Flake aggregation is usually avoided due to its significant contribution to gradation and its ability to reduce the entanglement characteristics of aggregates within asphalt mixtures. In practice, it is recommended that the amount of flaking aggregate be limited to 25% or less. Aggregates are considered flaky and elongated when they meet both of the above conditions, and those with more than 10-15% elongation are generally not desirable. The shape and texture of the aggregate have a greater effect on the properties of fresh concrete than on hardened concrete.
Smooth, rounded aggregates are more workable than rough or angular ones, and natural sands and gravel from riverbeds or seashores make excellent aggregates. Crushed stone, however, produces much more angular and elongated aggregates, which have a higher surface-to-volume ratio and better bonding characteristics, but require more cement paste to produce a workable mix. Flaky and elongated aggregates should be limited to 10-15% in concrete. Rounded aggregates can slide relative to each other easily, resulting in concrete with maximum workability.
The shape of crushed aggregate depends not only on the base material but also on the type of crusher and its reduction ratio. Elongated particles are those whose length is much greater than their thickness and width. These different aggregate properties give designers and contractors the flexibility to meet their design and construction requirements. Rounded aggregates are obtained when they are completely worn out by water or when they are completely shaped by wear.
Smooth surfaces improve workability, but rougher surfaces generate a stronger bond between the paste and the aggregate, resulting in greater strength. The weight of the aggregate depends on its moisture content, so a constant moisture content is required. Although aggregate is considered inert filler, it plays an important role in defining the thermal and elastic properties of concrete and its dimensional stability. Cuboidal aggregates can be used to increase workability without compromising strength and durability.
All aggregates contain some porosity, and their specific gravity value depends on whether these pores are included in the measurement. Similarly, angular aggregates have a larger surface area, which means that a greater adhesive force can be developed between them. Therefore, angular aggregates are ideal for high-strength concrete and pavements under stress. When determining the strength of normal concrete, most concrete aggregates are several times stronger than other components of concrete, so they do not affect its strength.
The angularity number is calculated based on the percentage of voids in the aggregates after compaction following certain criteria. The flaking and shape of coarse-grained aggregate generally have an appreciable effect on the workability of concrete. Other physical and mineralogical properties of the aggregate must also be known before mixing concrete to obtain a desirable mix.
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