9.4 : Superplasticizers

Superplasticizers are advanced admixtures that enhance the workability of concrete by lowering the water content without compromising the strength of the material. These substances are highly effective water reducers, improving concrete flow, making it easier to work with, and enabling concrete to reach inaccessible areas or densely reinforced sections without mechanical vibration. The key components in superplasticizers are either sulfonated melamine or naphthalene formaldehyde condensates, which induce a negative charge on cement particles, leading to mutual repulsion and dispersing them within the mix.

Incorporating superplasticizers in concrete increases the slump significantly, allowing the mix to remain cohesive while avoiding the problems of segregation and bleeding. This property is especially beneficial for creating high-strength concrete with low water-to-cement ratios. By reducing water content by 25 to 35 percent, concrete strengths of up to 15,000 pounds per square inch after 28 days can be achieved. However, the improved workability offered by superplasticizers is short-lived, usually lasting about 30 to 90 minutes, which requires precise timing in their addition to the concrete mix just before placement. While re-dosing superplasticizers is generally not recommended, adjustments can be made to maintain workability for a limited time. Despite their high cost, superplasticizers are valuable for their ability to produce high-strength, durable concrete efficiently.