Accelerator (concrete admixture)
As accelerator (short: BE ) are concrete admixtures referred to the solidification or curing of the concrete or mortar accelerate. A distinction is therefore made between setting accelerators and hardening accelerators . Special forms of the first-mentioned means are also the shotcrete accelerators .
Use and properties
Accelerators are used to achieve the required concrete strength as quickly as possible. This can be wanted for various reasons.
In the case of shotcrete, as well as concreting in flowing water or the repair of water ingress, the concrete should solidify immediately after processing in order to enable further layers to be applied or to prevent it from being washed out by water movements.
When building in winter, accelerators are also used as "antifreeze". The faster development of strength means that it is possible to expose the young concrete to frost earlier. Another advantage of the use is the shortening of the stripping times and thus a shorter construction time.
Accelerators have a disadvantageous effect on the final strength of the concrete. In addition, post-hardening and water impermeability are usually reduced. An incorrect dosage can also lead to a “reversal” of the effect, so that the agent acts like a retarder .
A common accelerator for unreinforced concrete is calcium chloride . In order to avoid corrosion, chloride-free accelerators such as alkaline carbonates (e.g. sodium carbonate ) or aluminates such as tricalcium aluminate are used for reinforced concrete . Calcium chloride was first used as an accelerator in 1873. The problem of corrosion effects was recognized as early as 1919, but calcium chloride was added to the concrete with up to 2% (based on the cement content in the concrete) until the early 1960s. In 1963, the use of calcium chloride as a hardening accelerator was banned in Germany.
Individual evidence
- ↑ a b Peter Grübl: Concrete. John Wiley & Sons, 2002, ISBN 978-3-433-01340-3 , p. 108 ( limited preview in Google book search).
- ↑ a b Cement leaflet concrete technology: Concrete additives - admixtures and additives. (PDF; 444 kB) Association of German Cement Works e. V., September 2005, accessed October 17, 2013 .
- ↑ Jochen Stark: Durability of concrete. Springer-Verlag, 2013, ISBN 978-3-642-35278-2 , p. 263 ( limited preview in Google book search).