Dowel Dowel Load Load Transfer Transfer Systems Systems Their - - PowerPoint PPT Presentation

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”

Concrete Pavement Deflections

12 ft Lanes Outside Pavement Edge Longitudinal Centerline (acts as tied PCC Shoulder)

Undoweled Transverse Joint Doweled Transverse Joint

2 Di 3 Di 5 Di 3 Di Di Di

Effects of Dowel Load Transfer on Pavement Behavior

Wheel Load 0% Load Transfer Direction of Traffic Approach Slab Leave Slab 100% Load Transfer Approach Slab Wheel Load Direction of Traffic Leave Slab

Load Transfer Efficiency (Deflection-based)

Unloaded LT (%) = Loaded

X 100 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

Source: Shiraz Tayabji, Fugro Consultants, Inc.

1 mil ~ 0.025mm

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!

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

A Brief History of U.S. Dowel Design

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

• r 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

• f 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

• btained 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 has varied with environment, coating properties, construction practice and other factors

Concerns with reliability

• ver long performance

periods

Photo credit: Tom Burnham, MnDOT Photo credit: Washington State DOT

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

Dowel Epoxy Coatings

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

Recommendations: Epoxy Coating

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

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.

Dowel LT Design Considerations

σb = Ky0 = KPt(2 + βz)/4β3EdId

β = (Kd/4EdId)0.25 Id = πd4/64 for round dowels Id = bh3/12 for rectangular dowels

Friberg’s Dowel-Concrete Bearing Stress

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

0.05 0.1 0.15 0.2 0.25 0.3 0.35 50 100 150 200 250 300 350 Age, months Faulting for 10 inch slab, ins

1" dia dowel 1.25" PCC 1.375" PCC 1.5" dia dowel

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

Reduce bearing stress while holding cross- sectional area constant (or reducing it) Examples: Hollow Dowels (fill or use end caps) Elliptical Dowels Plate Dowels “Optimized” Dowel Designs

(Photos: Greenstreak, PNA Construction Technologies, Glen Eder)

Free download at: apps.acpa.org

ELIMINATI ELIMINATION OF ON OF JOINT RESTRAINT AND JOINT RESTRAINT AND ALIGNMENT P ALIGNMENT PROBLEMS ROBLEMS

Reduction of Pullout Forces

• Limits on Dowel Pullout Force:
• AASHTO M 254 “Standard Specification for Corrosion-Resistant

Coated Dowel Bars”: maximum pullout load shall not exceed 1360 kg (3000 lb)

• Kansas DOT limits dowel pullout force to 1550 kg (3400 lbs)
• Michigan DOT limits bond stress (initial load divided by

embedded surface area) to 420kPa (60 psi)

• Example: For a 38mm (1.5-in) diameter dowel with 22.5 cm

(9 in) of embedment), maximum allowable initial load is 1160 kg (2545 lb) .

• Some dowel manufacturer’s claim that their coatings need no

lubricant to meet pullout force requirements – a cost savings (materials, labor)

Potential Dowel Misalignment Problems

Restraint of Movement in Area Pavements

Source: PNA Construction Technologies

Source: PNA Construction Technologies

Isolated Circle

Restraint of Odd-shaped Panels and Roundabouts

Center line Tolerance line

Formed void space on vertical sides of plate

Tolerance line

Plate Dowel Geometries for Contraction Joints

“Covex” Dowels

Other Factors Driving the Use of Alternate Dowels Reduced Material and/or Shipping Costs

Lighter weight dowels = more dowels/baskets per truck

Ease of Handling (Installation)

Less worker fatigue for lighter dowels Less potential for handling damage of baskets

Use in Thin Slabs Eliminate Magnetic Interference

Tollway gantry areas Magnetic inductance loops

DETERMININ DETERMINING STRUCTURA G STRUCTURAL L ADEQUAC ADEQUACY

Underlying question: what do we really need from load transfer systems? Answer: I don’t think we really know what we need! (… but I think we know what has worked in the past …) Therefore, easiest path to acceptance: prove comparable behavior and performance of alternate dowel system to current standard (generally cylindrical steel dowels). Long-term field performance Accelerated testing in labs Analytical equivalence

Structural Acceptance of Alternative Dowel Systems

Accelerated Load Testing

50 55 60 65 70 75 80 85 90 95 100 0.001 0.01 0.1 1 10 100

Applied Load Cycles (in millions) Load Transfer Efficiency (%)

Slab 1 - Epoxy-coated Steel Dowel Bars Slab 2 - Fiber Reinforced Polymer Dowel Bars Slab 3 - Grouted Stainless Steel Pipe Dowel Bars

Deflection-based Criteria

LTE Joint Stability Others?

Bearing Stress

Typically determined analytically High significance in many faulting models Includes influence of slab stiffness, foundation stiffness (through l)

Basis for System Equivalency

LTE is a measure of system behavior, not dowel equivalence.

Affected by joint width, total deflection, foundation stiffness, etc.

LTE has little meaning without an overall deflection reference

Example #1: dUL = 0.02mm, dL = 0.04mm, LTE = 50% … but is this joint bad? Example #2: dUL = 0.64mm, dL = 0.80mm, LTE = 80% … but is this joint good?

LTE as a measure of equivalence?

ACI 360 definition: “… a joints ability to limit differential deflection of adjacent slab panel edges when a service load crosses the joint … (t)he smaller the measured differential deflection number the better the joint stability.” Joint Stability

ACI 360.R-10):

< 0.010 in. (0.25 mm) (small, hard-wheeled lift truck traffic) < 0.020 in. (0.51 mm) (larger, cushioned rubber wheels)

What is appropriate for road pavements? Should the criterion vary with functional applications (e.g., streets vs highways)? Should the criterion vary with foundation design and environmental conditions (e.g., stabilized vs unbound base, and wet vs dry climate)?

Joint Stability Limits

Bearing Stress Basis An Analytical Approach Estimate critical dowel load

Linear distribution of load Structural modeling (i.e., finite element analysis)

Estimate bearing stress

Friberg’s analysis Structural modeling

Currently best option for dowels with nonuniform cross-section

Estimating Critical Dowel Load

l = (ECh3/12k(1 – μ2))0.25

Typical critical dowel load < 1350 kg (3000 lbs)

Free web app (Friberg Group Dowel Analyzer) at: apps.acpa.org

σb = Ky0 = KPt(2 + βz)/4β3EdId

β = (Kd/4EdId)0.25 Id = πd4/64 for round dowels Id = bh3/12 for rectangular dowels

Friberg’s Dowel-Concrete Bearing Stress

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

Effects of Embedment Length

22.5cm embedment: peak bearing stress = 17.3MPa 12.5cm embedment: peak bearing stress = 19.2 Mpa – an 11.6% increase

Effects of Embedment on Shear Load Capacity

ACI 325 (1956)

fb = f’c(4 – d)/3

where:

fb = allowable bearing stress, psi f’c = PCC 28-day compressive strength, psi d = dowel diameter, inches Provided factor of safety of 2.5 to 3.2 against bearing stress-related cracking Withdrawn from ACI 325 in 1960s, no replacement guidance provided Still commonly cited today …

Example Comparison of Alternate Dowel Behavior

Dowel Type Diameter (mm) Dowel Modulus, E (GPa) Applied Shear Force (kg) Dowel Deflection at Joint Face (mm) Bearing Stress (MPa)

Metallic 38 200 880 (30cm spacing) 0.023 9.69 Sch 40 Pipe 42 200 880 (30cm spacing) 0.023 9.80 FRP 38 39 880 (30cm spacing) 0.038 15.1 FRP 49 39 880 (30cm spacing) 0.023 9.60 FRP 38 39 590 (20cm spacing) 0.025 10.1

FRP, GFRP are relatively low-modulus products (<20% of steel) FRP is anisotropic - modulus varies across and along section Same diameter as steel will result in much higher bearing stresses, higher deflections, lower initial LTE values, more rapid loss of LTE under repeated loads Theory is borne out by lab tests and field experience

Structural Considerations for GFRP and FRP Dowels

Davis and Porter (1998)

Similar joint LTE for 44mm FRP @ 20cm and 38mm steel @ 30cm

Melham (1999)

38mm FRP performed comparably to 25mm steel

• Univ. of WV (2009)

FRP performance OK with good support, close spacing, narrow joints LTE dropped from 94% to 72% for 25mm FRP at 15cm spacing after 2M load cycles when joint width increased from 6.5mm to 13mm.

Many studies of FRP dowels …

“OPTIMIZATION” OF DOWEL LOCATION

Trend toward reducing standard dowel installations from 12 dowels per 3.7m lane to 11

Increase distance from lane edge to

• utside dowels to reduce incidence of

paver-induced misalignment

Concentrated dowels in wheel paths

Common in dowel bar retrofit applications Some trends for new construction

Evaluate bearing stresses for alternate spacings using DowelCAD software

“Optimized” Dowel Spacing

Example Dowel Layout

Smooth dowels 38 mm dia. 3.7 m 0.6 m 0.3 m typical 1.8 m minimum Mid-depth slab Traffic Direction 3 – 5 dowels/wheel path (typical)

Common for repairs; some agencies use this concept for new construction (e.g., Utah DOT uses 4 dowels/wheel path)

Source: CP Preservation Guide

Closure Dowels play an important role in the performance of concrete pavements

They are essential for long life in pavements carrying any significant heavy truck traffic!

Economics, sustainability and competition are driving a rapid increase in the development of alternate dowel materials and shapes. We must be open to improvements in pavement dowel technology, but must ensure that new dowels will meet the structural and durability requirements of

• ur pavements.

Acknowledgments

American Concrete Pavement Association Composite Rebar Technology (Jim Olson) Construction Materials, Inc. (MN) Dayton Superior (Glenn Eder, retired) Federal Highway Administration Fugro Consultants, Inc. (Shiraz Tayabji, now with ARA, Inc.) Jarden Zinc Products (Chris Schenk) Minnesota Department of Transportation (Tom Burnham and Maria Masten) National Concrete Consortium (Maria Masten and Tom Cackler) National Concrete Pavement Technology Center (Dale Harrington) PNA Construction Technologies (Nigel Parkes) University of Pittsburgh (Julie Vandenbossche) University of Minnesota (Lev Khazanovich)

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Questions? Discussion?