Aggregates

Classification of Aggregates

Aggregates form an essential part of many constructions projects, from large-scale commercial to smaller domestic works. Whether you need aggregates to form a sub-base for foundations or paving, decorative aggregates for driveways and footpaths – or simply need something to fill in unsightly holes – you should know which kind of aggregates will work best.

In this article, we’ll run through the different classifications of aggregates, based on their varying properties.

Contents:

·         Classification of aggregates based on: Geographical Origin

·         Classification of aggregates based on: Grain Size

·         Classification of aggregates based on: Density

 

A) Classification of aggregates based on Geographical Origin

Another way to classify aggregates is by their origin. You can do this with two groups:

·         Natural – 

  • Aggregates took from natural sources, such as riverbeds, quarries and mines. Sand, gravel, stone and rock are the most common, and these can be fine or coarse.

 


Fig. Natural Aggregates


·         Processed – 

  • Also called ‘artificial aggregates’, or ‘by-product’ aggregates, they are commonly taken from industrial or engineering waste then treated to form construction aggregates for high-quality concrete.
  • Commonly processed aggregates include industrial slag, as well as burnt clay.
  •  Processed aggregates are used for both lightweight and high-density concrete mixes.
    

Fig. Artificial Aggregates


 

B) Classification of aggregates based on Density

The variability in aggregates density can be used to classify aggregates of widely different unit weight. The most common classification of aggregates on the basis of bulk specific gravity is 

There are three weight-based variations of aggregates:

·         Lightweight

·         Standard (Normal Weight)

·         High density (heavyweight)

 

Different density aggregates will have many different applications. Lightweight and ultra-lightweight aggregates are more porous than their heavier counterparts, so they can be put to great use in green roof construction, for example. They are also used in mixes for concrete blocks and pavements, as well as insulation and fireproofing.

 

High density aggregates are used to form heavyweight concrete. They are used for when high strength, durable concrete structures are required – building foundations or pipework ballasting.

 

Category

Bulk specific gravity

Example

Typical application

Lightweight

 

Ceramic

Can be swelled or nailed, Also used for its insulating properties

ultra-lightweight

Gs<2.4

Shale or slate, Burnt clay and Crushed bricks

Structural lightweight concrete 1350 to 1850 Kg/ M ³

Masonry unit

Also used for its insulating properties

Normal Weight

2.4<Gs<2.8

Crushed limestone, River gravel, Crushed recycled concrete

Used for normal concrete work, Crushing strength at 28 days between 15 to 40 MPa are termed normal weight concrete,

Produced normal-weight concrete 2200 to 2400 Kg/ M ³

heavyweight

2.8>Gs

Steel or iron shots, Steel or iron Pellets

Produced normal-weight concrete upto 6400 Kg/ M ³

 

 

C) Classification of aggregates based on Grain Size

Aggregates are available in nature in different sizes. The size of aggregate used may be related to the mix proportions, type of work etc. the size distribution of aggregates is called grading of aggregates. Following is the classification of aggregates based on size:

  • Fine aggregate
  • Coarse aggregate

Fine Aggregate

When the aggregate is sieved through 4.75mm sieve, the aggregate passed through it called as fine aggregate. Natural sand is generally used as fine aggregate; silt and clay also come under this category. The soft deposit consisting of sand, silt and clay is termed as loam. The purpose of the fine aggregate is to fill the voids in the coarse aggregate and to act as a workability agent.

Fine aggregate

Size variation

Coarse Sand

2.0mm – 0.5mm

Medium sand

0.5mm – 0.25mm

Fine sand

0.25mm – 0.06mm

Silt

0.06mm – 0.002mm

Clay

<0.002




Coarse Aggregate

When the aggregate is sieved through 4.75mm sieve, the aggregate retained is called coarse aggregate. Gravel, cobble and boulders come under this category. The maximum size aggregate used may be dependent upon some conditions. In general, 40mm size aggregate used for normal strengths and 20mm size is used for high strength concrete. the size range of various coarse aggregates given below.

Coarse aggregate

Size

Fine gravel

4mm – 8mm

Medium gravel

8mm – 16mm

Coarse gravel

16mm – 64mm

Cobbles

64mm – 256mm

Boulders

>256mm

 


Fig. Coarse Aggregates

D) Classification of aggregates based on Shape

The shape is one of the most effective ways of differentiating aggregates. The shape of your chosen aggregates will have a significant effect on the workability of your concrete. Aggregates purchased in batches from a reputable supplier can be consistent in shape if required, but you can also mix aggregate shapes if you need to. The different shapes of aggregates are:

·         Rounded – Natural aggregates smoothed by weathering, erosion and attrition. Rocks, stone, sand and gravel found in riverbeds are your most common rounded aggregates. Rounded aggregates are the main factor behind workability.


Fig. Rounded Aggregates


·         Irregular – These are also shaped by attrition, but are not fully rounded. These consist of small stones and gravel and offer reduced workability to rounded aggregates.



Fig. Irregular Aggregates


·         Angular – Used for higher strength concrete, angular aggregates come in the form of crushed rock and stone. Workability is low, but this can be offset by filling voids with rounded or smaller aggregates.




·         Flaky – Defined as aggregates that are thin in comparison to length and width. Increases surface area in a concrete mix.


·         Elongated – Also adds more surface area to a mix – meaning more cement paste is needed. Elongated aggregates are longer than they are thick or wide.




·         Flaky and elongated – A mix of the previous two – and the least efficient form of aggregate with regards to workability.



Fig. Flat and elongated Aggregates

Knowing the various aggregate classifications is a good starting point when planning a concrete construction project. If you’re in need of specific advice for the kind of aggregates required for your needs, it helps to talk to the pros. A reliable aggregates supplier will be able to provide the perfect selection of aggregates for your project, ensuring they meet the quality requirements to form an integral part of your concrete mix.

 

 










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