fib phd symposium 2012 presentation on "cracking risk in early-age rc walls"

Cracking risk in early-age RC walls

MSc. Eng. Agnieszka KNOPPIK-WRÓBEL

Silesian University of Technology, Gliwice, PolandFaculty of Civil Engineering

Department of Structural Engineering

Karlsruhe, 22-25 July 2012

Agenda

1 Development of cracks in RC wallsThermal–shrinkage crackingFactors of influence

2 Numerical modelThermal and moisture analysisThermal–shrinkage strainsStress analysisImplementation

3 Analysis of RC wallThermal–moisture analysisStress analysisDamage intensity analysis

4 Parametric studyInfluence of ambient temperature and temperature differenceInfluence of time of formwork removalInfluence of concrete mix composition

5 Conclusions

Development of cracks in RC wallsNumerical model

Analysis of RC wallParametric study

Conclusions

Thermal–shrinkage crackingFactors of influence

Thermal–moisture effects

Figure 1: Hoover Dam, USA

concretewater + cement + aggregate

cement hydrationhighly exothermic process

heat and moisture transporttemperature and moisture gradients

stressesthermal–shrinkage stresses in structure

Agnieszka Knoppik-Wróbel Cracking risk in early-age RC walls



Conclusions


Internal restraint vs. external restraint

internal restraintresult of temperature andmoisture gradients withinthe element

self-induced stresses

predominant: massive structuresblock foundations

gravity dams

massive retaining walls

external restraintlimitation of deformation bymature concrete of previouslycast layers

restraint stresses

predominant: restrained structurestank walls

nuclear containment walls

bridge abutments




Conclusions


Cracking pattern in RC walls

Figure 2: Cracking pattern observed in a real RC wall.




Conclusions


Cracking pattern in RC walls

h

1/3-

2/3

hhh

l

21

cr

2

cr lcr

wk,maxwk,max

Figure 3: Typical cracking pattern in RC wall.




Conclusions


Factors affecting the risk of early-age cracking

Factors contributing to the complex process of thermal–shrinkagecracking of RC walls:

1 thermal properties of concrete dependent on concrete mixcomposition

2 conditions during casting and curing of concrete3 technology of concreting4 environmental conditions5 dimensions and geometry of concrete structure




Conclusions





2 conditions during casting and curing of concrete

3 technology of concreting4 environmental conditions5 dimensions and geometry of concrete structure




Conclusions





2 conditions during casting and curing of concrete3 technology of concreting

4 environmental conditions5 dimensions and geometry of concrete structure




Conclusions





2 conditions during casting and curing of concrete3 technology of concreting4 environmental conditions

5 dimensions and geometry of concrete structure




Conclusions





2 conditions during casting and curing of concrete3 technology of concreting4 environmental conditions5 dimensions and geometry of concrete structure




Conclusions

Thermal and moisture analysisThermal–shrinkage strainsStress analysisImplementation

General assumptions

1 phenomenological modelfull coupling of thermal and moisture fieldsdecoupling of thermal–moisture and mechanical fields

2 stress state determined under the assumption thatthermal–moisture strains have distort character

3 viscoelasto–viscoplastic material model of concrete




Conclusions


Thermal and moisture analysis

Coupled thermal–moisture equations

T = div(αTT gradT + αTW gradc) +1

cbρqv

c = div(αWW gradc + αWT gradT )− Kqv

Initial conditions

T (xi , t = 0) = Tp(xi , 0)

c(xi , t = 0) = cp(xi , 0)

Boundary conditions

nT (αTT gradT + αTW gradc) + q = 0

nT (αWW gradc + αWT gradT ) + η = 0




Conclusions


Thermal–shrinkage strains

Imposed thermal–shrinkage strains εεεn:

volumetric strains

dεεεn =[dεnx dεny dεnz 0 0 0

]calculated based on predetermined temperature and humidity

dεnx = dεny = dεnz = αT dT + αW dW

W = f (c)




Conclusions


Stress analysis

viscoelastic area

σσσ = Dve(εεε− εεεn − εεεc)

viscoelasto–viscoplastic area

σσσ = Dve (εεε− εεεn − εεεc − εεεvp)

failure surface

stress path

τoct

τoct

τoct

f

σm

Figure 4: Damage intensity factor.

possibility of crack occurrence

sl =τoct

τ foct

Figure 5: Failure surface development.




Conclusions


Implementation

pre-processor & post-processordata preparation & presentationwith ParaView

processorTEMWILthermal–moisture fieldsMAFEM_YOUNGstress analysis




Conclusions

Thermal–moisture analysisStress analysisDamage intensity analysis

Basic case

concrete class C30/37, steel class RB400cement type CEM I 42.5R, 375 kg/m3,ambient temperature Tz = 25◦C, initial temperature of concrete Tp = 25◦C,wooden formwork of 1.8 cm plywood removed after 28 days,no insulation, protection of top surface with foil.

20.0 m0.7

m

4.0

m

4.0 m

0.7 m

ZY

X

Figure 6: Geometry and finite element mesh of analysed wall.




Conclusions


Thermal fields

40

45

50

55ature [°C] interior

surface

20

25

30

35

0 2 4 6 8 10 12 14 16 18 20

tempera

time [days]

Figure 7: Temperature development in time.




Conclusions


Moisture fields

15

16

17

18ontent (x100)

3/m

3]

interior

surface

12

13

14

0 2 4 6 8 10 12 14 16 18 20

moisture co

[m3

time [days]

Figure 8: Moisture content development in time.




Conclusions


Stress development & deformations

0.6

1.2

1.8

MPa]

interior

surface

‐1.8

‐1.2

‐0.6

0.0

0 2 4 6 8 10 12 14 16 18 20

stress [M

time [days]

Figure 9: Stress development in time.




Conclusions


Stress distribution & damage intensity

-0,7

-0,2

0,3

0,8

1,3

1,8

2,3

2,8

3,3

3,8

-2,5 -1,5 -0,5 0,5 1,5 2,5

he

igh

t [m

]

stress [MPa]

interior

surface

Figure 10: Distribution of stress at the height of the wall.




Conclusions

Influence of ambient temperature and temperature differenceInfluence of time of formwork removalInfluence of concrete mix composition

Chosen factors

Influence of the following parameters analysed:

1 ambient temperature and temperature differenceTz = Tp = 25◦C (basic), 20◦C or 15◦Cpre-cooling by 5◦C or 10◦C

2 time of formwork removalafter 28 days (basic)after 3 days

3 concrete mix composition (type and amount of cement)CEM I 42.5R 375 kg (basic), 325 kg or 425 kgCEM II B-S 42.5N, CEM III/A 42.5N or CEM V/A 32.5R




Conclusions


Damage intensity maps (after 20 days) comparison

(a)Tz=Tp=15◦C (b)Tz=Tp=20◦C (c)Tz=Tp=25◦C

Figure 11: Damage intensity maps in the interior of the wall (ambient temperature).

(a)Tz=25◦C, Tp=25◦C (b)Tz=25◦C, Tp=20◦C (c)Tz=25◦C, Tp=15◦C

Figure 12: Damage intensity maps in the interior of the wall (temp. difference).




Conclusions


Maximum damage intensity factor comparison

0.53

0 37

0.54

0 39

0.57

0 390.45 0.45 0.470.5

0.6

0.7

0.8

0.9

1.0

tensity factor interior

surface

0.37

0.23

0.39

0.25

0.39

0.260.32

0.22

0.34

0.23

0.34

0.23

0.0

0.1

0.2

0.3

0.4

dam

age int

Figure 13: Influence of ambient temperature and temperature difference on damageintensity factor.




Conclusions



(a)Tz=Tp=25◦C, 28 days (b)Tz=Tp=25◦C, 3 daysFigure 14: Damage intensity maps in the interior of the wall.

(a)Tz=Tp=25◦C, 28 days (b)Tz=Tp=25◦C, 3 daysFigure 15: Damage intensity maps on the surface of the wall.




Conclusions



0.53 0.56 0.54 0.57 0.57 0.59

0.45

0.79

0.45

0.79

0.47

0.81

0.5

0.6

0.7

0.8

0.9

1.0

tensity factor interior

surface

0.0

0.1

0.2

0.3

0.4

dam

age in

Figure 16: Influence of time of formwork removal on damage intensity factor.




Conclusions


Hydration heat of cements

200

250

300

350

ration, [J/g]

0

50

100

150

0 10 20 30 40 50 60 70 80

heat of hydr

time, [h]

CEM I 42,5R

CEM II/B‐S 42,5N

CEM III/A 42,5N

CEM V/A (S‐V) 32,5R

Figure 17: Development of hydration heat of different types of cements.




Conclusions



(a)CEM I 325kg/m3 (b)CEM I 375kg/m3 (c)CEM I 425kg/m3

(d)CEM II 375kg/m3 (e)CEM III 375kg/m3 (f)CEM V 375kg/m3

Figure 18: Damage intensity maps in the interior of the wall.




Conclusions



0.49

0.570.64

0.530.57

0.490.470.52

0 44 0.470 5

0.6

0.7

0.8

0.9

1.0

ensity factor

interior

surface

0.400.44

0.41

0.0

0.1

0.2

0.3

0.4

0.5

CEM I 42.5R 325kg/m3



CEM II B‐S 42.5N 375kg/m3

CEM III/A 42.5N 375kg/m3

CEM V/A 32.5R 375kg/m3

dam

age inte

Figure 19: Influence of concrete mix composition on damage intensity factor.




Conclusions

Research importance

Importanceneed to ensure desired service life and function of thestructure

on-going examination of early-age cracking problem

Numerical modelqualitatively and quantitatively proper results

conformation with present knowledge and experience

Contributionmulti-parameter numerical model of thermal–moisture effects inearly-age concrete and its implementation




Conclusions

Discussion of results

Technology and curing conditionsmoderate ambient temperatures

positive influence of initial cooling

surface cracking risk if formwork removed early

Concrete mix compositionlow-heat cements: lower hydration temperatures vs. lowerrate of strength development


9th fib International PhD Symposium in Civil Engineering22–25 July 2012

Karlsruhe Institute of Technology, Germany

fib phd symposium 2012 presentation on "cracking risk in early-age rc walls"

Education

rc wallsfigure

age rc wallsmsc

real rc wall

risk of early

agenda1 development

typical cracking pattern

hoover dam

moisture transporttemperature