Chemical admixtures - Concrete Mix Design, Quality Control and Specification - page 65

Chemical admixtures
Since the third edition of this topic, 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 prod-
uct 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 proper-
ties. 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 apprecia-
tion of the interaction of different factors on concrete properties. For exam-
ple, the early promotion of superplasticisers for flowing concrete sometimes
failed to account for the limited ability to control water content in general
concreting.
As set out on his website (http://www.kenday.id.au), Ken Day experienced
a situation in which his mix submitted as a competition entry actually
completely failed to set at all. The cause was a complex interaction of the
admixture, the particular cement, and a large proportion of Type C fly ash.
The effect was predictable by the most senior researcher of the admixture
supplier but unknown to senior company technical representatives in both
Australia and the United States. The product is described on the web