Durability of Concrete Expose to Weather Condition
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Transcript of Durability of Concrete Expose to Weather Condition
DURABILITY OF CONCRETE EXPOSE TO WEATHER CONDITION
DEFINITION
Concrete durability has been defined by the American Concrete Institute (ACI) as its resistance to weathering action, chemical attack, abrasion and other degradation processes
DEFINITION The durability of concrete depends
on its quality, which can be increased by proper choice of materials, proportioning, placing and curing
Concrete with higher strength and lower permeability is more durable
THESE CAUSES WILL AFFECT THE DURABILITY OF CONCRETE:1. Spalling due to corrosion of
reinforcement 2. Alkali-aggregate Reaction (AAR) 3. Chemical Attack 4. Surface Deterioration 5. Cracking 6. Freeze/Thaw Action 7. Efflorescence
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EXPOSURE CONDITIONS AND DETERIORATION MECHANISMS Durability Aspect/Exposure Mechanism
Alkali-Aggregate Reaction
Chemical Resistance
Alkali-Silica Reaction
Alkali-Carbonate Reaction
Sulfates
Seawater Acids
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EXPOSURE CONDITIONS AND DETERIORATION MECHANISMS (CONT’D)Durability Aspect/Exposure Mechanism
Corrosion of Reinforcement
Chloride Resistance
Carbonation
Corrosion
Freeze-Thaw
Freezing and Thawing
Deicer Scaling
D-Cracking
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Durability Aspect/Exposure Mechanism
EXPOSURE CONDITIONS AND DETERIORATION MECHANISMS (CONT’D)
Miscellaneous
Abrasion
Erosion
Fire Resistance
Efflorescence
CONCRETE in MARINE ENVIRONMENT
DURABILITY REQUIREMENTS (ACI 318-08, SEC.4)
Max. water-cement ratios (w/cm) : 0.40 – 0.50 Recommended f’c : 27.50 – 35.00 MPa The licensed design professional shall assign
exposure classes based on the severity of the anticipated exposure category according to ACI 318-08 Sec.4.2.1
The Code addresses four exposure categories that affect the requirements for concrete to ensure adequate durability, i.e. : Exposure Category F, S, P and C
EXPOSURE CATEGORIES and CLASSES
Exposure Category F applies to exterior concrete that is exposed to moisture and cycles
of freezing and thawing, with or without deicing chemicals
Category Severity Class Condition Max w /cm Min f'c (MPa) Examples
Not applicable
F0Concrete not exposed to freezing-and-thawing cycles
N/A 17,5
Moderate F1Concrete exposed to freezing-and-thawing cycles and occasional exposure to moisture
0,45 30Exterior walls, beams, girder, and slabs not in direct contact with soil
Severe F2Concrete exposed to freezing-and-thawing cycles and in continuous contact with to moisture
0,45 30Exterior water tank or vertical members in contact with soil
Very severe F3
Concrete exposed to freezing-and-thawing cycles and in continuous contact with to moisture and exposed to deicing chemicals
0,45 30horizontal members in parking structures
F
EXPOSURE CATEGORIES and CLASSES
Exposure Category S applies to concrete in contact with soil or water containing
deleterious amounts of water-soluble sulfate ions
Category Severity ClassWater soluble sulfate (SO4) in soil, percent
by weight
Dissolved sulfate (SO4) in water, ppm
Max w /cm Min f'c (MPa)
Not applicable
S0 SO4 < 0,10 SO4 < 150 N/A 17,5
Moderate S1 0,10 < SO4 < 0,20150 < SO4 < 1.500
Seawater0,5 27,5
Severe S2 0,20 < SO4 < 2,00 1500 < SO4 < 10.000 0,45 30
Very severe S3 SO4 > 2,00 SO4 > 10,000 0,45 30
S
EXPOSURE CATEGORIES and CLASSES
Exposure Category P applies to concrete in contact with water requiring low permeability
Exposure Category C applies to concrete exposed to conditions that require additional protection against corrosion of reinforcement
Category Severity Class Condition Max w /cm Min f'c (MPa)
Not applicable
P0In contact with water where low permebility is not required
N/A 17,5
Moderate P1In contact with water where low permebility is required
0,50 27,5
Not applicable
C0 Concrete dry or protected from moisture N/A 17,5
Moderate C1Concrete exposed to moisture but not to external sources of chlorides
N/A 17,5
Severe C2
Concrete exposed to moisture and an external sources of chlorides from deicing chemicals, salt, brackish water, seawater, or spray from these sources
0,45 35
P
C
CONCRETE COATING Another way of enhancing the durability of
concrete is by applying a coating For many years, coatings have been applied for
aesthetic reasons. Depending on design and environmental
requirements, weatherproofing treatments have also been widely used and more durable paints have evolved.
The increasing use of steel-reinforced concrete in modern building has led, however, to widespread deterioration problems associated with reinforcement corrosion.
CONCRETE COATING These are related to the specification and quality of
concrete, the depth of protective cover, efficiency in placing and curing, or to factors in design or environmental exposure.
Coatings are increasingly being used to protect reinforced concrete structures against the penetration of carbon dioxide, water and other aggressive agents, such as chlorides, to ensure a satisfactory service life.
Suitable barrier coatings provide a logical option. The number of cases requiring such protection is
growing.
ADVANTAGES of COATING Decoration Cleanability Dust reduction Water proofing Enhanced slip resistance Protection against reinforcement corrosion Resistance to chemical attack Protect from damage caused by frost, abrasion,
mechanical stress, slat penetration, water and from solar heat
COATING COMPOSITION The primary ingredients used to formulate coatings
can be placed into one of three basic categories - solvent, resin, and pigment.
Historically, the first paints utilized fish or vegetable (e.g., linseed) oils as binders and natural earth pigments.
The first solvents were from trees (e.g., turpentine). Now most resins and solvents are derived from
petroleum, and many pigments are derived from organic synthesis or modification of natural minerals
SOLVENT Organic solvents are used to dissolve the resin material and
reduce the viscosity of the product to permit easier application.
They also control leveling, drying, durability, and adhesion. The blend must completely dissolve the total binder system
and be balanced to ensure compatibility and stability during all stages of curing.
Improper blends may result in cloudy films, pigment float to the wet film surface, or reduced film durability.
Paint solvents evaporate into the air and contribute to the production of photochemical smog.
Thus, there is a great pressure to reformulate coatings to reduce the solvent content of paints.
RESIN & PIGMENT Resins, also called binders, are the filmforming portions of
coatings. They are usually high molecular weight solid polymers (large
molecules with repeating units) in the cured film. In some cases, lower molecular weight units in two liquid
components react with each other upon mixing to polymerize into the higher molecular weight solid.
The pigment constitutes the solid portion of a wet paint. Pigments are insoluble in the vehicle and are generally heavier
than the liquid vehicle portion. They may settle to the bottom of a container upon prolonged
standing. Natural earth pigments are generally much more stable to light
than synthetic organic pigments.
GENERAL TYPES of COATINGS Alkyds & other Oil-containing coatings Water emulsion (latex) coatings Lacquers Epoxy coatings Coal-tar epoxy coatings Polyurethane coatings Polyester coatings Inorganic zinc coatings Zinc-Rich organic coatings
ALKYDS & OTHER OIL-CONTAINING COATINGS
The unmodified drying oil coatings initially developed were very easily applied, did not require a high level of surface preparation, and had good flexibility; they could readily expand and contract with the substrate.
They did, however, have several drawbacks: they were slow to dry, had residual tack, and provided a limited period of protection.
They cannot be used in sea water immersion service or on alkaline substrates (e.g. concrete), because they are easily hydrolyzed (deteriorated by reaction with water) by alkalinity.
They are used most on wood and steel surfaces.
ALKYDS & OTHER OIL-CONTAINING COATINGS
Advantages LimitationsEasy to apply/repair/topcoat Relatively high in VOCs
Good initial flexibility possible Poor performance in severe environment
Good surface wetting/adhesion Poor chemical/solvent resistance
Good gloss retention Poor immersion resistance
Relatively inexpensive Poor alkali resistance
Based on renewable source Poor heat resistance
Become brittle with extended aging
WATER EMULSION (LATEX)COATINGS Water emulsion coatings, commonly called latex
coatings, have been successfully used for many years to coat wood and masonry structures.
The porous nature of their films allows water vapor to pass through them
This porosity reduces their durability on steel. Thus, much effort is being made to develop more
durable products because of the great advantages of their low VOC contents and ease of application and clean-up.
Water-emulsion coatings have excellent flexibility and low cost, and are easily topcoated and repaired.
WATER EMULSION (LATEX)COATINGS Drawbacks include poor solvent and heat resistance (as
with all thermoplastics), poor immersion resistance, and difficulty in bonding to smooth oil/alkyd coatings and chalky surfaces.
The poor bonding is due to insufficient content of organic solvents to soften and wet the binder in the existing paint film.
Because of this limited adhesion, it is necessary to sand smooth enamels and/or use a surface conditioner before topcoating with latexcoatings.
Latex paints do not cure well at temperatures below 50 degrees F, as the emulsion does not coalesce to form a good film.
WATER EMULSION (LATEX)COATINGS
Advantages LimitationsEnvironmental acceptability Limited durability
Easy to apply/repair/topcoat Poor chemical/solvent resistance
Excellent flexibility and color and gloss retention Poor wetting of surfaces
Low cost Poor immersion service
Available in wide range of color and gloss Must cure above 50 degrees F
LACQUERS Lacquers (e.g., vinyls, chlorinated rubbers, and acrylics)
form durable films that have good water and chemical resistance but, being thermoplastics, poor solvent and heat resistance.
They have a low film build but dry so fast that they can be quickly topcoated.
When used on steel, they require a blast-cleaned surface, and in some cases wash priming, for good adhesion.
They are easy to topcoat and repair and can be formulated for good gloss retention.
The good weathering of acrylic lacquers is duplicated in acrylic water emulsion coatings.
LACQUERSAdvantages Limitations
Rapid drying and recoating High in VOCs
Good chemical resistance Poor solvent/heat resistance
Good in water immersion Low film build
Good gloss retention possible Blasted surface necessary
Good durability Occasional poor adhesion
Easy to topcoat and repair
Can be applied at low temperatures
EPOXY COATINGS Epoxy films are tough and relatively inflexible. Thus, they cannot expand or contract much without
cracking. However, they bond well and are very durable in
most environments. They require a blasted steel surface, and they chalk
freely in sunlight. An aliphatic polyurethane finish coat is usually
applied when the coating is exposed to sunlight. Epoxies can be formulated to be low in VOCs, some
actually solvent-free.
EPOXY COATINGSAdvantages Limitations
Low in VOCs Limited pot life
Good solvent/water resistance Chalk in sunlight
Tough, hard, smooth film Cure best above 50 degrees F
Good adhesion Top coating is a problem
Good abrasion resistance Blasted surface needed
COAL-TAR EPOXY COATINGS Coal-tar epoxy coatings are basically epoxies (with all
properties of epoxies) to which coal tar has been incorporated.
The coal tar reduces cost, improves water resistance, and provides for greater film builds.
Because of the coal tar, coatings tend to become brittle in sunlight, and there is great concern about toxic effects of the coal tar.
They are used primarily on steel piling and other buried structures.
The catalyst component is usually either a polyamide or an amine
COAL-TAR EPOXY COATINGSAdvantages Limitations
Low in VOCs Toxic; personal protection needed
Good water/chemical resistance Limited pot life
Good film build Blasted surface needed
Good abrasion resistance Top coating is a problem
Available only in black, dark red, or aluminum
POLYURETHANE COATINGS Polyurethane coatings are one or two-package systems. For two-package systems, one component is an isocyanate and the
other a polyol component. Polyurethanes are moisture sensitive, and the gloss may drop when the
wet film is exposed to high humidity. The toxicity of the isocyanate component is of great concern, and
personal protection, including respirators, must be used when applying them.
They require skilled applicators. Polyurethane coatings are available in a variety of formulations, giving
rise to a variety of properties (e.g., may be tough or elastomeric). They perform well in most environments. Aliphatic polyurethanes have excellent weathering in sunlight; aromatic
polyurethanes do not, but they have better chemical resistance. Both types can readily be formulated to be low in VOCs.
POLYURETHANE COATINGSAdvantages Limitations
Low in VOCs Highly toxic; need personal protection
Good solvent resistance Moisture sensitive; gloss may drop
Good hardness or flexibility Skilled applicator needed
May have excellent gloss Limited pot life
Good durability Blasted surface required
Good abrasion resistance High cost
POLYESTER COATINGS Polyester coatings are used most with
fiberglass or glass flake reinforcement. They can be very tough and durable
but are seldom used today except with glass reinforcement.
INORGANIC ZINC COATINGS Inorganic zinc coatings usually have a silicate resin The silicate film is very hard and abrasion resistant. They provide cathodic protection to steel, require greater steel
surface cleanliness, and must be applied by a skilled applicator Inorganic zinc silicate coatings frequently do not bond well to each
other, and it is safest to repair them using a zinc-rich organic coating.
Inorganic zinc coatings are extremely durable in an atmospheric environment, the steel preferentially receiving cathodic protection from the zinc.
The zinc is attacked, however, by acid and alkali Inorganic zinc coatings have not been used often in continuous
water immersion because of concern for their limited period of protection.
INORGANIC ZINC COATINGSAdvantages Limitations
Can be low in VOCs Needs clean, blasted surface
Excellent abrasion resistance Requires skilled applicator
Excellent heat resistance Constant agitation needed
Good atmospheric durability Difficult to topcoat
Useful as shop primer Attacked by acid and alkali
ZINC-RICH ORGANIC COATINGS Zinc-rich organic coatings utilize an organic resin rather
than an inorganic silicate binder. Zinc-rich organic coating films can be of the
thermoplastic (e.g., utilize vinyl or chlorinated rubber resins) or the thermosetting type (e.g., utilize epoxy or polyurethane resins).
Film properties of zinc-rich organic coatings are similar in most respects to those of zinc-free organic coatings using the same resin.
Organic zinc-rich coatings do not require as high a level of blast-cleaned steel surface as do zinc-rich inorganic coatings, and they are easier to topcoat.
ZINC-RICH ORGANIC COATINGS
Advantages Limitations
Can be low in VOCs Requires skilled operator
Good durability Constant agitation necessary
Relatively easily topcoated Unsuitable for acid or alkali
Moderate surface preparation needed Costly
SELECTION of SURFACE TREATMENT
SURFACE PREPARATION Surface treatments are designed to provide lasting
protection in any number of environments. This, almost inevitably, will mean that the choice
and application of surface treatment may vary considerably from one locale to another.
Concrete is not any different in terms of preparation than, say, timber or steel.
The surface must be clean, free from grease, flaking paint, eflorescence, fungal growth, corrosion products, mould release agents, curing membranes and, most importantly, be in a good state of repair.
SURFACE PREPARATION Most signs of degradation will be
apparent from the general surface appearance, such as reinforcement corrosion, spalling or mechanical damage.
The level of breakdown must be assessed before attempting to decide on the final surface preparation.
SURFACE PREPARATIONSome suggested methods for cleaning are:1. For small areas, mechanical wire brushing.2. High-pressure water jetting (provided
adequate, suitable drainage is present).3. A fungicidal wash.4. Wet, dry or vacuum abrasive blasting.5. Mechanical impact techniques, such as
needle gunning or bush hammering.6. Mechanical abrasion.
SURFACE PREPARATION Mechanical abrasion methods of cleaning are effective
for removing deeply ingressed contamination but may remove unacceptable quantities of surface concrete.
Needle gunning and bush hammering are extremely effective but are often too aggressive and lead to micro-cracking and form deep textures in previously smooth concrete, thus rendering the final surface unacceptable for coating without carrying out expensive skimming.
The use of washing techniques may also be flawed, as the correct choice of detergent is not always obvious and an incorrect material may just spread surface contaminants.
SURFACE PREPARATION Using solvent-based or sodium hydroxide-based
products are often more effective, but may lead to health and safety problems for operatives and surrounding periphery such as alumunium and glass.
Removal of these products efficiently is also vital, as remaining traces may interfere with the application of surface treatments at a later stage.
Whichever method appears to be the most suited to any project, it is always advisable to carry out trials on sample areas, with reference to material suppliers, contractors and or applicators before proceeding.
APPLICATION METHODS No matter what choice of coating has
been made, there are three main methods of application to post-fabricated concrete structures; brush, roller or spray.
Environmental considerations, surface area to be coated, accessibility and final choice of finish will be the main criteria to be considered when making a decision on the method of application.
APPLICATION METHODS The environment is of increasing concern and many owners
or local councils are beginning to move away from solvent-based products in favor of waterborne or high solids systems in enclosed areas or town centers
Due to solvent emissions from the atomized paint, this is of increasing importance if a sprayed finish is desired.
These emissions will be reduced if a roller is used but the method will be very labor intensive if large areas require coating and the use of a brush in such circumstances is not recommended.
Textured coatings may be sprayed or rolled to a desired pattern as required and the emissions from such systems are generally much lower than standard paint products.
APPLICATION METHODS When spraying smooth coatings, either an airless or air
assisted unit may be used. Most contractors favor the use of airless systems which
are quicker touse, reduce wastage and almost eliminate over-spray if set correctly.
Spray equipment is now available to apply two-pack materials without the necessity of pre-mixing the components, as all mixing takes place in the nozzle prior to atomization.
Whichever spray system is used, the final environmental consideration is the immediate locale and great effort must be made to mask off all surrounding areas.
APPLICATION METHODS Brushing and rolling may be used on small
surface areas, for ‘cutting-in’ or where access is limited and spraying is impossible.
Where a two-pack epoxy of polyurethane has been specified and the use of specialized spray equipment is impracticable, the utilization of a roller is recommended, but only sufficient product should be mixed at one time as can be applied within the ‘pot life’.