Mortar repair for cracks
Portland cement mortar may be
used for repairing defects on surfaces not prominently exposed. where the
defects are too wide for dry-pack filling or where the defects are too shallow
for concrete filling, and no deeper than the far side of the reinforcement that
is nearest the surface. repairs may be made either by use of shotcrete or by
hand application methods, although hand application methods are generally
recommended for areas subject to public view in historic preservation
applications.
Replacement mortar can be used to
make shallow, small-size repairs to new or green concrete, provided that the
repairs are performed within 24 hours of removing the concrete forms.
Accomplishing successful mortar repairs to old concrete without the use of a
bonding resin is unlikely or extremely difficult. Evaporative loss of water
from the surface of the repair mortar, combined with capillary water loss to
the old concrete, results in unhydrated or poorly hydrated cement in the
mortar.
Additionally, repair mortar bond
strength development proceeds at a slower rate than compressive strength
development. This causes workers to mistakenly abandon curing procedure
prematurely, when the mortar seems strong. Once the mortar dries, bond strength
development stops, and bond failure of the mortar patch results. For these
reasons using cement mortar without a resin bond coat to repair old concrete is
discouraged. A Portland cement mortar patch is usually darker than the
surrounding concrete unless precautions are taken to match colours. A leaner
mix will usually produce a lighter colour patch.
1 Preparation and materials
Concrete to repaired with
replacement mortar should first have all the deteriorated or unsound areas
removed. After preparation, the areas should be cleaned, roughened if necessary
and surface-dried to a saturated surface condition. The mortar should be
applied immediately thereafter. Replacement mortar contains water, Portland
cement and sand. The water and sand should be suitable for use in concrete, and
the same should pass through a no.16 sieves. Only enough water should be added
to the cement sand mixture to permit placing.
2Curing
Failure to cure properly is the most common cause of failure
of replacement mortar.
It is essential that mortar
repairs receive a through water cure starting immediately after initial set and
continuing for 14 days. In no event should the mortar be allowed to become dry
during the 14 day period following placement. Following the 14 day water cure
and while the mortar is still saturated, the surface of the mortar should be
coated with two coats of a wax-base curing compound meeting reclamation
specifications.
3 Applications
The success of this method
depends on complete removal of all defective and affected concrete, good
bonding of the mortar to the concrete, elimination of shrinkage of the patch
after placement, and thorough curing. Replacement mortar repairs can be made
using an epoxy bonding agent; this technique is highly recommended.
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