Aggregate materials are essential for creating concrete mixes that are more compact and reduce the consumption of cement and water. They also contribute to the mechanical strength of concrete, making them an indispensable ingredient in the construction and maintenance of rigid structures. The aggregate is also very important for the strength, thermal and elastic properties of concrete, dimensional stability and volume stability. Cement is more likely to be affected by shrinkage. The inclusion of aggregates in the mixture can control the level of shrinkage and prevent cracking.
The surface texture of the aggregate can be smooth or rough. A smooth surface can improve workability, but a rougher surface generates a stronger bond between the paste and the aggregate creating greater strength. Aggregate in precast concrete is a structural filler, but its role is much more than that. It occupies most of the volume of concrete and is the material that cement paste covers and binds. The composition, shape and size of the aggregate have a significant impact on the workability, durability, strength, weight and shrinkage of concrete.
Aggregate can also influence the appearance of the cast surface, which is an especially important consideration in precast concrete countertop mixes. The gradation of aggregates, whether in a concrete with mortar or in a traditional concrete mix, implies a balance between strength and workability and is always a delicate balance. It is important to use aggregates that meet ASTM C 33, Standard Specifications for Concrete Aggregates. The alkali content and type of aggregate in the system are likely to be unknown and therefore, if mixed with unsuitable materials, a risk of alkali-silica reaction is possible. Both gravel and crushed stone are generally acceptable for manufacturing quality concrete (Photo), although gravel is generally preferred for exposed aggregate. If there is a deficiency in a locally available fine aggregate, concrete may benefit from the addition of air entrainment, additional cement, or a supplemental cementitious material (SCM) to address these deficiencies.
Most natural sands and gravel from riverbeds or seashores are smooth and rounded and make excellent aggregates. If a concrete countertop is to be ground with diamond tools, the aggregate will show, so aesthetics will also affect the choice of aggregates. The purpose of aggregate dosing and sizing is to maximize the volume of aggregate in concrete (and, therefore, minimize the volume of the cement paste) while preserving strength, workability and aesthetics. The density of the aggregates is necessary in the dosing of the mixture to establish weight-volume ratios. The size distribution of fine to coarse-grained aggregates plays an important role in the workability and performance of concrete. This is usually achieved by properly storing aggregates and reprocessing stockpiles to counteract excessive segregation (Photo).
If the aggregates are too humid, excess moisture should be subtracted from the expected amount of mixing water. A track record of good performance of a local aggregate also provides an indication of the material's performance in service. Understanding the implications of aggregate gradation is especially important when creating a mix from scratch and will ultimately help you produce a better precast concrete countertop. This is a summary of the most important factors to consider when selecting and dosing concrete aggregate.