Dominguez Lab 8

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Student: Luis Dominguez

A-number: A01834041

Course: Experimental methods in structural

engineering, cee 6050

Professor: Marvin Halling, PhD

Assignment: Lab 8

Date: 04/29/2013


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Team #8: Edyson Rojas, Luis Dominguez, Ivan Quezada

Lab 8: NDT on Slabs

CEE 6050

Experimental MethodsApril 2013

Utah State University


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Table of Contents

Theoretical Background ............................................................................................................................ 4

Impact-Echo NDT Method ................................................................................................................... 4

Weaknesses ................................................................................................................................................. 5

Experiment .................................................................................................................................................. 5

Objectives ................................................................................................................................................ 5

Equipment ............................................................................................................................................... 6

Slabs ..................................................................................................................................................... 6

Spectral Analyzer ............................................................................................................................... 7

Accelerometer ..................................................................................................................................... 7

Impactor (Hammer) ........................................................................................................................... 7

Procedure ................................................................................................................................................ 8Analysis & Report .................................................................................................................................. 9

Conclusion ............................................................................................................................................ 16

Pictures .................................................................................................................................................. 17

References ................................................................................................................................................. 19

Appendix A ............................................................................................................................................... 20

Lab Notes .............................................................................................................................................. 20

Appendix B ............................................................................................................................................... 25

Calculations Tables .............................................................................................................................. 25

Table of FiguresFigure 1, Slab to be analyze ...................................................................................................................... 6

Figure 2, Spectral Analyzer....................................................................................................................... 7

Figure 3, Accelerometer ............................................................................................................................ 7

Figure 4, Hammer ...................................................................................................................................... 7
http://c/Users/Eng.%20Edyson%20Rojas/Google%20Drive/Utah/Spring%202013/Experimental%20Methods%20in%20Structural%20Dynamics/Lab%208/Lab%208%20WriteUp.docx%23_Toc353733846http://c/Users/Eng.%20Edyson%20Rojas/Google%20Drive/Utah/Spring%202013/Experimental%20Methods%20in%20Structural%20Dynamics/Lab%208/Lab%208%20WriteUp.docx%23_Toc353733847http://c/Users/Eng.%20Edyson%20Rojas/Google%20Drive/Utah/Spring%202013/Experimental%20Methods%20in%20Structural%20Dynamics/Lab%208/Lab%208%20WriteUp.docx%23_Toc353733848http://c/Users/Eng.%20Edyson%20Rojas/Google%20Drive/Utah/Spring%202013/Experimental%20Methods%20in%20Structural%20Dynamics/Lab%208/Lab%208%20WriteUp.docx%23_Toc353733848http://c/Users/Eng.%20Edyson%20Rojas/Google%20Drive/Utah/Spring%202013/Experimental%20Methods%20in%20Structural%20Dynamics/Lab%208/Lab%208%20WriteUp.docx%23_Toc353733847http://c/Users/Eng.%20Edyson%20Rojas/Google%20Drive/Utah/Spring%202013/Experimental%20Methods%20in%20Structural%20Dynamics/Lab%208/Lab%208%20WriteUp.docx%23_Toc353733846


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Theoretical Background

Delamination occurs when the corrosion in the steel rebar induces cracks and the cracks

joined together to cause the concrete cover to separate from the substrate concrete. It results in

the loss of structural strength and facilitates a rapid deterioration of the deck. The delamination

impairs both the appearance and the serviceability of the structure, and repairs can be very

costly. It is estimated that annual maintenance and repair costs related to corrosion for concrete

infrastructure approach $100 billion worldwide. As a corrosion-induced problem, delamination

is of great concern for bridges, and routine inspection is necessary.

Many methods have been developed to detect concrete delamination. These methods

include the conventional chain drag method, impact-echo (the one we will use in this lab),

ultrasonic tests, ground penetrating radar, imaging radar, and infrared thermography. Efforts

have been made to expand, improve, and combine currently available techniques.

Impact-Echo NDT Method

IE is a widely used NDT method, which has been demonstrated to be effective in

detecting delaminations in bare (without asphalt overlay) concrete structures (Sansalone 1996;

Tawhed andGassman 2002; Zhu and Popovics 2007). IE is a mechanical-wave method based on

the transient vibration response of a platelike structure subjected to mechanical impact. The

mechanical impact generates body waves (P waves, or longitudinal waves, and S waves, or

transverse waves), and surface guided waves (e.g., Rayleigh surface waves) that propagate

within the solid material. The multiply reflected and mode converted body waves eventually

set up sets of infinite vibration resonance modes within the solid material (Tolstoy and Usdin1953). The transient time response of the solid structure is measured with a contact sensor (e.g.,

displacement sensor or accelerometer) mounted on the surface close to the impact source. The

Fourier transform (amplitude spectrum) of the time signal will show maxima (peaks) at certain

frequencies, which represent particular resonant modes. Two sets of vibration modes are

relevant to this study, the thickness stretch modes and the flexural drum modes. The thickness

stretch mode family normally dominates the spectral response of a platelike structure that does

not contain any near-surface defects. In that case, the frequency of the fundamental thickness

stretch mode (also called the, IE frequency: ) can be related to the thickness of the plate.

Knowing the P-wave velocity Cp of concrete, the plate thickness D is related to the IE frequency

by:

Where is the correction factor, and ranges from 0.945 to 0.96 for the normal concrete

(Gibson and Popovics 2005) and we are going to use 0.96, D is in inches, Cp is 4100 m/s (need to

convert), and the frequency is in Hertz. The correction factor is based on elastic-dynamic effects

related to a S1 Lamb wave mode with zero group velocity, which depends on the Poissons


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ratio. The thickness stretch mode may also dominate the spectral response when the test is

carried out over a relatively deep defect or delamination. However, the flexural modes tend to

dominate the spectral response when a test is carried out over a shallow (nearsurface) defect or

delamination. These modes normally have much lower frequency than the stretch modes.

Unfortunately, a simple analytical expression that relates the fundamental flexural frequency to

the defect depth has not yet been established because this frequency value depends on the plategeometry, shape, and boundary condition. Despite this problem, IE test data provide valuable

information about the presence of delaminations in concrete plates; areas that show a dominant

response frequency with a relatively low value are likely located above the shallow

delamination defects, although the depth of the defect cannot be directly estimated.

Weaknesses

Data can be difficult to interpret; especially on thick plates or on layered materials(overlays, soil).

Small voids can be missed. Is limited by size of wavelength. Complicated geometries pose difficulties. Flaws beneath sensed flaw must be evaluated from the opposite side.

Experiment

This section will explain the procedure that will be done at the lab, what measurements

to take, what to calculate, and will contain relevant information regarding the experiment.

Objectives

1. Understanding of the Impact-Echo method to detect delamination on slabs.2. Use the IE method to determinate various frequencies values at a given number of

points on a slab.

3. With the obtained frequencies at each point, compute D (depth) at each point. (Depthwill change if theres delamination at that point or not.)

4. Compare the obtained depth with the measurements done in the lab, and calculate thepercentage of error.5. Calculate the area of delamination on the slab based on the calculated depths. Compare

with actual area of delamination and compute the percentage of error.


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Equipment

Slabs

Four reinforced concrete plates were constructed in the SMASH Lab. Each concrete platehas the same size, and same layout of #6 steel rebar. All the concrete was placed from the same

ready mix concrete batch in order to achieve similar concrete strengths between specimens. On

each slab, plexiglass sheets were used to simulate the effect of a delamination. Plexiglass is

unaffected by moisture and offers a high strength-to-weight ratio, which is good for preventing

the bonding of concrete above and below it. For the purpose of this experiment we are only

going to analyze one of the delaminated slabs.

For this experiment we are going to analyze just one slab.

Figure 1, Slab to be analyze


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Spectral Analyzer

This piece of equipment is

used to read the data from the

accelerometer and convert it to

frequency as it is displayed on thescreen. The user is then able to find

the frequency values for each point,

each time it is hit with the hammer.

Accelerometer

The accelerometer takes the vibration and transforms it into an

electrical signal which is then sent to the Spectral Analyzer to be

converted into the frequency and time domains.

Impactor (Hammer)

This tool is used to hit the slab at the desired point and

generate waves; the stress waves that propagate within the

structure and are reflected by flaws or external surfaces.

Figure 2, Spectral Analyzer

Figure 3, Accelerometer

Figure 4, Hammer


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Procedure

The procedure for this lab will be the following:

1. Measure the slab and the location of the points that are going to be tested.2. At each point indicated on the slab, the participants of the lab are going to do an

Impact-Echo Test (5 taps) and write down the value of the frequency read on the

Spectral Analyzer.

3. Proceed to compute the depth at each point, and compare it to the onesmeasured in the lab.

4. Based on its value, determine if the calculated depth corresponds to the bottomof the slab or the delamination.

5. Calculate the percentage of error of the value obtained from Impact-Echo incontrast with the measured value.

6. Compute the percentage of delamination of the slab with the actual value of thedelamination. %Delamination=(Adelamination/ASlab)*100.

7. Determine the area of the delamination based only of the information obtainfrom the Impact-Echo and the location of the tested points and calculate the

corresponding percentage of delamination. %Delamination=(Adelamination/ASlab)*100.

8. Calculate the percentage of error of the %Delamination.


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Analysis & Report

Data acquired at the lab.

X Dimension of slab: 9 ft

Y Dimension of slab: 6 ft

Distance between the points in the X direction: 10 in between points and 4 in from the edge

Distance between the points in the Y direction: 6 in between points and 3 in from the edge

Thickness of slab: 5.625 in

Point Frequency (kHz)

1x 18.312 Hz

2x 13.84 Hz

3x 13.84 Hz

4x 14.112 Hz

5x 14.368 Hz

6x 37.984 Hz

7x 35.775 Hz

8x 32.416 Hz

9x 34.496 Hz

10x 38.528 Hz

11x 13.952 Hz


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Point Frequency (kHz)

1y 14.48 Hz

2y 14.688 Hz

3y 14.384 Hz

4y 12.128 Hz

5y 14.432 Hz

6y 34.592 Hz

7y 34.336 Hz

8y 32.704 Hz

9y 32.96 Hz

10y 33.632 Hz

11y 34.56 Hz

12y 14.16 Hz


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I started this lab transforming the velocity that the waves can travel in concrete from m/s to in/s.

Using

I was able to calculate de depth of the slab.

0.96

CP (in/s) 161417.3228 in/s

Point Frequency (Hz) Depth (in) Slab or Delamination?

1x 18312 Hz 4.231122486 in Slab2x 13840 Hz 5.598288653 in Slab

3x 13840 Hz 5.598288653 in Slab

4x 14112 Hz 5.49038513 in Slab

5x 14368 Hz 5.392560896 in Slab

6x 37984 Hz 2.039814526 in Delamination



9x 34496 Hz 2.246066644 in Delamination10x 38528 Hz 2.011013158 in Delamination

11x 13952 Hz 5.553348263 in Slab


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Point Frequency (Hz) Depth (in) Slab or Delamination?

1y 14480 Hz 5.350850481 in Slab

2y 14688 Hz 5.27507591 in Slab

3y 14384 Hz 5.386562497 in Slab

4y 12128 Hz 6.388548397 in Slab

5y 14432 Hz 5.368647101 in Slab

6y 34592 Hz 2.239833342 in Delamination






12y 14160 Hz 5.471773655 in Slab


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In this table we compared the calculated depth based on the frequency measured versus the

actual measures of the slab.

PointDepth (in)

% Error

From IE Measured1x 4.23112249 in 5.625 in 24.78%

2x 5.59828865 in 5.625 in 0.47%

3x 5.59828865 in 5.625 in 0.47%

4x 5.49038513 in 5.625 in 2.39%

5x 5.3925609 in 5.625 in 4.13%

6x 2.03981453 in 2 in 1.99%

7x 2.16576701 in 2 in 8.29%

8x 2.39018741 in 2 in 19.51%9x 2.24606664 in 2 in 12.30%

10x 2.01101316 in 2 in 0.55%

11x 5.55334826 in 5.625 in 1.27%


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PointDepth (in)

% ErrorFrom IE Measured

1y 5.35085048 in 5.625 in 4.87%2y 5.27507591 in 5.625 in 6.22%

3y 5.3865625 in 5.625 in 4.24%

4y 6.3885484 in 5.625 in 13.57%

5y 5.3686471 in 5.625 in 4.56%

6y 2.23983334 in 2 in 11.99%

7y 2.25653294 in 2 in 12.83%

8y 2.36913879 in 2 in 18.46%

9y 2.35073771 in 2 in 17.54%10y 2.30376769 in 2 in 15.19%

11y 2.24190726 in 2 in 12.10%

12y 5.47177366 in 5.625 in 2.72%


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ElementDimension ft

Area ft2

% of

Delamination% of Error

X Y

Slab 9 6 54

Actual

Delamination 4 3 12 22.22%

IE Delamination 3.333 2.5 8.333 15.43% 30.56%


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Conclusion

This experiment taught us a very important way to realize a nondestructive testing of a

concrete slab of any type of structure. We had the chance to perform an impact echo

test on a slab and see for ourselves the relation between the theoretical background we

had acquired so far and the actual values that were obtained and see how close these

were to one another.

My views on this experiment are that the results were pretty close considering some

conditions that might have been a factor in the larger percentage of error that were

obtained in some cases. One of the reasons that could mess with the results might have

been the use of the wax to place the accelerometer on the slab, we could observe that

this was not an easy task and that it required wax which didnt always hold the

accelerometer as straight as we would have liked it to be and this may have presented alittle bit of an issue in the final readings gotten by the machine. Also I think it is

imperative that we understand that the wax, if too much was present between the

accelerometer and the concrete face may have also mess a little bit with the results.

Another reason we get in some cases a really different frequency reading at really close

points might be the way the aggregates are placed on the concrete, since this could

factor in the velocity of the wave through the concrete and therefore in the frequency.

I believe that this experiment could have presented better results if more modern

accelerometers could have been used, if said apparatus could have been placed on the

slab without the use of the wax and maybe just to be on the safe side we could have

performed a different type of test to compare this results.

All in all I think this proved to be an excellent experience for the structural engineer

who may have to practice this test in the future and now they possess important

knowledge about how to go about it.


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Pictures


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References

Seong-Hoon, T., John S. Popovics, M., & Ralf and Jinying Zhu, A. (2012). NondestructiveBridge Deck Testing with Air-Coupled Impact-Echo and Infrared Thermography. Univ

of At - Austin, 1-12.

Shutao Xing, M. W. (2012). Delamination Detection of Reinforced Concrete Decks UsingModal Identification.Journal of Sensors, 17.


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Appendix A

Lab Notes

Dimension of the Slab

Thickness: _______________in

Distance from Surface to Delamination: ______2_________in


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Location of the points to be evaluated


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Frequency readings

Point 1x: ______________KHz Point 7x: ______________KHz






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Point 6x: ______________KHz

Frequency readings

Point 1y: ______________KHz Point 7y: ______________KHz






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Appendix B

Calculations Tables

=

Cp (in/s)=

Point

Frequency

(Hz) Depth (in)

Slab or

Delamination?

1x

2x

3x

4x

5x

6x

7x

8x

9x

10x

11x

Point

Frequency

(Hz) Depth (in)

Slab or

Delamination?

1y

2y

3y

4y

5y6y

7y

8y

9y

10y

11y


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12y


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Point

Depth (in)


1x

2x

3x

4x5x

6x

7x

8x

9x

10x

11x

PointDepth (in)


1y

2y

3y

4y

5y

6y

7y

8y

9y

10y

11y

12y

Element

Dimension

Area

% of

Delamination % of ErrorX Y

Slab -------------------- ---------------

Actual

Delamination

IE

Delamination

Dominguez Lab 8

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