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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. Motivation: Early slab cracks. Early age pavement cracking is a persistent problem

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In search of crack free concrete current research on volume stability and microstructure

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


Motivation early slab cracks
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


Motivation: Slab curling

SLAB CURLING

P

HIGH STRESS

Material (I)

Material (II)


Material properties are key
Material properties are key

  • Properties are time-dependent

  • Stiffness develops sooner than strength

Ref: After Olken and Rostasy, 1994


A materials approach
A “materials” approach

  • Understand…

    • Cement

    • Microstructure

    • Source of stress

    • Nature of restraint

    • Structural response


Overview
Overview

Early Age Volume Change

Thermal

Shrinkage

Creep

Swelling

External Influences

Heat release from hydration

Autogenous shrinkage

External drying shrinkage

Basic creep

Drying creep

Redistribution of bleed water or water from aggregate

Early hydration

Chemical shrinkage

Cement hydration


Now put them all together
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
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

Applying knowledge to potential materials

Methods for quantifying material properties that affect volume change and thus cracking potential


Methods of measurement
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
Measurements (cont’d)

  • Creep

    • Tensile – uniaxial loading frames

    • Compressive – creep frames





Summary
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
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


In search of crack free concrete emerging approaches
In search of crack free concrete: Emerging approaches

  • Shrinkage reducing admixtures

    • Reduces drying or autogenous shrinkage

  • Saturated light-weight aggregate

    • Reduces autogenous shrinkage

  • Fibers

    • 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
Thermal dilation

  • Some sources of thermal change:

    • Ambient temperature change

    • Solar radiation

    • Hydration (exothermic reaction)


Heat of hydration
Heat of hydration

Hardening

Setting

Dormant


Mechanisms of thermal dilation
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



Thermal problems
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
Thermal gradient issues

  • Highly restrained slab

     Cracking

  • Low restraint in slab

     Curling + Wheel Load  Cracking


Can construction practices counteract thermal stress
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
Shrinkage

  • Usually divided into components:

    • Chemical shrinkage

    • Internal drying shrinkage

      • Known as Autogenous Shrinkage

    • External drying shrinkage


Chemical shrinkage
Chemical shrinkage

Typical values for PC: 7-10%

Ref: Neville, 1995


Autogenous shrinkage particularly a problem of hpc
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




The traditional shrinkage external drying shrinkage
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

Hydration product

Hydration product

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


Rh stress relationship

Internal Drying

Shrinkage Red.

External Drying

Adm. (SRA)

Hydration

Surface tension

Temperature

Pore Radius

Radius of meniscus

Salt Concentration

curvature

Mechanical

Physicochemical

equilibrium

Equilibrium

g

-

g

2

ln(

RH

)

RT

2

=

-

=

p

"

p

'

r

v

'

r

Underpressure in

Internal Relative

Kelvin

-

Laplace

pore fluid

Humidity Change

Equation

-

ln(

RH

)

RT

-

=

p

"

p

'

v

'

RH-stress relationship

  • Kelvin-Laplace equation allows us to relate RH directly to capillary stress development

    • Drying shrinkage

    • Autogenous shrinkage

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
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
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
Evidence of surface drying damage

Hwang & Young ’84

Bisshop ‘02


External restraint stress superposed

Overall stress gradient

in restrained concrete

Free shrinkage

drying stresses

Applied restraint

stress

T=0

ft

+

+

+

+

+

-

External restraint stress superposed


Time to fracture under full restraint related to gradient severity
Time to fracture (under full restraint) related to gradient severity

Failed at 7.9 days

Failed at 3.3 days


Fracture related to gradient severity
Fracture related to gradient severity severity

Load removed from

B-44 prior to failure

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
Creep: our friend? severity

  • 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
Creep comes in two flavors severity

  • 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
Swelling severity

  • 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


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