By Terry Harris and Ara A. Jeknavorian, Ph.D.
There can be little doubt that some of the most significant changes in concrete material technology are attributed to advancements in chemical admixtures for concrete in the past 20 years. The precast concrete industry has been at the heart of the new engineering properties enabled by chemical admixtures such as moving from 4-in. slump concrete to self-consolidating concrete (SCC). Before we look at what may be ahead in chemical admixtures, let’s take a look at where we’ve been and where we are now.
A little history
Admixtures have been used in concrete and mortar since at least the Roman Empire. The Romans found that certain materials such as milk, blood and lard, as well as organic materials such as molasses, eggs and rice paste allow greater workability in cementitious mixtures.
While the first patent for calcium chloride in concrete goes all the way back to 1873 in Germany, modern admixture technology started with basic air-entraining agents, retarders, accelerators and water reducers in the 1930s in North America.
However, it was not until the 1950s that these types of products began to see widespread use in concrete. ASTM first published its C494 standard in 1962, now titled “Historical Standard: Standard Specification for Chemical Admixtures for Concrete,” which set performance criteria for five types of admixtures: A, B, C, D and E. Types F and G, high-range water-reducing admixtures, were not added to the C494 standard until 1980. In 1962, only 36 states required or allowed the use of admixtures in concrete.
ACI Committee 212 publishes the “Report on Chemical Admixtures for Concrete,” which did not include high-range water reducers (HRWRs) in their document until 1981. While the 1970s saw a sharp increase in the use of admixtures in concrete, a 1982 survey found that only 71% of the concrete produced in the United States contained water-reducing admixtures, and that less than 2% contained HRWRs.
In 1979, the first corrosion-inhibiting admixture was introduced to help mitigate the impact of chloride salt (NaCl) attack on steel reinforcement. Almost 20 years later (1996), shrinkage-reducing admixtures followed and helped to address cracking issues associated with autogenous drying in high-performance concrete.
The 1980s and ’90s continued to see increased use of admixtures in concrete, which included significantly more frequent projects specifying the use of HRWRs as the placement benefits of higher slumps and improved durability of lower water-cementitious material (w/c) ratio concretes were realized.
Still, the biggest change in concrete in North America occurred with the introduction of a new HRWR technology that greatly expanded the plastic and hardened properties of concrete and, in the case of SCC, created a new concrete terminology.
Polycarboxylates and SCC
In the mid 1990s, polycarboxylates(i) in HRWR admixtures were introduced in North America, thus initiating a dramatic paradigm change in our understanding of how to design and use highly workable concrete mixtures. Because of the flexibility, enhanced workability, workability retention with minimal set retardation, and very good finishing characteristics, the acceptance curve for these admixtures was much shorter than anything previously introduced.
Shortly after the introduction of polycarboxylate-based HRWRs, concrete producers began experimenting with SCC in all segments of concrete production; however, nowhere was SCC more rapidly accepted than in the precast market. The ability to fill a mold quickly without vibration, while still maintaining or even improving the plastic and hardened properties of the concrete, made SCC a perfect match for precast concrete producers.
Precasters began using terms like slump flow, viscosity, filling ability, passing ability and rheology to describe this revolutionary type of self-consolidating and non-segregating concrete. Unlike earlier HRWR technologies such as naphthalene (C10H8) and melamine sulfonate (CH3SO2O) condensates, which have fixed-chemistry and limited-performance capability, polycarboxylate technology is highly flexible, meaning that the polycarboxylate polymer can be designed and optimized for a wide range of performance requirements, from high early strength to extended slump life to SCC.
With the emergence of SCC, another class of chemical admixtures – viscosity-modifying admixtures (VMAs) – has been commercialized to address the need for improving the water tolerance and segregation resistance of this highly flowable concrete. The rapid acceptance of VMAs prompted the inclusion of a new admixture category, Type S, in the ASTM C494 standard to assure users that VMAs have no impact on common concrete properties (workability, set, strength and shrinkage).
Where are we now?
A list of the most recent chemical admixtures relevant to precast concrete is as follows:
- General Purpose polycarboxylate-based HRWRs – A HRWR for high flow (≤10 in.) and/or low w/c ratio concretes (≥ 0.40). These superplasticizers would not necessarily be applied for a special performance such as high early strength or self consolidation.
- High Early Strength – A polycarboxylate-based HRWR designed and formulated to enable increased strength at early ages.
- Extended Slump Life HRWR – These admixtures are typically blends of different polycarboxylates, which are engineered to activate or increase slump at different times. The goal of these polycarboxylate-blended products is to maintain concrete mixtures at target workability levels until concrete is placed and consolidated without delaying set times. Several HRWR products have been recently introduced, comprised solely of slowly activating polycarboxylates, which can be added to almost any concrete mix to extend slump life as desired.
- Rheology Modifying Admixtures – These admixtures are designed to impart lubricity to the concrete, especially at very low slumps, resulting in improved formed finish, increased productivity and improved surface texture.
- Viscosity Modifying Admixtures (VMAs) – Viscosity modifying admixtures are most commonly used in SCC when the batch-to-batch variations in aggregate gradation, particle shape and density, and water content make it difficult to consistently produce a stable, non-segregating SCC mix. VMAs function by building a network structure within the pore water that helps minimize water and paste movement, especially once the concrete is in a static state. VMAs have also been used with conventional concrete mixes where the concrete mix may have a tendency to segregate.
- Shrinkage Reducing Admixtures (SRAs) – SRAs help provide some measure of reducing the potential for autogenous shrinkage cracking with high-performance concrete (w/c ratio < 0.40).
Where do we go from here?
Polycarboxylate HRWRs will continue to be the dominant admixture technology, especially in precast concrete. As a result of continually improving the dose efficiency of polycarboxylates, they are already penetrating into normal water-reducing and water-reducing/retarding chemical admixtures. Engineering or designing polycarboxylates to create or modify a specific performance attribute of the concrete mix will also likely be expanded.
Other products that may be on the horizon:
1. Universal Air-Entraining Admixtures – The search continues for “set it and forget it” air entrainment. Some work has been successful, and so admixture providers may not be far away from a product that will allow precast manufacturers to add an admixture to achieve a specific air target and be far less sensitive to the many factors that affect air content.
2. HRWRs – Polycarboxylate technology will continue to be exploited to push the concrete performance envelope. New polycarboxylate-based HRWRs can include admixtures that allow for higher levels of water reduction without compromising concrete workability; admixtures with shrinkage reducing capability; and admixtures for high early strength.
3. SCM Activators – In the push to use more fly ash and slag in precast, admixtures will be required to offset the early strength loss experienced when using fly ash or slag.
4. Nanotechnology(ii) – This has become quite the buzzword in many industries, and the concrete industry is no exception. A few products, based on aqueous suspensions of nano silica and nano C-S-H, have already been commercially introduced into the concrete market. These products are designed to significantly improve early strength (< one-day) development. The benefits from nanotechnology are realized from the very high surface area that nano particles provide versus ordinary portland cement.
5. Internal Curing Admixtures (ICAs) – Controlled release of ICAs for high-performance concretes (> 0.40 w/c ratio) is expected to help mitigate cracking due to autogenous drying shrinkage. The combination of ICA and SRA could represent an interesting synergistic approach to controlling cracks in low w/c ratio concrete.
While the past 20 years have seen amazing changes in the precast concrete industry, new innovative chemical admixtures are coming on stage to meet the increased demand for sustainable concrete, faster construction cycles and growing shortages of quality raw materials.
Terry Harris is North American technical services manager, W. R. Grace & Co., Cambridge, Mass. He has 32 years of experience in the concrete industry, including ready-mix and precast concrete, block and admixtures. Contact him at [email protected]
Dr. Ara Jeknavorian of Jeknavorian Consulting Services is an independent consultant serving the concrete construction industry and is an expert in cementitious systems and chemical admixtures for concrete. He served a 34-year career with the Construction Products Division of W.R. Grace in Cambridge, Mass., and spearheaded the introduction of polycarboxylate-based superplasticizers to the North America concrete market.
“History of Chemical Admixtures,” ACI International, April 1984, by Richard C. Mielenz
(i) Polycarboxylates, in particular, are gaining wide acceptance as dispersants in admixtures. The polymer chemistry can be used to customize admixtures by regulating flow rate, drying time and other variables to meet the needs of specific construction jobs. Admixture makers say the new chemistry supports strong and durable concrete at lower admixture usage levels than traditional dispersants such as ß-naphthalene sulfonate and lignosulfonates.
(ii) Nanotechnology is the engineering of functional systems at the molecular scale. For a discussion of nanotechnology, see “The Next Big Thing in Concrete is Not Big at All” in the January-February 2014 issue of Precast Inc. magazine.