20-09-2014, 09:53 AM
METHODS OF REPAIRING CONCRETE
METHODS OF REPAIRING.ppt (Size: 909 KB / Downloads: 98)
INTRODUCTION
3 Basic symptoms of distress in a concrete structure
Cracking, Spalling and Disintegration
Reasons for their development may be poor materials, poor design, poor construction practice, poor supervision or a combination
repair of cracks usually does not involve strengthening
repair of a structure showing spalling and disintegration, it is usual to find that there have been substantial losses of section and/or pronounced corrosion of the reinforcement
Repairing cracks
In order to determine whether the cracks are active or dormant, periodic observations are done utilizing various types of telltales
by placing a mark at the end of the crack
a pin or a toothpick is lightly wedged into the crack and it falls out if there is any extension of the defect
Types of cracks
active cracks and dormant cracks
the proper differentiation between active and dormant cracks is one of magnitude of movement, and the telltales are a measure of the difference
If the magnitude of the movement, measured over a reasonable period of time (say 6 months or 1 year), is sufficient to displace or show significantly on the telltales, we can treat the crack as an active one.
If the movements are smaller, the crack may be considered as dormant.
Methods of repairing cracks
Bonding with epoxies
Cracks in concrete may be bonded by the injection of epoxy bonding compounds under pressure
Usual practice is to
drill into the crack from the face of the concrete at several locations
Stitching
Concrete can be stitched by iron or steel dogs
A series of stitches of different lengths should be used
bend bars into the shape of a broad flat bottomed letter U between 1 foot and 3 feet long and with ends about 6 inches long
The stitching should be on the side, which is opening up first
External stressing
cracks can be closed by inducing a compressive force, sufficient to overcome the tension and to provide a residual compression
The principle is very similar to stitching, except that the stitches are tensioned; rather than plain bar dogs which apply no closing force to the crack
Some form of abutment is needed for providing an anchorage for the prestressing wires or rods
Grouting
same manner as the injection of an epoxy
cleaning the concrete along the crack
installing built-up seats at intervals along the crack
sealing the crack between the seats with a cement paint or grout
flushing the crack to clean it and test the seal; and then grouting the whole
Blanketing
similar to routing and sealing
applicable for sealing active as well as dormant cracks
Preparing the chase is the first step
Usually the chase is cut square
The bottom should be chipped as smooth to facilitate breaking the bond between sealant and concrete
Guniting
Gunite is also known as shotcrete or pneumatically applied mortar
It can be used on vertical and overhead, as well as on horizontal surfaces and is particularly useful for restoring surfaces spalled due to corrosion of reinforcement
Gunite is a mixture of Portland cement, sand and water, shot into the place by compressed air
Conclusions
When repairing cracks, do not fill the crack with new concrete or mortar
A brittle overlay should not be used to seal an active crack
The restraints causing the cracks should be relieved, or otherwise the repair must be capable of accommodating future movements
Cracks should not be surface-sealed over corroded reinforcement, without encasing the bars
The methods adopted for repairing spalling and disintegration must be capable of restoring the lost strength