CEMENT 071500baalbaki.ppt

© Holcim Group Support Ltd 2008
Concrete shrinkage
Moussa Baalbaki
Product Innovation and Development
Course for Cement Applications 2008

2
26.09.2008/BMO/hod
7. Shrinkage of concrete.ppt
© Holcim Group Support Ltd 2008 Course for Cement Applications 2008
Learning objectives
 Significance of shrinkage cracking in concrete
 Why concrete shrinks
 Concrete volume change and type of shrinkage
 Effect of concrete constituents and design

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Service life / cost ratio
Heavy cost
of repair
y/$
+
-
Repair-free concrete
Concrete durability
Significance of shrinkage cracking in concrete
 In order to reap the benefits of durable concrete it is
important to prevent cracks

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 13 km
 low alkali Silica Fume cement
 72 MPa average strength
 air entrained HPC
Designed for 100 years
Confederation Bridge - Canada
Weight of Eiffel Tower (~ 7’500 tons)

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Learning objectives

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Why concrete shrinks
Capillary tension
Pore walls
a radius
F
Contraction forces between particles
Capillary meniscus
SRA
 Result of the build up of tensile forces due to the formation of
water menisci within the concrete pore system
 inversely proportional to the diameter of the pores

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0
0.1
0.2
0.3
0.4
1
10
100
1000
Pore Diameter (nm)
Penetration
volume
(cc/g)
Voids < 50 nm are detrimental to shrinkage
w/c = 0.30
w/c = 0.40
w/c = 0.50
w/c = 0.60
Pore size distribution

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Learning objectives

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Concrete volume change and type of shrinkage
 Most commonly, concrete volume changes deals with
linear contraction due to moisture cycles and
temperature
 Plastic shrinkage
 Drying shrinkage
 Autogenous shrinkage
 Thermal shrinkage or contraction
 Chemical attacks
 Carbonation
 Sulfate
 AAR
 Creep
 Deformation caused by sustained stress or load

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(fct = 2 ~ 3 MPa)
 For practical purposes,
shrinkage is usually
described by the amount of
shrinkage in one dimension
as expressed by the
following formula:
ε (t) = (lt - I0)/I0 = Dl/l0
being:
I0 : initial length
lt :length at time t
Concrete volume change and type of shrinkage

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Green
concrete
Young
concrete
Hardening
concrete
Plastic
settlement
Plastic & Autogenous
Shrinkage
Autogenous, Drying
& Thermal Shrinkage
Time
Fresh
concrete
Hardened
concrete
2 h 24 h 2 - 3 d
Critical
Time of occurrence

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Plastic shrinkage cracking
 Usually associated with hot weather concreting or any
time when ambient conditions produce rapid
evaporation
 Occurs when water is lost from concrete during plastic
state (water evaporation > bleeding water)
 by evaporation (bleeding, humidity, wind, T°C)
 by suction of underlying dry concrete or soil
 Important in ordinary and HPC

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Plastic shrinkage (ACI 305R-91 nomograph)
 The Canadian code nominates
0.75 kg/m2/hr as the critical
value while Australian
references quote 0.5 kg/m2/hr
as a value at which
precautions should be taken
 According to ACI 305R-91, the
risk of plastic cracking is the
same at the following
combinations of temperature
and relative humidity:
 41°C and 90%
 35°C and 70%
 24°C and 30%

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Plastic shrinkage
 Special precautions in hot weather concreting
1. Moisten concrete aggregate that are dry and absorptive
2. Keep concrete temperature low by cooling aggregate and
mixing water
3. Dampen the subgrade and fog forms prior to placing concrete
4. Erect temporary windbreaks to reduce wind velocity over the
concrete surface
5. Erect temporary sunshades to reduce concrete surface
temperatures
6. Protect the concrete with temporary coverings such as
polyethylene during delay between placing and finishing or
spray aliphalic alcohol
7. Fog the slab immediately after placing and before finishing
8. Add plastic fibres
9. Night time concreting

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Plastic shrinkage
 Mineral components
 Have little effect except:
- silica fume – low bleeding rate (0.25 kg/m2/h)
- Some pozzolans increase water demand
- And those increasing significantly setting time

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Occurs when water is lost from hardened concrete
exposed to air with a low relative humidity
This is the only type of shrinkage which
develops significantly in ordinary concrete
Drying Shrinkage

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Drying shrinkage on concrete – effect of MIC
-500
-400
-300
-200
-100
0
0 14 28 42 56 70 84
Time (day)
Shrinkage
(mm/m)
Control
30% slag
50% slag
-500
-400
-300
-200
-100
0
0 14 28 42 56 70 84
Time (day)
Shrinkage
(
m
m/m)
Control
15% FA
30% FA
-1200
-1000
-800
-600
-400
-200
0
0 14 28 42 56 70 84
Time (day)
Shrinkage
(
m
m/m)
B50-Control
B67-15% PZ
B78-30% PZ
W/C = 0.50
[14D - 95% RH] - After 50% RH

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-600
-500
-400
-300
-200
-100
0
100
0 14 28 42 56 70 84
Time (day)
Shrinkage
(m
m/m)
OPC
50% slag
50% slag + SC
30% fly ash
Long term drying shrinkage same for all cements
 W/C = 0.30 - [14D - 95% RH] - After 50% RH
Drying shrinkage on concrete – effect of MIC

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Phenomenon in which cement paste shrinks at a constant
temperature without any change in weight ,consequence
of Le Chatelier contraction.
Very important in low W/C concrete
Autogenous Shrinkage
CONCRETE SELF-DESICCATES

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Hydration
Volumetric contraction
External
supply of
water
No Yes
Self desiccation
Menisci
Autogenous
shrinkage
Pores
and capillaries
connected?
No menisci
Noautogenous
shrinkage
No
Yes
Autogenous and Drying Shrinkage

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Set-up measurement for early shrinkage

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Holcim (Netherlands) – Concrete test

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• OPC suffers from autogenous shrinkage
• MIC suppress autogenous shrinkage
• Long term durability affected by autogenous shrinkage
-600
-500
-400
-300
-200
-100
0
100
0 4 8 12 16 20 24
Time (hour)
OPC
50% slag
50% slag + SC
30% fly ash
Shrinkage
(
m
m/m)
Autogenous Shrinkage – effect of MIC
• W/C = 0.30

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 Thermal shrinkage is a consequence of cement heat of hydration.
 After 2 or 3 hours (dormant period), temperature of concrete is
increasing up to a maximum value after about 10 hours or more,
corresponding to the end of setting time, which is depending on
many parameters
 Concrete tends to expand during this first phase and behaves like a
relative soft paste.
 After that, a cooling phase of the already hardening and stiffer
concrete is beginning
 This second phase is more or less long and steep, depending on
many parameters which are :
 mass of concrete (thickness of construction)
 type of formwork and removal time
 environmental conditions and curing measures
 type and dosage of cement
Thermal Shrinkage

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Thermal shrinkage
0
10
20
30
40
0 1 2 3 4 5 6 7 8
Days
Temperature
rise,
°C
Inside
Surface
Unprotected surface
cools fast
Form
removal
DT > 20°C (surface cracking)
DT < 20°C
no cracking

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Learning objectives

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Factors influencing concrete shrinkage
Actual situation
 Minimize shrinking inducing components
 water content
 volume of paste
 shrinking aggregates
 Reduction of shrinkage with admixtures
 SRA, SCA
 limited (lower strength and more expensive)
 Good curing

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Concrete design parameters
H2O
Cement
Modern concrete

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Effect of mineral components on water demand
0.3
0.35
0.4
0.45
0.5
0.55
0.6
30% PZ 15% PZ OPC
3200
15% FA 30% FA 30%
Slag
50%
Slag
Binder type
W/B
ratio
Same flow 110 +/-5%

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Aggregate volume

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0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
S
a
n
d
s
t
o
n
e
S
l
a
t
e
G
r
a
n
i
t
e
L
i
m
e
s
t
o
n
e
Q
u
a
r
t
z
Drying
shrinkage
after
1
year
(%)
 Aggregates with a higher
modulus of elasticity and
lower compressibility
respectively give a lower
shrinkage
 Aggregate with high
absorption may have high
shrinkage drying
 Clay coatings on aggregate
can increase shrinkage by
up to 70%.
Aggregate type

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(Breugel and Vries, 1998)
 Water saturated lightweight aggregates
Lightweight aggregates

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(Weiss and all, 1998)
Admixtures
 Shrinkage reducing admixture

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Effect of cement admixtures - 15% FA - W/C = 0.39
-800
-600
-400
-200
0
200
400
600
800
1000
1200
7d 14d 28d 90d
Time (day)
Shrinkage
(mm/m)
15% FA
SCA
SRA

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Effect of cement admixtures - 30% slag - W/C = 0.38
-800
-600
-400
-200
0
200
400
600
800
1000
1200
7d 14d 28d 90d
Time (day)
Shrinkage
(mm/m)
30% slag
SCA
SRA

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Compressive strength - 15% FA - W/C = 0.39
0
10
20
30
40
50
60
70
80
1d 7d 28d 90d
Time (days)
Compressive
strength
(MPa)
15% FA
SCA
SRA

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Compressive strength - 30% slag - W/C = 0.38
0
10
20
30
40
50
60
70
80
1d 7d 28d 91d
Time (days)
Compressive
strength
(MPa)
30% slag
SCA
SRA

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-0.02
-0.01
0
0.01
0.02
0.03
0.04
0 14 28 42 56
NC-1
HPC-1
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0 14 28 42 56
NC-2
HPC-2
28-D Water curing CC + 14-D Water curing
(Baalbaki, 1996)
Curing
 Effect of water curing and curing compound
on shinkage

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Curing
 Proper curing of all concrete, especially concrete
containing MICs should commence immediately after
finishing
 7 days moist cure or membrane cure should be applied
for concretes with normal dosages of most MICs
 Moist-cured surfaces should dry out slowly after the
curing period to reduce surface crazing

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How to get a low shrink concrete?
 The role of the following players is important to ensure a
low shrink concrete:
 Designer: consider geometry of concrete element, restraint
conditions to avoid tensile stress
 Materials technologist: select materials & design concrete
with minimum volumetric changes, adequate workability
and strength
 Constructor: place, consolidate and cure the concrete in a
proper way

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Where are we today ?
Two routes to reduce shrinkage
 Shrinkage reducing
and compensating
admixture
+ core business
+ low cost
+ high value added
+ differentiation
- standards
Trend Shrinkage value
$/MPa
$/performance
0 mm/m
300 mm/m
1000 mm/m
today
 Low shrink
MIC cements
+ introduced
+ easy to use
- experience needed
- expensive

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Findings on cement design
 Less drying shrinkage with coarser cement
 2 – 3% of entrained air reduce overall concrete shrinkage
 Drying shrinkage increases, with slag content above 30%, having a
Blaine fineness of 5000 cm2/g
 Up to 30% of fly ash the drying shrinkage is not influenced
 Higher compressive strength at 2 and 28d gives more shrinkage
[Product Handbook, DMA-Holcim (FBNL)]
 Drying shrinkage increase when using bad pozzolan
 SCA cancel out the drying shrinkage
 SRA reduces of about 30% the drying shrinkage but lower also the
strength of about 20%

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