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.