The classification of fine aggregates within the limits of ASTM C 33 (AASHTO M) is generally satisfactory for most concretes. The size and gradation of aggregates are the most important factors when selecting the right material for a concrete mix. Aggregates can range from fist-sized rocks to fine sand, with anything larger than ¼ of an inch classified as coarse-grained and anything smaller as fine aggregate. 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.
For example, the largest piece of aggregate allowed for a 1 ½ inch thick countertop slab is ½ inch. Coarse-grained aggregate is typically mixed with finer aggregates (such as sand) to fill the gaps between large parts and “lock” larger parts together. This reduces the amount of cement paste required and decreases the amount of shrinkage that could occur. The classification must be such that it can give reasonable workability with minimal segregation, as segregation can lead to a weak and variable final product. Lightweight aggregates are natural such as pumice, diatomite, sawdust, rice husk, volcanic ash, slag, or artificial such as foamed slag, sintered fly ash, swollen clay, coke breeze, expanded perlite, etc. Magnetite (Fe30), barites (BaSo) and scrap metal are used in the manufacture of heavy concrete.
The rounded aggregate has rounded particles with minimum voids ranging from 32 to 33 percent. Even with a fully sand mix, the gradation of the aggregate remains an important factor to consider and affects strength, workability and aesthetics. Coarse-grained aggregates with a size of 20 mm or less are commonly used for most concrete constructions. This type of aggregate is a mixture of rounded and angular pieces, requiring more cement paste for a given workability. The ideal construction aggregate should have a rough surface texture and should be clean, strong and free of coating and other dirt particles. In the case of fine aggregates, the density of the aggregate considered together with the volume of voids or voids between the particles.
Using larger coarse-grated aggregates generally reduces the cost of a concrete mix by reducing requirements for cement, the most expensive ingredient. If a concrete countertop is to be ground with diamond tools, the aggregate will show up, so aesthetics will also affect the choice of aggregates. For fine aggregates, a total deviation of 5% from the zone boundaries is allowed, but not beyond the coarser boundary of Zone I or the thinnest boundary of Zone IV. The shape of the aggregate influences strength but has a more immediate impact on the workability of plastic concrete. 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.
OD monitors the moisture content of coarse-grained and fine aggregates on a regular basis to promote consistency and uniformity from batch to batch. Good quality aggregate must be clean, hard, strong, have durable particles and be free of absorbed harmful chemicals, clay coatings or other contaminants that may affect cement hydration or reduce paste-aggregate bonding. The size distribution of fine to coarse-grained aggregates plays an important role in the workability and performance of concrete. Aggregate can also influence the appearance of the cast surface which is an especially important consideration in precast concrete countertop mixes. The purpose of aggregate dosing and sizing is to maximize the volume of aggregate in concrete (and therefore minimize the volume of cement paste) while preserving strength, workability and aesthetics. 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.
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