What is Coarse Aggregate?
The Aggregates which will get retained on the 4.75 mm (No. 4) sieve and will pass through a 3-inch sieve are known as Coarse aggregate.
The aggregates used in the production of concrete are inert granular materials such as gravel, crushed stone, sand, slag, and recycled concrete. The aggregates might be natural, manufactured, or recycled.
Usually, the aggregates are obtained by crushing the naturally occurring rocks.
The properties of the rocks mainly rely on the type of rock whether it is sedimentary rocks, igneous rock, or metamorphic rocks. Coarse Aggregate is one of the most important and greatly used building materials in the Construction Sector. Coarse aggregates are mainly used in concrete, railway track ballast, etc.
Requirements of Good Coarse Aggregate
A good coarse aggregate should meet the following requirements:
- The shape is of the aggregate preferably cubical or angular.
- The coarse aggregate should be durable.
- The coarse aggregate must be hard and strong.
- It must be tidy and free from dirt and organic materials otherwise it will decrease the bonding of the aggregate with concrete.
- The aggregates should not react with the cement after blending.
- Coarse aggregates must not be soft and permeable.
- Coarse Aggregates should not absorb water by more than 5%.
- Aggregates should be chemically inert.
Classification of the Aggregates
The coarse Aggregates are specifically classified are as follows:
- According to the source of the Aggregate
- According to the shape of the Aggregate
Classification According to the Source or Nature of Formation
- Natural Aggregates
- Manufactured Aggregates
Coarse aggregates are obtained from the stone quarries and the stone crushers. Crushed gravel or stone, natural disintegration of rock are the major sources of natural aggregates.
The broken bricks or blast furnace slag are the manufactured aggregates that are also utilized for the various concreting work.
Classification According to the Shape of the Aggregates
- Rounded Aggregates
- Angular Aggregates
- Flaky Aggregates
- Irregular Aggregates
Natural aggregates are smoothed by weathering, erosion, and attrition. Rocks, stone, sand, and gravel found in riverbeds are your most popular rounded aggregates.
Rounded aggregates are the main factor behind workability.
Angular aggregates have a greater specific surface area than the smooth rounded aggregate. With a greater specific surface area, the angular aggregate might show higher bond strength than rounded aggregates.
Also, angular aggregates exhibit a better interlocking effect in concrete that contributes to the strength of concrete.
An aggregate is called flaky when its minimum dimension (thickness) is less than three-fifths of its mean dimension.
The mean dimension of aggregate is the average of the sieve sizes through which the particles pass and are required, respectively.
These are also shaped by attrition but are not fully rounded. These contain little stones and gravel and provide decreased workability to rounded aggregates.
The Irregular and angular aggregates give higher bond strength as compared to the rounded and flaky Aggregates because of their shape because there is more interlocking effect in the aggregates.
Following are the different types of aggregate tests conducted to determine the suitability of aggregates:
- Crushing Test on Aggregates
- Abrasion Test of Aggregates
- Impact Test of Aggregates
- Soundness Test of Aggregates
- Shape Test of Aggregates
- Specific Gravity and Water Absorption Test of Aggregates
Crushing Test of Aggregate
The aggregate crushing value test shows the strength of aggregate which is one of the most important properties of coarse aggregate. The aggregate crushing value gives a related action of resistance to crushing under a gradually applied compressive load.
For achieving a premium quality of pavement, an aggregate of higher crushing strength such as reduced crushing value should be selected. The reduced crushing value indicates greater strength as it shows a reduced crushed fraction under load. This provides a longer service life and a more affordable performance.
Abrasion Value Test
This test is carried out to figure out the abrasion value of the coarse aggregate. The concept of the Los Angeles abrasion test is to find the percentage wear due to relative rubbing action between the aggregate and steel balls used as an abrasive charge.
Abrasion test on aggregate shows the hardness of aggregate. It shows the capability of aggregate to resist wear and tear.
Impact value of Aggregate
The impact value of the coarse aggregate is the test that gives a relative action of the resistance provided by the aggregate to the suddenly applied load which is also known as impact load.
Soundness Test of Aggregates
A soundness test on aggregate is conducted to realize the resistance of aggregates to weathering action. Aggregates are subjected to the physical and chemical action of rainwater, groundwater, and the other impurities of the atmosphere. This can cause a change in the volume which is not good and substandard for the durability of construction. The property of aggregates to withstand the damaging action of weather is called soundness of aggregate.
Shape Test of Aggregates
Aggregates are attainable in various sizes and shapes, such as rounded, cubical, and angular flaky or elongated. It is confirmed that the flaky and elongated particles have reduced strength, reduced bonding, less interlocking, and durability as compared with cubical, angular, or rounded particles of similar size aggregates.
It can be said that aggregates are flaky when aggregates are too thin contrasted to their mean dimension. Elongated aggregate indicates aggregates are too long compared to their normal size. Flakiness index test and elongation index test are performed to identify flaky and elongated aggregates.
a) Flakiness of Coarse Aggregates
The flakiness index of the aggregate is the percentage by the weight of the particles whose least size or the thickness is less than 3/5th of their mean dimension.
The flakiness index of the aggregate will provide the concept regarding the thickness of the aggregate.
b) Elongation of Coarse Aggregate
The elongation index of the aggregate is the percentage by the weight of the particles whose greatest dimension or the length is greater than 1.8 times of their mean dimensions.
Specific Gravity and Water Absorption Test of Aggregates
The specific gravity of aggregate is specified as the proportion of the weight of aggregate to the weight of an equal volume of water. It reveals the strength of aggregate. Aggregates having a low specific gravity are generally weaker than those with having high specific gravity.
Water absorption of aggregate provides thought about the inner structure of aggregate. If aggregates have more absorption capacity, they are called porous in nature. Such aggregates are unsuitable and normally not preferred for any construction.
The specific gravity and water absorption test on aggregates mean the strength or quality of the aggregate. This property allows in general identification of aggregates.
Characteristics of Coarse Aggregates
Coarse aggregates are extensively used in construction so it is truly essential that we must understand about the properties of the coarse aggregates.
The characteristics of the coarse aggregates are as follows:
- Water Absorption
- Surface Appearance
- Bulk Density
- Specific Gravity
The size of the coarse aggregate depends, upon the reason for which the concrete is used in the construction. For the mass concreting work the bigger aggregates are used. For the construction of home building generally, 15mm to 20 mm size aggregates are used.
Water absorption is one of the most essential properties of the coarse aggregate which is determined in the percentage.
If the aggregate absorbs water then, the water mixed in the concrete will get increased and the concrete may entail a higher water-cement ratio.
Water absorption of the coarse aggregate depends upon the porosity of the aggregate. If the water absorption is more, subsequently it will affect the workability and the durability of the concrete.
The soundness of the aggregate is defined as the resistance which is delivered by the aggregates to any kind of volume change.
Porous aggregates don’t have sound and are more reliant on the attacks by the chemicals. The foundation of the aggregates is determined by putting the aggregates in the sodium or magnesium sulfate and then use oven drying it.
The shape of the aggregate is one of the most critical elements which impact the workability of concrete. The shape of the aggregates also influences the bonding strength.
Usually, the angular and uneven shape of aggregates is used because they have more interlocking effects. The total area of the angular aggregate is even more in the angular aggregates as contrasted to the rounded aggregates.
The surface appearance or texture of the aggregate relates to the roughness and the level of smoothness of the aggregate.
The coarse aggregate with a rough appearance is preferred because it develops a good bond with the concrete and provides a lot more bonding strength. However, the Rough textured aggregate will give reduced workability.
The surface texture of the rounded aggregate with a smooth surface will require less quantity of cement and hence raised the output per bag.
The bulk density of the aggregate is specified as the ratio of the clear weight of the aggregate to the volume of the aggregate.
Specific gravity is specified as the proportion of the dry weight of the aggregate to the weight of an equal volume of water.
The specific gravity of the aggregate is the sign of Strength. Usually, the aggregates with the specific gravity range from 2.5 to 3.0 are taken in the construction work. Greater specific gravity means, even more, will be the strength.