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Non-Destructive Methods

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

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Motivation (2)

In 1991 the U.S. Department of Transportationreported that $90.9 billion dollars were requiredfor the rehabilitation and repair of the highwayinfrastructure system.

By 1997, the estimated cost had risen to $212 billion.

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SURFACE HARDNESS METHODS

Essentially, the surface hardness method consists ofimpacting a concrete surface in a standardmanner with a given energy of impact and thenmeasuring the size of indentation or rebound.

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SURFACE HARDNESS METHODS

The most commonly used method employs the

Schmidt rebound hammer which consists of aspring-controlled hammer that imparts on aplunger.

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Schmidt rebound hammer

Watch the video

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Penetration Resistance Techniques

The equipment used to determine the penetration resistance of concrete

consists of a powder-activated device.

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Windsor probe

The Windsor probe uses a powder-activated driver to fire a

hardened-alloy probe into the concrete. The exposed length of theprobe is a measure of the penetration resistance of concrete. Thestandard test procedure is described in ASTM C-803

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Windsor probe

The type and amount of aggregate play an important role in the penetration resistance.

The variation in the Windsor probe-test results is higher when comparedwith the variation in standard compressive strength tests oncompanion specimens.

This method is excellent for measuring the relative rate of strengthdevelopment of concrete at early ages, especially for determiningstripping time for formwork.

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Windsor probe

Watch the video

Compressive strength as a funct ion of exposed probe

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PULLOUT TESTS

A pullout test consists of casting a specially-shaped steel insert with anenlarged end into fresh concrete.

This steel insert is then pulled-out from the concrete and the forcerequired for pullout is measured using a dynamometer.

A bearing ring is used to confine failure to a well-defined shape.

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PULLOUT TESTS

As the steel insert is pulled out, a cone of concrete is also removed,

thereby damaging the concrete surface (which must be repaired afterthe test).

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PULLOUT TESTS

Schematic diagram of the pullout test

Watch the video

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Maturity Tests

Since the degree of cement hydration depends on both time and

temperature, the strength of concrete may be evaluated from theconcept of maturity, which is expressed as a function of the time andthe temperature of curing.

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MATURITY METHOD

It is assumed that batches of the same concrete mixtures of same

maturity will attain the same strength regardless of the time-temperature combinations leading to that maturity.

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Maturity

Influence of curing temperature at early ages on thestrength-maturity relationship when equation (1) is used withT0 = - 10 C. This early-age difference can be reduced whenbetter maturity functions are used.

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Absortion and Permeability Tests

The rate of water absorption by capillary suction is a good measure ofthe quality of a concrete and its potential durability when exposed toaggressive environments. Low values of absorption indicate thataggressive ions will have difficulty penetrating the concrete.

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Absortion and Permeability Tests

The rate of water absorption by capillary suction is a good measure of

the quality of a concrete and its potential durability when exposed toaggressive environments.

Low values of absorption indicate that aggressive ions will have

difficulty penetrating the concrete.

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ABSORPTION AND PERMEABILITY

Experimental research indicates that the water absorption values are

reduced which decrease in the water-to-cement ratio; increase in thecuring time; decrease in curing temperature; and increase in the degreeof consolidation.

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Absorption and Permeability

The air permeability of concrete increases when the moisture iseliminated, which in turn increases the connectivity of the pores.

The water absorption also increases when the capillary pores are empty.

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Initial Surface Absorption Test

Methods to measure the water absorption under field conditions

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Configuration of the Figg test

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STRESS WAVE PROPAGATION METHODS.The maximum displacement is the amplitude A, the time between two successive wave crests is the

period T and the distance between two successive wave crests is the wavelength .

D i s p

l a c e m e n t

Time

A

D i s p

l a c e m e n t

Distance

A

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Reflection and refraction

2

1

2

1VVsinsin = =

2

11ic V

Vsin

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Ultrasonic Pulse Velocity Methods

Watch the video

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Many receivers

T r a n s

i e n

t t i m e

Distance from transmitter

slope: 1/V

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Detect the presence of layers

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Method to determine the thickness of the layer

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Impact Methods

A simple method of assessing the condition of concrete is to tap thesurface with a hammer and listen to the resulting tone.

A high-frequency pitch indicates a sound concrete and a low-frequency pitch indicates the presence of flaws.

h d

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Impact Methods

A trained operator can delineate zones of high and low pitch using thismethod.

The disadvantage of the method is that it is dependent on the skill level

of the operator and does not provide quantitative information on theamount of damage in the interior of the concrete.

N D i M h d

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Impact Methods

To overcome these limitations, different methods were developed

To control the duration of the impact force so as to assure thereproducibility of the test and

(b) to characterize the surface displacement generated by the impact onconcrete.

N D t ti M th d

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Impact-echo

Impact forces generated by steel spheres

Use of sensitive broadband transducer at the surface.

Analysis of the waveforms in frequency domain

Non Destructive Methods

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Impact-echo


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Spectral Analysis of Surface Waves

Surface waves are not confined to the surface but, rather, are capable of penetrating a finite depth inside the material, sensing its properties.

Waves with short wavelength may not be able to sense a discontinuitydeep in the interior of a structure, however waves with longwavelengths will be affected by its presence and their velocity willchange.

This change of velocity with wavelength can be used to establish layersof high and low velocities.


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Set up for the SASW method


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Acoustic Emission

Acoustic Emission (AE) is a noninvasive, nondestructive method thatanalyzes the noises created when materials deform or fracture. Eachacoustic emission event is a signature of an actual mechanism, adiscrete event that reflects a given material response.


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Acoustic Emission

Acoustic emission waves propagate through the material and can bedetected on the surface by a sensor, which turns the vibrations intoelectrical signals. The sound of fracture propagation was originallycalled acoustic emission since it is acoustic and audible, however thefrequency of these emissions can range from the audible range to

many megahertz.


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Generation, propagation, and detection of Acoustic Emission


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Acoustic Emission

There is a critical difference between acoustic emission and ultrasonicmethods. In the former, a known signal is imparted into a materialand the materials response to on the signal is studied while in thelatter the signal is generated by the material itself.


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Typical Result


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Resistivity

The relationship between current, i, and potential,V, is given by Ohms law:

RV

i =where R is the resistance of the system.

Resistance is not a material property as it depends on thedimensions of the system. The resistance is normalized toestablish resistivity, r, as a material property

A L

R =

where L is the length and A is the cross-section .


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Wenner array (a=c=b/2)

i

V a = 2


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CEB (Comite Euro-International du Beton) recommendations

Concrete Resistivity ( . m) Likely Corrosion rate

> 200 Negligible100 to 200 Low50 to 100 High

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Corrosion Potential

Can be measured as the voltage difference between the steel and areference electrode in contact with the surface of the concrete.

Half-cell measurements may be made using only a high impedancevoltmeter and a standard reference electrode, such as a copper-coppersulfate electrode.


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Corrosion Potential


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Polarization Resistance


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Typical polarization resistance for steel in concrete

Rate of corrosionPolarization resistance,

R p (k .cm 2)Corrosion penetration,

p (m/year)Very high 0.25 < R p < 2.5 100 < p < 1000

High 2.5 < R p < 25 10 < p < 100Low/moderate 25 < R p < 250 1 < p < 10

Passive 250 < R p p < 1


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Advantages in using linear polarization technique

The determination of the corrosion rate is an important parameter in theassessment of the life cycle of the structure.

Commertial equipment is available to measure the polarization resistancein existing structures.


d l l h

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Advantages in using linear polarization technique

It has been verified experimentally that the magnitude of the corrosionrate measured by polarization resistance is similar to that measured bygravimetry.

The measurement times are small.

The method applies small pertubations that do not interfere with theexisting electrochemical processes.


Li i i

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Limitations

The whole reinforcing bar or the steel mat along which uniformcorrosion is rare, must be polarized. Therefore, the measured Rp isessentially an average result for all of the steel in the structureswhereas the actual corrosion is non-uniform.


Li it ti

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Limitations

This method assumes that the concrete resistivity is low, when in realitythe concrete resistivity is usually high. Linear polarization is a DCmethod. Because of the electrical capacitance across the steel/concrete

interface, it takes time of obtain a full response. As a result, thescanning rate plays an important role in obtaining accuratemeasurements.


Eq i t

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Equipment


El t h i l I d S t

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Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy, or AC impedance, is aninformative method because not Rp only is measured, but also the

physical processes in concrete and steel/ concrete interface areassessed.

Impedance measurement employs small-amplitude alternating (AC)signals in a wide range of frequency as a perturbation.


V l V i id l i l i

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Voltage response, V, to sinusoidal current signal i

time

V (t)

i (t)

V or i


Model of equivalent circuit

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Model of equivalent circuit.


Limitations

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Limitations

The equipment used is quite bulky and complex.

The entire reinforcing bar or network must be polarized.

Similar to the linear polarization, this method also requires a physicalconnection to the steel embedded in concrete. AC impedancemeasurements can have lengthy data acquisition times, especially forlow frequency measurements.


Covermeter

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Covermeter

Covermeter is the generic term for equipment used to locate steelreinforcing bar in concrete and to estimate the thickness of theconcrete cover over the reinforcement. Unlike concrete, steel barsinteract strongly with low-frequency electromagnetic waves applied atthe surface of the concrete, making it easy to identify their location.


Equipment I

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Equipment I


Equipment II

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q p


Ground Penetrating Radar

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g

Ground Penetrating Radar (GPR) methods use electromagnetic energy,typically at frequencies of 50-1500 MHz, to probe the subsurface.

This method has been used in concrete structures to detect voids anddelaminations, locate the reinforcing bars, measure the pavementthickness, and monitor structural changes.


Ground Penetrating Radar

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g

Recently, field research has been performed on the materialcharacterization of concrete, such as water content, degree of cement

hydration, and presence of chlorides.


Method

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Example

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p


To Find Delamination

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Tomography of Reinforced Concrete

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g p y

Tomography comes from the Greek word tomos (slice) and it has thegoal of imaging an object by taking measurements from slices of itscross-section.


X-ray Computed tomography

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In computed tomography, the image of an object is reconstructed from projections of the object.

Most commonly the projections are obtained by using penetrating x-rays,although other modalities for measuring projection data are alsoavailable.


X-ray Computed tomography

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A radiation source (x-rays or -rays) is rotated a full 360o around thestructure under inspection, and at each source position, the attenuationof the radiation penetrating through the material is measured with alinear array, or a 2D array of detectors at the opposite side.


Configuration

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Examples

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Radiographic (left) and CT image (right) images of the unloaded fiber-reinforced concrete cylinder


Examples

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Radiographic (left) and CT image (right) images of the loaded fiber-reinforced concrete cylinder


Examples

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Radiographic (left) and CT (right) image .

Microtomography (courtesy from Prof. Landis)

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Examples

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Backscattering Microwave Tomography

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To obtain higher resolution, it is necessary to employ electromagneticradiation with shorter wave-length, leading to the use of microwavefrequencies in the range of 300 MHz to 300 GHz.

This frequency range corresponds to wavelengths in the air of 1 m to 1mm, respectively.

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