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What solutions can be used to overcome challenges
with EcoSmart™ concrete?
With today's competitive schedule demands in concrete
construction, there may be situations when using EcoSmart concrete
presents challenges to workers. Some challenges that may be experienced
and what you can do to overcome them:
Setting time
EcoSmart concrete may take slightly longer to set
than conventional Portland cement concrete. This is due to the relatively
low content of Portland cement, low heat of hydration, and slow
pozzolanic reaction. The longer setting time is particularly important
when stripping cycle times for concrete forms are of concern, and
there are techniques to address this.
Lower ambient temperatures, higher carbon content
of SCM requiring larger doses of air-entraining admixtures, and
increased coarseness of SCM particles all contribute to increasing
the setting time of EcoSmart concrete.
Setting time is typically less of a concern in vertical
elements (columns, walls) than in horizontal elements (suspended
slabs). Portland cement replacement in horizontal elements may be
reduced compared to vertical elements.
Strength development
Long-term strength development is not affected negatively
by slower early-age strength development. Actually, EcoSmart concrete
becomes stronger and more impermeable over a long period of time.
Slow early strength can be addressed by a number of options:
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Keep water and water-to-cementing
materials ratio low. This may involve the use of superplasticizers.
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Reschedule so that forms can be left in
place longer.
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Reschedule so that finishing
activities can be delayed without increasing costs for non-active
crew waiting on site. For example, pour concrete in the afternoon,
hydrate over night, and finish the next morning. |
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Apply proper and timely curing techniques.
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Redesign concrete mixture proportioning
to ensure that an optimum SCM content is used in concrete
to meet required strength levels at selected ages.
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Adjust specifications to avoid requiring
strength more quickly than is actually needed. (Extend the
age of specified strength requirement from 28 to 56 days.)
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Modify construction procedures to suit
EcoSmart concrete's increased set times. Use permanent formwork
such as insulated concrete forms (ICF) or steel deck and cast-in-place
concrete composite flooring system.
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Use high quality SCMs. |
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Use a cement with a more rapid
strength development. |
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Use an accelerator to decrease
the setting time. |
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Lower the dosage of water reducer
to decrease the setting time. |
Curing
To fully develop its excellent long-term durability
properties, EcoSmart concrete, like any type of concrete, needs
adequate curing. The importance of applying wet curing or curing
compounds for sections or elements made of EcoSmart concrete that
are subjected to more severe environmental conditions should not
be underestimated.
The decrease in bleed water may also cause plastic
shrinkage at the concrete surface if the surface not kept moist
starting immediately after placing and during finishing operations.
To achieve proper curing on a project, the contractor
should be required to submit and follow a plan for protection and
curing of EcoSmart concrete, and the quality control on curing should
be increased.
Cold weather
Placement of concrete in temperatures less than 5°C
usually requires alternate measures to maintain production and quality.
This could include reducing SCM content on cold days, increasing
cycle times during cold weeks, using accelerating admixtures on
cold days, or all of the above.
Finishing
Steel trowelled finishes of EcoSmart concrete require
slightly different techniques than conventional Portland cement
concrete. Due to the lower water content, less bleed water rises
to the surface than would in a conventional mix. Finishers may not
be able to rely on bleed water for finishing and may have to adjust
their techniques. There may also be a delay in the time when finishers
can start their work.
When finishers are not able to rely on bleed water,
the use of a high-pressure washer to mist the air and keep the sheen
on the surface has proven effective.
Keeping the crew well informed and trained increases
the success with EcoSmart concrete and raises the satisfaction of
workers on site.
Construction standards
Concrete strength acceptance in North America has
historically been based on a 28-day test. To take into consideration
the slower strength development of EcoSmart concrete, 56-day compressive
strength specification is recommended. In fact, the current CSA
A23.1-04 requires that for EcoSmart mixes with fly ash content >40%,
or slag content >45% (% mass of total cementing materials), or
a combination of fly ash / 40% + slag / 45% > 1.00, the minimum
strength requirements shall be specified at 56 days.
Carbonation
EcoSmart concrete, particularly one containing high
volumes of fly ash, is more susceptible to carbonation, which may
lead to carbonation-induced corrosion of reinforcing steel.
Proper curing, adequate depth of concrete cover to
reinforcing steel, higher concrete strength and lower water-to-cementing
materials ratio.
Scaling
At higher Portland cement replacement levels (e.g.,
>30%), certain surface finishing techniques, the quality of the
concrete, and the curing regime may cause EcoSmart concrete to be
more susceptible to surface scaling particularly when exposed to
freezing and thawing and deicing salts.
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