Concrete Technology - FAQs
For more than 50 years, Grace Construction Products has improved the quality of concrete by developing innovative, value-added Concrete Admixtures and backing that up with a superb level of technical service and support for its customers. The following list of frequently asked questions has been developed to address some of the many issues surrounding the use of concrete admixtures. For additional help or inquiries, contact firstname.lastname@example.org.
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 minimise cracking due to plastic and drying shrinkage?
The use of a properly designed concrete mix with carefully selected materials, as recommended by ACI Committee report 224 on cracking, will help minimise drying shrinkage cracking. Adequate jointing and proper curing will also serve to minimise 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 fibres such as Grace MicroFiberTM will give additional protection.
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, minimises 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, utilisation of retarding admixtures, rapid transport and placement, and proper curing. See ACI Committee report 305 on Hot Weather Concreting.
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.
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, utilisation of low alkali cement, proper curing, and reductions of water flow to the concrete surface will minimise the potential for efflorescence. In addition, the use of a damp proofing product such as Darapel® can help reduce the incidence of effloresence.
How do I minimise 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 to segregate.
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