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| Retardation/Hot Weather Concrete |
The concrete
should be protected from drying out during handling.
Concreting
is only possible at high temperatures if special protective measures are
provided. These must be in place from the start of concrete production to the
end of curing. They are dependent on the outside temperature, air humidity,
wind conditions, fresh concrete temperature, heat development and dissipation
and the dimensions of the pour.
The fresh
concrete must not be hotter than +30°C during placement and installation
without these protective measures.
Possible
problems
Working with
non-retarded concrete can become a problem at air temperatures over 25°C.
·
Hydration is the chemical reaction of the
cement with the water. It begins immediately on contact, continues through
stiffening to setting (initial set) and finally to hardening of the cement
paste.
·
Each chemical reaction is accelerated at a
higher temperature.
This can
mean that correct and complete compaction is no longer possible.
The normal
counter measures are the use of retarded super plasticizers or super plasticizers
combined with a set retarder.
Retardation terms
and dosing tables
Purpose of
retardation: To extend
the working time at a specific temperature.
Working time: The time after mixing during
which the concrete can be correctly vibrated.
Free
retardation: The
initial set is certain to start only after a specific time.
Targeted
retardation: The
initial set is started at a specific time.
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