Classification of aggregate is essential to determine the particle size distribution for the material. This is because the level of limits and maximum aggregate size affects the amount of aggregate used, as well as cement and water requirements, workability, pumpability, and concrete durability. Generally, fine aggregates are defined as particles that are 4.75 mm or less in size. This means that they can be passed through a No.
4 screen with a mesh size of 4.75 mm. Fine aggregates can include sand, silt, clay, and crushed stone or gravel. It is important to know the density of the aggregates in order to accurately dose the mixture and establish weight-volume ratios. Specific gravity can be calculated by determining densities through water displacement. There are two terms used to distinguish this measurement: absolute specific gravity (ASG) which refers to solid material excluding pores, and apparent specific gravity (BSG) which includes pore volume. The BSG of an aggregate does not necessarily indicate its performance, although it is often specified to meet minimum density requirements.
Although aggregate is considered inert filler, it plays an important role in defining the thermal and elastic properties of concrete and dimensional stability. All aggregates contain some moisture depending on the porosity of the particles and the moisture state of the storage area. The shape and texture of the aggregate have a greater effect on the properties of fresh concrete than hardened concrete. Bulk density measures the volume that the graded aggregate will occupy in the concrete, including solid aggregate particles and the voids between them. Lightweight and ultra-light aggregates are more porous than their heavier counterparts, making them suitable for green roof construction. Surface moisture in fine aggregate creates a thick film on the surface of the particles, increasing their apparent volume.
Other physical and mineralogical properties of the aggregate must be known before mixing concrete to obtain a desirable mix. This is a summary of the most important factors to consider when selecting and dosing concrete aggregate. The permitted percentage of harmful substances for fine and coarse-grained aggregates are listed in Tables 1 and 3 of ASTM C 33, respectively. The content of uncompacted voids is calculated using the difference between the volume of the cylinder and the total volume of fine aggregate and specific gravity of mass material. Natural aggregates are usually extracted from larger rock formations through open excavation (quarry). Optimized grading based on aggregate availability and project requirements will result in cost-effective concrete with good workability and finishability.
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