Aggregates are one of the important building materials used in construction. Aggregates are mainly used to produce concrete which is produced by mixing a certain amount of cement, aggregates (coarse & fine aggregates), and water. In this article, we will discuss different types of aggregates used in construction and their classification.
What Is Aggregate?
Aggregates are small pieces of rock that are mixed with a binding material to produce concrete or mortar. The binding material used is cement, bitumen, etc. Thus, cement concrete & bituminous concrete are produced, respectively.
The size of the concrete quality aggregate depends upon the usage of concrete. This concept shall be elaborated on later in this article. The aggregate plays an important role in defining concrete properties.
The texture, shape, size, chemical nature, and gradation of aggregate affect the concrete quality. The choice of the wrong aggregate for concrete can lead to the degradation of concrete quality that, ultimately results in poor performance or failure, or both.
The aggregate-cement compatibility should be checked before use. The high silica aggregate reacts with the alkali of cement and forms deleterious compounds that adversely affect the concrete quality.
The scope of this article is limited to the basic understanding of aggregates, their purpose of usage, and their types depending on their origin and composition.
Types of Aggregates
The aggregates may be classified on the basis of sources, shape, and weight. The different types of aggregates used in construction are briefly discussed below.
Classification Of Aggregates Based Geological Origin
Natural & Artificial Aggregates
The rock fragments that need minimal processing before use are called natural aggregates. The natural aggregates are generally available at the earth’s surface. They are collected by excavating pits in-ground, on riverbeds, and on the seabed.
Before use, the natural aggregates must be washed to clean the foreign materials else the binding material doesn’t work properly. IS 383:1970 (Reaffirmed 2002) lays the guidelines for coarse and fine natural aggregates.
Artificial aggregates are produced from materials that are produced through an artificial process. Examples of artificial aggregates are broken bricks, burnt-clay aggregate, blast furnace slag, and synthetic aggregates.
Crushed & Recycled Aggregates
Concrete grade aggregate is not available everywhere, and hence it needs to be produced. The rocks are quarried and crushed with mechanical means to produce the aggregate. Such aggregate is called crushed aggregate.
They are high-quality aggregates as they are produced under controlled mechanical equipment. The recycled aggregate serves multiple purposes. It fulfills its’ desired duties, and it is sustainable as it is produced from concrete waste.
The debris of the old concrete structures is crushed to produce recycled aggregate. Hence, no quarry is required. The recycled aggregates are an excellent alternative to conservative aggregates in backfilling, drainage, and embankment.
Classification Of Aggregates Based On Grain Size
Fine & Coarse Aggregates
As per IS 383:1970 (Reaffirmed 2002), the aggregates passing through IS Sieve of 4.75mm are termed as fine aggregates, and aggregates retained on the 4.75 mm sieve are identified as coarse aggregates.
The criteria form the basis of segregation between the fine & coarse aggregate. Both types differ largely in properties and find their uses for different purposes.
The general maximum coarse aggregate size used in cement concrete is 40mm, while 4.75-20mm is the most used size. For bituminous concrete, <10mm size aggregate is an apt choice. For mass concrete works, the permitted coarse aggregates sizes are classified as follows-
Sr. No. Class Size
The fine aggregate is generally termed as sand composed of silicon dioxide in the form of quartz. The sand is further classified into 4 zones as per the material passing through IS sieves. The average particle size increases in moving from Zone 1 to Zone 4. Hence, the fineness modulus decreases.
Classification Of Aggregates Based On Shape
Rounded & Angular Aggregates
The rounded and angular correspond to the shape of aggregate. The shape of the aggregate affects the strength of concrete and is hence a point of consideration when selecting the aggregate.
The round aggregates tend to roll over each other and are hence unsuitable for the embankment. They have voids ranging from 30-35%.
The cement paste requirement is low. Due to lesser interlocking and more sliding tendency, the round aggregates are avoided as much as possible.
The angular aggregates are interlocked with each other. Therefore, the angular aggregates offer better strength as compared to the rounded aggregates. Only angular aggregates are used in construction to maintain the quality of concrete.
Pit sands and gravel are examples of irregular aggregates. They have partly nature-made shapes with 35- 37% of voids. Irregularly shaped aggregate offers lesser workability and has bond strength slightly good compared to rounded aggregate but not as required for high-strength concrete.
Flaky & Elongated Aggregates
Flaky aggregates are thin aggregates. Their thickness is less than 60% of the particle’s mean dimension. The flaky aggregates are undesirable as they have more voids and need more cement slurry to bind them together.
Elongated aggregates have a length greater than 180% of the particle mean dimension. Flaky and elongated aggregates offer lower workability and hence should be avoided.
Classification Of Aggregates Based On Shape Weight
Lightweight aggregates have a unit weight of up to 121kn/m3. They are available in either natural such as pumice, diatomite, sawdust, rice husk, volcanic cinders, scoria, or artificial forms such as foamed slag, sintered fly ash, bloated clay, coke breeze, expanded perlite, etc.
Normal Weight Aggregate
Normal weight aggregate produces concrete with unit weight ranging from 23 to 26 KN/m3 with a specific gravity between 2.5 and 2.7. The crushing strength at 28 days of these aggregates ranges between 15 to 40 MPa.
Examples: Sand, gravel, and crushed rocks such as granite, basalt, quartz, sandstone, and limestone.
Heavy Weight Aggregate
Heavyweight aggregates are those having specific gravities ranging from 2.8 to 2.9 and unit weights from 28 to 29 kN/m3.
Quality Of Aggregates
- The aggregate should be hard, clean, and well graded.
- The fineness modulus of natural sand should be in the range of 2.2-3.0. and that of artificial sand should be in the range of 2.4-2.8.
- The moisture content of aggregate should be stable and in artificial sand, it should not exceed 6%.
Uses Of Aggregate In Construction
In construction, the aggregates have multiple uses. Some of the uses are with binding materials and some are independent uses.
- The aggregate is the base material for concrete and provides bulk (70-80% of cement concrete volume is coarse aggregate).
- It limits the shrinkage of cement concrete upon drying.
- The coarse and fine aggregate mixture is used as a base and sub-base course in road construction.
- The coarse aggregate mixed with bitumen is used in wearing coats of roads.
- In railways, the high-strength crushed coarse aggregate is used as ballast to safely transfer the loads and vibration to the subgrade.
- A graded mixture of fine and coarse aggregate is used as filtration media in water and sewage treatment.
- The aggregates are often used as backfilling material.
- Sometimes, the aggregates are used below the foundations as an alternative to boulder soling.
The aggregates have been classified under various heads such as origin, chemical composition, shape & size. An aggregate can fall under multiple categories and hence shouldn’t be confused. For example, an aggregate can be artificial, angular, and coarse at the same time.
While selecting the type of aggregate, the other properties such as bulking, compatibility with cement, thermal insulation, specific gravity, water absorption, cleanliness, etc. should also be given weightage. Also, the economy is a major factor too.
Article By – Shazeb Ali Khan