Unit 5 maintenance and repair methods for rcc mrs

Unit 5: Maintenance and repair methods for RCC
4.1 Probable location of cracks in RCC elements :
 Location of cracks in RCC column
1. Diagonal Cracks in Concrete Column 2. Splitting cracks in RCC column
3. Horizontal crack in RCC column 4. Corrosion cracks in concrete columns
 Location of cracks in RCC beam
1. Cracks in concrete beams due to increase in shear stress and Shear stress is maximum
at a distance of 12 from the support where d is the effective depth of beam.
2. Cracks due to corrosion of reinforcement appear at the bottom of the beam.
3. Cracks due to increased bending stress in beams appear near the center of span of the
beam at an angle of 45 degree with horizontal as the bending moment is maximum at that
point.
4. Cracks due to compression failure in beams appear at top if the beam is over reinforced.
Prepared by- Abhay Abhale

5.1 Probable location of cracks in RCC elements :
 Location of cracks in RCC Slab
1. Shrinkage cracks originating at re-entrant corners
2. Re-entrant corners are corners that point into a slab, into the slab.
3. To combat random shrinkage cracks, control joints (often mistakenly referred to as
expansion joints) are incorporated into the slab.
4. Crack across the unsupported concrete slab.
5. Cracks Caused by Overloading the Slab.
6. A heavy truck drove over this sidewalk, cracking the edge.
7. Crusting cracks often happen during the concrete stamping process. They usually occur on
sunny or windy days when the top of the slab dries out sooner than the bottom.
 Location of cracks in RCC footing
1. Horizontal cracks in foundation walls.
2. Vertical and Diagonal Foundation Cracks located in the middle section of walls and are less
than (1/8) in wide.

5.1 Various causes of RCC failure :
The main factors that affect the failure of concrete structures are:
1. Incorrect selection of materials
2. Errors in design calculation and detailing
3. Improper construction techniques
4. Insufficient quality control and supervision
5. Chemical attacks on concrete structures
6. External mechanical factors

5.2 Causes of dampness in roof slab and its repair techniques such as mud
phuska with brick tile topping :
 Causes of dampness in roof slab:
The sources of dampness in the roof slab of a building are as follows.
1. Rainwater
2. Overhead water tank
3. Snowfall
4. Atmospheric moisture
5. Improper design of roof slab
6. Poor construction of porous roof slab.
7. Porous parapet wall over roof slab

5.2 Repair techniques of dampness in roof slab :
As it is necessary to repair the defects caused by damage of there are following
techniques useful to repair the same.
1. Mud phuska with brick tile topping 6. Tar-felting
2. Lime concrete terracing 7. Screed concreting
3. Brick coba 8. Grading of lime concrete with tiles
4. Ferro-cement topping
5. Ethylene Propylene Diene Monomer (EPDM)

1. Mud phuska with brick tile topping :
An insulating medium in roof finish of a layer of compacted soil underlying
another layer of soil mixed with building material such as cow dung and fibrous
reinforcing ingredient such as wheat straw.

1. Mud phuska with brick tile topping :
Procedure :
1. After surface preparation a coat of hot bitumen at the rate of 1.7 kg per sq. m. of roof area
or polyethylene sheet is laid over the substrate flat roof slab.
2. Mud phuska is laid for 100 mm thickness with material consisting of puddle clay mixed
with chopped straw (Bhusa) for about 8 to 10 kg per cubic meter of clay to a slope shall
not less than 1 in 40.
3. Mud plaster is laid to slope consisting of puddled clay mixed with chopped straw at about
30 to 35 kg per cubic meter of earth clay.
4. One or two coats of rendering by cow dung slurry (gobar) is laid to slope consisting of fine
clay and cow dung in equal amount along with adequate water.
5. Burnt Brick tiles in one or two layers is laid to slope on a bed of mud mortar and jointed
and pointed properly in cement mortar 1:3
6. Top surface pointing is done with highly plastic cement mortar prepared by adding water
proofing admixtures to sand mortar.

1. Mud phuska with brick tile
topping :
Layer sequence:
1. Bottommost Cement Plaster
2. Roof Slab (120 mm Thick)
3. Hot Bitumen (Rate 1.7 Kg/m2)
4. Mud Phuska (at a Slope 1:40)
(100 mm Thick)
5. Mud Plaster (12 mm Thick)
6. Brick Tile with Joints

2. Lime concrete terracing :
The lime concrete should be prepared by mixing the brick ballast and lime-surkhi
mortar thoroughly in proportion of 2.5:1 parts by volume.

2. Lime Concrete Terracing :
Procedure :
1. Burnt brick aggregate is thoroughly soaked in water for at least 6 hours, before using in
concrete.
2. Lime concrete is laid in single layer of about 100mm spread and compacted with wooden
rammers to the required slope.
3. To avoid surface cracking in lime concrete, fibrous materials like jute fibers, jaggery water
and fenugreek powder is sprinkled on the top surface.
4. The concrete is further consolidated by beating manually with wooden or bamboo
'Chappies.
5. Special care is needed to consolidate the concrete properly at junctions with parapet wall.
6. Beating is continued for 3 to 4 days until the mortar is almost set and wooden thapies'
rebound from the surface.
7. While beating, the surface is liberally sprinkled with fenugreek powder and a solution of
'gur' and boiled bael fruit in the ratio of 1.75 kg of 'gur' to 1 kg of bael fruit boiled in 60
litres of water.

2. Lime Concrete Terracing :
Lime concrete terracing should be repaired after an interval of 5 years by scrapping top
surface and relaying the same with lime cream and composite mortar. This is done by
thoroughly wetting the surface and compaction by beating with wooden thappies. The
mortar is mixed thoroughly and solution sprinkled with fibres and fenugreek powder and
gur-bael solution. The surface is finished smooth to the required slope.
Layer sequence:
3. Painted with jot Bitumen
(Rate 1.7 Kg/m2)
4. Lime Concrete (at a Slope 1:60)
(100 mm Thick)

3. Brick coba :
It consists of putting brick bats on roofs, to give a slope and then grouting the
same with mortar admixed with various proprietary chemicals most in the nature
of waterproofing compounds.

3. Brick coba :
Procedure :
Roof slabs constructed either by RC or RCC needs insulation for thermal comfort and
waterproofing treatment to prevent leakage of water. Both these requirements are effectively
full fill by brick bat coba treatment, the details of which are being below:
1. Firstly, remove the existing coatings using chisel and clean the slab surface by using hard
wire brush. Wash the surface using water to remove the present oil, grease if any.
2. Fill the minor structural cracks by applying remedial treatment in the form of painting by
cement slurry.
3. Expansions joints should be treated as per standard practice. All non-structural cracks
more than 0.5 mm wide and construction joints if any, should be cut in "V" shape, cleaned
with wire brush and washed, the cracks are then filled by polymer modified cement or
mortar using acrylic polymer, with addition cement slurry mix is spread upon cleaned roof
surface. Over this 15 mm thick cement, sand mortar, 1:4 admixed, with water proofer is
laid.

3. Brick coba :
Procedure :
Roof slabs constructed either by RC or RCC needs insulation for thermal comfort and
waterproofing treatment to prevent leakage of water. Both these requirements are effectively
full fill by brick bat coba treatment, the details of which are being below:
4. Soak the brick bats in water overnight and then lay them on the above green mortar with
an average thickness of about 110 mm, about 70 mm near rain water pipe and 150 mm at
ridge. Keep the gap between the brick bats about 15 and 20 mm.

3. Brick coba :
Procedure :
Layer sequence:
3. Elastomeric Waterproofing Layers
4. Screed Layer (12.5 mm Thick)
5. Brick Bat coba
6. Cement Coating

4. Ferro-cement topping :
Ferrocement is a composite material composed of a mortar reinforced with light
steel fabric/mesh, used to form thin sections.

Procedure :
1. The entire roof surface including parapet walls are to be cleaned with wire brushes and
washed with water. Any cracks found on the surface should be investigated, if they are not
static suitable remedial measures be provided before undertaking any water proofing
treatment.
2. The porous and honeycombed surfaces of the roof should be grouted by Non-Shrink grout.
3. Any cracks of more than 1 mm should be chased out to “V” shaped groove and filled with
Polymer modified mortar. For entire work the roof surface and parapet walls be kept in
saturated and surface dry condition.
4. On the above saturated and surface dry surface thin cement slurry be applied by brush.
Mixing water of which admixed with super plasticizer at a dosage of 200 ml / 50 kg of
cement.

Procedure :
5. Upon the wet slurry a mortar layer as per mix ratio 1 : 3 to be laid and provide a suitable
slope towards the drains.
6. The layer surface is to be made rough with small hard coconut brush.
7. For fixing wire mesh, wire nails were fixed in this mortar layer at appropriate spacing with
projection from the plaster surface of about 4 mm. Proper curing should be done for 3
days.
8. Two layer of 20 gauge 12 mm x 12 mm galvanized wire mesh are stretched over the above
prepared surfaces. At the joints of wire meshes, 150 mm overlap is provided.
9. The joints in the two layers of wire mesh are staggered. The mesh then fixed to nails. 5 mm
thick cover block or 5 mm dia G.I. wire pieces are inserted between roof and the mesh and
in between the two meshes for keeping the meshes at desired positions and for
maintaining proper cover.

Procedure :
10. Thin cement slurry, the mixing water of which admixed with 200 ml super plasticizer 50
kg of cement is sprayed over the surface and then immediately apply the mortar. The
mortar shall be as 1 : 2.5.
11. The mortar is thoroughly mixed and applied with trowels to finished thickness of about 20
mm.
12. While applying the mortar, the wire mesh layers are lifted up using a hook so that a cover
of about 4 mm could be provided below the bottom layer of the wire mesh and between
the two meshes.
13. Hard trowelling is carried out over the surface for leveling and finishing is done using a
wooden float. At the junction of roof slab with parapet walls or at roof projections, water
proofing treatment is done as shown in Fig. 1.

Procedure :

5.3 Repair methods for cracks in RCC structures :
Methods of repair the cracks in RCC structures
1. Epoxy injection method
2. Grooving & sealing method
3. Stitching method
4. Rebaring method
5. Grouting method
6. Spalling replacement method
7. Jacketing method
8. Shotcrete and guniting method
9. Polymer impregnation method
10. Autogenous healing method

5.3 Repair methods for cracks
in RCC structures :
1. Epoxy Injection Process :
i. Identify cracks for repair.
ii. Optimum port spacing and
location.
iii. Surface preparation.
iv. Seal cracks at surface.
v. Inject epoxy resin.
vi. Remove seal after 12 hours.
vii. Restore surface by grinding or by
other means.

2. Grooving & Sealing:
i. This method is useful for the dormant cracks.
ii. The cracks may be of fine pattern or isolated cracks.
iii. The method should not be applied for the active cracks or cracks subjected to
hydrostatic pressure.

2. Grooving & Sealing:
i. In this method the crack is enlarged along its exposed face by making a groove.
ii. The surface of the grooved joint is cleaned by blowing air and allowed to dry
iii. The sealant to be used is selected depending on tightness and permanency of the
seal.
iv. The sealants which are hot poured are better when water tightness and appearance
are not so important.
v. Urethanes remain flexible through large temperature variation, therefore can be
used in cracks up to 3 to 4mm width and of more depths.
vi. The selected sealant is placed in to the grooved crack.
vii. The aim of sealant is to create barrier to water and to join the cracked surfaces

3. Stitching :
i. Stitching the crack will tend to cause its migration elsewhere in the structure. For
this reason strengthening the adjacent areas of cracks have to be made to take care
of additional stresses.
ii. Moreover the stitching dogs should be of variable length, orientate and so located
that the tension transmitted across the crack does not devolve on a single plane of
the section but is spread over an area. Strengthening of the adjacent sections of
concrete may consist of external reinforcement embedded in a suitable overlay
material.
iii. Where there is a problem of leakage of water, the crack should be sealed as well as
stitched so that dog pins stitches are not corroded.

3. Stitching :
iv. Materials and equipment used in stitching
(1) Appropriate epoxy material
(2) Steel rod of 8mm diameter for making dog pins of different sizes.
(3) Drill bits of 10mm diameter and drilling machine.
v. Stitching involves drilling holes on both sides of the beam and a pin of c-shape with
short legs in the drilled holes are placed across the crack.
vi. The stitching procedure consist of drilling holes on both sides of the crack, cleaning
the holes and anchoring the legs of the stapes in the holes, with either a non shrink
grout or epoxy resin based bonding system.

3. Stitching :

4. Rebaring :
1. When a concrete slab is to be extended on the side of beam or slab where the dowel
bars are not provided for extension purpose, the rebaring of the reinforcement can
done.
2. The process involves drilling of holes along the length of the extension side on the
tensile layer of the member.
3. After cleaning the holes with compressed air and with water the dowel bars are
fixed in the holes with suitable bonding agents.
4. The old and new surfaces of concrete members are bonded together and made
water tight with the help of suitable epoxy resin mortar or by grouting.

4. Rebaring :

5. Epoxy Grouting :
1. The method of crack repair depends on whether the crack is live or dormant.
2. Active cracks may not have adequate provision of movement joints in the structure.
3. The repair process involves converting the crack into a movement joint, by the use
of a suitable sealant of a low Viscosity epoxy.
4. This is the best possible repair method for cracks between 0.02 mm to 6mm in
width.
5. For epoxy joint to be effective, the cracks must be free of dirt, grease, etc. before
injection can being, the crack where it appears on the surface of the member must
be sealed.
6. External flexural, shear and torsion reinforcement for beams and girders may be
provided bonding deformed bars or plates to the surface of concrete girders with
shotcrete, cast-in place concrete, or epoxy and polymer concrete.
7. Anchors or proper bonds are necessary in the repair method to en composite
action.

6. Spalling Replacement :
Cracks resulting in spalling of concrete and exposing the reinforcement can be repaired
in following ways.
1. Surface preparation: In order to ensure good bonding of the existing material with
new material the preparation can be done by chipping, air or water jetting or using
wire brush etc.
2. The pneumatic chipping also can be done when hand chipping is not possible.
3. There in for cement which is badly corroded need to be replaced by additional
reinforcement by welding to the existing reinforcement.
4. When more number of bars are to be replaced then weld mesh may be fixed by
lungs.
5. The lungs of the mesh are fixed in the drilled holes by suitable bonding agents. If the
bars are least rusted then the rust can be removed by sand blasting. The sand
blasting may help in making the concrete surface rough to receive the fresh
concrete.

6. Spalling Replacement :
Cracks resulting in spalling of concrete and exposing the reinforcement can be repaired
in following ways.
6. Concreting : The new concrete is made of rich mix 1 : 1.5 : 3 with sand of 3mm size
and stone chips of 6mm and down.
7. The water ratio is kept as low as 0.3 to 0.4.
8. After applying the concrete by and it is dried and hardened the surface may be
plastered.
9. If the area to be replaced is large then pneumatic pressure grouting can be applied.

7. Jacketing & Collaring :
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.
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.

5.3 Repair methods for cracks
in RCC structures :
8. Shotcreting & Guniting:
i. IS 9012 defines shotcrete as “mortar
or concrete (conveyed through a
hose) and pneumatically projected
at high velocity onto a surface.
ii. As the extra time in OPC provides
high PH around reinforcement OPC
is generally used preparing
Shotcrete.
iii. Gunite is less permeable so it
provides a more corrosion
resistance to the RCC Structures

8. Shotcreting & Guniting:
i. Thoroughly clean all surfaces to receive shotcrete by removing loose materials and
dust and pressure washing. Dampen the surface to a saturated surface dry condition.
ii. Fix wire mesh to the concrete surface. The steel wire mesh has to be placed in
position keeping the mesh within 10-15 mm from the surface.
iii. Suitable fixing pins are to be inserted to keep the mesh in proper position and to
ensure that the weld mesh is not disturbed during concrete spray.
iv. Prepare a cement/water mix. Pour this mix into Pump hose for lubrication before
starting to pump the production mixture.
v. When the pumped mixture reaches the nozzle, turn on the compressed air.
vi. Apply shotcrete evenly to targeted surfaces. Built-up the desired thickness of
shotcrete in layers of about 30 mm thick each. The presence of voids can be found by
hollow hammering sound after the shotcrete has attained strength after around 3
days.
vii. Clean all discharge lines with a through water flush

5.4 Repair of corroded RCC Element :
1. Exposing & Undercutting Rebar
2. Cleaning Reinforcing Steel
3. Compensating Reinforcement
4. Protective Coating

1. Exposing & Undercutting Rebar
Procedure :
i. All loose or delaminated concrete above corroded reinforcing steel shall be removed
until corrosion Free locations along the bar are reached.
ii. Care shall be taken not to open bond between the non corroded bar and concrete.
iii. The exposed corroded rebar's are undercut Undercutting exposes the blind side of
the rebar cleaning and will allow the repair material to fully surround the rebar,
securing the repair material structurally. 15mm clearance between exponent rebar's
and the surrounding concrete shall provided.
iv. Place the loose reinforcement in place by joining it to other secured bars or by other
appropriate methods

2. Cleaning Reinforcing Steel
Procedure :
i. To promote maximum bond of the repair material, remove all heavy corrosion and
scale from the bar.
ii. It can be done preferably by abrasive blasting method.
iii. A tightly bonded with light rust on the Surface of rebar is usually not detrimental to
bond.
iv. Manufacturers recommendations shall be checked for rebar preparation and
protective coating.

3. Compensating Reinforcement
Procedure :
i. A structural engineer should be consulted if reinforcing steel has reduced cross
section significantly.
ii. The bar can be replaced completely or supplementary bars can be placed over the
affected section.
iii. The additional bars may be mechanically spliced to old bars or placed parallel to old
bars at approximately 15mm from existing bars.
iv. Lap lengths must be checked in accordance with latest IS specifications and other
design guidelines.

4. Surface Conditioning of Concrete
Procedure :
After removing damaged concrete and undercutting reinforcing steel, remove
additional concrete as required to provide the minimum required thickness of the
repair material. At edge location, provide right angle cuts to the concrete surface with
either of the following methods :
i. Saw cut 12mm or less as required. Avoid cutting reinforcing steel. Avoid leather
edges.
ii. Repair configuration should be squared corners.
iii. Remove all dirt, concrete laitance, and loose aggregates by abrasive blasting or high-
pressure water blasting with or without abrasive.
iv. Cement and particulate slurry must be removed from the prepared surfaces before
the slurry

5.5 Repair Methods for Honeycomb and Larger Voids
Procedure :
The repair techniques vary greatly. Some authorities do not require a minimum
thickness, while others set limits, such as 25-50mm.
The need for a bonding agent, whether an epoxy or cementitious grout depends upon
the condition of honeycomb.
i. Use light chipping hammer weighing 7 kg or less to remove the affected voided
concrete.
ii. Follow chipping with sandblasting or water blasting to remove fractured surface to
safeguard the remaining concrete.
iii. Simply troweling the repair material it into the void will serve the purpose.
iv. A small pneumatic mortar gun to apply a grout similar to hand applied dry pack can
also be applied.

5.5 Repair Methods for Honeycomb and Larger Voids
Procedure :
The repair techniques vary greatly. Some authorities do not require a minimum
thickness, while others set limits, such as 25-50mm.
The need for a bonding agent, whether an epoxy or cementitious grout depends upon
the condition of honeycomb.
v. A suitable rich mix contains 1 part Portland cement and 3 parts sand, adequate
admixture and water cement ratio of about 0.35.
vi. For the repair of deeper than 25mm, apply the mortar in layers no thicker than 15-
20mm to avoid sagging and loss of bond.
vii. Each layer should be set before placing the next layer.
viii.To complete the repair, overfill the void slightly and finish smoothly but extreme care
should be taken to avoid impairing the bond.

Unit 5 maintenance and repair methods for rcc mrs

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