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(Mechanical Load Transfer ­Dowel Bars)
(Mechanical Load Transfer ­Dowel Bars)
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at Joints in Concrete Pavements," Highway Research Record No. 189, Highway Research Board, National Research Council, Washington D.C., 1967.</ref>.  
 
at Joints in Concrete Pavements," Highway Research Record No. 189, Highway Research Board, National Research Council, Washington D.C., 1967.</ref>.  
  
''Figure 7 Typical dowel placement in highway pavements. ''
+
[[File:DowelPlacement.png|thumb|left|250px| Typical dowel placement in highway pavements.]]
  
 
=Stabilized Subbases=
 
=Stabilized Subbases=

Revision as of 14:48, 2 March 2017

Load transfer is the ability of a joint to transfer a portion of an applied load from one side of the joint to the other. It can be measured by "joint effectiveness." If a joint is 100 percent effective, it will transfer one-half of the applied load, effectively sharing the load evenly between two slabs. Zero percent effectiveness means that no load is transferred across the joint. Field evaluation of load transfer is made by measuring the deflections on each side of a joint from an applied load. The equation below is used to rate joint effec­tiveness, E:

Effectiveness of load transfer.
2 dU

E = ---------------- 100

dL + dU

where: dL = deflection of the loaded side and dU= deflection of the unloaded side.

Load transfer is necessary for jointed concrete pavements to perform well. Adequate load transfer lowers deflections, and reduces faulting, spalling, and corner breaks. A joint effectiveness of 75% or more is considered adequate for medium and heavy truck loadings [1].

The following factors contribute to load transfer across joints:

  • Aggregate interlock - the interlocking action be­tween aggregate particles at the face of the joint.
  • Mechanical load transfer devices, i.e. dowel bars.
  • Stabilized subbases.


Aggregate Interlock

Aggregate interlock relies on the shear interaction bet­ween aggregate particles on the joint crack faces that form below a saw cut. It is most effective for slabs built with short joint spacings and non-erodible stabilized or permeable subbases which will experience low truck volumes.

A Federal Highway Administration study concluded that the following be considered to increase aggregate interlock load transfer and minimize faulting [2]:

Sensitivity of joint effectiveness to joint opening for laboratory and field data on undoweled joints [3].
  • Thicker slabs (larger area for interlock provides better load transfer).
  • Shorter joint spacings (e.g. 15 ft or less).
  • Stiff subbases (higher effective k-value).
  • Edge support (e.g. tied concrete shoulders or ex­tended lanes).
  • Coarse grained subgrade soils (drainage).
  • Improved drainage (pipe collection system/permeable subbase).


Slab length affects joint opening and effectiveness of aggregate interlock. Figure 5 shows how joint effec­tiveness varies with subbase type in laboratory tests [4]. However, the actual load transfer developed on in-service pavements is much higher [5]. Typical field measurements show joint effectiveness of about 58 percent for 15- to 25-year old undoweled pavements [6].

In dry, arid, nonfreeze environments, temperature variations and joint movement (opening) are small. Therefore, load transfer through aggregate interlock can accommodate greater truck volumes. However, short joint spacing is required.

Aggregate size is critical to load transfer [7]. Small (1/2 in.) aggregates provide only marginal interlock. Large (greater than 1 in.), durable aggregates are helpful in maintaining load transfer, especially for larger joint openings [8]. Generally crushed stone performs better than natural gravels because crushed aggregates create a rougher joint face. A rough face wears more slowly than a rounded smooth joint face created by natural aggregates. Likewise, early cracking increases the joint face roughness because cracks form around aggregates instead of through them.

Studies have found that, with short joint spacings, ag­gregate interlock provides acceptable joint perfor­mance when truck traffic volume is low (generally less than 80 to 120 trucks per day) [9][10]. This varies depending on aggregate type and support conditions. Other studies indicate that at least four to five million accumulated AASHTO 18,000 pound Equivalent Single Axle Loads (ESAL's) are required to produce objec­tionable faulting [11][12]. (AASHTO is the American Association of State Highway and Transportation Officials).

Mechanical Load Transfer ­Dowel Bars

Sensitivity of bearing stress to dowel diameter with vary­ing dowel spacing [13].

Aggregate interlock, alone, will not provide enough load transfer for good long-term performance for most highway pavements due to heavy truck traffic. Dowel bars should be used to provide added mechanical load transfer where truck traffic exceeds 120 per day or the accumulated design traffic exceeds four to five million AASHTO ESAL's. Typically, this truck traffic level will require at least an eight in. thick slab. For eight-inch slabs or greater, dowels are recommended for most highway applications.

Dowel bars are smooth round bars placed across joints to transfer loads without restricting horizontal joint movement. They also keep slabs in horizontal and ver­tical alignment. Since dowels span the joint, daily and seasonal joint opening does not affect load transfer across doweled joints as much as it does undoweled joints.

Dowel bars lower deflection and stress in the concrete slab and reduce the potential for faulting, pumping, and corner breaks [14][15]. This is true for short panel lengths and longer joint spacings. Performance evalua­tions of in-service concrete highway pavements show that the use of dowels effectively reduces faulting [16][17][18][19].

Dowels also increase pavement service life by reduc­ing deflections and stresses in the slab by effectively transmitting the load across the joint. For example, a 10-in. doweled slab with 80% load transfer will have the same deflection as a 12-in. undoweled slab with only 40% load transfer. For highway pavements less than 10 in. thick, 1-1/4 in. diameter dowels are recommended. One and one-half inch dowels should be used for pavements 10 in. thick or greater. A minimum dowel diameter of 1-1/4 to 1-1/2 in. is needed to control faulting on highway pavements. Loads on smaller diameter dowels induce higher bearing stresses and cause the concrete matrix around the dowel to deteriorate or elongate (socket). Elongation of the dowel hole reduces load transfer capabilities [20][21][22][23][24][25][26]. Figure 6 shows how dowel diameters 1-1/4 to 1-1/2 in. are subject to con­siderably lower bearing stresses than smaller diameter bars.

Dowel lengths are commonly in the range of 15 to 18 in. with shorter dowels being used in shorter slabs. An embedment length 6 times the dowel diameter is ade­quate for load transfer [27][28]. To account for joint opening and construction tolerances, 2 to 3 in. is add­ed to twice the embedment length to arrive at a prac­tical dowel length. Most agencies specify 18 in. long dowels for typical highway pavements.

Dowels are typically placed at mid-depth in the slab and spaced 12 in. apart (center to center) (Figure 7). However, some research shows that nonuniform spac­ing may provide satisfactory performance for larger diameter dowels in short slabs [29][30][31].

It is important to remember that aggregate interlock also helps load transfer where dowels are used. There­fore considerations that improve aggregate interlock should not be entirely neglected when using dowel bars. Large crushed aggregate and stronger subbases will also improve doweled joint performance [32].

Typical dowel placement in highway pavements.

Stabilized Subbases

Stabilized subbases reduce joint deflection, and im­prove and maintain joint effectiveness under repetitive loads. Stabilized subbases also provide an all-weather working platform for the paving contractor and a stable, smooth trackline.

Increased support provides better joint performance. Studies show that subbase strength significantly affects long-term load transfer [33][34][35]. Cement stabilized subgrades and lean concrete subbases produce greater slab support, reduce deflections, and increase fatigue life. Figure 8 shows that stabilized support more than doubles joint effectiveness and that loss of load transfer with a cement-treated subbase occurs slowly and stabilizes around one-half million load applications.

Figure 8 Joint Effectiveness with various subbase types (based on 9 in. slab after 1 million load applications).

References

  1. Smith, K.D., Peshkin, D.G., Darter, M.I., Mueller, A.L., Carpenter, S.H. Performance of Jointed Pavements, Vol. I, Evaluation of Concrete Pavement Performance and Design Features, Federal Highway Administration, FHWA-RD-89-136, March 1990.
  2. Henrichs, K.W, Liu, M.J., Darter, M.I., Carpenter S.H., and loannides, A.M. "Rigid Pavement Analysis and Design," FHWA-RD-88-068, Federal Highway Administration, Washington D.C., June 1989.
  3. Colley, B.E., and Humphrey, H.A., "Aggregate Interlock at Joints in Concrete Pavements," Highway Research Record No. 189, Highway Research Board, National Research Council, Washington D.C., 1967.
  4. Colley, B.E., and Humphrey, H.A., "Aggregate Interlock at Joints in Concrete Pavements," Highway Research Record No. 189, Highway Research Board, National Research Council, Washington D.C., 1967.
  5. Colley, B.E., and Humphrey, H.A., "Aggregate Interlock at Joints in Concrete Pavements," Highway Research Record No. 189, Highway Research Board, National Research Council, Washington D.C., 1967.
  6. Smith, K.D., Peshkin, D.G., Darter, M.I., Mueller, A.L., Carpenter, S.H. Performance of Jointed Pavements, Vol. IV Appendix A, Project Summary Reports and Summary Tables, Federal Highway Administration, FHWA-RD-89-139, March 1990.
  7. Colley, B.E., and Humphrey, H.A., "Aggregate Interlock at Joints in Concrete Pavements," Highway Research Record No. 189, Highway Research Board, National Research Council, Washington D.C., 1967.
  8. Colley, B.E., and Humphrey, H.A., "Aggregate Interlock at Joints in Concrete Pavements," Highway Research Record No. 189, Highway Research Board, National Research Council, Washington D.C., 1967.
  9. Kelleher, K. and Larson, R.M., "The Design of Plain Doweled Jointed Concrete Pavement," Proceedings from the 4th International Conference on Concrete Pavement Design and Rehabilitation, Purdue University, West Lafayette, April, 1989.
  10. Packard, R.G., "Design Considerations for Control of Joint Faulting of Undoweled Pavements," Proceedings from the First International Conference on Concrete Pavement Design, Purdue University, West Lafayette, IN, February 1977.
  11. Hudson, et al. Aggregate and Paved Surface Design and Rehabilitation Manual for Low-Volume Roads, Federal Highway Administration.
  12. Darter, M.I., Becker, J.M., Snyder, M.B., and Smith, R.E. "Portland Cement Concrete Pavement Evaluation (COPES)," National Cooperative Highway Research Program Report 277, Transportation Research Board, Washington D.C., 1985.
  13. Smith, K.D., Peshkin, D.G., Darter, M.I., Mueller, A.L., Carpenter, S.H. Performance of Jointed Pavements, Vol. I, Evaluation of Concrete Pavement Performance and Design Features, Federal Highway Administration, FHWA-RD-89-136, March 1990.
  14. Smith, K.D., Peshkin, D.G., Darter, M.I., Mueller, A.L., Carpenter, S.H. Performance of Jointed Pavements, Vol. IV Appendix A, Project Summary Reports and Summary Tables, Federal Highway Administration, FHWA-RD-89-139, March 1990.
  15. Construction and Maintenance of Rigid Pavements General Report, XVIII Brussels World Road Congress, PIARC, September 1987.
  16. Smith, K.D., Peshkin, D.G., Darter, M.I., Mueller, A.L., Carpenter, S.H. Performance of Jointed Pavements, Vol. IV Appendix A, Project Summary Reports and Summary Tables, Federal Highway Administration, FHWA-RD-89-139, March 1990.
  17. Construction and Maintenance of Rigid Pavements General Report, XVIII Brussels World Road Congress, PIARC, September 1987.
  18. Smith, L.L. "Load Transfer at Contraction Joints in Plain Portland Cement Concrete Pavements," Florida Research Bulletin No. 90, 1965.
  19. Gulden, W. "Pavement Faulting Study Investigation Into the Causes of Pavement Faulting on the Georgia Interstate System," Interim Report 2, Project 7104, Georgia Department of Transportation, Atlanta, GA, 1974.
  20. Smith, K.D., Peshkin, D.G., Darter, M.I., Mueller, A.L., Carpenter, S.H. Performance of Jointed Pavements, Vol. I, Evaluation of Concrete Pavement Performance and Design Features, Federal Highway Administration, FHWA-RD-89-136, March 1990.
  21. Smith, K.D., Peshkin, D.G., Darter, M.I., Mueller, A.L., Carpenter, S.H. Performance of Jointed Pavements, Vol. IV Appendix A, Project Summary Reports and Summary Tables, Federal Highway Administration, FHWA-RD-89-139, March 1990.
  22. Tabatabaie, A.M., and Barenberg, E.J. "Structural Analysis of Concrete Pavements Systems,"Transportation Engineering Journal, ASCE, Vol 106, No. TE5, Proceedings Paper 15671, pp 493-506.
  23. Tabatabaie, A.M., Barenberg, E.J., and Smith, R.E. "Longitudinal Joint Systems in Slip-Formed Rigid Pavements: Vol II. - Analysis of Load Transfer Systems for Concrete Pavements," Federal Highway Administration, FHWA/RD-86/042, Washington D.C., November, 1979.
  24. Kilareski, W.P., Ozxbeki, M.A., Anderson, D.A., "Fourth Cycle of Pavement Research at the Pennsylvania Transportation Research Facility - Vol. 4: Rigid Pavement Joint Evaluation and Full-Depth Patch Designs," Report No. FHWA/PA-84-026, Pennsylvania State University, University Park, PA, December 1984.
  25. Kilareski, W.P., Ozbeki, M.A., Anderson, D.A., "Computer Simulation and Field Evaluation of Transverse Joints in Rigid Pavements," Proceedings from the 3rd International Conference on Concrete Pavement Design and Rehabilitation, Purdue University, West Lafayette, April, 1985.
  26. Construction and Maintenance of Rigid Pavements General Report, XVIII Brussels World Road Congress, PIARC, September 1987.
  27. Teller, L.W. and Cashel, H.D. "Performance of Doweled Joints under Repetitive Loadings," Bulletin 217, Highway Research Board, Washington, D.C., 1968.
  28. Kelleher, K., Larson, R.M., "The Design of Plain Doweled Jointed Concrete Pavement," Proceedings from the 4th International Conference on Concrete Pavement Design and Rehabilitation, Purdue University, West Lafayette, IN, April 1989.
  29. Kelleher, K., Larson, R.M., "The Design of Plain Doweled Jointed Concrete Pavement," Proceedings from the 4th International Conference on Concrete Pavement Design and Rehabilitation, Purdue University, West Lafayette, IN, April 1989.
  30. Anderson, D.A., Kilareski, W.P., Luhr, D.R.A. "Fourth Cycle of Pavement Research at the Pennsylvania Transportation Research Facility - Vol. 7," Fourth Cycle of Pavement Research Summary Report, FHWA/PAS-84-029, Pennsylvania Department of Transportation, Harrisburg, PA, December 1984.
  31. Black, K.N., Larson, R.M., Staunton, L.R. "Evaluation of Stainless-Steel Pipes for Use as Dowel Bars," Public Roads, Vol. 52 No. 2, Federal Highway Administration, Washington, D.C., September 1988.
  32. Colley, B.E., and Humphrey, H.A., "Aggregate Interlock at Joints in Concrete Pavements," Highway Research Record No. 189, Highway Research Board, National Research Council, Washington D.C., 1967.
  33. loannides, A.M. and Korovesis, G.T. "Aggregate Interlock: A Pure-Shear Load Transfer Mechanism" Paper No. 89-0189, presented to the 1990 Annual Meeting of the Transportation Research Board, Washington, D.C., March 1990.
  34. Colley, B.E., and Humphrey, H.A., "Aggregate Interlock at Joints in Concrete Pavements," Highway Research Record No. 189, Highway Research Board, National Research Council, Washington D.C., 1967.
  35. Henrichs, K.W, Liu, M.J., Darter, M.I., Carpenter S.H., and loannides, A.M. "Rigid Pavement Analysis and Design," FHWA-RD-88-068, Federal Highway Administration, Washington D.C., June 1989.