Top 7 Concrete Admixtures20 Jul 2021, Posted by Concrete Facts in
Whether your current project has you searching for laborers, the best plywood for sale, or high-quality concrete, you want to ensure you are using the best possible products. When it comes to concrete, the addition of admixtures before or during mixing can improve quality, manageability, acceleration, or retardation of setting time. Many concrete mixes available today contain at least one of these admixtures, while some will offer multiple benefits. If you are deciding what admixture is best for your concrete project, read on for a guide to some of the top choices on the market today.
1. Set-Retarding Admixtures
When it is necessary to delay the chemical reaction that occurs when concrete begins the setting process, set retarding admixtures can be a good choice. They are commonly used to reduce the impact of high temperatures, which may lead to a faster initial setting of the concrete poured into concrete forms. A set-retarding concrete admixture is often found in concrete pavement construction products, as the additional time to finish reduces the cost of placing a new concrete batch plant and eliminates cold joints.
Retarders can also be used to resist cracking because of the form deflection that may happen when horizontal slabs are placed into sections. Most also function as water reducers and may entrain some of the air in concrete.
2. Air-Entrainment Admixtures
Air-entrained concrete is used to increase the freeze-thaw durability of concrete. This form of admixture produces concrete that is more workable, while also reducing bleeding and segregation of fresh concrete. This also offers improved resistance of concrete to severe frost action and freeze/thaw cycles. In addition, air-entrainment admixtures also offer high resistance to cycles of wetting and drying, with high workability and durability.
The entrained air bubbles act as a physical buffer against the cracking that can be caused by stresses like water volume augmentation in freezing temperatures. These admixtures are compatible with almost all other concrete admixtures.
3. Water-Reducing Admixtures
A water-reducing admixture is a chemical product that can be added to concrete to create the desired slump at a lower water-cement ratio than what is normally designed. This allows you to obtain a specific concrete strength with lower cement content, which results in lower CO2 emissions and energy usage per volume of concrete produced.
Water-reducers are often used in bridge decks, low-slump concrete overlays, and patching concrete. With recent advancements in admixture technology, it is also possible to find mid-range water reducers.
4. Accelerating Admixtures
When it is necessary to increase the rate of concrete strength development or reduce concrete setting time, an accelerating concrete admixture can be used. Calcium chloride is the most common accelerator component, though it is known to promote corrosion activity of steel reinforcement. However, with concrete best practices like proper consolidation, adequate cover, and proper concrete mixture design, these corrosion issues can be avoided. An accelerating admixture is also useful for modifying the properties of concrete in cold weather.
5. Shrinkage Reducing Admixtures
Added during initial mixing, a shrinkage-reducing admixture can reduce early and long-term drying shrinkage. These are good for situations where shrinkage cracking could lead to durability problems, or where large numbers of shrinkage joints are not desirable for economic or technical reasons. In some cases, a shrinkage reducing admixture can reduce strength development both at early and later ages.
The main reason superplasticizers are used is to produce flowing concrete with a high slump, in the range of seven to nine inches, to be used in heavily reinforced structures and in placements where adequate consolidation by vibration is not easily achieved. Another major application is the production of high-strength concrete at w/c’s from 0.3 to 0.4. For most types of cement, superplasticizer is known to improve workability.
One potential drawback to using a high-range water reducer in concrete is slump loss. High workability concrete that contains superplasticizer can be made with a high freeze-thaw resistance, however air content must be increased relative to concrete without superplasticizer.
7. Corrosion-Inhibiting Admixtures
Used to slow corrosion of reinforcing steel in concrete, corrosion-inhibiting admixtures fall into the category of specialty admixtures. These can significantly reduce the maintenance cost for reinforced concrete structures throughout a typical service life of 30 to 40 years. Corrosion inhibitors often have little effect on strength at a later age, but can accelerate strength in early development.
Calcium nitrate-based corrosion inhibitors do accelerate the setting times of concretes over a range of curing temperatures, unless they have been formulated with a set retarder to offset this effect.