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Repair and Rehabilitation
Concrete Technology
Syllabus
• Distress in structure: Causes and
precautions, damage assessment of structural
elements, repairing techniques and repairing
materials.
Distress In Structure
• Distress means Damage
• Concrete may suffer distress or damage during
its life period due to a number of reasons.
Because of the varying conditions under which
it is produced at various locations, the quality
of concrete suffers occasionally either during
production or during service conditions
resulting in distress.
Distress In Structure
Causes of distress of concrete:
• Structural causes
– Externally applied loads
– Environmental loads
– Accidents
– Subsidence's, etc.
• Error in design and detailing
• Poor Construction practices
• Construction Overloads
• Drying Shrinkage
• Thermal Stresses
• Chemical Reactions
• Weathering
• Corrosion
Distress In Structure
In addition to the distress in hardened concrete, the
plastic concrete may also suffer damage due to,
• Plastic Shrinkage
• Settlement Cracking
• Early removal of formwork
• Improper design of formwork.
Distress In Structure
Evaluation Procedure for Repair and
Strengthening of Concrete Structures
• Before finalizing any scheme for repairs and
rehabilitation of a distressed concrete structure, the
concerned engineer has to be fully aware of the
causes of causes of distress, extent of damage to the
structure and the present condition of the concrete
in the structure for repairs to be effective and
lasting. The extent of distress has to be categorized
so that the repair schemes can be formulated
according to the distress in a particular structural
element. So that, pre-repair evaluation and
assessment of a structure is pre-requisite for
working out effective repair schemes.
Evaluation Procedure for Repair and
Strengthening of Concrete Structures
• Once the repairs have been carried out on a
distressed structure, the post repair evaluation and
assessment of the structure can be carried out for
checking the efficacy of the repair. The post repair
assessment is a tool with the engineer to evaluate
whether the parent material and the repair
material have obtained bond or whether the
cracks or the voids have been filled up by the
grouting materials. Thus, any scheme for effective
repairs can be based on the pre-repair and post-
repair evaluation of concrete structures
Tools for Evaluation of Concrete
Structures
• The various tools available for evaluation of concrete
structures are as follows:
• Visual inspection and observations
• Questioning of concerned personnel
• Scrutiny of field data and records
• Design Checks
• Non- destructive testing (NDT)
• Extraction of cores and testing
• Supplementary laboratory techniques
• Load testing of a structural member
• The general approach adopted for pre-repair evaluation
of distressed concrete structures is given below:
Visual Inspection and Observations
• The first step in the process of evaluation of a
distressed concrete structure is visual
inspection and observations. A through visual
inspection and observations. A through visual
inspection leads to proper approach to be
adopted during investigation. It determines the
number of field and laboratory tests required to
be carried out. Visual inspection generally
include the study of the following.
Visual Inspection and Observations
• Ambient conditions
• Crack width and patterns
• Spelling of Concrete
• Color, texture and rust stains
• Sinking of columns
• Failure of beam-Column junctions.
• Mal-functioning of machinery, structural
components etc.
• Condition of fixtures
• Deposits/ splashes on structural components.
Questioning of Personnel/ Scrutiny
of field Data and Records
• The questioning of personnel and the
scrutiny of field data and records is carried
out for the following:
• Grade of concrete adopted
• Cube test results
• Type of material and sources
• Constructional details
• Environmental Conditions
Questioning of Personnel/ Scrutiny
of field Data and Records
• The Scrutiny of the grade of concrete and cube
test results brings out adequacy of strength of
concrete and the degree of quality control
exercised during constructions.
• The study of the type of materials used
particularly cement, coarse aggregate, fine
aggregate, additives etc. also focuses the direction
of investigations. The scrutiny of other
constructional details e.g. removal of formwork,
shifting of formwork for slip form construction,
the height of pouring of concrete, use of
compaction devices etc. are useful information for
further investigation in many cases.
• Rebound hammer test
• Ultrasonic pulse velocity test
• Pull- out test
• Pull- off test
• Carbonation test
• Cover measurement
• Break off test
• Endoscopy
• Radar techniques
• Rapid chloride, alkali and sulphate kits, etc.
Questioning of Personnel/ Scrutiny
of field Data and Records
• Scrutiny of field data and records:
• Design checks:
• Non-Destructive testing (NDT):
• Any visual inspection and scrutiny of the field
data, the in-situ testing is carried as per the
approach finalized. Various in-situ non-
destructive tests available are:
• X-ray diffraction analysis
• Differential thermal analysis
• Chloride permeability test
• Optical and Scanning microscopy
• Chloride permeability test.
• Compressive strength, density and modulus of
elasticity determination on core samples, etc.
• Load testing of a structural member:
• Extraction of Cores and testing:
In addition to various in-situ tests carried out, it
becomes necessary to support the findings with
laboratory tests
• The laboratory tests generally adopted are:
• Cement Content of Hardened Concrete
• Chemical Analysis
• Chlorides
• Sulphates
• pH
• Nitrates
Corrosion of Reinforcement in
Concrete
The damage to the concrete due to corrosion of
reinforcement is considered to be one of the most
serious problems. It is an universal problem and
property worth of crores of rupees is lost every
years. Due to corrosion problem in bridges,
buildings and other RCC structures, India incurs
heavy loss of about Rs. 1500 cores annually.
This paper deals with various causes of corrosion
and remedial measures thereon.
Corrosion of Reinforcement in
Concrete
Corrosion of Reinforcement in
Concrete
Corrosion Process and Mechanism:
• Corrosion of reinforcement steel is a complex
phenomenon involved chemical, electrochemical
and physical process. When reinforcement steel
rusts, the volume of iron oxide formed is 2-3
times greater than the steel corroded, which
results in bursting stresses in the concrete
surrounding the bar. This causes cracking,
spalling and delimination of concrete. Another
consequence of corrosion is reduction in cross-
sectional area of the steel at anode, thus reducing
its loads carrying capacity.
Causes of Corrosion and Remedial
Measures
• Various causes of corrosion and remedial measures are
discussed below:
• Presence of Cracks in Concrete: Certain amount of
cracking always occurs in the tension zone of RCC
depending upon the stresses in the reinforcing steel.
Through these cracks, oxygen or sea water ingress into
concrete and set up good environment for corrosion of
reinforcement. Maximum permissible width of elastic
cracks in RCC members would depend upon
environmental and other factors. For normal
environmental conditions, a maximum crack width of
0.3 mm for protected internal members and 0.2 mm for
unprotected external members may be recommended.
Presence of Cracks in Concrete
Causes of Corrosion and Remedial
Measures
• Presence of moisture: Presence of moisture is a
precondition for corrosion to take place because
concrete can act as electrolyte in electrochemical cell
only if it contains some moisture in pores. Corrosion
can neither occurs in dry concrete or in submerged
concrete.
• The worst combination for corrosion to process is when
the concrete is slightly drier than saturated i.e. about 80
- 90 % relative humidity with a low resistivity and the
oxygen can still penetrate to the steel. Hence in high
humidity areas like coastal India, low permeability
concrete is recommended.
Causes of Corrosion and Remedial
Measures
Permeability of Concrete:
• This is also an important factor affecting corrosion of reinforcement.
Ingress of moisture, sea water, oxygen, CO2 etc. is easier in porous
concrete than in dense and impermeable concrete. It is worth
mentioning than with each increases of W/C ratio 0.1, permeability of
concrete increases 1.5 times. Poor curring increases permeability 5 to
10 times in comparison to good cured concrete and poor compaction
increases permeability 7 to 10 times in comparison to good compacted
concrete.
• For this consideration, quantity of cement in concrete should not be
less than 350 kg/ m3 and W/C ratio should not exceed 0.55 for ordinary
structures and 0.45 for marine structures. All other normal
requirements of good quality concrete, namely, grading, and
cleanliness of aggregates, through mixing, proper compaction and
curing should be taken care of.
Permeability of Concrete
Causes of Corrosion and Remedial
Measures
• Carbonation: Hydrated cement paste forms a
thin passivity layer of Gamma iron oxide (Fe2
O3) strongly adhering to the underlying steel
and gives complete protection from reaction
with the oxygen and water, that is from
corrosion.
• Hydration of cement liberates some calcium
hydroxide which sets up a protective alkaline
medium inhibiting electrochemical cell action
and preventing corrosion of reinforcement.
Carbonation
• Carbon Dioxide (CO2) from the atmosphere
diffuse inside the concrete, react with calcium
hydroxide (Ca (OH)2) to form calcium carbonate
which is water soluble. This reaction is known as
carbonation. Carbonation lowers the alkalinity of
concrete and reduce its effectiveness as protective
medium. The pH value of pore water in concrete
is generally between 10.5 to 12.0 but if due to
carbonation it is lowered to 9.0 and below, the
medium converts to acidic type and corrosion of
reinforcement begins.
Carbonation
Causes of Corrosion and Remedial
Measures
• Chlorides: Chlorides can enter in the concrete
during concreting or during service conditions.
During concreting, the chloride can enter via
aggregates, gauging water and admixtures like
(CaCl2) In service conditions, the chloride ions
entry is due to ingress of sea water, de-icing and
other salts. The chloride ions (Cl-) attack the iron
oxide film leading to corrosion. Chloride ions
activate the surface of the steel to form an anode,
the passivated surface being the cathode. The rate
of corrosion depends upon chloride ion
concentration.
Chlorides
Causes of Corrosion and Remedial
Measures
• Sulphate Attack: Solubility sulphates like sodium,
potassium, magnesium and calcium are sometimes present
in soil, ground water or clay bricks, react with tricalcium
aluminate- 3CaO,Al2O3 (C3A) content of cement and
hydraulic lime in the presence of moisture and from
products which occupy much of cement and hydraulic lime
in the presence of moisture and from products which
occupy much bigger volume than the original constituent.
This, expansive reaction results in weakening of concrete,
masonry and plaster and formation of cracks as well as
corrosion of reinforcement.
• Severity of sulphate attack depends upon amount of soluble
sulphate present in soil, water or clay bricks, permeability
of concrete, amount of C3 A content in cement and duration
for which concrete remains damp.
Sulphate Attack
Causes of Corrosion and Remedial
Measures
• Alkali Aggregate Reactions: OPC contains alkalies
like sodium oxide (Na2 O) and potassium Oxide (K2O)
to some extent these alkalies chemically reacts with
reactive siliceous minerals in some aggregate and cause
expansion, cracking and disintegration of concrete give
rise to the corrosion of reinforcement.
• Preventive Measure Consists of:
• Avoid use of alkali-reactive aggregate in concrete.
• Cement with alkali content more than 0.6 % should not
be used.
• Portland Pozzolana cement is recommended.
Alkali Aggregate Reactions
Causes of Corrosion and Remedial
Measures
• Inadequacy of Cover: If Concrete cover to
reinforcement is inadequate, reinforcement is liable to get
corroded soon due to various factors such as
Carbonation, ingress of sea water, moisture penetration
etc. It is therefore necessary that RCC works should have
a minimum clear cover as recommended by IS 456:
2000. Reinforcement shall have concrete cover and
thickness of such cover (exclusive of plaster or other
decorative finish) shall be as follows:
Causes of Corrosion and Remedial
Measures
• At each end of reinforcing bars not less than 25
mm nor less than twice the dia. of such bar.
• For longitudinal reinforcing bars in column, not
less than dia. of such bar. In case of columns of
dimensions 200 mm or under, whose reinforcing
bars do not exceed 12 mm in dia. a cover of 25 mm
may be used.
• For longitudinal reinforcing bars in beams, not less
than dia. of such bar.
• for tensile, shear, compressive or other
reinforcement in a slab, not less than 15 mm, not
less than dia. of such bar.
Causes of Corrosion and Remedial
Measures
• Increased cover thickness may be provided when
surface of concrete members are exposed to the
action of harmful chemicals (as in case of
concrete in contact with earth faces contaminated
with such chemicals) acid vapours, saline
atmosphere, sulphurous smoke etc. and such
increase of cover may be between 15 mm to 40
mm beyond.
• For RCC members totally immersed in sea water,
cover shall be 40 mm more than the normal cover.
Inadequacy of Cover
Causes of Corrosion and Remedial
Measures
• In all such cases cover should not exceed 75 mm.
Based on part research, concrete, cover more than 50
mm is, however, not recommended as it give rise to
increase crack width which may further allow direct
ingress of deleterious materials to the reinforcement.
• Apart from the remedies discussed above other
preventive measures suggested in various literature are:
• Application of protective coating
• Modification of concrete
• Change in metallurgy of reinforcing steel
• Cathodic protection system
Causes of Cracks in Concrete
• The principle causes of cracking are discussed below. It will be
benefit of informative to professional working either for
design, construction or maintenance and repair.
• Temperature and Plastic Shrinkage:
• It is often seen relatively straight parallel with the span of
floors. This is mainly with one way slabs for corridors of large
length and is due to inadequate provision of distribution steel.
IS 456-2000, suggests that minimum reinforcement in slabs in
either direction shall not be less than 0.15 % for mild steel
reinforcement and 0.12 % for high strength deformed bars, of
the total cross sectional area, to avoid shrinkage cracks.
Causes of Cracks in Concrete
• Plastic shrinkage cracking occurs when subjected to a very rapid loss of
moisture caused by combination of factors which include air and concrete
temperature, relative humidity, and wind velocity at the surface of the
concrete. These factors can combine to cause high rates of surfaces
evaporation in either hot or cold weather.
• When moisture evaporates from the surface of freshly placed concrete
faster than it is replaced by bleeding water, the surface concrete shrinks,
Due to the restraint provide by the concrete below the drying surface
layer, tensile stresses develop in the weak, stiffening plastic concrete,
resulting in shallow cracks of varying depth which may form are often
fairly wide at surface. Plastic shrinkage cracks begins as shallow cracks
but can become full depth cracks.
• It is usual to see a crack parallel to main steel 4 to 7 m apart. This
particularly creates problem when the slab is for terrace as leakage starts
from these cracks only.
Cracks Repair By Routing and
Sealing
• The Crack sealers should ensure the structural
integrity and service ability. In addition they
provide protection from the ingress of harmful
liquids and gases.
• Routing and sealing of cracks can be used in
condition requiring remedial repair and where
structural repair is not necessary. The method
consists of enlarging remedial repair and where
structural repair is not necessary. The method
consists of enlarging the crack along its length on
the exposed surface, called chasing or routing,
and sealing it with a suitable joint sealant.
Cracks Repair By Routing and
Sealing
• This is a common technique for crack treatment and is
relatively simple in comparison to the procedures and
the training required for epoxy injection. The procedure
is most applicable to flat horizontal surfaces such as
floors and pavements. However, this method can be
accomplished on vertical surfaces as well as on curved
surfaces.
• This method is used to repair both fine pattern cracks
and larger, isolated cracks. A common and effective use
is for waterproofing by sealing cracks on the concrete
surface where water stands, or where hydrostatic
pressure is applied.
Cracks Repair By Routing and
Sealing
• The sealant may be of several materials, including
epoxies, silicones, urethanes, polysulfides,
asphaltic materials polymer mortars. Cement
grouts should be avoided due to the likelihood of
cracking. For floors, the sealant should be
sufficiently rigid to support the anticipated traffic.
• The procedure consists of preparing a groove at
the surface ranging in depth, typically from 6 to
25 mm. A concrete saw, hand tools or pneumatic
tools may be used. This groove is then cleaned by
plastic or sir blasting and allowed to dry. A sealant
is placed into the dry groove and allowed to cure.
Cracks Repair By Routing and
Sealing
Cracks Repair By Routing and
Sealing
Crack Repair by Stitching
• The stitching procedure consists of drilling holes on
both sides of the cracks, cleaning the holes and
anchoring the legs of the stitching dogs that span the
crack, which either a non-shrink grout or an epoxy-
resin-based bonding system. The stitching dogs should
be variable in length and orientation or both, and should
be so located that the tension transmitted across the
crack is not applied to a single plane but spread over
area.
• Stitching may be used when tensile strength must be
reestablished across major cracks. Stitching a crack
tends to stiffen the structure and the stiffening may
increase the overall structural restraint, causing the
concrete to crack elsewhere.
Crack Repair by Stitching
Providing Additional Reinforcement
• The cracked reinforced concrete bridge gird can
be successfully repaired by using epoxy injection
and reinforcing bars. This techniques consists of
sealings the crack, drilling holes of 20 mm
diameter that intersect the crack plane at
approximately 90 0, filling the hole and crack with
injected epoxy and placing a reinforcing bar into
drilled hole. Typically, 12 to 16 mm diameter bars
extending at least 500 mm on each side of the
crack are used. The epoxy bonds the bar to the
sides of the hole. The epoxy used to rebond the
crack should have a very low viscosity.
Drilling and Plugging
• This method consists of drilling down the length of the
crack and grouting it to form a key. A hole, typically 50 to
75 mm in diameter should be drilled, centered on and the
following the crack. The drilled hole is then cleaned, made
tight and filled with grout. The grout key prevents
transverse movements of the sections of concrete adjacent
to the crack. The key will also reduce heavy leakage
through the crack and loss of soil from behind a leaking
wall.
• When structural strength is not the criteria but water-
tightness is essential, the drilled hole, should be filled with a
resilient material of low modulus in lieu of grout. If the
keying effect is essential, the resilient material can be
placed in a second hole, the first being grouted.
Drilling and Plugging
Cracking Repair by Prestressing Steel
• When a major portion of a member is to be
strengthened, or a crack is to be closed, post-
tensioning is often the desirable solution. The
technique uses prestressing strands or bars to
apply a compressive force. Adequate
anchorage must be provided for the
prestressing steel. The method of correction
crack in slab and beam.
Cracking Repair by Pre-stressing
Steel
Cracking Repair By Grouting
• Based on grouting material used, there are
three methods:
• Portland Cement Grouting
• Chemical Grouting
• Epoxy Grouting
Portland Cement Grouting
• Wide Cracks, particularly in gravity dams and thick walls
may be repaired by filling with portland cement grout. This
method is effective in preventing water leakage, but will not
structurally bond cracking sections. The procedure consists
of cleaning the concrete along the crack by air jetting or
water jetting; installing grout at suitable intervals, sealing
the crack between the seats with sealant; flushing the crack
to clean it and test the seal; and then grouting the whole
area. Grout mixtures may contain cement and water or
cement plus sand and water, depending upon the width of
the crack. Water reducers or admixtures may be used to
improve the properties of the grout. For large volumes, a
pump is used and for small volumes, a manual injection gun
may be used. After the crack is filled, the pressure should be
maintained to ensure proper penetration of grout.
Portland Cement Grouting
Chemical Grouting
• Chemicals used for grouting are sodium
silicates, urethanes and acrylamides. Two or
more chemicals are combined to form gel, a
solid precipitate or a foam as opposed to
cement grouts that consists of suspension of
solids particles in a fluid. The advantages of
chemical grouts include applicability in moist
environments and their ability to be applied in
very fine cracks.
Chemical Grouting
Epoxy Grouting
Column Jacketing
• Column Jacketing is done to improve the load carrying
capacity of the column. The procedure followed is:
• Open the footing of the column by excavating soil around
it.
• Remove the plaster from the surface of the column.
• Make the surface of column concrete rough by sand
blasting.
• Remove the corroded bars by cutting them. Add new bars
from footing to the slab as per the instruction of engineers.
• Apply bonding agent on the old concrete for proper
bonding between old and new concrete.
• Erect necessary shuttering around the column.
• Pour minimum M-25 grade of concrete, vibrate and cure it.
Column Jacketing
Beam Jacketing
• Before taking up the strengthening of a beam, the
load acting on it should be reduced by removing
the flooring tiles and bed mortar from the slab.
Props are erected to support the slab. After clipping
off the existing plaster on the beam, additional
longitudinal bars at the bottom of the beam to-
geather with new stirrups are provided. Stirrups are
inserted by making holes from the slab. The
longitudinal bars are passed through the supporting
columns through holes of appropriate diameter
drilled in the columns. The spaces between bars
and surrounding holes are filled with epoxy grout
to ensure a good bond.
Beam Jacketing
• The surface of old concrete is cleaned by air
jetting. Expanded wire mesh is fixed on the two
sides and bottom of the beam. To ensure a good
bond between old concrete and new polymer
modified concrete, an apoxy bond coat is applied
to the old concrete surface. The polymer modified
mortar is applied, while the bond coat is still
fresh. Sometimes 2 to 3 coats of polymer
modified mortar are applied to achieve desired
thickness. The mortar is cured for appropriate
period in water. Epoxy resin grout is injected in
the cracks along top of beams.
Beam Jacketing
Questions
• State causes and precautions for distress in structure.
• State new repair system or products.
• Write a SN on properties of repair material and material for
repair.
• Describe the causes of cracks in concrete.
• What is meant by jacketing? Discuss different types of
jacketing?
• Explain in short the procedure for the damage assessment of
structural element.
References
• Concrete Technology by: R.P. Rethaliya
Atul Prakashan
• Concrete Technology by . M.S. Shetty
• Internet websites
Thanks
Prepared and Presented By:
Prof. Gaurav.H.Tandon
Sal Institute of Technology and Engineering Research

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Repair and Rehabilitation

  • 2. Syllabus • Distress in structure: Causes and precautions, damage assessment of structural elements, repairing techniques and repairing materials.
  • 3. Distress In Structure • Distress means Damage • Concrete may suffer distress or damage during its life period due to a number of reasons. Because of the varying conditions under which it is produced at various locations, the quality of concrete suffers occasionally either during production or during service conditions resulting in distress.
  • 4. Distress In Structure Causes of distress of concrete: • Structural causes – Externally applied loads – Environmental loads – Accidents – Subsidence's, etc. • Error in design and detailing • Poor Construction practices • Construction Overloads • Drying Shrinkage • Thermal Stresses • Chemical Reactions • Weathering • Corrosion
  • 6. In addition to the distress in hardened concrete, the plastic concrete may also suffer damage due to, • Plastic Shrinkage • Settlement Cracking • Early removal of formwork • Improper design of formwork.
  • 8. Evaluation Procedure for Repair and Strengthening of Concrete Structures • Before finalizing any scheme for repairs and rehabilitation of a distressed concrete structure, the concerned engineer has to be fully aware of the causes of causes of distress, extent of damage to the structure and the present condition of the concrete in the structure for repairs to be effective and lasting. The extent of distress has to be categorized so that the repair schemes can be formulated according to the distress in a particular structural element. So that, pre-repair evaluation and assessment of a structure is pre-requisite for working out effective repair schemes.
  • 9. Evaluation Procedure for Repair and Strengthening of Concrete Structures • Once the repairs have been carried out on a distressed structure, the post repair evaluation and assessment of the structure can be carried out for checking the efficacy of the repair. The post repair assessment is a tool with the engineer to evaluate whether the parent material and the repair material have obtained bond or whether the cracks or the voids have been filled up by the grouting materials. Thus, any scheme for effective repairs can be based on the pre-repair and post- repair evaluation of concrete structures
  • 10. Tools for Evaluation of Concrete Structures • The various tools available for evaluation of concrete structures are as follows: • Visual inspection and observations • Questioning of concerned personnel • Scrutiny of field data and records • Design Checks • Non- destructive testing (NDT) • Extraction of cores and testing • Supplementary laboratory techniques • Load testing of a structural member • The general approach adopted for pre-repair evaluation of distressed concrete structures is given below:
  • 11. Visual Inspection and Observations • The first step in the process of evaluation of a distressed concrete structure is visual inspection and observations. A through visual inspection and observations. A through visual inspection leads to proper approach to be adopted during investigation. It determines the number of field and laboratory tests required to be carried out. Visual inspection generally include the study of the following.
  • 12. Visual Inspection and Observations • Ambient conditions • Crack width and patterns • Spelling of Concrete • Color, texture and rust stains • Sinking of columns • Failure of beam-Column junctions. • Mal-functioning of machinery, structural components etc. • Condition of fixtures • Deposits/ splashes on structural components.
  • 13. Questioning of Personnel/ Scrutiny of field Data and Records • The questioning of personnel and the scrutiny of field data and records is carried out for the following: • Grade of concrete adopted • Cube test results • Type of material and sources • Constructional details • Environmental Conditions
  • 14. Questioning of Personnel/ Scrutiny of field Data and Records • The Scrutiny of the grade of concrete and cube test results brings out adequacy of strength of concrete and the degree of quality control exercised during constructions. • The study of the type of materials used particularly cement, coarse aggregate, fine aggregate, additives etc. also focuses the direction of investigations. The scrutiny of other constructional details e.g. removal of formwork, shifting of formwork for slip form construction, the height of pouring of concrete, use of compaction devices etc. are useful information for further investigation in many cases.
  • 15. • Rebound hammer test • Ultrasonic pulse velocity test • Pull- out test • Pull- off test • Carbonation test • Cover measurement • Break off test • Endoscopy • Radar techniques • Rapid chloride, alkali and sulphate kits, etc.
  • 16. Questioning of Personnel/ Scrutiny of field Data and Records • Scrutiny of field data and records: • Design checks: • Non-Destructive testing (NDT): • Any visual inspection and scrutiny of the field data, the in-situ testing is carried as per the approach finalized. Various in-situ non- destructive tests available are:
  • 17. • X-ray diffraction analysis • Differential thermal analysis • Chloride permeability test • Optical and Scanning microscopy • Chloride permeability test. • Compressive strength, density and modulus of elasticity determination on core samples, etc. • Load testing of a structural member: • Extraction of Cores and testing:
  • 18. In addition to various in-situ tests carried out, it becomes necessary to support the findings with laboratory tests • The laboratory tests generally adopted are: • Cement Content of Hardened Concrete • Chemical Analysis • Chlorides • Sulphates • pH • Nitrates
  • 19. Corrosion of Reinforcement in Concrete The damage to the concrete due to corrosion of reinforcement is considered to be one of the most serious problems. It is an universal problem and property worth of crores of rupees is lost every years. Due to corrosion problem in bridges, buildings and other RCC structures, India incurs heavy loss of about Rs. 1500 cores annually. This paper deals with various causes of corrosion and remedial measures thereon.
  • 22. Corrosion Process and Mechanism: • Corrosion of reinforcement steel is a complex phenomenon involved chemical, electrochemical and physical process. When reinforcement steel rusts, the volume of iron oxide formed is 2-3 times greater than the steel corroded, which results in bursting stresses in the concrete surrounding the bar. This causes cracking, spalling and delimination of concrete. Another consequence of corrosion is reduction in cross- sectional area of the steel at anode, thus reducing its loads carrying capacity.
  • 23. Causes of Corrosion and Remedial Measures • Various causes of corrosion and remedial measures are discussed below: • Presence of Cracks in Concrete: Certain amount of cracking always occurs in the tension zone of RCC depending upon the stresses in the reinforcing steel. Through these cracks, oxygen or sea water ingress into concrete and set up good environment for corrosion of reinforcement. Maximum permissible width of elastic cracks in RCC members would depend upon environmental and other factors. For normal environmental conditions, a maximum crack width of 0.3 mm for protected internal members and 0.2 mm for unprotected external members may be recommended.
  • 24. Presence of Cracks in Concrete
  • 25. Causes of Corrosion and Remedial Measures • Presence of moisture: Presence of moisture is a precondition for corrosion to take place because concrete can act as electrolyte in electrochemical cell only if it contains some moisture in pores. Corrosion can neither occurs in dry concrete or in submerged concrete. • The worst combination for corrosion to process is when the concrete is slightly drier than saturated i.e. about 80 - 90 % relative humidity with a low resistivity and the oxygen can still penetrate to the steel. Hence in high humidity areas like coastal India, low permeability concrete is recommended.
  • 26. Causes of Corrosion and Remedial Measures Permeability of Concrete: • This is also an important factor affecting corrosion of reinforcement. Ingress of moisture, sea water, oxygen, CO2 etc. is easier in porous concrete than in dense and impermeable concrete. It is worth mentioning than with each increases of W/C ratio 0.1, permeability of concrete increases 1.5 times. Poor curring increases permeability 5 to 10 times in comparison to good cured concrete and poor compaction increases permeability 7 to 10 times in comparison to good compacted concrete. • For this consideration, quantity of cement in concrete should not be less than 350 kg/ m3 and W/C ratio should not exceed 0.55 for ordinary structures and 0.45 for marine structures. All other normal requirements of good quality concrete, namely, grading, and cleanliness of aggregates, through mixing, proper compaction and curing should be taken care of.
  • 28. Causes of Corrosion and Remedial Measures • Carbonation: Hydrated cement paste forms a thin passivity layer of Gamma iron oxide (Fe2 O3) strongly adhering to the underlying steel and gives complete protection from reaction with the oxygen and water, that is from corrosion. • Hydration of cement liberates some calcium hydroxide which sets up a protective alkaline medium inhibiting electrochemical cell action and preventing corrosion of reinforcement.
  • 29. Carbonation • Carbon Dioxide (CO2) from the atmosphere diffuse inside the concrete, react with calcium hydroxide (Ca (OH)2) to form calcium carbonate which is water soluble. This reaction is known as carbonation. Carbonation lowers the alkalinity of concrete and reduce its effectiveness as protective medium. The pH value of pore water in concrete is generally between 10.5 to 12.0 but if due to carbonation it is lowered to 9.0 and below, the medium converts to acidic type and corrosion of reinforcement begins.
  • 31. Causes of Corrosion and Remedial Measures • Chlorides: Chlorides can enter in the concrete during concreting or during service conditions. During concreting, the chloride can enter via aggregates, gauging water and admixtures like (CaCl2) In service conditions, the chloride ions entry is due to ingress of sea water, de-icing and other salts. The chloride ions (Cl-) attack the iron oxide film leading to corrosion. Chloride ions activate the surface of the steel to form an anode, the passivated surface being the cathode. The rate of corrosion depends upon chloride ion concentration.
  • 33. Causes of Corrosion and Remedial Measures • Sulphate Attack: Solubility sulphates like sodium, potassium, magnesium and calcium are sometimes present in soil, ground water or clay bricks, react with tricalcium aluminate- 3CaO,Al2O3 (C3A) content of cement and hydraulic lime in the presence of moisture and from products which occupy much of cement and hydraulic lime in the presence of moisture and from products which occupy much bigger volume than the original constituent. This, expansive reaction results in weakening of concrete, masonry and plaster and formation of cracks as well as corrosion of reinforcement. • Severity of sulphate attack depends upon amount of soluble sulphate present in soil, water or clay bricks, permeability of concrete, amount of C3 A content in cement and duration for which concrete remains damp.
  • 35. Causes of Corrosion and Remedial Measures • Alkali Aggregate Reactions: OPC contains alkalies like sodium oxide (Na2 O) and potassium Oxide (K2O) to some extent these alkalies chemically reacts with reactive siliceous minerals in some aggregate and cause expansion, cracking and disintegration of concrete give rise to the corrosion of reinforcement. • Preventive Measure Consists of: • Avoid use of alkali-reactive aggregate in concrete. • Cement with alkali content more than 0.6 % should not be used. • Portland Pozzolana cement is recommended.
  • 37. Causes of Corrosion and Remedial Measures • Inadequacy of Cover: If Concrete cover to reinforcement is inadequate, reinforcement is liable to get corroded soon due to various factors such as Carbonation, ingress of sea water, moisture penetration etc. It is therefore necessary that RCC works should have a minimum clear cover as recommended by IS 456: 2000. Reinforcement shall have concrete cover and thickness of such cover (exclusive of plaster or other decorative finish) shall be as follows:
  • 38. Causes of Corrosion and Remedial Measures • At each end of reinforcing bars not less than 25 mm nor less than twice the dia. of such bar. • For longitudinal reinforcing bars in column, not less than dia. of such bar. In case of columns of dimensions 200 mm or under, whose reinforcing bars do not exceed 12 mm in dia. a cover of 25 mm may be used. • For longitudinal reinforcing bars in beams, not less than dia. of such bar. • for tensile, shear, compressive or other reinforcement in a slab, not less than 15 mm, not less than dia. of such bar.
  • 39. Causes of Corrosion and Remedial Measures • Increased cover thickness may be provided when surface of concrete members are exposed to the action of harmful chemicals (as in case of concrete in contact with earth faces contaminated with such chemicals) acid vapours, saline atmosphere, sulphurous smoke etc. and such increase of cover may be between 15 mm to 40 mm beyond. • For RCC members totally immersed in sea water, cover shall be 40 mm more than the normal cover.
  • 41. Causes of Corrosion and Remedial Measures • In all such cases cover should not exceed 75 mm. Based on part research, concrete, cover more than 50 mm is, however, not recommended as it give rise to increase crack width which may further allow direct ingress of deleterious materials to the reinforcement. • Apart from the remedies discussed above other preventive measures suggested in various literature are: • Application of protective coating • Modification of concrete • Change in metallurgy of reinforcing steel • Cathodic protection system
  • 42. Causes of Cracks in Concrete • The principle causes of cracking are discussed below. It will be benefit of informative to professional working either for design, construction or maintenance and repair. • Temperature and Plastic Shrinkage: • It is often seen relatively straight parallel with the span of floors. This is mainly with one way slabs for corridors of large length and is due to inadequate provision of distribution steel. IS 456-2000, suggests that minimum reinforcement in slabs in either direction shall not be less than 0.15 % for mild steel reinforcement and 0.12 % for high strength deformed bars, of the total cross sectional area, to avoid shrinkage cracks.
  • 43. Causes of Cracks in Concrete • Plastic shrinkage cracking occurs when subjected to a very rapid loss of moisture caused by combination of factors which include air and concrete temperature, relative humidity, and wind velocity at the surface of the concrete. These factors can combine to cause high rates of surfaces evaporation in either hot or cold weather. • When moisture evaporates from the surface of freshly placed concrete faster than it is replaced by bleeding water, the surface concrete shrinks, Due to the restraint provide by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks of varying depth which may form are often fairly wide at surface. Plastic shrinkage cracks begins as shallow cracks but can become full depth cracks. • It is usual to see a crack parallel to main steel 4 to 7 m apart. This particularly creates problem when the slab is for terrace as leakage starts from these cracks only.
  • 44. Cracks Repair By Routing and Sealing • The Crack sealers should ensure the structural integrity and service ability. In addition they provide protection from the ingress of harmful liquids and gases. • Routing and sealing of cracks can be used in condition requiring remedial repair and where structural repair is not necessary. The method consists of enlarging remedial repair and where structural repair is not necessary. The method consists of enlarging the crack along its length on the exposed surface, called chasing or routing, and sealing it with a suitable joint sealant.
  • 45. Cracks Repair By Routing and Sealing • This is a common technique for crack treatment and is relatively simple in comparison to the procedures and the training required for epoxy injection. The procedure is most applicable to flat horizontal surfaces such as floors and pavements. However, this method can be accomplished on vertical surfaces as well as on curved surfaces. • This method is used to repair both fine pattern cracks and larger, isolated cracks. A common and effective use is for waterproofing by sealing cracks on the concrete surface where water stands, or where hydrostatic pressure is applied.
  • 46. Cracks Repair By Routing and Sealing • The sealant may be of several materials, including epoxies, silicones, urethanes, polysulfides, asphaltic materials polymer mortars. Cement grouts should be avoided due to the likelihood of cracking. For floors, the sealant should be sufficiently rigid to support the anticipated traffic. • The procedure consists of preparing a groove at the surface ranging in depth, typically from 6 to 25 mm. A concrete saw, hand tools or pneumatic tools may be used. This groove is then cleaned by plastic or sir blasting and allowed to dry. A sealant is placed into the dry groove and allowed to cure.
  • 47. Cracks Repair By Routing and Sealing
  • 48. Cracks Repair By Routing and Sealing
  • 49. Crack Repair by Stitching • The stitching procedure consists of drilling holes on both sides of the cracks, cleaning the holes and anchoring the legs of the stitching dogs that span the crack, which either a non-shrink grout or an epoxy- resin-based bonding system. The stitching dogs should be variable in length and orientation or both, and should be so located that the tension transmitted across the crack is not applied to a single plane but spread over area. • Stitching may be used when tensile strength must be reestablished across major cracks. Stitching a crack tends to stiffen the structure and the stiffening may increase the overall structural restraint, causing the concrete to crack elsewhere.
  • 50. Crack Repair by Stitching
  • 51. Providing Additional Reinforcement • The cracked reinforced concrete bridge gird can be successfully repaired by using epoxy injection and reinforcing bars. This techniques consists of sealings the crack, drilling holes of 20 mm diameter that intersect the crack plane at approximately 90 0, filling the hole and crack with injected epoxy and placing a reinforcing bar into drilled hole. Typically, 12 to 16 mm diameter bars extending at least 500 mm on each side of the crack are used. The epoxy bonds the bar to the sides of the hole. The epoxy used to rebond the crack should have a very low viscosity.
  • 52. Drilling and Plugging • This method consists of drilling down the length of the crack and grouting it to form a key. A hole, typically 50 to 75 mm in diameter should be drilled, centered on and the following the crack. The drilled hole is then cleaned, made tight and filled with grout. The grout key prevents transverse movements of the sections of concrete adjacent to the crack. The key will also reduce heavy leakage through the crack and loss of soil from behind a leaking wall. • When structural strength is not the criteria but water- tightness is essential, the drilled hole, should be filled with a resilient material of low modulus in lieu of grout. If the keying effect is essential, the resilient material can be placed in a second hole, the first being grouted.
  • 54. Cracking Repair by Prestressing Steel • When a major portion of a member is to be strengthened, or a crack is to be closed, post- tensioning is often the desirable solution. The technique uses prestressing strands or bars to apply a compressive force. Adequate anchorage must be provided for the prestressing steel. The method of correction crack in slab and beam.
  • 55. Cracking Repair by Pre-stressing Steel
  • 56. Cracking Repair By Grouting • Based on grouting material used, there are three methods: • Portland Cement Grouting • Chemical Grouting • Epoxy Grouting
  • 57. Portland Cement Grouting • Wide Cracks, particularly in gravity dams and thick walls may be repaired by filling with portland cement grout. This method is effective in preventing water leakage, but will not structurally bond cracking sections. The procedure consists of cleaning the concrete along the crack by air jetting or water jetting; installing grout at suitable intervals, sealing the crack between the seats with sealant; flushing the crack to clean it and test the seal; and then grouting the whole area. Grout mixtures may contain cement and water or cement plus sand and water, depending upon the width of the crack. Water reducers or admixtures may be used to improve the properties of the grout. For large volumes, a pump is used and for small volumes, a manual injection gun may be used. After the crack is filled, the pressure should be maintained to ensure proper penetration of grout.
  • 59. Chemical Grouting • Chemicals used for grouting are sodium silicates, urethanes and acrylamides. Two or more chemicals are combined to form gel, a solid precipitate or a foam as opposed to cement grouts that consists of suspension of solids particles in a fluid. The advantages of chemical grouts include applicability in moist environments and their ability to be applied in very fine cracks.
  • 62. Column Jacketing • Column Jacketing is done to improve the load carrying capacity of the column. The procedure followed is: • Open the footing of the column by excavating soil around it. • Remove the plaster from the surface of the column. • Make the surface of column concrete rough by sand blasting. • Remove the corroded bars by cutting them. Add new bars from footing to the slab as per the instruction of engineers. • Apply bonding agent on the old concrete for proper bonding between old and new concrete. • Erect necessary shuttering around the column. • Pour minimum M-25 grade of concrete, vibrate and cure it.
  • 64. Beam Jacketing • Before taking up the strengthening of a beam, the load acting on it should be reduced by removing the flooring tiles and bed mortar from the slab. Props are erected to support the slab. After clipping off the existing plaster on the beam, additional longitudinal bars at the bottom of the beam to- geather with new stirrups are provided. Stirrups are inserted by making holes from the slab. The longitudinal bars are passed through the supporting columns through holes of appropriate diameter drilled in the columns. The spaces between bars and surrounding holes are filled with epoxy grout to ensure a good bond.
  • 65. Beam Jacketing • The surface of old concrete is cleaned by air jetting. Expanded wire mesh is fixed on the two sides and bottom of the beam. To ensure a good bond between old concrete and new polymer modified concrete, an apoxy bond coat is applied to the old concrete surface. The polymer modified mortar is applied, while the bond coat is still fresh. Sometimes 2 to 3 coats of polymer modified mortar are applied to achieve desired thickness. The mortar is cured for appropriate period in water. Epoxy resin grout is injected in the cracks along top of beams.
  • 67. Questions • State causes and precautions for distress in structure. • State new repair system or products. • Write a SN on properties of repair material and material for repair. • Describe the causes of cracks in concrete. • What is meant by jacketing? Discuss different types of jacketing? • Explain in short the procedure for the damage assessment of structural element.
  • 68. References • Concrete Technology by: R.P. Rethaliya Atul Prakashan • Concrete Technology by . M.S. Shetty • Internet websites
  • 69. Thanks Prepared and Presented By: Prof. Gaurav.H.Tandon Sal Institute of Technology and Engineering Research