abstract - university of houstonstructurallab.egr.uh.edu/.../downloads/reu-knc-report.pdfreu smart...

REU Smart Aggregates for Concrete Structures Choi

Abstract

As undergraduate researchers at the University of Houston, Choi and Mulkern are given the

topic “Smart Aggregates for Concrete Structures”. Smart aggregates are designed to monitor the

early age concrete strength development, impact detection, and structural health. During the 10-

week research period, the students have successfully made several smart aggregates from a simple

pipe cap. 3-D and RFI shielding smart aggregates are also made for sensing vibrations in three

different planes and blocking radio frequency interference respectively. Some smart aggregates are

embedded into 6” diameter, 12” height concrete cylinder specimens for compression test on the

Universal Compression Testing Machine. The purpose of the test is to find out whether the smart

aggregates will affect the strength of the concrete structures. The result is shown as a stress strain

curve for each testing specimen in this report.

1. Problem Statement

There are two main goals for this 10-week research project. One is to find an economic

way to build a smart aggregate using a reusable mold. The second goal is to find out whether the

strength of the concrete structure will be changed by embedded smart aggregates inside it.

2. Introduction & Background

What is a Smart Aggregate?

A smart aggregate is a small piece of concrete embedded with a small sensor in it. The

sensor is a small piece of patch made out of lead zirconate titanate (PZT), which is one of

the most common used piezoelectric materials. “The piezoelectric material will generate electric

charge when it is subjected to a stress or strain (direct effect); the piezoelectric material will also

produce the stress or strain when an electric field is applied to a piezoelectric substance in its poled

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direction (converse effect). Due to this special piezoelectric property, piezoelectric material can be

utilized as both an actuator and a sensor.” (Song et al. 2006) Moreover, they are light weighted,

cheap, and response. However, they are very fragile. This is why a small piece of concrete is

needed to protect the PZT patch from damaging due to the vibrations when sensing. For this

research project, the patches are all 10 ×10 square millimeters.

What is the function of Smart Aggregates?

Smart aggregates are designed for civil structures such as buildings and bridges. They are

designed to monitor the early age concrete strength development, impact detection, and structural

health. All three tasks are very important for safety purpose due to the frequent use of the

structures. Fatigue loading or sudden impact like earthquake will contribute to failure and may

cause collapse of the structures. This is why smart aggregates are so useful that they give out signal

when the civil structures crack and fail.

How is a Smart Aggregate made?

In this research, each smart aggregates is composed of a 10×10 square millimeters PZT

patch, a cable with BNC connector, and a piece of concrete. First, a 10×10 square millimeters PZT

patch is cut from a larger patch originally from the manufacturer. It is then connected to a 24/2 (24

American Wire Gauge (AWG), 2 conductors) shielded communication cable by soldering. The

other side of the cable is connected to a BNC connector, which is a common connector used for

linking sensors and instrument such as an oscilloscope. Since concrete consists of water, the PZT

patch is coated with a black insulating coating to keep it waterproofed.

Then the PZT patch is placed into the mold, in which a 2 inch pipe cap is used with an

aluminum plate fitting at the bottom of the cap. A hole is drilled in the bottom of the cap for taking

out the smart aggregate once it is set. Figure 1 shows the six steps to make a smart aggregate.

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Figure 1: Six steps to make a smart aggregate.

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1. Cut a 10×10 square millimeter PZT patch.

2. Solder the patch to the cable.

3. Solder the other side of the cable to a BNC connector.

4. Coat with insulating coating.

5. Place the PZT patch in the mold*.

6. Place an aluminum plate at the bottom of the mold and pour the concrete** into the mold.

* The mold is modified that a slot is cut out on the side for fitting a communication cable. Tape is used to seal the opening to prevent concrete from leaking out when casting. A hole is drilled on the bottom of the mold for the purpose of poking out the smart aggregate once it is set.

** The concrete is mixed with the components according to the ratio in Table 1.

Table 1: The ratio of each component of the concrete use for making smart aggregates.

Final ProductComponents

20 lbs Concrete 5 lbs Concrete 2.5 lbs Concrete

Type 3 Cement 7 lbs 1.75 lbs 0.875 lbsFine Aggregate (Sifted sand) 10.5 lbs 2.625 lbs 1.3125 lbs

Water 2.59 lbs 0.6475 lbs 0.32375 lbsBASF Super-plasticizer ~25 g ~6.25 g ~3.125 g

Then the smart aggregate is left at room temperature for 48 hours. This will give the smart

aggregate 6000 PSI (pound per square inch) of strength.

For artistic purpose, a University of Houston’s logo made from card board was placed on

the aluminum plate so the smart aggregate will have an UH logo engraved on it.

Figure 2: (a) A UH logo made from card board is

placed on the aluminum plate for an engraving on the

smart aggregates.

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(b) At the end of the research, the students are able to

get an acrylic plate with UH logo engraved on it. The

plate will replace the aluminum plate in this case.

Each smart aggregate is tested for its sensing feasibility on the oscilloscope and its

capacitance. Basically a hammer is used as an impact input to the smart aggregate to see if it

responds to the impact. And a healthy smart aggregate should have a capacitance from 6.0-7.0 nF

(10-9 Farad).

As more smart aggregates are made, some of them are made for difference purposes among

the regular ones. The students not only tried to put a UH logo engraved on the smart aggregates,

they also tried to make some 3-D ones and some with RFI (Radio Frequency Interference)

shielding ability. Each 3-D smart aggregate basically has three PZT patches lying on three different

planes: xy-, yz-, and xz-plane. (See Figure 3)

Figure 3: 3-D Smart Aggregates

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As for the RFI shielding smart aggregates, they are just regular smart aggregates with wire

mesh wrapped around its surface. In this case, a 100 Mesh copper 0.0022" wire diameter 48 inches

wide was picked for our project. It is cut into three pieces as shown in Figure 4 and placed in the

mold before the concrete is cast. It is believed that by placing an electromagnetic wave generator

or a radio frequency generator near a couple of smart aggregates, one with wire mesh and the other

without, the former one will have less noise than the latter one. However, since the generator is not

available during the research period, this hypothesis will have to be confirmed in later studies.

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xz-plane

yz-plane xy-plane

x

z

y


Figure 4: 100 Mesh copper 0.0022" wire diameter 48” wide cut in the shape that will fit around the pipe cap

A graph of the shielding effectiveness of the wire mesh that was used for the smart

aggregates is shown in Figure 5. For 100 Mesh copper 0.0022" wire diameter 48 inches wide, its

radio frequency interference shielding effectiveness is very good up to 1 Mhz.

Figure 5: Shielding Effectiveness of Copper Wire Meshes to Plane Waves. (TWP inc. 2003)

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Table 2 shows some of the properties of 100 mesh copper.

Table 2: Specification of 100 Mesh Copper. (TWP inc. 2003) Mesh per inch = 100 Wire diameter = .0045 inch (.114mm) pening size = .0055x0.0055 inch

(.14x0.14mm) Percentage of open area = 30.3% Weight = .14 pounds per square foot (.684 kg

per square meter)

Standard rolls are 100 feet in length TWP part number for 36" width

=100X100C0045W36T TWP part number for 72" width

=100X100C0045W72T

Figure 6 shows some of the smart aggregates that were made during the research period.

Figure 6: All four kinds of smart aggregates

Cost of a Smart Aggregate

Several materials are needed to build a smart aggregate. Table 3 shows a list of the

materials needed as well as the costs of each material.

Table 3: Costs of the materials for building a smart aggregate

Materials Costs Regular 3-DRFI

ShieldingUH Logo Engraved

Pipe Cap $0.78 Each One time One time One time One timeWire Mesh $1.06 per SA 0 0 1 0PZT Patch $2.50 / patch 1 3 1 1

Cable $0.40 / 3 ft for 1 SA 1 3 1 1BNC Connector $0.50 Each 1 3 1 1

Concrete 4.71 in3 per SA 1 1 1 12" Acryllic Plate with UH Logo (Optional)

$15 Each 0 0 0 One time

2" Aluminum Plate $10 Each One time One time One time 0Total (for 1 SA) $3.40 $10.20 $4.46 $3.40+ Concrete, Pipe Cap (One time only), and bottom plate (One time only)

* SA: Smart Aggregates

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Regular w/ UH logo Engraved 3-D RFI Shielding


3. Experimental Setup

How are Smart Aggregates being put into a civil concrete structure?

Due to limited resources and time, the smart aggregates cannot be used in a full scale civil

structure. Instead, they are embedded into some concrete cylinders, which can be used for a

loading test. The purpose of the loading test is to get a stress-strain relationship of the concrete

cylinders to see whether the strength of the cylinders will be affected by embedding smart

aggregates. Several concrete cylinders are cast, some with smart aggregates embedded. Each

cylinder mold has 6 inch diameter and 12 inch height. In order to embed two smart aggregates into

each concrete cylinder, two holes are drilled on the side of the mold at 4” and 8” height for the

BNC connectors to go through. (See Figure 7)

Figure 7: 6” diameter, 12” height cylinder mold

As discussed before, PZT is one kind of piezoelectric materials. “Piezoelectric material will

generate electric charge when it is subjected to a stress or strain (direct effect); the piezoelectric

material will also produce the stress or strain when an electric field is applied to a piezoelectric

substance in its poled direction (converse effect). Due to this special piezoelectric property,

piezoelectric material can be utilized as both an actuator and a sensor.” (Song et al. 2006) By

inputting a sweep sine signal to the smart aggregate that acts as an actuator, the other smart

aggregate, which acts as a sensor, will respond to the small force that it sends out a signal showing

the health of the concrete cylinder.

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As for the concrete for the cylinders, the components listed in Table 4 are used. Each

cylinder can hold about 30 pounds of concrete. Considering some waste on the mixer and

containers, a 200 lbs concrete is used to make six concrete cylinders instead of 180 lbs.

Table 4: The ratio of each component of the concrete use for making concrete cylinders.

(a) (b)

First, one third of the volume of the concrete for one cylinder mold is poured into the mold.

Then one smart aggregate is placed horizontally on the concrete. Another one third of concrete is

poured on top of the smart aggregate. The second smart aggregate is placed horizontally on top of

the concrete. Finally, the rest of the concrete is poured into the mold on top of the second smart

aggregate.

For normal concrete without plasticizer, it is tamped 25 times with a rod before pouring

concrete into the mold for the second time. The second smart aggregate is then placed on top of the

concrete horizontally. Again it is tamped 25 times before casting the rest of the concrete into the

mold. Note that when tamping the concrete, it is best not to tamp on the smart aggregates to

prevent them from tilting to one side. The smart aggregates are best when placed horizontally to

sense the vibrations in the up-down direction.

As for self compacting concrete with plasticizer, tamping is not needed.

Figure 9: Three Normal Concrete Cylinders and three Self Compacting Concrete Cylinders

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According to the Proceeding of 4th China-Japan-US Symposium on Structural Control and

Monitoring by Song, Gu, and Mo, “The compressive strength of concrete will become stable after

28 days.” (Song et al. 2006) However, the strength of the concrete increase at a slower rate after

seven days. (See Figure 10) As a result, it is all right to do a compression test on the concrete

cylinders after seven days.

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Self Compacting Concrete Cylinders

Normal Concrete Cylinders


Figure 10: Compressive strength vs. age (Song et al. 2006)

In order to find out whether the smart aggregates will affect the strength of the concrete

cylinders, they are tested using the Universal Compression Testing Machine.

However, before testing a concrete cylinder, it is necessary to put caps on both ends of the

concrete cylinder to make sure both ends are flat. This will improve the accuracy of the testing.

(See Figure 11)

Figure 11 (a), (b), and (c): 3 steps to make a cap for the compression test

(a) (b)

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Pouring cap concrete into the cap mold

Place a concrete cylinder on the cap concrete before it dries and becomes solid.


(c)

The finished concrete cylinders with caps on are shown in Figure 12.

Figure 12: Concrete Cylinders with caps on for compression test.

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Wait until the concrete is dry. Then take out the cap mold.


On the other hand, a frame with strain gage has to be installed to it. (See Figure 13) Each

concrete cylinder has to be placed at the center of the frame for best testing results.

Figure 13: A frame with strain gage has to be installed to the concrete cylinder before testing.

Figure 14: Testing Concrete Cylinder specimens on the Universal Compression Testing Machine(Left: Concrete Cylinder without Smart Aggregates; Right: Concrete Cylinder with Smart Aggregates)

After the setup is done, the test on the concrete cylinders on the Universal Compression

Testing Machine will begin. A steady load rate of 500 lbs/second is applied to each cylinder and a

set of data consists of the load and the displacement is recorded for each cylinder.

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4. Results

A total of six cylinders are tested using the Universal Compression Testing Machine. Two

of them are embedded with smart aggregates. The recorded data is listed in Tables A1-A5, and the

stress strain curve for each specimen is shown in Figures A1-A5 in Appendix A.

The equation for calculating the stress of the concrete cylinders from the load is

π4

62

Load

AreaSurface

LoadStress ==

.

And the equation for calculating the strain of the concrete cylinders from the displacement is

16

Deflection

LengthOriginal

DeflectionStrain == .

One concrete cylinder with smart aggregates is tested for health monitoring purpose. It is

concrete cylinder 6 with smart aggregates, and the data is shown in Appendix A6.

Table 5 shows a summary of the ultimate strength of each cylinder, with and without smart

aggregates embedded. And Figure 15 shows the graph of the ultimate strengths of all six concrete

cylinders. Note that cylinders 1, 2, and 3 are without smart aggregates, while cylinders 4, 5, and 6

are embedded with smart aggregates.

Table 5: Summary of the ultimate strength of all the concrete cylinders, with and without smart aggregates.

Concrete Cylinders without Smart Aggregates Concrete Cylinders with Smart AggregatesCylinder # Age (days) Ultimate Strength (KSI) Cylinder # Age (days) Ultimate Strength (KSI)

1 13 3.62 4 18 3.862 18 3.63 5 18 3.723 18 3.73 6 18 3.81

Average 3.66 Average 3.80

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Figure 15: Ultimate Strength of all five concrete cylinders.

Ultimate Strength of the Concrete Cylinders

0

0.5

1

1.5

2

2.5

3

3.5

4

1 2 3 4 5 6

Cylinder No.

Ult

imat

e S

tren

gth

(K

SI)

Figure 16 shows some of the concrete cylinders after the compression test.

Figure 16: Concrete Cylinders after the compression test

(a) (b)

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The results show that the concrete cylinders with smart aggregates embedded is slightly

stronger than the ones without smart aggregates. It is thought that this is due to the concrete used

for the smart aggregates is self-compacting. This will give the concrete cylinders a little more

strength. If the same mixture is used for both the smart aggregates and the concrete cylinders, their

ultimate strengths should be very close.

5. Conclusion

Smart aggregates are mainly used for concrete structures and are designed for monitoring

its early age strength development, impact detection, and structural health. It is composed by a 10

×10 square millimeters PZT patch, a communication cable, a BNC connector, and concrete. By

using a simple pipe cap as a mold and an aluminum plate fitting, a smart aggregate can easily be

made. Moreover, the mold and the metal plate are both reusable. On the other hand, in order to test

whether the smart aggregates will affect the strength of the concrete structures that are embedded

with them, a compression test on five concrete cylinders have been executed. Two of the concrete

cylinders are embedded with smart aggregates, while the rest of them do not have smart aggregates

embedded. The testing results show that the strength of the concrete structures does not change

with smart aggregates embedded inside the structures.

6. Future Work

One of the difficulties that the students encountered is to take out the smart aggregate from

the mold. Lubrication oil such as WD-40 and motor oil 10W-30 are applied on the inside surface of

the mold. However, it does not seem to ease the process of removing the mold. Of course it will be

easier to use some disposable plastic cup as a mold, so one the smart aggregate is set, the plastic

cup can just be cut off. But then it will lose the meaning of finding a reusable mold for making

smart aggregates. It is recommended to try form release and Vaseline on the mold for building

smart aggregates in the future.

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On the other hand, as mentioned before, the RFI shielding smart aggregates are supposed to

block RFI and allow user to get a signal with less noise. Since the generator for this test is not

available, it is recommended to connect two smart aggregates, one with wire mesh, and the other

without to an oscilloscope to see whether the noise level of the one with wire mesh is less than the

one without.

As for the 3-D smart aggregates, tests are still needed to see whether the three different

PZT patches sense three different directions of vibration. Testing them on the oscilloscope when

impacting the smart aggregate with a hammer was done during the research. However, since some

patches are closer to the impact surface than the other, it is hard to tell if each of them is sensing

the right vibration. Also, method of putting in the three patches in three different directions into the

mold needs to be developed. The only thing that was used to hold the patch lying on the right plane

was wire tape.

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Appendix A1: Testing data & stress strain curve for Normal Concrete Cylinder 1 without Smart Aggregates

Table A1: Testing Data for Normal Concrete Cylinder 1 without Smart Aggregates

Normal Concrete Cylinder 1 without Smart AggregatesCast on 6/29/2007 Tested on 7/12/2007

Load (Kips) Displacement (x10-5 inch) Stress (KSI) Strain10 90 0.354 0.00005620 215 0.707 0.00013430 315 1.061 0.00019740 455 1.415 0.00028450 600 1.768 0.00037560 750 2.122 0.00046970 925 2.476 0.00057880 1120 2.829 0.000790 1380 3.183 0.000863100 1800 3.537 0.001125101 1900 3.572 0.001188102 2000 3.608 0.00125103 2100 3.643 0.001313101 2200 3.572 0.001375101 2300 3.572 0.001438100 2400 3.537 0.001598 2500 3.466 0.00156396 2600 3.395 0.00162594 2700 3.325 0.00168889 2800 3.148 0.0017585 2900 3.006 0.001813

Ultimate Load 102.3 Kips Ultimate Strength 3.62 KSI

Figure A1: Stress-Strain Curve for Normal Concrete Cylinder 1 without Smart Aggregates

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Load (Kips) Displacement (x10-5 inch) Stress (KSI) Strain10 75 0.354 0.00004720 190 0.707 0.00011930 330 1.061 0.00020640 460 1.415 0.00028850 600 1.768 0.00037560 775 2.122 0.00048470 975 2.476 0.00060980 1195 2.829 0.00074784 1300 2.971 0.000813

86.7 1400 3.066 0.00087588.8 1500 3.141 0.00093891.4 1600 3.233 0.00100094.5 1700 3.342 0.00106395.9 1800 3.392 0.00112596.8 1900 3.424 0.001188

100.1 2000 3.540 0.001250100.9 2100 3.569 0.001313102.6 2200 3.629 0.001375102.5 2300 3.625 0.001438101.7 2400 3.597 0.00150098.2 2500 3.473 0.00156393.4 2600 3.303 0.001625



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Load (Kips) Displacement (x10-5 inch) Stress (KSI) Strain10 85 0.354 0.00005320 195 0.707 0.00012230 340 1.061 0.00021340 495 1.415 0.00030950 665 1.768 0.00041660 845 2.122 0.00052870 1060 2.476 0.00066380 1310 2.829 0.000819

83.4 1400 2.950 0.00087586 1500 3.042 0.000938

88.4 1600 3.127 0.00100090.2 1700 3.190 0.00106392.6 1800 3.275 0.00112595 1900 3.360 0.001188

96.4 2000 3.409 0.00125098.3 2100 3.477 0.001313100 2200 3.537 0.001375

101.5 2300 3.590 0.001438102.7 2400 3.632 0.001500103.4 2500 3.657 0.001563104.1 2600 3.682 0.001625104.6 2700 3.699 0.001688106.2 2800 3.756 0.001750106.2 2900 3.756 0.001813106.1 3000 3.753 0.001875105.9 3100 3.745 0.001938105.6 3200 3.735 0.002000105.4 3300 3.728 0.002063104.8 3400 3.707 0.002125103.9 3500 3.675 0.002188102.7 3600 3.632 0.002250100 3700 3.537 0.002313


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Appendix A4: Testing data & stress strain curve for Normal Concrete Cylinder 4 with Smart Aggregates

Table A4: Testing Data for Normal Concrete Cylinder 4 with Smart Aggregates

Normal Concrete Cylinders with Smart AggregatesCast on 6/29/2007 Tested on 7/17/2007


84.7 1300 2.996 0.00081388.1 1400 3.116 0.00087591.7 1500 3.243 0.00093893.2 1600 3.296 0.00100096.4 1700 3.409 0.00106399.9 1800 3.533 0.001125

102.5 1900 3.625 0.001188103.6 2000 3.664 0.001250105.3 2100 3.724 0.001313106.9 2200 3.781 0.001375108 2300 3.820 0.001438

108.6 2400 3.841 0.001500109 2500 3.855 0.001563109 2600 3.855 0.001625

109.1 2700 3.859 0.001688108.7 2800 3.844 0.001750108 2900 3.820 0.001813

107.2 3000 3.791 0.001875106.1 3100 3.753 0.001938104.8 3200 3.707 0.002000103.4 3300 3.657 0.002063101 3400 3.572 0.00212595.6 3500 3.381 0.00218895.4 3600 3.374 0.00225088 3700 3.112 0.00231385 3800 3.006 0.00237582 3900 2.900 0.002438


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Figure A4: Stress-Strain Curve for Normal Concrete Cylinder 4 with Smart Aggregates

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Appendix A5: Testing data & stress strain curve for Normal Concrete Cylinder 5 with Smart Aggregates

Table A5: Testing Data for Normal Concrete Cylinder 5 with Smart Aggregates

Normal Concrete Cylinder 5 with Smart AggregatesCast on 6/29/2007 Tested on 7/17/2007


83.1 1400 2.939 0.00087585.7 1500 3.031 0.00093888.6 1600 3.134 0.00100090.5 1700 3.201 0.00106393.3 1800 3.300 0.00112595.2 1900 3.367 0.00118897.4 2000 3.445 0.00125098.6 2100 3.487 0.001313

100.2 2200 3.544 0.001375101.1 2300 3.576 0.001438101.5 2400 3.590 0.001500102.9 2500 3.639 0.001563103.9 2600 3.675 0.001625104 2700 3.678 0.001688

104.2 2800 3.685 0.001750104.3 2900 3.689 0.001813104.9 3000 3.710 0.001875105 3100 3.714 0.001938

104.7 3200 3.703 0.002000104.3 3300 3.689 0.002063103.8 3400 3.671 0.002125103.1 3500 3.646 0.002188102.4 3600 3.622 0.002250101.6 3700 3.593 0.002313100.9 3800 3.569 0.002375100 3900 3.537 0.00243898.6 4000 3.487 0.00250097 4100 3.431 0.002563

95.6 4200 3.381 0.00262593.3 4300 3.300 0.00268890.5 4400 3.201 0.00275087.3 4500 3.088 0.00281384.6 4600 2.992 0.00287581.4 4700 2.879 0.00293877.3 4800 2.734 0.00300069.3 4900 2.451 0.003063


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Figure A5: Stress-Strain Curve for Normal Concrete Cylinder 5 with Smart Aggregates

Appendix A6: Health Monitoring Test data & stress strain curve for Normal Concrete Cylinder 6 with Smart Aggregates

Table A6: Health Monitoring Test Data for Normal Concrete Cylinder 6 with Smart Aggregates

Normal Concrete Cylinder 6 with Smart AggregatesCast on 6/29/2007

Tested on 7/17/2007Test Load (Kips) Stress (KSI)

2 6.9 0.244043 33.8 1.19544 43.1 1.52445 55 1.94526 64.2 2.27067 73.5 2.59958 82.4 2.91439 92.3 3.264410 103.2 3.650011 Cracked /12 Surface Lifted /

Ultimate Load 107.07 KipsUltimate Strength 3.81 KSI

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