Causes of concrete reinforcement corrosion and their repair

PowerPoint PresentationM N Ramesh MD, Talrak Construction Chemicals Pvt. Ltd., Bengaluru [email protected] web: www.Talrak.co.in Mobile : +919740030156
•Chemical Action
Compression member
Too less cover to sides
Corrosion and Spalling
Marine Environment
Extensive Corrosion in the soffit of the slab
Chemical Attack
Structural Problem
Wrong Detailing
Board stage
Fire damage
Earth Quake
War
Terrorism
Carbonation
What is Carbonation?
• Cement paste contains 25-50 wt% calcium hydroxide (Ca(OH)2), which mean that the pH of the fresh cement paste is at least 12.5.
• Carbonation occurs in concrete because the calcium bearing phases present in concrete are attacked by carbon dioxide of the air and converted to calcium carbonate.
Ca(OH) 2 + CO2 CaCO3 + H2O
• The pH of a fully carbonated paste is about 7.
Carbonation
• When Ca(OH)2 is removed from the paste CSH will liberate CaO which will also carbonate.
• The rate of carbonation depends on porosity & moisture content of the concrete.
• The carbonation process requires the presence of water because CO2 dissolves in water forming H2CO3 – Carbonic Acid
CO2 + H2O H2CO3
• If the concrete is too dry (RH <40%) CO2 cannot dissolve and no carbonation occurs.
• If it is too wet (RH >90%) CO2 cannot enter the concrete and the concrete will not carbonate.
• Optimal conditions for carbonation occur at a RH of 50% (range 40- 90%).
Carbonated paste
appears orange-
Advantage & disadvantage of carbonation
in the cement paste making the carbonated
concrete stronger. Carbonation is therefore an
advantage in unreinforced concrete.
as pH of carbonated concrete drops to about 7 - a
value below the passivation threshold of steel.
Bi-carbonation - what is that?
• Occasionally concrete may suffers from the so called bi-carbonation process.
• Bi-carbonation may occur in concrete with very high water- cement ratio due to formation of hydrogen carbonate ions at pH lower than 10.
• Contrary to normal carbonation, bi-carbonation results in an increase in porosity making the concrete soft and friable.
• Bi-carbonation may be recognized by the presence of large "pop-corn" like calcite crystals and the highly porous paste
Pop-corn" like calcite crystals present in carbonated paste.
Bi-Carbonation
Relative volume of Corrosion Products of Steel ( Oxides of Iron)
Rebar corrosion in carbonated concrete
Chloride ions cause
cell in reinforced concrete
Fe >> Fe++ + 2e- O2 + 2H20 + 4e- >> 4OH-
Cl- Cl- e- Corrosion
attack
HALF CELL POTENTIAL TEST FOR DETECTION AND RATE OF CORROSITION OF REBARS
ASTM C-876 criteria for corrosion of steel
Severe Corrosion < -406 mV <-500 mV
High- > 90% of corrosion
Corrosion
form expansive tri-calcium
43
rectify local defects
• Rehabilitation : Rendering unserviceable structures, serviceable
• Retrofitting : Measures carried out to the structures to make them fit to perform under conditions those not contemplated originally in the designs
• Restoration : Correcting defects structural/ otherwise
• Monitoring : Evaluating the structural performance of new or remediated structure
Conventional repair of chloride
repair / concrete interface
Zinc is more anodic than steel
i.e. more reactive as defined in the
electrochemical
series
Zinc is more anodic than steel
i.e. more reactive as defined in the
electrochemical
series
Cathode
Cathode
are O2 and Cl2
negative ions, such as Cl- and OH--
will migrate towards the anode,
thus drawing the chloride ions
away from the reinforcement.
Cathodic Protection (cont) (cont)
concrete structures
chloride ions to the level of reinforcement
Cathodic
prevention
Negative
Concrete
Rebar
marine structure
structure
Concrete
of marine structure
Corrosion Diagram
ZONE 1
ZONE 2
ZONE 3
ZONE 4
concrete
Vinyl
Styrene
Chlorinated
Pure Aliphatic
Polyurathenes
Expoxies
Performance of resins
Materials and Techniques – Case studies
Advanced
Composite
Polymers
Composites
• Flexural cracking, Cover concrete crushing • Buckling of longitudinal reinforcement • Debonding of laps of reinforcement • Lack of ductility
• The most critical mode of failure is the column shear failure
–Inclined cracking leads to the concrete cover spalling, rupture or
opening of stirrups
• Failure due to shear,
• Failure due to flexure both bottom and top of the member
Glass Fiber Wrapping for Axial Load Enhancement and Ductility
Improvement
ACM Jacket
De-bonding of terracotta tile cladding
• Slender columns
• Called for enhanced axial load capacity of the columns
Friday evening after work
During Saturday and Sunday
Sprayed concrete shear walls
Sprayed concrete shear walls
External post-tensioning
Vierendeel girder supported slab
Increased supporting area by top plate to reduce stress concentration
Floating stub column to reduce span
Summary
• ‘A stitch in time saves nine’
• Advent of new generation materials have opened new vistas in Remedial Engineering possibilities
• Remedial engineering is a multi-disciplinary domain.
• The new generation materials will only help to make the good constructing better
• They are not a substitute for bad quality construction and unsound engineering practices.
for your attention……