1
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
18
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
25
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
28
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
30
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
32
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
17
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)
19
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
40
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
42
Acknowledgements
J. Weiss
P. Fontana
C. Di Bella
A. Leemann