Civil Engineering Reference
In-Depth Information
Air entrainment
In freezing conditions, the pore moisture in concrete can freeze and the
resulting expansion of the ice can cause scaling and delamination of the
concrete surface. This is called freeze-thaw damage and is distinct from frost
damage, which occurs when the water in concrete which has not yet hardened,
freezes. Air-entraining agents are added to wet-sprayed concretes to create
a hardened concrete with small, well-distributed air pores, consequently
improving freeze-thaw durability and de-icing chemical scaling resistance.
The ACI Guide to shotcrete 506R (2005) specifies that wet-mix shotcrete
exposed to severe freeze-thaw conditions must be air-entrained, but up
to half the air content can be lost on impact during the spraying process
(Nordstrom, 1996). It can therefore be difficult to achieve an
in situ
air
content above 4% although there is some evidence to suggest that sprayed
concretes require a lower air content (2-3%) for freeze-thaw resistance than
conventionally placed concrete (Austin and Robins, 1995). Airentrainment
can also reduce the flow resistance in a pump line, to some extent acting
as a lubricant between the particles. The entrained air is then forced out
as the mix impacts on the substrate, thus stiffening the mix. However, air
entrainment can sometimes cause difficulties when pumping under high
pressure as the bubbles can be compressed or destroyed, increasing the
possibility of blockage. A pre-construction pumping trial should highlight
any problems with the air entrainment.
Polymers
Latex solutions (such as styrene-butadiene rubber, SBR) are added to improve
bleeding resistance, pumpability and adhesion. It is also claimed that they
can improve the tensile and flexural strength, permeability, abrasion and
chemical resistance of the hardened concrete (Warner, 1995). Research has
recently been completed on the addition of polysaccharide gums in sprayed
concrete, which appear to produce a balance between pumpability and
sprayability by reducing apparent viscosity at high shear rates (e.g. in the
mixer), but have less affect at low shear (i.e. after spraying) and hence allow
good build (Ghio and Monteiro, 1998).
Accelerators
Accelerators are rarely required in low-volume applications but can be
essential when rapid strength development is necessary. They are available in
powdered form (mainly for dry spraying) or liquid form (generally added at
the nozzle). It is important to keep the accelerator dosage as low as possible
to achieve the desired results as strength and durability reduce with age and
dosage level. Accelerators can reduce the 28-day strength of control mixes
without accelerators. Prediction of the setting time and strength gain is
