Classification of fine aggregates within the limits of ASTM C 33 (AASHTO M) is generally satisfactory for most concretes. The size and gradation of the aggregate are the most important factors when selecting the aggregate. The aggregate can be large or small, from fist-sized rocks to fine sand. Aggregates larger than ¼ of an inch are classified as coarse-grained aggregates, while any amount smaller than ¼ of an inch is referred to 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 “block larger parts”. 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. Segregation is very important in the production of good concrete.
A workable mix that can produce a strong and economical concrete will result in a final honey-shaped product that is weak, durable and variable if segregation occurs. For RCC work, a 20 mm aggregate is usually used. 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. Rounded aggregate has rounded particles with minimum voids ranging from 32 to 33 percent.
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 graduation curve of any fine aggregate that is completely within the limits of any zone is considered appropriate. Igneous rock aggregates are typically hard, hard and dense with a glassy texture, while sedimentary rock aggregates can range from soft to hard, porous to dense, and light to heavy. Poorly graded concretes generally require excessive amounts of cement paste to fill voids, making them uneconomical. The ideal construction aggregate should have a rough surface texture and be clean, strong and free of coating and other dirt particles. 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.
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. 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. Each region has its own deficiencies in aggregates, but once a combined aggregate gradation (percentage retained) is plotted againsta set of sharp, angular, and rough particles (crushed rock) is called angular aggregate, more cement paste is required to produce high-strength concrete. Aggregate types have little effect on flexural strength; however other researchers claim that higher strength coarser aggregates produce more flexural strength than lower strength aggregates.
These types of aggregates are the mixture of rounded aggregate and angular aggregate; while more cement paste is required for a given workability. Ensure your concrete producer purchases good quality aggregates as verified by regular aggregate test results in accordance with ASTM C 33 Standard Specifications for Concrete Aggregates. This is usually achieved by properly storing aggregates and reprocessing stockpiles to counteract excessive segregation (Photo).