ABSTRACT
Since the third edition of this book, no area of concrete technology has seen greater change than chemical admixtures. These advances in concrete admixtures have facilitated the use of concrete in ever-increasing applications. Table 4.1 from the “Report on Chemical Admixtures” (ACI 212.3R-10) from the American Concrete Institute summarises the vast array of materials available to change the fresh or hardened properties of concrete. Indeed, manufacturers are now able to specifically modify their polymers within these generic groups to further modify certain properties. On the Burj Khalifa project, the technical requirements for concrete included adequate retardation and workability retention for single-stage pumping to 600 m with ambient temperatures up to 50°C as well as achieving over 10 MPa compressive strength at 12 hours and 80 MPa at 28 days. The admixture supplier modified an existing product to achieve the required performance. This “just-in-time” admixture development is far removed from the tortuous testing procedures that used to be required to get a product approved for use. Up to the 1980s, many specifications excluded the use of admixtures, which was clearly not the best way to deal with what was to become one of the most important methods of modifying concrete properties. However, some caution is warranted when using complex chemicals in a very complex chemical system such as concrete. Many of the materials problems with concrete have occurred because of an inadequate appreciation of the interaction of different factors on concrete properties. For example, the early promotion of superplasticisers for flowing concrete sometimes failed to account for the limited ability to control water content in general concreting.
