Table of Contents
Fine and coarse aggregates,cyclopean aggregates
Fine and coarse aggregates,fine aggregate are inert materials mixed with a binding material like cement, lime or mud in the preparation of mortar or concrete. Fine and coarse aggregates shall consist of naturally occurring stones, gravel and sand and shall be hard, strong, dense, durable, clear and free from veins, adherent coating and injurious amounts of disintegrated pieces and deleterious substances. Aggregates shall not contain in excess harmful materials such as pyrites, laminated material, alkali, sea-shells, organic impurities and those which may attack the reinforcement. Aggregates shall not be chemically reactive with alkali of cement. Depending upon the size of their particles the aggregates are classified as fine aggregates, coarse aggregates and cyclopean aggregates.
Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel, and crushed stone are used mainly for this purpose. Recycled aggregates (from construction, demolition, and excavation waste) are increasingly used as partial replacements for natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.
The size distribution of the aggregate determines how much binder is required. Aggregate with a very even size distribution has the biggest gaps whereas adding aggregate with smaller particles tends to fill these gaps. The binder must fill the gaps between the aggregate as well as pasting the surfaces of the aggregate together, and is typically the most expensive component. Thus variation in sizes of the aggregate reduces the cost of concrete. The aggregate is nearly always stronger than the binder, so its use does not negatively affect the strength of the concrete.
Redistribution of aggregates after compaction often creates in-homogeneity due to the influence of vibration. This can lead to strength gradients.
Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative “exposed aggregate” finish, popular among landscape designers.
In addition to being decorative, exposed aggregate may add robustness to a concrete.