Specification - Concrete Mix Design, Quality Control and Specification - page 120

are water cured for 7 days (AS) or 28 days (ASTM) virtually eliminating
the effect of autogenous shrinkage as well as making the concrete quality
significantly better than would be expected on site. Autogenous shrink-
age is a very important component of cracking in higher-strength concrete
because it results in rapid through-section strain, which is often combined
with thermal shrinkage. As a result of more restrictive shrinkage require-
ments in an attempt by the specifier to reduce cracking, many producers
submit higher strength concrete mixtures that will have lower shrinkage in
the standard test procedure but will have higher autogenous shrinkage and
often higher thermal shrinkage due to the increased cementitious content.
Therefore the concrete may have a greater probability of cracking. If the
producer had decided to use a shrinkage-reducing admixture to achieve the
specification, he would have reduced both autogenous and drying shrink-
age, but it would have been more expensive. Reliance on an unrealistic test
procedure or one that does not account for an important factor is always a
danger in performance specification.
Another important issue in specifications concerning shrinkage is the
value of pour strips to mitigate drying shrinkage in slabs. Pour strips are
often proposed to allow the concrete to shrink significantly before closure.
As mentioned earlier, virtually all of the shrinkage that will occur in the
early curing period is the result of thermal and autogenous shrinkage and
this has largely occurred in the first week or two. The different models
would suggest a nominal reduction in drying-shrinkage-induced tensile
stress even with extended closing time.
The introduction of pour strips interferes with the construction sequence,
results in the accumulation of debris, and constitutes a safety hazard on site
as well as resulting in an extra construction joint and a strip of relatively
poor quality concrete at each pour strip. And at the same time provides no
significant reduction in expected drying shrinkage stresses. Accordingly,
as long as the construction delay between casting accommodates early
thermal (and autogenous) movement, we would suggest that pour strips
not be used, particularly for raft slabs where drying shrinkage is greatly
reduced due to one-sided drying and the thickness of the concrete element.
6.6 RHEOLOGY AND
SELF-CONSOLIDATING CONCRETE
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