Gravels, stone and sands form the granular structure, which must have its voids filled as completely as possible by the binder glue. They makeup approximately 80 % of the weight and 70–75% of the volume. Optimum use of the aggregate size and quality improves the concrete quality.
Aggregates can occur naturally (fluvial or glacial); for high-quality concrete they are cleaned and graded in industrial facilities by mechanical processes such as mixing together, crushing, screening and washing (mechanical preparation).
Suitable as concrete aggregates are materials which do not interfere with the cement hardening, have a strong enough bond with the hardened cement paste and do not put the resistance of the concrete at risk.
Standard and special aggregates
Standard aggregates: Density: 2.2–3 kg/dm³. From natural deposits, e.g. river gravel, moraine gravel etc.Material rounded or crushed (e.g. excavated tunnel)
Heavy weight aggregates: Density: > 3.0 kg/dm³. Such as barytes, iron ore, steel granulate.For the production of heavy concrete (e.g. radiation shielding concrete)
Light weight aggregates: Density< 2.0 kg/dm³. Such as expanded clay, pumice,polystyrene.For lightweight concrete,insulating concretes
Hard aggregates: Density> 2.0 kg/dm³. Such as quartz, carborundum;e.g. for the production of granolithic concrete surfacing
Recycled granulates: Density approx. 2.4 kg/dm³. From crushed old concrete etc.
Important terms from the standard (with additional notes):
- Natural aggregate: Comes from mineral deposits; it only undergoes mechanical preparation and/or washing.
- Aggregate mix: Aggregate consisting of a mixture of coarse and fine aggregates (sand).An aggregate mix can be produced without prior separation into coarse and fine aggregates or by combining coarse and fine aggregates (sand).
- Recycled aggregate: Aggregate made from mechanically processed inorganic material previously used as a building material (i.e. concrete).
- Filler (rock flour): Aggregate predominantly passing the 0.063 mm sieve, which is added to obtain specific properties.Particle size group: Designation of an aggregate by lower (d) and upper (D) sieve size, expressed as d/D.
- Fine aggregate (sand): Designation for smaller size fractions with D not greater than 4 mm.Fine aggregates can be produced by natural breakdown of rock or gravel and/or crushing of rock or gravel, or by the processing of industrially produced minerals.
- Coarse aggregate: Designation for larger size fractions with D not less than 4 mm and not less than 2 mm.
- Naturally formed aggregate 0/8 m: Designation for natural aggregate of glacial or fluvial origin with D notgreater than 8 mm (can also be produced by mixing processed aggregates).
- Fines: Proportion of an aggregate passing the 0.063 sieve.
- Granulometric composition: Particle size distribution, expressed as the passing fraction in percent by weight through a defined number of sieves.
Passing fraction, particle size distribution curves
The particle size is expressed by the hole size of the test sieves just passed by the particle concerned.
- Optimum grain shape, crushed/round: Cubic/spherical shapes have proved more suitable than linear forms, which can affect the consistence. Crushed aggregate has a slightly higher water requirement for the same consistence because of its larger specific surface area, but higher concrete compressive and particularly tensile strengths can be obtained due to better interlocking.
- Predominantly crushed aggregates: The surface of crushed materials from rock, large blocks etc. consists only of broken surfaces, while the surface of crushed round material also includes natural rounded areas.Crushed rock material is now mainly used in tunnelling, the motto being:“Extraction point = installation point”.
- Quarry sands: These are angular and also longish or flattish depending on the rock. They are not conducive to a good consistence and their water requirement is generally higher.
- Harmful contaminants: Loam, humus, marl, clay, gypsum and aggregates containing sulphates,chlorides and alkalis are all potentially harmful and their presence and possible consequences must be clarified.