REPAIR AND REHABILITATION OF STRUCTURE
REPAIR AND
REHABILITATION OF STRUCTURE
ABSTRACT:
Generally speaking concrete
structures will deteriorate over time from exposure to wetting and drying
cycles, freeze thaw cycles, abrasion, chloride intrusion, chemical attack, etc.
Concrete structures in the water and waste water industry are under constant
attack from the very processes they are built to serve. Combine these specific
processes with the exposures listed above, and the fact that many of our water
and waste water plants are beyond their designed life and capacity you have a
unique set of circumstances that must be addressed on a regular basis to
provide protection, repair, and rehabilitation of these concrete structures
In
the last few years have seen greater investment growth in the building repair
and rehabilitation sector than in that of new building homes. The most advanced
societies show great concert for the quality of their urban spaces and the
comfort of their users .This demand applies to both new constructions and old building. Consideration regarding
energy efficiency and sustainability tend to prioritize the rehabilitation of
existing building over the construction of new ones. In this paper we discuss
about this concepts and techniques used for repair and rehabilitation of
different Structures
Why does
Concrete need Repairs?
There are many factors which lead to the need of
repairs such as:
·
Corrosion of reinforcement due to carbonation, chlorides
·
Effects of Sulphates
·
Alkali silica reaction
·
Environmental pollution
·
De-icing salts
·
Acid rains
·
Oils
·
Freeze thaw cycles
·
Abrasion or erosion from wind or water borne agents
·
Plants or micro organisms
·
Overloading
·
Physical settlement
·
Impact
·
Earthquake
·
Fire
·
Chemical attack by aggressive chemicals, in industrial or marine environment
sewerage or even soft water.
Also the deterioration gets aggravated due to
errors/mistakes/ poor workmanship during construction such as:
·
Higher w/c ratio
·
Honeycombs and compaction voids
·
Bleeding and segregation
·
Plastic shrinkage & hardening stage shrinkage cracks
·
Inadequate or no curing
·
Insufficient concrete cover
·
Cast-in chlorides from contaminated water/aggregates/ /admixtures
·
Inadequate or excessive vibration during concreting
·
Shutter work or reinforcement movement during placement of concrete
Generally, concrete structure requires repairs in the
two cases– New construction and during the service life. Repairs in the new
construction require a different approach then the repairs during service life
and we shall deal one by one for better understanding. The repairs during
service life have more steps and we will deal this first. The repairs during
service life arise due to certain deterioration taken place and understanding
of the same is very vital in the design of the repair solution.
Why Concrete
Deteriorate?
The reinforced concrete was designed with a basic
understanding that it’s a marriage of two carrying spouses – concrete and
steel. Concrete protects steel from getting corroded and steel protects
concrete from getting cracked due to bending. The marriage was designed to last
forever, but the environment facilitates entry of many agents who leads the
marriage to divorce… Major agents and their actives are described as under.
Carbonation: The high pH of concrete passiveness steel reinforcement from getting corroded. The carbon dioxide/sulphur dioxide present in the atmosphere gets dissolved in to water and forms weak carbonic/ sulphuric acid and enters the concrete through micro pores reducing the pH, resulting in to loss of passivation layer around the reinforcement. The reinforcement starts getting corroded resulting in to the rust. The rust times of the original volume of the metal creating bursting pressure in the concrete mass. The build-up of the pressure eventually cracks the concrete and makes the access for ingress of water and other water dissolved agents easy. The quicker access aggravates the corrosion and structure starts deteriorating rapidly. Spalling of the concrete cover and formation of brown colour rust is a visual indication of the carbonation attack. The carbonation depth can be assessed by phenolphthalein liquid. The reaction is at its best at 50 – 75% relative humidity. The corrosive reaction is expressed as:
CO2+H2O+Ca(OH)2 >CaCO3+H2O
Carbonation: The high pH of concrete passiveness steel reinforcement from getting corroded. The carbon dioxide/sulphur dioxide present in the atmosphere gets dissolved in to water and forms weak carbonic/ sulphuric acid and enters the concrete through micro pores reducing the pH, resulting in to loss of passivation layer around the reinforcement. The reinforcement starts getting corroded resulting in to the rust. The rust times of the original volume of the metal creating bursting pressure in the concrete mass. The build-up of the pressure eventually cracks the concrete and makes the access for ingress of water and other water dissolved agents easy. The quicker access aggravates the corrosion and structure starts deteriorating rapidly. Spalling of the concrete cover and formation of brown colour rust is a visual indication of the carbonation attack. The carbonation depth can be assessed by phenolphthalein liquid. The reaction is at its best at 50 – 75% relative humidity. The corrosive reaction is expressed as:
CO2+H2O+Ca(OH)2 >CaCO3+H2O
Chloride
Attack: The main source of chlorides is the contaminated water or aggregates
used during construction and the marine environment – direct contact with sea
water or through wind borne chlorides in the splash zone. Chlorides ions first
attack Fe2O3 - the passivating ferrous oxide layer on the steel reinforcement.
Once reinforcement looses its passivation layer, it’s highly susceptible to
electro-chemical corrosion further induced by chlorides ions. The water
dissolved chloride ions forms electrochemical corrosion cell and establishes
anodic and cathodic sites on the rebar.
The electro-chemical corrosion results in to pitting corrosion – reduction in the cross section of the rebar at specific sites without noticeable deterioration of the concrete cover. The hidden reduction in the cross-section of the reinforcement can results in to sudden failure of the structural member - making this as one of the most dangerous deterioration in the concrete structure. The corrosive reactions are expressed as:
Fe++ + 2Cl -> FeCl2
FeCl2 + H2O + OH ->Fe(OH)2 + H+ + 2Cl - 2Fe(OH)2 + ½O2 > Fe2O3 + 2H2O
(ANODE)
½O2 + H2O + 2e -> 2OH-
(CATHODE)
There is no “net use” of chloride ions during the corrosion process. Therefore, once enough chloride ions reach the steel to break the passivation layer only water, oxygen and a conductive medium is needed to maintain the corrosion reaction. Also note that since corrosion is a chemical reaction, temperature plays an important role in the process. The higher the temperature the faster the corrosion reaction occurs. The general rule for the rate of chemical reactions is that for every 25oF increase, the reaction rate doubles.
Sulphate attack: The main source of sulphates is the ground water. The sulphates attack on concrete, by reacting with the C3A in the concrete. The reactive product is larger in the volume resulting in to the expansive cracking in the concrete mass. The Spalling and cracking of concrete takes places without any deterioration of the reinforcement to start with. With the time other forms of corrosion such as carbonation, chlorides becomes aggravated due to quicker access to the reinforcement. The sulphate attack can be reduced by using sulphate resistant cement which has low C3A content; but this also reduces the resistance of chloride attack and hence not preferred in the marine situation.
The electro-chemical corrosion results in to pitting corrosion – reduction in the cross section of the rebar at specific sites without noticeable deterioration of the concrete cover. The hidden reduction in the cross-section of the reinforcement can results in to sudden failure of the structural member - making this as one of the most dangerous deterioration in the concrete structure. The corrosive reactions are expressed as:
Fe++ + 2Cl -> FeCl2
FeCl2 + H2O + OH ->Fe(OH)2 + H+ + 2Cl - 2Fe(OH)2 + ½O2 > Fe2O3 + 2H2O
(ANODE)
½O2 + H2O + 2e -> 2OH-
(CATHODE)
There is no “net use” of chloride ions during the corrosion process. Therefore, once enough chloride ions reach the steel to break the passivation layer only water, oxygen and a conductive medium is needed to maintain the corrosion reaction. Also note that since corrosion is a chemical reaction, temperature plays an important role in the process. The higher the temperature the faster the corrosion reaction occurs. The general rule for the rate of chemical reactions is that for every 25oF increase, the reaction rate doubles.
Sulphate attack: The main source of sulphates is the ground water. The sulphates attack on concrete, by reacting with the C3A in the concrete. The reactive product is larger in the volume resulting in to the expansive cracking in the concrete mass. The Spalling and cracking of concrete takes places without any deterioration of the reinforcement to start with. With the time other forms of corrosion such as carbonation, chlorides becomes aggravated due to quicker access to the reinforcement. The sulphate attack can be reduced by using sulphate resistant cement which has low C3A content; but this also reduces the resistance of chloride attack and hence not preferred in the marine situation.
Step-by-step
process to successful repairs:
Essential steps for successful repairs
Evaluation
Evaluate the current condition of the concrete
structure. Structural analysis of the structure in its deteriorated condition,
review of records of any previous repair work accomplished, review of
maintenance records, visual examination, destructive and non-destructive
testing, and any lab analysis of concrete samples. Some of the popular tests
used during the evaluation are summarized as under:
·
Visual inspection & recording
·
Hammer sounding/Rebound hammer test
·
Phenolphthalein test for carbonation depth
·
Silver-nitrate test for chloride attack
·
Half-cell potential measurement
·
Core-cutting
·
Chemical analysis of concrete at different depths
Repair
Philosophy
It is most important to consider the full load envelope, which has been
acting on the structure during the completed service life and in the future.
The repair materials must have compatibility with the existing structure. The
Compatibility may be defined as a balance (equilibrium) of physical, chemical ,
electrochemical and dimensional properties between the repair material and the
existing substrate in structural exposure conditions for a determined period of
time.
1st Compatibility: Physical/ Permeability:
· Allow substrate to breath
· Prevent entry
of water & water Bourne salts – Sulphates, Chlorides, SO2, CO2 Chlorides, SO2, CO2
2nd Compatibility: Chemical:
2nd Compatibility: Chemical:
· No negative chemical interaction with the substrate
· Absence of potentially dangerous substances such as chlorides, alkalise...
· No expansive stringier formation with sulphates
3rd Compatibility: Electro- Chemical:
3rd Compatibility: Electro- Chemical:
· Higher resistance to corrosion current
· must have conductivity and should not isolate substrate
·Effective passivation of rebar’s
4th Compatibility: Dimensional stability:
4th Compatibility: Dimensional stability:
· Coefficient of Thermal Expansion: Different Coefficients of Thermal
Expansion causes differential movement, and hence shall be avoided
· Modulus of Elasticity: Under compression materials of different module
will causes stress at the interface, and hence shall be avoided
· Drying shrinkage: Drying shrinkage of fresh mortar causes stresses at
interface; hence needs to be controlled to minimum.
Treatment of
cracks
The most important criteria for selecting the right
material for crack injection is based on the structural status of the crack. Is
the crack alive or dormant? Can be checked by monitoring the crack width. If
the crack is live, stresses are still like to relieve and hence to avoid
further cracking at any other location, it is important to inject and seal the
live crack with flexible injection resin such as polyurethane based. The
dormant structural cracks can be sealed with epoxy/polyurethane resins meant
for structural bonding. In case if the sealing is only meant for water
tightness same can be achieved by injecting with the re-swell able acryl ate
injection resins. The surface cracks found within the concrete covers can be
open, routed and sealed used acrylic sealers as they are superficial nature but
needs effective sealing as can leads to other form of corrosion/ deterioration.
Many hairline cracks formed on the surface of concrete cannot be opened and
sealed and can be coated and sealed with high-elongation, flexible acrylic
protective coatings instead.
Surface
Preparation for Volume Replacements
The surface preparation is a prerequisite for an
effective volume replacement job. Following components explains the surface
preparation tasks:
·
Remove all identified defective concrete.
·
Saw cut perimeters - 15mm depth.
·
Expose steel until no corrosion is evident.
·
Expose the full circumference of the steel and beyond by 25mm.
·
Priming of the rebar’s: Prime immediately after cleaning. Apply a
continuous coat of active zinc rich epoxy primer or appropriate corrosion
protection system. Attention must be paid to the underside of the bars.
Priming of the concrete: Depending upon the need of the volume replacement
materials, apply right primer. In case of chloride contaminated area use epoxy
bonding agent.
Cosmetic Volume
Replacements
While replacing the concrete volume within the cover
is defined as cosmetic repairs. The aim is to replace defective, deteriorated
concrete cover with impervious polymer-modified mortar. The most cost effective
repair materials are ready to use Re-Profiling mortars or to use site-batched
polymer modified repair mortars with 1:5:15 proportion of polymer: cement:
sand. These mortars are not capable of achieving the high compressive strengths
but are able to provide effective corrosion barrier at economical costs. As the
repair is within the cover zone, it does not have significant barring on the
overall strength of the structure. The ready to use reprofiling mortars are
shrinkage compensated and have good thixotropy enabling up to 50mm thickness
built-ups in single operations; while polymer modified site-batched mortars may
need multi-layer applications on thick applications.
Structural
Volume Replacements
While replacing the volume of the concrete beyond
concrete cover, it is very important that the member under repair is relived
from the imposed loads by supports during repair period. Surface preparation is
very important and any negligence may cost the success of the structural
repairs. There are many options available for volume replacement such as – micro
concrete, single component patch repair mortars, two-component patch repair
mortars, spray applied micro-concrete, site-batched polymer modified mortars, self
compacting concrete, shot Crete, pre-placed aggregate concrete. Large volume
concrete repairs can be conducted using self compacting concrete, shot Crete or
pre-placed aggregate concrete provided the interface between new and old
concrete is taken care properly. Generally, in India the 1st choice for volume
replacement remains as form and pour micro concrete while spray applied
micro-concrete is very popular internationally due to reduced needs for
shuttering and ready availability of spray applied micro concretes. Patch
repair mortars if need to be used must be ready to use, one component type with
fibre modification to avoid chances of cracking. Product like two component
polymer modified mortars and site-batched polymer modified mortars known for
their low compressive strengths and can not be used to repair high grade
concrete. In most parts of India, it’s not practical to produce site batched
polymer modified mortars with > M20 grade compressive strengths due to
limitation of local fine aggregate gradation! If such low strength mortars are
used to replace concrete, it leads to higher stress concentration on the rest
of the structure resulting in to further distress and deterioration.
Special
Applications
Application such as under-water, chemical exposures,
pavement repairs, abrasion/impact damage, and heritage restorations requires
additional considerations while repairs and now many specialized products are available
for such applications some of them are summarized as below:
Underwater
Repairs
·
Micro-concrete: Emaco S46UW – Anti-washout grout
·
Patch repair mortar: Emaco S90 – Thixotropic patching mortar
·
Crack Injection resins: Concresive 1316 – water insensitive resin
·
Re-profiling/sealing mortar: Subcote S – Resin based
Chemical
Resistance Situations
·
Epoxy concrete: Concresive 2929
·
Patch repair mortar: Concresive ERL – Epoxy liner
·
Re-profiling/sealing mortar: Concresive 2200 – Resin based
Pavement
repairs
·
Patch repair mortar: Emaco R650 – Rapid hardening, cementitious
·
Re-profiling mortar: Concresive 1418T – Rapid hardening, resinous
More details on individual products can be available
from local BASF offices.
Corrosion
Control
While repairing the deteriorated structure it’s of
utmost importance to prevent the rest of sound structure from getting
deteriorated by implementing proper corrosion control measures. There are
various options available; the right selection needs to be based on the need of
the situation and the practicality of the options. Following is a basic
introduction with the advantages and disadvantages of some of the most popular
options:
A. Film forming
coatings – Acrylics & resin based coatings:
·
Limited life expectancy ( 5 ~ 10 years)
·
Limited crack bridging limits performance on cracked substrate
·
No passivation of corroding Reinforcement
B. Migrating
corrosion inhibitors - surface applied method
·
Easy to use
·
Effectiveness depends on chemical types & concrete porosity
·
Not effective against Carbonation, sulphates, ASR.
C. Cathodic
Protection – Impress Current method
·
Good life expectancy (> 20 years)
·
Very expensive
·
Destructive, Slow and time consuming
·
High application skills required
·
Power break-down can disrupt the protection
·
No protection against Carbonation, sulphate, ASR attacks
D. Cathodic
Protection – Impress Current method
·
Low to medium life expectancy ( 5 - 10 years)
·
Destructive method, requires replacements in future
·
Lump-sum applications, Design guidelines not followed
·
Not practical in heavy reinforced members
·
Corrosion continues > instead of steel embedded zinc corrodes
·
Not effective against Carbonation, sulphates, ASR.
E. Protectosil
CIT –Organo-functional silane based system
·
Spray applied - Easy to use
·
Organo functional Silane based - deep penetrative
·
Monomer structure – chemically reacts, no degradation in UV
·
Corrosion inhibition effect – repassivates rebar
·
Effective in all 4 types of corrosion as it hydrophobize concrete
·
Well established & tested worldwide
·
Non destructive method
·
Measurable reduction of corrosion current Effective till treated
concrete thickness lasts!
Other Important
Aspects of Repairs
While right diagnosis and selection of materials is
the core of the repair project; other aspects such as Preparation of drawings and
specifications, Selection of right contractor, Execution of the work to the
specifications, adequate Quality control are equally important. Repair project
is more specialized and good site management and construction practices have to
be followed. Also at the end of the job all the records relating to the
diagnosis, material selection and execution of the work will be maintained and
preserved for any reference needs in future.
CONCLUSION
With the right focus on the causes, evaluation and
selection of correct repair materials; the resultant repairs would be long
lasting and we would be able to avoid expensive repetitions of repairs in the
future.
REFERENCE:
1. www.google.com
2. Assessment and repair of the bearing structure of a multi-storey parking
garage.
Journal of Building Appraisal (2007) 2, 335–354. doi:10.1057/palgrave.jba.2950053
No comments
Post a Comment