Thermocouples Pavements
-
Upload
umar-sharif -
Category
Documents
-
view
234 -
download
0
Transcript of Thermocouples Pavements
-
8/8/2019 Thermocouples Pavements
1/14
Thermocouple Applications in Pavement Systems
Jake Hiller
Graduate Research Assistant
Presentation for CEE 398 KUC Experiments in Structures and Materials
March 6, 2002
-
8/8/2019 Thermocouples Pavements
2/14
Outline of Presentation
Background on Thermocouples
Rigid Pavement Applications
Flexible Pavement Applications
Summary
-
8/8/2019 Thermocouples Pavements
3/14
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)
-
8/8/2019 Thermocouples Pavements
4/14
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)
-
8/8/2019 Thermocouples Pavements
5/14
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.
-
8/8/2019 Thermocouples Pavements
6/14
Temperature Limitations
-
8/8/2019 Thermocouples Pavements
7/14
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
-
8/8/2019 Thermocouples Pavements
8/14
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
-
8/8/2019 Thermocouples Pavements
9/14
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
-
8/8/2019 Thermocouples Pavements
10/14
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
-
8/8/2019 Thermocouples Pavements
11/14
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
-
8/8/2019 Thermocouples Pavements
12/14
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
-
8/8/2019 Thermocouples Pavements
13/14
Flexible Pavement Applications
TCs placed near: pressure cells, strain gages, or
FWD test locations to correlate with temperature
-
8/8/2019 Thermocouples Pavements
14/14
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