enhancing homeland security by using self-sensing concrete a contemporary topic
TRANSCRIPT
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Enhancing Homeland Security by
Using Self-Sensing Concrete
A contemporary topic
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TsunamiTsunami
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Tsunami
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Hurricane Katrina
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Levee breach
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Homeland security
National security against manmade and natural
disasters
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Technological approaches
Border monitoringBuilding security enhancementBuilding/city evacuation monitoringStructural improvement Chemical sensingDisaster prediction
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Self-sensing concrete
Concrete that can sense its own condition
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Self-sensing
Ability of the structural material to sense itself
without any embedded or attached sensor
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Advantages of self-sensing
Low costHigh durabilityLarge sensing volumeAbsence of mechanical
property loss
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Types of self-sensing
Strain/stress sensingDamage sensing
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Applications of strain/stress sensingTraffic monitoring Border securityBuilding facility managementBuilding securityStructural vibration controlWeighingEarthquake prediction
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Applications of damage sensing
Structural health monitoringHazard mitigation
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Border security
Vehicle monitoringPedestrian monitoring
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Building security
Room occupancy monitoringEvacuation monitoring
Intruder detectionDamage monitoring
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Self-sensing concrete material
Cement-matrix composite containing discontinuous, randomly oriented and
well-dispersed carbon fiber
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Carbon fiber is not the sensor.
The composite is the sensor.
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Effects of fiber on concrete
Increase the flexural strengthIncrease the flexural toughnessDecrease the drying shrinkage.Increase the electrical conductivityRender the self-sensing ability
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Why not continuous fiber?
High costCannot be incorporated in mixProvides less effective self-sensing
than discontinuous fiber
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Carbon fiber
15 μm diameter5 mm longAmorphous (turbostratic)Isotropic pitch based
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Percolation threshold
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Reasons for low fiber content
High conductivity is not required for self-sensing
WorkabilityLow costCompression strength
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Below the percolation threshold
Poor fiber dispersion Good fiber dispersion
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Fiber dispersion techniques
Fine particulate admixture (silica fume, 0.1 μm)
Surfactant (methylcellulose)Fiber surface treatment (ozone)Rigorous premixing
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Scientific origin of the self-sensing of strain
Piezoresistivity(not piezoelectricity)
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PiezoresistivityChange of electrical resistivity due to
strainGage factor = fractional change in
resistance per unit strain (more than 2)Gage factor up to 700 attained in
carbon fiber reinforced cement
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Crack
Fiber
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Method
Measure the electrical resistance using a meter
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Uniaxial tension
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With carbon fiber
Uniaxial tension
Longitudinal effect
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With carbon fiber
Uniaxial tension
Transverse effect
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Without carbon fiber
Tension
Longitudinal effect
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Uniaxial compression
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Stress
Strain gageA B C D
A
B
C
D
d
dd
k
h
d
d
h k
Uniaxial compression
d = 13, 25 and 51 mm
Cured while the specimen is lying down
Fiber length = 5 mm
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d = 13 mm (small size)
Longitudinal effect
0.95 vol.% fiber
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d = 13 mm (small size)
Transverse effect
0.95 vol.% fiber
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Damage sensing
Structural health monitoring
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Stress
Strain gageA B C D
A
B
C
D
d
dd
k
h
d
d
h k
Uniaxial compression
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d = 25 mm (medium size)
0.48 vol.% fiber Longitudinal effect
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Compressive testing up to failure
Before loading
After initial 3 cycles of loading
Damage indeed occurred.
Cubic specimens
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Flexure
3-point bending
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A1 A2 A3 A4
B4B3B2B1
160
40
40
140
20 20 20 2080
Flexure
Dimensions in mm
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With carbon fiber
Flexure
Surface resistance at compression side
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Surface resistance at tension side
With carbon fiber
Flexure
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Conventional concrete
Self-sensing cement coating
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Alternate scheme for flexural sensing
Coating the tension or compression side of a conventional concrete slab with self-sensing cement
Coating on the tension side gives higher sensitivity than coating on the compression side.
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Self-sensing implementation in buildings
Coat the ceiling with
self-sensing cement.
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Carbon fiber vs. carbon nanofiber
Nanofiber is less effective as a reinforcement.
Nanofiber fails to provide
self-sensing.
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Conclusion 1Multifunctional cement-based
materials have been attained without compromising the structural performance.
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Conclusion 2
Carbon fiber cement is effect for the self-sensing of strain and damage, due to the reversible effect of strain on the electrical resistivity and the irreversible effect of damage on the resistivity.