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’.