Page 22 - Masonry and Concrete

Understanding Concrete
21
UNDERSTANDING CONCRETE

of batching and mixing in a controlled
environment with precisely calibrated
equipment. The admixtures most com-
monly used in residential construction
are chemical admixtures, air-entraining
agents, and coloring pigments. The three
most commonly used chemical admix-
tures are set accelerators, set retarders, and WATER
MAX.
water reducers.
1
/ 8 " SEDIMENT
Chemical Admixtures: Set accelerators
speed up the setting time and early strength
2" SAND
development of concrete. This can be help-
ful in winter weather to reduce the length
of time required for curing and protection
and to compensate for the effects of low FIGURE 2-9
temperatures on strength development. Set-
Field test for sand contamination.
ting time can be reduced by one-third to
one-half. Calcium chloride is the most widely used chemical accelera-
tor, but it has a corrosive effect on embedded steel reinforcement and
should never be used in concentrations exceeding 2% of the weight of
the cement. Other chemicals such as calcium nitrite and calcium for-
mate have a less corrosive effect but are not as widely available. So-
called “antifreeze compounds” for concrete are actually set accelerators.
Antifreeze mixtures manufactured for the automotive industry will
severely damage concrete and should never be used.
Set retarders slow down the hydration process so that the concrete
stays plastic and workable for a longer time after mixing. This can be
helpful in hot weather where high temperatures tend to speed up the
normal setting time, and for complicated pours where placement takes
a little longer than usual.
Water reducers lower the amount of mixing water required without
decreasing workability. This can be helpful when the available materi-
als simply will not produce concrete of adequate workability and con-
sistency without exceeding recommended water-cement ratios. In
practice, water-reducing admixtures are typically used only on com-
mercial projects because they require the testing of trial batches of
concrete to determine their effect on other properties.

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