BSM Design Models:PN vs Stellenbosch ME function

Reconciliation?

Fenella JohnsRubicon Solutions

Introduction

History of BSM structural design

Pavement Number• 2009 & 2019

Mechanistic-Empirical • New Stellenbosch function

Design Equivalent Material Classification• DEMAC

History of BSM Structural DesignSABITA Manuals (~1990s)• GEMS• ETBs

TG2 2002 (Asphalt Academy)• Foam Bitumen• ME Design Functions• Conservative, developed on limited data

History of BSM Structural DesignTG2 (2009)• Both foam bitumen and bitumen

emulsion• Introduction of BSM term• Still not enough data for ME

design model• Wanted to use field performance• Pavement Number and DEMAC

born• PN used routinely now

TG2 (2019)

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

PN 2009 - Data

Data Sets

• 20 field sites

• 7 HVS Sites (22 test sections)

• Construction, maintenance & performance information

• TRH4 Catalogues

PN - How does it work?Climate• Dry, moderate, wet

Subgrade• Material class (DEMAC)• Subgrade cover thickness

Layer materials• Material class (DEMAC)• Modular Ratio• Maximum allowed stiffness• Thickness adjustment factor for C3 & C4• Base confidence factor

CBR 7-15%

150 mm G3

200 mm G6

40 mm AC

E = 300

E = 150= 2

119 MPa

6. Calculate Layer ELTS Values

119 MPa

4. Adjust for cover

Example, Moderate Region1. Material Classes

2. Determine subgrade stiffness

3. Adjust for climate

CBR 7-15%

180 mm G6

200 mm C4

150 mm BSM2

G7

5. Assign modular ratio’s and max stiffness

MR = 1.8, EMax = 180

MR = 3, EMax = 400

MR = 2, EMax = 450

MR = 2, EMax = 800

7. Layer PN = thickness * ELTS * BCF * TA

8. PN = Σ layer PN

6. ELTS = min (Esupport * MR , Emax)

ELTS = min(214,180)ELTS = 180

ELTS = 400Thickness Adj = 0.4

ELTS = 450BCF = 0.7

ELTS = 800

180 mm G6

200 mm C4

150 mm BSM2

Seal

PN 2009 – Design Frontier• Considers climate

0

5

10

15

20

25

30

35

0 10 20 30 40 50Pavement Number

Min

imum

Str

uctu

ral C

apac

ity

(MES

A)

Category ACategory B

Not a transfer function

Rather, design frontier

PN 2009 - IssuesOver contribution of asphalt

Under contribution of cemented materials

Asphalt layers• Limited to < 50 mm

No asphalt bases

Increase in PN, no increase in life

Limited to 30 MESA

PN 2019 – Data 69 LTPP Sections

TRH 4 Catalogues

SATCC Catalogues

PN 2009 - IssuesOver contribution of asphalt

Under contribution of cemented materials

Asphalt layers• Limited to < 50 mm

No asphalt bases

Increase in PN, no increase in life

Limited to 30 MESA

40

20 – 200 mm

20 – 100 mm

2019

PN 2019 – What Else is New?Thickness adjustment factor for stabilized layers• Asphalt• Cemented• BSMsMaterials• Asphalt bases• EG 4 & 5Seals are 10 mm thickNew Frontier Curve

PN 2019 – Frontier CurveNew Frontier Curve

0

10

20

30

40

50

0 10 20 30 40 50

Min

imum

Allo

wed

Tra

ffic

Cap

acity

(MES

A)

Pavement Number

Category A

Category B

• Increase in PN, increase in life• Maximum 40 MESA

PN 2019 – Calibration

0

10

20

30

40

50

0 10 20 30 40 50

Pave

men

t Cap

acity

(Exp

ecte

d Va

lues

), M

ESA

Pavement Number

MECHANISTIC EMPIRICAL DESIGN

𝑙𝑙𝑙𝑙𝑙𝑙𝑁𝑁 = 𝐴𝐴 − 𝐵𝐵.𝐷𝐷𝐷𝐷𝐷𝐷3 + 𝐶𝐶.𝑃𝑃𝑃𝑃𝑙𝑙𝑃𝑃.𝐷𝐷𝑅𝑅𝑅𝑅𝐶𝐶 + 0.86753

Pmod = % Mod.AASHTO Plastic Strain % (a/b)

Retained CohesionDeviator Stress Ratio

a b

Biermann, Stellenbosch University

Built-in

Reliability A Rut Limit (mm) 95% 0.8436 10 90% 0.9312 15 80% 1.0198 20 50% 1.1369 25

Mechanistic-Empirical Design Function

MECHANISTIC EMPIRICAL DESIGN - BSM2 DEFAULTS

A A B A A BB B B+ B B+

Class Percent of Reclaimed

Asphalt

ITS (kPa) Triaxial

ITSDRY ITSWET Cohesion (kPa) Friction Angle (°)

Retained Cohesion (%)

BSM 1 < 50% 225 125 250 – 300 (250) 40 – 50 (40) 70-85 (75) 50 – 100% 225 125 265 – 350 (265) 38 – 45 (38) 75-90 (75)

BSM 2 < 50% 175 100 200 – 250 (225) 38 – 40 (39) 65-75 (70) 50 – 100% 175 100 225 – 250 (238) 35 – 40 (37) 70-85 (75)

ME – Recommended Inputs

MECHANISTIC EMPIRICAL DESIGN - PAVEMENT 5

0

5

10

15

20

25

Desig

n Li

fe (M

ESAs

)

Moderate Climate & Category (on bar)

PN (old) PN (new) PN (new) Stell (single) Stell (sub) Stell (sub)

A A B A A B

35mm HMA

250mm BSM1

250mm G5

180mm G7

Infinite G8

3500 MPa

720 MPa

240 MPa

120 MPa

90 MPa Note: Single layer & Sub-layer Analysis at ¼ depth

Comparison of Results

MECHANISTIC EMPIRICAL DESIGN - PAVEMENT 8

Note: 1. Single layer & Sub-layer Analysis at ¼ depth2. B+ uses default stiffness for BSM2 (not min)

Using only Min for C, φ and Ret.C is over-conservative

A A B A A B

20mm HMA

280mm BSM2

250mm G6

150mm G8

Infinite G8

2500 MPa

500 MPa

200 MPa

100 MPa

90 MPa

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

1

Desig

n Li

fe (M

ESAs

)

Moderate Climate & Category (on bar)

PN (old) PN (new) PN (new) Stell (single) Stell (single) Stell (sub)

B B B+ B B B+

Comparison of Results

DEMAC SYSTEM

Cert

aint

y pe

r Cla

ss

Cert

aint

y Fa

ctor

s (CF

)

Adju

st fo

r sam

ple

size

DCP

Visuals

Test Pits

FWD

Materials/ Pavement Tests

Cumulative Certainty

10th% Median 90th%

G7 with Certainty of 0.7

DEMACDesign Equivalent Material Classification System

FOCUS OF SYSTEM REVISION

• Update with latest performance studies and with industry standards

• Validation of Certainty Factors

DEMAC System Update

FOCUS OF SYSTEM REVISION

• Updated and new Material Classification Rules

Cemented materials – Broader spectrum of parameters– Refined DE-EG Class

• Recommended design application

Crushed stone (COTO DS, 2019) – Refined gradings– Additional indicators

DEMAC System Update

Final Certainty Recommended Application

< 0.3 Inadequate

0.3 to 0.5 Category D Roads

0.5 to 0.6 Category C Roads

0.6 to 0.7 Category B Roads

> 0.7 Category A Roads

ConclusionAdvances in BSM structural design• Pavement Number revisions• New ME function, with recommended inputs• DEMAC revisions

Applicable to all pavement design, not just BSM