Civil Engineering Reference
In-Depth Information
tions, moderate sulfate resistant cements, Types II or MS,
should be used along with a low water-cement ratio.
Sodium or potassium in salts present in seawater
used for mix water can aggravate alkali-aggregate reactiv-
ity. Thus, seawater should not be used as mix water for
concrete with potentially alkali-reactive aggregates.
Seawater used for mix water also tends to cause efflo-
rescence and dampness on concrete surfaces exposed to
air and water (
Steinour 1960
). Marine-dredged aggregates
are discussed in Chapter 5.
provided quick set is not induced. Higher concentrations,
however, may reduce concrete strength.
Potassium hydroxide in concentrations up to 1.2% by
mass of cement has little effect on the concrete strength
developed by some cements, but the same concentration
when used with other cements may substantially reduce
the 28-day strength.
The possibility for increased alkali-aggregate reactiv-
ity should be considered.
WASH WATER
ACID WATERS
The U.S. Environmental Protection Agency and state agen-
cies forbid discharging into the nation's waterways
untreated wash water used in reclaiming sand and gravel
from returned concrete or mixer washout operations. Wash
water is commonly used as mixing water in ready mixed
concrete (Fig. 4-4) (
Yelton 1999
). Wash water should meet
the limits in Tables 4-2 and 4-3.
Acceptance of acid mixing water should be based on the
concentration (in parts per million) of acids in the water.
Occasionally, acceptance is based on the pH, which is a
measure of the hydrogen-ion concentration on a log scale.
The pH value is an intensity index and is not the best
measure of potential acid or base reactivity. The pH of
neutral water is 7.0; values below 7.0 indicate acidity and
those above 7.0 alkalinity (a base).
Generally, mixing waters containing hydrochloric,
sulfuric, and other common inorganic acids in concentra-
tions as high as 10,000 ppm have no adverse effect on
strength. Acid waters with pH values less than 3.0 may
create handling problems and should be avoided if possi-
ble. Organic acids, such as tannic acid, can have a signifi-
cant effect on strength at higher concentrations (Fig. 4-3).
INDUSTRIAL WASTEWATER
Most waters carrying industrial wastes have less than
4000 ppm of total solids. When such water is used as
mixing water in concrete, the reduction in compressive
strength is generally not greater than about 10%-15%.
Wastewaters such as those from tanneries, paint factories,
coke plants, and chemical and galvanizing plants may
contain harmful impurities. It is best to test any waste-
water that contains even a few hundred parts per million
of unusual solids.
ALKALINE WATERS
Waters with sodium hydroxide concentrations of 0.5% by
mass of cement do not greatly affect concrete strength
100
28 day tests
Concrete cylinders:
75 mm x 150 mm
(3 in. x 6 in.)
80
60
40
20
0
0
1.0
2.0
3.0
4.0
5.0
Tannic acid, percent of mass of mixing water
Fig. 4-4. Reclaiming system allows immediate reuse of
wash water in batching. (69901)
Fig. 4-3. Effect of tannic acid on the strength of concrete
(
Abrams 1920
).
