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.