SPECIAL REPORT 96-12

PCC Airfield Pavement Response

During Thaw-Weakening Periods

A Field Study

Vincent C. Janoo and Richard L. Berg May 1996
Abstract

This investigation is part of CRREL’s on-going characterization of pavement

performance in seasonal frost areas. As part of the research, CRREL conducted several field studies for the Federal Aviation Administration (FAA) on the

response of airport pavements during thaw-weakening periods at three civil

airports in Wisconsin, where the design freezing index in the area was around

900 to 1100°C-days and frost penetration ranged between 1250 to 2000

mm. This study focused on the performance of Portland Cement Concrete

(PCC) pavements during the spring thaw-weakening period. The sites were

instrumented with subsurface thermocouples and Falling Weight Deflectometer (FWD) tests were conducted during the spring thaw period at the center of

the slab and across the joints. An analysis of the FWD data and backcalculation of the layer moduli using ILLIBACK and WESDEF was conducted. Unique

relationships between the FWD deflections and the subgrade modulus and

coefficient of subgrade reaction were obtained. Additional relationships were

developed using the FWD deflections, PCC thickness and the horizontal tensile stress at the bottom of the PCC layer. A relationship between load transfer

across joints and FWD deflections was also developed. On the basis of the

relationships obtained in this study, a methodology for evaluating PCC pavements during spring thaw was developed. However, this methodology needs

to be verified for other subgrade types and areas with other design freezing

indices.

For conversion of SI units to non-SI units of measurement consult Standard

Practice for Use of the International System of Units (SI), ASTM Standard E380-

93, published by the American Society for Testing and Materials, 1916 Race St.,

Philadelphia, Pa. 19103.

This report is printed on paper that contains a minimum of 50% recycled

material.
Special Report 96-12

PCC Airfield Pavement Response

During Thaw-Weakening Periods

A Field Study

Vincent C. Janoo and Richard L. Berg May 1996

Prepared for

FEDERAL AVIATION ADMINISTRATION

and

OFFICE OF THE CHIEF OF ENGINEERS

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
ii

PREFACE

This report was prepared by Dr. Vincent C. Janoo and Dr. Richard L. Berg, Research Civil

Engineers, Civil and Geotechnical Engineering Research Division, Research and Engineering

Directorate, U.S. Army Cold Regions Research and Engineering Laboratory. Funding was provided by the Federal Aviation Administration and the Office of the Chief of Engineers. The OCE

portion was funded through DA Project 4A762784 AT42, Design, Construction and Operations

Technology for Cold Regions; Mission Area, Base Support; Work Unit BS/036, Improved Pavement Design Criteria in Cold Regions.

Technical review of the manuscript of this report was provided by Dr. Raymond Rollings

(U.S. Army Engineer Waterways Experiment Station) and Michel Hovan (FAA). The authors

express special thanks to L. Barna and F. Carver for assisting in the data reduction and for their

patience, and to R. Guyer and C. Berini for gathering the data.

The contents of this report are not to be used for advertising or promotional purposes. Citation

of brand names does not constitute an official endorsement or approval of the use of such commercial products.
iii

CONTENTS

Preface ................................................................................................................................... ii

Introduction ........................................................................................................................... 1

Description of airfields.......................................................................................................... 1

Central Wisconsin Airport ................................................................................................ 1

Outagamie County Airport ............................................................................................... 3

Field instrumentation and testing program ........................................................................... 4

Environmental data analysis ................................................................................................. 5

FWD data analysis ................................................................................................................ 6

Bearing capacity analysis ................................................................................................. 10

Backcalculation of layer moduli ...................................................................................... 14

Load transfer efficiency ........................................................................................................ 29

Proposed pavement evaluation procedure ............................................................................ 37

Conclusions ........................................................................................................................... 38

Literature cited ...................................................................................................................... 38

Abstract ................................................................................................................................. 39

ILLUSTRATIONS

Figure

1. Location of airfields ....................................................................................................... 1

2. Pavement structure at Central Wisconsin Airport ......................................................... 2

3. Pavement structure at Outagamie County Airport ........................................................ 3

4. FWD, temperature and moisture sensor locations......................................................... 4

5. Daily minimum and maximum temperatures ................................................................ 5

6. Air-freezing indices ....................................................................................................... 6

7. Frost and thaw depths calculated from subsurface temperature measurements ........... 7

8. Location of FWD sensors across joints and corner of a PCC slab ............................... 7

9. Changes in basin area and impulse stiffness modulus during spring thaw at

Outagamie County Airport ....................................................................................... 10

10. Changes in basin area during spring thaw at Central Wisconsin Airport ..................... 11

11. Changes in impulse stiffness modulus during spring thaw at Central Wisconsin

Airport ...................................................................................................................... 12

12. Relationship between surface temperature and basin area ............................................ 13

13. Relationship between surface temperature and impulse stiffness modulus .................. 14

14. Idealized pavement structures........................................................................................ 14

15. Effect of PCC modulus on WESDEF absolute error, Outagamie County Airport ....... 15

16. Effect of PCC modulus on change in subgrade modulus from WESDEF during

spring thaw at Outagamie County Airport ............................................................... 16

17. Backcalculated base course modulus using WESDEF, Outagamie County Airport ..... 17

18. Change in subgrade modulus during spring thaw ......................................................... 18

19. Change in base course modulus during spring thaw ..................................................... 19

20. Relationship between subgrade moduli backcalculated by WESDEF and ILLIBACK. 20

21. Typical backcalculated PCC modulus from ILLIBACK ............................................... 20

22. Relationship between measured total basin area and calculated subgrade modulus .... 21

23. Relationship between measured partial basin area and calculated subgrade modulus

at Outagamie County Airport ................................................................................... 21

24. Relationship between measured impulse stiffness modulus and calculated subgrade

modulus .................................................................................................................... 22
iv

Figure

25. Relationship between total basin area and subgrade modulus at Central Wisconsin

Airport and Outagamie County Airport ................................................................... 22

26. Relationship between total basin area and coefficient of subgrade reaction calculated using ILLIBACK at Outagamie County Airport and Central Wisconsin

Airport ...................................................................................................................... 23

27. Relationship between impulse stiffness modulus and subgrade modulus at

Outagamie County Airport and Central Wisconsin Airport .................................... 24

28. Configuration and location of stress calculations for Boeing 757 and MD-DC9 ......... 25

29. Amount of damage during spring thaw at Central Wisconsin Airport .......................... 25

30. Effect of pavement thickness on damage at Central Wisconsin Airport ....................... 25

31. Amount of damage during spring thaw at Outagamie County Airport ......................... 27

32. Effect of subgrade modulus on the horizontal tensile stress at the bottom of the PCC

layer at Outagamie County Airport and Central Wisconsin Airport ....................... 27

33. Effect of the coefficient of subgrade reaction on the horizontal tensile stress at the

bottom of the PCC layer .......................................................................................... 28

34. Effect of PCC modulus on the horizontal tensile stress at the bottom of the PCC

layer .......................................................................................................................... 29

35. Linear relationship between total basin area and maximum horizontal tensile stress

at bottom of PCC layer ........................................................................................... 30

36. Relationship between impulse stiffness modulus and horizontal tensile stress at

bottom of PCC layer ................................................................................................ 30

37. Load transfer efficiency across a joint ........................................................................... 30

38. Placement of FWD sensors for load transfer efficiency test ......................................... 31

39. Relationship between air temperature and transverse joint transfer efficiency ............ 31

40. Change of transverse joint efficiency with subsurface temperature ............................. 34

41. Effect of subgrade modulus calculated with ILLIBACK on transverse joint

efficiency .................................................................................................................. 34

42. Change in transverse joint transfer with time on taxiway B, Central Wisconsin

Airport ...................................................................................................................... 35

43. Relationship between AREA and l for a dense liquid foundation ................................ 35

44. Interpolated relationships between joint efficiency and load transfer as a function

of a /l ........................................................................................................................ 36

45. Relationship between JTE and LTE for transverse joints ............................................. 36

46. Relationship between total basin area and subgrade modulus ...................................... 37

47. Relationship between JTE and LTE............................................................................... 37

TABLES

Table

1. Pavement structures at Central Wisconsin Airport ........................................................ 2

2. Temperature sensor locations under pavement surface ................................................. 4

3. Types of falling weight deflection tests conducted ....................................................... 8

4. Pavement surface temperatures at time of falling weight deflection test ..................... 9

5. Thickness of subgrade at backcalculated falling weight deflection locations .............. 15

6. Backcalculated modulus at Central Wisconsin Airport using WESDEF ....................... 17

7. Effect of change in PCC modulus on base and subgrade modulus ............................... 18

8. Gear loading for the MD-DC9 and Boeing 757 ............................................................ 24

9. Gear information for computer simulations of the MD-DC9 and Boeing 757 ............. 24

10. Ratio of maximum horizontal tensile stress to flexural strength during spring thaw ... 26

11. Joint transfer efficiencies at Central Wisconsin Airport ................................................ 32

12. Joint transfer efficiencies at Outagamie County Airport ............................................... 33
INTRODUCTION

In the spring of 1986, CRREL conducted a field

study for the Federal Aviation Administration (FAA)

on how airport pavements responded to frost action.

The emphasis was on thaw weakening. The study was

conducted at three regional airports in Wisconsin—

Central Wisconsin Airport (CWA), Outagamie County

Airport (OCA) and Wittman Field. The pavement surfaces at CWA and OCA were predominately Portland

Cement Concrete (PCC), whereas at Wittman they

were mostly Asphalt Concrete (AC). The results of a

study on AC airfield pavement structures during thawweakening periods can be found in a previously published report (Janoo and Berg 1991). This report examines PCC airport pavements during thaw periods.

It is accepted that in the winter the load carrying

capacity of pavements increases dramatically because

of freezing of the pavement structure. This is more dramatic in AC pavements because of the stiffening of the

asphalt at low temperatures. This increase is also seen

in PCC pavements because of a similar stiffening of

the base, subbase and subgrade.

During thaw periods, the pavement structure below

the PCC layer thaws and can become saturated with

water from the melting ice lenses and infiltration of

surface water from rain or melting snow. This saturation of the material reduces the strength of the base,

subbase or subgrade, or all three, leading to reduced

bearing capacity of the entire pavement structure. In

addition, the large temperature differentials during

thawing periods can cause curling of the corners and

edges of slabs of PCC pavements, thus affecting load

transfer across joints.

The objective of the study was to determine any

structural changes in PCC airport pavements during

thaw-weakening periods. To evaluate these changes,

CRREL conducted Falling Weight Deflectometer

(FWD) measurements to quantify the changes in the

stiffness of the pavement structure and the load transfer efficiency of the joints. In addition, subsurface

PCC Airfield Pavement Response

During Thaw-Weakening Periods

A Field Study

VINCENT C. JANOO AND RICHARD L. BERG

pavement temperatures were measured at selected locations at each airport. This report gives a general description of the airports and the pavement structures

and a comprehensive analysis of the FWD measurements.

DESCRIPTION OF AIRFIELDS

Central Wisconsin Airport (CWA)

CWA is located in Mosinee, Wisconsin (Fig. 1).

The subsurface soils at CWA are silts, sandy silts and

clayey silts and can be classified as either SM or ML

using the Unified Soil Classification System, and as

F3 and F4 with respect to frost-susceptibility (Stark

and Berg 1989). F3 and F4 soils are very susceptible

to frost heave and thaw weakening. Stark and Berg

(1989) also reported that the subgrade was not uniFigure 1. Location of airfields.

* Mean Air Freezing Index (°C-days)

CWA

Mosinee

Appleton

OCA
2

Figure 2. Pavement structure at Central Wisconsin Airport (CWA).

Table 1. Pavement structures at Central Wisconsin Airport.

Slab

Date Length Width size Longitudinal Transverse

Pavement constructed (m) (m) (m) joint joint

Runway 8/26

(original) 1968 2042 46 3.8 × 6.1 keyed & dummy doweled

Runway 8/26

(extension) 1973 244 46 7.6 × 7.6 keyed doweled

Runway 17/35 1972 1737 46 7.6 × 7.6 keyed doweled

Taxiway A 1969 297 23 3.8 × 6.1 keyed & tied doweled

Taxiway B

(ramp to taxiway C) 1975 139 23 7.6 × 7.6 keyed & tied doweled

3.8 × 7.6

Taxiway B

(taxiway C to 17/35) 1977 954 15 3.8 × 7.6 Butt, tied doweled & dummy

Taxiway C 1973 2256 15 7.6 × 7.6 keyed & tied doweled

3.8 × 7.6

Taxiway D 1973 88 20 7.6 × 7.6 keyed & tied doweled

3.8 × 7.6

Taxiway E 1973 88 20 3.8 × 5.3 keyed & tied doweled

form, having clusters of rocks and boulders incorporated into the finer grained soils. Bedrock was reported at uneven depths and at some locations it was

close to the pavement surface.

The airport pavements consist of two intersecting

runways, five taxiways and three ramps (Fig. 2). The

original airfield—runway 8/26, taxiway A and an air

carrier ramp—was constructed in 1968 and 1969. Between 1972 and 1973, runway 17/35 and taxiways C,

D and E were constructed. Also in 1973, runway 8/26

was extended by 245 m and the air carrier ramp expanded. Taxiway B (between the ramp and taxiway C)

was constructed in 1975 and was extended to connect

the two runways in 1977. Portions of runway 8/26

were reconstructed in 1987. A summary of the construction history, length, width and types of joints of

the different pavement structures at CWA is presented

in Table 1.

The airfield basically was constructed with PCC.

The original runway, taxiway and ramp, constructed
in 1968 and 1969, had 254 mm of PCC over 229 mm

of crushed stone base over subgrade. Later construction mostly used 305 mm of PCC over 229 mm of

crushed stone base over subgrade. The structure of the

different pavement sections as of spring 1986 is also

shown in Figure 2.

The slabs sizes were primarily 7.6 by 7.6 m; however, in some areas, the slabs were 3.8 by 3.8 m. Other

sizes used are shown in Table 1. Loads are transferred

across the transverse joints by dowels and aggregate

interlocks (Table 1) (CMT 1984). At longitudinal

joints, loads are transferred through keyways, aggregate interlocks and tiebars (Table 1) (CMT 1984). The

primary types of aircraft using the airport are Convair

580 (24,766 kg), MD DC-9 (44,452 kg) and Boeing

757 (115,666 kg) (CMT 1984).

Outagamie County Airport (OCA)

OCA is located near Appleton, Wisconsin (Fig. 1).

The subgrade at the airport consists mostly of a low

plasticity clay (CL), some sand (SP) and silty sand

(SM). At OCA, bedrock was estimated to be 4.0 m

deep or more, on the basis of boring logs. ERES consultants (1985) reported the subgrade under runway

3/21 as a highly frost-susceptible red silty clay. They

also reported that the subbase material may be frostsusceptible because of a high amount of fines passing

the no. 200 sieve (8–10 %). Runway 3/21 has severe

frost heave problems (ERES 1985). Mead and Hunt

(1988) reported that the subgrade under runway 11/29

was a heavy clay (USC classification–CL; FAA classification–E7) with a design California Bearing Ratio

(CBR) of 4. They also reported clay migration into the

base course and trapped water under the pavement.

The airport pavements consist of two intersecting

runways, five taxiways and three ramps (Fig. 3). Runway 3/21 was constructed in 1967 and 1968, with 203

mm of PCC (254 mm in critical areas) over 203 mm of

crushed gravel over subgrade. Runway 11/29, reconstructed in 1988 and 1989, had 178 mm PCC (229 mm

in critical areas) over 203 mm of crushed aggregate

base course over subgrade.

The PCC slabs were mostly 3.8 m wide by 6.1 m

long; but, in some areas, the slabs were 3.8 m wide by

5.3 m long. A typical transverse joint used aggregate

interlocks and dowels for load transfer.* Across longitudinal joints, keyways, aggregate interlocks and tiebars were used for load transfer (ERES Consultants

1985, Mead and Hunt 1988). Richardson stated that

on the basis of a pavement evaluation done prior to

1986, the gross allowable aircraft weights on runway

11/29 were 27,200-kg single, 40,800-kg dual and

74,860-kg dual tandem. On runway 3/21, the gross allowable aircraft weights were 38,570-kg single,

81,670-kg dual and 95,280-kg dual tandem.

3

Figure 3. Pavement structure at Outagamie County Airport (OCA).

N

3

29

T/W E

T/W C

T/W B

Runway 3-21

Runway 11-29

T/W D

21

T/W A

11

Outagamie County Airport

Base

200 mm Gravel

203 mm Gravel

200 mm Gravel

203 mm Gravel

Surface

178 mm PCC

203 mm PCC

229 mm PCC

254 mm PCC

* Personal Communication, with K. Richardson, Wisconsin Department of Transportation, 1991.
FIELD INSTRUMENTATION

AND TESTING PROGRAM

In the summer of 1985, several locations along the

airport runways and taxiways were instrumented with

moisture sensors and copper-constantan thermocouples

as temperature sensors. At CWA, six locations were instrumented for temperature measurement (Fig. 4a). At

OCA, there were two temperature measurement sites

(Fig. 4b). With a few exceptions, thermocouples were

placed to depths of approximately 5 m below the pavement surface. The spacings of the sensors are given in

Table 2. At TC4, the hole could not be held open past

2.5 m from the surface.

The temperature measurements were made periodically by airport personnel during the winter months and

by CRREL personnel during the FWD testing period in

the spring. The measured temperatures at the two airports are given in Janoo and Berg (1996).

In the spring of 1986, non-destructive testing using a

Dynatest 8000 Falling Weight Deflectometer (FWD)

4

b. Outagamie County Airport.

Table 2. Temperature sensor locations under pavement surface (cm).

Sensor TC1, TC2 (CWA)

no. TC1, TC2 (OCA) TC3 (CWA) TC4 (CWA)

1 30.5 30.5 30.5

2 45.7 45.7 45.7

3 61 106.7 61

4 91.4 137.2 91.4

5 121.9 167.6 121.9

6 152.4 198.1 152.4

7 182.9 228.6 182.9

8 213.4 259.1 213.4

9 243.8 289.6 243.8

10 304.8 350.5 259.1

11 365.8 411.5 137.2

12 487.7 472.4 106.7

a. Central Wisconsin Airport.

Figure 4. FWD, temperature and moisture sensor locations.

was conducted at selected sites at the two airports.

The FWD test sites covered a large area of the airports

and included both AC and PCC pavements. As mentioned earlier, the analysis presented here is for only

the PCC slabs. Deflection measurements were made
5

a. Central Wisconsin Airport.

Temperature (C)

–40

–20

0

20

40

20 Sep 20 Oct 19 Nov 19 Dec 18 Jan 17 Feb 19 Mar 18 Apr 18 May

 Outagamie County Airport, Appleton

 Maximum

 Minimum

1985 1986

b. Outagamie County Airport, Appleton.

Figure 5. Daily maximum and minimum temperatures.

at the center of the slab, at transverse joints, longitudinal joints and at a corner. The FWD test program consisted of one drop at each of four height levels producing loads of 600, 900, 1100 and 1600 kN, respectively,

on the pavement surface. The deflection sensors were

located at 0, 300, 600, 900, 1200, 1500 and 1800 mm

from the center of the loading plate. The FWD loading

plate used was 300 mm in diameter. The location of the

FWD tests at the two airports are shown in Figure 4.

ENVIRONMENTAL DATA ANALYSIS

The environmental data used in this analysis were

the air temperature and subsurface temperatures. The

daily maximum and minimum air temperature at the

airports between 1 October 1985 and 30 April 1986 are

presented in Figure 5. The air temperature data used at

CWA were obtained from the nearest weather station,

which was located approximately 24 km north of the

airport at Wausau, since there was no meteorological

station at CWA. These air temperatures were used to

determine air-freezing indices (Fig. 6). The air-freezing index was calculated in Celsius degree-days using

Air-freezing index = T T

n

max min +

=

∑ 2 1

181

whereTmax = daily maximum temperature (°C)

Tmin = daily minimum temperature(°C)

(n = 1) = 1 October 1985

n = 181 = 30 April 1986.

The air-freezing index range was 1100 and 920°C

days at CWA and OCA respectively. The design freezTemperature (C)

–40

–20

0

20

40

20 Sep 20 Oct 19 Nov 19 Dec 18 Jan 17 Feb 19 Mar 18 Apr 18 May

 Wausau, Marathon County

 Maximum

 Minimum

1985 1986
6

Air Freezing Index (°C degree-days)

–800

–400

0

400

30 Sep 30 Oct 29 Nov 29 Dec 28 Jan 27 Feb 29 Mar 28 Apr 28 May

CWA, Mosinee

 Beginning of Freezing 

Period (17 Nov)

 Beginning of Spring Thaw 

(29 Mar)

 Air Freezing Index (1100 °C degree-days)

1985 1986

Air Freezing Index (°C degree-days)

–800

–400

0

400

30 Sep 30 Oct 29 Nov 29 Dec 28 Jan 27 Feb 29 Mar 28 Apr 28 May

 Appleton

 Beginning of Freezing 

Period (19 Nov)

 Beginning of Spring Thaw 

(24 Mar)

 Air Freezing Index (920 °C degree-days)

1985 1986

a. Central Wisconsin Airport.

b. Outagamie County Airport.

Figure 6. Air-freezing indices.

ing index published in Army TM 5-818-2 for these

areas ranges between 830 and 1110°C days, which

means that the air-freezing index in the winter of

1985–1986 was close to that. The air-freezing season,

as indicated in the above two figures, ended around 24

March 1986 at OCA and 29 March 1986 at CWA.

Using the temperature measurements gathered

from the various thermocouple locations, we determined frost and thaw depth, which we assumed to be

where the temperature was 0°C. Frost depths at the

beginning of the winter were calculated for some of

the locations and are also presented in Janoo and Berg

(1966). A summary of the frost and thaw penetration

depths at CWA and OCA is presented in Figure 7. The

maximum measured frost penetration at CWA ranged

around 2000 mm and at OCA around 1300 mm. The

measured subsurface temperatures indicate that thaw

began at CWA around 29 March. The ground was

completely thawed by the second week of April. At

OCA, when subsurface temperature measurements

were made in the spring, thaw had already started. The

ground was completely thawed by the first week of

April.

FWD DATA ANALYSIS

Falling weight deflection measurements were conducted at four locations on a slab. These were at 1) the

middle, 2) across the transverse joint, 3) across the

longitudinal joint and 4) across the diagonal on one
7

0

500

1000

1500

2000

2500

29 Nov '85 29 Dec 28 Jan '86 27 Feb 29 Mar 28 Apr 28 May

Frost Depth (mm)

TC1

TC2

TC4

TC6

a. Central Wisconsin Airport.

0

500

1000

1500

2000

29 Nov '85 29 Dec 28 Jan '86 27 Feb 29 Mar 28 Apr

Frost Depth (mm)

Assumed

TC1

TC2

b. Outagamie County Airport.

Figure 7. Frost and thaw depths calculated from subsurface temperature measurements.

Transverse Joint

Corner Joint

Direction

of Travel

Longitudinal

Joint

D1

D0 D0

D1 D0

D1

corner of the slab. The placement of the sensors across

the joints and corner is illustrated in Figure 8. The

FWD measurements were alternated between the two

airports. At CWA, FWD deflection measurements began on 18 March and continued to 24 April 1986 (Table

3a). At OCA, FWD testing started on 15 March and

continued to 26 April 1986 (Table 3b). The FWD deflection measurements taken at both airports are presented in Janoo and Berg (1966).

The pavement surface temperatures at the time of

FWD testing for both airports are presented in Table 4.

Surface temperatures were measured with a thermocouple attached to a wooden dowel. The thermocouple

was placed against the pavement by the FWD operator.

At the time of FWD testing, the pavement surface at

CWA was dry except on 18 March and 14 April. At

OCA, the pavement surface was dry except on 5 April.

Subsurface temperature measurements indicated that

the pavement structures at CWA were frozen at the beginning of FWD testing and completely thawed by theFigure 8. Location of FWD sensors across joints and

corner of a PCC slab.
8

Table 3. Types of FWD tests conducted.

a. Central Wisconsin Airport.

Date 1 2 3 4 5 6 7 8 9 10

18 Mar 86 ——————— — — 1,2,3,4

20 Mar 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 —

24 Mar 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

27 Mar 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

4 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

7 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

10 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

14 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2,3,4

17 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2

21 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

24 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

Date 11 12 13 14 15 16 17 18 19 20

18 Mar 86 1,2,3,4 1,2 1,2,3,4 — 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

20 Mar 86 ——————— — ——

24 Mar 86 1,2 1,2 1,2,3,4 — 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3

27 Mar 86 1,2 1,2 1,2 — 1,2 1,2 1,2 1,2 1,2 1,2,3,4

4 Apr 86 1,2 1,2 1,2,3,4 — 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

7 Apr 86 1,2 1,2 1,2,3,4 — 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

10 Apr 86 1,2 1,2 1,2 — 1,2 1,2 1,2 1,2 1,2 1,2

14 Apr 86 1,2 1,2 1,2,3 — 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

17 Apr 86 1,2 1,2 1,2 — 1,2 1,2 1,2 1,2 1,2 1,2

21 Apr 86 1,2 1,2 1,2,3 — 1,2 1,2 1,2 1,2 1,2 1,2

24 Apr 86 1,2 1,2 1,2 — 1,2 1,2 1,2 1,2 1,2 1,2

Date 21 22 23 24 25 26 27

18 Mar 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2

20 Mar 86 ———————

24 Mar 86 ———————

27 Mar 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2

4 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2

7 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2

10 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2

14 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2

17 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2

21 Apr 86 1,2 1,2 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2

24 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2

b. Outagamie County Airport.

Date 1 2 3 4 5 6 7 8 9 10 11

15 Mar 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

26 Mar 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

29 Mar 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

5 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

9 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

12 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

16 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

23 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

26 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

Date 12 13 14 15 16 19 20 21 22 23 24

15 Mar 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

26 Mar 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

29 Mar 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

5 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

9 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

12 Apr 86 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

16 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

23 Apr 86 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

1–Center of slab; 2–Transverse joint; 3–Longitudinal joint; 4–Corner.
9

Table 4. Pavement surface temperatures (°C) at times of FWD tests.

a. Central Wisconsin Airport.

Date 1 2 3 4 5 6 7 8 9 10

18 Mar 86 — — — — — — — — — 2.2

20 Mar 86 –1.1 –1.7 –2.8 –2.8 –2.8 –2.8 –2.8 –2.8 –2.8 —

24 Mar 86 1.1 1.7 1.7 2.2 1.1 2.8 2.2 2.2 3.3 2.8

27 Mar 86 8.9 8.9 6.7 5.6 7.2 5.6 6.1 6.1 5.0 6.7

4 Apr 86 11.1 12.2 11.7 4.4 11.7 4.4 4.4 4.4 4.4 11.7

7 Apr 86 20.0 20.0 15.6 15.6 15.6 15.6 15.6 18.9 18.9 18.9

10 Apr 86 11.1 8.3 8.3 8.3 8.3 7.2 7.2 7.2 7.2 8.9

14 Apr 86 2.8 2.8 2.2 2.8 2.8 2.8 2.8 2.8 2.8 4.4

17 Apr 86 12.2 11.1 13.3 12.8 13.3 13.3 13.9 14.4 14.4 16.1

21 Apr 86 0.6 0.6 0.6 0.6 0.6 0.6 2.2 2.2 1.7 1.7

24 Apr 86 16.1 16.1 16.1 16.7 16.1 16.7 17.2 18.3 18.3 18.9

Date 11 12 13 14 15 16 17 18 19 20

18 Mar 86 2.2 2.2 2.2 — 6.1 6.1 3.3 2.2 1.7 1.7

20 Mar 86 — — — — — — — — — —

24 Mar 86 2.2 3.3 3.9 — 5.6 5.0 4.4 4.4 5.0 2.2

27 Mar 86 7.8 8.3 9.4 — 10.0 10.0 12.8 11.7 12.8 13.3

4 Apr 86 12.2 11.7 11.7 — 10.0 8.9 8.9 10.0 9.4 9.4

7 Apr 86 18.9 18.9 18.9 — 19.4 18.9 19.4 18.9 18.9 18.9

10 Apr 86 11.1 11.1 12.2 — 17.2 17.2 16.1 16.1 15.6 16.7

14 Apr 86 5.0 4.4 3.9 — 8.3 6.1 5.6 5.0 5.0 5.6

17 Apr 86 17.8 17.8 17.8 — 18.9 18.9 18.3 18.3 18.3 18.3

21 Apr 86 3.9 3.3 4.4 — 8.9 8.9 8.9 9.4 8.9 8.3

24 Apr 86 19.4 20.0 20.6 — 25.0 25.0 23.3 23.9 25.6 26.7

Date 21 22 23 24 25 26 27 28 29 30

18 Mar 86 1.7 1.7 1.7 1.7 1.7 1.7 2.2 3.3 3.3 3.3

20 Mar 86 — — — — — — — — — —

24 Mar 86 — — — — — — — 6.1 4.4 4.4

27 Mar 86 13.3 11.7 11.7 12.8 13.3 12.8 10.0 14.4 14.4 12.2

4 Apr 86 9.4 9.4 11.7 11.7 12.2 13.3 13.3 12.8 12.8 12.2

7 Apr 86 18.3 19.4 18.9 20.0 21.7 19.4 22.8 21.7 21.7 21.7

10 Apr 86 16.1 15.6 15.6 16.1 16.7 16.7 16.1 16.1 20.6 20.6

14 Apr 86 6.7 5.6 5.6 5.6 5.6 5.6 5.0 4.4

17 Apr 86 18.3 18.3 18.3 18.9 19.4 19.4 19.4 22.2 21.7 21.7

21 Apr 86 8.9 9.4 7.8 7.8 7.8 8.9 8.9 11.1 11.7 11.7

24 Apr 86 27.2 25.6 20.6 20.6 20.0 21.1 20.6 26.1 26.1 26.1

b. Outagamie County Airport.

Date 1 2 3 4 5 6 7 8 9 10 11

15 Mar 86 2.8 4.4 3.3 6.1 3.3 7.2 4.4 4.4 4.4 4.4 6.7

26 Mar 86 11.1 11.1 11.1 12.2 11.7 13.9 15.6 16.1 16.1 16.1 16.7

29 Mar 86 21.1 21.7 23.3 23.9 23.9 25.6 25.6 23.9 23.9 25.0 25.0

5 Apr 86 10.0 10.0 9.4 9.4 10.0 10.0 10.6 10.6 11.1 11.1 12.2

9 Apr 86 7.8 9.4 10.0 9.4 10.0 10.0 10.6 10.6 10.6 10.6 11.1

12 Apr 86 6.7 7.2 7.2 7.2 7.2 7.2 8.9 8.3 7.8 7.8 7.8

16 Apr 86 7.8 7.8 8.3 8.3 8.3 8.9 8.9 8.9 8.9 8.9 10.0

23 Apr 86 16.7 16.7 16.1 17.2 17.2 17.2 17.8 17.8 17.8 17.8 19.4

26 Apr 86 23.3 23.3 23.3 23.3 23.9 23.9 24.4 24.4 24.4 24.4 24.4

Date 12 13 14 15 16 19 20 21 22 23 24

15 Mar 86 4.4 4.4 4.4 4.4 4.4 4.4 4.4 4.4 3.9 5.6 5.6

26 Mar 86 16.7 12.2 10.6 12.8 13.3 13.3 12.2 10.6 15.6 11.1 11.1

29 Mar 86 25.0 22.2 23.3 23.3 23.3 26.7 27.2 27.2 27.2 25.6 25.6

5 Apr 86 11.7 10.6 10.0 10.0 8.9 10.0 10.0 10.0 10.6 10.0 10.0

9 Apr 86 10.6 8.9 8.3 9.4 10.0 8.3 8.9 10.0 10.6 8.9 9.4

12 Apr 86 8.3 6.1 6.1 7.2 6.7 10.0 9.4 9.4 9.4 6.7 6.7

16 Apr 86 11.1 8.3 8.3 11.1 11.1 11.1 11.1 10.6 6.7 8.3 8.3

23 Apr 86 18.9 13.3 13.9 15.0 15.0 18.9 18.9 19.4 17.8 14.4 14.4

26 Apr 86 25.0 27.8 26.1 27.8 27.8 — — — 26.7 26.7 26.7
end. At OCA the subsurface temperature measurements indicated that the pavement structures were

partially thawed at the beginning of FWD testing and

completely thawed prior to the end. It should be noted

that no FWD data were obtained during the 6-day period from 29 March through 4 April, which was undoubtedly the critical thaw-weakening period. This

was unfortunate; however, we will work with the data

that we have.

Bearing capacity analysis

Any change in the structural capacity of the pavement during the thaw-weakening period was inferred

from the FWD data. We used both the deflection basin

10

Figure 9. Changes in basin area and impulse stiffness modulus during spring thaw at Outagamie County Airport.

b. Change in ISM.

FWD 3

FWD 7

FWD 22

FWD 23

FWD 24

30

20

10

0

1000

800

600

400

200

0

10 Mar 15 20 25 30 4 Apr 9 14 19 24 29

ISM (MN/m)

Temperature (°C)

Outagamie County Airport

a. Change in basin area.

area concept and the Impulse Stiffness Modulus

(ISM) to characterize the changes in pavement performance. The basin area method was a good indicator of

AC pavement response during thaw periods under

controlled conditions (Janoo and Berg 1990, 1991).

The deflection basin obtained from the seven-deflection sensor system is used to calculate the basin area

by the following equation

Basin Area = + [ ] ( ) i i+1 i+1 i ( ) −

=

∑1

2 0

6

δ δ r r

i

where δi is deflection at sensor (i), and ri is sensor (i)

distance from the center of the loading plate.

1000

800

600

400

200

0

10 Mar 15 20 25 30 4 Apr 9 14 19 24 29

30

20

10

0

Basin Area (mm ) 2

Temperature (°C)

Outagamie County Airport

FWD 3

FWD 7

FWD 22

FWD 23

FWD 24
11

400

300

200

100

0

14 Mar 19 24 29 3 Apr 8 13 18 23 28 3 May

Temperature 

Loc 3

Central Wisconsin Airport 20

10

0

Surface Temperature (°C)

Basin Area (mm ) 2

400

600

200

0

14 Mar 19 24 29 3 Apr 8 13 18 23 28 3 May

Temperature 

Central Wisconsin Airport

20

10

0

Surface Temperature (°C)

Basin Area (mm ) 2

Loc 2

Loc 6

Loc 9

400

600

200

0

14 Mar 19 24 29 3 Apr 8 13 18 23 28

Central Wisconsin Airport

Basin Area (mm ) 2

30

20

10

0

Surface Temperature (°C)

Temperature

Loc 15

Loc 16

Loc 17

Loc 18

Loc 19

b. FWD locations 2, 6 and 9.

a. FWD location 3 .

c. FWD locations 15 to 19.

Figure 10. Changes in basin area during spring thaw at Central Wisconsin Airport.

The ISM, used by the Corps of Engineers for characterizing pavement structures, is analogous to the

spring constant (k) of a spring mass system. The ISM

is calculated from

ISM = P

δ0

where P is applied FWD load and δ0 is deflection under the center of the plate.

Typical changes in ISM values and basin areas at

the centers of selected slabs are presented in Figures

9–11. The measured surface temperatures at the FWD

sites are also plotted in these figures.
12

1600

1200

800

400

0

20 25 30 5 10 15 20 25 30

Mar Apr May

ISM M( N/m)

20

15

10

5

0

–5

Surface Temperature (°C)

 Location 3 Location 4

ISM

Temperature

2000

1600

1200

800

400

0

20 25 30 5 10 15 20 25 30

Mar Apr May

ISM M( N/m)

20

15

10

5

0

–5

Surface Temperature (°C)

ISM

Temperature

 Location 2 Location 6 Location 9

b. FWD locations 2, 6 and 9.

a. FWD locations 3 and 4.

Figure 11. Changes in ISM during spring

thaw at Central Wisconsin Airport.

c. FWD locations 15 to 19.

ISM M( N/m)

2000

1600

1200

800

400

0

20 25 30 5 10 15 20 25 30

Mar Apr May

20

15

10

5

0

–5

 Location 15 Location 16 Location 17 Location 18 Location 19

ISM

Surface Temperature (°C)

Temperature
800

600

400

200

Basin Area (mm ) 2

Surface Temperature (°C)

Outagamie County Airport

1000

0 10 20 30

Outagamie County Airport

At the beginning of spring thaw, the basin areas are

low, indicating a stiff pavement structure. As expected, we observed that as the pavement structure

thawed, the basin areas increased. As mentioned

above, we have used plots of the deflection basin area

versus time to identify periods of thaw weakening and

recovery.

At FWD locations 3 and 7 (Fig. 9a), the basin areas

increase somewhat at the beginning of thaw and then

level off after 5 April. This suggests that the pavement

structure at these locations does not undergo substantial thaw weakening. However, at other locations,

such as FWD sites 22, 23 and 24, the pavement structure undergoes more substantial

thaw weakening and recovery, as

shown by the shape of the basin

area curves in Figure 9a. If total

recovery is assumed to occur on 23

April, the bearing capacity of the

pavement structure at locations 22,

23 and 24 was reduced by 30 to

40%.

The changes in the ISM can

also be used to identify periods of

thaw weakening and recovery. The

changes in ISM with time for the

same FWD locations discussed

previously are shown in Figure 9b.

At the beginning of spring thaw,

the value of the ISM is large, indicating a stiff structure. During

thaw, the ISM drops and at locations 3 and 7 it levels off, indicating no substantial spring thaw

weakening (Fig. 9b). As indicated

above, no FWD data were obtained

during the most critical period

from 29 March through 5 April. A

similar trend was seen in the basin

area (Fig. 9a). At the other locations in Figure 9b, some thaw

weakening was observed. If the

pavement structure is considered

to have fully recovered on 23

April, the amount of thaw weakening shown by the ISM is approximately 17 to 25%. This is lower

than that predicted by the basin

area method.

Central Wisconsin Airport

The same observations of the

increase in basin area as thaw pro13

gressed were also made at CWA (Fig. 10a,b). At some

locations, we observed that the basin areas are a function of the pavement surface temperature during the

recovery period (Fig. 10c). As the temperature

changed, so did the basin area.

A study was made to see if there was any relationship between surface temperature and ISM or basin

area. Figures 12a and 13a show that there is no relationship between either basin area or ISM with surface temperature at OCA. The data around 5°C indicate some form of relationship. However, we felt that

the decrease in basin area or increase in ISM was

probably attributable to the presence of frozen subsurface layers.

Figure 12. Relationship between surface temperature and basin area.

b. Central Wisconsin Airport.

600

400

200

Basin Area (mm ) 2

Surface Temperature (°C)

Central Wisconsin Airport

10 20 30

0

0

Runway 17-35

Runway 8-26

a. Outagamie County Airport.
2000

1600

1200

800

400

0

5 0 10 5 15 20 25 30

Surface Temperature ( °C)

ISM (MN /m)

Central Wisconcin Airport

 Runway 08 -26

 Runway 17 -35

800

600

400

200

Basin Area (mm ) 2

Surface Temperature (°C)

Outagamie County Airport

1000

0 10 20 30

14

At CWA, however, it was apparent from Figure

12b that the basin area on both runways increased

with increasing surface temperature. The ISM (Fig.

13b) was fairly indifferent to surface temperatures

greater than 10°C. The earlier effects could be explained by the changes in subsurface layers as thaw

progressed.

Initially, we thought that the effect on the basin

area was attributable to curling of the PCC pavements

with temperature gradients, since the slab area on runway 17/35 was 7.6 m square and that on runway 8/26

was 6.1 by 3.8 m. However, we found that the slab

dimensions at OCA were also 6.1 by 3.8 m. The big

difference between OCA and CWA is the subsurface

conditions. At CWA, bedrock is

close to the surface in some locations. It is possible that, because

of the shallow bedrock locations,

the frozen and thawed layers have

a greater effect on the basin area,

as reflected by the surface temperature. This does not explain the

change in the basin area in the recovery period, where the temperatures were above 0°C.

Backcalculation of

layer moduli

 Layer moduli were backcalculated using WESDEF and ILLIBACK.

WESDEF was developed by the

Corps of Engineers (Waterways Experiment Station). Although it is

considered to be a five-layer analytical tool, it can backcalculate a maximum of four layers at one time. The

fifth (bottom) layer has been “fixed”

to act as a rigid layer. From our experience, reasonable results are

obtained if the number of backcalculated layers is kept to three. We

attempted to backcalculate the

layer moduli of the PCC pavement

structures at OCA and CWA. The

pavement structure at OCA was

idealized as shown in Figure 14a.

The idealized pavement structure

used at CWA is shown in Figure

14b. The depth of subgrade to bedrock at CWA was estimated from

drawings found in a pavement

evaluation report (CMT 1984).

 In the first attempt at OCA, the

moduli of all three layers during

the spring thaw period were backcalculated. We found,

in most cases, that errors between the calculated and

measured deflections were large (more than 50%). We

also found that, in many instances, the PCC modulus

exceeded the upper limit of 69,000 MPa set in the program (the modulus of PCC is usually in the range of

27,000 to 42,000 MPa) and the base course modulus

was near zero (in some cases it was zero). In the subsequent attempt, the PCC layer modulus was fixed at

27,580, 34,475 or 41,370 MPa. The following observations were made from the results:

1. Increasing the PCC stiffness from 27,580 to

41,370 MPa reduced the errors to more reasonable

(AA < 20%) differences between the measured and

Figure 13. Relationship between surface temperature and ISM.

b. Central Wisconsin Airport.

a. Outagamie County Airport.

Runway 8-26

Runway 17-35
15

calculated the deflections (Fig. 15). The Absolute

Arithmetic (AA) error is the absolute sum of the errors

between measured and calculated deflections for the

seven sensors.

2. Changing the PCC modulus within the range did

not significantly affect the subgrade modulus (Fig. 16).

We also found that the subgrade modulus obtained

from the first attempt, i.e., when the modulus of the

PCC layer was not established at some particular value, was very similar to that obtained when it was. This

infers that, as far as the subgrade modulus is concerned, large differences between the measured and

calculated deflections can be tolerated for PCC pavement.

Changing the PCC modulus generally had a significant effect on the base course modulus. Results generated by increasing the PCC modulus showed a decrease in the backcalculated base course modulus.

However, in general at OCA, the backcalculated moduli were quite low, as shown in Figure 17. With the

exception of a few locations, the results were too erratic to make any meaningful conclusions.

At CWA, we selected a number of sites (FWD sites

2, 6 and 9 on runway 8/26 and 15 to 19 on runway

17⁄35) and attempted to backcalculate the layer modulus during the thaw period. While at OCA bedrock

was at least 4 m deep, at CWA bedrock was close to

the surface at some of the FWD locations. The sites

selected represented bedrock at various depths. Thicknesses of the subgrade layers are shown in Table 5. We

also attempted to backcalculate the base course modulus under FWD location 3. Location 3 was different

from the other sites because the base course layer was

founded directly on top of the bedrock. The structure

at FWD location 3 was 330 mm of PCC over 1220

mm of base over bedrock. We were unable to get reasonable agreement between the calculated and mea6.1 m

h1

Rigid Layer

PCC, E , 1 ν = 0.15

Subgrade, E , 3 ν = 0.40

h2

h3

Base, E , 2 ν = 0.35

a. OCA. b. CWA.

h1

Bedrock

h2

h3

PCC, E , 1 ν = 0.15

Subgrade, E , 3 ν = 0.40

Base, E , 2 ν = 0.35

Figure 14. Idealized pavement structures.

30

40

20

10

0

14 Mar 19 24 29 3 Apr 13 18 8 23 28

Error (%)

Outagamie County Airport

E = 27,580 MPa

E = 34,475 MPa

E = 41,370 MPa

Figure 15. Effect of PCC Modulus on WESDEF absolute error (OCA).

Subgrade

FWD thickness

location (mm)

2 2435

6 1092

9 5613

15 610

16 1829

17 3785

18 3073

19 2743

Subgrade

FWD thickness

location (mm)

Table 5. Thickness of subgrade at backcalculated FWD locations.
16

a. FWD location 3.

b. FWD location 9.

c. FWD location 22.

Figure 16. Effect of PCC modulus on change in subgrade modulus from

WESDEF during spring thaw at Outagamie County Airport. Comparison

of subgrade modulus from ILLIBACK and WESDEF.

400

300

200

0

14 Mar 19 24 29 3 Apr 13 18 8 23 28

Subgrade Modulus (MPa)

Outagamie County Airport

100

No Fix

27,580 MPA

34,475 MPa

41,370 MPa

ILLIBACK

400

500

300

200

0

14 Mar 19 24 29 3 Apr 13 18 8 23 28

Subgrade Modulus (MPa)

Outagamie County Airport

100

No Fix

27,580 MPA

34,475 MPa

41,370 MPa

ILLIBACK

400

300

200

0

14 Mar 19 24 29 3 Apr 13 18 8 23 28

Subgrade Modulus (MPa)

Outagamie County Airport

100

27,580 MPa

34,475 MPa

41,370 MPa

ILLIBACK
FWD PCC Base Subgrade AA Error

Date location (MPa) (MPa) (MPa) (%)

20 Mar 2 27,580 54,148 433 —

24 Mar 2 27,580 6,807 214 26.7

27 Mar 2 27,580 5,096 97 12.0

4 Apr 2 27,580 2,123 70 12.3

7 Apr 2 27,580 1,832 56 17.1

10 Apr 2 27,580 2,002 70 16.0

14 Apr 2 27,580 2,791 65 21.4

17 Apr 2 27,580 2,284 56 16.9

21 Apr 2 27,580 4,087 56 15.0

24 Apr 2 27,580 2,402 47 16.3

20 Mar 9 27,580 4,965 825 13.3

24 Mar 9 27,580 1,935 455 17.4

27 Mar 9 27,580 1,373 162 20.6

4 Apr 9 27,580 643 139 14.5

7 Apr 9 27,580 526 62 14.4

10 Apr 9 27,580 1,050 107 16.9

14 Apr 9 27,580 1,495 111 9.5

17 Apr 9 27,580 1,200 81 13.8

21 Apr 9 27,580 1,547 116 16.9

24 Apr 9 27,580 1,612 99 18.3

18 Mar 17 27,580 5,956 425 31.6

24 Mar 17 27,580 4,802 589 12.8

27 Mar 17 27,580 2,071 157 11.6

4 Apr 17 27,580 2,821 131 5.9

7 Apr 17 27,580 1,747 51 10.9

17

8000

6000

4000

0

14 Mar 19 24 29 3 Apr 13 18 8 23 28

Base Course Modulus (MPa)

Outagamie County Airport

2000

Loc 3

Loc 9

Loc 22

Loc 23

Figure 17. Backcalculated base course modulus using WESDEF (OCA).

10 Apr 17 27,580 1,520 58 8.4

14 Apr 17 27,580 3,717 133 3.9

17 Apr 17 27,580 2,416 57 8.7

21 Apr 17 27,580 3,560 50 8.9

24 Apr 17 27,580 18,212 18 129.2

18 Mar 18 27,580 149 595 —

24 Mar 18 27,580 3,216 367 23.0

27 Mar 18 27,580 3,645 32 10.8

4 Apr 18 27,580 4,027 39 9.6

7 Apr 18 27,580 3,499 26 4.6

10 Apr 18 27,580 4,713 29 9.1

14 Apr 18 27,580 2,611 82 5.2

17 Apr 18 27,580 2,035 39 7.0

21 Apr 18 27,580 — — —

24 Apr 18 27,580 2,572 35 8.9

18 Mar 19 27,580 5,648 272 11.4

24 Mar 19 27,580 5,133 316 —

27 Mar 19 27,580 — — —

4 Apr 19 27,580 — — —

7 Apr 19 27,580 2,163 29 8.2

10 Apr 19 27,580 1,852 32 7.0

14 Apr 19 27,580 — — —

17 Apr 19 27,580 1,482 29 9.3

21 Apr 19 27,580 — — —

24 Apr 19 27,580 1,891 30 8.8

FWD PCC Base Subgrade AA Error

Date location (MPa) (MPa) (MPa) (%)

Table 6. Backcalculated modulus at CWA using WESDEF.

lack of values in the base modulus, subgrade modulus

and error columns in Table 6 indicate that either it was

not possible to converge to a solution or that the base

course modulus was extremely low (less than 1 MPa).

When only the AA error column lacks values, it

means that we used the backcalculated results from

WESDEF but ignored the controlling layer modulus

range, i.e., the backcalculated modulus was either

above or below the prescribed range.

The effect of changing the PCC modulus is shown

in Table 7 for FWD locations 9, 17, 18 and 19. As

sured deflections. The AA error was in the vicinity of

450%.

Solutions could not be obtained for locations 6, 15

and 16. In many cases, WESDEF printed a “THIS

MATRIX HAS NO SOLUTION” message. At other

times, the backcalculated base course modulus was

zero, or very close to zero. It is interesting to note that

at these locations the bedrock was quite close to the

surface (less than 2 m). At the other locations, with a

few exceptional days, the error between the calculated

and measured values was acceptable (Table 6). The
18

Table 7. Effect of change in PCC modulus on base and subgrade modulus.

FWD PCC Base Subgrade AA error PCC Base Subgrade AA error

Date location (MPa) (MPa) (MPa) (%) (MPa) (MPa) (MPa) (%)

20 Mar 9 27,580 4,965 825 13.3 34,475 3617 828 12.3

24 Mar 9 27,580 1,935 455 17.4 34,475 1045 463 12.8

27 Mar 9 27,580 1,373 162 20.6 34,475 529 165 17.7

4 Apr 9 27,580 643 139 14.5 34,475 81 155 10.3

7 Apr 9 27,580 526 62 14.4 34,475 16 81 11.5

10 Apr 9 27,580 1,050 107 16.9 34,475 255 110 14.3

14 Apr 9 27,580 1,495 111 9.5 34,475 537 113 8.9

17 Apr 9 27,580 1,200 81 13.8 34,475 387 82 11.7

21 Apr 9 27,580 1,547 116 16.9 34,475 685 118 15.1

24 Apr 9 27,580 1,612 99 18.3 34,475 749 100 16.2

18 Mar 17 27,580 5,956 425 31.6 34,475 4092 429 32.2

24 Mar 17 27,580 4,802 589 12.8 34,475 3038 595 9.7

27 Mar 17 27,580 2,071 157 11.6 34,475 — — —

4 Apr 17 27,580 2,821 131 5.9 34,475 1455 134 5.8

7 Apr 17 27,580 1,747 51 10.9 34,475 639 52 9.4

10 Apr 17 27,580 1,520 58 8.4 34,475 316 59 8.3

14 Apr 17 27,580 3,717 133 3.9 34,475 2282 135 4.2

17 Apr 17 27,580 2,416 57 8.7 34,475 — — —

21 Apr 17 27,580 3,560 50 8.9 34,475 2350 49 7.8

24 Apr 17 27,580 18,212 18 129.2 34,475 — — —

18 Mar 18 27,580 149 595 — 34,475 — — —

24 Mar 18 27,580 3,216 367 23.0 34,475 1750 373 19.4

27 Mar 18 27,580 3,645 32 10.8 34,475 2335 33 9.2

4 Apr 18 27,580 4,027 39 9.6 34,475 — — —

7 Apr 18 27,580 3,499 26 4.6 34,475 2241 26 3.5

10 Apr 18 27,580 4,713 29 9.1 34,475 2609 30 2.1

14 Apr 18 27,580 2,611 82 5.2 34,475 — — —

17 Apr 18 27,580 2,035 39 7.0 34,475 708 40 7.2

21 Apr 18 27,580 — — — 34,475 — — —

24 Apr 18 27,580 2,572 35 8.9 34,475 1473 36 9.7

18 Mar 19 27,580 5,648 272 11.4 34,475 3839 278 8.6

24 Mar 19 27,580 5,133 316 — 34,475 3195 319 11.5

27 Mar 19 27,580 — — — 34,475 — — —

4 Apr 19 27,580 — — — 34,475 — — —

7 Apr 19 27,580 2,163 29 8.2 34,475 830 30 8.0

10 Apr 19 27,580 1,852 32 7.0 34,475 551 33 7.4

14 Apr 19 27,580 — — — 34,475 — — —

17 Apr 19 27,580 1,482 29 9.3 34,475 444 30 8.2

21 Apr 19 27,580 — — — 34,475 — — —

24 Apr 19 27,580 1,891 30 8.8 34,475 591 31 8.3

Figure 18. Change in subgrade modulus during spring thaw (FWD location 9, CWA).

800

600

400

200

0

19 Mar 24 29 3 Apr 8 13 18 23 28

Central Wisconsin Airport

27,580 MPa

34,475 MPa

Subgrade Modulus (MPa)
found at OCA, changing the PCC modulus did not significantly affect the backcalculated subgrade modulus

(Fig. 18 for location 9). However, as was also found at

OCA, changing the PCC modulus had a significant effect on the base course modulus (Fig. 19). Trends in the

data indicate that the base was in a weakened state from

the end of March through about mid-April. The values

reported here for the base course modulus are still considered too large. By combining the change in the PCC

modulus with changing the different layer thicknesses,

singularly or simultaneously, we found that, in some

cases, we were able to reduce the base course modulus

to more reasonable values. However, we felt that

changing the layer thicknesses was introducing another

unknown variable into the analysis. Therefore, although the base course moduli are high, we have opted

to present the results obtained for the reported

thicknesses. It is very clear that a small coring program

should be conducted with FWD testing to verify thicknesses. Irwin et al. (1989) reported that a 6-mm change

in layer thickness has a large impact on the backcalculated layer modulus. Also note that, although changing

the PCC modulus affected the base course modulus, the

change produced very small changes in the absolute error (Table 7).

ILLIBACK was also used to backcalculate the layer

moduli. This procedure was developed by Ioannides

et al. (1989) as a closed form of backcalculation for

a two-layer rigid pavement system using principles of

dimensional analysis. For a given deflection sensor distribution, they found a unique relationship between the

FWD deflection basin area and the radius of relative

stiffness of the pavement subgrade system (l). They

then developed relationships between the ratios of nondimensional deflections and actual FWD deflections

and l for a constant FWD loading plate radius (300

mm). These relationships were used with the applied

load to calculate the coefficient of subgrade reaction

(k), PCC modulus (Ec) and subgrade modulus (Es).

19

The elastic modulus of concrete (Ec), elastic subgrade modulus (Es) and the coefficient of subgrade reaction (k) can be determined from

E

h

d

D

Pl c i

i

= ( ) − × ×

12 1 2

3

2 μ

E d

D

P

l s i

i

= − ( ) 1 × ×

2 2 ν

 k d

D

P

l

= × i

i

2

where μ = Poisson’s ratio for concrete

ν = Poisson’s ratio for subgrade

P = applied FWD load

Di = FWD deflection measurement at sensor i

di = nondimensional deflection at sensor i

h = PCC layer thickness.

Additional information on this method can be found

in Ioannides et al. (1989) and Barenberg and Ioannides

(1989). Barenberg and Ioannides have developed figures to determine l from the deflection basin area; l is

then used to determine di. The base and subgrade were

modeled as a single composite structure at OCA. At

CWA, the base, subgrade and bedrock were combined

into a single layer.

Typical changes in the ‘subgrade’ modulus during

spring thaw at OCA are shown in Figure 16. The ‘subgrade’ modulus backcalculated from ILLIBACK was

approximately 25% larger than that from WESDEF, as

shown in Figure 20. The backcalculated PCC modulus

for some of the FWD sites is shown in Figure 21. These

are typical results and are also representative of other

sites at OCA. They are also within the range of reported

values, varying between 21,000 to 42,000 MPa. The

data also indicate that the PCC modulus increased by

about 15% over the duration of the monitored period.

Based on the results, relationships between the subgrade modulus and the basin area or ISM were devel5000

4000

3000

2000

0

19 Mar 24 29 3 Apr 8 13 18 23 28

Central Wisconsin Airport

27,580 MPa

34,475 MPa

Base Modulus (MPa)

1000

Figure 19. Change in base course modulus

during spring thaw (FWD location 9, CWA).
20

600

400

200

0

Subgrade Modulus from WESDEF (MPa)

Subgrade Modulus from ILLIBACK (MPa)

200 400 600

Outagamie County Airport

R = 0.8 2

0

14 Mar 19 24 29 3 Apr 8 18 23 28

PCC Modulus (MPa)

Outagamie County Airport

Loc 7

Loc 9

Loc 23

40 x 103

30

20

10

13

Figure 20. Relationship between subgrade moduli backcalculated by

WESDEF and ILLIBACK.

Figure 21. Typical backcalculated PCC modulus from ILLIBACK (OCA).

oped. Basin area results from ILLIBACK and WESDEF

are shown for both OCA and CWA in Figures 22 and

23. In Figure 22, the total basin area was calculated using all seven deflections. In Figure 23, the basin area

was calculated using the third to seventh deflections.

Previously, we had determined that the thaw depth

under AC pavements could be related to the partial basin area calculated using the third to seventh deflections (Janoo and Berg 1990). The current data indicate

that this partial basin area (A3-7) may be related to the

subgrade modulus. The figures show that as the total

basin area or partial basin area increased, the subgrade

modulus decreased. A power relationship was found to

best fit the total basin area or partial basin area and the

subgrade modulus. We also found that a better correlation existed with the subgrade modulus obtained from

ILLIBACK. The use of the partial basin area (A3-7)

(Fig. 23) did not significantly increase the correlation

at OCA. We used other combinations of deflections to

calculate the partial basin areas and found that none significantly increased the correlation with the calculated

subgrade modulus.

A similar relationship between ISM and subgrade

modulus is shown in Figure 24. A second-order polynomial function provides the best fit to the data. Again, a

better correlation is obtained using the results from ILLIBACK.

The results from OCA and CWA are combined in

Figures 25 and 26. In Figure 25a presents a relationship

developed between the measured total basin area and

subgrade modulus calculated from ILLIBACK for both

sites. An excellent correlation between total basin area

and subgrade modulus is found. In Figure 25b, the relationship developed between the total basin area and

subgrade modulus calculated from WESDEF for both

sites is presented. There are two distinct trends. This is

probably attributable to the separation of the base and

subgrade in WESDEF. Again, a power curve seems to
21

0

Subgrade Modulus (MPa)

Outagamie County Airport

WESDEF

ILLIBACK

200 400 600

 Basin Area (mm ) 2

600

400

200

R = 0.97 2

– 1.1175 SM = 86156A3-7

R = 0.65 2

SM = 35778A– 1.0096

3-7 Figure 23. Relationship between measured

partial basin area (third–seventh deflections) and calculated subgrade modulus at

Outagamie County Airport.

Figure 22. Relationship between measured total basin area and calculated subgrade modulus.

600

400

200

0 200 400 600 800

Total Basin Area (mm2 )

Subgrade Modulus (MPa )

Outagamie County Airport

 ILLIBACK

 WESDEF

y = 43006x –0.9954

R2 = 0.60

y = 134316x – 1.1412

R2 = 0.96

1600

1200

800

0

Subgrade Modulus (MPa)

200 400 600

Outagamie County Airport

WESDEF

ILLIBACK

Total Basin Area (mm ) 2

400

y = 154319x – 1.1708

R = 0.99 2

y = 116238x – 1.3034

R = 0.70 2

Central Wisconsin Airport
22

0

Subgrade Modulus (MPa)

Outagamie County Airport

WESDEF

ILLIBACK

200 400 600

ISM (MN/m)

600

400

200

800 1000

SM = 0.0004ISM + 0.2968ISM 2

R = 0.92 2

SM = 0.0001ISM + 0.2918ISM 2

R = 0.51 2

ILLIBACK

WESDEF

ISM (MN/m)

Subgrade Modulus (Mpa)

0

400

800

1200

1600

2000

400 800 1200 1600 2000

y = 0.0209x1.4753

R2 = 0.9601

y = 0.0008x1.8606

R2 = 0.8512

Central Wisconsin Airport

Figure 24. Relationship between measured ISM and calculated subgrade modulus at Outagamie County Airport and Central Wisconsin

Airport.

a. Calculated using ILLIBACK.

Figure 25. Relationship between total basin area and subgrade modulus at

Outagamie County Airport and Central Wisconsin Airport.

CWA

OCA

Total Basin Area (mm2)

Subgrade Modulus (MPa) 400

800

1200

1600

0 200 400 600 800

y = 240185AT –1.248

R2 = 0.91
23

b. Calculated using WESDEF.

c. Calculated using WESDEF with a single power trend.

Figure 25 (cont’d).

Total Basin Area (mm2)

Coefficient of Subgrade Reaction (MN/m3)

400

800

1200

0 200 400 600 800

CWA

OCA

y = 741480x–1.6662

R2 = 0.86

Figure 26. Relationship between total basin area and coefficient of subgrade

reaction calculated using ILLIBACK at Outagamie County Airport and Central Wisconsin Airport.

0

Subgrade Modulus (MPa)

200 400 600

200

800

400

1000

 Total Basin Area (mm ) 2

WESDEF

R = 0.70 2

y = 116238x– 1.3034

y = 43006x– 0.9954

R = 0.59 2

600

OCA

CWA

800

0

Subgrade Modulus (MPa)

200 400 600

200

800

400

1000

 Total Basin Area (mm ) 2

WESDEF

y = 10773x– 1.796

600

OCA

CWA

800

R = 0.91 2
24

Table 9. Gear information for computer simulations of the

MD-DC9 and Boeing 757.

Contact Contact Load on

Aircraft Tire Radius area pressure tire X-cord Y-cord

type no. (mm) (m2) (kPa) (kN) (mm) (mm)

MD-DC9 1 181.4 0.103 1103 114.1 –330.2 0

2 181.4 0.103 1103 114.1 330.2 0

Boeing 757 1 189.5 0.113 1172 132.1 –431.8 0

2 189.5 0.113 1172 132.1 431.8 0

3 189.5 0.113 1172 132.1 –431.8 1143

4 189.5 0.113 1172 132.1 431.8 1143

best fit the data, so for estimation, a single power

trend was applied to the data as shown in Figure 25c.

Figure 26 shows the relationship between the measured total basin area and the coefficient of subgrade

reaction (k) computed from ILLIBACK for both sites

The following equations have been developed from

the ILLIBACK results and can be used to estimate the

subgrade modulus and the coefficient of subgrade reaction (k)

SM A = 240 185 , T

–1.248 (R2 = 0.91)

k A = 741 480 , T

–1.6662 (R2 = 0.86)

where SM = subgrade modulus (MPa)

AT = total basin area (mm2)

k = coefficient of subgrade reaction

(MN/m3).

2000

1600

1200

400

0 400 800 1200 1600 2000

ISM (MN/m)

Subgrade Modulus (MPa)

800

WESDEF

ILLIBACK OCA

WESDEF

ILLIBACK CWA

2 y = 0.0004 ISM + 0.1378 ISM + 41.57

R = 0.96 2

y = 0.0003 ISM – 0.0365 ISM + 72.57 2

R = 0.73 2

Figure 27. Relationship between ISM and

subgrade modulus at OCA and CWA.

Table 8. Gear loading for the MD-DC9 and Boeing 757.

Aircraft Design load % of design load Load on main gear

 type (MN) on each main gear (MN)

MD-DC9 480 47.5 228.2

Boeing 757 1112 47.5 528.2

In Figure 27, the relationship of ISM and subgrade

modulus for both OCA and CWA is shown. Again, a

single power curve can be fitted to the data.

Current criteria for PCC pavements state that failure

occurs when the horizontal tensile stress at the bottom

of the PCC layer is equal to or greater than the flexural

strength of the slab. The flexural strength reported at

OCA and CWA was 4.5 MPa. The two types of aircraft

used for these simulations were the MD-DC9 and the

Boeing 757. The gear loads, tire spacings and radii

were obtained from the FAALEA computer program

and are presented in Tables 8 and 9.

Several FWD locations were selected from each airport for computing the tensile stress at the bottom of the

PCC layer on different days during the monitoring period. The damage (D) reported here is defined as the ratio

of maximum horizontal (tensile) stress to the flexural
25

r = 1814

mm

3

2 1

4

86.8

mm

228.6

mm

330.2

mm

330.2

mm

b. MD: DC 9

13

4

3 2 1

r = 189.5

mm

1143

254 mm

mm

101.6

mm 218.9

mm

431.8

mm

431.8

mm

a. Boeing 757

Figure 28. Configuration and location of stress calculations for Boeing 757 and MD-DC9.

80

60

40

20

0

D, Damage (%)

100

14 Mar 19 24 29 3 Apr 8 13 18 23 28

LOC 2

LOC 3

LOC 6

LOC 9

LOC 16

LOC 17

LOC 19

LOC 24

Boeing 757

Central Wisconsin Airport

254 mm

305 

330

Figure 29. Amount of damage during spring thaw at Central Wisconsin Airport.

Figure 30. Effect of pavement thickness on damage at Central

Wisconsin Airport.

strength of the PCC layer

D = σ

σ

tensile

flexural

.

Layer moduli from ILLIBACK were used

to represent the pavement structures. The

computer program BISAR was used to calculate the stresses at locations shown in Figure

28. The stress calculation points are the same

as those used in FAALEA. The results are

tabulated in Table 10 and damage is shown

for CWA in Figure 29. As thaw progresses,

the amount of damage increases until thawing is complete; then it levels off with time.

The results also indicate that the damage is a

function of pavement thickness, a linear relationship being found in the 24 April data

from CWA (Fig. 30).

The thinner pavements (178 to 203 mm)

at OCA showed potential near-failure conditions during the spring thaw (Fig. 31). Some

pavements recovered somewhat, as typified

by location 9 (Fig. 31). The thicker section

(location 23, 254 mm) had a similar amount

of damage as those sections of similar thickness at CWA; however, the OCA sections did

not exhibit the loss during the thawing period observed at CWA.

The horizontal stresses are plotted as

functions of the subgrade modulus, coefficient of subgrade reaction and the PCC modulus in Figures 32–34. With respect to the

PCC modulus, no trends were seen (Fig. 34),

and nonlinear trends were observed for all of

0.8

0.6

0.4

0

 Damage

1.0

0.2

200 220 240 260 280 300 320 340

PCC Thickness (mm)

D = – 0.004t + 1.6356

R = 0.95 2
FWD Damage ratio

Date location Boeing757 MD DC9

a. Central Wisconsin Airport

20 Mar 2 0.29 0.32

24 Mar 2 0.34 0.38

27 Mar 2 0.46 0.50

4 Apr 2 0.56 0.58

7 Apr 2 0.62 0.62

10 Apr 2 0.60 0.61

14 Apr 2 0.64 0.63

17 Apr 2 0.62 0.62

21 Apr 2 0.62 0.62

24 Apr 2 0.63 0.63

20 Mar 3 0.25 0.27

24 Mar 3 0.26 0.28

27 Mar 3 0.27 0.29

4 Apr 3 0.35 0.35

7 Apr 3 0.45 0.42

10 Apr 3 0.30 0.32

14 Apr 3 0.25 0.27

17 Apr 3 0.33 0.34

21 Apr 3 0.20 0.22

24 Apr 3 0.25 0.27

20 Mar 6 0.25 0.26

24 Mar 6 0.32 0.35

27 Mar 6 0.49 0.52

4 Apr 6 0.50 0.53

7 Apr 6 0.60 0.60

10 Apr 6 0.58 0.59

14 Apr 6 0.63 0.62

17 Apr 6 0.66 0.65

21 Apr 6 0.61 0.61

24 Apr 6 0.58 0.59

20 Mar 9 0.25 0.27

24 Mar 9 0.38 0.41

27 Mar 9 0.55 0.57

4 Apr 9 0.53 0.55

7 Apr 9 0.67 0.65

10 Apr 9 0.61 0.61

14 Apr 9 0.55 0.57

17 Apr 9 0.63 0.63

21 Apr 9 0.62 0.62

24 Apr 9 0.61 0.61

18 Mar 16 0.22 0.24

24 Mar 16 0.24 0.26

27 Mar 16 0.48 0.46

4 Apr 16 0.37 0.39

7 Apr 16 0.46 0.45

10 Apr 16 0.51 0.48

14 Apr 16 0.46 0.45

17 Apr 16 0.41 0.42

21 Apr 16 0.39 0.40

24 Apr 16 0.39 0.40

18 Mar 17 0.23 0.26

24 Mar 17 0.20 0.22

27 Mar 17 0.30 0.33

4 Apr 17 0.34 0.37

7 Apr 17 0.46 0.45

10 Apr 17 0.42 0.42

14 Apr 17 0.34 0.36

17 Apr 17 0.46 0.45

21 Apr 17 0.46 0.45

24 Apr 17 0.48 0.46

24 Mar 19 0.24 0.26

27 Mar 19 0.36 0.38

4 Apr 19 0.42 0.42

7 Apr 19 0.48 0.46

10 Apr 19 0.45 0.44

14 Apr 19 0.39 0.40

17 Apr 19 0.49 0.47

21 Apr 19 0.44 0.43

24 Apr 19 0.47 0.45

b. Outagamie County Airport

15 Mar 1 0.52 0.55

26 Mar 1 0.59 0.60

29 Mar 1 0.56 0.57

5 Apr 1 0.00 0.61

9 Apr 1 0.64 0.63

12 Apr 1 0.58 0.59

16 Apr 1 0.63 0.63

23 Apr 1 0.48 0.51

26 Apr 1 0.61 0.61

15 Mar 4 0.55 0.60

26 Mar 4 0.78 0.83

29 Mar 4 0.92 0.94

5 Apr 4 — 1.00

9 Apr 4 0.99 0.98

12 Apr 4 0.97 0.97

16 Apr 4 1.00 0.99

23 Apr 4 0.97 0.97

26 Apr 4 0.94 —

15 Mar 9 0.64 0.70

26-Mar 9 0.81 0.86

29 Mar 9 0.97 0.97

5 Apr 9 0.95 0.95

9 Apr 9 0.85 0.88

12 Apr 9 0.78 0.83

16 Apr 9 0.84 0.87

23 Apr 9 0.84 0.87

26 Apr 9 0.85 0.88

15 Mar 23 0.79 0.87

26 Mar 23 0.80 0.88

29 Mar 23 0.91 0.99

4 Apr 23 0.96 1.04

9 Apr 23 0.99 1.06

12 Apr 23 1.00 1.07

16 Apr 23 0.97 1.04

23 Apr 23 0.90 0.97

26 Apr 23 0.96 1.03

FWD Damage ratio

Date location Boeing757 MD DC9

Table 10. Ratio of maximum horizontal tensile stress to flexural strength during

spring thaw.

26
27

a. Boeing 757.

b. MD-DC9.

Figure 32. Effect of subgrade modulus on the horizontal tensile

stress at the bottom of the PCC layer (OCA and CWA).

0.8

0.4

0

Damage

1.2

14 Mar 19 24 29 3 Apr 8 13 18 23 28

Boeing 757

Outagamie County Airport

LOC 1

LOC 4

LOC 9

LOC 23

178, 203 mm

203

254

Figure 31. Amount of damage during spring thaw at Outagamie

County Airport.

5000

4000

3000

2000

0 400 800 1200 1600

Tensile Stress (kPa)

Subgrade Modulus (MPa)

1000

Boeing 757

y = 41732x – 0.5493

R = 0.69 2

Tensile Stress ( kPa)

Subgrade Modulus (MPa)

5000

4000

3000

2000

1000

0 400 800 1200 1600

Boeing 757

y = 41732x – 0.5493

R2 = 0.69
28

Tensile Stress ( kPa)

Subgrade Modulus ( MPa)

5000

4000

3000

2000

1000

0 400 800 1200 1600

Boeing 757

y = 41732x – 0.5493

R2 = 0.69

a. Boeing 757.

b. MD-DC9.

Figure 33. Effect of the coefficient of subgrade reaction, k, on the horizontal

tensile stress at the bottom of the PCC layer.

Since we found damage to be a function of thickness, σh

was developed as a function of Es or k and the coefficient of thickness. We found that the correlations increased when thickness was taken into consideration.

σh= 7360 – 1.5Es – 17t

or

σh= 7389 – 13.02k – 17.5t

where t is PCC thickness (mm).

The relationship between the total basin area and the

horizontal stresses (σh) at the bottom of the PCC layer is

shown in Figure 35. A linear trend was applied to the

the other relationships. Small differences were obtained

for the Boeing 757 and the MD-DC9.

The following equation could be used to estimate the

horizontal stress σh as a function of the subgrade modulus Es (MPa) during the spring thaw

σh s–0.549 = 41 732 , . E

We observed a similar trend between the horizontal

strain and the coefficient of subgrade reaction. The following equation could be used to estimate the horizontal

stress (σh) as a function of the coefficient of subgrade

reaction, k (MN/m3) during the spring thaw

σh

–0.411 = 12 436 , . k

5000

4000

2000

0 200 400 600 800

Tensile Stress (kPa)

MD DC 9

y = 10973x– 0.3744

R = 0.54 2

k, Coefficient of Subgrade Reaction (MN/m ) 3

3000

1000
29

4000

3000

2000

1000

0 10 20 30 40

PCC Modulus (MPa)

Tensile Stress (kPa)

50 x 103

Boeing 757

a. Boeing 757.

b. MD-DC9.

Figure 34. Effect of PCC modulus on the horizontal tensile stress at the

bottom of the PCC layer.

results. We used partial basin areas, but found no better correlation. The following equation can be used to

determine σh from the total basin area

σh = 7.35 × AT R2 = 0.7.

The ISM can also be used to estimate σh, as shown

in Figure 36. A power relationship was found to best

describe the relationship between ISM and (σh)

σh = 357,068 × ISM–0.8165 R2 = 0.81.

LOAD TRANSFER EFFICIENCY (LTE)

Joints are generally weak points in jointed concrete

pavements. Although the load transfer across joints has

a great effect on the stress at the bottom of the slab, and

therefore on the performance of the pavement, it is

largely ignored in most evaluation schemes because of

the difficulty of measuring these stresses (Foxworthy

1985). The load transfer can be inferred from the

amount of deflection on both sides of the joint when a

load is applied on one side of the joint. The FWD, simulating a wheel load on one side of a joint, can be used

4000

3000

2000

1000

0 10 20 30 40

PCC Modulus (MPa)

Tensile Stress (kPa)

50 x 103

MD DC 9

5000
30

4000

3000

2000

1000

0 200 400 600

Maximum Tensile Stress (MPa)

5000

Total Basin Area (mm ) 2

Boeing 757

MD DC 9

σ H = 7.03 x AT

r = 0.7 2

Figure 35. Linear relationship between total basin area and maximum horizontal tensile stress at bottom of PCC Layer.

Figure 37. Load transfer efficiency across a joint.

BOEING 757 - CWA

MD DC9 - CWA

BOEING 757 - OCA

MD DC9 - OCA

ISM (MN/m)

Tensile Stress (kPa)

1000

2000

3000

4000

5000

0 400 800 1200 1600 2000

y = 357068x–0.8165

R2 = 0.81

Figure 36. Relationship between ISM and horizontal tensile stress at

bottom of PCC Layer.
either side of the joint from an applied load. Load (or

stress) transfer is the ratio of amount of stress that is

transferred across the joint. For a perfect transfer,

50% of the stress is transferred across the joint.

From his study, Foxworthy (1985) reported the following:

1. JTE increased with increasing temperature and

the relationship between JTE and temperature was the

same whether he used air or pavement temperatures.

2. The type of joint construction affected the rate at

which LTE changed with temperature.

3. The JTE was not affected by the FWD load used

during the test.

4. The JTE was the same across transverse joints

when measured from the approach or leave slab on

airfield pavements (see Fig. 37). However, across longitudinal joints, the LTE values were higher when

measured from the leave slab.

Foxworthy (1985) developed relationships for predicting the LTE. He found that, in general, an S type

curve could be used to fit his results concerning load

transfer efficiency and temperature (Fig. 39). The

JTE–temperature relationships obtained by Foxworthy were from tests conducted at different times of the

day where the only changes that were occurring were

in the PCC layer. Since the base and subgrade

strengths were changing very little with time, these

results may not be applicable to pavements during

thaw-weakening periods. So, we used the limited data

collected at OCA and CWA to study the effects of

changing subbase–subgrade stiffness during spring

thaw on JTE.

Loads were transferred across transverse joints

with dowels or aggregate interlocks and across longitudinal joints by the use of keyways, aggregate interlocks or tiebars. Typical changes at CWA are shown in

Figures 40 and 41. We found two basic trends at CWA

and only the first at OCA:

1. In general, the transverse JTE increased with increasing temperature, similar to that reported by Fox31

Figure 39. Relationship between air temperature and transverse joint

transfer efficiency (after Foxworthy 1985).

100

80

60

40

20

0

4.4 26.7 48.9

Air Temperature (°C)

Joint Load Transfer Efficiency (%)

Slab No. 1

 2

 3

 4

Figure 38. Placement of FWD sensors for load transfer efficiency test.

to determine the amount of deflection on either side of

the joint as illustrated in Figure 37.

The Corps of Engineers’ method (TM 5-826-5, U.S.

Army, no date) for calculating the Joint Transfer Efficiency (JTE) using the deflections obtained with the

FWD is

JTE = δ

δ

1

0

where JTE = joint transfer efficiency

δ0 = center plate deflection

δ1 = deflection measured across joint.

With the FWD, the center sensor was placed on one

side of the joint and the second sensor, located 300 mm

away from the center of the loading plate, was placed

across the joint. This is illustrated in Figure 38. The

JTE values across the transverse, longitudinal and corner joints, as calculated from the equation above for

both CWA and OCA, are presented in Tables 11 and

12. The JTE values presented in the tables are the average from the four drop heights (joint and load transfer

have been used interchangeably in the literature). In

this report, a distinction is made between joint (JTE)

and load transfer efficiency (LTE). Joint transfer efficiency as shown above is the ratio of the deflections on
32

Date 10 11 12 13 15 16 17 18 19

18 Mar 86 50.2 67.2 — — 9.3 62.2 15.2 11.0 65.424 Mar 86 57.3 — — — 24.6 43.0 42.3 59.1 41.34 Apr 86 62.7 — — 98.8 45.4 63.0 66.2 43.5 35.77 Apr 86 76.6 — — 98.2 44.0 49.5 68.8 52.1 39.014 Apr 86 15.5 — — 98.3 73.5 77.2 27.9 86.3 45.421 Apr 86 14.4 — — 92.4 — — — — —

Date 20 21 22 23 24 25 26 27 28

18 Mar 86 60.8 49.1 90.0 22.5 61.3 87.1 54.5 — —27 Mar 86 42.0 109.3 68.3 33.4 58.7 87.2 72.6 — —4 Apr 86 67.2 52.1 79.3 25.0 26.0 91.7 79.6 — —7 Apr 86 78.1 49.2 80.5 — 43.2 88.9 79.6 — —14 Apr 86 53.2 71.8 83.4 21.5 33.9 98.5 88.6 — —21 Apr 86 — — — 18.5 30.6 90.9 80.6 — —

c. Across corner joints.

Date 1 2 3 4 5 6 7 8 9

20 Mar 86 61.0 23.1 99.4 85.0 33.6 30.5 78.6 64.2 33.024 Mar 86 77.0 24.6 100.0 84.0 68.3 65.6 61.3 66.0 29.34 Apr 86 73.4 56.7 94.8 90.7 62.7 39.0 84.3 82.6 64.77 Apr 86 69.4 65.0 93.6 94.5 61.6 53.2 67.6 55.1 81.617 Apr 86 62.5 73.9 94.5 91.4 54.4 53.9 71.5 63.0 65.021 Apr 86 57.3 78.0 99.8 84.4 51.9 73.3 85.6 84.7 79.2

Date 10 11 12 13 15 16 17 18 19

18 Mar 86 66.5 69.7 — 116.3 79.8 60.6 44.5 64.9 64.324 Mar 86 73.5 — — — 61.1 49.8 51.2 46.4 46.527 Mar 86 0.0 — — — — — — — —4 Apr 86 95.3 — — 79.3 84.3 69.7 41.9 52.5 71.47 Apr 86 91.6 — — 77.4 86.2 59.1 45.2 56.2 79.514 Apr 86 94.2 — — — 72.0 72.9 71.0 63.1 76.921 Apr 86 90.2 — — — — — — — —

Date 20 21 22 23 24 25 26 27 28

18 Mar 86 80.0 70.9 83.9 88.4 64.5 64.7 42.9 — —27 Mar 86 64.9 58.9 69.9 91.4 55.4 71.0 45.2 — —4 Apr 86 82.2 80.9 77.3 92.8 64.5 68.6 57.4 — —7 Apr 86 80.7 75.3 83.0 89.7 57.4 69.0 53.9 — —14 Apr 86 89.6 79.3 86.1 92.9 73.1 78.9 67.3 — —21 Apr 86 — — — 89.3 64.7 69.9 53.0 — —

Table 11. Joint transfer efficiencies at Central Wisconsin Airport (%).

a. Across transverse joints.

Date 1 2 3 4 5 6 78 9

20 Mar 86 59.0 51.9 81.6 86.4 55.0 49.7 59.0 60.7 42.524 Mar 86 63.4 32.2 89.4 85.4 69.9 64.0 63.2 86.5 67.027 Mar 86 68.8 49.8 94.5 95.2 47.6 60.8 75.0 86.7 58.14 Apr 86 77.1 78.4 92.0 92.5 47.1 60.7 74.4 85.1 78.07 Apr 86 77.6 82.1 93.2 94.7 49.3 90.1 71.7 90.4 92.310 Apr 86 71.9 71.6 92.9 89.5 59.1 61.3 80.4 77.0 74.114 Apr 86 75.0 76.6 92.3 94.5 67.8 65.4 84.4 91.8 82.817 Apr 86 71.3 72.6 91.7 94.0 59.8 90.5 71.2 71.9 91.721 Apr 86 74.3 75.2 89.2 89.5 71.1 82.4 83.7 84.3 91.124 Apr 86 70.9 91.8 90.3 91.6 70.4 92.6 80.9 80.2 92.9

Date 10 11 12 13 15 16 17 18 19

18 Mar 86 81.6 88.8 75.6 94.3 46.3 51.3 36.1 33.4 56.624 Mar 86 76.0 58.2 55.1 86.4 62.7 68.6 34.1 50.3 45.827 Mar 86 88.6 73.7 66.0 91.9 61.5 67.4 37.0 54.8 56.24 Apr 86 90.7 67.0 63.6 102.6 54.8 79.5 50.0 62.2 60.67 Apr 86 96.5 90.4 102.9 103.1 64.3 86.8 74.3 76.8 86.910 Apr 86 94.7 73.2 63.4 99.6 60.8 82.9 50.6 67.1 63.814 Apr 86 94.6 77.0 67.0 93.9 43.1 94.6 57.5 59.4 69.817 Apr 86 95.6 72.1 61.7 95.5 58.7 89.9 67.2 67.7 78.021 Apr 86 92.4 74.1 59.1 91.2 57.8 69.8 58.7 61.4 70.424 Apr 86 93.8 68.5 81.6 92.4 66.2 84.0 89.6 82.0 89.9

Date 20 21 22 23 24 25 26 27 28

18 Mar 86 77.1 86.0 75.5 80.0 51.5 65.2 70.6 77.6 35.924 Mar 86 62.6 — — — — — — — 78.727 Mar 86 81.3 76.8 85.8 90.9 53.9 68.7 66.0 85.8 59.54 Apr 86 79.6 80.9 80.3 87.3 57.6 65.6 65.6 94.5 73.57 Apr 86 89.5 87.7 88.9 87.0 59.4 68.9 72.7 92.0 32.810 Apr 86 80.7 77.3 80.7 84.6 57.4 67.7 70.4 91.7 72.814 Apr 86 84.5 89.6 78.0 88.7 69.2 75.0 74.2 94.3 25.917 Apr 86 85.8 81.3 77.5 89.7 57.2 63.4 64.2 93.0 30.421 Apr 86 80.7 80.2 72.0 82.8 59.7 68.6 64.7 89.4 26.624 Apr 86 92.7 84.3 91.5 83.2 58.3 71.8 67.4 92.7 29.4

b. Across longitudinal joints.

Date 1 2 3 4 5 6 78 9

20 Mar 86 47.5 56.8 40.7 42.7 14.2 31.6 40.5 36.7 24.024 Mar 86 28.6 74.5 35.7 85.1 32.9 74.4 31.5 25.2 27.04 Apr 86 15.8 23.7 46.1 82.8 6.9 78.1 82.1 7.4 12.27 Apr 86 19.0 20.1 52.4 92.9 7.5 47.2 71.6 31.1 27.317 Apr 86 14.9 18.8 23.5 88.1 14.6 57.7 59.1 11.0 40.821 Apr 86 11.4 23.7 50.0 — 13.5 77.2 64.8 6.8 21.3
33

Table 12. Joint transfer efficiencies at OCA (%).

a. Across transverse joints.

Date 1 2 3 4 5 6 7 8 9 10 11

15 Mar 86 38.1 22.4 57.1 30.3 27.4 19.5 39.6 69.7 57.9 36.8 48.3

26 Mar 86 58.3 16.2 64.7 — 28.1 11.6 37.6 96.8 76.1 36.3 58.8

29 Mar 86 71.7 25.8 62.9 84.0 88.5 25.3 48.2 98.3 94.8 91.5 93.6

5 Apr 86 64.4 22.9 63.3 70.8 73.8 21.3 42.3 44.5 66.6 40.1 81.0

9 Apr 86 54.4 15.0 57.8 20.5 39.4 12.6 42.1 12.3 33.0 22.7 61.8

12 Apr 86 75.3 13.8 62.0 31.0 51.0 13.3 45.7 12.9 51.2 50.2 82.5

16 Apr 86 63.9 20.0 56.7 42.3 43.0 49.1 37.7 21.8 47.2 58.0 77.2

23 Apr 86 77.2 27.7 63.5 93.3 85.7 27.9 42.8 34.1 89.1 90.2 97.2

26 Apr 86 94.3 41.2 77.8 94.7 94.7 76.3 54.2 76.2 92.6 93.7 100.6

Date 12 13 14 15 16 19 20 21 22 23 24

15 Mar 86 41.4 24.1 42.2 7.1 65.8 77.2 54.4 51.0 30.5 52.3 34.0

26 Mar 86 66.5 21.3 65.6 18.8 80.7 92.2 59.1 55.3 40.2 61.0 50.0

29 Mar 86 87.6 69.4 90.7 29.6 92.3 94.8 87.1 62.0 67.9 92.4 75.5

5 Apr 86 74.6 37.0 48.6 14.0 78.9 91.4 65.4 54.6 76.6 30.2 75.5

9 Apr 86 77.4 19.7 28.5 16.9 76.3 90.9 61.2 54.2 52.6 76.5 33.4

12 Apr 86 82.2 18.9 36.9 — 78.6 92.2 63.6 54.1 55.7 15.7 116.2

16 Apr 86 75.6 36.6 41.4 29.4 63.3 89.4 66.9 56.8 50.4 28.3 21.0

23 Apr 86 91.3 20.4 35.3 18.0 76.8 91.2 73.7 56.9 49.7 18.0 15.0

26 Apr 86 92.8 67.6 94.0 89.7 89.9 — — — 58.2 45.8 81.7

b. Across longitudinal joints.

Date 1 2 3 4 5 6 7 8 9 10 11

15 Mar 86 24.9 61.2 67.7 34.8 17.7 20.1 80.7 45.9 50.6 11.0 27.4

29 Mar 86 16.6 73.5 86.1 83.3 17.0 27.5 98.9 72.3 87.5 15.8 46.4

5 Apr 86 15.8 80.1 89.0 59.6 18.7 19.6 92.8 77.8 74.7 23.3 17.9

12 Apr 86 11.7 82.1 83.1 89.8 17.2 14.5 99.5 84.0 49.1 17.2 18.3

26 Apr 86 80.1 106.4 86.3 88.1 25.0 49.4 92.9 84.3 91.6 39.5 82.5

Date 12 13 14 15 16 19 20 21 22 23 24

15 Mar 86 14.4 14.6 11.1 14.7 70.3 70.3 64.8 20.7 45.2 32.7 74.4

29 Mar 86 16.8 49.8 13.0 21.5 87.3 89.7 76.1 76.8 93.5 90.0 99.4

5 Apr 86 31.7 32.3 26.4 27.2 53.9 81.8 62.7 68.0 80.7 88.6 99.4

12 Apr 86 16.1 32.4 24.8 20.5 99.0 83.7 37.4 89.1 20.9 75.9 89.6

26 Apr 86 28.1 35.0 27.3 64.0 81.8 — — — 52.4 93.4 97.8

c. Across corner joints.

Date 1 2 3 4 5 6 7 8 9 10 11

15 Mar 86 33.6 27.2 50.7 84.9 17.5 16.6 41.3 29.3 13.8 15.6 20.9

29 Mar 86 52.7 76.3 53.1 90.5 32.3 13.4 63.7 87.4 42.5 75.6 52.6

5 Apr 86 56.0 41.8 59.2 85.5 25.3 10.2 57.4 66.1 31.0 32.6 103.6

12 Apr 86 47.5 26.3 49.3 36.0 13.7 6.6 42.0 17.7 47.4 33.4 40.6

26 Apr 86 46.9 95.8 66.7 92.4 55.3 — 68.2 91.7 90.1 85.4 70.1

Date 12 13 14 15 16 19 20 21 22 23 24

15 Mar 86 27.4 5.2 87.0 4.4 52.7 84.6 31.1 27.5 22.4 13.2 29.3

29 Mar 86 73.5 85.1 93.7 11.9 72.8 90.0 71.7 29.9 61.1 21.3 69.2

5 Apr 86 66.3 17.8 69.8 4.2 79.2 94.6 65.5 113.1 23.2 41.7 69.2

12 Apr 86 82.1 29.7 47.4 5.1 87.2 96.5 66.9 53.3 80.5 22.4 18.2

26 Apr 86 90.4 78.4 96.7 31.4 84.4 — — — 90.0 91.9 72.0
Central Wisconsin Airport

Surface Temperature (°C)

Transverse Joint Efficiency (%) 

0

20

40

60

80

100

–5 0 5 10 15 20 25

 24 March

 27 March

 21 April

 14 April 7 April

24 April

10 April 17 April

4 April

20 March

Location 2

 Thaw Period 29 March to 10 April

a. FWD location 2.

Figure 40. Change of transverse joint efficiency with surface temperature.

Central Wisconsin Airport

Surface Temperature (°C)

Transverse Joint Efficiency (%) 

20

40

60

80

100

0 5 10 15 20 25 30

Location 19

 Thaw Period 29 March to 10 April

18 March

24 March

27 March

4 April

7 April

10 April

14 April

17 April

21 April

24 April

b. FWD location 19.

Figure 41. Effect of subgrade modulus calculated with ILLIBACK on transverse joint efficiency.

34

200 400 600 800 1000 1200 1400

Subgrade Modulus (MPa)

0

100

80

60

40

20

Transverse Joint Efficiency (%)

Central Wisconsin Airport

Transverse Joints

 Location 02

 Location 05

 Location 09

 Location 17

Location 2

Location 5

Location 9

Location 17
35

0

100

80

60

40

20

Transverse Joint Efficiency (%)

15 20 25 30 5 10 15 20 25

Mar Apr

Central Wisconsin Airport

Transverse Joints

 Location 27

 Location 26

 Location 24

 Location 23

Figure 42. Change in transverse joint transfer with time on taxiway B.

worthy (1985) (Fig. 40). In addition, we determined

that increases in temperature did not cause increases in

JTE during the early thaw period; the JTE in most cases decreased. As thaw progressed, the JTE recovered.

In the late thaw period, the JTE was basically a linear

function of temperature (Fig. 40b). The most significant effect of thawing was related to the base–subgrade

modulus (Fig. 41).

2. At the CWA FWD locations where the base

course was 1.2 m thick, the effect of temperature on the

transverse joint efficiency was negligible (Fig. 42). It

should also be noted that the thickness of the PCC layer

is 330 mm compared to the other sites, where it ranged

from 254 to 305 mm. This increase in thickness may

also contribute to the negligible effect of surface temperature on the JTE. This was not observed at OCA.

3. With a few exceptions, as shown in Tables 11b

and 12b, the JTE was low across the longitudinal

joints at CWA and OCA. The data infer that, in general, the longitudinal joints were weak with respect to

load transfer.

The LTE was determined for the transverse joints

both at CWA and OCA using the procedure outlined

in Hammons and Pittman (1993). The LTE across a

joint is estimated as a function of the radius of relative

stiffness (l) and the JTE. The JTE is determined from

FWD deflection data as discussed above. The radius

of relative stiffness (l) is determined from the normalized basin area (AREA) from a unique relationship developed by Ioannides (1989). This relationship for a

dense liquid foundation is reproduced in Figure 43. A

sixth-order polynomial equation was developed for

Radius of Relative Stiffness (in.)

15 20 25 30 35

Area (in.)

0

20

40

60

80

100

Dense Liquid Foundation

a = 5.9055

Figure 43. Relationship between AREA and l for a dense liquid foundation, a = 5.90655 in. (after Hammon and Pittman 1993).
36

Figure 44. Interpolated relationships between joint efficiency and load

transfer as a function of a/l (after Hammon and Pittman 1993).

Figure 45. Relationship between JTE and LTE for transverse joints.

Central Wisconsin Airport

OCA

CWA

Joint Transfer Efficiency (%)

Transverse Joint Efficiency (%)

10

20

30

40

50

0 20 40 60 80 100

 Transverse Joints

this relationship and is in the form of

l AREA AREA AREA

AREA AREA

AREA

=− + −

+ −

+

0 5 1 24 1 21 0 1803

0 011098 0 0003075

0 000003198

2 3

4 5

6

. (. ) . .

. .

.

and

AREA = + 



 



 + 



 



 + 



 





















61 2 2 1

0

2

0

3

0

δ

δ

δ

δ

δ

δ

where l = radius of relative stiffness (inches)

δi= FWD deflections (inches)

AREA = normalized deflection basin area

i = deflection sensor number.

With the ratio of radius of the FWD loading plate (a) to

the radius of relative stiffness (a/l) and the joint efficiency, Figure 44 was used to calculate the load transfer

efficiency. Figure 44 was developed by Tabatabaie et

al. (1979) and modified by Hammons and Pittman

(1993). The results of the load transfer across transverse joints were determined at both CWA and OCA

and are presented in Figure 45. We found that the relationship below can be used to estimate the LTE of transverse joints using FWD JTE data.

LTE (%) = 1.912e3.115(JTE)

where JTE is joint transfer efficiency (decimal).

We did not attempt to develop similar relationships

for the longitudinal joint because of the low JTE values
37

Joint Transfer Efficiency (%)

Transverse Joint Efficiency (%)

10

20

30

40

50

0 20 40 60 80 100

Figure 47. Relationship between JTE and LTE.

Figure 46. Relationship between total basin area and subgrade modulus.

and also because of the small database. However, because of the unique relationship between JTE and LTE

shown in Figure 45, it may be possible to apply the

same relationship to longitudinal joints.

To meet the criterion that the LTE of the joint be not

less than 25%, the JTE should exceed 80%. If this is the

case, a review of Tables 11a and 12a indicates that

many of the joints at CWA and OCA were below capacity during the thawing period. At the end of thaw, nearly

all of them met the 25% criterion. The results indicate that

most of the damage to transverse joints probably occurs

during the winter and spring thawing period.

PROPOSED PAVEMENT

EVALUATION PROCEDURE

A methodology is proposed for evaluating pavement performance during the thaw-weakening periods

using FWD deflection data. From this study, we found

that, for PCC pavement, the composite subgrade modulus (base and subgrade) can be estimated from the deflection basin area and Figure 46 or from the following

equation

Esub = 154,319∆–1.1708

where Esub is the subgrade modulus (MPa), and ∆ is

deflection basin area (mm2). The horizontal tensile

stress at the bottom of the PCC layer was found to be a

function of pavement thickness and Esub or the coefficient of subgrade reaction (k) from the following equations for either a Boeing 757 or for a MD-DC9.

σtensile = 7360 – 1.5Esub – 17t

σtensile = 7389 – 3.02k – 17.5t

or, with respect to total basin area, an estimate could be

made using

σtensile = 7.35AT

where σtensile = horizontal tensile stress at the bottom

of the PCC layer (kPa)

k = coefficient of subgrade reaction

(MN/m3)

t = PCC layer thickness (mm).

The LTE across transverse joints can be estimated from

the JTE from Figure 47 or from the following equation

LTE (%) = 1.912e3.115(JTE)

where JTE is the joint transfer efficiency (decimal)

JTE = δ

δ

u

i

and δu is FWD deflection on an unloaded slab and δi is

FWD deflection on a loaded slab.

2000

1500

1000

500

0 200 400 600

Subgrade Modulus (MPa)

2500

Total Basin Area (mm ) 2
CMT, Inc. (1984) Pavement evaluation program—

Central Wisconsin Airport.

ERES Consultants (1985) Structural evaluation of

runway 3-21.

Foxworthy, P.T. (1985) Concepts for the Development

of a Nondestructive Testing and Evaluation System for

Rigid Airfield Pavements. Ph.D. Dissertation, University of Illinois at Urbana-Champaign (unpublished).

Hammons M.I. and D.W. Pittman (1993) Effectiveness of load transfer devices. Draft Final Report. USA

Waterways Experiment Station, Vicksburg, Mississippi.

Ioannides, A.M., E.J. Barenberg and J.A. Lary (1989)

Interpretation of falling weight deflectometer results

using principles of dimensionless analysis. Advanced

Construction Technology Center, Document No. 89-

51-01.

Irwin L.H., W.S. Yang and R.N. Stubstad (1989) Deflection reading accuracy and layer thickness accuracy

in backcalculation of pavement layer moduli. In Proceedings, Nondestructive Testing of Pavement and Backcalculation of Moduli (A.J. Bush III and G.Y. Baladi,

Ed.). ASTM STP 1026, p. 171–188.

Janoo, V.C. and R.L. Berg (1990) Predicting pavement

response during thaw weakening periods using the falling weight deflectometer. In Proceedings, Third International Conference on Bearing Capacity of Roads

and Airfields, Trondheim, Norway, vol. 1, p. 31–40.

Janoo, V.C. and R.L. Berg (1991) Asphalt concrete airfield pavement evaluation during thaw weakening

periods using the falling weight deflectometer. USA

Cold Regions Research and Engineering Laboratory,

CRREL Report 91-7.

Janoo, V.C. and R.L. Berg (1996) PCC airfield pavement response to thaw weakening periods: Data. USA

Cold Regions Research and Engineering Laboratory,

Internal Report 1155.

Mead and Hunt, Inc. (1988) Runway 11/29 strengthening design study.

Stark, J. and R.L. Berg (1989) Performance of pavement at Central Wisconsin Airport. In Proceedings, 5th

International Conference on Cold Regions Engineering. New York: American Society of Civil Engineers.

CONCLUSIONS

During the spring thawing period, the JTE was a

function not only of the surface temperature but also

of subgrade stiffness. Near the end of thaw, the JTE

was basically solely a function of the surface temperature.

Unique relationships were developed between

FWD deflections and the subgrade modulus (Esub)

and between the FWD deflections and the coefficient

of subgrade coefficient (k). Additional relationships

were developed between the PCC layer thickness and

either Esub or k and the horizontal tensile stress (σtensile)

at the bottom of the PCC layer.

A relationship was also developed for determining

the load transfer across transverse joints on the basis

of JTE calculated from FWD deflection data. This relationship could be also used for longitudinal joints.

The developed relationships allowed us to propose

a methodology for evaluating PCC pavement performance during winter and thaw-weakening periods.

However, this methodology could be used at other

times of the year.

The relationships developed from this study should

be verified at other sites. In particular, although there

are only small differences in the horizontal stress relationships for the Boeing 757 and the MD-DC9, additional horizontal stress relationships may be needed

for other aircraft.

Field measurements of horizontal stress or strain

are necessary to verify these relationships.

LITERATURE CITED

Barenberg, E.J. and A.M. Ioannides (1989) Structural

evaluation of concrete slabs using falling weight deflectometer results, Urbana-Champaign: Advanced

Construction Technology Center, University of Illinois.

U.S. Army (no date) Nondestructive procedures for

airfield pavement evaluation. Draft Technical Manual

TM 5-826-5.

38
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May 1996

PCC Airfield Pavement Response During Thaw-Weakening Periods: PR: 4A762784AT42

A Field Study MA: BS

WU: BS/036

Vincent C. Janoo and Richard L. Berg

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road Special Report 96-12

Hanover, New Hampshire 03755-1290

Federal Aviation Administration and Office of the Chief of Engineers

Technical Center Washington, D.C. 20314-1000

Atlantic City International Airport

New Jersey 08405

Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161.

This investigation is part of CRREL’s on-going characterization of pavement performance in seasonal frost areas. As part

of the research, CRREL conducted several field studies for the Federal Aviation Administration (FAA) on the response of

airport pavements during thaw-weakening periods at three civil airports in Wisconsin where the design freezing index in

the area was around 900 to 1100°C-days and frost penetration ranged between 1250 and 2000 mm. This study focused on

the performance of Portland Cement Concrete (PCC) pavements during the spring thaw-weakening period. The sites were

instrumented with subsurface thermocouples and Falling Weight Deflectometer (FWD) tests were conducted during the

spring thaw period at the center of the slab and across the joints. An analysis of the FWD data and backcalculation of the

layer moduli using ILLIBACK and WESDEF was conducted. Unique relationships between the FWD deflections and the

subgrade modulus and coefficient of subgrade reaction were obtained. Additional relationships were developed using the

FWD deflections, PCC thickness and the horizontal tensile stress at the bottom of the PCC layer. A relationship between

load transfer across joints and FWD deflections was also developed. On the basis of the relationships obtained in this

study, a methodology for evaluating PCC pavements during spring thaw was developed. However, this methodology

needs to be verified for other subgrade types and areas with other design freezing indices.

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Evaluation Pavement Thaw-weakening

Load transfer PCC pavements