Concrete future - Concrete Mix Design, Quality Control and Specification - page 282

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, it should be noted
that most “shrinkage” cracking is due to plastic, thermal, and autogenous
shrinkage (in that order) not drying shrinkage. Drying shrinkage tests are
conducted on well-cured small specimens 75 mm × 75 mm (3 inches × 3
inches) in cross-section dried at 50% relative humidity and therefore not
representative of standard concrete elements exposed to drying in most envi-
ronments. In situ drying shrinkage is a slow process. Pour strips interfere
with construction and do virtually nothing to accommodate drying shrink-
age strains in thicker slabs. Higher-strength concrete with higher cementi-
tious contents tends to exhibit lower shrinkage in these tests. However, such
mixes may have greater movement due to higher peak temperatures and
more autogenous shrinkage, which are not measured in the test.
The well-meaning but poorly thought through use of a performance
criterion may reduce both sustainability and concrete performance.
13.1.5 Regulations
Standard concrete production in Australia has essentially been based on
compressive strength performance for more than 20 years. The Australian
system resulted in good concrete producers, with well-equipped, suit-
ably staffed, and accredited laboratories, designing and controlling a
range of mixes to meet the specified strength. Concrete producers often
prepared monthly reports on the mixes on the ConAd system, which were
circulated to the purchasers. In the event of any marginally low result being
predicted from early tests, the producer was expected to inform the pur-
chaser of the concrete in question. The better producers generally use a
graphical and statistical control system on concrete and input materials
data, which helped identify any problem at an early stage.
The result of this system has been that typical concrete in Australia has
a standard deviation of strength of between 2 and 3 MPa, well below most
other countries. A lower standard deviation means a lower target mean
strength and lower cementitious contents with reduced cost and environ-
mental impact. Investment in quality control and quality testing was effec-
tively incentivised.
The situation could not be more different for concrete from the same
suppliers to various projects that must comply with prescriptive specifica-
tion requirements, particularly when additional performance requirements
have been added. Some state authorities have prescriptive specification
requirements to which performance requirements have been added, such
as chloride diffusion, sorptivity, or volume of permeable voids. Mixture
proportions and material suppliers have to be registered with the state
authority. The considerable time and cost involved in obtaining registration