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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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