August 2013
CONSTRUCTION WORLD
61
The Concrete InstituteMD, Bryan
Perrie, this month concludes
Construction World’s
two-
par t discussion on the
importance of water quality
in the concrete mix:
precast concrete products
THE QUALITY OF THE
mixing water for pro-
duction of concrete can influence the setting
time, the strength development of concrete
and the protection of the reinforcement
against corrosion.
When assessing the suitability of water
of unknown quality for the production of
concrete, both the composition of the water
and the application of the concrete to be
produced should be considered.
SANS 10100-2:1992 states, in clause 4.2.1:
“Water shall be clean and free from injurious
amounts of acids, alkalis, chlorides, organic
matter and other substances that could im-
pair the strength or durability of concrete or
metal embedded in the concrete. (It should
be noted that sea water contains injurious
amounts of chlorides and alkalis.)”
It is recommended that SANS 51008; 2006
be consultedwhen assessing the suitability of
water for use in the production of concrete.
Types of water
In general the suitability of water for the pro-
duction of concrete depends upon its origin.
The following types may be distinguished:
Potable water
This water is considered as suitable for use in
concrete and does not need testing.
Water recovered from processes in
the concrete industry
This water will normally be suitable for use in
concrete, but should be assessed.
Water from underground sources
Thiswatermay be suitable for use in concrete,
but should be tested.
Natural surface water and
industrial waste water
Thiswatermay be suitable for use in concrete,
but should be tested.
Sea water or brackish water
This water may be used for concrete without
reinforcement or other embeddedmetal, but
is in general not suitable for the production of
reinforced or prestressed concrete. For con-
crete with steel reinforcement, or embedded
metal, the permitted total chloride content in
the concrete is the determining factor.
Sewage water
This water is not suitable for use in concrete.
Recommended limits for
various substances in mixing
water
SANS 51008 recommends a series of prelimi-
nary tests which assess the water acidity and
for the presence of:
• Oils and fats
• Detergents
• Suspended matter
• Odours
• Humic matter
Chlorides
The chloride content of the water expressed
as Cl-should not exceed the following levels:
• Prestressed concrete or grout – 500 mg/l
• Concrete with reinforcement or
embedded metal - 1 000 mg/l
• Concrete without reinforcement or
embedded metal - 4 500 mg/l
Sulphates
The sulphate content of the water, expressed
as SO42-, should not exceed 2 000 mg/l.
Alkali
If alkali-reactive aggregates are expected to
be used in the concrete, the water should be
tested for its alkali content. The equivalent
sodium oxide content of the water shall not
normally exceed 1 500 mg/l. If this limit is
exceeded, the water may be used only if it
can be shown that actions have been taken
to prevent deleterious alkali-silica reactions.
Harmful contamination
If contamination is suspected, qualitative
tests for sugars, phosphates, nitrates, lead
and zinc may be carried out. If the qualita-
tive tests show a positive result, either the
quantity of the substance concerned should
be determined or tests should be carried to
determine whether setting time and com-
pressive strength are compromised.
The limits for harmful substances are as
follows:
• Sugars – 100 mg/l
• Phosphates; expressed as P2O5 – 100
mg/l
• Nitrates; expressed as NO3-– 500 mg/l
• Lead; expressed as Pb2+ 100 mg/l
• Zinc; expressed as Zn2+ 100 mg/l
When tested then:
• The initial setting time obtained on
specimens made with the water shall
be not less than 1 hour and not differ by
more than 25% from the initial setting
time obtained on specimens made with
distilled or de-ionised water.
• The final setting time shall not exceed
12 hours and not differ by more than
25% from the final setting time obtained
on specimens made with distilled or
de-ionised water.
• The mean compressive strength
at 7 days of the concrete or mortar
specimens, prepared with the water,
shall be at least 90% of the mean
compressive strength of corresponding
specimens prepared with distilled or
de-ionised water.
For further information, contact The Con-
crete Institute on telephone 011 315 0300 or
or visit the
website
●
Concrete solutions from the
Concrete Institute
Seawater is not suitable for making concrete with steel reinforcement and should not be used in
prestressed concrete due to the risk of corrosion of the steel.
