Concrete Products

MAR 2018

Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.

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6 • March 2018 GUEST EDITORIAL BY JOHN KIM Systems and ideologies, such as the three sigma rule or lean six sigma, are used by companies to improve the quality and efficiency of their prod- uct manufacturing while minimizing defects and waste. Usage of the word "sigma" traces back to the mathematical concept of standard deviation (σ), and it is a way to define outliers or, in manufacturing cat- egories, defects. The implementation of these "sigma methodologies" evokes a sense of quality and excel- lence, and therefore combining the word "sigma" and concrete is a creative way to represent high quality concrete. Furthermore, there are six specific attributes of concrete, all starting with the letter "S", that directly contribute to high quality. Begin with cement Portland cement concrete is, perhaps, the most ubiquitous building material in the world. This technology has stood the test of time and it is still a mainstay in our modern-day infrastruc- ture. However, while its success as a material cannot be undermined, there has been limited innovation and improvement with the product since its discovery by the Aspdin family in the 1800s. If the task of improving portland cement were undertaken, each industry and individual would probably come up with different attributes tailored to their specific needs. If the overall improvement of portland cement were to happen, six specific attributes must be considered: speed, strength, shrinkage, sustainability, silica reac- tion, and sulfate resistance. A material or system that embodies all six of the characteristics would be considered a six sigma concrete. Belitic calcium sulfoaluminate cement Some or perhaps all of the qualifications of six sigma concrete can be achieved with a combina- tion of chemical admixtures and or supplementary cementitious materials. However, one interesting solution that embodies all six characteristics is belitic calcium sulfoaluminate (BCSA) cement. This innovative powder was developed in the 1970s, following the discovery of the calcium sulfoaluminate (CSA) compound in the 1950s by University of California, Berkeley Professor Alex- ander Klein. The primary constituents of BCSA cement are belite, a portland cement compound, and CSA. It is a standalone cement that is most often characterized by its extraordinary rapid strength development capabilities. The trade- marked Rapid Set product, manufactured by CTS Cement, is chemically a BCSA cement. Sigma 1 – Speed In any portland cement concrete application, there is always this inevitable bottleneck: curing time. If the strength gain of the concrete could be quickened, it would result in a much more efficient material. In portland cement, alite is the chemical compound that is primarily responsible for the onset of early strength. The more alite you have, the higher the early strength of the cement. For example, ASTM C150 Type III product has the highest concentration of alite among the port- land cement family. The problem with alite is that it takes a few days to begin hydrating and to develop adequate strength. In BCSA cement, there is no alite at all because it has been com- pletely replaced with the CSA compound, which hydrates very quickly, and exhibits a chemi- cal process practically completed in 24 hours. Essentially, BCSA cement is capable of produc - ing structural strength within one hour, while a 28-day portland cement-grade strength is often attained within one day. Sigma 2 – Strength Strength, especially long-term strength, is a criti- cal component in concrete durability. In portland cement, the chemical compound responsible for late age strength is belite. It hydrates significantly slower than alite, but continues contributing to strength development for many years. The more belite you have, the higher ultimate strength you will achieve with the concrete. BCSA cement has a significant amount of belite—about 45 percent by weight. This belite content is much higher than in Type I or Type II portland cements, which are the most commonly used among the ASTM C150 products. The combination of the high belite content along with the strength that comes from the hydration of CSA produces a highly durable material, capable of achieving long service life. Sigma 3 – Shrinkage Shrinkage has long been the bane of portland cement concrete. The use of control joints has somewhat mitigated the effects of shrinkage, but uncontrolled cracking still remains a big issue. From a cement chemistry perspective, there is one primary reason why portland cement con- crete shrinks. Both alite and belite hydrate in order to form calcium silicate hydrate (CSH). CSH gel is a metastable product that will shrink over time as the water leaves the system. BCSA cement has about half of the silicates as found in port- land cement; such reduction drastically lowers the potential for drying shrinkage. Also, the hydration of CSA is very water dependent. With no excess water of convenience, a BCSA concrete will almost never have bleed water that rises to John Kim is Senior Research Engineer at CTS Cement Manufacturing Corp. in Cypress, Calif. He holds bachelor's and master's degrees in Materials Science and Engineering from the University of California, Los Angeles, along with an EDP Certificate from the Wharton School of Business. He has more than eight years of experience in materi- als science, product development, re- search, cement chemistry, pavement design and business development. Six Sigma Concrete: A new standard for high quality

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