Should air-entrainment be specified in all concrete?
Air-entrainment should be specified for all exterior concrete
placed in environments subjected to freezing and thawing. Air-entraining
admixtures provide freeze-thaw resistance and improved durability,
along with finishability enhancements and yield control. Air-entraining
admixtures impart these benefits to concrete by purposely entraining
microscopic air bubbles into the paste of the concrete.
How do I minimize cracking due to plastic and drying
The use of a properly designed concrete mix with carefully selected
materials, as recommended by ACI Committee report 224 on cracking,
will help minimize drying shrinkage cracking. Adequate jointing and
proper curing will also serve to minimize drying shrinkage cracking.
A new concrete admixture classified as a "shrinkage reducing
admixture" can reduce drying shrinkage by up to 50%. See our
section on Eclipse®. Protecting
the concrete surface from drying before final set will help to prevent
plastic shrinkage cracking. The use of polyproylene fibers such as
Grace MicroFiberTM will give additional protection.
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Why are air-entraining admixtures now being used in
Controlled Low Strength Material (CLSM)?
The inclusion of 15-35% entrained air in CLSM improves flowability,
minimizes segregation and settlement, lowers unit weight (90-110
pcf), and controls ultimate strength development. In addition,
air, gas and water permeability properties will improve as CLSM
air contents increase.
How do I successfully pour concrete in hot weather?
Pre-pour planning and special precautions are required to assure
successful placement of concrete in hot weather. These precautions
may include lowering material temperatures, utilization of retarding
admixtures, rapid transport and placement, and proper curing.
See ACI Committee report 305 on Hot Weather Concreting.
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Is there such a thing as an "anti-freeze admixture"?
At low temperatures, all concrete can potentially freeze. If
it does not have sufficient strength before it freezes, permanent
damage can occur. The use of chloride and non- chloride accelerating
admixtures at high addition rates can help the concrete achieve
sufficient early strength to prevent the damaging effects of freezing
temperatures. The recommendations of ACI Committee report 306,
Cold Weather Concreting should be followed.
How can I prevent concrete steel reinforcement from
corroding due to the presence of chlorides?
A well designed, durable, low permeability concrete mix, with
an appropriate water to cementitious materials ratio (w/cm) using
a high range water reducer will provide some protection against
corrosion of embedded metals due to the presence of chlorides.
Additional protection can be obtained by further reducing the
permeability of the concrete with the use of Force 10,000® microsilica admixture and including DCI® Corrosion Inhibitor for long term durability.
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Why is proper curing of concrete so important?
Because water is essential for the hydration of cement in concrete,
curing is the single most important action that a contractor or
home owner can take to prolong the life of a concrete structure.
Curing significantly impacts long term concrete properties including:
durability, strength, surface integrity, watertightness, and resistance
to freezing and thawing and deicer salts. In addition, proper
curing can prevent common surface defects such as: dusting, plastic
shrinkage cracking, and crazing or pattern cracking.
How do I prevent efflorescence on concrete surfaces?
Designing low permeability concrete, utilization of low alkali
cement, proper curing, and reductions of water flow to the concrete
surface will minimize the potential for efflorescence. In addition,
the use of a damp proofing product such as Darapel can help reduce
the incidence of effloresence.
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How do I minimize the potential for segregation to occur?
Keep concrete within recommended slump ranges, use properly
air-entrained concrete, and use sufficient vibration for proper
consolidation. The use of high range water reducer rather than
water to produce high slumps will reduce the tendency for concrete
What types of chemical admixtures are available today?
ASTM C494 specifies the requirements for seven chemical admixture
types. They are:
Type A: Water-reducing admixtures
Type B: Retarding admixtures
Type C: Accelerating admixtures
Type D: Water-reducing and retarding admixtures
Type E: Water-reducing and accelerating admixtures
Type F: Water-reducing, high range admixtures
Type G: Water-reducing, high range, and retarding admixtures