Page 90 - ICJ Jan 2023

POINT OF VIEW COLLECTOR’S EDITION

COMPARED TO OLD CONCRETE with the current durability requirements for
MIXTURES, MODERN CONCRETE TENDS concrete in our codes, as discussed below.
TO CRACK MORE EASILY DUE TO • When considering the service life of actual
LOWER CREEP AND HIGHER THERMAL structures, the results of laboratory tests
SHRINKAGE, DRYING SHRINKAGE, on concrete durability should be used with
AND ELASTIC MODULUS caution because the cracking behaviour of
concrete is highly dependant on the specimen
LESSONS FROM 20TH CENTURY size, curing history, and environmental
EXPERIENCE conditions. Laboratory specimens are small
and usually not re-strained against volume
The authors have drawn the following conclusions
from the 20th century concrete construction change. Laboratory tests of rich mixtures
containing a fast-hydrating cement may yield
practice.
low permeability values. The same concrete
mixture when used in an actual structure
• A number of field surveys during the
may not prove to be durable if exposed to
20th century have shown that since 1930
whenever cement and concrete strengths frequent cycles of wetting-drying, heating-
cooling, and freezing-thawing. Under similar
were raised, this was generally followed by
a corresponding increase in deterioration circumstances, inadequately cured concrete
containing a high volume of fly ash or slag
problems.
will also crack and deteriorate in the field,
• A gradual increase in the C S content whereas well-cured specimens may have
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and fineness of general-purpose portland given excellent performance in a laboratory
cements have enabled these cements to test on permeability.
develop very high strengths at early ages.
There is a general trend now to produce DURABILITY REQUIREMENTS IN THE
correspondingly high early-strength concrete CODES OF RECOMMENDED PRACTICE
mixtures containing large proportions of
modern portland cement. Compared to old The current concrete construction practice for
concrete mixtures, modern concrete tends structural concrete in the U.S. is governed by the
to crack more easily due to lower creep and ACI Building Code 318 or modified versions of it. The
higher thermal shrinkage, drying shrinkage, code was reformatted in 1989 to emphasize that,
and elastic modulus. There is a close, inverse when durability requirements are important, the
relation between high strength and early-age selection of mixture proportions shall be governed
cracking in concrete. primarily by the durability considerations. Although
the goal is well-intentioned, the recommended
• There is a close relationship between cracking practice to pursue this goal has become
and deterioration of concrete structures counterproductive from the standpoint of building
exposed to severe exposure conditions.
durable and environmentally sustainable concrete
• Premature deterioration of concrete structures. To illustrate this point, an analysis
structures has occurred even when state-of- of how the ACI 318-99 durability requirements
the-art construction practice was followed. would affect the mixture proportions of concrete
This shows that there is something wrong for a reinforced structure, with 3000 lb/in (20
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The Indian Concrete Journal | November 2018 91