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It
is common for concrete specifications to require strength in 28 days,
but often
strength is not needed at that time. In fact, 28-day strength
requirements are
a limiting factor to durable concrete. Organizations like
Headwaters Resources, USA
are working to promote the concept of extending the 28-day strength
requirement
in the governing specifications to exceed 28 days (e.g. 42 day
strength, 56 day
strength etc).
Typically,
concrete made with fly ash will be slightly lower in strength than
straight cement
concrete up to 28 days, equal strength at 28 days, and substantially
higher
strength within a year’s time. Thus, fly ash
concrete achieves significantly
higher ultimate strength than can be achieved with conventional
concrete.
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(1
psi = 0.007 N/mm2 )
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Fly
Ash concrete is more durable than normal concrete made with ordinary
Portland
Cement. However, present
specifications of 28 day strength do not allow taking
full advantage of benefits that Fly Ash concrete offers.
Developing sustainable
concrete structures to last 100 years or more can require extending the
common
28-day strength specifications. With extended age strength parameters,
better,
more durable concrete can be achieved.
This
extended age acceptance has been approved in some countries
(like in
USA’s “Greenbook, Standard Specifications for
Public Works Construction”, which
is recognized completely or in part across Southern California and into
San
Diego); where previously only 28-day specifications were allowed, now
35, 42,
and up to 56 day strengths are accepted.)
It
is important to note that extended strength parameters are not suitable
for
every application. However, if extended age strengths are
acceptable, higher
percentages of fly ash can be used. With the right expertise, mix
designs can
be amended to reflect a percentage of strength at designated
time-frames with
an ultimate strength overall. Utilizing the experience of the local
ready mix
producer, proper mix designs can be developed to optimize the projects
timeline
in order to achieve the highest quality concrete for the project.
Listed
below are just a few
projects (in U.S.A) driven by the need for long lasting,
high quality concrete that
have allowed for 35-, 42-, and 56-day or more
strength specifications/requirements.
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