CE 555-L18,19-NDT of concrete-4-nuc radar therm ae.pdf

8/17/2019 CE 555-L18,19-NDT of concrete-4-nuc radar therm ae.pdf

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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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CE 555-L18,19-NDT of concrete-4-nuc radar therm ae.pdf

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