The Trump International Hotel and Tower in Chicago pushes the capabilities of high-strength concrete. Self-consolidating concrete up to 16,000 psi was specified. The 92-story, 1134-foot structure (1362 feet including the spire) is the tallest structurally reinforced concrete building in the U.S. Crews topped off the building in August 2008.
One of the highest-strength concretes used in any large-scale commercial application has been concrete attaining a target compressive strength of 19,000 psi in the 58-story, 720-foot Two Union Square in Seattle.
Are there fair measurements?
There are extraordinary differences when comparing the properties of a very high-strength concrete (HSC) having a compressive strength of 20,000 psi to that of a conventional strength structural concrete with a compressive strength of 4000 psi.
When considering adjustments to the principles of mix proportioning necessary to satisfy mixture performance requirements, it's interesting to note that no abrupt change in material technology occurs at any particular strength level or particular water-binder (w/b) ratio. Rather, the changes that occur when progressing up the strength ladder are quite subtle with each advancing step.
As with w/b ratio changes, so do the principles governing mix proportioning progress, which in turn establishes strength and other mechanical properties. To develop an intuitive understanding of how it is possible to produce a concrete four to five times stronger than conventional concrete, any beliefs that the principles governing concrete proportioning change little should be abandoned from this point on.
It is only natural that hydraulic cement concrete would be viewed as a single material. But in reality, concrete is much better understood when viewed as a composite material comprised of two fundamentally different materials—filler (aggregate) and binder (paste). Material properties, principally those mechanical in nature, are fundamentally derived from the relative similarities (or differences) in the properties of the aggregate and paste.
Thus, proportioning guidelines that might be viewed as “best practice” for one strength level might be quite inappropriate for concrete of a different strength level. The requisite properties of constituents and material proportions will subtly vary from one w/b ratio to another.
This article is excerpted from the book,High-Strength Concrete: A Practical Guide, by Michael A. Caldarone. The author is a professional engineer with more than 20 years experience in concrete material engineering and has been involved with many of the world's most notable projects involving high-strength concrete. He is the current chairman of ACI Committee 363 on High-Strength Concrete. You can buy the book from the bookstore at World of Concrete, Feb. 3-6, in Las Vegas.