Concrete future - Concrete Mix Design, Quality Control and Specification - page 283

is a disincentive to ongoing mix development and upgrading to more
advanced admixtures and so on. The standard deviation of these registered
mixes with minimum cementitious content and other requirements can be
up to double that of the standard mixes, which are controlled by the premix
supplier to achieve a strength requirement only. When the variability in
strength increases so does the variation in other properties.
An unexpected consequence of durability performance specification has
been the submission of inappropriate concrete mixtures just because they
had the necessary test data so the producer did not have to conduct addi-
tional trial mixes and long or expensive testing. For a structural element
with minimum thickness varying from 0.45 m to 1.8 m and a specified
strength of 50 MPa, the premix company proposed a mix with a cementi-
tious content of 635 kg/m³ incorporating 25% fly ash. The mix was pro-
posed because it had been approved by the appropriate statutory body and
the performance criteria had been met. Modeling the proposed mix showed
that he estimated peak temperature was 98°C (208°F) and the differential
was more than 60°C (108°F). The mix may have achieved the required
chloride diffusion but, if it had been used in this application, it would
have resulted in severe cracking and significant delayed ettringite forma-
tion potential. In addition, the mix used over 200 kg/m³ more cementitious
material than was necessary. In this situation, a performance requirement
intended to improve durability and a registration procedure intended to
ensure compliance could have resulted in the use of a totally inappropriate
concrete mixture with serious consequences in terms of premature deterio-
ration and waste of resources. This was simply to avoid additional testing
and paperwork caused by the specification and regulations.
The Heart of Doha is an urban redevelopment in the historic center of
the city. It will transform the district into a network of sustainable inter-
connecting buildings, public squares, courtyards, and landscaped streets.
I was involved in helping improve the sustainable use of concrete for the
project, which is targeting LEED™ Gold. Qatar had regulations prevent-
ing the establishment of a batching plant within the city—the “not in my
backyard” rule. A comparison of the transport requirements for off-site
compared with on-site concrete production of the estimated 1.25 million
cubic metres showed a reduction of 55% in terms of truck kilometers for
on-site production. Other important sustainability benefits were the abil-
ity to reduce loads on road infrastructure, reduce rejection, of noncompli-
ant concrete, and reduce disruption to city traffic. Based on these benefits,
permission was given to have a site plant.
13.1.6 Standards
Standards have necessarily been developed from the prevalent construc-
tion practices. Indeed, the time taken to develop standards means that they