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
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In Search of Crack-Free Concrete · Motivation: Early slab cracks Early age pavement cracking is a persistent problem Runway at Willard Airport (7/21/98) Early cracking within 18
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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
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