Dowel Dowel Load Load Transfer Transfer Systems Systems – Their Their Evolution Evolution and and Curr Current ent Innovations Innovations for for Sust Sustainabl ainable Pavements Pavements presented by Mark B. Snyder, Ph.D., P .E. Staff Consultant to American Concrete Pavement Association Past President of ISCP
Presentation Outline Introduction: The Need for Mechanical Load Transfer A Brief History of Pavement Dowels in the U.S. from 1917 – present The Drive to Use Alternate Dowel Materials/Shapes Determining Structural Equivalency Consideration of Shear, Bending and Bearing Stress Dowel Design: Dimensions, Placement and Materials “Optimization” of Dowel Location Dowel Structural Testing and Evaluation
INTRODUCTION: INTRODUCTION: THE THE NEED NEED FOR FOR MECHANICAL MECHANICAL LOAD LOAD TRANSFER TRANSFER
Dowels: Critical Structural Components of JCP Provide Load Transfer Reduce slab stresses Reduce slab deflections, potential for erosion of support Restraint of Curl/Warp Deformation Influence Dowel-Concrete Bearing Stress Need to last for expected pavement service life (requires corrosion resistance, other durability) 20 – 35 years for conventional pavement and repairs 40 - 100 years for long-life pavements
Load Transfer Ability of a slab to share load with neighboring slabs through shear mechanism(s) Typically quantified in terms of “Load Transfer Efficiency” (LTE), a deflection -based value Many factors affect LTE: Load transfer mechanisms Aggregate Interlock Dowels/Tie Bars Keyways Edge support Widened lanes, tied concrete shoulders or curb and gutter Decrease edge & corner stresses & deflections Foundation stiffness and shear resistance
Aggregate Interlock Shear between aggregate particles below the initial saw cut May be acceptable for: • Few heavy loads • Hard, abrasion-resistant coarse aggregate • Joint opening <0.03”
Effects of Dowel Load Transfer on Pavement Behavior Concrete Pavement Deflections Outside Pavement Edge 5 D i 3 D i 12 ft Lanes D i D i 3 D i 2 D i Longitudinal Centerline (acts as tied PCC Shoulder) Undoweled Transverse Joint Doweled Transverse Joint
Load Transfer Efficiency (Deflection-based) 0% Load Wheel Transfer Load Direction of Traffic Leave Slab Approach Slab Unloaded X 100 LT (%) = 100% Load Loaded Transfer Wheel Load Direction of Traffic Approach Slab Leave Slab Typically specify around 70% as threshold for action
Jo Joint int Lo Load ad Tra Trans nsfer fer Con Conside sidera ration tions LTE vs. Relative Deflection 1 mil ~ 0.025mm Source: Shiraz Tayabji, Fugro Consultants, Inc.
A BRIEF A BRIEF HISTORY HISTORY OF PAVEMENT OF PAVEMENT DOWELS DOWELS IN THE IN THE U.S. U.S. (1917 (1917-PR PRES ESENT) ENT)
A Brief History of U.S. Dowel Design First U.S. use of dowels in PCCP: 1917-1918 Newport News, VA Army Camps Two 19mm dowels across each 3m lane joint Rapid (but non- uniform) adoption through ‘20s and ‘30s 1926 practices: two 13mm x 1.2m, four 16mm x 1.2m, eight 19mm x 0.6m By 1930s, half of all states required dowels!
A Brief History of U.S. Dowel Design Numerous studies in ‘20s, ’30s, ‘40s and ‘50s Westergaard, Bradbury, Teller and Sutherland, Teller and Cashell, and others Led to 1956 ACI recommendations that became de facto standards into the ‘90s: Diameter – D/8, 30 cm spacing Embedment to achieve max LT: 8*dia for 19mm or less, 6*dia for larger dowels. 45 cm length chosen to account for joint/dowel placement variability. Recent practices: Trend toward increased diameter, some shorter lengths
Current Dowel Bars (Typical) Cylindrical (round) metallic dowels Typical length = 45 cm Typical diameter Roads: 25 – 32mm Airports: 32 – 50mm Typically spaced at 30 cm across transverse joints or wheel paths Epoxy coating or other corrosion-protection typically used in harsh climates (deicing or sea salt exposure) for corrosion protection
THE THE DRIVE DRIVE TO TO USE USE ALT ALTERNA ERNATE TE MATER MATERIALS IALS AND AND SHAP SHAPES ES
Driving Factors for Using Alternate Materials and Shapes Improved Corrosion Resistance (Increased Service Life) Improved Performance through Reduced Bearing Stress Elimination of Joint Restraint and Alignment Problems Economy (Reduced Cost of Raw Materials, Shipping) Facilitate Construction Ease of Handling Lighter Weight Products (e.g., FRP, Pipe Dowels, Plate Dowels) Ease of Installation (e.g., plate dowel slot formers, “Covex” plate dowel slot cuts, etc.) Use in Thin Slabs Eliminate Magnetic Interference
PREVENTI PREVENTION OF ON OF CORR CORROSIO OSION-RELATE RELATED PROBLE D PROBLEMS MS
The Corrosion Problem Corrosion - the destruction or deterioration of a metal or alloy substrate by direct chemical or electrochemical attack. Corrosion of reinforcing steel and dowels in bridges and pavements causes cracking and spalling. Corrosion costs an estimated $276B per year in the U.S. alone! Corroded dowels obtained from 19- year-old jointed concrete pavement
Effects of Corrosion on Dowels Loss of Cross-Section at Joint Poor Load Transfer Reduced Curl-Warp Restraint Loss of Joint Function (Restraint) Spalling Crack Deterioration Premature Failure
Dowel Corrosion Solutions Barrier Techniques Form Oil, Grease, Paint, Epoxy, Plastic Coating breach corrosion failure FRP Encasement Stainless Steel Cladding and Sleeves Relatively expensive Corrosion at coating breaches (including ends), accelerated due to galvanic reaction.
Dowel Epoxy Coatings Most common approach to corrosion prevention since 1970s Long-term performance Photo credit: Washington State DOT has varied with environment, coating properties, construction practice and other factors Concerns with reliability over long performance periods Photo credit: Tom Burnham, MnDOT
Dowel Epoxy Coatings Typical product: AASHTO M254/ASTM 775 (green, “flexible”) ASTM 934 (purple/grey, “nonflexible”) has been suggested Perception of improved abrasion resistance (but green meets same spec requirement) Mancio et al. (2008) found no difference in corrosion protection What is needed: Durability, resistance to damage in transport, handling, service Standardized coating thickness
Recommendations: Epoxy Coating Remains least expensive, potentially effective option Only effective if durable and applied with sufficient and uniform thickness Consider use of improved epoxy materials 0.25mm nominal minimum thickness meets or exceeds requirements of all surveyed states Would allow individual measures as thin as 0.2mm if average exceeds 0.25mm Probably not necessary to specify upper limit Self-limiting due to manufacturer costs Potential downside is negligible for dowels
Dowel Corrosion Solutions Corrosion-Resistant and Noncorroding Material 316/316L Stainless Steel (Solid, Tubes) Superior corrosion resistance! Expensive (solid bars and, to a less extent, grout- filled tubes) Deformation and slab cracking concerns (hollow tubes only) “Microcomposite” Steel and Lower -grade Stainless Steel Sufficient corrosion resistance? GFRP, FRP (Solid, Tubes) Noncorroding! Not yet widely adopted Concerns over structural behavior
Dowel Corrosion Solutions Cathodic Protection Impressed Current Useful in bridge decks, impractical for pavement dowels Galvanic (Sacrificial) 1mm zinc alloy cladding or bonded sleeve Inexpensive and self-regulating
IMPROVED PERFORMANC IMPROVED PERFORMANCE E THROUGH THROUGH REDU REDUCED BEAR CED BEARING STRESS ING STRESS
Dowel LT Design Considerations LT achieved through both shear and moment transfer, but moment contribution is small (esp. for joint widths of 6mm or less), so bending stress is not critical . Typical critical dowel load < 1350kg, so shear capacity of dowel is usually not critical (by inspection). What about bearing stress between dowel and supporting concrete at the joint face? Varies with load transferred, joint width, relative stiffness of dowel and concrete, etc. Maximum load transferred varies with slab thickness, foundation support, dowel layout, load placement, etc. Bearing stresses can be critical to performance! Pumping, faulting, fatigue, corner breaks, etc.
Friberg’s Dowel -Concrete Bearing Stress σ b = Ky 0 = KP t (2 + βz)/4β 3 E d I d β = (Kd/4E d I d ) 0.25 I d = πd 4 /64 for round dowels I d = bh 3 /12 for rectangular dowels Assumes sufficient embedment to match behavior of Timoshenko 1925 analysis (semi-infinite embedded bar). Free web app (Friberg Single Dowel Analyzer) at: apps.acpa.org
COPES Model: Bearing Stress vs Joint Faulting
Impact of Dowel Diameter on Joint Faulting 0.35 1" dia dowel 0.3 1.25" PCC Faulting for 10 inch slab, ins 1.375" PCC 0.25 1.5" dia dowel 0.2 0.15 0.1 0.05 0 0 50 100 150 200 250 300 350 Age, months Example for 10- in slab with specific traffic and climate … not a design chart!
Dowel Design Factors That Affect Bearing Stress Dowel Shape Round Elliptical Plates (Various Shapes) Dowel Stiffness Elastic Modulus Shape and Size Number and Placement of Dowels
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