Concrete is the second most consumed material in the world and results from a mix of cement, water, and aggregates. It is the aggregates that are responsible for much of the material's strength and durability. Choosing the right aggregates is essential to ensure the quality of the concrete produced: you must consider their physical and chemical properties, along with the grain size distribution and shape, in order to obtain concrete with adequate strength, good workability, and durability. In this article, we explain what aggregates are and why they matter for the quality of concrete.
What aggregates are
We call aggregates the materials that, in the early development of concrete, were added to the cement and water mix to give it body, making it more economical. Today they account for about eighty percent of the weight of concrete. Beyond their beneficial influence on shrinkage and strength, the size, density, and shape of the grains can define several of the desired characteristics of a concrete.
It is important to keep in mind that good concrete is not the strongest one, but the one that meets the needs of the project for the element being cast. Therefore, consistency and method of application are, together with strength, factors that define the choice of suitable materials for the mix, which should combine workability with the most economical proportioning.
Origin and size of aggregates
Within this cost-benefit logic, aggregates should have a varied grain size distribution curve and come from deposits near the batching site. This leads to a regionalization in the types of crushed stone, sands, and pebbles that can be part of the mix design.
Regarding grain size, aggregates can be divided into coarse and fine. Coarse aggregate is any aggregate retained on the number 4 sieve (a square mesh with 4.8 mm sides), while fine aggregate is the one that passes through that sieve.
Classification of aggregates
Aggregates can also be classified as artificial or natural. Artificial aggregates are the sands and stones produced by crushing rocks, since they require human action to modify the size of the grains. As examples of natural aggregates, we have sands extracted from rivers or banks and rounded pebbles, which are stones from riverbeds.
Another factor that defines the classification of aggregates is their apparent specific mass, which allows them to be divided into lightweight (expanded clay, pumice, vermiculite), normal (crushed stone, sands, pebbles), and heavy (hematite, magnetite, barite).
Because of the importance of aggregates within the mix, several tests are required before use, which serve to define aspects such as:
- Grain size distribution
- Real specific mass
- Apparent specific mass
- Fineness modulus
- Clay lumps
- Organic impurities
- Powdery materials
- Among others
The ABNT (Brazilian Association of Technical Standards) is the body that defines these tests and how they are carried out. Their results determine whether the aggregates are approved for use in concrete.
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