Significant strides have been made in concrete construction over the years largely because of advancements in concreting materials—in particular, chemical admixtures. Since the development of the first modern-era chemical admixture in the 1930s, admixtures have transformed concrete more rapidly in the past few decades.
In combination with supplementary cementitious materials and the use of advanced mixture proportioning techniques, it is now possible to produce high-performance concretes to meet the ever-increasing demands of owners and the design community. These demands include improved workability and control of the setting characteristics of concrete to facilitate concreting operations, high strength, improved durability and sustainability in concrete construction.
Ever since the introduction of the first water reducer in 1932 by Master Builders, Inc., now a part of BASF Corporation, water reducers have played a central role in concrete production. Mid-range water reducers, typically used in the mid-slump range of 5 to 8 inches, help reduce stickiness and improve pumpability and the finishability of concrete mixtures, including mixtures containing silica fume, or concrete made with manufactured or harsh sand.
Consequently, mid-range water reducers are used quite a bit in flatwork. Advancements in water reducers have predominantly been in high-range water reducers and, currently, polycarboxylate-ethers (PCEs) have led to increased use of higher slump concretes, in particular, self-consolidating concrete (SCC). SCC is proportioned to flow and consolidate under its own weight with minimal, if any, vibration.
As a result, SCC is helping to minimize consolidation-related issues and improving overall surface finish, especially in structures with heavily congested reinforcement. SCC has widespread acceptance in the precast concrete industry and its use in cast-in-place applications is increasing. Iconic buildings, such as the Burj Khalifa in Dubai and the World Trade Center buildings in New York City, are examples of structures where SCC has been used to facilitate construction.
High-range water reducers are also an integral part of high-strength concrete mixtures and concretes proportioned for low permeability and durability, because they provide significant reductions in total mix water content. Concrete structures can now be designed for increased service life in aggressive environments because of durability-enhancing admixtures.
These admixtures include air entrainers for concretes that will be exposed to repeated cycles of freezing and thawing, corrosion inhibitors to protect embedded reinforcement from chloride-induced corrosion, lithium-based admixtures for mitigation of alkali-silica reaction (ASR), shrinkage reducers and a recently introduced crack-reducing admixture.
In addition, there are other specialty admixtures such as anti-washout admixtures for underwater concreting, permeability reducing admixtures for “waterproofing” of concrete and coloring admixtures to improve aesthetics.
Among their many uses, chemical admixtures have also facilitated the production of very high early strength-on-demand concretes for overnight replacement of deteriorated sections of concrete pavements, concretes for long-haul applications through the use of hydration-controlling admixtures and/or workability retaining admixtures that make the 90 minute/300 revs limitation for concrete discharge obsolete, and high-performance environmentally preferable concretes to meet the needs of sustainable professionals.
Overall, the real benefit of chemical admixtures is that they make it possible to produce concrete for challenging applications.
By Charles Nmai, Ph.D., P.E., FACI – BASF Corporation (Admixture Systems).