Design Methods • Highway Pavements
AASHTO The Asphalt Institute Portland Cement Association
• Airfield Pavements
FAA The Asphalt Institute Portland Cement Association U.S. Army Corps of Engineers
Objectives of Pavement Design To provide a surface that is:
• Strong
Surface strength Moisture control
• Smooth • Safe
Friction Drainage
• Economical
Initial construction cost Recurring maintenance cost
Pavements are Designed to Fail !!
Pavement Design Methodologies • Experience • Empirical
Statistical models from road tests
• Mechanistic-Empirical
Calculation of pavement stresses/strains/deformations stresses/strains/deformations Empirical pavement performance models
• Mechanistic
Calculation of pavement stresses/strains/deformations stresses/strains/deformations Mechanics-based pavement performance models
Empirical vs. Mechanistic Design P
d
L
Mechanistic:
Wood Floor Joist
Empirical “Rule of 2”: d in inches= (L in feet / 2) + 2
σbending =
PL 4S
≤ σallowable
1993 Version
AASHTO Pavement Design Guide • Empirical design methodology • Several versions:
1961 (Interim Guide) 1972 1986
1993
Refined material characterization characterization Version included in Huang (1993) More on rehabilitation More consistency between flexible, rigid designs Current version
2002
Under development Will be based on mechanistic-empirical mechanistic-empirical approach
AASHO Road Test (late 1950’s)
(AASHO, 1961)
One Rainfall Zone...
(AASHO, 1961)
One Temperature Zone...
(AASHO, 1961)
One Subgrade...
A-6 / A-7-6 (Clay) Poor Drainage
(AASHO, 1961)
Limited Set of Materials... • One asphalt concrete
3/4” surface course 1” binder course
• One Portland cement concrete (3500 psi @ 14 days) • Four base materials
Well-graded crushed limestone (main experiment) Well-graded uncrushed gravel (special studies) Bituminous-treated base (special studies) Cement-treated Cement-treated base (special studies)
• One uniform sand/gravel subbase
1950’s Construction Methods...
(AASHO, 1961)
(AASHO, 1961)
1950’s Vehicle Loads...
Limited Traffic Volumes...
) s d n a s u o h T ( s d a o L e l x A
1.1M 1.1MAxles Axles
22Years Years
Time (Months)
(AASHO, 1961)
1950’s Data Analysis... (AASHO, 1961)
Some Failures...
(Some pavements too!)
(AASHO, 1961)
AASHTO Design Design Based on Serviceability Decrease
(AASHTO, 1993)
What is Serviceability? Serviceability? • Based upon Present upon Present Serviceability Serviceability Rating (PSR)
• Subjective rating by individual/panel
Initial/post-construction Various times after construction
• 0 < PSR < 5 • PSR < ~2.5: Unacceptable Unacceptable (AASHO, 1961)
Present Serviceability Index (PSI) via Present • PSR correlated to physical pavement measures via Present Serviceability Index (PSI):
PSI
2
= 5.03 − 1.91log(1 + SV ) − 1.38 RD − 0.01(C + P )1/ 2
SV = slope variance (measure of roughness) RD = average rut depth (inches) C + P = area of cracking and patching per 1000 ft 2 PSI
≈ PS R
Empirical!
AASHTO Design Guide (1993) Part I: Pavement Design Des Desiign gn and Management Principles
• Introduction and Background • Design Related to Project Level Pavement Management • Economic Evaluation of Alternative Design Strategies • Reliability
AASHTO Design Guide (1993) Part II: Pavement Design D esign for New Des ign Procedures for Construction or Reconstruction
• Design Requirements • Highway Pavement Structural Design • Low-Volume Road Design
AASHTO Design Guide (1993) Part III: Pavement Design for Design Procedures for Rehabilitation Rehabilitatio n of of Existing Pavements
• Rehabilitation Concepts • Guides for Field Data Collection • Rehabilitation Methods Other Than Overlay • Rehabilitation Methods With Overlays
Design Scenarios Included in AASHTO Guide Guide
(AASHTO, 1993)
AASHTO Design Design Based on Serviceability Decrease
(AASHTO, 1993)
Flexible Pavements
Design Equation log10 (W18 ) = Z R So + 9.36 log10 ( SN + 1) − 0.20
Structural Number
+
∆ PSI 4.2 − 1.5 + 2. 2.32 log10 ( M R ) − 8.07
log10 0.40 +
1094
( SN + 1)
5.19
W 18 = design traffic (18-kip ESALs) Z R = standard normal deviate S o = combined standard error of traffic and performance prediction = difference between initial and terminal serviceability index ∆ PSI = M R = resilient modulus (psi) SN = = structural number
(AASHTO, 1993)
Traffic vs. Analysis Period
(AASHTO, 1993)
Analysis Period
(Also basis for life-cycle cost analysis)
(AASHTO, 1993)
Design Traffic (18K ESALs ESALs)
(AASHTO, 1993)
Design Traffic (18K ESALs ESALs) • DD = 0.5 typically • DL:
(AASHTO, 1993)
Reliability
(AASHTO, 1993)
Recommended Values for Standard Error S o
• Rigid Pavements: 0.30 - 0.40 • Flexible Pavements: 0.40 - 0.50
Standard Normal Deviate ZR
(AASHTO, 1993)
Recommended Reliability Levels
(AASHTO, 1993)
Serviceability
∆ PSI =
po
− pt
• PSI = Pavement Serviceability Index, 1 < PSI < PSI < < 5 • po = Initial Serviceability Index
Rigid pavements: 4.5 Flexible pavements: 4.2
• pt = Terminal Serviceability Index
(AASHTO, 1993)
Adjustment of Adjustment Roadbed (Subgrade) M R for Seasonal Variations
(AASHTO, 1993)
Structural Number n
SN
= a1 D1 + ∑ ai Di mi i=2
SN = = structural number = f (s tructural capacity) ai = ith layer coefficient Di = ith layer thickness (inches) mi = ith layer drainage coefficient n = number of layers (3, typically)
No Unique Solution!
(AASHTO, 1993)
Layer Coefficient a1: Asphalt Asphalt Concrete Concrete
(AASHTO, 1993)
Layer Coefficient a2 : Granular Base
a2
≅ 0.249 ( log10 E base ) − 0.977
E base in psi
(AASHTO, 1993)
Layer Coefficient a2 : Cement Treated Base
(AASHTO, 1993)
Layer Coefficient a2 : Bituminous Treated Base
(AASHTO, 1993)
Layer Coefficient a3: Granular Subbase
a3
= 0.227(log10 E subbase ) − 0.839
E subbase in psi
(AASHTO, 1993)
Quality of Drainage
(AASHTO, 1993)
Drainage Coefficient mi mi increases/decreases the effective value for ai
(AASHTO, 1993)
Next Slide (AASHTO, 1993)
Traffic vs. Analysis Period
(AASHTO, 1993)
(AASHTO, 1993)
Effect of Frost on Performance
PSI = Pavement Servicability Index 1 < PSI < 5 “Failure”: PSI < 2+
(AASHTO, 1993)
Frost Heave Rate
φ = f (-0.02mm)
(AASHTO, 1993)
Maximum Serviceability Loss
∆PSImax = f (frost depth, drainage)
(AASHTO, 1993)
Effect of Swelling on Performance
PSI = Pavement Servicability Index 1 < PSI < 5 “Failure”: PSI < 2+
(AASHTO, 1993)
Swell Rate Constant
θ = f (moisture supply, soil fabric)
(AASHTO, 1993)
Maximum Potential Heave V R
VR = f (PI, compaction, thickness) thickness)
(AASHTO, 1993)
Rigid Pavements
Design Equation log10 (W18 ) = Z R S o
+ 7.35 log10 ( D + 1) − 0.06
PCC Thickness
∆ PSI log10 Sc' Cd ( D 0.75 − 1.132 ) 4.5 − 1.5 + + ( 4. 4 .22 − 0.32 pt ) log10 7 1.64x10 215.63 J D 0.75 − 18.42 1+ 8.46 0.25 ( D + 1) ( Ec / k )
W 18 = design t raffic (18-kip ESALs) Z R = standard normal deviate
S c’ = = modulus of rupture (psi) for Portland cement concrete
S o = combined standard error of traffic and performance prediction prediction
J = load transfer coefficient
D = D = thickness (inches) of pavement slab
E c = modulus of elasticity (psi) for Portland cement concrete
∆ PSI = =
difference between initial and terminal serviceability indices
pt = terminal serviceability value
C d = drainage coefficient
= modulus of subgrade reaction (pci) k =
(AASHTO, 1993)
(AASHTO, 1993)
Design Inputs W 18 = design traffic (18-kip ESALs) Z R = standard normal deviate S o = combined standard error of traffic and performance prediction ∆ PSI = =
difference between initial and terminal serviceability indices
pt = terminal serviceability serviceability index (implicit in flexible design)
All consistent with flexible flexible pavements!
Additional Design Inputs • S ′ ′c = modulus of rupture for concrete = joint load transfer coefficient • J = • C d = drainage coefficient (similar in concept to flexible pavement terms) terms)
• E c = modulus = modulus of elasticity for concrete • k = modulus of subgrade reaction
Additional inputs inputs reflect differences in materials and structural behavior.
Modulus of Rupture S c ’
(AASHTO, 1993)
Joint Load Transfer Coefficient J Pavement Type (no tied shoulders) JCP/JRCP w/ load transfer devices JCP/JRCP w/out load transfer devices CRCP
J 3.2 3.8-4.4 2.9
Joint Load Transfer Coefficient J Additionall benefits of Additiona of tied shoulders: shoulders:
(AASHTO, 1993)
Drainage Coefficient C d • Two effects:
Subbase and subgrade strength/stiffness Joint load transfer effectiveness effectiveness
(AASHTO, 1993)
PCC Modulus of Elasticity E c • Measure directly per ASTM C469 • Correlation w/ compressive strength: E c = 57,000 ( f f c’ )0.5 E c = elastic modulus (psi) f c’ = = compressive strength (psi) per AASHTO T22, T140, or ASTM C39
Effective Subgrade Modulus k • Depends on:
Roadbed (subgrade) resilient modulus, MR Subbase resilient modulus, ESB
• Both vary by season
Determining Effective k (See Table 3.2) • Identify:
Subbase types Subbase thicknesses Loss of support, LS (erosion potential of subbase) Depth to rigid foundation (feet)
• Assign roadbed soil resilient modulus ( M M R) for each season • Assign subbase resilient modulus ( E E SB) for each season
15,000 psi (spring thaw) < E < E SB < 50,000 psi (winter freeze) E SB < 4( M M R)
(AASHTO, 1993)
Determining Effective k (cont’d) • Determine composite k for for each season
For D For DSB = 0: k = M = M R/19.4 For D For DSB > 0: Use Figure 3.3
for effect of • If depth to rigid foundation < 10 feet, correct k for rigid foundation near the surface (Figure 3.4)
• Estimate required thickness of slab (Figure 3.5) and determine relative damage ur for each season
• Use average ur to determine effective k (Figure 3.5) • Correct k for for potential loss of support LS support LS (Figure (Figure 3.6)
Composite Modulus of Subgrade Reaction k = f (M R , E SB , DSB )
(AASHTO, 1993)
Rigid Foundation Correction
(AASHTO, 1993)
Relative Damage u r = f ( k, D)
(AASHTO, 1993)
(AASHTO, 1993)
Loss of Support, LS
Subbase/subgrade erosion at joints causes Loss of Support, impairs load transfer.
(AASHTO, 1993)
Loss of Support
(AASHTO, 1993)
(AASHTO, 1993)
Next Slide
Consistent with flexible pavement approach! (AASHTO, 1993)
Traffic vs. Analysis Period
(AASHTO, (AASHTO,1993) 1993)
Joint Design • Joint Types
Contraction Expansion Construction Longitudinal
• Joint Geometry
Spacing Layout (e.g., regular, skewed, randomized) Dimensions
• Joint Sealant Dimensions
Types of Joints • Contraction
Transverse For relief of tensile stresses
• Expansion
Transverse For relief of compressive stresses Used primarily between pavement and structures (e.g., bridge)
• Construction • Longitudinal
For relief of curling and warping stresses
Typical Contraction Joint Details
(Huang, 1993)
Typical Expansion Joint Detail
(Huang, 1993)
Typical Construction Joint Detail
(Huang, 1993)
Typical Longitudinal Joint Detail
Full Width Construction
(Huang, 1993)
Typical Longitudinal Joint Detail
Lane-at-a-Time Construction (Huang, 1993)
Joint Spacing • Local experience is best guide • Rules of thumb:
JCP joint spacing (feet) < 2D (inches) W/L < 1.25
Joint Dimensions • Width controlled by joint sealant extension • Depths:
Contraction joints: D/4 Longitudinal joints: D/3
• Joints may be formed by:
Sawing Inserts Forming
Joint Sealant Dimension Governed by expected joint movement, sealant resilience
(AASHTO, 1993)
Design Inputs Z
c
(AASHTO, 1993)
Reinforcement Reinforcemen Reinforcementt Design (JRCP)
• Purpose of reinforcement is not to prevent cracking, but to hold tightly closed any cracks that may form
• Physical mechanisms:
Thermal/moisture contraction Friction resistance from underlying material
• Design based on friction stress analysis
(Huang, 1993)
Dowel Bars: Transverse Joint Load Transfer
• “…size and spacing should be determined by the local agency’s procedures and/or experience.”
• Guidelines:
Dowel bar diameter = D/8 (inches) Dowel spacing: 12 inches Dowel length: 18 inches
Friction Stresses
Induces tensile stresses in concrete Causes opening of transverse joints
(Huang, 1993)
Applies to both longitudinal longitudinal and transverse steel reinforcement (Generally, P s=0 for L< ~15 feet) (AASHTO, 1993)
Friction Factor
(AASHTO, 1993)
Steel Working Stress
Based on preventing fracture and limiting permanent deformation. (AASHTO, 1993)
Transverse Tie Bars
(AASHTO, 1993)
Transverse Tie Bars
(AASHTO, 1993)
