and 45% for other projects.
Abrasion test
The abrasion test focuses primarily on the hardness of the aggregate and helps decide whether a certain aggregate is suitable for use in construction or not. The Los Angeles abrasion test is the most preferred form of abrasion test of aggregates and has been standardised as the test for aggregate hardness in India.
Impact test
The impact test of aggregates helps in determining the resistance of an aggregate to impact forces.
Soundness test
The soundness test is conducted to determine the resistance of aggregates to wear and tear due to weather forces. This is done through a simulation of weather cycles. Porous aggregates that are exposed to freezing and thawing over and over again are more likely to disintegrate prematurely. To increase the durability of such aggregates, they are exposed to a series of soundness tests in order to select the highest quality aggregate.
Shape tests
An aggregate, whether naturally occurring or artificially manufactured, has flaky or elongated parts in it. These flaky components help determine the flakiness of the aggregate. Aggregates with a higher number of flakes seem to be detrimental to higher workability and the stability of mixes. This is known as the flakiness index.
Another metric that helps determine the stability of an aggregate is the elongation index of an aggregate. The elongation index of an aggregate is defined as the percentage index by weight particles whose greatest dimension is 1.8 times their mean dimension. This test is only applicable to aggregates larger than 6.3mm.
Specific gravity and water absorption test
The design of concrete and bituminous mixes must take into account an aggregate’s specific gravity and water absorption. A solid’s specific gravity is determined by how much mass it has in relation to an equivalent amount of distilled water at a given temperature. There is a possibility of the aggregates containing water-permeable voids. Hence, to avoid this, two measures of the specific gravity of aggregates are used, which are known as apparent specific gravity and bulk specific gravity.
Whereas, apparent specific gravity is calculated on the basis of the net volume of aggregates or the volume of aggregates excluding the water-permeable voids.
Bulk-specific gravity, on the other hand, is calculated on the basis of the total volume of aggregates, including the water-permeable voids.
Bitumen adhesion test
Bitumen is a well-known compound that is widely used in construction. When picking the ideal aggregate for a given use case, it’s crucial to consider bitumen’s adherence in cold and wet conditions. This test is conclusive of the adhesion of aggregate when wet and cold.
Test of aggregates: Importance
Aggregates are compound mixtures that are most commonly used in civil projects for the public. Thorough testing of aggregates is crucial because the quality of an aggregate determines the quality of the concrete that will be used in a particular project and hence, determines the structural integrity of the said project as well. It is one of the most important constituents of concrete and helps give it form and structure.
FAQs:
What type of aggregate is used for pavements and roads?
Gravel is a type of aggregate mainly used in the making of pavements and roads.
How do I know if my aggregate is suitable for wet surfaces?
If an aggregate has to face wet surfaces for longer periods of time, its water absorption range should be between 1-2%.
How long does a structure last if the aggregate soundness is 16%?
Pavement structures with a moderate soundness result can last anywhere between 30-50 years if taken proper care of.