AGGREGATE

   Construction aggregate, or simply "aggregate", is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are a component of composite materials such as concrete and asphalt concrete; the aggregate serves as reinforcement to add strength to the overall composite material. Due to the relatively high hydraulic conductivity value as compared to most soils, aggregates are widely used in drainage applications such as foundation and french drains, septic drain fields, retaining wall drains, and road side edge drains. Aggregates are also used as base material under foundations, roads, and railroads. To put it another way, aggregates are used as a stable foundation or road/rail base with predictable, uniform properties (e.g. to help prevent differential settling under the road or building), or as a low-cost extender that binds with more expensive cement or asphalt to form concrete.
     
    
     The American Society for Testing and Materials publishes an exhaustive listing of specifications for various construction aggregate products, which, by their individual design, are suitable for specific construction purposes. These products include specific types of coarse and fine aggregate designed for such uses as additives to asphalt and concrete mixes, as well as other construction uses. State transportation departments further refine aggregate material specifications in order to tailor aggregate use to the needs and available supply in their particular locations.Sources for these basic materials can be grouped into three main areas: Mining of mineral aggregate deposits, including sand, gravel, and stone; use of waste slag from the manufacture of iron and steel; and recycling of concrete, which is itself chiefly manufactured from mineral aggregates. In addition, there are some (minor) materials that are used as specialty lightweight aggregates: clay, pumice, perlite, and vermiculite.

Strength

Normal concrete strengths are lower than natural aggregates. Most aggregates are stronger than the concrete designed or specified strength. In fact, moderate and low strength aggregates can reduce the stress in cement paste and increase the durability of concrete.

ABSORPTION, POROSITY AND PERMEABILITY

In aggregate, the internal pore characteristics are very important. The size and the continuity of the pores in an aggregate particle are always relate to the strength of aggregate, abrasion resistance, surface texture, specific gravity, bonding capabilities, and resistance to freezing and thawing action. Absorption relates to the particles ability to accommodate liquid. Porosity is a ratio of volume of void to the volume of the solid particle. Permeability refers to the particle’s ability for liquids to pass through. If the rock pores are not connected, a rock may have high porosity and low permeability.

Grading of aggregates

Aggregates which are retained on a 5mm BS sieve and bigger are termed coarse aggregates, while those passing 5mm sieve are termed fine aggregates.

Aggregates are described by their maximum size, graded down, eg 14mm, 20mm or 40mm.

It is important to use well graded aggregates in a concrete mix to achieve the following:

-                    the various sizes of particles interlock, leaving the minimum volume of voids to be filled with cement

-                    the particles flow together readily, ie the mix is workable

-                    a lower water/cement ratio resulting in higher strength of  hardened concrete

-                    maximum density for good strength and durability

BS 882: 1992, Specification for Aggregates from natural sources for concrete, British Standard, gives grading limits for the various particle sizes (see Table 16.1). Aggregates for use in concrete must fall within the limits of the grading curves for coarse and fine aggregate. Fig 16.2  shows the grading limits for the various particle sizes; both for coarse and fine aggregates.

The coarse aggregate can be either;

-single size where nearly all of the particles are within 2 successive sizes,
        eg 5-10mm, 10-20mm or 20-40mm

- graded where the smallest size is 5mm, 
        e.g. 5-14mm, 5- 20mm or 5-40mm.

The BS standard subdivides this into 3 divisions:

-                    fine

-                    medium

-                    coarse

Is there an ideal grading which is applicable to all aggregates?

No, for important work, tests need to be carried out to determine the grading (for the particular type of aggregate)  
which gives maximum workability, economy, density, strength and durability in the concrete.

As a guide, 
a ratio of 1 fine aggregate : 1½ to 3 coarse aggregate is 

satisfactory.