What makes up concrete?
Concrete is a mixture of sand, gravel, crushed rock and/or other aggregates that are held together by a hardened paste of cement and water. The properties of concrete vary depending on the ingredients used and their proportions in the mix.
The Five Concrete Components
1. Cement/Binder
Cement is the hydraulic binder (hydraulic = hardening when combined with water) which is used to produce concrete. Cement paste (cement mixed with water) sets and hardens by hydration, both in air and under water. The main base materials, e.g. for Portland cement, are limestone, marl and clay, which are mixed in defined proportions. This raw mix is burned at about 1,450 °C to form clinker which is later ground to the well-known fineness of cement.
In Europe, cements are covered by the standard EN 197-1 (composition, specifications and conformity criteria). The standard divides the common cements into five main types, as follows:
- CEM I Portland cement
- CEM II Composite cements (mainly consisting of Portland cement)
- CEM III Blast furnace cement
- CEM IV Pozzolan cement
- CEM V Composite cemen
2. Concrete Aggregates
Concrete aggregates, consisting of sand and gravel, represent the grain skeleton of the concrete. All cavities within this skeleton have to be filled with binder paste as much as possible. Concrete aggregates sum up to approximately 80% of the concrete weight and 70% of the concrete volume. Optimum use of the aggregate size and quality improves the concrete quality.
Aggregates can occur naturally (fluvial or glacial), industrially produced like lightweight aggregates as well as recycled aggregates. For high-quality concrete they are cleaned and graded in industrial facilities by mechanical processes such as crushing, washing, screening and mixing together. Concrete aggregates should have a strong bond with the hardened cement paste, should not interfere with the cement hardening, and should not have negative effect on concrete durability.
3. Concrete Admixtures
Concrete admixtures are liquids or powders which are added to the concrete during mixing in small quantities. Dosage is usually defined based on the cement content. Concrete admixtures have significant impact on the fresh and/or hardened concrete properties and Admixtures can act chemically and/or physically.
4. Concrete Additions and Supplementary Cementitious Materials (SCM)
SCM according to European Standards are defined as finely divided materials used in concrete in order to improve or to obtain desired fresh and hardened concrete properties. EN 206-1 lists two types of inorganic additions:
- Nearly inert additions (type I)
- Pozzolanic or latent hydraulic additions (type II)
5. Water
The suitability of water for concrete production depends on its origin.
Water according to European Standard (EN 1008) lists the following types:
Drinking water
Suitable for concrete. Does not need to be tested.
Ground water
May be suitable for concrete but must be checked.
Water recovered from processes in the concrete industry (e.g. wash water)
Generally suitable for concrete but the requirements in Annex A of the standard must be met (e.g. that the additional weight of solids in the concrete occurring when water recovered from processes in the concrete industry is used must be less than 1% of the total weight of the aggregate contained in the mix).
Sea water or brackish water
May be used for non-reinforced concrete but is not suitable for reinforced or pre-stressed concrete. The maximum permitted chloride content in the concrete must be observed for concrete with steel reinforcement or embedded metal parts.
Natural surface water and industrial process water
May be suitable for concrete but must be checked.
Waste water
Not suitable for concrete.Combined water is a mixture of water recovered from processes in the concrete industry and from a different source.
