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Thermocouple Applications in Pavement Systems

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Thermocouple Applications in Pavement Systems. Jake Hiller Graduate Research Assistant. Presentation for CEE 398 KUC – Experiments in Structures and Materials March 6, 2002. Outline of Presentation. Background on Thermocouples Rigid Pavement Applications Flexible Pavement Applications

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Thermocouple Applications in Pavement Systems

Jake Hiller

Graduate Research Assistant

Presentation for CEE 398 KUC – Experiments in Structures and Materials

March 6, 2002

outline of presentation
Outline of Presentation
  • Background on Thermocouples
  • Rigid Pavement Applications
  • Flexible Pavement Applications
  • Summary
background on thermocouples
Background on Thermocouples
  • Two conductor cables
  • Composed of metal alloys
  • Ends of wires are soldered to form a couple
  • Resistance of couple changes with temperature
    • Seebeck Effect – 1822
  • Resistance is then correlated with temperature
    • Standardized correlations
    • Correlations can change over time (deformation, corrosion)
seebeck effect
Seebeck Effect
  • Discovered by Thomas Seebeck, 1822
  • Electrons flow from one wire to other
    • Due to different energy potentials of alloys
  • As temperature changes, current flows
  • Voltage is measured between the two alloys
    • Small voltage (less than 10 mV)
many types of thermocouples
Many types of Thermocouples
  • Type K – cheap, general purpose
    • Positive 90% Ni, 10% Cr Negative 95% Ni…Al, Mn, Si
  • Type T – Good accuracy in pav’t temp range
    • Positive 100% Cu Negative 55% Cu, 45% Ni (constantan)
    • Low corrosion potential
  • Type J
    • Positive 100% Fe Negative 55% Cu, 45% Ni (constantan)
  • Type N
    • Positive 85% Ni…Cr, Si Negative 96% Ni… Si, etc.
thermocouples options
Thermocouples Options
  • Insulation/sheathing to protect from outside factors
  • Gage of wire is related to performance
  • Pre-assembled with connectors to fit thermometer
    • Multiple TC’s pre-assembled
  • Unassembled wire
    • Color code by type
    • Differs between some countries
problems with thermocouples
Problems with Thermocouples
  • Accuracy
    • Often between 0.5 and 2.2ºC, depending on TC type
  • Noise
    • Long leads can attract electrical signals
    • Already low signal from thermocouple
  • Thermal shunting
    • Heating of wire mass can affect measurements by absorbing energy
  • Corrosion
    • High alkali or water environments can modify calibration
rigid pavement applications

L

DL

Rigid Pavement Applications
  • Three types of thermal movements in rigid pav’ts
    • Curling
      • Thermal gradient in slab
    • Expansion/Contraction
      • Uniform temperature change
    • Soil frost heave
      • Lifting of slab due to increasing volume of underlying layers

Upward curling: Top contracts relative to bottom

Downward curling: Bottom contracts relative to top

calibration instrumentation of thermocouples
Calibration/Instrumentation of Thermocouples
  • Typically tested in hot and cold baths
    • Confirmation and sway in readings
  • Placed in two ways
    • Set at different depths along wooden dowel or bracket
    • Placed in by hand as paving is occurring (less reliable)
  • Minimum of 0.5” of cover needed
rigid pavement testing
Rigid Pavement Testing
  • Used in conjunction with other sensors to evaluate pavement performance
  • Include vibrating wire, moisture resistance sensors, psychrometers, etc.
  • Typically placed at either corner, edge, or middle of slabs
flexible pavement applications
Flexible Pavement Applications
  • Determination of Viscoelastic Properties
    • Rutting potential increases with temperature
    • Lower modulus - Higher deflections
  • Thermal Cracking
    • Low temperatures – thermal stresses increase
    • Stress can surpass tensile strength of material
    • Fatigue of material can also occur
  • Soil frost heave
flexible pavement applications1
Flexible Pavement Applications
  • TC’s placed near: pressure cells, strain gages, or FWD test locations to correlate with temperature
summary
Summary
  • TC’s based on energy potential differences of alloys
  • Each TC type has distinct advantages
    • Type T and K most used in pav’t field testing
    • Accuracy is sometimes questionable
    • Corrosion can be a potential problem
  • Used in rigid pav’t to assess curling and expansion
    • Installed before paving typically
  • Used in flexible pav’t to determine seasonal variability and frost action
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