Chemical Admixtures
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admixtures are the ingredients in concrete other than portland cement,
water, and aggregate that are added to the mix immediately before
or during mixing. Producers use admixtures primarily to reduce the
cost of concrete construction; to modify the properties of hardened
concrete; to ensure the quality of concrete during mixing, transporting,
placing, and curing; and to overcome certain emergencies during concrete
operations.
 Successful
use of admixtures depends on the use of appropriate methods of batching
and concreting. Most admixtures are supplied in ready-to-use liquid
form and are added to the concrete at the plant or at the jobsite.
Certain admixtures, such as pigments, expansive agents, and pumping
aids are used only in extremely small amounts and are usually batched
by hand from premeasured containers.
The effectiveness of an admixture depends on several factors including:
type and amount of cement, water content, mixing time, slump, and
temperatures of the concrete and air. Sometimes, effects similar to
those achieved through the addition of admixtures can be achieved
by altering the concrete mixture-reducing the water-cement ratio,
adding additional cement, using a different type of cement, or changing
the aggregate and aggregate gradation.
Five Functions
 Admixtures
are classed according to function. There are five distinct classes
of chemical admixtures: air-entraining, water-reducing, retarding,
accelerating, and plasticizers (superplasticizers). All other varieties
of admixtures fall into the specialty category whose functions include
corrosion inhibition, shrinkage reduction, alkali-silica reactivity
reduction, workability enhancement, bonding, damp proofing, and
coloring. Air-entraining admixtures, which are used to purposely
place microscopic air bubbles into the concrete, are discussed more
fully in "Air-Entrained Concrete."
Water-reducing admixtures usually reduce the required
water content for a concrete mixture by about 5 to 10 percent. Consequently,
concrete containing a water-reducing admixture needs less water
to reach a required slump than untreated concrete. The treated concrete
can have a lower water-cement ratio. This usually indicates that
a higher strength concrete can be produced without increasing the
amount of cement. Recent advancements in admixture technology have
led to the development of mid-range water reducers. These admixtures
reduce water content by at least 8 percent and tend to be more stable
over a wider range of temperatures. Mid-range water reducers provide
more consistent setting times than standard water reducers.
Retarding admixtures, which slow the setting
rate of concrete, are used to counteract the accelerating effect
of hot weather on concrete setting. High temperatures often cause
an increased rate of hardening which makes placing and finishing
difficult. Retarders keep concrete workable during placement and
delay the initial set of concrete. Most retarders also function
as water reducers and may entrain some air in concrete.
Accelerating admixtures increase the rate of early
strength development, reduce the time required for proper curing
and protection, and speed up the start of finishing operations.
Accelerating admixtures are especially useful for modifying the
properties of concrete in cold weather.
 Superplasticizers,
also known as plasticizers or high-range water reducers (HRWR),
reduce water content by 12 to 30 percent and can be added to concrete
with a low-to-normal slump and water-cement ratio to make high-slump
flowing concrete. Flowing concrete is a highly fluid but workable
concrete that can be placed with little or no vibration or compaction.
The effect of superplasticizers lasts only 30 to 60 minutes, depending
on the brand and dosage rate, and is followed by a rapid loss in
workability. As a result of the slump loss, superplasticizers are
usually added to concrete at the jobsite.
Corrosion-inhibiting admixtures fall into the specialty
admixture category and are used to slow corrosion of reinforcing
steel in concrete. Corrosion inhibitors can be used as a defensive
strategy for concrete structures, such as marine facilities, highway
bridges, and parking garages, that will be exposed to high concentrations
of chloride. Other specialty admixtures include shrinkage-reducing
admixtures and alkali-silica reactivity inhibitors. The shrinkage
reducers are used to control drying shrinkage and minimize cracking,
while ASR inhibitors control durability problems associated with
alkali-silica reactivity.
More on chemical
admixtures. |
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Related Topics:
How Cement is Made
Aggregrate
Supplementary
Cementing Materials
Air
Entrained Concrete
Placing
& Finishing Concrete
Curing Concrete |