Determining Frost Depth in Pavement Systems

Using a Multi-Segment Time Domain Reflectometry Probe

Ruth RobersonJohn Siekmeier

Minnesota Department of TransportationOffice of Materials and Road Research

Mn/ROADOffice of Materials and Road Research

Applications in Pavement Research and Engineering

• Seasonal Load Limit – Optimize timing of:

• Winter load increase• Spring load decrease

• Mechanistic Empirical Design– Seasonal Changes in Strength Properties

• Material properties database

Objectives

• Evaluate Mult-Segment TDR probe for improved frost depth measurements within pavement structures.

• Implement field testing at designated Road and Weather Information (R/WIS) System sites.

Summary of Methods

Frost Tube Resistivity Probe

Moisture Block

TDR Probe

Data Collection

Manual Primarily Manual

Automated Automated

Data Interpretation

Subjective Subjective, requiring temperature data.

Subjective, requiring temperature data.

Potential for developing algorithm for automated analysis.

Installation Labor intensive. Extensive soil disturbance.

Labor intensive. Extensive soil disturbance.

Labor intensive. Extensive soil disturbance.

Minimal Labor. Minimal soil disturbance.

Using TDR to Estimate Frost Depth

• Significant decrease in dielectric constant (Ka) when water freezes.

• Result is abrupt decrease in the propagation time(∆t) measured by the TDR probe.

Multi-Segment TDR Probe

Type K Probe:•Four segment Probe•Each segment 15 cm•Two stainless steel rails separated by an epoxy and high density plastic.

•Segment boundaries defined by switching diodes.

1

2

3

4

Switching Diodes

• Frozen Soil Core – PVC column packed with Class 5 Special

resting on heater core base.

– Instrumentation• MP Probe• Thermocouples

– Automated Data collection• MP917• Campbell Scientific CR10X and AM416• Dell PC (Procomm script file)

Laboratory Experiment

Confine Freezing to the Surface of the Soil Column

3” R13 U

nfaced Fiberglass Insulation

1

2

3

4

Switching Diodes

Thermocouples

Heater Core

Segment 1

-10

0

10

20

30

40

90 110 130 150 170 190 210

Time (doy)

Tem

pera

ture

(C)

4

6

8

10

12

14

16

Diel

ectr

ic

Air_T Soil_T Ka

Freeze

Thaw

Frost Depth(Step-wise freezing)

Segment 3-10

0

10

20

30

40

90 110 130 150 170 190 210

Time (doy)

Tem

pera

ture

(C)

4

6

8

10

12

14

16

Diele

ctric

Air_T Soil_T Ka

Freeze

Thaw

Frost Depth(Rapid freezing)

Frost Depth(Resolution)

Segment 1

-10

0

10

20

30

40

90 110 130 150 170

Time (doy)

Temp

eratur

e (C)

4

6

8

10

12

14

16

Diele

ctric

Air_TSoil_T (2 cm)Soilt_T (13cm)Ka

RESULTS• Measured decrease in Ka near 0° C for each segment.

• Estimated frost depths agree with temperature measurements and physical test.

• Rate of freeze/thaw and initial moisture content affect magnitude of change in Ka near 0° C.

• Resolution is limited to the length of the segment.

Road and Weather Information System (R/WIS)

• Main elements of RWIS are– environmental sensor system (ESS) – advanced processing systems to develop forecasts and

tailor the information into an easily understood format – dissemination platforms on which to display the

tailored information.

Road and Weather Information System (R/WIS) cont’d

• Specific to Minnesota– Implementation of a system that is able to

accommodate various types of instruments. – Portion of the R/WIS project is dedicated to testing the

system's communication architecture.• Specific to Minnesota Road Research

– Investigate new methods for determining frost depth as well as looking for improved data transfer protocols.

STATE-WIDE INSTALLATIONS

ROCHESTERSolar Powered

MN/ROAD

MARSHALLSolar Powered

EFFE R/WIS Site

JACOBSON R/WIS Site

R/WIS SITES:EFFE and JACOBSON, MN

0

6

12

18

24

30

36

42

2-Mar 12-Mar 22-Mar 1-Apr 11-Apr 21-Apr 1-May 11-May 21-May

Date

Ka

(d

iele

ctri

c co

nst

an

t)

21 - 27 inches

0

6

12

18

24

30

36

42

2-Mar 12-Mar 22-Mar 1-Apr 11-Apr 21-Apr 1-May 11-May 21-May

Date

Ka

(die

lect

ric

con

stan

t)

27 - 33 inches

0

6

12

18

24

30

36

42

2-Mar-01 12-Mar-01 22-Mar-01 1-Apr-01 11-Apr-01 21-Apr-01 1-May-01 11-May-01 21-May-0

Date

Ka (

die

lectr

ic c

on

stan

t) 15 - 21 inchesR/WIS SITE: 52EFFE, MN

Spring Thaw 2001

CONCLUSIONS

• Multi-segment TDR probe shows promise as instrument for measuring frost depth within pavement structures.

• TDR methods provide a means for validating air temperature based models.

• Increase the number of field sites.• Continue research into TDR methods.

Continuing Research

• Temperature Effects on Dielectric Constant – Evidence of significant temperature effect on

measured Ka.– Temperature correction for moisture content

measurements.• Calibration for Moisture Content• Automation of data interpretation

– Ka and raw waveform interpretation

Acknowledgements

• Minnesota Local Road Research Board• Mn/DOT Engineering Services• Mn/DOT Maintenance Division (Ed Fleedge)• Office of Materials and Road Research• Environmental Sensors Inc.• Chad Millner