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CONSTRUCTION WORLD
MAY
2017
32
Reduce permeability – concrete more durable
structure being produced within the paste
with a resulting reduction in permeability.
Similar outcomes can be produced with
slag-based cements, especially with
longer curing times and post 28-day ages.
Silica fume and to a lesser extent finer
fly ash will also densify the usually less
dense transition zone between the paste
and aggregate so helping the concrete to
become less permeable. “The reduction
in permeability can be quite dramatic
with the use of silica fume,” Roxburgh
observes; and
• Finally, the use of well graded smooth
rounded sands that pack well will reduce
water resulting in a less permeable
concrete. Fundamental to producing a
cheap but durable concrete is to use a
good sand.
“Concrete mix design to reduce the
permeability of the concrete should also
always be considered along with the plastic
properties of the concrete. On site, the
formation of cracks, voids and bleed water
lenses and channels within the concrete
will cause permeable concrete so the
plastic properties of the concrete should
be designed to minimise these. Also the
transport, placing, compaction and curing
of the concrete plays just as an important
role in the reduction of permeability as the
original mix design,” Roxburgh adds.
DAMS AND RESERVOIRS
Roxburgh says air and water will carry
chemicals into the concrete where, in the
presence of water, chemical degradation
will start both in the concrete and the
reinforcing steel. Concrete should be
made less permeable right at the mix
design stage. Here Roxburgh provides
three guidelines:
• The first is to reduce the amount of voids
in the form of capillaries and pores in the
paste part of the concrete. This can be
done by either reducing the water content
or increasing the cement content. By
doing the above the water to cement ratio
(w/c) is lowered which leads to a stronger
concrete – and stronger concretes are
typically less permeable than weaker
concretes. “This is an older, well tested
and popular approach to making concrete
less permeable. However, it can be
an expensive approach if the cement
content is increased. So, with an optimal
amount of cement, mix designers should
concentrate on reducing the amount of
water. It is the excess water in concrete
that is responsible for the capillary and
pore sizes and overall volumes. Here
admixtures and especially the plasticising
admixture will play an important role
in the reduction of the water content in
concrete,” he advises;
• A more modern approach is to use
extenders as a portion of the binder.
Using fly ash will result in a finer pore
Many concrete durability problems can be prevented if the concrete is made
less permeable, says John Roxburgh, lecturer at The Concrete Institute’s
School of Concrete Technology.
LEFT:
Over 82 years after being
erected, these concrete intake
towers of America’s Hoover
Dam prove how durable properly
mixed concrete can be.
BELOW:
John Roxburgh, lecturer
at The Concrete Institute’s
School of Concrete Technology.
Many concrete durability problems can
be prevented if the concrete is made less
permeable, says The Concrete Institute.