Transcript Slide 1

In Search of Crack-Free Concrete:
Current Research on
Volume Stability and Microstructure
David A. Lange
University of Illinois at Urbana-Champaign
Department of Civil & Environmental Engineering
ILLINOIS
University of Illinois at Urbana-Champaign
Motivation: Early slab cracks

Early age pavement
cracking is a
persistent problem
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
Runway at Willard
Airport (7/21/98)
Early cracking within
18 hrs and additional
cracking at 3-8 days
Motivation: Slab curling
SLAB CURLING
P
HIGH STRESS
Material (I)
Material (II)
Material properties are key
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Properties are
time-dependent
Stiffness
develops
sooner than
strength
Ref: After Olken and
Rostasy, 1994
A “materials” approach

Understand…
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Cement
Microstructure
Source of stress
Nature of restraint
Structural response
Overview
Early Age Volume Change
Thermal
External
Influences
Heat release
from hydration
Shrinkage
Autogenous
shrinkage
Creep
External drying
shrinkage
Basic creep
Chemical
shrinkage
Cement
hydration
Drying creep
Swelling
Redistribution
of bleed water
or water from
aggregate
Early hydration
Now put them all together…
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…and you have a very complex problem
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All of the possible types of volume change are
interrelated. For example:
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Temperature change affects shrinkage, hydration
reaction (i.e. crystallization, chemical shrinkage, pore
structure)
Even worse, the mechanisms for each type
often share the same stimuli. For example:

Drying effects shrinkage and creep
The goal: optimization
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A challenging problem
Methods that improve performance in
regard to one issue may exacerbate
another. For example:
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Lowering w/c is known to reduce drying
shrinkage and increase strength, but…
Creep is reduced, autogenous shrinkage
is increased, and material is more brittle.
All BAD.
Applying knowledge to
potential materials
Methods for quantifying material
properties that affect volume
change and thus cracking
potential
Methods of measurement
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Volume change:
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Embedded strain gages
LVDT
Dial gage
Environmental stimuli
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Temperature
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Thermocouple or
thermistor
Internal or external RH
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Embeddable RH
sensor
Field ready!
Measurements (cont’d)
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Creep
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Tensile – uniaxial
loading frames
Compressive – creep
frames
Examples of field
instrumentation
Bridge Deck Temperatures –
1st week
I-70/Big Creek - Midspan, center
I-70/Big Creek - Pier, center
60
60
Air
55
A1
A2
A3
A4
A5
B1
B2
B3
B4
B5
50
Temperature (Deg C)
Temperature (Deg C)
50
45
40
35
30
25
45
40
35
30
25
20
20
15
15
10
8/27
Air
55
8/28
8/29
8/30
8/31
Date
9/1
9/2
9/3
10
8/27
8/28
8/29
8/30
8/31
Date
9/1
9/2
9/3
Strain in bridge deck
100
70
B1 - Bot
B2 - Middle
0
B3 - Top
60
B4 - Trans
Temperature
Strain (me)
50
-200
40
-300
30
-400
20
-500
10
-600
8/30
9/6
9/13
9/20
9/27
Date
10/4
10/11
10/18
0
10/25
Temperature (Deg C)
-100
Summary
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The primary causes of volume change have
been discussed
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Along with ideas for minimization and
optimization
The goal of our research is to provide info
that aids in the development of specs that
minimize problems due to concrete volume
change
Ultimate goal: crack free concrete
Immediate goal: maximizing joint spacing
and minimizing random cracking
In search of crack free concrete:
Basic principles
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Limit paste content
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Use moderate w/c
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Aggregates usually are volume stable
Limits overall shrinkage (autogenous +
drying)
Avoids overly brittle material
Use larger, high quality aggregates
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Improves fracture toughness
In search of crack free concrete:
Emerging approaches
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Shrinkage reducing admixtures
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Saturated light-weight aggregate
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Reduces drying or autogenous shrinkage
Reduces autogenous shrinkage
Fibers
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Reduces drying or autogenous shrinkage
END
Upcoming events sponsored by CEAT:
Brown Bag Lunches -April 7 -- Marshall Thompson
May 5 -- Jeff Roesler
June 9 -- Erol Tutumluer
July 7 -- John Popovics
Workshop on Pavement Instrumentation & Analysis
May 17 at UIUC with FAA participants
Thermal dilation
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Some sources of thermal change:
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Ambient temperature change
Solar radiation
Hydration (exothermic reaction)
Heat of hydration
Hardening
Dormant
Setting
Mechanisms of thermal dilation
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3 components:
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Solid dilation – same as dilation of any solid
Hygrothermal dilation – change in pore fluid
pressure with temperature
Delayed dilation (relaxation of stress)
Linked to moisture content, but dominated
by aggregate CTD
CTD of concrete ~10 x 10-6/C
Timing of set & early heat
Thermal problems
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Hydration heat  early age cracking
on cool-down
Thermal gradients
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High restraint  stresses at top of pavement  cracking
Low restraint  curling  cracking under wheel loading
Buckling
Thermal gradient issues
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Highly restrained slab
 Cracking
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Low restraint in slab
 Curling + Wheel Load
 Cracking
Can construction practices
counteract thermal stress?
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Construct during low ambient heat
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Morning hours, moderate seasons
Use wet curing
Use low fresh concrete temperatures
Use blankets or formwork that reduce RATE of cooling
Reduce joint spacing in pavements and reduce restraint
of structure
Avoid early thermal shock upon form removal
Shrinkage
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Usually divided into components:
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Chemical shrinkage
Internal drying shrinkage
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Known as Autogenous Shrinkage
External drying shrinkage
Chemical shrinkage
Volumic percentage
Typical values for PC: 7-10%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
3.7
7.4
12
24
60
30.8
33.5
61.6
20
7
50%
100%
40
0%
Hydration degree
Ref: Neville, 1995
voids
gel water
Hydrates
Capill ary water
Anhydrous Cement
Autogenous shrinkage:
Particularly a problem of HPC
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Internal drying (self-desiccation)
associated with hydration
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Only occurs with w/c below ~ 0.42
Same mechanism as drying shrinkage
Reason to place LOWER limit on w/c
Traditional curing NOT very effective
Autogenous Shrinkage
Autogenous Shrinkage (10-6 m/m)
50
OPC1, w/c = 0.40
SCC1, w/c = 0.39
SCC2, w/c = 0.33
SCC3, w/c = 0.41
SCC4, w/c = 0.32
HPC1, w/c = 0.25
SCC2-2
SCC2-slag
0
-50
-100
-150
-200
-250
0
20
40
60
Age (d)
80
100
Autogenous shrinkage: why
only low w/c?
“Extra” water remains in
small pores even at =1
0.50
w/c
Cement grains
initially separated by
water
Initial set locks in
paste structure
Chemical shrinkage
ensures some porosity
remains even at 
0.30
w/c
Autogenous
shrinkage
Pores to 50 nm
emptied
Increasing degree of hydration
Internal RH and pore fluid
pressure reduced as smaller
pores are emptied
The “traditional” shrinkage:
external drying shrinkage
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Occurs when pore water diffuses to
surface
Risk increases as diffusivity (porosity)
goes up
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Reason to place UPPER limit on w/c (or
have minimum strength requirement)
Mechanism of shrinkage
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Both autogenous and
drying shrinkage dominated
by capillary surface tension
mechanism
As water leaves pore
system, curved menisci
develop, creating reduction
in RH and “vacuum”
(underpressure) within the
pore fluid
Hydratio
n
product
Hydration
product
RH-stress relationship
Internal Drying
Shrinkage Red.
Adm. (SRA)
External Drying
Hydration
Surface tension
Temperature
Radius of meniscus
curvature
Mechanical
equilibrium
p"- p ' 
2g
r
Underpressure in
pore fluid
Pore Radius
Salt Concentration
Physicochemical
p” = vapor pressure
Equilibrium
p’ = pore fluid pressure 2g - ln( RH ) RT

RH = internal relative humidity
r
v'
R = Universal gas constant
v’ = molar volume of water
T =Kelvin-Laplace
temperature in kelvins
Internal Relative
Equation
p"- p ' 
- ln( RH ) RT
v'
Humidity Change
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Kelvin-Laplace
equation allows
us to relate RH
directly to
capillary stress
development
 Drying
shrinkage
 Autogenous
shrinkage
Visualize scale of mechanism
Capillary stresses present in pores with radius between 2-50 nm
Note the
dimensions
•C-S-H makes up ~70% of hydration product
•Majority of capillary stresses likely present within C-S-H network
*Micrograph take from Taylor “Cement Chemistry” (originally taken by S. Diamond 1976)
Shrinkage problems
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Like thermal dilation…
Shrinkage gradients
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High restraint  tensile stresses on top
of pavement  micro and macrocracking
Low restraint  curling  cracking under
wheel loading
Bulk (uniform) shrinkage  cracking
under restraint
Evidence of surface drying
damage
Hwang & Young ’84
Bisshop ‘02
External restraint stress
superposed
Free shrinkage
drying stresses
Applied restraint
stress
Overall stress gradient
in restrained concrete
ft
+
-
+
+
T=0
+
+
Time to fracture (under full restraint)
related to gradient severity
6
A-44
A-44 Average
B-44
B-44 Average
C-44
C-44 Average
D-44
D-44 Average
41
41 Average
38
38 Average
32
32 Average
5
Stress (MPa)
4
Failed at 7.9 days
3
2
1
Failed at 3.3 days
0
0
10
20
30
40
Specimen Width (mm)
50
60
70
Fracture related to gradient
severity
6
A-44
B-44
C-44
D-44
41
38
32
Differential Stress (MPa)
5
4
Load removed from
B-44 prior to failure
3
2
1
0
2
3
4
5
6
7
8
9
Failure Age (Days)
Grasley, Z.C., Lange, D.A., D’Ambrosia, M.D., Internal Relative Humidity and Drying Stress Gradients in
Concrete, Engineering Conferences International, Advances in Cement and Concrete IX(2003).
Creep: our friend?
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In restrained concrete, creep alleviates
tensile stresses
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Reduces tendency to crack
Many possible mechanisms including
moisture movement, microscale particle
“sliding”, microcracking
Difficult to measure, quantify, and account
for in pavement and mixture design
Creep comes in two flavors
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Basic creep
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Time-dependent deformation that occurs
in all loaded concrete
Drying creep
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Additional creep that occurs when load is
present during drying
Occurs for both tensile and compressive
loads
Swelling
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Bleed water readsorption
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As water is consumed during hydration,
bleed water may be sucked back in
Crystallization pressure
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Certain hydration products force
expansion during formation