Special concrete and mortar
ü High
Aluminates Cement Concrete
ü Shrinkage
Compensated Concrete
ü Polymer
Cement concrete
ü Polymer
Impregnsed Concrete
ü Epoxy
Concrete
ü Latex
modified concrete
Necessity of adding concrete
chemicals
ü To
improve the performance of concrete
ü To have
the early strength gain as early as possible
ü To
accelerated the setting time of concrete
ü To make
the structure waterproof
Special elements for accelerated
strength
In repairs of certain structures,
particularly roadways and bridges, it may be desired that early strength gain
should be rapid as possible. The engineer may, as a first approach, consider
using admixtures, so that ordinary types of Portland cement can be used. The
chief chemical admixture
Now used for this purpose is
Super plasticizer of one type or another. Formerly, high doses of calcium
chloride were advocated, but this producer has been rejected on the basis of
corrosion, problems associated with calcium chloride were advocated, but this
procedure has been rejected on the basis of corrosion, problems associated with
calcium chloride use. The time of setting of Portland cement concrete and its
strength gain may be shortened by the use of calcium aluminates cement, because
of problems associated with the conversion under hot humid conditions, of the
calcium aluminates hydrates from one from to another, and the resultant
strength losses, other types of cements have been preferred.
Regulated set cement is a modified Portland
cement, which contains a substantial amount of calcium fluro-aluminate. This
cement contains a substantial amount of fluorite as a substitute for limestone.
The burning process is not without problems, due to the release of small amount
of fluoro compounds.
When prepared and ground, the
initial and final set of this type of cement occurs almost simultaneously, and
therefore the time between mixing and set is often referred to as the handling
time.
As a rule, this varies between 2
and 45minutes. The strength level is adjusted by controlling the amount of
calcium fluoro-alminate in the cement. The time of set
Is reduced, and compressive
strength gain increased in regulated cement mortars and concrete by an increase
in the cement content of mix, reduction of the w/c ratio, increased temperature
of the mix and increase in curing temperature. The chemical reactions of this
type of cement are much more energetic, than those of Portland cements. For
that reason, retardation is necessary.
Conventional retarders for
Portland cement are not effective in controlling the set of regulated set
cement. However, citric acid is used in the mix as a retarder. Where practical,
the setting action can be effectively controlled by reducing the mix
temperature. Such reductions in the temperature of the mix are also
advantageous, as the heat of hydration is considerably higher than that of
Portland cement concrete. For this reasons, all exposed surfaces of newly
placed regulated set Portland cement concrete must be protected from moisture
loss. Chlorinated rubber, butadiene styrene sealing as well as polyethylene
sheets or wet burlap are recommended for this purpose.
Special cements based on chemical
reactions, which are completely different from those of normal Portland or
similar cements, are now part of the technology. These include fast-setting
magnesium phosphate and aluminum-phosphate cements, which used for concrete
patching of pavements, allow traffic flow after only 45 minutes.
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