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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
hydratization 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
Source: Zementtaschenbuch 2008
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
hydratization of cement
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
Source: Zementtaschenbuch 2008
hydratization of cement
SEM 24 hours of hydratization CA(OH)2 CSH
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009
Source: Zementtaschenbuch 2008
hydratization of cement
SEM 28 days of hydratization CA(OH)2 CSH
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009
Cement phases
SEM Ettringite-needles
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Volumes in hardened cement paste
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Source: Zementtaschenbuch 2008
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
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Source: Zementtaschenbuch 2008
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
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Typical crack pattern plastic shrinkage
ⓒJMR 09Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009
Initial movement
after few hours
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
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
What can you do against early shrinkage ?
Keep the water in the concrete
secondary compaction in plastic status (1….2 hours after mixing)
Shrinkage
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Cementclass climate
Chemical Shrinkage Drying shrinkage
Shrinkage
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Source: Zementtaschenbuch 2008
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)
ⓒJMR 09Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009
Primary, secondary and tertiary crack systems
Deformation without confinement
confined
unconfined
Stress with confinement
Stress by confined deformation
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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)
ⓒJMR 09Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009
Confined deformation (here by friction) lead to stress
Omnidirectional stresses lead to „craquelee pattern“
Typical crack pattern (asphalt)
ⓒJMR 09Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009
p
st
Typical crack pattern drying shrinkage
ⓒJMR 09Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009
Shrinkage cracks in concrete
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Weakest lineShrinkage causes cracking
ⓒJMR 09Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009
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
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Joints to avoid cracks
• Joint distance 4 to 6 m
Concrete wall on thickfoundation concrete slab(perfect confinement at wall toe)
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Avoid thermal cracks
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Avoid thermal cracks
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Avoid thermal cracks
Avoid thermal cracks
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Avoid thermal cracks
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Avoid thermal cracksThermal expansion of roof
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Avoid thermal cracks
Thermal expansion of roof shifts the walls
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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
ⓒJMR 09Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009
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!!
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Pozzolanic reaction
Blast furnace slag
Clinker
Fly ashMikrosilicaTrass Limestone-
Quartz-flour
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Hydrauliclime
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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
ⓒJMR 09Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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
ⓒJMR 09Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009
Statistic fast food: same average, different std dev
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
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?
Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 ⓒJMR 09
Statistic fast food: same quantile, different mean&std dev
5%-Quantile means 5% of production may be worse