Properly applied chemical ad-mixtures can enhance the properties of concrete in its plastic and hardened state. Some ad-mixtures make the mix easy to pump or make it self-consolidating. Others make the hardened concrete stronger or corrosion-resistant. Many building techniques and high-performance uses of concrete simply wouldn't be possible without admixtures. To better understand these important products and how they can be employed in concrete, the best place to start is ACI 212.3 “Chemical Admixtures in Concrete.”
Water-reducing and set-controlling admixtures represent one of the more important groups of chemical admixtures used in concrete. These admixtures reduce the water requirement of a concrete mixture for a given slump or increase the slump for a given water content. Decreasing the water content reduces the water-cement ratio, thereby increasing the resulting concrete's strength.
These admixtures can be broken into three main categories. Conventional water-reducing admixtures—also known as low-range water reducers—represent the first generation of water-reducing admixtures that have been around for more than 50 years. ACI 212.3 describes several different chemical varieties, though they generally consist of organic compounds such as lignosulfonates, naphthalene, and hydroxylated carboxylic acids salts.
Low-range water reducers can decrease the amount of water by at least 5%, though typically they come with side effects, including entraining excessive amounts of air and retarding the concrete's setting time.
To combat the retarding effect, water-reducing admixtures have been combined with accelerating admixtures to make a new type of admixture with greater abilities. These admixtures, marketed as midrange water reducers, can provide water reduction up to 12% and often meet the requirements of ASTM C 494 for Type A water-reducing admixtures.
High-range water reducers (HRWR), also known as superplasticizers are covered in ACI 212.4.
When used properly, they can increase the strength of the concrete and provide greatly increased workability without adding more water. HRWR are commonly used to produce flowing, self-consolidating, or high-performance concrete. They should reduce the water requirement by at least 12%—with ACI reporting possible reductions as high as 30%—and meet the requirements of ASTM C 494 for Type F and G water-reducing admixtures or ASTM C 1017.
A new class of high-range water reducers, is currently revolutionizing the marketplace. Introduced in the early 1990s, polycarboxylates have proven extremely useful in many concrete mixes.
ACI 212.4 stresses the use of field trial batches when using polycarboxylates, or any kind of superplasticizer to identify and correct potential problems, including rapid slump loss, excessive bleeding, or segregation. It also is important to be aware of when the HRWR is added to the mix (either at the batch plant or onsite), because this can significantly affect slump loss and setting time.
How superplasticizers work
Naphthalene and melamine superplasticizers involve surface chemistry. The polymers stick to the cement grains, increasing their negative charge. The grains then repel one another, dispersing through electrostatic repulsion. Polycarboxylates provide even better cement dispersion through electrostatic repulsion and steric hindrance.
This dispersing effect means there is more cement surface area available for hydration, so there is better cement efficiency. The results are higher strengths with the same cement and water content.
The end result of using polycarboxlyates is increased productivity-through faster placing, finishing, and form stripping and improved quality.
While the effects are often less with low- and midrange water reducers, superplasticizers' uses in different types of concrete projects demonstrate the important role of water-reducing admixtures.
Superplasticizers can be effective, for example, when making high-strength concrete (6000 psi or more) and are added to mixes with water-cement ratios below 0.35 to ensure control of the water and again in the field for placing purposes. HRWR's effect of early strength gain in concrete and easy placement has been recognized by the pre-cast industry. They are also commonly found in self-consolidating concrete. Added to architectural concrete mixes, they increase the concrete's workability and prevent bug holes.
Parking structures, which must endure constant exposure, often require low water-cement ratio, low permeability, air-entrained concrete with HRWR. Commercial and high-rise concrete buildings—which employ gang forming or tunnel forming—can benefit from the use of HRWR in the mix because its quick strength gain permits early form stripping and rapid cycling of the formwork.