Apidays New York 2024 - Scaling API-first by Ian Reasor and Radu Cotescu, Adobe
Aashto design
1. 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
1
2. Pavements are Designed
to Fail !!
Pavement Design Methodologies
• Experience
• Empirical
Statistical models from road tests
• Mechanistic-Empirical
Calculation of pavement stresses/strains/deformations
Empirical pavement performance models
• Mechanistic
Calculation of pavement stresses/strains/deformations
Mechanics-based pavement performance models
2
3. Empirical vs. Mechanistic Design
P
d
L
Wood Floor Joist
Empirical “Rule of 2”:
d in inches= (L in feet / 2) + 2
Mechanistic: σbending =
PL
≤ σ allowable
4S
1993 Version
3
4. AASHTO Pavement Design Guide
• Empirical design methodology
• Several versions:
1961 (Interim Guide)
1972
1986
Refined material characterization
Version included in Huang (1993)
1993
More on rehabilitation
More consistency between flexible, rigid designs
Current version
2002
Under development
Will be based on mechanistic-empirical approach
AASHO Road Test (late 1950’s)
(AASHO, 1961)
4
10. What is Serviceability?
• Based upon Present
Serviceability Rating (PSR)
• Subjective rating by
individual/panel
Initial/post-construction
Various times after
construction
• 0 < PSR < 5
• PSR < ~2.5: Unacceptable
(AASHO, 1961)
Present Serviceability Index (PSI)
• PSR correlated to physical pavement measures via Present
Serviceability Index (PSI):
2
PSI = 5.03 − 1.91log(1 + SV ) − 1.38 RD − 0.01(C + P)1/ 2
SV = slope variance (measure of roughness)
Empirical!
RD = average rut depth (inches)
C + P = area of cracking and patching per 1000 ft 2
PSI ≈ PSR
10
11. AASHTO Design Guide (1993)
Part I: Pavement Design 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 Procedures for New
Construction or Reconstruction
• Design Requirements
• Highway Pavement Structural Design
• Low-Volume Road Design
11
12. AASHTO Design Guide (1993)
Part III: Pavement Design Procedures for
Rehabilitation 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
(AASHTO, 1993)
12
19. Serviceability
∆PSI = po − pt
• PSI = Pavement Serviceability Index, 1 < PSI < 5
• po = Initial Serviceability Index
Rigid pavements: 4.5
Flexible pavements: 4.2
• pt = Terminal Serviceability Index
(AASHTO, 1993)
Adjustment of
Roadbed (Subgrade)
MR for Seasonal
Variations
(AASHTO, 1993)
19
20. Structural Number
n
SN = a1 D1 + ∑ ai Di mi
i =2
SN = structural number = f (structural 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)
20
31. Design Inputs
W18 = design traffic (18-kip ESALs)
ZR = standard normal deviate
So = combined standard error of traffic and performance prediction
∆PSI = difference between initial and terminal serviceability indices
pt = terminal serviceability index (implicit in flexible design)
All consistent with flexible pavements!
Additional Design Inputs
• S′c = modulus of rupture for concrete
• J = joint load transfer coefficient
• Cd = drainage coefficient (similar in concept to flexible
pavement terms)
• Ec = modulus of elasticity for concrete
• k = modulus of subgrade reaction
Additional inputs reflect differences in
materials and structural behavior.
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32. Modulus of Rupture
Sc’
(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
32
33. Joint Load Transfer Coefficient J
Additional benefits of tied shoulders:
(AASHTO, 1993)
Drainage Coefficient Cd
• Two effects:
Subbase and subgrade strength/stiffness
Joint load transfer effectiveness
(AASHTO, 1993)
33
34. PCC Modulus of Elasticity Ec
• Measure directly per ASTM C469
• Correlation w/ compressive strength:
Ec = 57,000 (fc’)0.5
Ec = elastic modulus (psi)
fc’ = 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
34
35. 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 (MR) for each season
• Assign subbase resilient modulus (ESB) for each season
15,000 psi (spring thaw) < ESB < 50,000 psi (winter freeze)
ESB < 4(MR)
(AASHTO, 1993)
35
36. Determining Effective k (cont’d)
• Determine composite k for each season
For DSB = 0: k = MR/19.4
For DSB > 0: Use Figure 3.3
• If depth to rigid foundation < 10 feet, correct k for effect of
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 potential loss of support LS (Figure 3.6)
Composite Modulus
of Subgrade Reaction
k = f (MR , ESB , DSB )
(AASHTO, 1993)
36
40. Next Slide
Consistent with flexible pavement approach!
(AASHTO, 1993)
Traffic vs. Analysis Period
(AASHTO, 1993)
(AASHTO, 1993)
40
41. 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
41
43. Typical Construction Joint Detail
(Huang, 1993)
Typical Longitudinal Joint Detail
Full Width Construction
(Huang, 1993)
43
44. 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
44
45. 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)
45
46. Design Inputs
Z
αc
(AASHTO, 1993)
Reinforcement 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)
46
47. 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)
47
48. Applies to both longitudinal
and transverse steel reinforcement
(Generally, Ps=0 for L< ~15 feet)
(AASHTO, 1993)
Friction Factor
(AASHTO, 1993)
48
49. Steel Working Stress
Based on preventing fracture and limiting permanent deformation.
(AASHTO, 1993)
Transverse
Tie Bars
(AASHTO, 1993)
49
