INTRODUCTION
Aspasa members supply aggregates to Sarma (South African Readymix Association) members and Sarma members have complained that often they get aggregates that are not up to standard.

Some information sharing is therefore needed on the supply of aggregates to readymix companies.

Aggregates generally occupy 70 to 80 percent of the volume of concrete.  Hence, their characteristics influence the properties of concrete.  Aggregates also influence concrete mix proportions.  They must conform to certain requirements and should consist of clean, hard, strong and durable particles free of chemicals, coatings of clay or other fine materials that may affect hydration and bond of the cement  paste.

Weak or laminated aggregate particles are undesirable.  Aggregates containing natural shale or shale particles, soft and porous particles, may have poor resistance to weathering.  Visual inspection will often disclose weaknesses in coarse aggregates.  All aggregates that do not have reasonably reliable service records should be tested for compliance with requirements.

CHARACTERISTICS OF AGGREGATES
Abrasion Resistance:  The abrasion resistance of an aggregate is often used as a general index of aggregate quality.  In abrasion tests, a specified quantity of aggregate is placed in a steel drum that is rotated; the percentage of material worn away during the test is then determined.  Specifications often set an upper limit on this weight loss percentage.

Resistance to Freezing and Thawing:  An important characteristic in exposed concrete, the freeze-thaw resistance of an aggregate is related to its porosity, absorption, and pore structure.  If an aggregate particle absorbs so much water that insufficient pore space remains, it will not accommodate water expansion that occurs during freezing.

Chemical Stability: Aggregates possessing chemical stability with neither neither react chemically with cement in a harmful manner nor be affected chemically by other external influences.

Particle Shape and Surface Texture:  The particle shape and the surface texture of an aggregate influence the properties of fresh concrete more than they affect the properties of hardened concrete.  Rough-textured or flat and elongated particles require more water to produce workable concrete than do rounded or cubicle aggregates.  Hence, aggregate particles that are angular require more cement and water.  Crushed and uncrushed aggregates generally give essentially the same strength for the same cement content.

Aggregate particles should be chunky and fee of excessive amounts of flat and elongated pieces.  Long, slivery aggregate pieces should be avoided.  This requirement is equally important for crushed fine aggregate, since sand made by crushing stone often contains more flat and elongated particles.

Grading:  Grading or particle size distribution of aggregate is determined by a sieve analysis.  There are several reasons for specifying grading limits and maximum aggregate size.  The grading and maximum size of aggregates affect relative aggregate proportions as well as cement and water requirements, workability, economy, porosity, and shrinkage or concrete.  Variations in grading may seriously affect the uniformity of concrete from one batch to another.  Very fine sands are often uneconomical and very coarse sands may produce harsh, unworkable mixes.  In general, aggregates that do not have a large deficiency or excess of any size and give a smooth grading curve produce the most satisfactory results.

Absorption and Surface Moisture:  The absorption and surface moisture of aggregates should be determined so that the net water content of the concrete can be controlled and correct batch weights determined.  Aggregate particles are made up of solid matter and voids that may or may not contain water.

The moisture conditions of aggregates are designated as :

1.  Oven-dry – fully absorbent
2. Air-dry – dry at the particle surface but containing some interior moisture, thus somewhat absorbent.
3. Saturated-surface-dry – neither absorbing water from nor contributing water to the concrete mix.
4. Damp or wet – containing an excess of moisture on the surface; contributes water to the mix.

Batch weights of materials must be adjusted for moisture conditions of the aggregates.  Bulking of sand occurs when fine aggregate is damp and is shovelled or otherwise moved.  Surface moisture holds the particles apart, causing an increase in volume known as “bulking”.  The amount of bulking varies with the moisture content and the grading; fine sands bulk more than coarse sands for a given amount of moisture.  Since most sands are delivered in a damp condition, wide variations can occur in batch quantities if the batching is done according to volume.  For this reason, proportioning by volume is not recommended.

Deleterious Substances in Aggregates
Harmful substances that may be present in aggregates include organic impurities, silt, clay, coal, lignite, and certain lightweight and soft particles.  Most specifications limit the permissible amounts of these substances in aggregates.

Handling and Storing Aggregates
Aggregates should be handled and stored to minimise segregation and prevent contamination with deleterious substances.  Stockpiles should be built up in layers of uniform thickness and not into high cone-shaped piles; this results in segregation.

Equipment used in stockpiling aggregates should not be operated on the stockpiles because this results in degradation of the material and dirt is tracked onto the materials.  Fine aggregate should be handled in a damp state to minimize the wind blowing fines.

Aggregate should be stockpiled so that subsequent movement into storage bins does not contaminate adjacent stockpiled aggregates.  A suitable base should be provided to prevent contamination from underlying materials, and provide proper drainage.

Stockpiles should have ample spacing or suitable wall to prevent intermingling of various aggregate stockpiles.  Stockpiles may require misting with water to compensate for high absorption, or to provide cooling or to prevent blowing by the wind.