Intrinsically smart cement-matrix composites

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Intrinsically smartcement-matrix composites

• Topic 5

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Reading assignment

• Chung, Composite Materials, Ch. 13.
• No. 130, under Publications cement in website
  http//www.wings.buffalo.edu/academic/department/e
  ng/mae/cmrl

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Functions

• Structural
• Strain/stress sensing
• Damage sensing
• Temperature sensing
• Electromagnetic interference (EMI) shielding
• Vibration reduction
• Self-heating

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Applications of strain-stress sensing

• Structural vibration control
• Traffic monitoring
• Weighing (including weighing in motion)
• Building facility management
• Security

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Strain/stress sensing

• Piezoresistivity
• Direct piezoelectricity

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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

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With carbon fiber
Tension
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With carbon fiber
Tension
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Without carbon fiber
Tension
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Applications of damage sensing

• Structural health monitoring
• Damage/microstructural evolution study

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Damage sensing methods

• Acoutic emission
• Electrical resistivity measurement
• Optical fiber sensor embedment

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Resistance measurement methods

• Volume resistance (for sensing the damage of a
  volume)
• Surface resistance (for sensing the damage of the
  surface)
• Contact resistance (for sensing the damage of an
  interface)
• Apparent volume resistance (for sensing the
  damage of an interface between dissimlar
  materials)

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Flexure
Dimensions in mm
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Surface resistance at compression side
With carbon fiber
Flexure
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Surface resistance at tension side
With carbon fiber
Flexure
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Through-thickness resistance
With carbon fiber
Flexure
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Oblique resistance
With carbon fiber
Flexure
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Oblique
Surface tension
Through-thickness
Surface - compression
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Mortar (without fiber) during freeze-thaw cycling
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Without freezing
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Carbon fiber concrete under repeated compression
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Interface between concrete and steel rebar under
cyclic shear
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Interface between old and new mortar under
cyclic shear
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Interface between unbonded mortar elements
under cyclic compression
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Interface between concrete and its carbon fiber
epoxy composite retrofit
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Applications of temperature sensing

• Thermal control
• Structural operation control
• Hazard monitoring

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Temperature sensing methods

• Thermocouples
• Thermistors

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Cement-based thermistor

• Carbon fiber reinforced cement
• Activation energy 0.4 eV

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Cement-based thermocouple

• Carbon fiber reinforced cement (p-type)
• Steel fiber reinforced cement
• (n-type)
• pn-junction
• 70 microvolts/degree C

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Thermoelectric cement-based materials

• Absolute thermoelectric power tailored by using
  conductive admixtures
• Carbon fiber for p-type behavior
• Steel fiber for n-type behavior

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Effect of stainless steel fiber (60 micron
diameter)
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Applications of electrically conducting
cement-based materials

• EMI shielding
• Electrostatic protection
• Lightning protection
• Cathodic protection
• Self-heating
• Lateral guidance in automatic highways

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Cement pastes (with 1 vol. conductive admixture)

• Steel fiber (8 microns)
  40 ohm.cm
• Carbon fiber (15 microns)
  830 ohm.cm
• Carbon nanofiber (0.1 micron) 12,000
  ohm.cm
• Graphite powder (0.7 micron) 160,000
  ohm.cm
• Coke powder (less than 75 microns) 38,000
  ohm.cm