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International Journal of Civil, Structural,Environmental and Infrastructure EngineeringResearch and Development (IJCSEIERD)ISSN 2249-6866Vol.2, Issue 2 June 2012 1-9© TJPRC Pvt. Ltd.,

LOAD TRANSFER EFFICIENCY OF DOWEL BAR SYSTEM IN

UNBODED CONVENTIONAL WHITETOPPING OVERLAY

D. R. JUNDHARE1, K. C. KHARE

2& R. K. JAIN

3

1Research Scholar, Civil Engineering Department, Sinhgad College of Engineering, Vadgaon (Bk.),Pune-411041. M. S. India. University of Pune.

2Professor, Civil Engineering Department, Sinhgad College of Engineering, Vadgaon (Bk.), Pune-411041.

M. S. India.

3Professor, Pad. Dr. D.Y.Patil Institute of Engineering and Technology, Pimpri, Pune - 411018. M. S.

India.

ABSTRACT

In unbonded conventional whitetopping overlay, the dowel bar system and the aggregate interlock

are two mechanisms for transferring wheel loads from one panel to the adjacent panel. The aggregate

interlocking load transfer mechanism is effective for narrow joints while the dowel bar system works

well for both narrow and wider joints. This paper discusses about Load Transfer Efficiency (LTE) of a

transverse joint with the help of field study by Falling Weight Deflectometer (FWD) for the analysis of a

dowelconventional whitetopping overlay in Pune city, Maharashtra State, India. LTE computed using

equations referred in book by Papagiannakis, A. T. and Masad, E. A. (2007) has been used to validate

FWD field test results.

KEYWORDS : Load Transfer , Dowel Bar system, Whitetopping.

INTRODUCTION

Whitetopping is a rehabilitation or structural strengthening alternative on bituminous pavement. It is

defined as a Portland Cement Concrete (PCC) overlay constructed on the top of an existing bituminous

pavement or Hot Mix Asphalt (HMA). Whitetopping has been classified into three categories, viz.,

Conventional whitetopping, Thin Whitetopping and Ultra -Thin Whitetopping which offer immense

potential as a rehabilitation strategy for Indian Roads (IRC:SP:76-2008). Unbonded PCC overlays, often

called as "classical or conventional whitetopping.” It consists of a PCC overlay of thickness 200 mm or

more. It is designed and constructed like a new rigid pavement without assuming any bond between the

concrete overlay and existing bituminous layer (Cole 1997). Studies on conventional whitetopping

proved that, this is a rehabilitation alternative for asphalt maintenance and repair due to its better

performance and durability (McGhee 1994). Load Transfer Efficiency (LTE) is important factor in

unbonded conventional whitetopping overlay.

Load Transfer Efficiency (LTE) of jointed concrete pavement is typically measured by the ratio of

unloaded and loaded slab deflection. Load transfer across joints in a concrete pavement is accomplished

mainly by: (1) aggregate interlock (2) dowel bar action or (3) by a combination of the two mechanisms.

In the aggregate interlocking mechanism, the load is transferred by shear interaction between individual



D. R. Jundhare, K. C. Khare & R. K. Jain 2

aggregates at a joint or crack. This type of load transfer mechanism is effective for slabs having small

joint widthand with short joint spacing. For pavements with heavy traffic volume, mild steel dowel bars

are placed across transverse joints to transfer load including those with small joint openings ≤1.0 mm.

The dowel bars transfer load without restricting the horizontal joint movement caused by thermal andmoisture contraction and expansion. They also help in maintaining the horizontal and vertical alignments

of slabs.

In the present study, for determining Load Transfer Efficiency (LTE) at transverse joint of the

conventional whitetopping of 320mm thickness overlay slab resting on 150mm HMA of modulus of

subgrade reaction (k)=12 MPa/mm and 3.65m x 4.50m slab size, non-destructive field testing using

FWD was performed. Load transfer reduces stresses in the slab and also reduces deflections at the joint.

Effective load transfer provides several benefits to conventional whitetopping or rigid pavements:

1.

Slows or reduces development of pumping and faulting by reducing slab deflections.2. Decreases cracking within the slab by reducing tensile stresses.

The most common and generally recommended method is to base LTE on measurement of the

deflections of slabs on either side of a crack or joint during wheel loadings. This paper also discusses

about LTE which has been computed using equations referred in book by Papagiannakis, A. T. and

Masad, E. A. (2007) to validate the field test results by FWD test.

MECHANISM OF LOAD TRANSFER BY DOWEL BAR SYSTEM

Dowel bars transfer the load across a pavement joint primarily by shear action. When one panel/slab

of the pavement is loaded, the panel is deflected along with the dowels connecting the loaded panel with

an adjacent panel and in the process the dowels transfer part of the load to the unloaded panel. The load

transfer mechanism between the dowel and the concrete is a complex phenomenon. This mechanism

depends mainly on a parameter known as the modulus of dowel support (k), the value of which can be

determined by load testing (Yoder and Witczak 1975). A high modulus of dowel support value indicates

a good contact between the surrounding concrete and the steel dowel. However, with repeated

application of wheel loads, the contact between the surrounding concrete and the dowel deteriorates,

particularly where the bar is seated and in the vicinity of the face of the joint. At these locations, the

concrete may be crushed over time and repeated loading when subjected to high bearing stresses. As thecrushed concrete particles are displaced, voids are created around the dowels causing dowel looseness

(DL). The amount of looseness may vary along the length of the dowel bars. However, near the joint

face, the looseness is generally more than those at the other locations of the dowel bar. DL is generally

composed of two parts—initial looseness and looseness from enlargement of the socket under repetitive

loading (Buch and Zollinger 1996). In real life pavements, looseness in dowels may occur on any side of

the joint due to repeated application of wheel loads on both sides of the pavement. The efficiency of a

joint in transferring the applied wheel load depends on a number of dowel-joint parameters like modulus

of dowel support, dowel diameter, embedded length of dowel, dowel spacing, DL, joint opening,

properties of both steel and concrete, and also to a lesser extent on subgrade strength(Maitra et al. 2009).



Load Transfer Efficiency of Dowel Bar System in Unboded Conventional Whitetopping Overlay3

Dowel bars provide mechanical connection between slabs in case of conventional whitetopping

overlays without restricting horizontal joint movement. They also keep slabs in horizontal and vertical

alignment. When loaded by heavy vehicles, dowel bars lower joint deflection and stress in the concrete

slab and reduce the potential for joint problems by increasing LTE as shown in “Figure 1”. For novertical load transfer due to absence of dowel bar or aggregate interlocking LTE is 0% as shown in

“Figure 2”.

Figure 1 : 100% load transfer efficiency Figure 2 : 0% load transfer efficiency

When dowels are not used, joints depend solely upon aggregate interlock for load transfer.

Aggregate interlock is the mechanical locking which forms between the fractured surfaces along the

crack below the joint saw cut. Reliance on aggregate interlock without dowels is acceptable on low-

volume or secondary road systems, Ultra-Thin and Thin White topping Overlays where truck traffic is

low. Shear between aggregate particles below the saw cut is shown in “Figure 3”.

Figure 3 : Aggregate interlock (LTE)

FALLING WEIGHT DEFLOCTOMETER (FWD) TEST

A Falling Weight Deflectometer (FWD) is commonly used to measure LTE. Impact deflection

testing by FWD for pavement nondestructive evaluation (NDE) is widely used testing devices among

many nondestructive testing technologies available for pavement condition evaluation (Hudson et al.

1987). The FWD device applies an impact load on a steel loading plate and measures peak deflections onthe pavement surface using seismic velocity transducers at the center and at the several locations away

from the loading plate.




Figure 4 : FWD test carried on study road

Figure 5 : Schematic of FWD deflection sensors

Use of FWD for evaluation of pavements is gaining popularity in many countries, as it is possible to

simulate the magnitude and duration of load applied by a fast moving vehicle on highways using this

equipment. In order to determine the applicability of utilizing the finite element method to analyze the

unbonded conventional whitetopping, non-destructive field testing of pavement using FWD was

performed on 14 locations in June 2008 in the morning hours on existing conventional whitetopping

overlay under study. In this study, Dynatest 8000 FWD model with 150 mm diameter load plate and nine

displacement measuring sensors, was used which is trailer mounted and have the capability to apply load

of 50 kN and 100 kN. For details of the testing procedure, construction report (Cable et al. 2003) has

been referred. One tranducer is located at the center of the load plate and remaining tranducers are placed

at varying intervals from the plate. “Figure 4” shows FWD test carried on study road and “Figure 5”

shows schematic showing the arrangement of the sensors of FWD test equipment used in this study.

“Figure 6”revealed sensor no. 2 and 3 across transverse joint to measure deflection in adjacent slabs.

The results obtained by this test are given in “Table 1 and 2” along with Load Transfer efficiency. The

bar charts of LTE for edge and corner loading in various panels have been shown in “Figure 7 and 8”

respectively.




Figure 6 : FWD deflectionmeasurement in adjacent slabs

Table 1 : FWD test stresses and deflections data at edge of overlay slab along with LTE

Sr. No.

Stress

in

MPa

Deflection in mm at distance in mm from load point

Test

load

(kN)

LTE

%

(Series) 0 -200 -300 450 600 900 1200 1500 1800

1 0.814 0.1223 0.1004 0.0902 0.0857 0.0778 0.0652 0.0585 0.0414 0.0392 51.53 89.84

2 0.912 0.1214 0.1002 0.0921 0.0899 0.0781 0.0609 0.0552 0.0463 0.0384 54.46 91.91

3 0.757 0.1229 0.1018 0.0982 0.0878 0.0794 0.0696 0.0518 0.0475 0.0368 50.51 96.46

4 0.811 0.1187 0.1004 0.0808 0.0715 0.0701 0.0622 0.0566 0.0450 0.0343 51.32 80.47

5 0.821 0.1115 0.0934 0.0827 0.0728 0.0690 0.0628 0.0586 0.0399 0.0294 53.02 88.54

6 0.798 0.1195 0.0995 0.0875 0.0724 0.0699 0.0655 0.0546 0.0399 0.0293 51.40 87.93

7 0.785 0.1183 0.0981 0.0851 0.0786 0.0695 0.0599 0.0502 0.0384 0.0297 51.48 86.74

Table 2 : FWD test stresses and deflections data at corner of overlay slab along with LTE

Sr.No.

Stress

in

MPa

Deflection in mm at distance in mm from load point

Test

load

(kN)

LTE

%

(Series) 0 -200 -300 450 600 900 1200 1500 1800

1 0.749 0.0861 0.0792 0.0674 0.0611 0.0595 0.0541 0.0485 0.0373 0.0287 52.93 85.10

2 0.761 0.0879 0.0710 0.0689 0.0605 0.0593 0.0548 0.0459 0.0350 0.0341 53.78 97.04

3 0.725 0.0893 0.0796 0.0686 0.0615 0.0570 0.0588 0.0455 0.0346 0.0365 51.24 86.18

4 0.781 0.0896 0.0761 0.0699 0.0581 0.0551 0.0544 0.0438 0.0333 0.0294 51.20 91.85

5 0.773 0.0791 0.0699 0.0649 0.0615 0.0557 0.0521 0.0405 0.0378 0.0295 54.63 92.84

6 0.812 0.0872 0.0798 0.0667 0.0619 0.0569 0.0498 0.0398 0.0391 0.0273 51.39 83.58

7 0.742 0.0832 0.0794 0.0698 0.0616 0.0561 0.0502 0.0399 0.0298 0.0236 52.44 87.90




Figure 7 : LTE for edge loading in various panels

Figure 8:LTE for corner loadingin various panels

COMPUTATION OF LTE USING EQUATIONS

The efficiency of a joint is generally expressed in terms of its ability to transfer load from one side of

the joint/ crack to the other side and is termed as Load Transfer Efficiency. LTE is expressed as a

percentage of the unloaded slab deflection to the loaded slab deflection.“Figure9”showsisometric view of

loaded and unloaded slabs of conventional whitetopping overlays with wheel loading.“Figure10”

revealed cross section of the transverse joint of unbonded conventional whitetopping overlays.The

equation 1 is most commonly used for calculating LTE (Papagiannakis, A. T. and Masad, E. A., 2007).

100×=l

ul LTE

δ

δ

(1)

Where,

δul =the surface vertical deflection at the unloaded edges of the joint of approach slab

δl = the surface vertical deflection at the loaded edges of the joint of leave slab




Figure 9 : Isometric view of loaded and unloaded slabs (LTE)

Figure 10 : Cross section of the transverse joint

LTE from dowel action is computed on the basis of the load transfer stiffness variable Jd. Initially it

is given by, h

d J

d

2120=

Where, Jd = load transfer stiffness variable, h= overlay slab thickness, d= diameter of dowel bar.

Finally, the LTE due to dowel action, LTEdowel is computed using equation 2.

849.02.11

100−

+=

d

dowel

J LTE

(2)

In the subsequent months, load transfer stiffness variable is computed considering the cumulative

damage of the cement concrete supporting the dowels.

LTE from FWD test it is ranging from 80.47% to 96.46 % when the load is applied at edge of

overlay slab and 83.58 % to 87.04 % when the load is applied at corner of overlay slab as shown in Table

1 and 2. Also if calculated using equations referred in book by Papagiannakis, A. T. and Masad, E. A.




(2007), it is 89.13%. This shows the good agreement among LTE values of FWD field test and computed

values using available equations.

LTE ranges from 0% for no vertical load transfer to 100% for perfect load transfer between adjacent

slabs. LTE values above 70%, between 50% - 70% and below 50% characterize load transfer as good,

fair and poor respectively (Papagiannakis, A. T. and Masad, E. A., 2007).

CONCLUSIONS

Through the present study following conclusions have been for calculating LTE at the transverse

joints of 320mm thick on in-service unbonded conventional whitetopping overlay constructed in Pune

city, Maharashtra State (India), for its performance evaluation subjected to various traffic and climatic

conditions relevant to Indian scenario.

• LTE from FWD test it is ranging from 80.47% to 96.46 % when the load is applied at edge of

overlay slab and 83.58 % to 87.04 % when the load is applied at corner of overlay slab as shown

in Table 1 and 2. Also if calculated using equations referred in book by Papagiannakis, A. T.

and Masad, E. A. (2007), it is 89.13%. This shows the good agreement among LTE values of

FWD field test and computed values using available equations.

• LTE measurements can vary significantly as the temperature of the pavement slabs change. At

high temperatures the thermal expansion of the slabs causes the joints to close, thus LTE is

increased by aggregate interlock. It is important that FWD deflection testing is conducted in the

morning hours when ambient air temperatures are less than 21°C. This is probably the best time

for measuring deflections and determining LTEs because the joints should not be as “tight” as

they will be when the temperature rises.

ACKNOWLEDGEMENTS

The authors are grateful to PimpriChinchwad Municipal Corporation, Pimpri-Pune, Maharashtra

State, India for kind permission and availing the required data. Also authors express their deep gratitude

to Prof. Dr. B.B. Pandey, Indian Institute of Technology,Kharagpur, India for his valuable suggestions

in calculating LTE values.

REFERENCES

1. Buch, N., and Zollinger, D. G. (1996). “Development of dowel looseness prediction model for

jointed concrete pavements.” Transportation Research Record. 1525, Transportation Research

Board, Washington, D.C., 21–27.

2. Cable, J. K., Anthony, M. L., Fanous, F. S., and Phares, B. M. (2003). “Evaluation of composite

pavement unbonded overlays: Phases 1 and 2.” Ames, IA: Center for Portland cement Concrete

Pavement Technology.

3. Cole, L W. (1997). “Pavement condition surveys of ultrathin whitetopping projects”, Proc., Sixth

Int. conf. on concrete pavements, Purdue University, West Lafayette, Indiana, Volume 2, 175-187.




4. Hudson, W.R., Elkins G.E., Uddin W., and Reilley K.T. (1987). “Evaluation of pavement deflection

measuring equipment.” FHWA-TS-87-208, Federal Highway Administration.

5. IRC: 58. (2002). “Guidelines for the design of rigid pavements for highways.” Indian Roads

Congress, New Delhi, India.6. IRC: SP: 76 – 2008. “Tentative guidelines for conventional, Thin and Ultra-Thin Whitetopping.”

Indian Road Congress, New Delhi. 2008.

7. Maitra, S. R., K. S. Reddy and L.S. Ramachandra (2009). “Load transfer characteristics of dowel bar

system in jointed concrete pavement”, Journal of Transportation Engineering, Vol. 135, No. 11, 813-

821.

8. McGhee, K. H. (1994). “NCHRP synthesis of highway practice 204: Portland cement concrete

resurfacing,” Transportation Research board, Washington, D. C., 73-79.

9. Papagiannakis, A. T., Masad, E. A. (2007). “Pavement design and materials”. John Wiley and Sons,

Inc., Hoboken, New Jersey.

10. Yoder and Witczak. (1975). Principles of pavement design, 2nd Ed., Wiley, New York.

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