evaluation of load transfer efficiency · pdf filei final report mid-atlantic universities...

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FINAL REPORT

MID-ATLANTIC UNIVERSITIES TRANSPORTATION CENTER

Evaluation of Load Transfer Efficiency Measurement

Samir N. Shoukry, Ph.D. Departments of Mechanical and Aerospace/

Civil and Environmental Engineering Tel: (304) 293-3111 Ext 2367

Fax: (304) 293-6689 Email: [email protected]

Gergis W. William, Ph.D., P.E.

Mourad Y. Riad, MSCE Department of Civil and Environmental Engineering

West Virginia University College of Engineering and Mineral Resources

The contents of this report reflect the views of the authors, who are responsible for the facts and accuracy of the information presented herein. This document is designated under the sponsorship of West Virginia Department of Transportation, Division of Highways in the interest of information interchange. The U.S. Government assumes no liability for the contents or use thereof. This report does not constitute a standard, specification, or regulation. The contents do not necessarily reflect the official views or policies of the State or Federal Highway Administration.

ii

Technical Report Documentation Page 1. Report No. WVU-2002-04 2. Government Accession No. 3. Recipient’s Catalog No.

5. Report Date July 2005 4. Title and Subtitle Evaluation of Load Transfer Efficiency Measurement 6. Performing Organization Code

7. Author(s) Samir N. Shoukry , Gergis W. William, Mourad Y. Riad

8. Performing Organization Report No.

10. Work Unit No. (TRAIS) 9. Performing Organization Name and Address West Virginia University, Department of Civil and Environmental Engineering Morgantown, WV 26505-6103. 11. Contract or Grant No.

13. Type of Report and Period Covered

12. Sponsoring Agency Name and Address Mid-Atlantic Universities Transportation Center MAUTC-WVU

14. Sponsoring Agency Code

15. Supplementary Notes Sponsored by Mid-Atlantic Universities Transportation Center 16. Abstract This report documents an evaluation of load transfer efficiency (LTE) of dowel jointed concrete pavements. Measurement of load transfer efficiency of transverse joints in concrete pavements is universally conducted using FWD device. LTE is an important parameter affecting pavement performance. Due to the importance of the results for maintenance decisions, the accuracy of the measurement technique is investigated in this report. The availability of instrumented dowel fitted concrete slabs in West Virginia Smart Road (Corridor H, Route 33 as well as Goshen Road, West Virginia), offers a unique opportunity to examine the accuracy of determining the load transfer efficiency of transverse joints using FWD. For this purpose, FWD tests were conducted on both pavement sites at different times during years 2003, 2004, and 2005. Thirty transverse joints were tested along the slab edges as well as along the wheel-path. Trend analysis was performed to evaluate the effect of design features and site conditions on LTE. Key findings from this study: 1. Load transfer efficiency was found to be a complex parameter that depends on many factors that include load

position, testing time, slab temperature, and load transfer device. 2. Testing time and season was found to have a significant effect on the measured load transfer efficiency. 3. The slabs fitted with 32 mm (1.25 in) diameter dowels displayed higher variability of the measured load

transfer efficiency than those fitted with 38 mm (1.5 in) diameter dowels. 4. Joint opening changes daily and seasonally as the ambient temperature changes. As the amount of joint

opening increases due to slab contraction during winter, the measured load transfer efficiency generally decreases.

5. Poor correlation was found between the deflection-based load transfer efficiency and the percentage of the load transferred through the load transferring devices mounted across the transverse joint.

17. Key Words Dowel bars, Transverse joints, Load transfer Efficiency, Joint Testing,

18. Distribution Statement

19. Security Classif. (of this report) Unclassified

20. Security Classif. (of this page) Unclassified

21. No. of Pages

22. Price

Form DOT F 17007.7 (8-72) Reproduction of completed page authorized

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TABLE OF CONTENTS

ABSTRCT TABLE OF CONTENT LIST OF FIGURES LIST OF TABLES CHAPTER ONE INTRODUTION

1.1 Background 1.2 Mechanistic Modeling of Load Transfer Efficiency 1.3 Measurement of Load Transfer Efficiency 1.4 Objective 1.5 Research Methodology

CHAPTER TWO PAVEMENT SECTION AND DATA COLLECTION

2.1 Pavement Test Section 2.2 FWD Loading Tests 2.3 Effect of Joint Opening 2.4 Effect of Slab Temperature 2.5 Effect of Temperature Gradient

CHAPTER THREE MEASUREMENT OF DOWEL SHEAR FORCES

3.1 Introduction 3.2 Goshen Road Pavement Site 3.3 FWD Testing 3.4 Dowel Bar Shear 3.5 Distribution of Shear Forces among Dowel Bars

CHAPTER FOUR SUMMARY AND CONCLUSIONS REFERENCES

ii iii iv v

1 1 3 5 6 6

8 8

13 20 20 21

22 22 22 23 24 25

31

32

iv

LIST OF FIGURES

Figure 2.1 Figure 2.2 Figure 2.3 Figure 2.4 Figure 2.5 Figure 2.6 Figure 2.7 Figure 2.8 Figure 3.1 Figure 3.2 Figure 3.3 Figure 3.4 Figure 3.5

Layout of Elkins Instrumented Pavement Site Typical Slab Instrumentation System Recorded Temperature Differential and Dowel Bending Moment Transverse Joint Opening along Instrumented Pavement Section Recorded Mean Slab Temperature and Joint Opening Load Transfer Efficiency Versus Joint Opening Load Transfer Efficiency Versus Slab Temperature Temperature Differential Versus Load Transfer Efficiency Construction of Instrumented Slabs FWD Testing of Transverse Joints Typical Shear Force History in Dowel Bars Shear Forces Distribution in Dowel Bars Shear Forces Distribution in Shokbars

9 10 10 11 12 20 21 21 23 24 25 27 28

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LIST OF TABLES

TABLE 2.1 TABLE 2.2 TABLE 2.3 TABLE 2.4 TABLE 2.5 TABLE 2.6 TABLE 3.1 TABLE 3.2 TABLE 3.3

Load Transfer Efficiency Data Measured on September 2, 2003 Load Transfer Efficiency Data Measured on March 22, 2004 Load Transfer Efficiency Data Measured on April 28, 2004 Load Transfer Efficiency Data Measured on June 15, 2004 Load Transfer Efficiency Data Measured on August 30, 2004 Load Transfer Efficiency Data Measured on June 15, 2005 Shear Forces in Instrumented Dowels and Shokbars Shear Forces in Dowel bars and Shokbars (N) Comparison of Deflection Load Transfer Efficiency and Actual Load Transfer Efficiency

14 15 16 17 18 19 26 29

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1

CHAPTER ONE

INTRODUCTION

1.1 Background

As a wheel load is applied near a transverse doweled joint in a PCC pavement, both loaded and unloaded slab deflect since a portion of the load applied to the loaded slab is transferred to the unloaded one through the dowel bars. As a result of the presence of load transfer devices, deflections and stresses in the loaded slabs may be significantly less than those induced in slab with free edge. The magnitude of reduction in stress and deflections by a joint depends on its load transfer efficiency.

The term load transfer efficiency is used to express the ability of a joint to transmit part of the applied load on the loaded slab to the adjacent unloaded one (Ioannides and Krovesis, 1992). Several formulae for calculating load transfer efficiency have been adopted by various researchers to provide quantitative measures of pavement-system response: 1. Deflection load transfer efficiency, LTEδ:

This is the most common measure for load transfer efficiency.

l

u

dd

LTE =δ ……………………….. (1.1)

where du and dl are the vertical displacements of the unloaded and loaded slabs respectively measured at the joint on the top of the slabs. Another definition for the deflection load transfer efficiency is the one first proposed by Teller (Teller and Sutherland, 1936; Teller and Cashell, 1959) and is still in use by researchers (American Concrete Paving Association, 1991, McGhee, 1995, Colley and Humphrey, 1967; Raja and Snyder, 1991; Khazanovich and Gotlif, 2003) as:

lu

u

ddd

LTE+

=∗ 2δ …………….………. (1.2)

If a joint exhibits a poor ability to transmit load, the deflection of the unloaded slab will be much less than that of the loaded one. In this case, both LTE indexes will have values close to zero. On the other hand, if the joint has a good ability to transfer load, the deflection of both sides will be close to each other and both indexes have values close to 1.0. The two indexes are related to each other by the equation:

+

−×=∗

δδ LTE

LTE1

112 …………. (1.3)

Therefore, these two indexes are equivalent, and if one is known, the other can be easily calculated. In this study, we will adopt the first index defined by Equation 1 since it is much

2

more widely used and accepted by the AASHTO Pavement Design Guide (1993) and its supplement for Concrete Pavement Design (AASHTO, 1998). 2. Stress Load Transfer Efficiency, LTEσ

Load transfer efficiency can be computed based on stress using formulae similar to those developed for deflection-based load transfer efficiency. The most commonly used equation for stress-based load transfer efficiency is given as:

l

uLTEσσ

σ = ……………………... (1.4)

where σu and σl are bending stresses of the unloaded and loaded slabs respectively. Also, the following expression was proposed by Sutherland and Cashell (1945):

if

jfLTEσσσσ

σ −

−= .……………….. (1.5)

Where σf = stress for a given load applied at a free edge, σj = stress for a given load applied at the crack or joint edge, σi = stress for a given load applied at the slab interior. The stress-based load transfer efficiency indicates the reduction in bending stress at the joint caused by the presence of load transfer devices. Previous studies indicated that there is no one-to-one relationship between stress-based and deflection-based load transfer efficiency indexes. Korbus and Barenberg (1979) developed a relationship that can be used to correlate these two efficiency parameters. Such a relationship was subsequently adopted by the AASHTO Pavement Design Guide (AASHTO, 1993). Because of the difficulty inherited in stress measurement in concrete slabs, the fact that stress in concrete slab is influenced by the slab geometry and applied load configuration, and the ease of rapidness of deflection measurement, the deflection-based load transfer efficiency is commonly used to measure the load transfer in concrete pavements. 3. Transferred Load Efficiency, TLE

TLE, a dimensionless variable, was introduced by Ioannides et al. (1990) to express the ratio of the load transferred across the entire length of joint PT to the total applied load P, and given by:

PP

TLE T= ……………………….. (1.6)

Additionally, Ioannides et al. (1990) presented the load distribution factor, fd, that indicates the load share of any given dowel bar from the total transferred load through a joint as expressed by the equation:

T

id P

Pf = ……………………….. (1.7)

3

where Pi is the load transferred by a particular dowel bar. 1.2 Mechanistic Modeling of Load Transfer Efficiency

Westergaard (1929) presented the first rational method of designing joints in concrete pavements by providing a theoretical means for computing the shearing force in each active dowel bar as well as the critical stresses that occur in the slab under the edge load. However, his analysis was limited to the hypothetical case in which the deflections of the two sides of the joint are equal (LTE=100%). Westergaard concluded that only the first two dowel bars from each side are active in transferring the load to the adjacent slab. Friberg (1938) assumed that dowels at a distance greater than 1.8 times the radius of the relative stiffness, l , from the center of the load are inactive in transferring any load through the joint. Westergaard (1926) defined the radius of the relative stiffness as:

42

3

112 k)(Ehlν−

= ………………… (1.8)

Where: E= Elastic modulus of concrete. h= Thickness of the concrete slab. ν= Poisson's ratio of concrete. k= Modulus of subgrade reaction.

Friberg (1938) also assumed that the shear force in an active dowel decreases as a linear function of distance. Stresses induced in concrete were calculated based on Timoshinko's solution for the dowel bar as an infinite beam encased in an elastic medium. The concrete surrounding the dowel bar was idealized with a single parameter, the modulus of dowel support, K. Values of K ranging between 300,000 and 3,000,000 pci were suggested. A value of 1,500,000 pci is commonly used as a typical value (Yoder and Witczak 1975 and Haung 1993). The relative stiffness, β, of a bar embedded in concrete is given by:

44EIKb

=β ……………….…….. (1.9)

Where: b= diameter of the dowel. E= modulus of elasticity of the dowel. I= moment of inertia of the dowel.

If the joint width is designated w, Friberg calculated the deflection of the dowel at the joint face, y0, and the bearing pressure (σb) on the concrete as:

( )wEI

Py t

o ββ

+= 24 3 ………..… (1.10)

4

( )wEIPK

yK too β

βσ +== 2

4 3 …. (1.11)

It is significant to point out that the above stress is a compressive stress that is well below the concrete compressive strength.

The lack of sound experimental or theoretical procedures for determining both the modulus of dowel bar support K and the amount of load transferred in any dowel made it impossible to accurately calculate the stresses or the deflection using the Firberg’s approach. Kushing and Fremont (1940) suggested that the range of distribution of load transfer hardly extended over more than π l each side of the load, while Sutherland (1940) reported experimental results that support Westergaard's conclusion that only the dowels near the load are effective in load transfer.

In an attempt to better understand the role of dowels in load transfer, Two Dimensional

Finite Element (2D-FE) was introduced. Several finite element programs have been developed including: ILLI-SLAB (Tabatbaie and Barenberg, 1978; Krovosis, 1990), JSLAB (Tayabji and Colley, 1986), KENSLABS (Huang and Wang, 1974), WESLIQUID (Chou, 1981), WESLAYER (Chou, 1981), KENLAYER (Huang, 1993), FEACONS IV (Tia et al., 1987; Wu and Larsen, 1993), and ISLAB2000 (Khazanovich et al., 2000, Beckemeyer et al., 2002). All these programs simulated the traffic loads as a static load. Earlier 2D FE programs utilized the single parameter K to simulate the dowel-concrete interaction. Using ILLI-SLAB, Tabaabaie and Barenberg (1980) supported linear decrease of shearing forces in dowels with distance from center of load application; however the distribution length was reduced to 1.0 l . The use of reduced distribution length was also supported by the results of Heinrichs et al. (1989) and Huanng (1993). Results from J-SLAB (Tayabji et al. 1986) indicated that only the dowel diameter, modulus of subgrade reaction, and modulus of dowel support significantly effected pavement stresses and deflections.

In 2D FE programs, dowel bars were simulated using beams elements or linear and

torsion spring elements. In such a representation, it is assumed that the displacements at the ends of the beam or spring element are equal to those slab elements. Nishizawa et al. (1989) rejected this assumption and suggested modeling dowels using a combination of linear and torsional springs that simulated both the dowel bar and the aggregate interlock. Guo et al. (1995) integrated the modulus of dowel support in a two-dimensional finite element formulation in order to simplify dowel bar simulation in 2DFE models of rigid pavements and to account for dowel looseness. The 2DFE-calculated shear force in the dowel bar is substituted into Equation 11 to calculate the bearing stress in the concrete surrounding that dowel. However, the triaxial state of stress that takes place around the dowel could not be viewed. Further, the horizontal friction force between the dowel bars and the surrounding concrete cannot be modeled.

Channakeshava et al. (1993) developed a nonlinear static 3D-FE model to study the

combined effect of a linear thermal gradient and a static wheel load. Dowel bars were simulated as beam elements mounted on elastic springs of constants equal to the modulus of dowel support K. Effect of different extents of loss of subgrade support and different dowel-concrete interface characteristics were considered. The study concluded that nighttime curling is a critical loading

5

case since truck traffic is heaviest during the night; in addition, it causes loss of support under the joints. They also reported the loss of support under transverse joint due to nighttime curling would result in an increase of the load transfer efficiency of the joint. The authors reported a large concentration of tensile stresses in the elements below the dowel bars. A finding that does not explain the small horizontal cracks observed in Friberg’s (1938) experiments.

Davids et al. (1997, 1998) developed a 3D-FE program specifically for rigid pavements.

Concrete slabs were modeled using 8-node solid brick elements. An embedded beam element formulation was developed to precisely locate the dowel bar within the finite element mesh irrespective of the slab mesh lines. This model also permits rigorous treatment of dowel looseness, which affects the load transfer efficiency. Later, the program was modified so that a Winkler foundation can be specified between the dowel and concrete instead of explicitly modeling dowel looseness (Davids, 2000). Examining the load transfer efficiency at doweled transverse joints for both curled and flat slabs revealed that slab curling has a large influence on dowel-concrete bearing stress (Davids, 2001). However, both simplified dowel modeling techniques employed in these models are still incapable of predicting the triaxial state of stress that develops in the concrete surrounding the dowel bars.

The tendency to simplify the dowel bar representation in 3DFE models using beam

elements supported on springs whose modulus is the modulus of dowel support became the common feature of the majority of 3DFE models developed during the past decade (Kuo et al., 1996; Kennedy and Everhart, 1996, 1997, 1998; Sargand and Breegle, 1998, Davids, 2001). This simplification is justified by the fact that such models were used to investigate the overall slab stresses and deflections induced under mechanical and/or thermal loads. However, when the modeling objective is to examine dowel-concrete contact stresses, detailed modeling of dowel bars and their interfaces with concrete becomes essential.

A 3DFE model of dowel-jointed concrete pavement that featured detailed three

dimensional modeling of dowel bars was developed by Shoukry and William (1998) to examine the effect of dowel looseness on load transfer efficiency and slab stresses. The general 3D-FE code LS-DYNA was used as the equation solver. All pavement layers as well as dowel bars were modeled using 8-node solid brick elements. Dowel-concrete interfaces as well as slab-base interface were simulated using sliding interfaces with friction that allows for slab-base separation. The results indicated that the localized high stresses induced in the concrete surrounding the dowels causes an elastic deformation. As a result, for a joint with intact dowel bars, the maximum recorded load transfer efficiency was 91 percent. The model results also indicated that such high stresses around dowels initiate cracks around the dowel, which is the main reason for dowel looseness (Shoukry et al., 2002).

1.3 Measurement of Load Transfer Efficiency Nondestructive deflection devices are currently used to evaluate the in situ load transfer efficiency of rigid pavement joints as well as cracks slabs (AASHTO, 1993). Such techniques allow the engineer to evaluate the actual joint performance in the field relative to their expected performance in the design phase.

6

All nondestructive deflection devices are suitable for evaluating the load transfer

efficiency at any joint provided that deflection sensors can be mounted close enough to each other across the joint to measure vertical displacement of both loaded and unloaded slabs. Once the unloaded and loaded slab deflections are measured, deflection-based load transfer efficiency can be directly determined using Equation 1.

Falling Weight Deflectometer (FWD) is the most commonly used nondestructive testing

device for load transfer efficiency measurements. Because of its ease of use, rapidness and accuracy of measurements, the AASHTO Pavement Design Guide (AASHTO, 1993) recommends the use of such a device for deflection measurements required for designing the required thickness of the asphalt overlay or bonded concrete overlay. The AASHTO Design Guide (1993) recommends measuring the joint deflection along the outer wheel-path. The loading plate should be placed on one side of the joint with its edge touching the joint. Deflections are measured at the loading plate center and at 12 inches from the center (6 inches from the joint edge on the adjacent slab). Deflection load transfer Efficiency can be computed from the following Equation:

B100 ×

×=

l

u

dd

LTEδ ……….. (1.12)

Where LTEδ = deflection load transfer efficiency, percent, du = unloaded side deflection, inches, dl = loaded side deflection, inches, B = slab bending Correction factor. The slab bending correction factor, B, is required since the deflections d0 and d12 are measured 12 inches apart, which would not be equal even if measured in the interior of a slab. An appropriate value for the correction factor may be determined from the ratio d0 to d12 for typical center slab deflection basin measurements. Typical values for B are within the range from 1.05 to 1.15 (AASHTO, 1993). 1.4 Objective

The main objective of this study is to examine the accuracy of measuring load transfer efficiency of transverse joints in dowel jointed concrete pavements using FWD and examine how such a deflection-based load transfer efficiency correlates with the actual shear force transmitted through mechanical load transfer devices (traditional dowel bars or shokbars) across transverse joints. 1.5 Research Methodology

The study was performed on West Virginia instrumented pavement sections located on Route 33 near Elkins and in Goshen Road, West Virginia. Each instrumented pavement section is fitted with thermistor tree that enables accurate measurement of the temperature gradient

7

profile at the time of load transfer efficiency measurement. The slabs are also fitted with instrumented dowels that allow measurement of the actual shear load transmitted by the dowels during the application of impact loading. FWD tests were conducted on slabs fitted with 1.5 inch diameter epoxy coated dowel bars and shokbars. The deflection-based load transfer efficiency was measured at the corner dowels, wheel-path dowels and center dowels. The results obtained from FWD sensors were compared with the direct measured results using bonded strain gauges on the dowel bar. Measurements were conducted during different times of the year to investigate the seasonal effect on the measured data.

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CHAPTER TWO

PAVEMENT SECTION AND DATA COLLECTION

2.1 Pavement Test Section A heavily instrumented concrete pavement section was constructed in September 2001 along the east bound lanes of Robert C. Byrd’s Highway (Route 33) near Elkins, West Virginia, USA. The 135 m (450 ft) long section consists of 30 consecutive dowel jointed concrete slabs. Each slab is 0.28 m (11 in.) thick, 4.5 m (15 ft) long, and 3.6 m (12 ft) wide. The slabs are placed on top of a 0.10 m (4 in.) asphalt-stabilized free-drainage base constructed over 0.15 m (6 in.) compacted gravel. Six slabs out of the thirty were fitted with systems of sensors designed for continuous monitoring of slab response to temperature variations as shown in Figure 2.1. The instrumentation plan was set in order to collect data for several key performance parameters that evaluate the behavior of pavement slabs due to diurnal, seasonal, and structural inputs. For this purpose, strains in various locations along slab centerlines, joint openings, temperature profiles, dowel bar moments and axial forces were recorded at specific intervals starting at the time of placing concrete. The instrumentation plan was aimed to focus sensors in selected slabs so as to capture the full behavior of slabs in a mechanistic fashion rather than collecting redundancy in data. The selection of the sensors was based on their functional characteristics, size, sensitivity, accuracy, and past experience with successful performance. Figure 2.2 illustrates the arrangement of the sensors installed in a typical instrumented slab prior to concrete placement. A detailed instrumentation plan as well as a description of the sensors and data acquisition systems could be found in (Shoukry et al., 2004 a, b).

The state of curling of the slab is recorded through dowel bending measurements of instrumented epoxy coated dowels 38.1 mm in diameter and 45.7 cm long located at the slab transverse joints. For collecting bending strains, two uniaxial strain gages were mounted on the top and bottom of the dowel within the bar center. Each instrumented dowel went through a careful procedure to mount the strain gages in their proper locations, with the goal of producing the instrumented dowel with almost no change in its characteristics or its surface texture. Each instrumented dowel was calibrated in the laboratory to check the accuracy of the collected readings and the theoretically calculated straining actions. This was achieved by applying pure bending moments of known magnitudes, and verifying the response of the bars against theoretical calculations. To calculate bending moments of the dowels the following equation was used:

yEI

M bt

×−

=2

)( εε ………………… (2.1)

Where: M = Moment of dowel E = Young’s Modulus of steel I = Moment of inertia of dowel εt = Collected strain on top of dowel

9

εb = Collected strain on bottom of dowel y = Distance to strain gage from neutral axis The calibration procedure showed a maximum variation of 10 % between the actual applied moment and that calculated from the gages for a range of bending moments up to 226 N.m (2000 lb.in). Temperature gradients across the slab depth are measured by a thermistor tree including 11 thermistors located at an interval of 2.54 cm (1 in) from the slab bottom. Figure 2.3 illustrates the time histories of the temperature gradient and the corresponding bending moment measured at the corner dowel.

Traf

fic D

irect

ion

(Tow

ard

East

)

Figure 2.1 Layout of Elkins Instrumented Pavement Site.

Shok bar steel 1.25”

Shok bar polymer conc. 1.25”

Shok bar polymer conc. 1.5”

Steel dowel 1.5”

Shok bar steel 1.5”

Steel dowel

Instrumentation box housing long term monitoring data

acquisition systems

Instrumentation box housing dynamic strain

gage connectors

Instrumented slab

Total of 30 slabs

10

Figure 2.2 Typical Slab Instrumentation System.

1400

1400

Figure 2.3. Recoded Temperature Differential and Dowel Bending Moment.

0

100

200

400

Tem

pera

ture

Diff

eren

tial.

°C

Dow

el B

endi

ng M

omen

t. N

.m

0 200 400 600 800 1000 1200

-200

Time, days

-100

0 200 400 600 800 1000 1200Time, days

-8

-4

0

4

8

12

(a) Temperature Differential

(b) Dowel Bending Moment

-300

300

11

Figure 2.4 Transverse Joint Opening Along Instrumented Pavement Section.

0 1 2 3 4 5

Joint Opening, mm

0

5

10

15

20

25

30

35

Join

t Num

ber

After 1 day After 2 daysAfter 3 daysAfter 7 days

12

Transverse joints were formed by saw-cutting a shallow groove approximately 50 mm in

depth five hours after concrete placement. As the concrete slab starts shrinking, cracks initiate from the tip of such opened groove and propagate downward. Visual inspection of the opened cracks at transverse joints during the seven days following concrete placement indicated that the crack width at transverse joints is not constant along the pavement section as illustrated in Figure 2.4. This indicates that the amount of edge constraint differs from one transverse joint to another along the road. Therefore, the concrete slab does not contract or expand symmetrically around its center. Figure 2.4 also indicates that the amount of joint opening at each transverse joint increases with time during the first week of pavement life.

To have a better insight into the change of the amount of joint opening with time, the

time-history of the joint opening recorded by the crack-meter installed at transverse joint No. 13 is shown in Figure 2.5 (b) together with the corresponding mean slab temperature shown in Figure 2.5 (a). It can be noticed that after an initial increase in the amount of joint opening

0 200 400 600 800 1000 1200 1400-10

0

10

20

30

40

Time, days

Tem

pera

ture

, °C

0 200 400 600 800 1000 1200 1400Time, days

0

0.4

0.8

1.2

1.6

Join

t Ope

ning

, mm

(a) Mean Slab Temperature

(a) Joint Opening

Figure 2.5 Recorded Mean Slab Temperature and Joint Opening.

13

during the first week to about 0.20 mm, it starts increasing as the temperature decreases during winter until it reaches a maximum value of about 0.6 mm after 180 days, it then decreases during summer to fluctuate around 0.2 mm (the initial opening) after 300 days. Such a change in the amount of joint opening with time indicates a change in the amount of constrains along the slab edges due to the presence of dowel bars. The constraining effect of the dowel bars at the transverse joint opening is evident from observing the change in recorded dowel bending moment due to the change in the temperature gradient. Figure 2.3 (a) illustrates the time history of the measured difference in slab top and bottom temperatures. Over the monitoring period of 400 days (from September 2001 to June 2005), the difference between slab top and bottom temperatures varied between –8 ºC to +10 ºC. The time history of dowel bending moments developed in the corner dowel nearest to the shoulder during the same period of pavement life is illustrated in Figure 2.3 (b).

The magnitude of the joint opening as well as the amount of its curling due to the

temperature gradient through its thickness will influence the joint load transfer efficiency. For example, if the joint is tested while it is fully opened, the measured load transfer efficiency is expected to be less than that measured if the joint is closed. Also, the load transfer efficiency measured while the joint suffers upward curling will differ from that measured if the joint suffers downward curling and is in contact with the base layer.

2.2 FWD Loading Tests

The thirty transverse joints were tested using FWD to measure load transfer efficiency. Deflection tests were conducted along the pavement edge as well as along the wheel-path. The tests were conducted on several days during the years 2003, 2004, and 2005. At each joint test, three load drops are applied and the deflection basin is recorded for each drop. The load transfer efficiency is calculated for each drop and an average value was taken as the measured load transfer efficiency.

The results of the load transfer efficiency obtained from different tests are summarized in

Tables 2.1 to 2.6. The following observations can be made from the results presented in Tables 2.1 to 2.6:

• The slabs fitted with 32 mm (1.25 in) diameter dowels displayed higher variability of the measured load transfer efficiency than those fitted with 38 mm (1.5 in) diameter dowels.

• Load transfer efficiencies measured along the wheel-path are generally higher than those measured along the slab edges.

• Shokpolymer out performs the shoksteel and traditional dowel bars. The lower values of load transfer efficiencies measured for shoksteel are expected because of the clearance between the sleeves and dowel.

• The value of the measured load transfer efficiency along the wheel-path is higher than that measured along the pavement edge.

• Both Shoksteel and Shokpolymer designs preserved the load transfer efficiencies. Although their measured load transfer efficiencies are close to those of the steel dowels,

14

they enabled a more uniform joint opening through their sections than regular steel dowels.

TABLE 2.1 Load Transfer Efficiency Data Measured on September 2, 2003.

Slab mean temperature = 22.84 °C. Temperature Differential = -1.069 °C.

Edge Wheel-path Type Joint

No. LTE Average Standard Deviation LTE Average Standard

Deviation T Test

1 88.07% 92.56% 2 51.22% 91.37% 3 88.07% 88.60% 4 77.90% 89.85% Sh

okst

eel

1.25

in.

5 78.72%

76.79% 15.11%

91.81%

90.84% 1.59% 0.0564

6 53.07% 85.11% 7 87.57% 88.28% 8 83.91% 90.61% 9 86.10% 91.52%

Shok

poly

mer

1.

25 in

.

10 -

77.66% 16.46%

87.83%

88.67% 2.52% 0.1050

11 83.81% 83.81% 12 86.81% 89.09% 13 92.90% 81.27% 14 85.36% 80.20%

Shok

poly

mer

1.

5 in

.

15 91.13%

88.00% 3.86%

80.12%

82.90% 3.77% 0.0718

16 90.45% 75.42% 17 90.45% 85.44% 18 88.83% 86.31% 19 84.44% 85.92%

Stee

l Dow

el

1.5

in.

20 69.47%

84.73% 8.87%

65.44%

79.71% 9.18% 0.0701

21 83.15% 80.41% 22 88.26% 80.12% 23 83.55% 67.00% 24 91.96% 83.16% Sh

okst

eel

1.5

in.

25 84.22%

86.23% 3.79%

55.70%

73.28% 11.66% 0.0222

26 91.29% 76.43% 27 83.35% 66.11% 28 89.12% 81.56% 29 94.60% 72.86%

Stee

l Dow

els

1.25

in.

30 87.16%

89.10% 4.24%

65.49%

72.49% 6.85% 0.0016

15

TABLE 2.2 Load Transfer Efficiency Data Measured on March 22, 2004.



Deviation T Test

1 77.4% 91.96% 2 87.2% 90.59% 3 88.5% 90.75% 4 68.0% 79.79% Sh

okst

eel

1.25

in.

5 86.8%

81.6% 8.80%

89.10%

88.4% 4.94% 0.0297

6 88.1% 86.61% 7 86.5% 86.90% 8 86.2% 88.43% 9 84.2% 81.26%

Shok

poly

mer

1.

25 in

.

10 85.2%

86.0% 1.47%

88.55%

86.4% 2.98% 0.3998

11 91.1% 88.42% 12 82.6% 86.83% 13 86.2% 92.38% 14 93.1% 93.35%

Shok

poly

mer

1.

5 in

.

15 95.5%

89.7% 5.23%

92.97%

90.8% 2.96% 0.2876

16 87.7% 90.87% 17 95.4% 90.66% 18 72.9% 80.98% 19 83.4% 90.17%

Stee

l Dow

el

1.5

in.

20 72.9%

82.5% 9.72%

86.79%

87.9% 4.20% 0.0752

21 75.7% 89.84% 22 75.7% 85.59% 23 63.9% 82.10% 24 86.3% 86.69% Sh

okst

eel

1.5

in.

25 88.3%

78.0% 9.78%

85.10%

85.9% 2.80% 0.0617

26 86.6% 84.63% 27 80.7% 88.47% 28 73.4% 84.63% 29 63.5% 71.36%

Stee

l Dow

els

1.25

in.

30 82.3%

77.3% 9.06%

87.45%

83.3% 6.89% 0.0267

Slab mean temperature = 4.30 °C. Temperature Differential = 1.05 °C.

16

TABLE 2.3 Load Transfer Efficiency Data Measured on April 28, 2004.



Deviation

T Test

1 74.2% 87.92% 2 77.6% 89.76% 3 44.9% 61.80% 4 65.1% 84.23% Sh

okst

eel

1.25

in.

5 78.9%

68.1% 14.07%

83.98%

81.5% 11.31% 0.0026

6 61.8% 79.09% 7 46.1% 51.81% 8 77.4% 85.03% 9 55.6% 81.68%

Shok

poly

mer

1.

25 in

.

10 61.7%

60.5% 11.41%

86.97%

76.9% 14.36% 0.0093

11 61.1% 83.68% 12 55.8% 78.32% 13 64.5% 87.19% 14 70.0% 87.70%

Shok

poly

mer

1.

5 in

.

15 70.3%

64.3% 6.16%

78.74%

83.1% 4.47% 0.0012

16 75.3% 89.43% 17 72.7% 86.14% 18 80.8% 93.66% 19 76.3% 94.51%

Stee

l Dow

el

1.5

in.

20 78.6%

76.7% 3.07%

87.76%

90.3% 3.66% 0.0004

21 54.9% 72.38% 22 69.4% 85.35% 23 53.0% 77.24% 24 68.7% 82.95% Sh

okst

eel

1.5

in.

25 61.1%

61.4% 7.57%

84.01%

80.4% 5.44% 0.0003

26 40.2% 72.27% 27 65.5% 72.27% 28 62.5% 86.68% 29 65.0% 84.09%

Stee

l Dow

els

1.25

in.

30 64.5%

59.5% 10.89%

85.51%

80.2% 7.26% 0.0037


17

TABLE 2.4 Load Transfer Efficiency Data Measured on June 15, 2004.



Deviation

T Test

1 57.9% 90.26% 2 86.5% 94.33% 3 62.1% 95.23% 4 80.2% 91.06% Sh

okst

eel

1.25

in.

5 57.6%

68.9% 13.54%

78.49%

89.9% 6.70% 0.0081

6 86.7% 91.44% 7 80.2% 88.49% 8 84.4% 91.29% 9 81.1% 89.92%

Shok

poly

mer

1.

25 in

.

10 87.0%

83.9% 3.14%

89.09%

90.0% 1.31% 0.0038

11 88.5% 91.80% 12 93.6% 90.19% 13 87.6% 90.13% 14 93.7% 95.25%

Shok

poly

mer

1.

5 in

.

15 90.6%

90.8% 2.83%

92.43%

92.0% 2.09% 0.1880

16 95.3% 97.72% 17 92.0% 91.27% 18 74.4% 92.32% 19 91.0% 93.28%

Stee

l Dow

el

1.5

in.

20 88.5%

88.2% 8.13%

91.87%

93.3% 2.58% 0.0998

21 86.7% 90.73% 22 91.1% 92.42% 23 83.0% 91.90% 24 88.7% 88.94% Sh

okst

eel

1.5

in.

25 84.0%

86.7% 3.31%

91.21%

91.0% 1.34% 0.0292

26 88.9% 86.35% 27 87.0% 91.87% 28 79.6% 91.71% 29 80.1% 91.13%

Stee

l Dow

els

1.25

in.

30 85.2%

84.2% 4.15%

89.60%

90.1% 2.30% 0.0436


18

TABLE 2.5 Load Transfer Efficiency Data Measured on August 30, 2004.



Deviation

T Test

1 90.1% 91.81% 2 59.3% 67.69% 3 88.5% 89.55% 4 72.9% 87.42% Sh

okst

eel

1.25

in.

5 72.3%

76.6% 12.82%

87.29%

84.8% 9.72% 0.0269

6 54.3% 61.68% 7 70.8% 88.26% 8 70.3% 84.42% 9 74.1% 89.78%

Shok

poly

mer

1.

25 in

.

10 81.1%

70.1% 9.86%

91.03%

83.0% 12.19% 0.0011

11 82.0% 82.47% 12 88.7% 89.78% 13 93.0% 90.75% 14 90.9% 84.61%

Shok

poly

mer

1.

5 in

.

15 89.5%

88.8% 4.15%

91.87%

87.9% 4.11% 0.2920

16 83.9% 92.06% 17 94.9% 93.69% 18 92.8% 91.74% 19 88.8% 91.01%

Stee

l Dow

el

1.5

in.

20 69.7%

86.0% 10.04%

87.50%

91.2% 2.29% 0.1110

21 84.4% 88.84% 22 75.1% 90.05% 23 84.8% 87.59% 24 88.8% 92.41% Sh

okst

eel

1.5

in.

25 76.7%

82.0% 5.81%

82.40%

88.3% 3.73% 0.0231

26 86.2% 89.58% 27 84.7% 84.77% 28 91.2% 91.36% 29 87.7% 90.99%

Stee

l Dow

els

1.25

in.

30 73.9%

84.7% 6.52%

87.09%

88.8% 2.79% 0.0851


19

TABLE 2.6 Load Transfer Efficiency Data Measured on June 15, 2005.



Deviation

T Test

1 79.1% 87.35% 2 52.6% 71.75% 3 69.5% 81.81% 4 64.0% 80.06% Sh

okst

eel

1.25

in.

5 76.1%

68.3% 10.52%

82.35%

80.7% 5.67% 0.0033

6 59.3% 53.71% 7 74.6% 83.45% 8 78.4% 82.54% 9 80.1% 88.14%

Shok

poly

mer

1.

25 in

.

10 84.2%

75.3% 9.59%

85.81%

78.7% 14.15% 0.1290

11 83.5% 78.70% 12 79.4% 89.48% 13 87.9% 83.77% 14 89.9% 83.92%

Shok

poly

mer

1.

5 in

.

15 87.2%

85.6% 4.16%

89.10%

85.0% 4.45% 0.4286

16 90.5% 91.47% 17 90.2% 92.60% 18 84.7% 85.87% 19 81.6% 85.94%

Stee

l Dow

el

1.5

in.

20 71.9%

83.8% 7.65%

80.72%

87.3% 4.81% 0.0361

21 82.6% 82.30% 22 82.5% 88.03% 23 87.0% 84.69% 24 86.4% 86.41% Sh

okst

eel

1.5

in.

25 81.6%

84.0% 2.47%

85.54%

85.4% 2.13% 0.1983

26 86.4% 90.46% 27 84.3% 86.62% 28 80.9% 87.39% 29 85.6% 82.19%

Stee

l Dow

els

1.25

in.

30 81.8%

83.8% 2.40%

83.40%

86.0% 3.30% 0.1257

20

• Loading position has a significant effect on resulting LTEs of individual joints especially if the joints were fitted with 32 mm diameter dowels (T-Test values are less than 0.1). The effect of the loading position on the load transfer efficiency is less in the case of joint fitted with 38 mm diameter bars.

2.3 Effect of Joint Opening

The effect of change in joint opening on LTE was examined. Figure 2.6 presents the joint opening recorded at the time of testing versus the deflection-based load transfer efficiency for the joint fitted with regular 38 mm diameter dowels. As the joint opening increases, the load transfer efficiency decreases. The limited amount of data available from this study indicates a linear relationship with a moderate correlation between LTE and joint opening.

2.4 Effect of Slab Temperature

Figure 2.7 presents the mean slab temperature recorded at the time of testing versus the deflection-based load transfer efficiency for the joint fitted with regular 38 mm diameter dowels. As the slab temperature increases, the load transfer efficiency increases. The limited amount of data available from this study indicates a linear relationship with a moderate correlation between LTE and joint opening.

0

10 20 30 40 50 60 70 80 90

100

0 0.2 0.4 0.6 0.8 1.0 1.2 Joint Opening, mm

LTE,

% LTE = -7.7352 ∆ + 94.565

R2 = 0.1738LTE = -18.143∆ + 94.994

R2 = 0.3502

Figure 2.6 Load Transfer Efficiency Versus Joint Opening.

Wheel-Path

Edge

21

2.5 Effect of Temperature Gradient

Figure 2.8 presents the temperature differential through slab thickness recorded at the time of testing versus the deflection-based load transfer efficiency for the joint fitted with regular 38 mm diameter dowels. As the temperature differential increases, the load transfer efficiency increases if the load is positioned at the corner, while it decreases if the load is positioned along the wheel-path. The limited amount of data available from this study indicates a linear relationship with a moderate correlation between LTE and joint opening.

0102030405060708090

100

0 5 10 15 20 25 30

Slab Temperature,

LTE,

%LTE = 0.3T + 83.332

R2 = 0.3423LTE = 0.4844T + 72.791

R2 = 0.3268

Figure 2.7 Load Transfer Efficiency versus Slab Temperature.

Wheel-Path

Edge

LTE = 1.9534∆T + 85.649 R2= 0.4297

LTE = -1.0994∆T + 83.833R2 = 0.0498

0 10 20 30 40 50 60 70 80 90

100

-2 -1 0 1 2 3 4Temperature Differential, °C

LTE,

%

Wheel-Path

Edge

Figure 2.8 Temperature Differential Versus Load Transfer Efficiency.

22

CHAPTER THREE

MEASUREMENT OF DOWEL SHEAR FORCES 3.1 Introduction

Both 38-mm diameter steel dowels as well as Shokbars are fitted with two miniature strain gages embedded on top and bottom to measure dowel bending as presented in Figure 2.3 (b) and strain rosette on the side to measure the shear force. Attempting to measure the dynamic shear force in the dowels in Elkins, West Virginia during the FWD tests failed since the signals were contaminated with a high noise level that prevented extracting any useful information out of them. The reasons for such a nose can be attributed to the long cable lengths as well as the aging of strain gages. For this reason, the dynamic tests were conducted on another newly constructed concrete pavement site in Goshen Road, West Virginia. This Chapter describes this pavement site as well as the data collected from the dynamic testing of dowel bars and shock bars.

3.2 Goshen Road Pavement Site

A set of seven test concrete slabs were cast for experimental study in a designated open area at the parking lot of the WVDOT Maintenance Shop at Goshen road, West Virginia. The slabs were instrumented with a variety of sensors for long-term monitoring of the slab response to various loading conditions including seasonal and daily temperature changes. The sensory system is designed to provide continuous data from key-performance parameters that formulate the behavior of the slabs such as the distribution of strains along and across the slab centerlines, joint openings, temperature profiles through the slab thickness, dowel bending moments, dynamic shear and normal forces, strains at the concrete-dowel interface along with a continuous record of weather conditions.

In total seven full scale slabs (4.57 m × 3.65 m × 0.25 m) were constructed. Three of the seven slabs were not instrumented and act as support or joint slabs. They were laid in September 2002. The remaining four slabs are instrumented and were poured in October 2003. Data collection began in October 2003 at 12:20 pm when the first of the four instrumented slabs was poured. The site has a total of seven slabs; five jointed slabs with their joints fitted with either regular dowels or Shokbars and two free slabs.

Special care was taken while constructing the base layer of the instrumented slabs in

order to provide the required base/slab friction characteristics. The base consisted of a 10 cm thick layer of crushed stones with an average size of 1.25 cm. Three bleeders about 50 cm in width were constructed along the 36 meters long base for the purpose of rain water drainage. The base layer was extended 30 cm from each side of the slab edges to provide uniform distribution to the slab support. The top surface of the base layer was carefully treated in order to provide minimum amount of slab/base friction under 6 slabs. A 2.5 cm thick layer of cement paste was spread on the top surface of the base layer, finished to a smooth surface and set to cure for 15

23

days. This layer was then drilled at staggered spacing of about 30 cm throughout the surface with openings 1 cm in diameter to allow better water drainage. A thick fabric sheet was then placed on top of the finished surface and concrete slabs were cast on top of the fabric sheets within wooden forms. In contrast, one free slab was placed directly on the crushed stones base layer for the purpose of studying the effect of slab/base friction. The construction of slabs was carried out through 2 stages. The first stage consisted of placing 3 slabs that served to provide joints to the instrumented ones. Those 3 slabs were anchored to the ground to simulate the extended continuity of pavements, i.e. maximum resistant to movement. In each of the fixed slabs, 4 steel guard rail posts 1.8 meters long were driven along the transverse center line of the slab. The extended portion of the posts above the finished surface of the base layer measured 25.4 cm which is exactly the thickness of the slabs. The second stage of construction consisted of placing the instrumentation system and casting the 4 instrumented slabs in between the wooden forms as shown in Figure 3.1.

3.3 FWD Testing The West Virginia Department of Transportation provided a Dynatest Falling Weight Deflectometer (FWD) to apply dynamic loads at transverse joints as shown in Figure 3.2. The tests were conducted along three lines: slabs edge, wheel-path, and slab centerline. The transverse joints in this instrumented pavement section were constructed to be fully opened using a foam separator. This construction technique ensures that dowel bars or shokbars are the only means of load transfer device between adjacent slabs. This means that the shear force measured

Figure 3.1 Construction of Instrumented Slabs.

Free slab on crushed stones

Free slab on Frictionless base

Doweled slabs on frictionless base

Weather station

24

in this study represents the highest value of the shear force that can be transferred by the dowels as the FWD is applied at the transverse joint edge.

The FWD impacts are applied at the locations of the instrumented dowels as the strain rosette output was recorded. The data acquisition system was programmed to collected data from all sensors at a rate of 1000 sample per second during the FWD testing. The system was activated as the FWD was about to drop and stopped after impact. Three FWD impacts were applied and recorded at each testing location. The deflection-based load transfer efficiencies were calculated from the surface deflections measured using the FWD device. The tests were performed in June 2005.

3.4 Dowel Bar Shear

Under the effect of the impact load of the FWD, dowel bars are subjected to bending moment and shear force. In this analysis, we will focus on the shear force since it represents the actual force transferred by the dowels to the unloaded slabs. Equation 3.1 was used to calculate vertical shear forces from the strain measured by the strain gage rosette as the FWD loads were applied at the joint edge:

( )2143

sidesideAGP εε −= ……………… (3.1)

Where P = Vertical shear force G = Shear modulus of the dowel A = Cross-sectional area of the dowel εside 1 = strain measured in the rosette leg directed 45° upward. εside 2 = strain measured in the rosette leg directed 45° downward.

Figure 3.2 FWD Testing of Transverse Joints.

25

Figure 3.3 illustrates a typical measured time history of the shear forces recorded in the

instrumented dowel bar located at the slab corner due to applying the FWD loading pulse at the same location. The maximum magnitudes of the shear forces for all tests at different positions are listed in Table 3.1. It can be noticed that the maximum shear forces are recorded in the corner dowels when the FWD loading plate is positioned just over these dowels. It can be noticed that the shear force recorded in the shokbar, located at the corner, is approximately five times that recorded in the corner dowel. This clearly demonstrates the ability of the shokbar in transferring shear forces between adjacent slabs.

3.5 Distribution of Shear Forces among Dowel Bars Figure 3.4 illustrates the distribution of the shear forces among the dowel bars at transverse joint due to the application of the FWD load at three locations: corner, wheel-path and joint center. It can be noticed that the magnitude of the dowel shear force decreases linearly as the distance from the loading position decreases on both sides, which agrees with the assumption of Friberg (1938) and the finding of Tabatabie and Bernberg (1980). The same observation can also be made on the shokbars presented in Figure 3.5. However, the magnitudes of the shear force transmitted through the shokbars are higher than those transmitted through traditional dowels and the number of the active bars is less. The magnitude of the shear forces induced in the

0 0.5 1.0 1.5 2.0 2.5 -100

-50

0

50

100

150

200

Time (seconds)

Dow

el sh

earin

g fo

rce

(lbs)

Shear Force

Figure 3.3 Typical Shear Force History in Dowel Bars.

26

uninstrumented dowels can be calculated through the linear interpolation of the forces shown in Figures 3.4 and 3.5.

TABLE 3.1 Shear Forces in Instrumented Dowels and Shokbars.

Location Force of

FWD Drop (kN)

Shear in Corner Dowel

(N)

Shear in wheel-path Dowel (N)

Shear in Center Dowel

(N) 70.03 1700 1250 670 70.27 1100 1400 700 Joint 1 Corner

(Regular Dowels) 70.34 860 1350 600 71.73 800 1250 550 71.70 800 1320 550 Joint 1 Wheel-path

(Regular Dowels) 71.82 750 1370 560 69.91 50 850 2200 69.95 50 820 2100 Joint 1 Center

(Regular Dowels) 69.75 50 900 2100 68.00 940 125 67.49 940 120 Joint 2 Corner

(Regular Dowels) 67.37 900 118 70.85 750 2400 350 70.78 700 2300 350 Joint 2 Wheel-path

(Regular Dowels) 70.66 680 2100 340 71.45 71.91 Joint 2 Center

(Regular Dowels) 71.93 69.03 10000 600 0 69.36 9200 750 0 Joint 3 Corner

(Shokbar) 69.10 9300 770 0 69.95 2300 3000 500 70.63 1220 1400 500 Joint 3 Wheel-path

(Shokbar) 70.51 1360 1360 500 68.97 400 800 1500 69.14 350 800 1200 Joint 3 Center

(Shokbar) 68.80 260 750 1500

27

y = 903.49x + 670R2 = 0.982 y = -905.6x + 2050

R2 = 0.9935

0200400600800

1000120014001600

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Distance along Joint, m

Shea

r Fo

rce,

N

y = -672.57x + 1787.5R2 = 0.9982

0

500

1000

1500

2000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8


Shea

r Fo

rce,

N

y = 1415.9x - 189.47R2 = 0.999

0

500

1000

1500

2000

2500

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8


Shea

r Fo

rce,

N

a. Corner Load

y = 903.49x + 670R2 = 0.982 y = -905.6x + 2050

R2 = 0.9935

0200400600800

1000120014001600

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8


Shea

r Fo

rce,

N

b. Wheel-path Load

c. Joint Center Load

Figure 3.4 Shear Forces Distribution in Dowel Bars.

28

y = -1E-12x + 1360R2 = #N/A

y = -984.25x + 2150R2 = 1

0

500

1000

1500

2000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8


Shea

r Fo

rce,

Ny = -13993x + 11433

R 2 = 1

0

2000

4000

6000

8000

10000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8


Shea

r Fo

rce,

N

0

500

1000

1500

2000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8


Shea

r Fo

rce,

N

c. Joint Center Load

Figure 3.5 Shear Forces Distribution in Shokbars.

a. Corner Load

b. Wheel-path Load

29

The magnitudes of the shear forces induced in the dowel bars at transverse joints are

listed in Table 3.2. The results in Table 3.2 show that the shear force transmitted through the shokbars due to corner loading is as twice as that transmitted by regular dowels. This indicates that the use of shokbars at transverse joints would reduce the corner breaks. The maximum shear force transmitted through the dowels or shokbars is obtained when the load is applied near the joint center. The results indicate the superiority of the shokbars in transmitting the corner and wheel-path loads.

TABLE 3.2 Shear Forces in Dowels and Shokbars (N).

Dowel Bar Location measured from Corner (m) Load Position 0.15 0.46 0.76 1.07 1.37 1.68 1.98 2.29 2.59 2.90 3.20 3.51

Sum (N)

Dowel Bars Corner 1685 1480 1275 1070 865 660 455 250 45 7785 Wheel-Path 808 1083 1358 1084 808 532 256 5929 Joint Center 26 458 889 1321 1753 2184 2184 1753 1321 889 458 26 13262

Shokbars Corner 9300 5035 770 15106 Wheel-Path 1400 1380 1360 1100 800 500 200 6740 Joint Center 258 506 754 1002 1250 1498 1498 1250 1002 754 506 258 10536

TABLE 3.3 Comparison of Deflection Load Transfer Efficiency and Actual Load Transfer Efficiency.

Load Position

Total Force Transmitted by

Dowels (KN)

Force of Drop (KN)

Load Transferred

(%)

Deflection-Based Load Transfer

Efficiency (%)

Dowel Bars Corner 7.785 70.21 11.1% 77.03% Wheel-Path 5.929 71.75 8.3% 89.84% Joint Center 13.262 69.87 19.0% 78.51%

Shokbars Corner 15.106 69.17 21.8% 81.16% Wheel-Path 6.74 70.29 9.6% 87.31% Joint Center 10.536 68.97 15.3% 77.37%

The percentages of the load transferred through the dowels and/or shokbars are compared with the deflection load transfer efficiency calculated based from the deflection basins as illustrated in Figure 3.3. Despite the high values of deflection-based load transfer efficiencies, the percentages of the actual shear forces transmitted through dowels/Shokbars are very low. This finding agrees with measurements reported by Sargand (2000), who reported low values for the shear forces measured in dowel bars irrespective to the high values of deflection-based load transfer

30

efficiency. The results in Table 3.3 indicate that there is no one-to-one relationship between load transfer efficiency and the deflection-based load transfer efficiency.

The large discrepancies between the deflection-based load transfer efficiencies and load

transfer efficiencies illustrated in Table 3.3 can be explained by the following reasons: • Slab deflections increase in the presence of dowel bar looseness while dowel shear forces

decrease. • The FWD tests were conducted between 10:00 AM and 12:00 PM. During this time, the

slabs are subjected to positive temperature gradient, which makes the joints in contact with the base layer. This reduces the shear forces transmitted by the dowel bars or the shokbars (Channakeshava et al., 1993).

31

CHAPTER FOUR

SUMMARY AND CONCLUSIONS

The analyses of the trends reported in this study were intended to calculate deflection-based load transfer efficiency (LTE) and the percentage of load transferred through the load transferring devices mounted across the transverse joint. The effects of the seasonal temperature variations and the position of load application were examined. The following are highlights of the results obtained from this study: 1. Load transfer efficiency was found to be a complex parameter that depends on many factors

that include load position, testing time, slab temperature, and load transfer device. 2. Testing time and season were found to have a significant effect on the measured load transfer

efficiency. For the same joint, the load transfer efficiency measured in winter was found to be less than that measured in summer.

3. The slabs fitted with 32 mm (1.25 in) diameter dowels displayed higher variability of the

measured load transfer efficiency than those fitted with 38 mm (1.5 in) diameter dowels. 4. Joint opening changes daily and seasonally as the ambient temperature changes. As the

amount of joint opening increases due to slab contraction during winter, the measured load transfer efficiency generally decreases.

5. As the slab temperature increases, the load transfer efficiency increases. 6. Poor correlation was found between the deflection-based load transfer efficiency and the

percentage of the load transferred through the load transferring devices mounted across the transverse joint.

32

REFERENCES

1. American Association of State Highway and Transportation Officials (1993). AASHTO Guide for Design of Pavement Structures.

2. American Association of State Highway and Transportation Officials (1998).

Supplement: AASHTO Guide for Design of Pavement Structures-Part II Rigid Pavement design and Rigid Pavement Joint Design.

3. American Concrete Pavement Association (1991). Design and Construction of Joints

for Concrete Highways. Technical Bulletin-010.0, American Concrete Pavement Association, Portland Cement Association.

4. Beckemeyer, C.A., L. Khazanovitch, and H.T. Yu (2002). Determining the Amount of

Built-in Curling in JPCP. Presented at the 81st Annual Meeting of Transportation Research Board, National Research Council, Washington, D.C.

5. Channakeshava, C., and F. Barzegar, and G.Z. Voyiajis (1993). Nonlinear FE Analysis

of Plain Concrete Pavements with Doweled Joints. Journal of Transportation Engineering, ASCE, Vol. 119, No. 5, pp.763-781.

6. Chou, Y.T. (1981). Structural Analysis Computer Programs for Rigid Multicomponent

Pavement Structures with Discontinuities: WESLIQUID and WESLAYER. Technical Report GL-81-6. Vicksburg, MS: U.S. Army Engineer Waterways Experiment Station.

7. Colley, B.E., and H.A. Humphrey. Snyder (1967). Aggregate Interlock at Joints in

Concrete Pavements. Highway Research Record No. 189, Transportation Research Board, National Research Council, Washington, D.C., pp. 1-18.

8. Davids, W.G., and G.M. Turkiyyah (1997). Development of Embedded Bending

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