Why Flake-Shaped Aggregate is Not Recommended in Concrete

Aggregates are known to be inert fillers in concrete, and their properties have a major impact on the strength, durability, workability and economy of the material. The shape and texture of the aggregate affect the properties of fresh concrete more than hardened concrete. Flake aggregation is normally avoided because it has a significant contribution to gradation and reduces the entanglement characteristics of aggregates within the asphalt mixture. In practice, it is recommended that the amount of flaking aggregate be limited to 25% or less. The flaky shape of the aggregate is when the thickness of the aggregate is much smaller than its length and width.

Elongated or flaky particles in excess of 10 to 15% are generally not desirable. Most natural sands and gravel in riverbeds or seashores are smooth and rounded and make excellent aggregates. 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. Rounded aggregates can slide relative to each other easily, which will produce concrete with maximum workability. Similarly, the larger surface area of angular aggregate means that a greater adhesive force can be developed.

The entanglement between aggregate particles is better than rounded aggregate, but it is not suitable for use in high-strength concrete and stressed pavements. Flaky and elongated aggregates should be limited to 10-15% in concrete. Therefore, smooth, rounded aggregate is used instead of a rough or elongated angular aggregate for better workability. The full role of the shape and texture of the aggregate in the development of concrete strength is unknown, but possibly a rougher texture will result in a greater adhesive force between the particles and the cement matrix. Bulk density measures the volume that the graded aggregate will occupy in the concrete, including solid aggregate particles and the voids between them. In the case of crushed aggregate, the shape of the particles depends not only on the nature of the base material, but also on the type of crusher and its reduction ratio. All aggregates contain some porosity and their specific gravity value depends on whether these pores are included in the measurement.

Therefore, it is important to consider both shape and texture when selecting an aggregate for use in concrete.

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