Specification - Concrete Mix Design, Quality Control and Specification - page 114

3. GGBS >35% or Type F Fly ash >20%
4. Silica fume ≥5%
5. Use cement with C 3 A between 4% and 10%
Some specifications appear to allow a peak temperature in excess of
70°C on the basis of using supplementary cementitious material (SCMs) in
the concrete, which would reduce susceptibility to DEF. However, DEF is a
very rare problem in concrete but thermal cracking is not. The default value
of 70°C in many specifications may coincide with the generally accepted
value above which DEF may occur, but it is also a reasonable value to
reduce the thermal restraint cracking and possible strength issues without
having to take extraordinary measures.
6.2 ALKALI-SILICA REACTION
Table 6.2 outlines the ASTM procedures to test for alkali-silica reactions
(ASR). The procedures range from rapid accelerated procedures such as
the ASTM C289 chemical test to longer-term procedures such as ASTM
C1293 on concrete, which requires a year or more of exposure. Perhaps the
most reliable procedure is petrographic analysis, but this is expensive and
samples may not include reactive materials.
Based on the problems with alkali aggregate testing, some authorities have
taken the precaution of specifying minimum pozzolanic content in areas where
potentially reactive aggregate may be present. Icelandic cement contains 7.5%
silica fume to help deal with potentially reactive materials. In Queensland,
the use of 20% fly ash is allowed as an alternative to testing for reactivity.
Although these are quite good examples of practical prescriptive specifica-
tions to overcome a technical problem, problems could arise. The Australian
standard for fly ash (AS 3582) does not mention calcium content. Therefore,
if a Type C fly ash from a nontraditional source were used, the assumed pro-
tection against ASR would be greatly reduced in comparison with the Type F
fly ash usually used in Australia. Iceland has its own ferrosilicon plant and a
good quality silica fume, however, if it were to import silica flour marketed as
silica fume (which is becoming more common), the benefit of 7.5% replace-
ment on ASR would evaporate. Some performance tests are still advisable to
protect against poor quality materials or changed circumstances.
6.3 AIR CONTENT
Air entrainment to prevent damage due to freeze-thaw has been widely
studied over many years. The use of air entrainment is widely used in
North America to reduce freeze-thaw damage. An air content of 6% ± 1%,