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
ILLINOISUniversity of Illinois at Urbana-Champaign
ILLINOISUniversity of Illinois at Urbana-Champaign
Motivation: Early slab cracks
Early age pavement
cracking is a
persistent problem
Runway at Willard
Airport (7/21/98)
Early cracking within
18 hrs and additional
cracking at 3-8 days
HIGH STRESS
SLAB CURLING P
Motivation: Slab curling
Material (I) Material (II)
Material properties are key
Properties are
time-dependent
Stiffness
develops
sooner than
strength
Ref: After Olken and
Rostasy, 1994
A “materials” approach
Understand… Cement
Microstructure
Source of stress
Nature of restraint
Structural response
Chemical
shrinkage
Overview
Early Age Volume Change
Thermal Shrinkage Creep Swelling
External
Influences Autogenous
shrinkage
External drying
shrinkage Basic creep Drying creep
Redistribution
of bleed water
or water from
aggregate
Early hydration Heat release
from hydration
Cement
hydration
Now put them all together…
…and you have a very complex problem
All of the possible types of volume change are
interrelated. For example:
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
A challenging problem
Methods that improve performance in regard to one issue may exacerbate another. For example:
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
Volume change: Embedded strain gages
LVDT
Dial gage
Environmental stimuli Temperature
Thermocouple or thermistor
Internal or external RH
Embeddable RH sensor
Field ready!
Measurements (cont’d)
Creep Tensile – uniaxial
loading frames
Compressive – creep frames
Examples of field
instrumentation
Bridge Deck Temperatures –
1st week
I-70/Big Creek - Midspan, center
10
15
20
25
30
35
40
45
50
55
60
8/27 8/28 8/29 8/30 8/31 9/1 9/2 9/3
Date
Tem
pera
ture
(D
eg C
)
Air A1 A2 A3 A4 A5
I-70/Big Creek - Pier, center
10
15
20
25
30
35
40
45
50
55
60
8/27 8/28 8/29 8/30 8/31 9/1 9/2 9/3
Date
Tem
pera
ture
(D
eg C
)
Air B1 B2 B3 B4 B5
-600
-500
-400
-300
-200
-100
0
100
8/30 9/6 9/13 9/20 9/27 10/4 10/11 10/18 10/25
Date
Str
ain
(m
e)
0
10
20
30
40
50
60
70
Tem
pera
ture
(Deg C
)
B1 - Bot
B2 - Middle
B3 - Top
B4 - Trans
Temperature
Strain in bridge deck
Summary
The primary causes of volume change have been discussed 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
Limit paste content
Aggregates usually are volume stable
Use moderate w/c
Limits overall shrinkage (autogenous + drying)
Avoids overly brittle material
Use larger, high quality aggregates
Improves fracture toughness
Shrinkage reducing admixtures
Reduces drying or autogenous shrinkage
Saturated light-weight aggregate
Reduces autogenous shrinkage
Fibers
Reduces drying or autogenous shrinkage
In search of crack free concrete:
Emerging approaches
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
Some sources of thermal change:
Ambient temperature change
Solar radiation
Hydration (exothermic reaction)
Heat of hydration
Setting
Hardening
Dormant
Mechanisms of thermal dilation
3 components: 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
Hydration heat early age cracking
on cool-down
Thermal gradients High restraint stresses at top of pavement cracking
Low restraint curling cracking under wheel loading
Buckling
Thermal gradient issues
Highly restrained slab
Cracking
Low restraint in slab
Curling + Wheel Load
Cracking
Can construction practices
counteract thermal stress?
Construct during low ambient heat
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
Usually divided into components:
Chemical shrinkage
Internal drying shrinkage
Known as Autogenous Shrinkage
External drying shrinkage
Chemical shrinkage
40
20
60
33.5
7
30.8
61.6
12
24
3.77.4
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0% 50% 100%
Hydration degree
Vo
lum
ic p
erc
en
tag
e
voids
gel water
Hydrates
Capillary water
Anhydrous Cement
Ref: Neville, 1995
Typical values for PC: 7-10%
Autogenous shrinkage:
Particularly a problem of HPC
Internal drying (self-desiccation)
associated with hydration
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
-250
-200
-150
-100
-50
0
50
0 20 40 60 80 100
Age (d)
Auto
genous S
hrin
kage (
10
-6 m
/m)
OPC1, w/c = 0.40SCC1, w/c = 0.39SCC2, w/c = 0.33SCC3, w/c = 0.41SCC4, w/c = 0.32HPC1, w/c = 0.25SCC2-2SCC2-slag
Autogenous shrinkage: why
only low w/c?
0.50 0.50
w/cw/c
0.30 0.30
w/cw/c
Cement grains
initially separated by
water
Initial set locks in
paste structure
Chemical shrinkage
ensures some porosity
remains even at
“Extra” water remains in
small pores even at =1
Pores to 50 nm
emptied
Internal RH and pore fluid
pressure reduced as smaller
pores are emptied
Autogenous Autogenous
shrinkageshrinkage
Increasing degree of hydration
0.50 0.50
w/cw/c
0.30 0.30
w/cw/c
Cement grains
initially separated by
water
Initial set locks in
paste structure
Chemical shrinkage
ensures some porosity
remains even at
“Extra” water remains in
small pores even at =1
Pores to 50 nm
emptied
Internal RH and pore fluid
pressure reduced as smaller
pores are emptied
Autogenous Autogenous
shrinkageshrinkage
Increasing degree of hydration
The “traditional” shrinkage:
external drying shrinkage
Occurs when pore water diffuses to
surface
Risk increases as diffusivity (porosity)
goes up
Reason to place UPPER limit on w/c (or
have minimum strength requirement)
Mechanism of shrinkage
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
Surface tension Temperature Pore Radius
Radius of meniscus
curvature
Underpressure in
pore fluid
Internal Relative
Humidity Change
Internal Drying
External Drying
Hydration
Physicochemical
Equilibrium
Mechanical
equilibrium
Kelvin - Laplace
Equation
Shrinkage Red.
Adm. (SRA)
Salt Concentration
r p p
g 2 ' " -
'
) ln( 2
v
RT RH
r
-
g
RH-stress relationship
Kelvin-Laplace
equation allows
us to relate RH
directly to
capillary stress
development
Drying
shrinkage
Autogenous
shrinkage
'
) ln( ' "
v
RT RH p p
- -
p” = vapor pressure
p’ = pore fluid pressure
RH = internal relative humidity
R = Universal gas constant
v’ = molar volume of water
T = temperature in kelvins
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
Like thermal dilation…
Shrinkage gradients
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
ft
+ + -
Free shrinkage drying stresses
+ +
Overall stress gradient in restrained concrete
+
Applied restraint stress
T=0
Time to fracture (under full restraint)
related to gradient severity
0
1
2
3
4
5
6
0 10 20 30 40 50 60 70
Specimen Width (mm)
Str
ess (
MP
a)
A-44A-44 AverageB-44B-44 AverageC-44C-44 AverageD-44D-44 Average4141 Average3838 Average3232 Average
Failed at 7.9 days
Failed at 3.3 days
Fracture related to gradient
severity
0
1
2
3
4
5
6
2 3 4 5 6 7 8 9
Failure Age (Days)
Diffe
ren
tia
l S
tre
ss (
MP
a)
A-44
B-44
C-44
D-44
41
38
32
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).
Load removed from B-44 prior to failure
Creep: our friend?
In restrained concrete, creep alleviates
tensile stresses
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
Basic creep
Time-dependent deformation that occurs
in all loaded concrete
Drying creep
Additional creep that occurs when load is
present during drying
Occurs for both tensile and compressive
loads
Swelling
Bleed water readsorption
As water is consumed during hydration,
bleed water may be sucked back in
Crystallization pressure
Certain hydration products force
expansion during formation
