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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