lecture 5 - willkommen an der fh münster€¦ · lecture 5 • intrinsic cracks early...

Lecture 5• Intrinsic cracks

early chemical/plastic shrinkagedrying shrinkagecarbonation shrinkage

• Concrete corrosion/deteriorationacid solutionsulfate swell

• Statistic „light“ⓒJMR 09Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009

hydratization of cement

• In concrete with quartzitic aggregatesvolume changes can only happen in the cement paste

Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09

hydration of cement


Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009

plastic hardenedUnstable structure

Base structure Solid structure

Pore volume

Long fibre

Short fibre

Hydration time Minutes hours days

volume

Source: Zementtaschenbuch 2008

CSH =calcium-silicate-hydrates

ⓒJMR 09

hydratization of clincergrain

Dry milled grain after watercontact fully hydrated




Scanning electron microscope (SEM) picture 3 hours hydratization needles of primary CSH and

short columns of ettringitAssessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009



SEM 24 hours of hydratization CA(OH)2 CSH




SEM 28 days of hydratization CA(OH)2 CSH


Cement phases

SEM Ettringite-needles


Volumes in hardened cement paste



Capillary voids

Solid cementpaste

airvoids

Unhydratedcement

Water/cement-ratio

Volume changes

• chemical volume decrease• Early“primary“capillary volume decrease

„plastic shrinkage“• Drying shrinkage• Carbonatization shrinkage


Chemical Volume decrease

• Chemical connection of water to cementparticles decreases volume of cementpaste by 8% .

• As aggregates confine the deformationa linear shrinkage of concrete of -0,1Common Strength) up to -0,25 (highstrength) is the result



Early shrinkage

• Caused by capillary stresses(water absorbed by porous aggregates or evaporating water)

• Cracks after less than 24 hours• Without curing up to 4mm/m


Typical crack pattern plastic shrinkage

ⓒJMR 09Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009

Initial movement

after few hours

Early shrinkage

Surface of a oncrete parking slab day after concreting

Early shrinkage

Drilled cores from the parkdeck, cracks fom surface ending in the center


What can you do against early shrinkage ?

Keep the water in the concrete

secondary compaction in plastic status (1….2 hours after mixing)

Shrinkage


Cementclass climate

Chemical Shrinkage Drying shrinkage

Shrinkage



Total shrinkage for 2 concretes

Age in days

Drying at 80% RF from day 15

Cement type32,5R or 42,5 N

mm/m

FAQ: How big is the drying shrinkage for common concrete

Typical crack pattern (clay)


Primary, secondary and tertiary crack systems

Deformation without confinement

confined

unconfined

Stress with confinement

Stress by confined deformation


Rough stress/crack width estimation:0,4 mm/m total shrinkage, ultimate tensile deformation of concrete 0,1…0,2 mm/mcalculated with Hooke equation σ=εE E ca 30.000MPa

σ=0,2x10-3x 30.000 Mpa = 6 MpaThis exceeds the tensile strength of normal concreteand leads to crackscracks release the stress, crack width = shrinkage deformation

confined

Typical crack pattern (clay)


Confined deformation (here by friction) lead to stress

Omnidirectional stresses lead to „craquelee pattern“

Typical crack pattern (asphalt)


p

st

Typical crack pattern drying shrinkage


Shrinkage cracks in concrete



Weakest lineShrinkage causes cracking


What can you do against drying shrinkage?Nothing, it will happen!Which structures are effected, how to avoid these cracks?Critical are geometric forms witha big surface:Roads, foundation slabs, storey slabs


Which structures are effected, how to avoid these cracks?

W/C > 0.4 is commonly usedW/C 0.2 is chemically needed,

there is enough water, ifthere is no evaporation.Reduce evaporation by -paraffinspray „antisol“ (used on concrete roads)-cover with polymer foil-cure with waterspray

Joints to avoid cracks

• Roads in Germanyno reinforcement, joints cut at 5m distance with steel bar „dowel shift elements“

Cuts made after 6 to 20 hours after placing the concrete to enforce cracking at this place


Joints to avoid cracks

• Joint distance 4 to 6 m

Concrete wall on thickfoundation concrete slab(perfect confinement at wall toe)


Stress by hydratization temperature rise

Concrete temperature

stress

Fully confinedAssessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09

compressiontension

ca 15K

crack

hours

time


Avoid thermal cracks



To get the settlement before connecting tubes to container, container was filled with water (T ca 12 C)

Cooling by coldwater, if T-difference >15K




Avoid thermal cracksThermal expansion of roof



Thermal expansion of roof shifts the walls


Avoid thermal cracks, but there isalso a defect sewage piping

Lecture 5 Conclusion• Cement hydratization causes volume

decrease (early plastic shrinkage)• Water evaporation of concrete causes

drying shrinkage• Hydratization increases temperature of

concrete, temperature differences cause deformation

• Confined deformation create stress and cracks


Concrete Corrosion/deterioration1 Solution of cement paste by acids2 Swelling by sulfates, reacting with alkalines

(Ca(OH)2 +SO4-- >CaSO4

Gipsum: CaSO4 x 2 H2OEttringite: CaSO4 x 16 H2O Thaumasite: CaSO4 x 32 H2O Cristals in voids/pores expand> tensile failureof hardened cement paste

3 Aggregate-alkali-reaction (only with amorphous silicium acid SiO2)Carbonatisation is no corrosion/deterioration!!


Concrete Deterioration

• Concrete Deterioration is in humid climates not the main reason for reinforced concrete problems

• Concrete Corrosion may be relevant for foundations (acids or sulfate in soil)or in wastewater collecting pipes with microbial amplification of chemical attack



Sulfate attack, type of degradation

Sewagewater

Concrepipe

Sulfobacter create a lower pH-value(higher concentration)than sewage water

Sulfates react with alcaline parts of cement pastecreating more volume


Sulfate attack, solutions

Sewagewater

Concretepipe

Low alcali, high sulfate resistant cement (NA, HS)

pozzolans need CaOH2

Clinker content of Cement EN197

Portland cement CEM I >95%

Composite cement CEM II >65%

Composite cement CEM III >15%low heat, low alcali, high sulfate resistance


Pozzolanic reaction

Blast furnace slag

Clinker

Fly ashMikrosilicaTrass Limestone-

Quartz-flour


Hydrauliclime


Filler functionFly ash av. size 10..30µm,Blaine 6200cm2/gmicrosilica av. size 1..3µm,Blaine 2200.000cm2/g

Concrete Corrosion Situation Riyadh / Jeddah

• The climate is too dry for carbonatisation (above soil)

• Main problem: sulfate salt creating swelling new minerals:„ettringite, the concrete cancer“increasing salt concentration by evaporation



What is the water consumption of Riyadh?How many cars can pass a bridge?how many people a Airbus 380 can carry?Each German drinks 80 l coffee per year!A car in Germany rides 13.000 km/year!What is the concrete strength of Gammarat bridge?How can you know?

By representative samples and

Statistic fast food: normal distribution, mean, std.deviation


Statistic fast food: same average, different std dev


Statistic fast food: how to calculate a Quantile/Fractile

Q%= mean ± k std. deviationk for large number, 5%=1.645,single sided only –ks

fc,5% = 24-1.645×3.21 =18.72 MPa

FAQ: How calculate a Quantil?


Statistic fast food: same quantile, different mean&std dev

5%-Quantile means 5% of production may be worse

lecture 5 - willkommen an der fh münster€¦ · lecture 5 • intrinsic cracks early...

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