Air Voids Characterization Of Concrete

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Air Voids Characterization Of Concrete

• Presented By
• Chetan Hazaree,
• Graduate Student (Geotech/Materials),
• Department of Civil, Construction and
  Environmental Engineering Iowa State
  University, Ames, Iowa
• Professors Drs. Kejin Wang And Halil Ceylan
• Presented At
• 2006 Area III/IV HEEP Conference Ames,
  Iowa April 11-13, 2006

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Presentation Outline

• Problem Statement
• Objectives
• Methods of characterizing the air content in
  concrete
• Conclusions

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Problem statement

• Freeze/thaw cycles lead to damage and
  deterioration of concrete and other materials
• Every year we are spending billions of tax
  payers dollars wither for maintaining or
  rebuilding our infrastructure
• Air entrainment in concrete is known to provide
  the necessary and effective protection to
  concrete in the freeze/thaw environment
• The mechanism by which the air provides
  protection is not fully known

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Problem statement(Hover)
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Problem statement

• Presence of air is not the only condition
  required for effective protection, but its
  morphology, size and spatial distribution are
  critical in providing the resistance
• Measurement of helpful parameters in concrete is
  critical in taking timely action from QA/QC
  perspective
• Hence timely quantification of such parameters is
  essential

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Problem statement(ACPA website)
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Problem statement(USBR, Concrete Lab Report,
C824)
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Objectives

• Review of the significant methods of air void
  characterization
• Scope for future work

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Need for comprehensive specs Characterizing air
voids parameters(References Price, Concrete96)

• Specs for effective protection of concrete will
  require
• Total air content (volume )
• Spacing factor Maximum distance in cement paste
  from the periphery of air void (mm or in.)
• Specific Surface Area of voids per unit volume
  of air voids (mm2/mm3 or mm-1 or in.-1)
• Paste/Air ratio Ratio of volume of hardened
  paste to the air voids
• Small air voids in paste with chord length
  gt0.35mm in paste (volume )

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Methods of characterizing air void system

• Hardened Concrete
• Stereological Microscopy
• Flatbed scanner
• Computer Tomography
• X-ray microscopy
• Fresh Concrete
• Air void analyzer
• Fiber optic airmeter

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Stereological Microscopy(References Snyder,
ACBM, 98 Attiogbe, ACI M J., 90, 93, 96
Philleo, CCA, 83 Pleau and Pigeon, CCA, 96 Lu
and Torquato, Phy Review, 91, 92 ASTM C457
EN480-11 CSA A23.1

• Air voids spacing or amongst air voids or the
  least water travel distance to the lab
  Freeze/Thaw tests
• ASTM C457 Linear traverse and Point count
• Equations based on Powers theory and then
  modified by Philleo, Attiogbe, Pleau and Pigeon,
  Lu and Torquato (refer to literature for details)
• Equations proposed on oversimplification of
  random distribution of air voids
• Cubic lattice model Proven to be unrealistic
• Effects due to aggregates are neglected

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Stereological MicroscopySpacing Equations (Cont)

• Nomenclature
• n no. of air voids per unit volume
• A air void volume fraction
• p Paste volume fraction
• a specific surface area of spheres
• r sphere radii
• f(r) sphere radii probability density function
• Rk expected value for radius distribution
• s spacing factor parameter

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Stereological MicroscopySpacing Equations (Cont)

• Powers spacing equation
• ? p/aA.(for p/A lt 4.342)
• ? 3/a1.4(p/A1)1/3-1.(for p/A 4.342)
• Provide ? less than specific value for
  protection
• Philleo spacing factor equation
• F(s) 1-exp-4.19x3-7.8x2ln(1/p)1/3-4.84xln(1/p)
  2/3
• Provide x sn1/3 for providing sufficient
  protection

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Stereological MicroscopySpacing Equations (Cont)

• Attiogbes total protected fraction (G) equation
• G 5.731(a3)/4a/(p/A1) 1
• Useful for air void radii distribution from
  mean spacing
• Pleau and Pigeon
• k(s) o?8 h (rs) f(r) T (rs) dr
• Integrate to obtain the probability density
  function using Hertz distribution

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Stereological MicroscopySpacing Equations (Cont)

• Lu and Torquato Equations
• Mapping of n-point correlation functions to the
  systems of poly-dispersed sphere radii an can be
  extended to mono-sized spheres
• Mean nearest surface-surface distance lp(R)
• lp(R) R?8 ep(?,R) d?

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Stereological MicroscopySpacing Equations
(References Snyder, ACBM, 98)

• Snyder compared significant spacing equations
  (Powers, Philleo, Attiogbe, Pleau and Pigeon, Lu
  and Torquato) using a computer model.
• Proximity of paste to voids and voids to one
  another
• Considerations for Air void radii distribution,
  Paste-void proximity, Void-void proximity,
  Particle dynamics and equilibrium and spatial
  statistics of air voids
• Lu and Torquato equation performed well
• Powers equation needs further testing and
  modifications

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Stereological Microscopy

• Disadvantages
• The basic model is not potent to exactly
  determine the required parameters
• Other equations require refinement for better
  prediction and modeling of the air voids system
• Operator specific, time consuming and incapable
  of being used for timely corrective action
• A spacing factor of 0.2mm (0.008 in.) is
  recommended for better protection
• Repeatability and reproducibility

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Automated Stereological Microscopy(Ansari, ACI M
J, 2005 Elsen, CCR, 2001 )

• Automated-Air-Void-Analyzer test equipment
• computerized control unit (PC)
• 19 color monitor
• a video camera
• a microscope objective mounted on a moving stage,
  
• analysis software operating
• Can perform linear traverse or modified count
  method
• Surface needs to be contrast enhanced

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Automated Stereological Microscopy (Cont)
(www.concrete-experts.com)
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Automated Stereological Microscopy (Cont)
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Automated Stereological Microscopy (Cont)

• European study (13labs) suggested that this
  technique
• is faster, more reliable and operator independent
• can be problematic for high amt of porous sand
• needs detailed guidelines for sampling and
  analysis
• Ansari, et. Al used different air content levels
  and found
• relative errors w/I acceptable limits w.r.t.
  Manual
• resolution of the system needs to be improved
• need for accelerated sample preparation methods

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Flatbed Scanner(Scott, 97 Carlson, et.al.,
TRB, 06 Zhang, et.al, ACI M J., 05
Kasperkiewicz and Zalocha, CCR, 05)

• The sample is placed on the glass plate of the
  flatbed scanner
• A camera identical to the previous method is used
• Modifications may be/have been done in the optics
  and graphics of the assembly
• Dieing is used for coloring the cement paste
  followed by application of zinc paste

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Flatbed Scanner (Cont) (Kasperkiewicz and
Zalocha, CCR, 05)
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Flatbed Scanner (Kasperkiewicz and Zalocha, CCR,
05)
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Flatbed Scanner Advantages

• Preparation of only one side of the specimen
• Misidentification and misinterpretation of
  certain transparent grains is avoided
• Uses a steady source of light
• Comparatively less costlier than the stereoscopic
  microscope
• It is possible to automatically measure the paste
  content on the surface of concrete

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Computed Tomography(Wong and Chau, CCR, 05
Fratta, et.al, GeoFrontier 05 Masad, et.al,
ASCE J of M in CE, 02 Shah, et.al,
ACI M J, 01 Weise, 00 Santamarina and Fratta
98 Dennis 97 Martz, et.al, ACI M J, 93 )

• Known as a useful NDT for analyzing various
  materials and civil engineering structures
• 3D images can be obtained to study concrete and
  obtain complementary information
• Provides an integration of slices taken
  throughout the specimen
• Employs X ray source and detector along with
  sample holding assembly capable of rotating in
  horizontal direction and moving in vertical
  direction
• Multiple projections could be obtained using
  modifications

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Computed Tomography (Cont) Working
Principle(Massad, et.al. 02)
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Computed Tomography (Cont)Concrete Image(Wong
and Chau 05)
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Computed Tomography (Cont)Conclusions

• Requires calibration to real time data
• Accuracy depends on the resolution of the CT
  scanner
• Can be applied to study the spatial distribution
  of aggregates and the anisotropic effect of
  loading on the air voids distribution
• No special sample preparation is required
• Sensitivity and calibration of the equipment are
  important
• Multiple 2D projections can be obtained
  simultaneously

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Other techniques(Monteiro, et. al, ACI M J, 02
Vaughan, et.al, ASCE J of Hydraulic Engg, 02
Lawler, et.al, ACI M J, 01)

• Computed microtomography can be used in obtaining
  high resolution images
• Using analytical X ray microscope for mapping and
  analyzing the air voids in concrete
• Air void morphology can be obtained using low
  temperature scanning electron microscopy
• Engineering applicability and calibration of
  these emerging techniques needs to be assessed

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Fresh State Air void Analyzer(Magura,
FHWA-SA-96-062, 96 Magura, CI, 96 Price,
Concrete, 96 Elsen, et.al, CCR, 94 FHWA
website ACPA website)

• Developed in 1990s by the Dans Beton Teknik
• Provides the air voids parameter of fresh
  concrete, hence useful in quality control and
  quality assurance of concrete
• Takes about 45 min for one sample (one specimen)
• Implemented by Denmark, Belgium, German, Czech
  Republic, Iceland, Switzerland, Italy, Spain

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Air void analyzer Working Principle

• Buoyancy principle
• Stokes law
• Larger bubbles will rise faster
• smaller bubbles will rise slowly

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Air void analyzer Working assembly
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Air void analyzer Procedure
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Air void analyzer Procedure
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Air void analyzer Procedure
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Air void analyzer Procedure
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Air void analyzer Results
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Air void analyzer Correlation
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Air void analyzer Comparison with ASTM C457
Parameter Value
Air content lt 2
Specific surface Similar
Spacing Similar
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FHWA Initiative

• Initiated the study in 90s
• As on date, at least 16 States used AVA on a
  limited basis in 2005, including Arkansas,
  California, Delaware, Iowa, Kansas, Minnesota,
  Missouri, Nebraska, Nevada, New York, North
  Carolina, North Dakota, Oklahoma, Pennsylvania,
  Texas, and Utah.

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Air void analyzer Drawbacks

• Sample excludes volume fractions greater than 6
  mm
• Sample quantity is very small
• Cannot be used directly on-site, needs some
  stable platform
• Sampling is critical
• Water temperature range is too narrow 21-25oC
• Repeatability, precision and accuracy of the
  method is yet to be established clearly

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Conclusions

• Various methods for characterizing air contents
  Hardened concrete Linear traverse, point count,
  automated digital image, flatbed scanner,
  Computed tomography, x ray microscopy
• Fresh concrete Air void analyzer
• Equations need modifications and remodeling
• Manual methods will not be used in near future
• Automated methods are time saving, but are based
  on the same models
• Flatbed scanner is resolution dependent, but
  offers better operational ease

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Conclusions (Cont)

• Computed tomography needs work on resolution and
  calibration
• X-ray microscopy has a potential for application.
  Needs further work
• Repeatability, precision and accuracy are still
  not resolved fully
• Air void analyzer has proved to be very helpful,
  but needs work on the precision, accuracy and
  repeatability

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Thank you!!!