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Neithalath , Spring 2006, CE 455/555 Structural Damage: Assessment, Repair, and Strengthening
NDT of Concrete - V
Nuclear Methods
Thermography
Radar
Acoustic Emission
Carbonation
Lectures 18,19
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Nuclear methods
Gaining information about a test object dueto interactions between high-energyelectromagnetic radiation and the material
Radiometry
measuring the intensity of electromagneticradiation (gamma rays) that has passedthrough the concrete
Radiography use of the radiation passing through the
concrete to produce a “photograph” of theinternal structure of the concrete
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Direct transmission Radiometry
A portion of the gammaradiation scattered by
free electrons (Comptonscattering)
Another portion
absorbed by atoms Another portion
transmitted
Relative amountsindicate density
Detector
Geiger-Muller
tube
Source
137Cs
Concrete density determination
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Schematic for concrete density
Source buried in freshconcrete or insertedinto a drilled hole inhardened concrete
Measures the averagedensity between thesource and thedetector
Radioactive – onlycertified personnelshould use
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Backscattered radiometry for
surface density Source, detector on the
same side
Bridge decks – one sidedaccess
All gamma rays are
scattered
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Radiography
Identify reinforcing bars, hidden conduits,post-tensioning ducts, pipes etc withinconcrete
Radiation is attenuated by differing amountsbased on the material through which itpasses
Emerging radiation strikes a special
photographic film High density regions shown light and voids
shown dark
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Schematic of Radiography
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Pros and Cons of Nuclear methods
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Infrared Thermography
Based on the poor thermal conductivity of
concrete Any discontinuity parallel to the surface
inhibits the transfer of heat to the concrete
Difference in temperatures between adelaminated area of concrete and a soundarea can be significant (2-5oC)
Using an infrared camera, this temperaturedifference can be recorded
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IR Thermography
Applied to the identification of internal voids,delaminations, and cracks in concrete structures
such as bridge decks highway pavements, parkinggarages, pipelines, and buildings
senses the emission of thermal radiation and
produces a visual image from this thermal signal
Thermography measures variation in surfaceradiance and does not directly measure surface
temperature Needs to heat the object using neon lamps (to
create a temperature difference)
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Two principles
(1) Surface emitsenergy in the form of
electromagneticradiation, and the rateof energy emitted per
unit surface area isgiven by the Stefan-Boltzmann law
(2) Sub-surfaceanomalies affect theheat flow throughconcrete
R=eσT4
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Schematic of the infrared scannersystem
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Effect of internal anomalies
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IR thermographs
Water ingress in an aircraft structure
http://www.infraredtraining.com/
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Bridge Deck IR
Optical photograph
IR thermograph
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Pros and Cons
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Ground Penetrating Radar
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Ground Penetrating Radar (GPR)
RADAR – RA dio Detection A nd R anging
Analogous to pulse-echo – except that radio
waves or micro waves are used instead of stresswaves
Short pulses – Micro wave
Can measure pavement layer thickness,delaminations, moisture content
Can locate underground utilities
Based on the principle of echos
Relies on the dielectric constant of materials
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Principle of GPR
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GPR theory explained
Depends on the relative energies reflected and transmitted
Material properties that influence these are the dielectricconstant and the conductivity
Dielectric constant (dielectric permittivity) is the amount ofelectrostatic energy stored per unit volume for a unitpotential gradient
Electrical conductivity (reciprocal of electrical resistivity) isa measure of the ease with which an electrical current canbe made to flow through a material
Ratio of a material’s dielectric constant (ε) to that of free
space (ε0) is defined as the relative dielectric constant εr
εr = ε /ε0 ε0 =8.854*10-12 F/m
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Wave speed and Dielectric constant
εr governs the speed ofEM waves (C) in a
materialr
C C
ε
0=
2
Ct D =
21
21
2,1r r
r r
ε ε
ε ε
ρ +
−
=
εεεεr1
εεεεr2
Dρρρρ1,2
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GPR
www.radar-solutions.com/pavement.html
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GPR for Repair Detection
http://www.sensoft.ca/products/noggin/noggin_roadmap.html
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GPR for thickness and rebardetection
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Differences between GPR andStress wave methods
Recall stress wave method
What happens at an air-concreteinterface?
Mismatch in dielectric constantsnot that great
GPR not as sensitive as stresswave methods in the detectionof voids
GPR penetrates below theinterface and “sees” the secondmaterial
12
12
Z Z
Z Z R
+
−=
21
212,1
r r
r r
ε ε
ε ε ρ
+
−=
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Factors to note
Influence of water in GPR measurements
Masking of signals Very strong reflections from metals may mask
other signals, like that from delaminations
Interpretation of data is very important
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Acoustic Emission (AE)
What happens when a material starts tocrack?
What happens when a crack propagates?
When does an acoustic activity start?
Related the rate at which acoustic activitystarts compared to the previous load level
Location determination - triangulation
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AE techniques
http://www.uic.edu/depts/cme/research/ssndtl/
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Terminology in AE
Counts Events Hits Duration
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Counts, Events, Hits, Duration, Amplitude
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Frequency Analysis
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Kaiser Effect
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Some facts
Is there any acoustic emission at a certain load level?If no , then no damage is occurring under theseconditions; if yes , then damage is occurring.
Is acoustic emission continuing while the load is heldconstant at the maximum load level? If no , no damagedue to creep is occurring; if yes , creep damage isoccurring. Further, if the count rate is increasing, thenfailure may occur fairly soon.
Have high amplitude acoustic emissions eventsoccurred? If no , individual fracture events have been
relatively minor; if yes , major fracture events haveoccurred.
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More facts
Does acoustic emission occur if the structure hasbeen unloaded and is then reloaded to theprevious maximum load? If no , there is nodamage or crack propagation under low cyclefatigue; if yes , internal damage exists and thedamage sites continue to spread even under lowloads
Does the acoustic emission occur only from aparticular area? If no , the entire structure is beingdamaged; if yes , the damage is localized.
Is the acoustic emission in a local area verylocalized? if no , damage is dispersed over asignificant area; if yes , there is a highly localizedstress concentration causing the damage.
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AE and associated fracture
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AE under cyclic loading
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What is Carbonation?
Recall Cement hydration…
C3S + H C-S-H + C-H
Calcium hydroxide (25-50% in cementitious
systems) is responsible for maintaining the
pH of concrete around 12.5 CH can be attacked by CO2 of the
atmosphere to produce calcium carbonate
Ca(OH)2 + CO2 CaCO3 + H2O
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Carbonation Process
Once the CH is carbonated, the C-S-H willliberate CaO which also will be converted to
CaCO3 in the presence of CO2 Rate of carbonation depends on
Moisture content of concrete
Porosity of Concrete Carbonation does not occur if
Concrete is too dry (RH90%) CO2 cannot enter the concrete in this case
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What does Carbonation do?
Carbonation results in a decrease in porosity
Decreasing porosity is always good!!!
So Is Carbonation Good?
Yes – for non-reinforced concrete
Carbonation is bad for reinforced concrete –it reduces the pH of concrete
When pH drops too low, the steel in
concrete is susceptible to corrosion loss of the depassivating layer
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Detecting Carbonation
Presence of a discolored zone in the concrete surface
Light grey to orange
Visualized by using Phenolphthalein
www.ndtjames.com/catalog/corrosionTesting
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