However, the nature of the chemistry that leads to polycarboxylates is rich with possibilities. It allows a chemist to design a dispersant that is an excellent water reducer versus a dispersant that may maintain high levels of workability over longer periods of time. However, the powerfully flexible chemistry behind polycarboxylates is helpful only if one truly understands the nuances of the other materials in the concrete mixture. Ultimately, differences seen in concrete behavior often can be traced back to mineralogical differences in the cements and aggregates. This opens up the door for chemists to optimize a dispersant’s performance based on the predominant mineralogies found in a given material. The design of these next generation dispersants may be based on careful and intelligent manipulation of any of the design parameters for polycarboxylates, or through customized formulations, or both. One thing is for certain—game changing performance is often the result.
The next generation
The latest generation of polycarboxylate admixtures is based on this cutting-edge molecular design and synthesis. One can now take into consideration a significant number of factors and create a dispersant molecule that is custom designed for that scenario. As has been previously mentioned, the concept of these polycarboxylate dispersants being labeled simply as “water reducers” is somewhat outdated. Polycarboxylates do reduce water, however, additional performance characteristics that benefit the concrete construction process also are possible via molecular design. These additional benefits include previously unreachable levels of slump retention, which can result in significant material and production efficiencies for the concrete producer and the contractor. Figure 2 shows the slump retention comparison between three polycarboxylates tailor made for specific applications.
Impressive levels of high early compressive strength also can be achieved, which will impact production efficiencies for contractors, as well as precast concrete producers. Some of these new molecules provide combinations of the previously mentioned characteristics resulting in never before seen concrete performance. For example, a molecule has been developed that will provide slump retention for 45 to 60 minutes while still providing high early strength. Historically, some level of retardation was required to provide slump retention to high early strength concrete mixtures, however, too much retardation would negatively impact the early compressive strength. Because of this need to balance retardation and strength gain, a compromise was required that would not allow one to capture the full benefits of both slump retention and early compressive strength. Figures 3 and 4 compare a first generation high early strength polycarboxylate molecule coupled with a retarder (for slump retention) versus a next generation polycarboxylate designed specifically for slump retention and high early strength without a retarder. The figures show slump retention, rate of hardening, and 14-hour compressive strength respectively.
These new performance combinations provide significantly greater value to the concrete producer and contractor than previous dispersants. In considering the concrete performance, one must take the next step and relate that to actual value for the industry. Performance characteristics, such as slump retention, can provide the following benefits:
- Eliminate or reduce retempering at the jobsite allowing for more consistent and efficient concrete placement
- Improved surface aesthetics due to consistent workability resulting in a reduction in surface patching
- Overall more consistent and higher quality concrete
These next generation polycarboxylate superplasticizers are being used across North America by those who understand the value of their performance. They aid in the development of new concrete mixture types, providing additional value to the contractor, as well as enhancing production efficiencies. For example, these new molecules are being used to produce high-performance mixtures, such as self-consolidating concrete (SCC).
These well engineered mixtures are being used in a variety of concrete applications and the new polycarboxylates have become an important component in these mixtures. Additionally, several large precast concrete producers have begun using the high early strength/slump retaining polycarboxylates to facilitate the placement of high-performing SCC mixtures. This results in more consistent concrete production as well as further improvements to the surface finish of the elements cast with concrete.
This next generation of polycarboxylate superplasticizers is being recognized as more than just a high-range water-reducing admixture. They are being recognized as “performance admixtures.” The incredible feat of this technology is to allow concrete producers to find new ways of producing concrete as well as creating concrete mixtures with new levels of performance. This is perfect timing for an industry moving toward performance-based concrete. One thing is for sure—this is just the beginning.
—Joseph A. Daczko is product manager for High Range Water Reducers and Michael L. Kerns, Ph.D., is product development manager, both for BASF Construction Chemicals. Daczko may be reached at 216-839-7044 or joseph.daczko@basf.com. Kerns can be reached at 216-839-7387 or mike.kerns@basf.com.