Upload
della-marsh
View
222
Download
0
Embed Size (px)
Citation preview
Enhancing Homeland Security by
Using Self-Sensing Concrete
A contemporary topic
TsunamiTsunami
Tsunami
Hurricane Katrina
Levee breach
Homeland security
National security against manmade and natural
disasters
Technological approaches
Border monitoringBuilding security enhancementBuilding/city evacuation monitoringStructural improvement Chemical sensingDisaster prediction
Self-sensing concrete
Concrete that can sense its own condition
Self-sensing
Ability of the structural material to sense itself
without any embedded or attached sensor
Advantages of self-sensing
Low costHigh durabilityLarge sensing volumeAbsence of mechanical
property loss
Types of self-sensing
Strain/stress sensingDamage sensing
Applications of strain/stress sensingTraffic monitoring Border securityBuilding facility managementBuilding securityStructural vibration controlWeighingEarthquake prediction
Applications of damage sensing
Structural health monitoringHazard mitigation
Border security
Vehicle monitoringPedestrian monitoring
Building security
Room occupancy monitoringEvacuation monitoring
Intruder detectionDamage monitoring
Self-sensing concrete material
Cement-matrix composite containing discontinuous, randomly oriented and
well-dispersed carbon fiber
Carbon fiber is not the sensor.
The composite is the sensor.
Effects of fiber on concrete
Increase the flexural strengthIncrease the flexural toughnessDecrease the drying shrinkage.Increase the electrical conductivityRender the self-sensing ability
Why not continuous fiber?
High costCannot be incorporated in mixProvides less effective self-sensing
than discontinuous fiber
Carbon fiber
15 μm diameter5 mm longAmorphous (turbostratic)Isotropic pitch based
Percolation threshold
Reasons for low fiber content
High conductivity is not required for self-sensing
WorkabilityLow costCompression strength
Below the percolation threshold
Poor fiber dispersion Good fiber dispersion
Fiber dispersion techniques
Fine particulate admixture (silica fume, 0.1 μm)
Surfactant (methylcellulose)Fiber surface treatment (ozone)Rigorous premixing
Scientific origin of the self-sensing of strain
Piezoresistivity(not piezoelectricity)
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
Crack
Fiber
Method
Measure the electrical resistance using a meter
Uniaxial tension
With carbon fiber
Uniaxial tension
Longitudinal effect
With carbon fiber
Uniaxial tension
Transverse effect
Without carbon fiber
Tension
Longitudinal effect
Uniaxial compression
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
d = 13 mm (small size)
Longitudinal effect
0.95 vol.% fiber
d = 13 mm (small size)
Transverse effect
0.95 vol.% fiber
Damage sensing
Structural health monitoring
Stress
Strain gageA B C D
A
B
C
D
d
dd
k
h
d
d
h k
Uniaxial compression
d = 25 mm (medium size)
0.48 vol.% fiber Longitudinal effect
Compressive testing up to failure
Before loading
After initial 3 cycles of loading
Damage indeed occurred.
Cubic specimens
Flexure
3-point bending
A1 A2 A3 A4
B4B3B2B1
160
40
40
140
20 20 20 2080
Flexure
Dimensions in mm
With carbon fiber
Flexure
Surface resistance at compression side
Surface resistance at tension side
With carbon fiber
Flexure
Conventional concrete
Self-sensing cement coating
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.
Self-sensing implementation in buildings
Coat the ceiling with
self-sensing cement.
Carbon fiber vs. carbon nanofiber
Nanofiber is less effective as a reinforcement.
Nanofiber fails to provide
self-sensing.
Conclusion 1Multifunctional cement-based
materials have been attained without compromising the structural performance.
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.