Thermocouples Pavements

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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


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Outline of Presentation

Background on Thermocouples

Rigid Pavement Applications

Flexible Pavement Applications

Summary


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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)


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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)


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Many types of Thermocouples

Type K cheap, general purpose Positive 90% Ni, 10% Cr Negative 95% NiAl, Mn, Si

Type T Good accuracy in pavt 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% NiCr, Si Negative 96% Ni Si, etc.


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Temperature Limitations


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ThermocouplesOptions

Insulation/sheathing to protect from outside factors

Gage of wire is related to performance

Pre-assembled with connectors to fit thermometer

Multiple TCs pre-assembled

Unassembled wire Color code by type

Differs between some countries


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Problems with Thermocouples

Accuracy Often between 0.5 and 2.2C, 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


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Three types of thermal movements in rigid pavts

Curling

Thermal gradient in slab

Expansion/Contraction

Uniform temperature change

Soil frost heave

Lifting of slab due to increasing volume of underlying layers

Rigid Pavement Applications

Downward curling:

Bottom contracts

relative to top

Upward curling: Top

contracts relative to

bottom

L (L


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Calibration/Instrumentation of Thermocouples

Typically tested in hot and coldbaths

Confirmation and sway in readings Placed in two ways

Set at different depths along woodendowel or bracket

Placed in by hand as paving isoccurring (less reliable)

Minimum of 0.5 of coverneeded


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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


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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


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TCs placed near: pressure cells, strain gages, or

FWD test locations to correlate with temperature


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Summary

TCs based on energy potential differences of alloys

Each TC type has distinct advantages

Type T and K most used in pavt field testing

Accuracy is sometimes questionable

Corrosion can be a potential problem

Used in rigid pavt to assess curling and expansion Installed before paving typically

Used in flexible pavt to determine seasonal

variability and frost action

Thermocouples Pavements

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