Advice to specifiers - Concrete Mix Design, Quality Control and Specification - page 4

1.3 SPECIFYING THERMAL LIMITS
Specifications for temperature rise and differentials in massive pours
require attention. A default peak temperature of 70°C is prudent as it
would virtually eliminate the possible problem of delayed ettringite for-
mation. Many specifiers focus on the temperature differential within the
concrete mass and a value of 20°C is often specified. Most concrete with
crushed aggregate can tolerate a much higher differential without cracking,
28°C would probably be a better default value. However, in our experience,
most thermal cracking has been caused by external restraint of massive
concrete elements by a rigid substrate during cooling. The attention on the
differential temperature requirement in temperate conditions often leads
to excessive insulation and increases both the peak temperature and the
volume of concrete that reached high temperature. Therefore, to reduce a
minor potential problem, the more likely problem is exacerbated.
1.4 SPECIFYING RHEOLOGY
There is a tendency to limit concrete workability in specifications based on
the assumption that lower workability produces better concrete. Although
often true when added water was the only way to increase workability, it
is certainly not true in the age of advanced admixtures. Poor workability
can lead to honeycombing, slower construction, and uncontrolled water
addition after compliance sampling. Resultant defects can lead to costly
repairs and litigation where the specification will come under scrutiny. The
problem of prescriptive specification of rheology can also occur with self-
consolidating concrete (SCC) where overzealous specifiers can require very
high workability parameters, which can lead to segregation. We would
suggest that the specification require that the contractor/premix company
confirm that the rheology of the concrete is satisfactory for the proposed
placement procedure and the mix developed complies with the performance
parameters. Site compliance testing would be used to confirm that the sup-
plier complied with their agreed rheology.
1.5 SPECIFYING DRYING SHRINKAGE
Many specifications include limits on drying shrinkage according to a
standard procedure such as ASTM C157 or AS 1012.13. Although this
may seem prudent and would be expected to reduce cracking, there are a
number of dangers of specifying stringent drying shrinkage limits. The test
procedures are conducted on well-cured small specimens 75 mm × 75 mm
(3 inches × 3 inches) in cross-section dried at 50% relative humidity