Lecture 5 Intrinsic cracks early chemical/plastic shrinkage drying shrinkage carbonation shrinkage

Transcription

1 Lecture 5 Intrinsic cracks early chemical/plastic shrinkage drying shrinkage carbonation shrinkage Concrete corrosion/deterioration acid solution sulfate swell Statistic light Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

2 hydratization of cement In concrete with quartzitic aggregates volume changes can only happen in the cement paste Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

3 hydratization of cement v o l u m e CSH =calcium-silicate-hydrates Pore volume hydration of cement Long fibre Short fibre Hydration time plastic Minutes hours days Unstable structure hardened Base structure Solid structure Source: Zementtaschenbuch 2008 Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

4 hydratization of clincergrain Dry milled grain after watercontact fully hydrated Source: Zementtaschenbuch 2008 Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

5 hydratization of cement Source: Zementtaschenbuch 2008 Scanning electron microscope (SEM) picture 3 hours hydratization needles of primary CSH and short columns of ettringit Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009

6 Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 hydratization of cement Source: Zementtaschenbuch 2008 SEM 24 hours of hydratization CA(OH) 2 CSH

7 Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 hydratization of cement SEM 28 days of hydratization CA(OH) 2 CSH

8 Cement phases SEM Ettringite-needles Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

9 Volumes in hardened cement paste airvoids Unhydrated cement Capillary voids Solid cementpaste Water/cement-ratio Source: Zementtaschenbuch 2008 Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

10 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 cjmr 09

11 Chemical Volume decrease Chemical connection of water to cementparticles decreases volume of cementpaste by 8%. As aggregates confine the deformation a linear shrinkage of concrete of -0,1Common Strength) up to -0,25 (high strength) is the result Source: Zementtaschenbuch 2008 Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

12 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 cjmr 09

13 Typical crack pattern plastic shrinkage Initial movement after few hours Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

14 Typical crack pattern plastic shrinkage Initial movement after few hours Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

15 Early shrinkage Surface of a oncrete parking slab day after concreting

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

17 What can you do against early shrinkage? Keep the water in the concrete secondary compaction in plastic status (1.2 hours after mixing) Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

18 Shrinkage Cementclass climate Chemical Shrinkage Drying shrinkage Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

19 Shrinkage Age in days Cement type32,5r or 42,5 N mm/m Source: Zementtaschenbuch 2008 Drying at 80% RF from day 15 Total shrinkage for 2 concretes FAQ: How big is the drying shrinkage for common concrete Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

20 Typical crack pattern (clay) Primary, secondary and tertiary crack systems Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

21 Deformation without confinement unconfined Stress with confinement confined

22 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/m calculated with Hooke equation σ=εe E ca MPa σ=0,2x10-3x Mpa = 6 Mpa This exceeds the tensile strength of normal concrete and leads to cracks cracks release the stress, crack width = shrinkage deformation confined Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

23 Typical crack pattern (clay) Confined deformation (here by friction) lead to stress Omnidirectional stresses lead to craquelee pattern Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

24 Typical crack pattern (asphalt) t s p Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

25 Typical crack pattern drying shrinkage Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

26 Shrinkage cracks in concrete Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

27 Shrinkage causes cracking Weakest line Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

28 What can you do against drying shrinkage? Nothing, it will happen! Which structures are effected, how to avoid these cracks? Critical are geometric forms with a big surface: Roads, foundation slabs, storey slabs Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

29 Which structures are effected, how to avoid these cracks? W/C > 0.4 is commonly used W/C 0.2 is chemically needed, there is enough water, if there is no evaporation. Reduce evaporation by -paraffinspray antisol (used on concrete roads) -cover with polymer foil -cure with waterspray Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

30 Joints to avoid cracks Roads in Germany no 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 cjmr 09

31 Joints to avoid cracks Joint distance 4 to 6 m Concrete wall on thick foundation concrete slab (perfect confinement at wall toe) Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

32 Stress by hydratization temperature rise Concrete temperature ca 15K stress compression tension time hours Fully confined crack Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

33 Avoid thermal cracks Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

34 Avoid thermal cracks Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

35 Avoid thermal cracks Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

36 Avoid thermal cracks To get the settlement before connecting tubes to container, container was filled with water (T ca 12 C) Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

37 Avoid thermal cracks Cooling by coldwater, if T-difference >15K Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

38 Avoid thermal cracks Thermal expansion of roof Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

39 Avoid thermal cracks hermal expansion of roof shifts the walls Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

40 Avoid thermal cracks, but there is also a defect sewage piping Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 JMR 09

41 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 Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

42 Concrete Corrosion/deterioration 1 Solution of cement paste by acids 2 Swelling by sulfates, reacting with alkalines (Ca(OH) 2 +SO -- 4 >CaSO 4 Gipsum: CaSO 4 x 2 H 2 O Ettringite: CaSO 4 x 16 H 2 O Thaumasite: CaSO 4 x 32 H 2 O Cristals in voids/pores expand> tensile failure of hardened cement paste 3 Aggregate-alkali-reaction (only with amorphous silicium acid SiO 2 ) Carbonatisation is no corrosion/deterioration!! Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

43 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 cjmr 09

44 Sulfate attack, type of degradation Sulfates react with alcaline parts of cement paste creating more volume Concre pipe Sulfobacter create a lower ph-value (higher concentration) than sewage water Sewage water Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

45 Sulfate attack, solutions Low alcali, high sulfate resistant cement (NA, HS) pozzolans need CaOH 2 Concrete pipe Sewage water Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

46 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 cjmr 09

47 Pozzolanic reaction Clinker Hydraulic lime Fly ash Mikrosilica Trass Blast furnace slag Limestone- Quartzflour Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

48 Filler function Fly ash av. size µm,Blaine 6200cm 2 /g microsilica av. size 1..3µm,Blaine cm 2 /g Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

49 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 Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

50 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 km/year! What is the concrete strength of Gammarat bridge? How can you know? By representative samples and Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

51 Statistic fast food: normal distribution, mean, std.deviation Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

52 Statistic fast food: same average, different std dev Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09

53 Statistic fast food: how to calculate a Quantile/Fractile Q%= mean ± k std. deviation k for large number, 5%=1.645, single sided only ks fc,5% = =18.72 MPa FAQ: How calculate a Quantil?

54 Statistic fast food: same quantile, different mean&std dev 5%-Quantile means 5% of production may be worse Assessment and Repair of Concrete structures Riyadh,Jeddah.Tabouk, may 2009 cjmr 09