Transcript CE2071_Unit 5

UNIT – V
REPAIR, REHABILITATION AND
RETROFFITNG OF STRUCTURES
REPAIRS
TO
OVERCOME
LOW
MEMBER STRENGTH:
These guidelines have the following objectives;
•To indicate appropriate methods of repair and
restoration taking into account the building type
and the type of damage.
•To
recommend
methods
of
seismic
strengthening to upgrade the strength of the
building in line with the requirements of the
seismic-zoning map of India (IS : 1893-1984)
and Earthquake Resistance Codes (13 : 43261993) and (18:13828-1993).
CONCEPTS OF REPAIR, RESTORATION AND
RETROFITTING:
REPAIR:
It consists of actions taken for patching up
superficial
defects,
re-plastering
walls,
repairing doors and windows and services such
as the following :
•Patching up of defects as cracks and fall of
plaster and re-plastering if needed.
•Repairing doors, windows and replacement of
glass panes.
•Checking and repairing electrical connections, gas
connections, plumbing, heating, ventilation
•Rebuilding non-structural walls, chimneys,
boundary walls.
•Relaying cracked flooring at ground level and
roofing sheets or tiles.
•Redecoration work (White or colour washing etc.)
•It would be seen that the repairing work carried out
as above does not add any strength
to the structure.
RESTORATION :
This includes actions taken for restoring the lost
strength of structural elements of the building. This is
done by making the columns, piers, beams and walls
at least as strong as originally provided as follows:
• Removal of portions of cracked masonry walls and
piers, and rebuilding them in richer mortar. Use of
non-shrinking mortar will be preferable.
• Addition of reinforcing mesh on both faces of the
cracked wall, holding it to the wall through spikes or
bolts and then covering it suitably with micro-concrete
or 1:3 cement -coarse sand plaster.
• Injecting neat cement slurry or epoxy like
material, which is strong in tension, into the
cracks in walls, columns, beams etc.
• If the structural restoration is properly
executed, the structure will be as strong as
before the-earthquake. It is also possible to
strengthen a structure to take increased vertical
loading, if required.
SEISMIC STRENGTHENING (RETROFITTING)
It will involve actions for upgrading the seismic
resistance of an existing building so that becomes
safer under the occurrence of probable future
earthquakes.
The seismic behavior of existing buildings is
affected by their original structural inadequacies,
material degradation due to aging and alterations
carried out during use over time.
The complete replacement of such buildings in a
given area is just not possible due to a number of
social, cultural and financial problems. Therefore,
seismic strengthening of existing undamaged or
damaged buildings is a definite requirement.
Seismic strengthening structural restoration
and cosmetic repairs may sometimes cost upto
25 to 30 per cent of the cost of rebuilding
although usually it may not exceed 12 to 15 per
cent. Hence justification of strengthening work
must be fully considered from cost point of
view.
The main items of seismic strengthening could
be some or all of that following actions:
• Modification of roofs,
• Substitution or strengthening of floors,
•
•
•
•
•
Modification in the building plan,
Strengthening of walls including provision
of horizontal and vertical bands or belts,
introduction of header stones in thick stone
walls, and injection grouting etc.,
Adding to the sections of beams and
columns by casing or jacketing etc.,
Adding shear walls or diagonal bracings,
Strengthening of foundations if found
necessary (but very difficult and
expensive).
STRENGTHENING OF FOUNDATIONS
Seismic strengthening of foundations before or after the
earthquake is the most involved task since it may require
careful underpinning operations. Some alternatives are given
below for preliminary consideration of the strengthening
scheme.
• Introducing new load bearing members including
foundations to relieve the already loaded members. Jacking
operations may be needed in this process.
• Improving the drainage of the area to prevent saturation of
foundation soil to obviate any problems of liquefaction which
may occur because of poor drainage.
• Providing apron around the building to prevent soaking of
foundation directly and draining off the water.
• Adding strong elements in the form of reinforced
concrete strips attached to the existing foundation part
of the building. These will also bind the various wall
footings and may be provided on both sides of the
wall.
• To avoid digging the floor inside the building, the
extra width could be provided only on the outside of
external walls.
• The extra width may be provided above the existing
footing or at the level of the existing footing. In any
case the reinforced concrete strips and the walls have
to be linked by a number of keys, inserted into the
existing footing.
CRACKING
•Cracking, like corrosion of reinforcing steel, is not
commonly a cause of damage to concrete. Instead,
cracking is a symptom of damage created by some
other cause.
•All Portland cement concrete undergoes some
degree of shrinkage during hydration. This
shrinkage produces multidirectional drying
shrinkage and curing shrinkage cracking having a
somewhat circular pattern. Such cracks seldom
extend very deeply into the concrete and can
generally be ignored.
•Plastic shrinkage cracking occurs when the
fresh concrete is exposed to high rates of
evaporative water loss which causes
shrinkage while the concrete is still plastic.
•Plastic shrinkage cracks are usually
somewhat deeper than drying or curing
shrinkage cracks and may exhibit a parallel
orientation that is visually unattractive.
•Thermal cracking is caused by the normal
expansion and contraction of concrete during
changes of ambient temperature.
•Concrete has a linear coefficient of thermal
expansion of about 5.5 milli inch per inch per
degree Fahrenheit (°F).
•This can cause concrete to undergo length changes
of about 0.5 inch per 100 linear feet for an 80 °F
temperature change.
•Thermal cracking can also be caused by using
Portland cements developing high heats of
hydration during curing.
•Such concrete develops exothermic heat and
hardens while at elevated temperatures.
•Cracking is also caused by alkali-aggregate
reaction, sulfate exposure, and exposure to
cyclic freeze-thaw conditions, as has been
discussed in previous sections, and by structural
overloads as discussed in the following section.
Successful repair of cracking is often very
difficult to attain.
•The selection of methods for repairing cracked
concrete depends on the cause of the cracking. First, it
is necessary to determine if the cracks are "live" or
"dead."
•If the cracks are cyclically opening and closing, or
progressively widening, structural repair becomes very
complicated and is often futile. Such cracking will
simply reestablish in the repair material or adjacent
concrete.
•If reflective cracking is intolerable, the repairs must
be designed as separate structural members not bonded
to the old existing concrete.