In Search of Crack-Free Concrete - University of Illinois at ...

frizzflowerUrban and Civil

Nov 29, 2013 (3 years and 10 months ago)

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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
ILLINOIS
University 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


1
st

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
Temperature (Deg 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
Temperature (Deg 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
Strain (
m
e)
0
10
20
30
40
50
60
70
Temperature (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

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)
Autogenous Shrinkage (10
-6
m/m)
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

Autogenous shrinkage: why
only low w/c?

0.50
0.50
w/c
w/c
0.30
0.30
w/c
w/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
shrinkage
shrinkage
Increasing degree of hydration
0.50
0.50
w/c
w/c
0.30
0.30
w/c
w/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
shrinkage
shrinkage
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

f
t

+

+

-

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)
Stress (MPa)
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
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)
Differential Stress (MPa)
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