To achieve a good concrete mix, it is essential to select the right aggregates. Aggregates are commonly considered inert filler, accounting for 60 to 80 percent of the volume and 70 to 85 percent of the weight of concrete. Although they are considered inert, they play a crucial role in defining the thermal and elastic properties of concrete and its dimensional stability. Aggregates are classified into two types: coarse-grained and fine.
Coarse-grained aggregates are usually greater than 4.75 mm (retained at No. The compressive strength of the aggregate is an important factor in its selection. For normal strength concrete, most concrete aggregates are several times stronger than the other components of concrete and therefore do not affect its strength. However, for lightweight aggregate concrete, the compressive strength of aggregates can have a greater influence.
The ideal construction aggregate should have a rough surface texture and be clean, strong and free of coating and other dirt particles. Its density is necessary in the dosing of the mixture to establish weight-volume ratios. Crushed stone produces much more angular and elongated aggregates, which have a higher surface-to-volume ratio, better bonding characteristics, but require more cement paste to produce a workable mix. Most natural sands and gravel in riverbeds or seashores are smooth and rounded and make excellent aggregates.
It is essential to select good quality coarse-grated aggregates for the production of concrete that meets the desired strength and durability requirements. Aggregates containing natural shale or shale-like particles, soft and porous particles and certain types of chert should be especially avoided, since they have poor weather resistance. Elongated, angular, rough-textured particles need more water to produce workable concrete than smooth compact aggregates. Concrete is more workable when a smooth, rounded aggregate is used instead of a rough or elongated angular aggregate.
Other physical and mineralogical properties of the aggregate must be known before mixing the concrete to obtain a desirable mix. Gravel and crushed stone are most commonly used as coarse-grained aggregates to be combined with sand, water and cement to produce concrete. Aggregates also affect concrete shrinkage levels and help prevent cracking, among other advantages. It is important that aggregates should not be reactive with cement alkalis. Bulk density measures the volume that the graded aggregate will occupy in the concrete, including solid aggregate particles and the voids between them.
Aggregates are extracted from natural sand or sand and gravel pits, hard rock quarries, dredging submerged deposits, or mining underground sediment. The ideal construction aggregate should have a rough surface texture and must be clean, strong and free of coating and other dirt particles. To obtain a good concrete mix, it is essential to select the right aggregates that meet all these requirements.
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