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(1)

FLEXIBLE PAVEMENT DESIGN (IRC:37-2012)

BACKGROUND IN BRIEF:

The guidelines on design of flexible pavement were first brought out in 1970

The guidelines were based on

CBR of subgrade soil Traffic in terms of no. of commercial vehicles (> 3 tonne laden wt.)

(2)

FLEXIBLE PAVEMENT DESIGN (IRC:37-2012)

BACKGROUND IN BRIEF:

Then IRC:37-1970 was revised in 1984 in which design

traffic was considered in terms of cumulative number of

equivalent standard axle load of 80 kN in msa

In addition, design charts were provided for traffic up to

30 msa using an empirical approach

Once again, IRC:37-1984 was revised in 2001

when pavements were required to be designed for

traffic as high as 150 msa

(3)

FLEXIBLE PAVEMENT DESIGN (IRC:37-2012)

BACKGROUND IN BRIEF:

Once again, IRC:37-1984 was

revised in 2001

when pavements were required to be designed for

traffic as high as 150 msa

This particular guidelines used a semi-mechanistic

approach based on the results of the MORT&H’s

research scheme R-56 implemented at IIT

Kharagpur

The software, FPAVE was developed for the

analysis and design of flexible pavements.

Multilayer elastic theory was adopted for stress

analysis of the layered elastic system.

(4)

FLEXIBLE PAVEMENT DESIGN (IRC:37-2012)

BACKGROUND IN BRIEF:

The traffic pattern has changed since then and so

has the technology

The volume of tandem, tridem and multi-axle

vehicles has increased manifold and heavier axle

loads are common

Experience has been gained on the use of new

form of construction and materials such as stone

matrix

asphalt,

modified

bitumen,

foamed

bitumen, bitumen emulsion, warm mix asphalt,

cementitious bases and sub-bases since the

publication of the last revision of the guidelines

Conventional

construction

material

like

aggregates is becoming progressively scarce on

account of environmental concerns as well as

legal

restrictions

on

quarrying

while

the

construction activity has expanded phenomenally.

This has shifted focus from large scale use of

conventional aggregates to use of local, recycled

and

engineered

marginal

aggregates

in

construction

(5)

FLEXIBLE PAVEMENT DESIGN (IRC:37-2012)

SCOPE OF THE GUIDELINES

The guidelines shall apply to the design of new flexible pavements for Expressway National Highways State Highways MDR

(6)

For the purpose of guidelines, flexible pavements

include pavements with bituminous surfacing

over:

1. Granular base and sub-base

2. Cementitious bases and sub-bases with a crack

relief layer of aggregate interlayer below the

bituminous surfacing

3. Cementitious bases and sub-bases with SAMI

in between bituminous surfacing and the

cementitious base layer for retarding the

reflection cracks into the bituminous layer

4. Reclaimed Asphalt Pavement (RAP) with or

without addition of fresh aggregates treated

with foamed bitumen/bitumen emulsion

(7)

A brief introduction to foamed asphalt

In 1956 Prof. Ladis Csanyi came up with the idea of introducing moisture into a stream of hot bitumen, which effects a spontaneous foaming of the bitumen (similar to spilling water into hot oil).

In the foam state the bitumen has a very large surface area and extremely low viscosity making it ideal for mixing with aggregates

Why is foamed asphalt only now gaining popularity?

Part of the answer lies in the fact that the original bitumen foaming process was a proprietary product, patented by Mobil Oil,

with the associated restrictions on the general use of the technology.

Furthermore, the lack of standardized mix design procedures meant that foamed asphalt was overlooked in preference for more well documented and familiar products.

(8)

Foamed bitumen is produced by injection of a

small amount of tap water into hot bitumen.

The fine droplets of water come into contact

with the hot bitumen (typically 160 °C to 170

°C).

After the nozzle (pressure reduction) the rapid

evaporation of water produces a very large

volume of foam:

Theoretically 1 liter of water forms about 1200

liters of steam.

How Foam Bitumen is manufactured?

The steam expands until a film of bitumen holds

the steam and air in bubbles.

(9)

An important factor in foaming is the

nozzle design and

the injection pressure in order to obtain a good water

droplet spray in contact with the hot bitumen

The foaming characteristics of a specific bitumen are

further influenced by numerous factors:

(10)

Temperature of the bitumen. For most bitumens

the foaming characteristics are improved with

higher temperature.

The expansion ratio increases with an increase in

the amount of water added, whilst the half-life

decreases.

The water helps in creating the foam, but the foam

can collapse rather fast due to rapidly escaping

steam.

It is known that e.g. silicone compounds can be

effective anti-foaming agents.

On the other hand, compounds have also been

identified that can increase the expansion ratio

and the half-life of the foam from seconds to

minutes.

(11)
(12)

Illustration Showing How Foamed Asphalt Works

The Wirtgen 2500 Is Used to Both Pulverize the Road Bed and Apply the Foamed Asphalt

Illustration Showing How Foamed Asphalt is Applied Inside the Mixing Chamber

(13)

What's so special about foamed asphalt?

Foamed asphalt epitomizes the asphalt industry drive towards energy efficient, environmentally friendly and cost effective solutions for road-building.

Some of the most striking advantages of foamed asphalt are as follows:

1.

The foamed bitumen increases the shear strength and reduces the moisture susceptibility of granular materials. The strength characteristics of foamed asphalt approaches that of cemented materials, but foamed asphalt is flexible and fatigue resistant.

2. Foam treatment can be used with a wider range of aggregate types than other of cold mix processes

(14)

3. Lower binder and transportation costs because foamed asphalt requires less binder and water than other types of cold mixing.

4. Saving in time because foamed asphalt can be compacted immediately and can carry traffic almost immediately after compaction is completed.

5. Energy conservation because only the bitumen needs to be heated while the aggregates are mixed in cold and damp (no need for drying).

6. Environmental side-effects of the evaporation of volatiles from the

mix is avoided since curing does not result in the release of volatiles.

(15)

7. Foamed asphalt can be stockpiled with no binder runoff or leeching. Since foamed asphalt remains workable for very extended periods the usual time constraints for achieving compaction, shaping and finishing the layer are avoided.

8. Foamed asphalt layers can be constructed even in adverse weather conditions, such as cold or light rain, without affecting the workability or the quality of the finished layer

(16)

Sub-base layer (cemented/unbound) Subgrade/Stabilised Subgrade Base layer (cemented/unbound) Aggregate interlayer for cemented

base/SAMI layer Bituminous layer

(17)

Stress Absorbing Membrane Interlayer (SAMI)

Basically, it is a reinforced layer. Researchers have used FiberMat as SAMI

FiberMat is a process that sandwiches strands of chopped fiberglass between two layers of polymer modified asphalt emulsion, and is applied using specialized equipment.

The first layer of emulsion provides a bond to the existing hard surface, with random interweaving of the fiberglass strands providing tensile strength to the mix, the second application of asphalt emulsion encapsulates the fiberglass, ensures the existing pavement is sealed, and is quickly covered with a thin veil of aggregate

(18)

The aggregate is seated into this second layer of

emulsion using traditional rolling techniques and the

SAMI is capable of accepting traffic in approximately 20

minutes

This reinforced layer can be used as a temporary

wearing surface, on high volume roads, and is usually

covered with a thin layer of hot mix asphalt within 14

days.

Once capped with hot mix, it becomes a true SAMI.

Its function is to seal the existing pavement with a

resilient waterproof membrane, reduce reflective cracking

through the new wearing surface, and ultimately prolong

the useful service life of the road.

(19)

Hot mix asphalt overlay SAMI Pre-existing pavement

SAMI within the pavement structure

(20)

1st layer of asphalt emulsion 2nd layer of asphalt emulsion Chopped fiber glass

(21)

Even distribution of materials

(22)

PRINCIPLES OF PAVEMENT DESIGN

A flexible pavement is modelled as an elastic structure.

Stresses and strain at critical locations are computed

using a linear layered elastic model.

IITPAVE has been used for the computation of stresses and strains in flexible pavements

Top Down Cracking in Bituminous Layer:

Fatigue cracking is conventionally considered as a “bottom-up cracking” phenomenon.

“Top down” cracking has also been observed on high volume roads in the country, because of excessive tensile stresses developing at the top surface due to heavy axle loads.

(23)

PRINCIPLES OF PAVEMENT DESIGN

Tensile strain, t, at the bottom of the bituminous layer and the

ͼ

vertical subgrade strain, v, on the top of subgrade are

ͼ

conventionally considered as critical parameters for pavement

design to limit cracking and rutting in the bituminous layers and

non-bituminous layers respectively.

(24)

DESIGN STIPULATIONS

1. TRAFFIC

2. TRAFFIC GROWTH RATE

3. DESIGN LIFE

4. VEHICLE DAMAGE FACTOR (VDF)

5. LANE DISTRIBUTION FACTOR

(25)

VEHICLE DAMAGE FACTOR(VDF)

It is a multiplier to convert the number of

commercial vehicles of different axle loads and axle

configuration to the number of standard axle load

repetition

It is defined as equivalent number of standard axles

per commercial vehicle

The VDF varies with the vehicle axle configuration,

axle loading, terrain, type of road and from region

to region

(26)
(27)

EXAMPLE 1: Design the pavement for construction of a new

flexible pavement

with the following data:

1. 4 lane divided carriageway

2. Initial traffic in the year of completion of construction = 5000

CV/day (both directions)

3. % of single, tandem & tridem axles are 45%, 45% and 10%

respectively

4. Traffic growth rate per annum (r) = 6.0 %

5. Design life = 20 years

6. Vehicle damage factor (based on axle load survey) = 5.2

7. CBR soil below the 500 mm of the subgrade = 3%

(28)

DESIGN CALCULATIONS

1. LDF for 4 lane divided carriageway = 0.75

2. Initial traffic = 2500 CVPD assuming 50% in each direction 3. VDF = 5.2 (given)

Cumulative no. of standard axles to be catered for in the design

N = 2500 x 365 x { ( 1+0.06)

20 – 1}

0.06

(29)

4. Since there is a large difference between CBR of the

embankment

material (3%) and CBR of 500 mm subgrade (10%),

effective CBR of the

subgrade should be obtained

(30)

Now, find the relevant resilient modulus for a known effective CBR : MR (MPa) = 10 x CBR for CBR 5 MR (MPa) = 17.6 x (CBR)0.64 for CBR > 5 (1) (2)

Since effective CBR > 7%, using eqn (2)

Resilient modulus (MR) is calculated as below:

(31)

Thickness of proposed Bituminous layer with VG 40

bitumen (40/60 as per IS:73-2006) with bottom DBM

layer having air void of 3% (0.5% to 0.6% additional

bitumen over OBC) over WMM and GSB = 185 mm at

reliability of 90 %

Two fatigue equations were fitted , one in which the computed strains in 80% of the actual data in the scatter plot were

higher than the limiting strains predicted by the model (and termed as 80% reliability level) and the other corresponding to 90% reliability level

Two equations for conventional bituminous mixes designed by Marshall method are as given below:

Nf = 2.21 x 10 – 04 x [1/ t] ͼ 3.89 x [1/M

R]0.854 (80% reliability) --- (1) Nf = 0.711 x 10 – 04 x [1/ t] ͼ 3.89 x [1/M

R]0.854 (90% reliability) ---

(2)

Nf = fatigue life in number of standard axles

t = maximum tensile strain at the bottom of the bituminous layer ͼ

(32)

As per the prevailing practice, the mixes used

in the pavements under study section were

generally designed for 4.5% air voids and

bitumen content of 4.5% by wt. of the mix

(which in terms of volume should come to

11.5%)

Most literature recommend a factor “C” to be

introduced in fatigue models to take into

account the effect of air voids (Va) and volume

of bitumen (Vb), which is given by the

relationships

C = 10

M

, and M = 4.84 (

Vb

Va +Vb - 0.69)

Corresponding to the values of Va & Vb as stated above, introduction of “C” in eqn (2) leads to Eqn (3)

Nf = 0.5161 x C x 10-04 x [ 1/ t]ͼ 3.89 x [1/M

R]0.854

--- (3)

recommended for 90% reliability

(33)

Rutting model also established and calibrated with the R-56 studies using the pavement performance data collected during the R-6 and R-19 studies at 80% and 90% reliability levels.

Two equations are given below:

N = 4.1656 x 10 – 08 x [1/ͼ v] 4.5337 (80% reliability) --- (4)

RUTTING MODEL

N = 1.41 x 10 – 08 x [1/ͼ v] 4.5337 (90% reliability) --- (5)

N = Cumulative no. of standard axles to produce rutting of 20 mm

(34)

Subgrade/Stabilised Subgrade Granular Sub-base (GSB) = 230 mm Wet Mix Macadam (WMM) = 250 mm Dense Bituminous Macadam (DBM) =

140 mm Bituminous concrete = 50 mm 90 mm 50 mm 150 mm 100 mm

For BC or SDBC = in no case a single layer thickness should be less than 25

mm & not more than 100 mm

For DBM or DGBM = in no case a single layer thickness should be less than 50 mm & not more than 100 mm

For WMM = in no case a single layer thickness should be less than 75 mm & not more than 200 mm

For GSB = in no case a single layer thickness should be less than 100 mm & not more than 225 mm

130 mm 100 mm

(35)
(36)

SELECTION OF SUBGRADE CBR FOR PAVEMENT DESIGN

The CBR values of the subgrade soil varies along a highway

alignment even on a homogenous section. 90th percentile

CBR is recommended in the guidelines.

Method of determination of the 90th percentile is shown below

Say Sixteen CBR values have been obtained from different chainages of the road section.

3.

(37)

SELECTION OF SUBGRADE CBR FOR PAVEMENT DESIGN

3.

5 5.2 8.0 6.8 8.8 4.2 6.4 4.6 9.0 5.7 8.4 8.2 7.3 8.6 8.9 7.6

Arrange the above 16 values in ascending order 3.

5 4.2 4.6 5.2 5.7 6.4 6.8 7.3 7.6 8.0 8.2 8.4 8.6 8.8 8.9 9.0

Now, Calculate the percentage greater than equal to each of the values:

For CBR value of 3.5, % of values greater than equal to 3.5 = 16/16 * 100 = 100 For CBR value of 4.2, % of values greater than equal to 4.2 = 15/16 * 100 = 93.75

(38)

SELECTION OF SUBGRADE CBR FOR PAVEMENT DESIGN

3.

5 4.2 4.6 5.2 5.7 6.4 6.8 7.3 7.6 8.0 8.2 8.4 8.6 8.8 8.9 9.0

For CBR value of 4.6, % of values greater than equal to 4.6 = 14/16 * 100 = 87.5 For CBR value of 5.2, % of values greater than equal to 5.2 = 13/16 * 100 = 81.25

Similarly for :

For CBR value of 5.7, % of values greater than equal to 5.7 = 12/16 * 100 = 75.00

For CBR value of 6.4, % of values greater than equal to 6.4 = 11/16 * 100 = 68.75

For CBR value of 6.4, % of values greater than equal to 6.4 = 10/16 * 100 = 62.50

(39)

Now a plot is made between percentages of values greater than equal and the CBR values versus the CBR as follows

The 90th percentile CBR value = 4.7, and 80th percentile CBR =

5.7.

According to the Asphalt Institute, USA, 87.5 percentile

subgrade modulus is recommended for design traffic greater than one msa

(40)

PAVEMENT DESIGN CATALOGUES

FOR

BITUMINOUS SURFACING WITH GRANULAR BASE & GRANULAR SUB-BASE

Five different combinations of traffic and material

properties have been considered for which

pavement composition has been suggested in the

form of design charts presented in plates

The five combinations are as follows:

1. Granular Base and Granular Subbase (Plate 1 to

8)

2. Cementitious Base and Cementitious Subbase

with aggregate interlayer for crack relief. Upper

100 mm of the cementitious subbase is the

drainage layer (Plate 9 to 12)

(41)

3. Cementitious base and subbase with SAMI at the

interface of base and the bituminous layer (Plate

13 to 16)

4. Foamed bitumen/bitumen emulsion treated RAP

or fresh aggregates over 250 mm cementitious

subbase (Plate 17 to 20)

5. Cementitious base and granular subbase with

crack relief layer of aggregate layer above the

cementitious base (Plate 21 to 24)

(42)

Treated as single granula r layer Sem i-infinite subgra de

CROSS SECTION OF BITUMINOUS PAVEMENT WITH GRANULAR BASE AND GRANULAR SUB-BASE

(43)
(44)
(45)
(46)
(47)
(48)

Note:

1. These charts are to be used for traffic of 2 msa and

above. For traffic below 2 msa, refer IRC SP

72-2007. City roads should be designed for minimum 2

msa traffic.

2. Thickness design for traffic between 2 and 30 msa

is exactly as per IRC 37-2001

3. In all cases of cementitious sub-bases, the top 100

mm thickness of sub-base is to be porous and act

as drainage layer

(49)

It is considered as a three layer elastic structure

consisting of bituminous surfacing, granular base

and subbase and the subgrade

The granular layers are treated as a single layer

Strain and stresses are required only for three

layer elastic system

The critical locations are shown in the Fig. above

For traffic > 30 msa, VG 40 bitumen is

recommended for BC as well as DBM for plains in

India

Thickness combinations up to 30 msa are the

same as those adopted in IRC:37-2001.

(50)
(51)

BITUMINOUS PAVEMENTS WITH CEMENTED

BASE AND CEMENTED SUBBASE WITH CRACK

RELIEF INTERLAYER OF AGGREGATE

Fig. shows a five layer elastic structure consisting of bituminous surfacing, aggregate interlayer layer, cemented base, cemented subbase and the subgrade

(52)

Important points:

Material properties such as modulus and poission’s ratio

are the input parameters apart from loads and geometry of

the pavement for the IITPAVE software.

For traffic > 30 msa, VG 40 bitumen is used for preventing

rutting

DBM has air void of 3% after rolling (bitumen content is 0.5

% to 0.6% higher than the optimum)

Cracking of cemented base is taken as the design life of a

pavement

(53)

For traffic > 30 msa, minimum thickness of bituminous

layer consisting of DBM and BC layers is taken as 100

mm (AASTHO-1993) even though the thickness

requirement may be less from structural consideration

Residual life of the bituminous layer against fatigue

cracking is not considered since it cracks faster after

the fracture of the cemented base.

Allowable horizontal tensile strain in bituminous layer is

153 x 10

-6

for VG 40 mixes whereas as per IRC: 37 2001

this value is 178 x 10

-6

(eqn – 1) for a mix with VG 30

Allowable vertical compressive strain on subrade is 291

x 10

-6

whereas as per IRC: 37-2001 this value is 370 x

10

-6

(eqn.4).

Allowable tensile strain in cementitious layer is 64.77 x

10

-6

(54)
(55)
(56)

Illustration

For traffic 150 msa Subgrade CBR = 10 %

Since CBR value is > 5% therefore, use eqn MR= 17.6 x (CBR)0.64

MR subgrade = 17.6 x (10) 0.64 = 75 Mpa

Pavement composition for 90% reliability is DBM + BC = 100 mm

Aggregate inter layer = 100 mm (MR = 450 MPa) Cemented base = 110 mm (E= 5000 MPa)

(57)

Design criteria adopted

Corresponding to the values of Va & Vb as stated above, introduction of “C” 1. Nf = 0.5161 x C x 10-04 x [ 1/ t]ͼ 3.89 x [1/M R]0.854

2. N = 1.41 x 10

– 08

x [1/ͼ

v

]

4.5337

(90%

reliability)

3. N = RF

(11300/E 0.0804 + 191) ͼt 12

(58)

Design criteria adopted

N = RF

(11300/E

0.0804 + 191)

ͼt

12

RF = Reliability factor for cementitious materials for failure against fatigue

= 1 for expressways, NHs & other heavy vol roads = 2 for others carrying less than 1500 trucks per day N= Fatigue life of the cementitious material

E= Elastic modulus of cementitious material t = tensile strain in the cementitious layer, microstrain ͼ

(59)

CEMENTED BASE AND CEMENTED

SUBBASE WITH SAMI AT THE INTERFACE

OF CEMENTED BASE AND THE

BITUMINOUS LAYER

(60)

Fig shows a four layer pavement consisting of bituminous

surfacing, cemented base, cemented subbase and the subgrade Upper 100 mm of the cemented subbase having the gradation 4 shown in Table below is provided over the cemented lower

subbase

For the given composition of pavement thicknesses, 90% reliability is adopted

(61)

The reduction in thickness of the cemented base increases the bending stresses considerably because it is inversely proportional to the square of the thickness. Hence, design should be checked against fatigue damage.

(62)

Design criteria adopted

Corresponding to the values of Va & Vb as stated above, introduction of “C” 1. Nf = 0.5161 x C x 10-04 x [ 1/ t]ͼ 3.89 x [1/M R]0.854

2. N = 1.41 x 10

– 08

x [1/ͼ

v

]

4.5337

(90%

reliability)

3. N = RF

(11300/E 0.0804 + 191) ͼt 12

(63)
(64)
(65)
(66)

FOAMED BITUMEN/BITUMEN EMULSION

TREATED RAP/ AGGREGATE OVER CEMENTED

SUBBASE

(67)

Fig shows four layer pavement consisting of bituminous surfacing recycled layer Reclaimed asphalt pavement, cemented subbase and the subgrade

(68)
(69)
(70)

Illustration:

Traffic 150 msa

Subgrade CBR = 10%, E subgrade = 17.6 (CBR).64 = 75 Mpa MR = 3000 Mpa, MR of RAP = 600 MPa, E of cemented

subbase = 600 MPa

From the plate shown above,

BC+DBM = 100, RAP = 160, Cemented subbase = 250 mm

(71)

Design criteria adopted

Corresponding to the values of Va & Vb as stated above, introduction of “C” 1. Nf = 0.5161 x C x 10-04 x [ 1/ t]ͼ 3.89 x [1/M R]0.854

2. N = 1.41 x 10

– 08

x [1/ͼ

v

]

4.5337

(90%

reliability)

(72)

CEMENTED BASE AND GRANULAR SUBBASE

WITH CRACK RELIEF LAYER OF AGGREGATE

(73)

Critical location for vertical

subgrade strain

(74)

For reconstruction of a highway , designers may

have a choice of bituminous surface, aggregate

interlayer, cemented base while retaining the

existing granular subbase.

The drainage layer in GSB is required to be restored

in area where rainfall may damage the pavements

Using IITPAVE, it has been modelled as five layer

elastic structure

The aggregate interlayer acting as a crack

relief should meet the specifications of Wet

Mix Macadam and if required, it may contain

about 1 to 2 % bitumen emulsion if the

surface of the granular layer is likely to be

disturbed by construction traffic

Emulsion can be mixed with water to make the

fluid equal to optimum water content and

added to the WMM during the processing.

(75)
(76)
(77)
(78)

Design criteria adopted

Corresponding to the values of Va & Vb as stated above, introduction of “C” 1. Nf = 0.5161 x C x 10-04 x [ 1/ t]ͼ 3.89 x [1/M R]0.854

2. N = 1.41 x 10

– 08

x [1/ͼ

v

]

4.5337

(90%

reliability)

3. N = RF

(11300/E 0.0804 + 191) ͼt 12

(79)
(80)
(81)

PERPETUAL PAVEMENT

The pavement having a life of 50 years or

longer is termed as a perpetual pavement.

If the tensile strain caused by the traffic in the

bituminous layer is less than 70 micro strains,

the endurance limit of the material, the

bituminous layer never cracks (Asphalt Institute,

MS-4, 7

th

Edition 2007).

Similarly, if vertical subgrade strain is less than

200 micro strains, there will be little rutting in

subgrade.

In such pavement design concept, different

layers are so designed and constructed that

only the surface layer is the sacrificial layer

which is to be scrapped and replaced with a

new layer from time to time.

References

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