Thermocouple Applications in Pavement Systems

1. Thermocouple Applications in Pavement Systems Jake Hiller Graduate Research Assistant Presentation for CEE 398 KUC – Experiments in Structures and Materials March 6, 2002

2. Outline of Presentation • Background on Thermocouples • Rigid Pavement Applications • Flexible Pavement Applications • Summary

3. 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)

4. 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)

5. 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.

6. Temperature Limitations

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. Flexible Pavement Applications • TC’s placed near: pressure cells, strain gages, or FWD test locations to correlate with temperature

14. 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