Aggregate materials are essential for making concrete mixes more compact and durable. They reduce the consumption of cement and water, and 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 important than what that simple statement implies. Aggregate occupies most of the volume of concrete. It 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. As a natural material, the aggregate will sometimes include worn or unstable particles in the supplied product. As a general rule, the largest aggregate should not have a diameter greater than one-third of the depth of the slab, or one-fifth of the smallest dimension of the shape. 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 mixture.
Recycled concrete will have higher absorption and lower specific gravity than conventional aggregates. This surface moisture in the fine aggregate creates a thick film on the surface of the particles, separating them and increasing the apparent volume. Although aggregate is considered inert filler, it is a necessary component that defines the thermal and elastic properties of concrete and dimensional stability. The ratios between coarse-grained and fine aggregates will change depending on the unique characteristics of each aggregate, the method of placement and the desired finish. While it is possible to mix different sands of different sizes in a manner similar to graded aggregates, generally only one type of sand is used. All aggregates contain some porosity and the specific gravity value depends on whether these pores are included in the measurement.
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. When determining the strength of normal concrete, most concrete aggregates are several times stronger than other components of concrete and are therefore not a factor in normal strength concrete. Good quality aggregate must be clean, hard, strong, have durable particles, and be free from absorbed harmful chemicals, clay coatings or other contaminants that may affect cement hydration or reduce paste-aggregate bonding. The density of aggregates is necessary for dosing mixtures to establish weight-volume ratios. Aggregate is commonly considered inert filler, accounting for 60 to 80 percent of volume and 70 to 85 percent of weight in concrete.
The purpose of aggregate dosing and sizing is to maximize volume while preserving strength, workability and aesthetics. If aggregates are too humid, excess moisture should be subtracted from expected amount of mixing water. When it comes to precast concrete countertop mixes, it's important to remember that aggregate plays an essential role in determining both strength and appearance. It's also important to consider porosity when selecting an aggregate for your project as this can affect both workability and strength. In conclusion, aggregate materials are essential for making concrete mixes more compact and durable while reducing consumption of cement and water. They also contribute to mechanical strength while influencing thermal properties as well as dimensional stability.
Aggregates also play an important role in controlling shrinkage levels while providing aesthetic appeal when used in precast countertop mixes.