Specification - Concrete Mix Design, Quality Control and Specification - page 123

Concrete Society 163 “Guide to the design of concrete structures in the
Arabian Peninsula” was published in 2008 and would classify all of
the elements in contact with the ground and up to 3 metres above
as extreme. For an intended life of only 30 years, the recommended
mix is C60, maximum w/cm = 0.35 and minimum cementitious
content is 400 kg/m 3 for a 70 mm nominal cover. This is based on
66%-75% GGBS or 36%-40% FA or a ternary blend. The guide
also suggests that special structures with extended design lives would
require special consideration and may need enhanced protection,
such as admixtures, corrosion resistant rebar, surface treatment, or
electrochemical methods. This guideline does advocate service life
modeling to refine recommendations.
Unlike the Australian and American codes, the European codes and
guidelines focus on the composition of the cementitious materials used
to achieve durability. Although the authors would agree that fly ash,
GGBS, and other supplementary cementing materials generally have a
significant beneficial effect on durability, there is considerable variation
among different commercially available products. Of particular relevance
to the chloride penetration is the aluminate component, which strongly
influences the chloride binding capacity. A particular problem encoun-
tered recently has been so-called silica fume with a high silicon dioxide
content but low pozzolanicity due to the presence of large quantities of
crystalline silica.
Therefore total reliance on a requirement for particular levels of cement
replacement based on assumed qualities of the materials has some risks in
terms of durability. In addition, confirming the quantity of GGBS, FA, or
silica fume in a hardened concrete sample is also difficult and expensive
in the event of dispute about the quantity used. This has not been consid-
ered a potential problem in most specifications requiring fly ash and GGBS
because they were considered less expensive. The reduced availability of
good products does make this assumption less likely.
In addition to the code requirements, there has been an increasing trend
to specify performance limits based on different transport properties.
Chlorides can penetrate concrete through capillary absorption into unsatu-
rated concrete, wick action, and diffusion through water filled pathways
within the matrix driven by a concentration gradient. However, unlike
compressive strength, there is little information available on the expected
variation in the results obtained as well as on the relationship between such
compliance tests and in situ properties/performance. Indeed, unlike air
entrainment to enhance freeze-thaw resistance, the required performance
for the different specified parameters to achieve the desired durability has
often not been established.