Building Design and Construction Handbook - Merritt - Ventech!

9.12 SECTION NINE
Proportions for Normal, Heavyweight, and Mass Concrete,’’ ACI 211.1, and ‘‘Standard
Practice for Selecting Proportions for Structural Lightweight Concrete,’’ ACI
211.2.
One common misconception relative to air entrainment is the fear that it has a
deleterious effect on concrete strength. Air entrainment, however, improves workability.
This will usually permit some reduction in water content. For lean, lowstrength
mixes, the improved workability permits a relatively large reduction in
water content, sand content, and water-cementitious materials ratio, which tends to
increase concrete strength. The resulting strength gain offsets the strength-reducing
effect of the air itself, and a net increase in concrete strength is achieved. For rich,
high-strength mixes, the relative reduction in the ratio of water to cementitious
materials, water-cementitious materials ratio, is lower and a small net decrease in
strength results, about on the same order of the air content (4 to 7%). The improved
durability and reduction of segregation in handling, because of the entrained air,
usually make air entrainment desirable, however, in all concrete except extremely
high-strength mixtures, such as for lower-story interior columns or heavy-duty interior
floor toppings for industrial wear.
Accelerators. Calcium chloride for accelerating the rate of strength gain in concrete
(ASTM D98) is perhaps the oldest application of admixtures. Old specifications
for winter concreting or masonry work commonly required use of a maximum
of 1 to 3% CaCl 2 by weight of cement for all concrete. Proprietary admixtures now
available may include accelerators, but not necessarily CaCl 2. The usual objective
for use of an accelerator is to reduce curing time by developing 28-day strengths
in about 7 days (ASTM C494).
In spite of users’ familiarity with CaCl 2, a number of misconceptions about its
effect persist. It has been sold (sometimes under proprietary names) as an accelerator,
a cement replacement, an ‘‘antifreeze,’’ a ‘‘waterproofer,’’ and a ‘‘hardener.’’
It is simply an accelerator; any improvement in other respects is pure serendipity.
Experience, however, indicates corrosion damage from indiscriminate use of chloride-containing
material in concrete exposed to stray currents, containing dissimilar
metals, containing prestressing steel subject to stress corrosion, or exposed to severe
wet freezing or salt water. The ACI 318 Building Code prohibits the use of calcium
chloride or admixtures containing chloride from other than impurities from admixture
ingredients in prestressed concrete, in concrete containing embedded aluminum,
or in concrete cast against stay-in-place galvanized forms. The Code also
prohibits the use of calcium chloride as an admixture in concrete that will be
exposed to severe or very severe sulfate-containing solutions. For further information,
see ‘‘Chemical Admixtures for Concrete,’’ ACI 212.3R.
Retarders. Unless proper precautions are taken, hot-weather concreting may cause
‘‘flash set,’’ plastic shrinkage, ‘‘cold joints,’’ or strength loss. Admixtures that provide
controlled delay in the set of a concrete mix without reducing the rate of
strength gain during subsequent curing offer inexpensive prevention of many hotweather
concreting problems. These (proprietary) admixtures are usually combined
with water-reducing admixtures that more than offset the loss in curing time due
to delayed set (ASTM C494). See ‘‘Hot Weathering Concreting,’’ ACI 305R, for
further details on retarders, methods of cooling concrete materials, and limiting
temperatures for hot-weathering concreting.
Superplasticizers. These admixtures, which are technically known as ‘‘high-range
water reducers,’’ produce a high-slump concrete without an increase in mixing water.
Slumps of up to 10 in. for a period of up to 90 min can be obtained. This