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Jacketing
and Collars for Strengthening of Concrete Structures
Gopal Mishra
Jacketing is the process whereby a section of an existing structural
member is restored to original dimensions or increased in size by encasement
using suitable materials.
A steel reinforcement cage or composite material
wrap can be constructed around the damaged section onto which shotcrete or
cast-in-place concrete is placed.
Collars are jackets that surround only for a part of a column or
pier. These are usually used to provide increased support to the slab or beam
at the top of the column.
The form for the jacket consists of timber,
corrugated metal, precast concrete, rubber, fiberglass, or special fabric; and
may be permanent in some cases.
The form must be provided with spacers to ensure
equal clearance between it and the existing member.
Materials, like
conventional concrete and mortar, epoxy mortar, grout, and latex-modified mortar
and concrete, are used as encasement materials.
For jacketing, the void between
the form and the existing member is filled using pumping, tremie, or preplaced
aggregate concrete.
Jacketing
is particularly used for the repair of deteriorated columns, piers, and piles
and may easily be employed in underwater applications.
The method is applicable for
protecting concrete, steel, and timber sections against further deterioration
and for strengthening.
Permanent
forms are preferred where protection against weathering, abrasion, and chemical
pollution is desired.
The collar provides increased
shear capacity for the slab, and it decreases the effective length of the
column.
Architecturally collars are
considered better than jacketing but performing the same structural function.
Before applying jackets or
collars, all deteriorated concrete must be removed, cracks must be repaired,
existing reinforcement must be cleaned, and surfaces must be prepared.
The surface preparation improves
the bond of the newly placed materials with the existing structure, which is
difficult for underwater repairs.
For underwater conditions, a
plastic shell may be applied at the splash zone to help minimize abrasion.
A drawback of jackets and collars
is that they occupy space that was earlier available for other uses.
Timber,
cardboard and corrugated steel forms may be used as temporary or permanent
forms.
Permanent fiberglass, rubber, and
fabric forms have gained considerable popularity because they provide
resistance to chemical attack after the repair is complete.
Jacketing
used for purposes other than covering the deteriorated concrete and providing
lateral confinement, such as to bear longitudinal loads, needs special
considerations.
The existing column may have
undergone full shrinkage and most of the creep and also has elastic strains due
to carried loads, whereas the shrinkage and creep of the new material has to
occur.
The
load transfer to jacketing is also a big issue. It is better to use jacking to
release the load on the member before jacketing, to use non-shrinking materials
for jacketing and to hammer steel shims at the transfer points of the jacketing
after curing.
If
the material used for jacketing is cement mortar or concrete, the cement
content must be exactly according to the requirements; both excessive and less
cement contents may be dangerous.
Use clean, stable and the largest
possible size aggregates. In order to reduce shrinkage, control the temperature
of the materials and the immediate surroundings during placing and curing.
Use
of admixtures such as plasticizers, air-entraining agents, retarders,
accelerators and waterproofing admixtures is more beneficial in repair than
even the ordinary construction.
Expanding
mortars / concretes can be made by adding aluminum powder to the matrix to
overcome the setting shrinkage and some part of the drying shrinkage.
The use of iron fillings or
powder can also perform this function if moisture and air are available.
In
case grout is used for filling the forms, it is allowed to settle for about 20
minutes after fulfilling and then is filled to overflowing condition.
The top of the jacket must be
finished with pneumatically projected or hand placed concrete.
Gopal Mishra is a Civil Engineer from NIT Calicut and has more than
10 years of experience in Civil Engineering and Construction. He is the founder
of The Constructor.
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