The Beginners Guide to Coarse Aggregate

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You can find coarse aggregates in different areas of the construction industry. They are present in the asphalt mix, they are present as a layer in the pavement under your feet, and they are present in the concrete used for your driveway.

Basically, aggregates are used as an economical option to lower down the cost of concrete usage. In simpler words, we can say that these are the fillers used in concrete mix. The majority of construction applications use different coarse aggregate size, but it is essential to keep the characteristics in mind to get uniform and strong output.

What are Coarse Aggregates?

The proper definition of coarse aggregate depicts that it is the important constituent of concrete and used to give body to the concrete mix while reducing the shrinkage. Around 70% to 80% of the total volume of the concrete is made up from coarse aggregates.

In lots of construction applications, you can see coarse aggregates are performing an integral role, for instance, as a granular base under a slab and as a component in a mixture, such as asphalt or concrete mixtures.

Categories of Coarse Aggregates

Coarse aggregates are generally categorized as per their shape and size. Depending on the shape they can be round, irregular, angular, flaky, and elongated. Apart from that, depending on the coarse aggregate size you can have gravels, cobble, and boulders in the category. From normal strength to high strength concrete, coarse aggregates can be used between the sizes 4mm to 256mm.

Properties of Coarse Aggregate

Because of the variability in the size and shape of coarse aggregates, several test methods are incorporated to identify the most relevant characteristics. The most important key characteristics that are used to describe the properties of coarse aggregates are as follows:

  • Composition: Coarse aggregates are checked for the presence of constituents that will react with alkalis in cement. The test methods to identify the composition help engineers to avoid the alkali reaction which may result in cracking, expansion and deterioration of the concrete mix.
  • Specific gravity: Coarse aggregates can have low and high specific gravity. It is important to identify the characteristic because low specific gravity means the material is porous, weak and absorptive; however, if the aggregate features high specific gravity, then it means the material is of good quality. It is advised to have gravity between the range of 2.6 to 2.9.
  • Bulk Density: The coarse aggregate density depends on the degree of compaction, grading of aggregates and shape of aggregate particles. Usually, it is expressed in kilograms per liter and used to define the weight of the aggregate.
  • Porosity and absorption: Coarse aggregates can also be categorized with the help of their moisture content. From very dry and dry, to saturated and moist categories aggregates are used in varied applications in the construction industry.

How to grade coarse aggregate?

The gradation of coarse aggregate is assessed by sieving the samples. It helps in producing workable concrete which includes all the required proportion of standard fractions of aggregates. The grading zone of coarse aggregate may be described as:

  • Uncrushed Gravel or Stone: Such aggregate is formed by natural disintegration of rocks.
  • Crushed Gravel or Stone: It is the outcome of crushing of gravel or hard stone.
  • Partially Crushed Gravel or Stone: It forms by blending of the above two aggregate.

The above classification is based on the source of aggregates, according to the size, the grading can be done in different zones becoming progressively finer.

Varied properties and standards of the aggregate contribute to the effectiveness of the high-performance concrete; therefore, you should keep the standards in mind while selecting the right category for your application.

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