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8/12/2019 Durabilit_ Pawer Piont

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2NEDYEAR MASTER CCS

DURABILITE OF CONCRETE

ACADEMIC YEAR2011-2012


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The design service life of most buildings is often 30

years, although buildings often last 50 to 100 years

or longer. Most concrete and masonry buildings aredemolished due to obsolescence rather than

deterioration. A concrete shell can be left in place if

a building use or function changes or when abuilding interior is renovated. Concrete, as a

structural material and as the building exterior skin,

has the ability to withstand natures normaldeteriorating mechanisms as well as natural

disasters.


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Definition :

Durability is the ability to last a longtime without significant deterioration. A durable

material helps the environment by conserving

resources and reducing wastes and theenvironmental impacts of repair and replacement.

Construction and demolition waste contribute to

solid waste going to landfills. The production ofnew building materials depletes natural resources

and can produce air and water pollution.


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High Humidity and Wind-Driven Rain:

Concrete is resistant to wind-driven rain and

moist outdoor air in hot and humid climates because it is

impermeable to air infiltration and wind-driven rain.

Moisture that enters a building must come through joints

between concrete elements. Annual inspection and repair ofjoints will minimize this potential. More importantly, if

moisture does enter through joints, it will not damage the

concrete. Good practice for all types of wall construction isto have permeable materials that breathe (are allowed to

dry) on at least one surface and to not encapsulate concrete

between two impermeable surfaces. Concrete will dry out

if not covered by impermeable treatments.


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Portland cement plaster (stucco) should not be confused with

the exterior insulation finish systems (EIFS) or synthetic

stucco systems that have become popular but may have

performance problems, including moisture damage and low

impact-resistance. Synthetic stucco is generally a fraction of

the thickness of Portland cement stucco, offering less impact

resistance. Due to its composition, it does not allow the insideof a wall to dry when moisture gets trapped inside. Trapped

moisture eventually rots insulation, sheathing, and wood

framing. It also corrodes metal framing and metal attachments.There have been fewer problems with EIFS used over solid

bases such as concrete or masonry because these substrates are

very stable and are not subject to rot or corrosion.


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Ultraviolet Resistance:

The ultraviolet portion of solar radiation does not harm

concrete. Using colored pigments in concrete retains the color inconcrete long after paints have faded due to the suns effects.

Inedible:

Vermin and insects cannot destroy concrete because it is

inedible. Some softer materials are inedible but still provide

pathways for insects. Due to its hardness, vermin and insects will not

bore through concrete. Gaps in exterior insulation to expose theconcrete can provide access for termite inspectors.


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Moderate to Severe Exposure Conditions for

Concrete:

The following are important exposure

conditions and deterioration mechanisms in

concrete. Concrete can withstand these effects whenproperly designed. The Pacifiers Guide for

Durable Concreteis intended to provide sufficient

information to allow the practitioner to selectmaterials and mix design parameters to achieve

durable concrete in a variety of environments.


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Resistance to Freezing and Thawing:

The most potentially destructive weathering factor is freezing and

thawing while the concrete is wet, particularly in the presence of deicing

chemicals. Deterioration is caused by the freezing of water and subsequent

expansion in the paste, the aggregate particles, or both.

With the addition of an air entrainment admixture, concrete is

highly resistant to freezing and thawing. During freezing, the water displacedby ice formation in the paste is accommodated so that it is not disruptive; the

microscopic air bubbles in the paste provide chambers for the water to enter

and thus relieve the hydraulic pressure generated. Concrete with a low water-

cementations ratio (0.40 or lower) is more durable than concrete with a high

water-cementations ratio (0.50 or higher). Air-entrained concrete with a lowwater-cementations ratio and an air content of 5 to 8% will withstand a great

number of cycles of freezing and thawing without distress.


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Chemical Resistance:

Concrete is resistant to most naturalenvironments and many chemicals. Concrete is

virtually the only material used for the

construction of wastewater transportation and

treatment facilities because of its ability to resist

corrosion caused by the highly aggressive

contaminants in the wastewater stream as well as

the chemicals added to treat these waste products.


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However concrete is sometimes exposed to substances that

can attack and cause deterioration. Concrete in chemical

manufacturing and storage facilities is especially prone to

chemical attack. The effect of sulfates and chlorides is

discussed below. Acids attack concrete by dissolving the

cement paste and calcareous aggregates. In addition to

using concrete with a low permeability, surface treatments

can be used to keep aggressive substances from coming in

contact with concrete.Effects of Substances on Concrete

and Guide to Protective Treatmentsdiscuss the effects ofhundreds of chemicals on concrete and provide a list of

treatments to help control chemical attack.


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Resistance to Sulfate Attack:

Excessive amounts of sulfates in soil or water can attack

and destroy a concrete that is not properly designed. Sulfates (forexample calcium sulfate, sodium sulfate, and magnesium sulfate)

can attack concrete by reacting with hydrated compounds in the

hardened cement paste. These reactions can induce sufficient

pressure to cause disintegration of the concrete.

Like natural rock such as limestone, porous concrete

(generally with a high water-cementations ratio) is susceptible to

weathering caused by salt crystallization. Examples of salts knownto cause weathering of concrete include sodium carbonate and

sodium sulfate.


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Sulfate attack and salt

Crystallization are more severe at locations

where the concrete is exposed to wetting and

drying cycles, than continuously wet cycles.

For the best defense against external sulfateattack, design concrete with a low water to

cementations material ratio (around 0.40)

and use cements specially formulated for

sulfate environments.


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Seawater Exposure:

Concrete has been used in seawater exposures for decades

with excellent performance. However, special care in mix design andmaterial selection is necessary for these severe environments. A

structure exposed to seawater or seawater spray is most vulnerable in

the tidal or splash zone where there are repeated cycles of wetting

and drying and/or freezing and thawing. Sulfates and chlorides in

seawater require the use of low permeability concrete to minimize

steel corrosion and sulfate attack. A cement resistant to sulfate

exposure is helpful. Proper concrete cover over reinforcing steel must

be provided, and the water-cementations ratio should not exceed

0.40.


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Chloride present in plain concrete that does not

contain steel is generally not a durability concern.

Concrete protects embedded steel from corrosionthrough its highly alkaline nature. The high pH

environment in concrete (usually greater than 12.5)

causes a passive and noncorroding protective oxidefilm to form on steel. However, the presence of

chloride ions from deicers or seawater can destroy

or penetrate the film. Once the chloride corrosion

threshold is reached, an electric cell is formed along

the steel or between steel bars and the

electrochemical rocess of carrions be ins.


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The resistance of concrete to chloride is good; however, for

severe environments such as bridge decks, it can be

increase by using a low water-cementations ratio (about

0.40), at least seven days of moist curing, and

supplementary cementations materials such as silica fume,

to reduce permeability. Increasing the concrete cover over

the steel also helps slow down the migration of chlorides.

Other methods of reducing steel corrosion include the use

of corrosion inhibiting admixtures, epoxy-coated

reinforcing steel, surface treatments, concrete overlays, andcathode protection.


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ASR is an expansive reaction between reactive forms of

silica in aggregates and potassium and sodium alkalis,

mostly from cement, but also from aggregates, pozzolans,

admixtures, and mixing water. The reactivity is potentially

harmful only when it produces significant expansion.

Indications of the presence of alkali-aggregate reactivity

may be a network of cracks, closed or spelling joints, ormovement of portions of a structure. ASR can be controlled

through proper aggregate selection and/or the use of

supplementary cementations materials (such as fly ash orslag cement) or blended cements proven by testing to

control the reaction.


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Concrete is resistant to the abrasive effects of

ordinary weather. Examples of severe

abrasion and erosion are particles in rapidly

moving water, floating ice, or areas where

steel studs are allowed on tires. Abrasion

resistance is directly related to the strength of

the concrete. For areas with severe abrasion,

studies show that concrete with compressivestrengths of 12,000 to 19,000 psi work well.


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At the end we can ask two questions :

1) Why does concrete crack?

Concrete, like most materials, will shrink slightly

when it dries out. Common shrinkage is about 1/16thof an

inch in a 10-foot length of concrete. The reason contractors

place joints in concrete pavements and floors is to allow the

concrete to crack in a neat, straight line at the joint, where

concrete cracks due to shrinkage are expected to occur.Control or construction joints are also placed in concrete

walls and other structures.


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1) Why do concrete surfaces spall?

Concrete spelling (or flaking) can

be prevented. It occurs due to one or more of

the following reasons.

1.) In cold climates subjected to freezing and

thawing, concrete surfaces have the potential

to spall if the concrete is not air-entrained.

2.) Too much water in the concrete mix willproduce a weaker, more permeable and less

durable concrete. The water-cementations

ratio should be as low as possible (0.45 or

less).


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3.) Concrete finishing operations should not

begin until the water sheen on the surface is

gone and the excess bleed water on the

surface has had a chance to evaporate. If this

excess water is worked into the concretebecause finishing operations have begun too

soon, the concrete on the surface will have

too high of a water content and this surface

will be weaker and less durable.


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