properties and deterioration processes in concrete.1.ppt

8/10/2019 PROPERTIES AND DETERIORATION PROCESSES IN CONCRETe.1.ppt

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ADVANCED CONCRETE TECHNOLOGY

Jahangir MIRZAVisiting Faculty

Dept. of Agriculture EngineeringUniversity of Engineering & Technology

Peshawar

Spring 2011

DEPT. of CIVIL ENGINEERING UET, PESHA WA


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Presentat ion

Concrete at early ages

Properties of concrete

Fresh concrete

Hardened concrete


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Batching, mixing and conveying concretemixture from the point it is made to job site

Placing the plastic concrete at the job site

Compacting and finishing while the concretemixture still workable

Moist curing to achieve desired maturity

Removal of formwork, if any

Con c rete at ear ly ages


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B atch ing of c on crete ingred ien ts

By weight of ingredients (problem damp sand)In Pakistan ??? (1:2:4 by volume)No problem with liquids (water, admixtures) batched accurately by volume or weight

In many countries, concrete today is mostlybatched and mixed by ready-mixed concreteplants, where batching is generally automatic orsemi-automatic1966 1980, manually batched concretereduced from 54% to < 25% in USA


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Mixing of co nc rete

Sequence of charging ingredients into mixers:very important in the uniformity of finishedproduct

Sequence can be varied depends onadjustments in time when water is added, totalnumber of revolutions of mixer drum, speed ofrevolution, etc.


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Con veying /t rans po r tat ion of co nc rete

As quickly as possible to minimize stiffeningUnder normal conditions, negligible consistencyloss during first 15-30 minutes after the beginningof cement hydration

Place and consolidate within 90-120 minutes

After placement, easy consolidation and proper

finishingLong distance, chemical admixtures for hot andcold climates


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Placing of c on cre te

Place as near or directly to its final positionConcrete deposited in horizontal layers ofuniform thickness

Each layer thoroughly compacted before thenext is placed

Rate of placement rapid while the layerimmediately below still plastic

Prevents flow lines, planes of weakness whenfresh concrete placed on hardened concrete


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Cons ol idat ion /com pact ion of c on crete

Process of molding concrete within forms andaround embedded parts to eliminateentrapped air

Hand rodding and tamping

Mechanical methods: power tampers andvibrators to place stiff mixtures with low W/Cor high coarse aggregate contents


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Fin ish ing of c on crete

To produce dense surfaces to keep them maintenance free

Sequence of steps, which must be carefully coordinatedwith thesetting and hardening of concrete mixture

Screeding: striking off excess concrete to bring top surfaceto the desired gradeFloating: Flat wood or metal blades to firmly embedaggregates, compacting the surface, and removing anyremaining imperfections

Brings the cement paste to surface, forms bleed-water sheen weakens surfaces when floated too early or for too long

Steel-troweling: very smooth and highly wear resistantsurface

Not done on surfaces already floatedBrooming or scoring: skid-resistant surfaces with a rake or asteel-wire broom before concrete fully hardened


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Curing o f con cre te

Strongly influence hardened concrete properties durability, strength, water-tightness, wear resistance,volume stability, resistance to freezing and thawing andwetting and drying cycles

To promote cement hydration, consisting of control oftime, temperature and humidity conditions immediatelyafter placement of fresh concrete, i.e.,

To prevent loss of moistureTo control the concrete temperature for a period sufficient toachieve desired strength level


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Curing o f con cre te (contd)

Under normal temperature conditions:

Some OPC constituents begin to hydrate as soon as water isadded

Hydration reactions slow down when hydration products coatthe anhydrous cement grains

Why? Hydration proceeds satisfactorily only under internalsaturation conditions

Hydration stops when water vapor pressure in pores falls < 80%of saturation humidity

Time and humidity important factors in hydrationprocesses controlled by water diffusion as well astemperature


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Curing o f con cre te (contd) Time and humidity

In thin concrete elements, water loss by evaporation ishigh, prevailing air curing conditions strength will notincrease with time as well as without humidity


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Tem perature con vers io n table


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Temperature It influences strength depending on time-temp.history of casting (placing) and curing:

Three casesCase 1: Concrete cast and cured at same temp.

Case 2: Concrete cast at different temp. but cured

at normal temp.Case 3: Concrete cast at normal temp. but curedat different temps.


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CASE 1Cast and cure at specific temperature

Temperature range 5 OC (40 OF) to 45 OC (115 OF) concrete cast and cured at specific constanttemperature

Up to 28 days, higher the temperature Higher the cement hydration Higher the strength gain


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CASE 2Cast at different temperatures, BUT cured atnormal temperatures

28-day strength at 40 OF (5 OC) will be around 80%of that at 75 OF (25 OC) to 115 OF (45 OC)

At later ages, differences in cement hydration

reduced, such differences in concrete strength notsustained, in fact, at higher temperatures,ultimate strength will be lower


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CASE 2

Cast at different temperatures, BUT cured atnormal temperatures

Concrete temperature after 2 hrs. making, variedbetween 40 (5 OC) and 115 OF (45 OC) , thereafter allmoist cured at constant temperature of 70 OF(22 OC)

Ultimate strength (180 days) of concrete cast at40 (5 OC) or 55 O (12-13 OC) higher than others


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Curing o f con cre te (contd) CASE 3

Concrete cast at normal temperature, BUT cured atdifferent temperatures

Concrete cast at 70 OF (22 OC) and subsequently cured

from below freezing to 70 OF (22 OC)

Lower the curing temperatures, lower the strength up to28 days

At curing temperatures near freezing, strength half than70 OF (22 OC)

At curing temperature below freezing, strength almost nil.


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Cur ing o f co nc rete (contd) - Summary

Influence of time, humidity and temperature onconcrete strength has several important applications

Since curing temperature far more important to strengththan the cast (placement) temperature

Ordinary concrete placed in cold weather must maintain

above a certain minimum temperature for a long time Concrete cured in summer or in tropical climate will have

higher early strength but lower ultimate strength than thesame concrete cured in winter or colder climate

In pre-cast concrete industry, steam curing is used to

accelerate strength to achieve quicker mold release (Qs)

CURING - VERY IMPORTANT FACTORFOR DURABLE CONCRETE


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Curing o f con cre te (contd) Curing time required to obtain

near impermeable cement paste

Initial W/C Wet cure duration

0.40 3 days

0.45 7 days

0.50 14 days

0.60 6 months

0.70 12 months

> 0.70 IMPOSSIBLE


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Cur ing m ethod s o f con crete

Methods that maintain the presence of mixingwater in concrete during early hardening period

Ponding or immersionSpraying or foggingSaturated wet coverings

Cooling through evaporation:Beneficial in hot weather


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Curing methods of concrete (contd)

Ponding or immersionOn flat surfaces pavements, sidewalks, floors,etc., curing accomplished by immersion of finishedconcrete, called ponding

Requires considerable labor and supervision,method generally used for small jobs

Common in the laboratory for curing concrete testspecimens


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Spraying or fogging

Continuous spraying or fogging water excellent

when ambient temperature well above freezingSoaking hoses for vertical surfaces

Disadvantages: requires ample water supply andcareful supervision


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Saturated wet coveringsburlaps, cotton mats, rugs, or other moist retaining fabrics commonly used for curing

Sand or sawdust often useful for small jobs about 51 mmthick layer evenly distributed over previously moistenedsurface and kept continuously wet

Wet hay or straw to cure flat surfaces. Layer at least152 mm thick, held down with wire, burlap, tarpaulin, orother means to prevent being blown off by wind

Disadvantages of moist sand, sawdust, hay or straw discoloring of concrete


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Methods that prevent loss of mixing waterfrom concrete by sealing the surface

Impervious papers or

plastic sheets or

applying membrane-forming curing compounds


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Impervious papers

2 sheets of kraft paper cemented together by abituminous adhesive with fiber reinforcement

Horizontal surfaces and structural concrete ofrelatively simple shapes

Advantage periodic additions of water notrequired


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Plastic sheet

Material polyethylene

Lightweight

effective moisture barrier

easily applied to complex and simple shapes

White and black White for exteriors during hot weather to reflect

suns rays

Black for cool weather or for interior locations


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Membrane-forming curing compounds

Waxes

Resins

Chlorinated rubber

2 types clear and white pigmented Clear contain dye to assure complete coverage of

concrete surface fades away after application

White-pigmented reduce solar-heat gain andtemperature good for hot, sunny days


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Steam curing

Advantageous

Early strength gain in concrete is important

Additional heat required for hydration as in cold weather

2 methods curing in live steam at atmospheric pressure (for enclosed

cast-in-place structures and pre-cast concrete units Curing in high-pressure-steam autoclaves (for small

manufactured units)


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Steam-curing cycle

An initial delay prior to steaming

A period for increasing temperatureA period for holding maximum temperature constant

A period for decreasing temperature

Optimum temperature shown as follows


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Insulating blankets or covers (cold weather)

Straw or hay used to provide insulation againstfreezing of concrete at temperatures 32 OF (0 OC)

Formwork insulated with blankets made byfiberglass, sponge rubber, cellulose fibers, mineralwool, vinyl foam, open-cell polyurethane foam

Framed enclosures of canvas, reinforcedpolyethylene film, etc., placed around structure andheated by space heaters or steam


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Cur ing per iod

Length of time required to protect concreteagainst moisture loss depends:

Type of cement

Required strengthRatio of exposed-surface area to volume ofconcrete

WeatherFuture exposed conditions


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Curing period (contd)

3 weeks or longer for lean concrete mixturescontaining pozzolanic materials (used formassive structures dams, piers, etc.)

Conversely, few days for richer mixtures(Type III cement) and much shorter for steamcured products


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Curing period (contd) Desired concrete properties improved by curing,it must be as long as possible for practicalpurposes

For concrete slabs ( floors, pavements, patios,parking lots, driveways, sidewalks ) and

For structural concrete ( cast-in-place walls, columns,beams, small footings, piers, retaining walls, bridgedecks )

Minimum 7 days above 4 OC or

Time necessary to attain 70% of specified compressive orflexural strength


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Hydration rate depends: cement composition andfineness, curing period

Prolonged for concrete made with cement having slowstrength gain ( Qs )

For mass concrete (large piers, locks, abutments, dams,heavy footings, massive columns, transfer girders), with nopozzolanic materials , curing of unreinforced sections minimum 2 weeks , with pozzolan 3 weeks

Heavily reinforced mass concrete minimum 7 days


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Cold weather

Additional temperature required to maintaincuring 50 OF to 70 OC (10 OC to 21 OC)

Supplied by vented gas- or oil fired heaters,heating coils or fans, or live steam

High early strength concrete (Qs) used in cold

weather to speed setting time and strengthdevelopment curing reduced from 7 to 3days, but minimum temperature 50 OC (10 OC)


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Cur ing m ethod s o f co nc rete (Summary)

Ponding or immersion (Temperature well abovefreezing) pavements, slabs, etc

Spraying or fogging or moisture retaining coveringssaturated with water burlaps, cotton (beneficial inhot-weather concreting)

Water-proof curing paper, polyethylene sheets, ormembrane forming curing compounds preventmoisture loss from concrete by sealing the surface

Insulating blankets in cold weather (to protectconcrete from freezing) strength gain rateaccelerated by steam, heating coils, or electricallyheated forms or pads


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Prop er t ies o f f resh co nc rete

Workability

Consistency

Consolidation or compaction

Segregation (Bleeding)


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Properties of fresh concrete (contd)

WorkabilityEase or difficulty of placing, consolidating,and finishing freshly mixed concrete

Should not segregate or bleed excessivelyASTM C125 Property determining the effortrequired to manipulate a freshly mixed

concrete with minimum loss of homogeneityManipulate includes the early age operationsof placing, compacting, and finishing


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Consistency

Index for mobility or flowability of freshlymixed concrete to flow

Differs in degree of plasticity for different usesin concrete

Measured by slump test (cone 300 mm high, 100mm diameter top, 200 mm diameter bottom)

Slump loss: Loss of consistency in fresh concretewith time (normal phenomenon gradualstiffening and setting of hydrated PC paste) ( Qs )


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Consolidation or compaction

Molding concrete within the form and around

embedded parts to eliminate pockets ofentrapped air

Hand rodding, tampering, vibration, etc.,


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Properties of hardened concrete (contd)

STRENGTH

Evaluated in Compression or in Flexure and values calledCompressive strength and flexural strength

Compressive strength of concrete primary property ofconcrete and a factor needed for designing bridges,buildings, and other structures

Compressive strength used as an index of flexuralstrength an empirical relationship

Flexural strength 8 to 10 times the sq. root ofcompressive strength; another is compressive strength tothe 0.79 power.


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Compressive strength

Measured by squeezing to break concrete anddivide by contact area

Generally expressed in psi or MPa at age of 28days

Tests on cylinders and/or cubes


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Properties of hardened concrete (contd) Effect of low water/cement ratio on strength


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Properties of hardened concrete (contd) Strength vs aggregate size, W/C


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Strength vs W/C, moist curing age


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f h d d ( d )


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Properties of hardened concrete (contd) Strength vs W/C, moist curing, cement type and air entrainmen

i f h d d ( d )


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Erosion resistanceAbrasionCavitationChemical

Closely related to strengthLow W/C and curing

P i f h d d ( d )


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Properties of hardened concrete (contd) Erosion resistance


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P i f h d d ( d )


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Freezing-thawing cyclesMost destructive weather factor in wet ormoist concrete Freezing-thawing cycles

Deterioration increases when combined withdeicing salts

SOLUTIONair entraining admixtures

P ti f h d d t ( td )


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Properties of hardened concrete (contd) Freezing-thawing cycles



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Freezing-thawing cycles and W/C



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Effect of entrained air and cementcontent on concrete performance

Without entrained air With entrained air



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Effect of weather on air entrainedand non-air entrained concrete

Non-air entrained Air entrained

Properties of hardened concrete (contd )


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Properties of hardened concrete (contd) Strength vs W/C of air and non-air entrained concrete

Reco m m end ed ai rcon ten t s


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Reco m m end ed ai r con ten t s

Normal maximumsize of coarse Total air contentaggregate

In. mm % vol. concrete

3/8 10 6 to 10 12.5 5 to 9 19 4 to 81.0 25 3 to 6

1 37.5 3 to 62.0 50 2 to 5 3.0 75 1 to 4

Durabi l i tyofco nc rete


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Durabi l i ty of co nc rete

Its ability to resist physical, chemical, thermal and biological process(es) of deteriorations (weatheringaction, chemical attack, abrasion, etc.), i.e., durableconcrete should retain its original form, quality andserviceability when exposed to its environment (ACIcommittee 201)

BUT, UNFORTUNATELY

ALL MATERIALS DO DETERIORATE

AND ONE OF THE GENERALLY INVOLVEDMATERIAL FOR DEGRADATION ISWATER

Durabi l i tyofco nc rete


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Durabi l i ty of co nc rete

WATER

Both creates and destructs many naturalmaterials (Pious (Q) and Poison (Q ))In porous solids physical process of deterioration

Vehicle for transportation of aggressive ions

sourceof chemical degradationPhysico-chemical phenomenon surface and internaldeteriorationsRate of deterioration affected by concentration ofions in water and chemical composition of solids(e.g. alkaline concrete (pH = 12 14) and acidic water)

Caus eso fco nc reteDeter iora t ion s


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Caus es o f co nc rete Deter iora t ion s

Physical

Chemical

Thermal

Biological

Causes of concrete Deteriorations contd )


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Causes of concrete Deteriorations cont d..)

PhysicalOverloads - uneven surfaces, collapseof structures

Deformation imposed by differentsettlement of foundations - crackingand/or collapsing of structures

Fire damage and weakens structures

Overloaded trac to r t ro l ley


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Overloaded t rac to r-t ro l ley

Causes of concrete Deteriorations contd )


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Causes of concrete Deteriorations cont d..)

PhysicalErosion -surface cavities and damage

Abrasion (photos 1, 2): Wearing away of concrete due to abrasive action of tires,

flowing water and fine and coarse particles of sand, silt,

gravel and other solids Dams, spillways (aprons), stilling basins, pipelines,

suction fans, slabs and cross joints, under lock gates,irregular surfaces, roads, bridges, canals, runways, etc.,

Cavitation: high head dams Collapse of vapor bubbles in flowing water Pitting of surface around the hard coarse aggregate particles

Exam p leab ras io n ero sio n ( h 1)


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Exam p le ab ras io n -ero s io n (photo 1)

Exam p leab ras io n -ero s io n ( h t 2)


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Exam p le ab ras io n -ero s io n (photo 2)

Causes of concrete Deteriorations (contd )


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Causes of concrete Deteriorations (cont d..) Chemical

Acid attack leaching of cement paste, surface erosionChloride attack corrosion, spalling and cracksDeicing salts - spalling and cracks severe scaling 2%and 1%CaCl 2 by weight of Q, non air-entrainedand air-entrained concrete, 2% NaCl for both concretes

(photo 3)Sea water - corrosion, spalling, cracks, other damages(photo 4)

Sulfate attack cracks, weakens concrete surface (photo 5)AAR or ASR cracks, spalling (photos 6, 7)MgO cracksCaO cracksCarbonation seepage, white deposition on surface(photo 8) (mauratic acid)

Effectofd eic ings al t son co nc reteperform anc


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Effect of d eic ing s al t s on co nc rete perform anc

Non-air entrained photo 3 Air entrained

Causes of concrete Deteriorations (contd )


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Causes of concrete Deteriorations (cont d..) Concrete cylinder exposed to seawater (photo 4)

Effecto fsu l fateso i lcon cre teperform anc e


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Effect o f su l fate so i l con cre te perform anc e

Without entrained air Photo 5 With entrained air

Typica lpo lygon alc racks du etoA AR pho to6


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Typica l po lygon al c racks du e to A AR pho to 6

Concre tedam ageduetoco r ros ion and AA R pho to7


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Concre te dam age due to co r ros ion and AA R pho to 7

Leach ingby carbo nat ion pho to8


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Leach ing by carbo nat ion pho to 8

Causes of concrete Deteriorations (contd..)


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Causes of concrete Deteriorations (cont d..)

Thermal

Expansion/contraction due to temperaturechange (deformation, cracking) (photo 9)

Freezing and thawing cycles (cracks, surfacedeterioration)

Wetting and drying cycles (volume or length change:moist - expansion, drying shrinkage)

External versus internal temperature variations- cracking (Q which structures?)

Deformat ion exp ans ion /co nt rac t ion pho to9


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Deformat ion exp ans ion /co nt rac t ion pho to 9



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Causes of concrete Deteriorations (cont d..)

Biological

Bacteria

Sulphur (H 2S to H 2SO 4) leaching andexpansion (photo 10)

Iron (Fe(OH) 3) corrosion, spalling

Algea - aesthetic

Seeweeds - aesthetic



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Causes of concrete Deteriorations (cont d..)Bacteria photo 10


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QUESTIONS???

properties and deterioration processes in concrete.1.ppt

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