Enhancing Homeland Security by Using Self-Sensing Concrete A contemporary topic
Tsunami
Tsunami
Hurricane Katrina
Levee breach
Homeland security National security against manmade and natural disasters
Technological approaches Border monitoring Building security enhancement Building/city evacuation monitoring Structural improvement Chemical sensing Disaster 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 cost High durability Large sensing volume Absence of mechanical property loss
Types of self-sensing Strain/stress sensing Damage sensing
Applications of strain/stress sensing Traffic monitoring Border security Building facility management Building security Structural vibration control Weighing Earthquake prediction
Applications of damage sensing Structural health monitoring Hazard mitigation
Border security Vehicle monitoring Pedestrian monitoring
Building security Room occupancy monitoring Evacuation monitoring Intruder detection Damage 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 strength Increase the flexural toughness Decrease the drying shrinkage. Increase the electrical conductivity Render the self-sensing ability
Why not continuous fiber? High cost Cannot be incorporated in mix Provides less effective self-sensing than discontinuous fiber
Carbon fiber 15 μm diameter 5 mm long Amorphous (turbostratic) Isotropic pitch based
Percolation threshold
Reasons for low fiber content High conductivity is not required for self-sensing Workability Low cost Compression strength
Below the percolation threshold Poor fiber dispersionGood 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)
Piezoresistivity Change of electrical resistivity due to strain Gage 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 gage A B C D A B C D d d d 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 gage A B C D A B C D d d d 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
A1A1 A2A2 A3A3 A4A4 B4B4 B3B3 B2B2 B1B 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 1 Multifunctional 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.