The Pavement Number as a Structural Design MethodThe Pavement Number as a Structural Design Method...
Transcript of The Pavement Number as a Structural Design MethodThe Pavement Number as a Structural Design Method...
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SARF/IRF 2014 | 2-4 September, South Africa
The Pavement Number as a Structural Design Method
Dr Fenella Johns Rubicon Solutions
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Background
• TG2 (Foamed bitumen) (2002) – Uncomfortable with structural design method
– Conservative designs
– Loose links between mix and structural design
• SAMDM – Powerful, but many flaws
– Loose guidelines for inputs
– Very difficult to validate new criteria (transfer functions) with real field data
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Background
• Relook at methods
– Sponsored by SABITA and Gauteng (GDPTRW)
– Started process in 2005
– Much discussion with relevant role players
• Priorities
– Use real field data to develop and validate structural design methods
– Robust systems, not open to abuse
– Strong links between mix design, structural design and specifications
– Clear guidelines for use
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Background
• Resulted in Material Classification and Pavement Number Methods
– Formally published in TG2 (2009)
– Not only applicable to BSMs
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Pavement Design Methods
• Empirical
– Primarily based on observations of performance
• Mechanistic-empirical
– Considers the pavement as a mechanism, with stresses and strains
– Links the calculated stresses and strains under loading to observations of performance
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Pavement Design Methods
• Empirical – AASHTO
– DCP
• Mechanistic-Empirical – South African Mechanistic Design Method
(SAMDM)
– With it’s associated failure criteria and transfer functions
- PAVEMENT NUMBER (knowledge based)
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Knowledge Based Approach
• Gather all available field performance data • Utilise best elements of mechanistic analysis • Robust and easy to use • Validated! • Develop clear, strong links to field testing
(material classification) and specifications • Data Sets
– 20 field sites – 7 HVS Sites (22 test sections) – Construction, maintenance & performance information – TRH4 catalogue
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Pavement Number
• Based on the Structural Number Concept
– Old AASHTO method
• Improved by incorporating
– Established design principles
– Local experience
• Calibrated for long term field performance
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Structural Number Revisited
CBR 7-15%
150 mm ETB
200 mm C4
40 mm AC Thick1 x A1 = SN1
Thick2 x A2 = SN2
Thick3 x A3 = SN3
SN = SN1 + SN2 + SN3
Adjusted for Subgrade Conditions
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Disadvantages of SN Approach
• Non-uniqueness of the index
– Switching base and subbase give same SN
• Insensitivity to placement of weak layers
CBR 7-15%
150 mm G1
150 mm C4
40 mm AC Thick1 x A1 = SN1
Thick2 x A2 = SN2
Thick3 x A3 = SN3
SN = SN1 + SN2 + SN3
These limitations are overcome by incorporating design rules of thumb that make the SN more “intelligent”.
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Pavement Number
• Structural Number
– modified with rules of thumb
• Calibrated & validated
– Field data
– Catalogues of design
– Experience
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Rules of Thumb
• Structural capacity is a function of the combined long term load spreading of all pavement layers
• Subgrade quality is the point of departure for design
• The type and quality of the base layer is critical
Says Who?
• Established knowledge in guidelines and textbooks
• Trends in well established design catalogues (TRH4)
• Trends in LTPP and HVS section performance
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Rules of Thumb
• Effective Long Term Stiffness (ELTS)
• Modular Ratio
• Maximum Allowable Stiffness
• Base Confidence Factor
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Modular Ratio
• The stiffness of one layer as a ratio of the layer underneath
CBR 7-15%
150 mm G3
200 mm G6
40 mm AC
E = 300
E = 150
= 2
MR for BSMs higher than granular
materials due to increased cohesion
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Effective Long Term Stiffness (ELTS)
• Represents the average stiffness of the material over the design life
• Depends on the material type/quality
• Model-specific parameter
– Not measured in the lab or field
• To calculate:
Modular Ratio x Support Stiffness
Maximum Allowable Stiffness Minimum
Given values for DEMAC
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Quality of Base Material
• Base Confidence Factor (BCF)
– Recognizes the importance of appropriate base material
– Ensures poor designs are disqualified
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Material Class Modular Ratio Max Allowable
Stiffness
Base
Confidence
Asphalt 5 2500 1
Seals 2 800 N/A
BSM1 3 600 1
BSM2 2 450 0.7
G1 3 700 1.1
G2 2 500 0.8
G3 2 400 0.7
G4 1.8 375 0.2
G5 1.8 320 0.1
G6 1.8 180 -2
G7 1.7 140 -2.5
G8 1.6 100 -3.0
G9 1.4 90 -4.0
G10 1.2 70 -5.0
C1 & C2 9 1500 0.8
C3 4 550 0.6
C4 3 400 0.4
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Subgrade Characterization
• Starting point for design
• Determine stiffness based on Material Class
Design equivalent
material class
Stiffness value
(MPa)
DE-G6 or better 180
DE-G7 140
DE-G8 100
DE-G9 90
DE-G10 70
• Adjust for climate
• Adjust for cover depth (stress-sensitivity)
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Climate Adjustment
Climate /
Weinert N
Values
Adjustment
Factor
Wet
(N < 2)
0.6
Moderate
(N = 2 to 5)
0.9
Dry
(N > 5)
1.0
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Subgrade Cover Adjustment
-15
-10
-5
0
5
10
15
400 450 500 550 600 650 700 750 800 850 900
Ad
jus
tme
nt
to S
ub
gra
de
Sti
ffn
es
s (
MP
a)
Thickness of Subgrade Cover (mm)
If Cover Thickness (in mm) is:
> 800, then Adjustment = +10 MPa
< 500, then Adjustment = -10 MPa
else:
Adjustment = -10 + [(Cover-500)/300] * 20 MPa
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Thickness Limits
Material type Layer situation Thickness limits
Minimum Maximum
Asphalt Surfacing 20 50
Seals Surfacing 5 5
BSMs Base and subbase 100 350
Cement stabilised Subbase 100 400
Granular (G1-G10) Base, subbase &
selected
100 300
• Must have 5 layers
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Traffic Limits
• PN applicable to
– 1 to 30 MESA
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PN example
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118 MPa
6. Calculate Layer ELTS Values
118 MPa
4. Adjust for cover
Example, Moderate Region
1. Material Classes
2. Determine subgrade stiffness
3. Adjust for climate
CBR 7-15%
180 mm G6
200 mm C4
150 mm BSM2
150 mm G7
5. Assign modular ratio’s and max stiffness
MR = 1.8, EMax = 180
MR = 3, EMax = 400
MR = 2, EMax = 450
7. Layer PN = thickness * ELTS
8. PN = layer PN
6. ELTS = min (Esupport * MR , Emax)
ELTS = min(212,180)
ELTS = 180
ELTS = 400
Thickness Adj = 0.4
ELTS = 450
BCF = 0.7
180 mm G6
200 mm C4
150 mm BSM2
150 mm G7
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Design Criteria
0
5
10
15
20
25
30
35
0 10 20 30 40 50
Pavement Number
Min
imu
m S
tru
ctu
ral C
ap
acit
y
(ME
SA
)
Category A
Category B
Not a transfer function
Rather, design frontier
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Calibration
0
5
10
15
20
25
30
35
40
45
0 5 10 15 20 25 30 35 40 45 50 55
Pavement Number
Str
uctu
ral C
ap
acit
y / T
raff
ic A
cco
mm
od
ate
d (
mesa)
TRH4 Cat A
TRH4 Cat B
LTPP Sound
LTPP Warning
LTPP Failed
HVS Sound
HVS Warning
HVS Failed
Criteria Cat A
Criteria Cat B
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Advantages of PN
• Simple method for determining structural capacity
• Valid for most SA materials
• Easy to understand & use
• Robust, and cannot be easily manipulated
• Utilizes well known rules of thumb
• Well and explicitly validated with TRH4, LTPP and HVS datasets
• Requires Material Class as design inputs
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PN Software
• www.asphaltacademy.co.za/bitstab
• Any changes in constants will be posted on website
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PN example Subgrade class G7
Initial stiffness
Climate Moderate
Climate adj
Cover depth
Cover adj.
SG ELTS
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5
BSM2 200
C4 200
G6 180
G7 N/A
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PN example Subgrade class G7
Initial stiffness 140
Climate Moderate
Climate adj 0.9
Cover depth
Cover adj. -4
SG ELTS
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5 5 2 800 1 N/A
BSM2 200 ✓ 2 450 1 1
C4 200 ✓ 3 400 0.4 N/A
G6 180 ✓ 1.8 180 1 N/A
G7 N/A N/A N/A 1 N/A
Design
equivalent
material
class
Stiffness
value (MPa)
G6 or better 180
G7 140
G8 100
G9 90
G10 70
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PN example Subgrade class G7
Initial stiffness 140
Climate Moderate
Climate adj 0.9
Cover depth
Cover adj. -4
SG ELTS
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5 5 2 800 1 N/A
BSM2 200 ✓ 2 450 1 1
C4 200 ✓ 3 400 0.4 N/A
G6 180 ✓ 1.8 180 1 N/A
G7 N/A N/A N/A 1 N/A
Climate /
Weinert N
value
Adjustment
factor
Wet (N < 2) 0.6
Moderate
(N = 2 to 5)
0.9
Dry (N > 5) 1.0
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PN example Subgrade class G7
Initial stiffness 140
Climate Moderate
Climate adj 0.9
Cover depth
Cover adj. -4
SG ELTS
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5 5 2 800 1 N/A
BSM2 200 ✓ 2 450 1 1
C4 200 ✓ 3 400 0.4 N/A
G6 180 ✓ 1.8 180 1 N/A
G7 N/A N/A N/A 1 N/A
DEMC BSM1 BSM2
Modular Ratio 3.0 2.0
Max Allowed
Stiffness
600 450
BCF 1.0 0.7
Thickness limit 100 to 350
mm
100 to 350
mm
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PN example
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5 5 2 800 1 N/A
BSM2 200 ✓ 2 450 1 1
C4 200 ✓ 3 400 0.4 N/A
G6 180 ✓ 1.8 180 1 N/A
G7 N/A N/A N/A 1 N/A
-15
-10
-5
0
5
10
15
400 450 500 550 600 650 700 750 800 850 900
Ad
justm
en
t to
Su
bg
rad
e S
tiff
ness (
MP
a)
Thickness of Subgrade Cover (mm)
If Cover Thickness (in mm) is:
> 800, then Adjustment = +10 MPa
< 500, then Adjustment = -10 MPa
else:
Adjustment = -10 + [(Cover-500)/300] * 20 MPa
Subgrade class G7
Initial stiffness 140
Climate Moderate
Climate adj 0.9
Cover depth 585
Cover adj. -4
SG ELTS
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PN example Subgrade class G7
Initial stiffness 140
Climate Moderate
Climate adj 0.9
Cover depth 585
Cover adj. -4
SG ELTS 122
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5 5 2 800 1 N/A
BSM2 200 ✓ 2 450 1 1
C4 200 ✓ 3 400 0.4 N/A
G6 180 ✓ 1.8 180 1 N/A
G7 N/A N/A N/A 1 N/A
140 * 0.9 - 4 = 122
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PN example Subgrade class G7
Initial stiffness 140
Climate Moderate
Climate adj 0.9
Cover depth 585
Cover adj. -4
SG ELTS 122
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5 5 2 800 1 N/A
BSM2 200 ✓ 2 450 1 1
C4 200 ✓ 3 400 0.4 N/A
G6 180 ✓ 1.8 180 1 N/A
G7 N/A N/A N/A 122 1 N/A
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PN example Subgrade class G7
Initial stiffness 140
Climate Moderate
Climate adj 0.9
Cover depth 585
Cover adj. -4
SG ELTS 122
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5 5 2 800 1 N/A
BSM2 200 ✓ 2 450 1 1
C4 200 ✓ 3 400 0.4 N/A
G6 180 ✓ 1.8 180 1 N/A
G7 N/A N/A N/A 122 1 N/A
Calculation of ELTS
122*1.8 = 220
180
But, max is 180
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PN example Subgrade class G7
Initial stiffness 140
Climate Moderate
Climate adj 0.9
Cover depth 585
Cover adj. -4
SG ELTS 122
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5 5 2 800 800 1 N/A
BSM2 200 ✓ 2 450 450 1 1
C4 200 ✓ 3 400 400 0.4 N/A
G6 180 ✓ 1.8 180 180 1 N/A
G7 N/A N/A N/A 122 1 N/A
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PN example Subgrade class G7
Initial stiffness 140
Climate Moderate
Climate adj 0.9
Cover depth 585
Cover adj. -4
SG ELTS 122
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5 5 2 800 800 1 N/A
BSM2 200 ✓ 2 450 450 1 1 9.0
C4 200 ✓ 3 400 400 0.4 N/A
G6 180 ✓ 1.8 180 180 1 N/A
G7 N/A N/A N/A 122 1 N/A
Calculation of layer PN
(450 * 200)/10000 * 1 = 9.0
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PN example Subgrade class G7
Initial stiffness 140
Climate Moderate
Climate adj 0.9
Cover depth 585
Cover adj. -4
SG ELTS 122
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5 5 2 800 800 1 N/A 0.4
BSM2 200 ✓ 2 450 450 1 0.1 9.0
C4 200 ✓ 3 400 400 0.4 N/A 3.2
G6 180 ✓ 1.8 180 180 1 N/A 3.2
G7 N/A N/A N/A 122 1 N/A 15.8
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Design Criteria
0
5
10
15
20
25
30
35
0 10 20 30 40 50
Pavement Number
Min
imu
m S
tru
ctu
ral C
ap
acit
y
(ME
SA
)
Category A
Category B
Not a transfer function
Rather, design frontier
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PN example Subgrade class G7
Initial stiffness 140
Climate Moderate
Climate adj 0.9
Cover depth 585
Cover adj. -4
SG ELTS 122
Mat
class
Thick T
check
MR Max E ELTS Thick
adj
BCF Layer
PN
Seal 5 5 2 800 800 1 N/A 0.4
BSM2 200 ✓ 2 450 450 1 0.1 9.0
C4 200 ✓ 3 400 400 0.4 N/A 3.2
G6 180 ✓ 1.8 180 180 1 N/A 3.2
G7 N/A N/A N/A 122 1 N/A 15.8
Cat A 3.0
Cat B 3.6