Flexible pavement thickness

design

by

Haryati Yaacob (fka, utm)

Flexible pavement thickness design

• Flexible Pavement Structure

• Factors to be Considered in Designing Flexible Pavement Thickness

• Thickness Design Methods

Pavement types

Type of Pavement & their Load Distribution

Pavement Types & How They Effect the Subgrade

Flexible pavement structureElements of a flexible pavement:

1. Sub-grade – upper layer of natural soil or fill, support load transmitted from overlaying layers.

2. Sub-base – specified material, secondary load spreading layer, prevent infiltration of sub-grade into pavement, construction platform for construction traffic, drainage layer

3. Road base – specified material, main load spreading layer, provide pavement with added stiffness and resistance to fatigue

4. Surfacing – uppermost layer, provide safe & comfortable riding surface, withstand traffic stresses, protect lower layers, impermeable and flexible, may consist of BC and WC, premix layer.

Failure Criteria

1. Failure mechanism – 2 categories:

- Permanent deformation- fail if rut (accumulation of permanent strain)> 20mm

- Crack (fracture under repeated or fluctuating stress)

Rut

Factors to be considered in designing pavement thickness

Traffic Loading

Axle wheel configurations

Tyre loads and pressure

Traffic distribution

Load repetitions-

unrecoverable damage

Vehicle speed

**Traffic Damaging Power – CV (BTM > 1.5 ton, 3 tonfor RN31), convert to SA (80 kN, 8160 kg, 18000 lb), fordesign life ESA/CSA

e = (L/Ls)4 - examples

Commercial vehicle 1

Commercial vehicle 2a

Commercial vehicle 2b

Commercial vehicle 3

Commercial vehicle 4

Commercial vehicle 4

Commercial vehicle 5

Factors to be considered in designing pavement thickness

Environmental

Temperature

Rheology

Expansion and contraction

Sunlight

Moisture

Safety

Rainfall-unbound material

Placing HMA in wet conditions

Pavement thickness design methodsObjective – to provide a structure that will be suitable in a specific

environment and able to sustain the anticipated traffic loading

Design methods

Empirical

Group Index, RN 29, RN 31 , 1993 AASHTO design method

Mechanistic Empirical

Shell, Chevron, Kentucky, TRRL 1132, Circly

Design process

Pavement Thickness Design Methods

ATJ 5/85ATJ 5/85 (2013)

RN31

ATJ 5/85

Pavement Thickness Design ATJ 5/85

Data required:

1. Design period – proposed 10 years

2. JKR Hierarchy

3. Average Daily Traffic (opening year) – ADT (PLH)

4. Percentage of Commercial Vehicle - Pc

5. Traffic Growth Rate - r

6. Sub-grade strength - CBR

7. Terrain

Design Procedure

1. Estimate Vo = ADT x (1/2) x 365 x (Pc/100)

2. Determine Vc= Vo [(1 + r)n - 1] / r

3. Convert to ESA, ESA = Vc x e (Table 3.1 or e = 2.52)

4. Check capacity (Table 3.2, 3.3, 3.4)

5. Determine Sub-grade CBR

6. Determine TA’ from nomograph

7. Calculate thickness for each layer (Table 3.5, 3.7, 3.8)

TA’ = SN = a1D1 + a2 D2 + ... + anDn

8. Sketch thickness obtained

Equivalence Factor- Table 3.1

ATJ 5/85 (Determine Sub-grade CBR)

In case of varying CBR for 1m depth of sub-grade, mean CBR is determined as

follows:

CBReff = [(h1CBR11/3 + h2CBR2

1/3 + … + hnCBRn1/3) / (1000)]3

where:

CBReff = effective CBR

CBR1, CBR2, … CBRn = CBR of soil strata

h1, h2, … hn = thickness of soil strata (mm)

h1 + h2 + … + hn = 1000 mm

Maximum Hourly capacity

Reduction Factor

Terrain Factor

Nomograph

Layer Coefficient, a

Standard and Construction Thickness, D

Minimum thickness of bituminous layer

ATJ 5/85- Design Example

Design Example:

JKR 05, carriageway width = 7.5m, shoulder = 2.0m

ADT = 6600

Pc = 15 %

r = 7 %

Sub-grade CBR = 5 %

Rolling Terrain

Material:

Surfacing = AC

Road base = wet mix Macadam

Sub-base = sand

ATJ 5/85 Design Method

(2013 revision)

Data required:

• Type and volume of commercial vehicles

• Design life

• Sub-grade type and strength

• Type and properties of paving materials

• Environment which pavement will be exposed to

Criteria

Traffic

• Data• Number of commercial vehicles during Year 1 of Design Period, which is the

expected year of completion of construction.

• Vehicle class and axle load distribution.

• Directional and lane distribution factors.

• Traffic growth factors.

Type of Terrain Terrain factor, T

Flat 1.0

Rolling 1.1

Mountainous/steep 1.3

3. Design period• 10 years for low volume and rural road.• 20 years for high volume and urban road

4. Design traffic (1st year of design period)

ESALY1 = ADT x 365 x PCV x LEF (3.7) x L x T

ESALY1 = number of ESALs for base year (design lane)ADT = Average Daily TrafficPCV = Percentage of CV (un-laden weight > 1.5 tons)VLF = Vehicle Load Equivalent Factor (including Tire Factor)L = Lane Distribution FactorT = Terrain Factor

If traffic distribution by vehicle type is available:

ESALY1 = [ADTcv1 x LEFcv1 + ADTcv2 x LEFcv2 +…+ ADTcv3 x LEFcv3] x 365 x L x T

5. Design Traffic (Number of ESALs) for the Design Period

ESALDES = ESALY1 x [(1 + r)n – 1)]/r

ESALDES = design traffic for the design lane in one direction

r = annual traffic growth rate factor for design period

n = number of years in design period

OR

ESALDES = ESALY1 x TGF

Total Growth Factor (TGF)

6. Decide traffic category

Normal distribution with single tailed analysis, the following normal deviate values shall apply:• 60% Probablility: Mean – 0.253 x STD•70% Probablility: Mean – 0.525 x STD• 85% Probablility: Mean – 1.000 x STD•statistical analysis shall be used to evaluate laboratory or field test results for use as input for pavement design (sub-grade, sub-base, road base and bituminous courses)

7. SG categories• Min 5% CBR for T1- T3

• If not, at least 0.3 meter of SG shall be replaced or stabilized to ensure the minimum value is met.

• Large volume traffic T4 and T5, min CBR 12%

8. Get T and S, choose from catalogue

• Mechanistic Design using Elastic Layer Programs• Asphalt Institute SW-1 (based on Manuals MS-1; MS-11; MS-17; MS-23)

• Pavement Design: A Guide to the Structural Design of Road Pavements, STANDARDS AUSTRALIA and AUSTROADS, 2004, in conjunction with CIRCLY Version 5.0

• SHELL SPDM Version 3.0

• Pavement Design and Analysis by Yang H. Huang, Second Edition, 2003 in conjunction with KENLAYER

• Layer Elastic Theory using RUBICON TOOLBOX Version 2.9.8.

• 3 types of pavement : • Conventional flexible pavement with granular base.

• Deep-strength flexible (composite) pavement with bituminous surface course(s) and a base stabilized with Portland cement, bituminous emulsion, or a combination of both.

• Full-depth asphalt pavement with bituminous base course

T1 : < 1 million ESALs

T2 : 1- 2 million ESALs

T3: 2 -10 million ESALs

T4 : 10 – 30 million ESALs

T5 : > 30 million ESALs

T5 : > 30 million ESALs ( Polymer Modified Asphalt)

Properties of Paving Materials

• Bituminous Wearing and Binder Courses

• Bituminous Road base• similar to binder and wearing course except a lower

temperature used for this layer

• Crushed Aggregate and Wet Mix Road Base• Performance -> shear strength, stiffness and by material

breakdown that may occur during construction and heavy traffic

• similar composition but construction practices are different

• Min CBR 80%, elastic modulus 350±100 Mpa

• Stabilized Road base• In situ or Plant

• 2 types: • STB 1 . Aggregates stabilised primarily with cement or lime . 3% to

5% Portland cement.

E = 1800 MPa; v = 0.40

• STB 2. Aggregates stabilised primarily with a bituminous emulsion/foamed bitumen + cementitious. Bituminous emulsion or foamed bitumen and a maximum of 2% Portland cement. E= 1200 MPa; v 0.35

Other options for Low Volume Roads

Example 1

• Traffic count data: ADT 2700 vehicles both directions (24 hour period)

• PCV: 16% ( no detailed break down by vehicle type)

• Terain : rolling

• Design life: 20 years

• Annual traffic growth: 4%

• CBR mean =18.5% , standard deviation= 4.4%, 85% probability

Example 2• Design a road pavement for a 4-lane freeway (concession toll-road) with an

average daily traffic of 7286 vehicles, of which 20% are commercial vehicles with an un-laden weight > 1.5 tons

CV 1 = 624

CV 2 = 456

CV 3 = 316

CV 4 = 102

Road Note 31

RN31

Emphasis on 5 aspects:

1. Influence of tropical climate on moisture conditions in road sub-grades

2. Severe conditions imposed on exposed bituminous surfacing by tropical climates

3. Interrelationship between design and maintenance

4. High axle load and tyre pressures

5. Influence of tropical climate on the nature of the soils and rocks used in the road building

RN31

• Designed for tropical and sub-tropical countries to carry up to 30M CSA

• Heavy vehicle > 3 ton

• Equivalence: e = (L/Ls)4.5

Design procedure:

1. Estimate CSA for design life >>> T (Table 3.9)

2. Assess sub-grade strength >>> S (Table 3.9, 3.10)

3. Select combination of material and thickness from structure catalogues based on T and S

Table 3.9 , T & S determination

S determination

Material Definition

Granular base, surface dressing

Granular base, structured surface

Road Note 31

Design Example 1:

ADT = 250/day.dir, Pc = 55 %, r = 5 %, CBR = 7 %

Design Example 2:

CSA = 12M, PI > 45, WT = 2m below formation

Example

• The predicted commercial vehicle of a proposed two lane urban highway is 31%. The ADT of highway in 2015 is 3500 and the highway is expected to complete and open to traffic in 208. If the traffic growth rate is estimated at 4% over the 20 year design life. Subgrade thickness and CBR value are 350mm (9%), 400mm(7%) and 250mm (4%). Determine the required thickness for the pavement layers using Road Note 31.

Example

• Design a two lane state road with the following known parameters using Road Note 31 method. Road will be opened to traffic in 2018. Clearly state all the assumptions used.

-ADT 2016= 12 000

-Directional spilt=60/40

-Percentage of heavy vehicles (Pc)= 20%

-Annual Average traffic growth rate (r)=7%

-Subgrade CBR and thickness: 12% (300mm), 8% (350mm) and 5% (350mm).

QUESTIONS?