Copy of Chloride Attack on Structures
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EFFECTS OF FREEZING,THAWING
AND CHLORIDE ON DURABILITY
CONCRETE
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Hareesh Haridasan
Ac 0310Structures 3
Faculty Prof. R.J Shah
Masters in Architecture and
Settlement Conservation
CEPT University
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CONCRETE
Concrete is versatile construction material with manyadvantage but with numerous disadvantage too.
It can be cast in place (cast in situ) or precast or pre
stressed. The cast in situ concrete is the most commonly used
form of this material. The concrete when it is mixed isin semi fluid form is poured into forms. When thispaste hardens , it attains the desired shape.
The versatility of forms makes possible the creation ofcast in place concrete in practically limitless shapes andsurface.
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Mill Owners Association , Ahmedabad
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Assembly Building ,Chandigarh
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DURABILITY OF CONCRETE
Durability is defined as the quality of materials orstructure to be in good state after an extendedperiod of time and usage.
Durability is the ability of concrete to resistweathering action, chemical attack, and abrasionwhile maintaining its desired engineeringproperties.
Different concretes require different degrees ofdurability depending on the exposureenvironment and the properties desired.
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DURABILITY OF CONCRETE
Types of chemical reaction in concrete affecting durability:
Reactions with larger products than the reactants,
causing expansion and as a result cracking.
(Transformation) reactions with products having alower strength (adhesion).
Dissolution (leaching/corrosion) or other chemical
reactions, leading to higher porosity and therefore
lower strength.
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CONCRETE ADDITIVES
Instead of using a special cement, it is possible toalter some of the properties of the cement byuse of suitable additives.
Additives can be classified as1. Accelerating admixture
2. Retarding admixture
3. Water reducing admixtures or super plasticizers.
Important feature of the majority of admixturesfor concrete is that they are primarily used onthe basis of experience or ad hoc tests
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CHEMICAL RESISTANCE OF CEMENT
Water :- When concrete is in permanent contactwith water, the calcium salts present tend tohydrolyze and leach out, leaving residues of silica,iron oxides and alumina. When acidic water actsupon concrete for long period of time the surfacebecomes very soft and starts to disintegrate.
All acids attack concrete. The strong mineral acidshave the most rapid effect if soluble calcium salts
are formed. Alkalis on the other hand do not generally effect
concrete.
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Sulphate solutions have effect on concrete. Itleaches out hydroxides and weakens thestructure. Steam curing can increase the
resistance to sulphate attack. Chlorides:- Sodium chloride in less
concentration has little effect on concrete. Theeffect of ammonium chloride and highconcentrated sodium chloride is same as anacid.
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FAILURE OF CONCRETE
Concrete is sensitive to material failure because it contains a greatnumber of ingredients.
Concrete failures occurs in two principal areas1. material itself2. in individual structural elements Material failure happen often during the manufacturing process
especially during casting or hardening. Deterioration of concrete in use is often caused byfreeze thaw
cycles. Concrete parking decks are particularly subject to freezeand thaw damage as they are exposed to snow, rain and lowtemperature.
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The application of deicing salts is harmfulbecause it accelerates the deterioration process.
Environmental conditions also effects concrete.
It freezes at low temperature and cracks at hightemperature.
Material failure is either a failure of selection or
a failure in the manufacture process. Materialsthemselves never fail as they follow laws ofphysics and nature.
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FAILURE OF CONCRETE
Design, Materials and Workmanship
EmbeddedMetal Corrosion-induced cracking
and spalling
Reduction in Structural Capacity
Chloride Penetration
Carbonation
Thermal andMoisture
Concrete is a highly alkalinity material (pH=12).
Corrosion-an electrochemical process is accelerated in an
acidic environment
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FAILURE OF CONCRETE
Corrosion Promoters
Oxygen (cracks, honeycombs)
Water (cracks, honeycombs) Acidic environment (carbonation)
Chlorides (salts, atmosphere, water)
Insufficient concrete covers (penetrationpath)
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FREEZING AND THAWING
Water has lots of ways entering in to the concrete volume; the main
reason is being the permeability and porosity of concrete.
Deterioration of concrete from freeze thaw actions may occur when the
concrete is critically saturated, which is when approximately 91% of its
pores are filled with water. The water already inside the capillary pore of cement paste and aggregate
will change to ice in cold weather thereby increasing the volume of the
ice; the ice occupies 9% more volume than that of water. This increase in
volume of ice will lead to expansion of the surrounding concrete.
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During warm seasons, the ice so formed will melt into water. This successive freezing (during cold
weather) and thawing (during warm weather) of the
concrete will lead to a cumulative expansion on the
concrete volume leading to the scaling ordisintegration of the concrete.
In cold climates, for example northern Europe and
Canada, deicing chemicals (various salts and
alcohols) are widely used to lower the freezing pointof water thereby aiding the removal of ice from the
concrete surface by melting the ice .
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FREEZING AND THAWING
The effect of freeze thawing depends on the
exposure of the concrete member to the
environment. Exposed areas such as tops of walls,
piers, parapets, and slabs enhance the vulnerabilityof concrete to the harmful effects of the repeated
cycles of freezing and thawing.
Most common chemical deicers do not chemically
attack concrete, but the use of these deicing
chemicals on concrete surfaces may accelerate
damage caused by freezing and thawing.
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Freeze-thaw resistant concrete should have 4-8% air-bubbles by volume and
the air-bubbles should be well distributed and have a distance between each
other of less than 0.25 mm in the cement paste matrix (air-entraining
agent).
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DE-ICING SALTS
De- icing is defined as a process of removal of ice,
snow or frost from a surface and continue the delay
in reformation of ice on the surface for a certain
extent of time. Sodium Chloride (common salt) is one of the most
frequently used salts for de-icing.
Apart from common salt other salts like Calcium
Chloride , Potassium Chloride are also used
depending on the freezing point of these chemicals.
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CHEMICALS USED TO MELT ICE
Name Formula Lowest
Practical
Temp
Pros Cons
Sodium chloride NaCl -9C
(15F)
Keeps sidewalks dry Corrosive, damages
concrete & vegetation
Ammonium sulfate (NH4)2SO4 -7C
(20F)
Fertilizer Damages concrete
Calcium chloride CaCl2 -29C
(-20F
Melts ice faster than
sodium chloride
Attracts moisture,
surfaces slippery below
-18C (0F)
Potassium chloride KCl -7C
(20F)
Fertilizer Damages concrete
Calcium
magnesium acetate
(CMA)
CaCO3, MgCO3,
and acetic acid
-9C
(15F)
Safest for concrete &
vegetation
Works better to prevent
re-icing than as ice
remover
Magnesium
chloride
MgCl2 -15C
(5F)
Melts ice faster than
sodium chloride
Attracts moisture
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PROBABLE SOLUTION
Using these methods, the deteriorated part shall be removedand substituted with a new concrete having the following
characteristics:
Air-entrained concrete: the air bubbles larger than the cement
pores will act as expansion vessels that will provide a space for theice to expand freely without causing any surface deterioration.
Surface treatment: the freeze thaw damage occurs if the concrete is
wet. Therefore, the better solution is to prevent the ingress of
water by applying a surface coat (hydrophobic agents) that will not
affect the free evaporation of water from the surface of concrete. Low water/cement (w/c) ratio with good curing, but according to
[7], this method is best for moderate climate regions.
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Freeze thaw may not be the only problem of the deteriorated
concrete as the cracks caused by the freeze thaw can
aggravate the ingress of chloride ,carbon dioxide, water,
oxygen and other aggressive agents that affects the steel
reinforcement. Whenever a repair for freeze thaw is taking
place, it is important to check the condition of the embedded
steel reinforcement whether it is corroded or not.
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CHLORIDE
Chloride attack is one of the main cause of structuraldamage in reinforced concrete structures.
Chlorides penetrate into concrete due:
Surface moisture Crack
Construction joint
Cast-in chloride
Corrosion begins when chlorides contact steel alongwith delamination and spalling.
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Chlorides aggravate corrosion of steel in concrete. Theyoriginate from several sources, some in the concrete mixitself and some from outside:
Calcium chloride that was added to the concrete mix incold weather to accelerate the setting process itsharmful effects were recognized and it is now no longerpermitted as an additive
Sea water and marine spray (seafront structures as well as
those in the sea)De-icing salts applied to highways and carried, mainly in
solution, into the fabric of bridges, car parks, etc.
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Breakdown of the passive layer and recycling chlorides
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Chloride induced reinforcement corrosion process
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Reinforcement corrosion caused by chloride ingress
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Chloride content limits, as recommended by some codes of practices
Country Recommended limits of chloride
content(% by mass of cement)
USA 0.15% for chloride exposure and o.3%
for chloride free exposures
UK 0.3%
India 0.15%
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PROBABLE SOLUTION
Severely cracked or spalled concrete and/or corrodingreinforcement will generally point to the need for treatmentto restore durability, even if the loss of section is not yetstructurally significant. Clearly a view has to be taken,particularly if the damage is widespread and the structure
is mundane, as to whether partial or total replacementwould not be a more cost-effective and neater solution.There are a variety of repair methods, among which are thefollowing:
Traditional repairs, cutting out the damaged concrete andreinforcement, cleaning corroded steel, splicing new steelinto place, and replacing the removed concrete with a mixoften containing a bonding additive to ensure that itadheres to the original concrete and provides the requiredcover.
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Applying an impermeable coating to the concrete(after repair) to enhance its future protection(often used on external concrete to mask
weathering, repel dirt and improve appearance). Electrochemical realkalisation of the concrete,
aiming to re-passivate the steel.
Electrochemical extraction of chlorides.
Cathodic protection using an impressedcurrent.
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Cathodic protection of steel in chloride-containing concrete
Reaction showing the chloride extraction
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REFERENCES
Properties Of Concrete A.MNeville
Design And Construction Failures Dov Kaminetzky
The Chemistry Of Building Materials R.M.E Diamant
Free And Bound Chloride Contents In CementitiousMaterials M.V.A.
Marinescu & H.J.H. Brouwers
Concrete Durability II ,Concrete corrosion, sulfate attack, salt scaling, and
strength reduction - Dr. Jan Bisschop
Durability problems of 20th century reinforced concrete heritage
structures and their restorations - Tekeste Teshome Gebregziabhier ; Pere
Roca I Fabregat
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