properties and deterioration processes in concrete.1.ppt
TRANSCRIPT
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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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Curing o f con cre te (contd)
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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Curing o f con cre te (contd)
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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Curing o f con cre te (contd)
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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Curing o f con cre te (contd)
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Curing o f con cre te (contd)
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)
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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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Curing o f con cre te (contd)
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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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Curing methods of concrete (contd)
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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Curing methods of concrete (contd)
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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Curing methods of concrete (contd)
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Curing methods of concrete (contd)
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Curing methods of concrete (contd)
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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Curing methods of concrete (contd)
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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Curing methods of concrete (contd)
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Curing methods of concrete (contd)
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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Curing methods of concrete (contd)
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Curing methods of concrete (contd)
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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Curing methods of concrete (contd)
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Curing methods of concrete (contd)
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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Curing methods of concrete (contd)
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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Curing methods of concrete (contd)
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Curing methods of concrete (contd)
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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Curing period (contd)
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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Curing period (contd)
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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Properties of fresh concrete (contd)
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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Properties of fresh concrete (contd)
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 fresh concrete (contd)
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Properties of fresh concrete (contd)
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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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Properties of hardened concrete (contd)
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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Properties of hardened concrete (contd)
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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Properties of hardened concrete (contd)
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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Properties of hardened concrete (contd)
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
P ti f h d d t ( td )
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Properties of hardened concrete (contd)
Freezing-thawing cycles and W/C
P ti f h d d t ( td )
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Properties of hardened concrete (contd)
Effect of entrained air and cementcontent on concrete performance
Without entrained air With entrained air
P ti f h d d t ( td )
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Properties of hardened concrete (contd)
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
Causes of concrete Deteriorations (contd..)
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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
Causes of concrete Deteriorations (contd..)
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Causes of concrete Deteriorations (cont d..)Bacteria photo 10
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QUESTIONS???