Civil Engineering Reference
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unprocessed surface or well water if it can be obtained at a lower cost than
processed water. However, impurities in the mixing water can affect con-
crete set time, strength, and long-term durability. In addition, chloride ions
in the mixing water can accelerate corrosion of reinforcing steel.
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6.10.1
Acceptable Criteria
The acceptance criteria for questionable water are specified in ASTM C94.
After 7 days, the compressive strength of mortar cubes made with the ques-
tionable water should not be less than 90% of the strength of cubes made
with potable or distilled water (ASTM C109). Also, the set time of cement
paste made with the questionable water should, as measured using the Vicat
apparatus (ASTM C191), not be 1 hour less than or 1-1/2 hours more than
the set time of paste made with potable or distilled water.
Other adverse effects caused by excessive impurities in mixing water in-
clude efflorescence (white stains forming on the concrete surface due to the
formation of calcium carbonate), staining, corrosion of reinforcing steel, vol-
ume instability, and reduced durability. Therefore, in addition to the compres-
sive strength and set time, there are maximum chemical limits that should not
be exceeded in the mixing water, as shown in Table 6.6. Several tests are avail-
able to evaluate the chemical impurities of questionable water. Over 100 dif-
ferent compounds and ions can exist in the mixing water and can affect
concrete quality; the more important effects are described in the Table 6.7.
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6.10.2
Disposal and Reuse of Concrete Wash Water
Disposal of waste water from ready-mixed concrete operations is a great con-
cern of the ready-mixed concrete producers (Chini and Mbwambo 1996).
T ABLE 6.6
Chemical Limits for Wash Water Used as Mixing Water (ASTM C94)
(Copyright ASTM, reprinted with permission)
Maximum Concentration
Chemical
(ppm)
Test Method
Chloride, as Cl
ASTM D512
Prestressed concrete or concrete in bridge
decks
500
Other reinforced concrete in moist
environments or containing aluminum
embedments or dissimilar metals or with
stay-in-place galvanized metal forms
1000
Sulfate, as
SO
4
3000
ASTM D516
Alkalis, as
Na
2
O
+
0.658 K
2
O
600
1
2
Total Solids
50,000
AASHTO T26
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