Civil Engineering Reference
In-Depth Information
Mixing water
The requirements for mixing water are the same as for conventional concrete.
Admixtures
Admixtures can add significant flexibility to wet-process sprayed concrete
and can overcome some potential difficulties, such as poor stop/start
flexibility. A water-reducing admixture can be used to reduce the w/c ratio
(and so increase the strength and durability) whilst maintaining the required
workability and pumpability. Stabilisers (sometimes called 'hydration
suspenders') can extend the workable life of the concrete, while accelerators
are added at the nozzle to initiate setting once the concrete has been sprayed.
However, using admixtures costs more and they can be caustic in nature and
may even be detrimental to some properties of the sprayed concrete (e.g.
accelerators can reduce long-term strength). All admixtures are complex
chemicals and advice should always be sought from the manufacturer on
dosage and application rates. Admixtures can alter their behaviour when
added in combination, so care should be taken to ensure compatibility.
Admixtures for sprayed concrete should comply with the requirements of
BS EN 934-2 and BS EN 934-6 or EFNARC (1996). BS EN 934-5:2007
covers admixtures for sprayed concrete.
Superplasticisers and water reducers
Superplasticisers are high-performance water reducers which disperse the
fines more effectively within a mix and therefore improve workability and
cohesion. They need to be used with care and trials are recommended to
determine the optimum dose and properties. The commonest types are
sulfonated melamine formaldehyde condensates (which form a lubricating
film on the particle surfaces) and sulfonated napthalene formaldehyde
condensates (which electrically charge the cement particles so that they repel
each other), although many hybrids now exist. Water reducers are commonly
based on lignosulfonates or hydroxylated carboxylic acids and are utilised in
a similar way to superplasticisers. The dosage of superplasticiser and water
reducer depends on the mix specification, w/c ratio, required workability,
and cement and aggregate types.
Superplasticisers and water reducers can reduce the water/cement ratio
for a given workability (thereby improving the compressive strength and
other properties) or increase the workability for a given w/c ratio (thereby
increasing the pumpability of a mix). The workability will usually return to
normal approximately 20 minutes after the addition of the superplasticiser
(depending on the type) and so it should be added immediately before
placing. They do not adversely affect the final hardened properties of the
concrete, although the setting time may be increased slightly.
