The Civil Engineering Contractor May 2018 | Page 23

TECHNOLOGY
One of the advantages of hydro demolition is that is leaves rebar untouched while concrete is removed from around it.
iron salts, inorganic salts, and sodium
can severely affect the product. These
are in fact so hazardous to concrete
that they reduce the initial strength
and, in some cases, jeopardise the
strength entirely. Rusting steel in
reinforced cement concrete (RCC)
is another hazard created by the
presence of any of these minerals in
the water.
The presence of acid, alkali, industrial
waste, sanitary sewage, and water,
along with supposedly innocuous
sugar, also reduce concrete’s strength.
Silt or suspended particles in water
is a strength-reducing culprit, as are
oils, such as linseed oil, vegetable oil,
or mineral oil. Any of these present in
water above 2% reduces the strength
of concrete up to 25%.   
Organics, such as plant material
or algae in water used for mixing
concrete, reduce the strength
substantially, while also decreasing
the bond between cement paste and
aggregate.
Before using it for construction, it is
recommended that the water source
is tested through a certified lab and
must conform to national standards.
In South Africa, that is SANS 51008:
“Mixing water for concrete —
Specification for sampling, testing
and assessing the suitability of water,
including water recovered from
processes in the concrete industry, as
mixing water for concrete.”
Water suitable for mixing is also
appropriate for curing. However,
the water used for curing should
not produce any stain or unsightly
deposit on the surface. The presence
of tannic acid or iron compounds in
water meant for curing is considered
objectionable. Seawater should not be
used for mixing or curing.
It is a common concept in
construction work that water fit
for human consumption is generally
acceptable for mixing mortar or
concrete and curing work. However,
the water must be tested before used
for this application. The negligible
cost spent on water testing should
not be a deterrent. Tested water or
treated water can be used, as this
will increase the strength of cement
concrete and enhance the life of a
building.   
Sometimes, after a building has gone
up, it needs to come down again.
Enter demolition, or rather, hydro
demolition.
Hydro demolition
Traditional demolition relies on a
combination of demolition using
explosive (controlled implosions) and
mechanical demolition. These methods
produce a significant amount of dust that
permeates the surrounding environment
and, because of the noise levels involved,
often must be conducted during
scheduled working hours, dependent on
the location of the demolition.
On the other hand, the might of water
is well documented, especially when
the aftermath of natural disasters such
as tsunamis are considered. Entire
buildings are reduced to rubble and
homes to matchsticks. Imagine taking
all that power and forcing it through
a nozzle or jet. This is what hydro
demolition entails.
CEC May 2018 - 21