Lecture3_5102011_PlasticShrinkage_slides.pdf

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Lecture 3Plastic shrinkage and cracking

Pietro Lura

Concrete & Construction Chemistry

Shrinkage and Cracking of Concrete: Mechanisms and Impact on Durability, ETHZ, 05.10.11

2

Contents

Introduction about plastic shrinkage cracking

Model for plastic shrinkage cracking in gels

Lab setup for settlement, pore pressure and cracking

Studies on non-reactive systems (fly ash pastes)

Studies on mortars and fly ash pastes with SRA

2

3

Plastic settlement cracks

Weiss REACCT 2009

Differential settlement causes stress concentrations and cracksMay be a problem in very fluid mixtures

4

Plastic shrinkage cracks

Galilee, Israel, 2001

Fast evaporation due to:

Low relative humidity

High wind speed

High temperature, surface exposed to sun

3

5

Hypothesis about cracking

Phase 1 = Concrete still workable

Phase 2 = Dormant period

Phase 3 = Acceleration period

Risk of cracking depends on the amount of water

that evaporates during the dormant period

Time

Tensile stressTensile strength

13

2

Crack

Lura and Leemann 2008

6

Climatic conditions

5

10

15

20

25

30

35

16.07.200700:00

16.07.200704:00

16.07.200708:00

16.07.200712:00

16.07.200716:00

16.07.200720:00

17.07.200700:00

Tem

pera

tur

(C)

20

30

40

50

60

70

80

rel.

Luftf

euch

tigke

it (%

)

Worst case:

- High evaporation rate

- Slow concrete hardening

Cool spring day or hot summer day?

Data about air temperature, relative humidity and wind velocity are available from weather stations

4

7

Plastic shrinkage cracks in concrete (1)

Photos by A. Leemann, 2008 Cracks go through concrete slab, water seeps through cracks

8

Plastic shrinkage cracks in concrete (2)

Schmidt et al. BUST 2007

Sunny and windy day, slab 25 cm thick. Cracking within the first 4 hours after placing the concrete, crack distance ~1 m

5

9

Active solution: fogging/wetting of concrete surface

Labor intensive, expensive, not always practicalCuring compound (slows down evaporation), similar problems

Atcin et al.ACI SP 220 2004

10

Passive solutions

It is preferable to make the concrete mixture less prone to plastic shrinkage cracking:

Fibers (i.e. fibrillated polypropylene), at low concentration help also with plastic settlement

Viscosity modifiers

Shrinkage reducing admixtures (SRA)

Internal curing (i.e., LWA or SAP)

Cement type (i.e., using relatively coarse cements)

Mix composition (i.e., low cement paste content)

Need for standard tests to assess cracking risk

6

11

Plastic shrinkage mechanisms - Capillary tension

r

From http://www.pharmainfo.net/reviews/pharmaceutical-micropellets-overview

12

Young-Laplace equation

Thomas Young (1773 1829) Pierre-Simon, marquis de Laplace (1749 - 1827)

r

cospcap

= 2

+=

21

11

RRp

(1805-06)

7

13

Suspension of cement particles

Cryo-nanotomography, fresh cement paste with superplasticizerZingg et al. CCR 2008

14

Phases of drying

r

Evaporationof bleed water

Constant rateperiod

Falling rateperiod

Brinker and Scherer Sol-gel science 1990

8

15

Constant rate period in gels

W

W

SL

S

mP

DJ

==

Flux to surface=

Evaporation

r

r

cos2P

=

Brinker and Scherer Sol-gel science 1990

Pressure gradient

depends on D

16

Critical point and cracking in gels

Hansen 1995

( ) PPP~x =

1

21

~

x

Scherer J Non-Crystalline Solids 1992

In gels, pressure gradient at surface causes crackingPressure gradient can be limited by reducing the maximum pressure (i.e., with surfactants) or slowing evaporation

9

17

Evaporation rate

Brinker and Scherer Sol-gel science 1990

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

405060708090100

Fraction of intial water (%)

rate

of w

ater

loss

(g

cm-1

min

-1)

Fly ash

Deionized water

Fontana, Di Bella and Lura 2010

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Critical Point and settlement Pressure release

Brinker and Scherer Sol-gel science 1990

Fly ash paste, w/s 0.28

-4

-3

-2

-1

0

0 1 2 3

Time (h)

Set

tlem

ent

(m

m)

Settlement -Laser

Settlement -Weight loss

Fontana, Di Bella and Lura 2010

10

19

Plastic cracking (ASTM C-1579)

20

Quantitative image analysis

Qi, Weiss and Olek J ASTM 2005

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21

Setup plastic settlement

Lasers mounted on movable holders for settlement measurementsFontana, Di Bella and Lura 2009

Final Specimen Surface

Change in CCDs

Measurement

Initial Specimen Surface

CMOS/CCDLaser

Source

Final Specimen Surface

Change in CCDs

Measurement

Initial Specimen Surface

CMOS/CCDLaser

Source

Final Specimen Surface

Change in CCDs

Measurement

Initial Specimen Surface

CMOS/CCDLaser

Source

22

320

100

230

320

new development

200

700

300

300 300

30

H=6

5

blower

150 150200 200

1620

vertival section

120

B=6

25

motor

800

grids

diffusorrelaxationchamber nozzle

horizontal section

elastic material fordecoupling of vibrations

320

100

230

320

new development

200

700

300

300 300

30

H=6

5

blower

150 150200 200

1620

vertival section

120

B=6

25

motor

800

grids

diffusorrelaxationchamber nozzle

horizontal section

elastic material fordecoupling of vibrations

mould (615x410x130)

balance

50

130

130

cover (with hand grip)

615 (mould)

section A-A

mould (615x410x130)

balance

50

130

130

50

130

130

cover (with hand grip)

615 (mould)

section A-A

A

A

410 mould)

blade for shaving the lowerboundary layer of the nozzle

deflector

nozzle

mould 1 mould 2

cover 1 cover 2

mould 3

A

A

410 mould)

blade for shaving the lowerboundary layer of the nozzle

deflector

nozzle

mould 1 mould 2

cover 1 cover 2

mould 3

vertical

Fontana, Di Bella and Lura 2009

Control of wind speed and climate above molds (1)

12

23

Control of wind speed and climate above molds (2)

20

25

30

35

40

1 2 3 4 5Time (h)

Tem

pera

ture

(C

)

20

30

40

50

60

70

TemperatureHumidity R

elat

ive

hum

idity

(%

)

20.5 31 41.5 62.5 73 83.5 104.5 115 125.5 136-15.75

0

15.75

Distance (cm)6.7-6.8 6.8-6.96.9-7.0 7.0-7.1

7.1-7.2 7.2-7.3

7.3-7.4 7.4-7.5

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Measurements of capillary pressure

Fontana, Di Bella and Lura 2009

amplifier / datalogger

atmospheric pressure pressure in pore liquid Tensiometer is measuring thepressure difference (depression)

concrete

mould

sensor

concrete

mould

sensor

rubber hosetube

(1.4301)

water (degassed)

2.

5 m

m

1.

8 m

mpressure sensor

com

pens

atio

nof

sup

ply

volta

ge(B

eat)

rubber hosetube

(1.4301)

water (degassed)

2.

5 m

m

1.

8 m

mpressure sensor

com

pens

atio

nof

sup

ply

volta

ge

13

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Influence on w/s on settlement

Fontana, Di Bella and Lura 2010

-2.0

-1.5

-1.0

-0.5

0.0

0 1 2 3 4

Time (h)

Set

tlem

ent (

mm

)

FA032

FA027

FA025

Increased water content leads to:

Increased settlement

Later occurrence of critical point

Increased water loss and settlement to compact the particles

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Capillary pressure measurements (1)

S3S4S5

S6

S7

S8

838

3534

43

150

150

25 42 62 21150

100

838

3534

43

150

50

S3, S4, S5

S6

S7

S8

S3S4S5

S6

S7

S8

838

3534

43

150

150

25 42 62 21150

100

838

3534

43

150

50

S3, S4, S5

S6

S7

S8

Fontana, Di Bella and Lura 2009

Multiple pressure sensors at different depths and locations

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27

Capillary pressure measurements (2)

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Time (h)

Set

tlem

ent (

mm

)

-450

-400

-350

-300

-250

-200

-150

-100

-50

0

50

Cap

illar

y pr

essu

re (

mba

r)

Hydrostatic pressure

Settlement

Capillary pressure

Laser

Calculated by weight loss30C / 50% RHw/s = 0.27

S3S4

S5

S6

S7

S8

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Time (h)

Set

tlem

ent (

mm

)

-450

-400

-350

-300

-250

-200

-150

-100

-50

0

50

Cap

illar

y pr

essu

re (

mba

r)

Hydrostatic pressure

Settlement

Capillary pressure

Laser

Calculated by weight loss30C / 50% RHw/s = 0.27

S3S4

S5

S6

S7

S8

Fontana, Di Bella and Lura 2009

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Critical point and cracking in concrete

Hansen 1995

( ) PPP~x =

1

21

~

x

Scherer J Non-Crystalline Solids 1992

Pressure gradient at surface seems of minor importance to cracking in cement paste and concreteRestraint to global shrinkage seems to cause cracking

15

29

Effect of SRA on evaporation

Lura et al. ACI Mat J 2007

-5

-4

-3

-2

-1

0

0 1 2 3 4 5 6

Time (hours)

Spe

cific

mas

s ch

ange

(kg

/m2 )

Plain Mortar

Mortar 5% SRA

Mortar 1% SRA

5% SRA solution

Deionized water

1% SRA solution

SRA reduce the surface tension of the pore solution

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Effect of SRA on plastic shrinkage cracking

Lura et al. ACI Mat J 2007

0 0.4 0.8 1.2 1.6 2.0

Crack Width (mm)

100

80

60

40

20

0

Cum

ulat

ive

Dis

trib

utio

n of

Mea

sure

d C

rack

s (%

)

Plain5% SRA

2% SRA

1% SRA

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Effect of SRA on settlement and capillary pressure

Fontana, Di Bella and Lura 2009

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Time (h)

Set

tlem

ent (

mm

)

-400

-300

-200

-100

0

Cap

illar

y pr

essu

re (

mba

r)

FA027

FA027+2%SRACapillary pressure

Settlement

FA027

FA027+2%SRA

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Critical point with and without SRA

rr

cos2P

= r is the same (particle

geometry)

is half with SRA Contact angle similar

Max capillary pressure is half for given particle geometry

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33

Effect of cement fineness (1)

Type of cement CEM I 32.5 N CEM I 42.5 N CEM I 52.5 R

Blaine fineness [cm2/g] 2530 3150 4510

Aggregate 0/16 mm [kg/m3] 1858 1858 1858

Cement content [kg/m3] 352 352 352

w/c 0.5 0.5 0.5

Superplasticizer [mass-% of cement] 0.2 0.2 0.2

Air content [volume-%] 4.0 3.9 3.4

Flow [cm] 40 40 40

Density [kg/m3] 2333 2336 2358

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-3.0

-2.0

-1.0

0.0

0 1 2 3 4 5Time (h)

Mas

s ch

ange

(kg

/m2 )

CEM I 42.5 N

CEM I 32.5 N

CEM I 52.5 R

0

1

2

3

4

5

6

7

0 1 2 3 4 5Time (h)

Ble

edin

g (%

)

CEM I 42.5 N

CEM I 32.5 N

CEM I 52.5 R

Effect of cement fineness (2)

18

35

-1.5

-1.2

-0.9

-0.6

-0.3

0.0

0 1 2 3 4 5Time (h)

Set

tlem

ent (

mm

)

CEM I 52.5 R

CEM I 42.5 N

CEM I 32.5 N

-400

-300

-200

-100

0

0 1 2 3 4 5Time (h)

Cap

illar

y pr

essu

re (

mba

r)

CEM I 52.5 R

CEM I 42.5 N

CEM I 32.5 N

Effect of cement fineness (3)

36

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.5 1.0 1.5 2.0

Crack width (mm)

Cum

ulat

ive

dist

ribut

ion

CEM I 52.5 R

CEM I 42.5 N

Effect of cement fineness (4)

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37

Pressure and particle size

r

r

cos2P

=

The pressure in the system is governed by the particle size

In binders with fine particles (e.g. fine cements, silica fume addition), pressure increases

38

Effect of w/c and binder volume

250

270

290

310

330

350

0.40 0.45 0.50 0.55 0.60

Cem

ent p

aste

vol

ume

(l/m

3 )

w/c

M2

M4

M9

M8

M14

M1

M3

M5M15

Red crosses indicate mixtures that cracked

Intermediate w/c with higher amount of pastes cracked

20

39

Need further study

Mechanism of action of fibers: perhaps they create some local porosity and dissipate stresses

Mechanism of action of viscosity modifiers: do they influence bleeding rate and transport of water to surface?

Mechanism of action of LWA and SAP: will be mentioned in later lecture

Effect of cement type

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Summary

During constant rate period, evaporation and settlement are not influenced by w/s, (particle size) or SRA addition

Critical point is anticipated by low w/s, (large particles) and low surface tension of pore fluid

Earlier critical point means lower evaporation, settlement and capillary stresses. In some cases also fewer and smaller plastic shrinkage cracks

Actual mechanism of cracking needs further study

21

41Alberto Burri, Grande Cretto Nero, 1977

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Acknowledgements

J. Weiss

P. Fontana

C. Di Bella

A. Leemann