HOW SUPER ARE SUPERPLASTICIZERS? by Concrete Construction Staff
Superplasticizers transform stiff, low-slump concrete into flowing, pourable, easily placed concrete. They can improve workability, speed finishing, increase strength, conserve cement and help reduce shrinkage and thermal cracking. However, because of their brief history, questions and doubts about their use still exist. Just how super are superplasticizers? Are they problem-solvers or problem-makers?
HOW THEY ARE USED
Superplasticizers can be used in three ways: to create flowing, self-leveling concrete without increasing water, without reducing cement and without sacrificing strength; to produce workable, high-strength concrete by reducing the water and thus the water-cement ratio; or, to save cement by reducing both the water and cement contents while maintaining the same water-cement ratio and the same workability. Although the lignosulfonate-based superplasticizer retards the setting time about an hour when used with certain cement types, superplasticizers generally do not affect the setting time. At the most, they may retard initial set about 15 minutes.
Finishing problems, however, can occur. Because superplasticized mixes, especially flowing superplasticized mixes, have a proportionately large volume of mix mortar, they tend to be sticky. The concrete pulls or drags on the trowel, the surface tears and the mix tends to move under the finisher's weight. Using coarser fine aggregate or a higher proportion of coarse aggregate stiffens the mix and provides a less-mortar-intensive appearance. Another remedy is simply to delay finishing. A superplasticizer is not a cure-all admixture; it has its own problems, which must be taken into account. Nonetheless, dramatic transformation of stiff concrete into flowing concrete by adding superplasticizer instead of water may be the most significant breakthrough in concrete technology since the development of air-entraining agents.