a critical review of bituminous paving mixes used in india

7/30/2019 A Critical Review of Bituminous Paving Mixes Used in India

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1. INTRODUCTION

The bituminous paving mixes as specified in MORTH

Specifications for Road and Bridge Works, Fourth

Revision, 200128 are commonly used in India. Some of

these mixes have evolved since 1960s, an era when the

present day hot mix asphalt plants were not common and

mixes were produced with small portable mixing plants

with limited aggregate heating, blending and mixing

capabilities. The proliferation of bituminous paving mixes

as specified in the MORTH publication basically manifest

the constraints of non-availability of modern hot-mix plant

besides likely cost reduction of lean bituminous mixes.Today, the scenario has substantially changed. There is a

growing concern to construct long lasting pavements with

minimum maintenance. It is accordingly, felt that only those

specifications for bituminous mixes be allowed, which are

durable, irrespective of their location (NH or SH or Rural

Road) because durability and long term performance is

central to all of them.

A critical review of commonly used bituminous

paving mixes in India has been accordingly attempted

keeping in perspective the following: (a) fundamentals

of mix selection based on their intended function and

location within the flexible pavement structure, and (b)

capabilities of the present day hot mix asphalt plants.

The gradations of some of the mixes also need to be

updated based on proven field performance of similar

mixes in the test tracks and in regions of developed

countries like USA with climate reasonably close to

that of India. This review is also expected to be helpful

to the contractors who are currently designing their

own projects (including flexible mix selection) under

the Public-Private-Partnership (PPP) projects. Thefuture trend of mix design improvements should finally

aim to achieve long-lasting perpetual pavements.

2. FUNDAMENTALS OF MIX SELECTION BASED

ON THEIR INTENDED FUNCTION AND LOCATION

WITHIN PAVEMENT STRUCTURE

Bituminous mixes are used in a flexible pavement to

serve the following three important functions:

* Associate Director Emeritus, National Center for Asphalt Technology (NCAT), Auburn University, USA. E-mail : [email protected]

* * Additional Director General, Ministry of Shipping, Road Transport & Highways, New Delhi.

** * Professor, Civil Engineering Department, Indian Institute of Technology, Madras, Chennai - 600 036. Written comments on this Paper are invited and will be received upto 15th November, 2008.

Paper No. 541

A CRITICAL REVIEW OF BITUMINOUS PAVING MIXESUSED IN INDIA

PRITHVI SINGH KANDHAL*, V. K. SINHA** & A. VEERARAGAVAN***

ABSTRACT

A critical review of bituminous paving mixes used in India in accordance with the current MORTH Specifications (2001)

has been made keeping in perspective the fundamentals of mix selection based on their intended functions in different

courses within the flexible pavement.

There is a proliferation of bituminous paving mixes in India. MORTH Specifications broadly provides 4 mixes for base

courses, 6 mixes for binder courses and 4 mixes for wearing courses. Further two grading, each of BM, DBM, SDBC and BC

are specified in the MORTH specifications. Too many options for a specific bituminous course have created confusion in mix

selection and are mainly responsible to a considerable extent for the poor performance of flexible pavements in India. A

case has been made out on technical grounds to have only 5 dense graded mixes of different nominal maximum aggregate

size (NMAS) in the specifications, as is the case in most developed countries of the world. The following 5 dense graded

mixes have been accordingly proposed along with their recommended rut resistant gradations:

37.5 mm NMAS DBM Base Course Grading 1 25 mm NMAS DBM Base Course Grading 2 19 mm NMAS BC Binder Course 12.5 mm NMAS BC Wearing Course Grading 1 (for heavy traffic) 9.5 mm NMAS BC Wearing Course Grading 2 (for light to medium traffic, urban areas and thin application)


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114 KANDHAL, SINHA & VEERARAGAVANON

Provide structural strength

Facilitate subsurface drainage

Provide surface friction especially when wet

2.1. Provide Structural Strength

Fig. 1(a) presents a typical cross-section of flexible

pavement in a developed country like USA. The

structural bituminous courses can consist of bituminous

binder course and bituminous surface or wearing course

as shown in Fig. 1(a). Providing structural strength is

the primary purpose of most bituminous mixes except

those used in very thin surfacing. The objective is to

disperse appropriately the dynamic and static effects

of traffic wheel loads to the underlying pavement layerssuch as bituminous/crushed stone base course. For low-

volume roads only a granular base and a bituminous

wearing course may suffice based on structural

requirements. Normally, lower layers of base courses

including those of bituminous base courses (as provided

in developed countries) should have desired stiffness

characteristics to act as good foundation, which should

be effective in dispersing the traffic loads to the lower

layers. Upper layers of bituminous binder course have

the requirement of being effective in re-bounding against

the dynamic effect of traffic load. In other words, thetop layers of bituminous binder courses should have

adequate stiffness to resist rutting coupled with the

flexibility to be effective in re-bounding. The flexibility

characteristics should, therefore, increase when going

from bottom to upwards layer.

From the perusal of the current literature and

practices, it is observed that the preceding requirements

can be fulfilled by using continuously dense-gradedbituminous mixes with nominal maximum aggregate size

(NMAS) decreasing from base course through binder

course to surface or wearing course30. The nominal

maximum aggregate size is defined as one sieve larger

than the first sieve to retain more than ten per cent of

combined aggregate31.

Base course mixes, which use relatively larger size

aggregate, are not only stiff/stable but also are economical

because they use relatively lower bitumen contents and

need less breaking and crushing energy/effort. Surface or

wearing course mixes with smaller aggregate on the otherhand have relatively higher bitumen contents, which not

only impart high flexibility but also increase their durability.

The binder (intermediate) course mix serves as a transition

between the base course and wearing course. Several

studies3,30,34 have shown that permanent deformation

(rutting) within flexible pavement is usually confined to the

top 100 to 150 mm of the pavement. This means both the

binder and wearing course mixes should be designed to be

resistant to rutting. That is why in extreme cases of heavy

traffic loads and high tyre pressures, it is considered prudent

to use Stone Matrix Asphalt (SMA) mix in which due tostone-on-stone contact the load is carried directly by the

coarse aggregate skeleton. An idealized cross-section for

a heavy-duty flexible pavement is shown in Fig.1(b).

Recently, the Indian Roads Congress (IRC) has adopted a

tentative SMA specification, (IRC SP 79:2008)which could

be used under such circumstances. Kandhal13 has published

a manual on design and construction of SMA mixes, which

is widely used in the USA.

Fig. 1. (a) Typical cross-section of flexible pavement in USA Fig. 1. (b) Idealized cross-section of flexible pavementusing SMA

SUBGRADE


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HIGHLIGHTSOFTHE 178THCOUNCILMEETING 115A CRITICAL REVIEWOF BITUMINOUS PAVING MIXESUSEDIN INDIA

2.2. Facilitate Subsurface Drainage

Typically, granular sub-base in a flexible pavement is

intended to provide subsurface drainage. However, in manysituations where granular sub-bases contain high percentages

of fines (less than 75 micron size material) such layers are

found to be not very effective. In developed countries like

USA, Permeable Asphalt Treated Base (PATB) has been

used extensively on major highways to provide positive

subsurface drainage. PATB basically does not constitute a

conventional base course. It is considered as a separate

course exclusively for subsurface drainage. From cost

consideration, PATB is not recommended for most

highways in India where GSB alone should suffice.

However, it is felt that a specification similar to those ofPATB should be available for use in exceptional drainage

problem situations. The lift or layer thickness of the PATB

generally ranges from 75 to 100 mm.

PATB is provided between the granular sub-base

(GSB) and the bituminous base course. Figures 2(a) and

2(b) shows two typical provisions of PATB. The concept

is to provide a two-layer drainage system. It is on the

presumption that water accumulated in the sub-base

always seeks least path of flow and some part of it flows

into the PATB rather than traveling altogether a long

distance to the edge of the sub-base. Water collected in

the PATB is then drained out in two ways. The PATB

can either be connected to a subsurface pavement edge

drain as shown in Fig. 2(a) or it can be extended all the

way to the edge of the embankment or daylighted as

shown in Fig. 2(b). This two-layer subsurface drainage

system is very effective in quickly removing water, which

may enter the pavement by any manner.

It is of paramount importance to ensure that the

PATB layer is not blocked in any way otherwise it will

be saturated with water and thus create a bath tub

scenario in the pavement. This phenomenon will not only

induce stripping21,22,24within the PATB (due to pore water

pressure buildup under traffic) but also in the bituminous

courses overlying it. Premature failures of pavements

from this phenomenon have been experienced in

California. In India, open-graded permeable Bituminous

Macadam (BM) is used as binder/base course in

pavements without any positive outlet for water, which

can result in such failures. This will be discussed laterwhen the BM mix is reviewed.

2.3. Provide Surface Friction

Mixes like bituminous concrete, semi-bituminous

concrete, besides premix carpet, mix-seal surfacing is

commonly provided as per MORTH Specification as

wearing course. Some bituminous wearing course mixes

are designed to provide high surface friction especially

when the pavement is wet. Open-Graded Friction

Courses (OGFC) usually 20-25 mm thick (Figure 1b)

are designed as an open graded mix with interconnectedvoids that provide drainage during heavy rainfall. OGFC

reportedly provides the following advantages12:

Reduce splash and spray

Reduce tyre-pavement noise14

Enhance visibility of pavement markings, and

Reduce night time surface glare in wet weather

It is important that the bituminous layer underneath

the OGFC is very dense, impermeable, and highly

resistant to stripping. The design, construction, and

maintenance of OGFC is fully developed and welldocumented by Kandhal in manual and papers12,18,27.

Fig. 2. (a) Permeable Asphalt Treated Base (PATB) connectedto pavement edge drain

Fig. 2. (b) Permeable Asphalt Treated Base (PATB)

daylighted


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The OGFC has been known to induce stripping in the

underlying bituminous layer15,22. OGFC basically is a

specification for cold country having skid problem. Itinvolves additional cost and hence may be used only on

highways, which are accident-prone during rains. This

course should be constructed with proper anti stripping

agents like hydrated lime.

If an existing pavement with OGFC needs to be

overlaid, it is necessary to first remove (mill off) the

OGFC and also examine the underlying layer for potential

moisture damage (stripping). Open graded wearing

courses like premix carpet, surface dressing and to some

extent semi-dense bituminous concrete falls under the

above category. In case of overlay these wearing courselayer should be totally removed, which is often not done

in India. Experience in the US has shown that if an

existing OGFC layer is not removed it will trap water

and cause premature moisture damage by stripping,

particularly when the underlying layer is a dense mix.

Photo 1 and 2 exhibits such failures in Australia and

Oklahoma (USA)15.

3. PERPETUAL PAVEMENTS AN EMERGING

CONCEPT

The concept of Perpetual Pavement35 was firstlaunched by the Asphalt Pavement Alliance (APA) in a

joint promotional effort with Asphalt Institute, National

Asphalt Pavement Association, and the State Asphalt

Pavement Associations of USA in 2003-04. Fig. 3 depicts

the perpetual design concept.

Photo 2. Premature OGFC related distress on ahighway in Oklahoma, US (Ref. 15)

Photo 1. Premature OGFC related distress on a

highway west of Sydney, Australia (Ref. 15)

Fig. 3. Perpetual pavement design concept (HMA = hot-mix

asphalt)

Source : US Department of Transportation FHWA

From the perusal of the Fig. 3, it will be observedthat the bituminous portion is divided into three zones

and bitumen mixes to be adopted for these three zones

are characteristically different. The first zone is wearing

course 40-75 mm thick. It could be high quality HMA or

OGFC. The succeeding zone/layer is high compression

zone with high modulus rut resistant mix 100-200 mm

thick. The third layer is to cater maximum tensile strain

and should be able to resist flexible fatigue. The design

of the bituminous layers is done on mechanistic principles

by keeping the strain within each layer less than

endurance limit. Thus no damage accumulation takes

place in any layer and the pavement layer constructed,

normally do not need any replacement/rehabilitation. It

is only the top layer which is required to be replaced in

case of any renewal/strengthening.

Figure 4 shows how high asphalt content improves

fatigue resistance. This proves why international

specifications are opting for dense graded mixes with

more bitumen content rather than open graded mixes

with less bitumen. This emerging concept has already

been followed, at least in principle, at many locations in

USA. For example, on I-710 (the Long Beach Freeway)in Los-Angeles County a mix comprising of 25 mm


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OGFC + 75 mm dense-graded HMA + 150 mm asphalt

has been used on the design concept of perpetual

pavement (Monismith and Long, 1999). The perpetualpavements last long, provided they are built on a solid

foundation. It is observed that rutting on such roads built

on sub-grade with the CBR greater than 5 per cent

originates mostly in the HMA layer. This suggests that a

sub-grade with a CBR greater than 5 per cent (MR

greater than 50 MPa) is considered adequate. Proper

construction techniques and quality control are essential

for perpetual pavements like any other pavement.

The concept of perpetual pavements is briefly

introduced to emphasize the current trend of

development in respect of specifications of bituminous

mixes. This also highlights the required change in

mindset of Indian engineers to review the existing

specifications of bituminous mixes from the point of

view of long-lasting pavements rather than on the

consideration of cost and conveniences. The subsequent

chapter describes the current specifications followed

in many developed countries. It is felt that even these

specifications in those countries might go further

changes consistent with the perpetual pavement

concepts or similar concepts emerging in future. Thepoint of concern is that long-lasting pavements going

to be future concern in India. The advantages of such

specifications lie not only in long life but also in the

reduced cost of travel with better serviceability

conditions. Figure 7. shows one such comparison.

Fig. 4. Improve fatigue resistance with high asphaltcontent mixes

Source: Idaho Project Development Conference 2007

Fig. 5 shows how the design concepts of the

perpetual pavements minimizes the tensile strain with

pavement thickness. Figure 6. shows how the design

concept of perpetual pavements deals with high

temperature encountered during summer in case of most

of our pavements in DBM layers.

Fig. 5. Minimize Tensile Strain with pavement thicknessSource: Idaho Project Development Conference 2007

Fig. 6. Impact of Temperature Gradient on Asphalt Grade


Fig. 7. Pavement serviceability comparison


4. EXPERIENCE IN DEVELOPED COUNTRIES (USA)

Most specifications for dense graded bituminous

mixes in the USA and other countries are intendedfor four pavement courses, namely, base course,

Years


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binder course, and two surface (wearing) courses (one

coarse and one fine). Table 1 shows four such

Marshall mixes used by the Georgia Department ofTransportation (GDOT) in the recent past, before

changing to Superpave designed mixes6. The table

shows nominal maximum aggregate size (NMAS),

minimum and maximum layer (lift) thickness, and

gradations of the four mixes. Unlike other highway

agencies, a narrow range of material passing the 2.36

mm sieve is specified by GDOT for developing the

job mix formula. The second sieve size has been

changed from 85-100 to 90-100 per cent in this table

to suit the specified NMAS of the mix.

GDOT has held the general reputation ofconstructing one of the best and most durable flexible

pavements in the US for several years. This time period

encompasses the use of both Marshall and Superpave

mixes in Georgia. Eleven projects consisting of Marshall

mixes were evaluated in Georgia recently35. The

average rut depth after about 6 years of heavy traffic

was determined to be 1.5 mm only. Georgias

experience should be of interest to India because

Georgia being in the southeastern US has climate similar

to north India. Air temperature up to 44.50 C has been

recorded in Georgia. That is why in the past, Georgia

used AC-30 viscosity grade bitumen, which is equivalent

to VG-30 grade (50-70 penetration) used in India.Georgias experience is, therefore, quite relevant in

reviewing and revising Indian dense graded mix

specifications in terms of NMAS, gradation and layer

(lift) thickness.

As shown in Table 1, the base course consists of a

25 mm NMAS mix; the binder course consists of a 19

mm NMAS mix; and the surface (or wearing) course

consists of 12.5 mm or 9.5 mm NMAS mix. The 12.5

mm wearing course mix is used for heavy traffic roads,

whereas the 9.5 mm wearing course mix is used for low

to medium traffic roads, in urban areas, and in thin (25mm) applications. Any of the four dense graded mixes

can be used for leveling or profile corrective course

(PCC) depending upon the required thickness. Only

dense graded mixes are used in the US in PCC30.

The Georgia DOT and some other state DOTs in

the US attempted to use 37.5 mm NMAS mix for base

course, which was only marginally more stable than the

25 mm mix, but had the following disadvantages:

The 37.5 mm NMAS mix was found very prone

Mix Type Base Course Binder Course Wearing Course Wearing Course

Lift Thickness 75-125 mm 45-75 mm 35-60 mm 25-50 mm

NMAS 25 mm 19 mm 12.5 mm 9.5 mm

Sieve Size, mm Per cent Passing

37.5 100 - - -

25 90-100 100 - -

19 - 90-100 100 -

12.5 60-80 - 90-100 100

9.5 - 55-75 70-85 90-100

4.75 - - - 55-75

2.36 32-40 30-36 44-48 44-50

1.18 - - - -

0.600 - - - -

0.300 11-19 11-19 10-25 14-25

0.150 - - - -

0.075 4-7 4-7 4-7 4-7

Bitumen Content 4.0 5.5 4.0 - 5.5 5.0 - 7.0 5.2 - 7.5

TABLE 1. GRADATIONOF GEORGIA DOT MARSHALL MIXES (REF. 6)


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to segregation resulting in honeycombing

(Photo 3).

The 37.5 mm NMAS mix becomes permeableat relatively lower air void content levels as

compared to the 25 mm NMAS mix, thereby

increasing the potential for water/moisture

related damage. Fig. 8 clearly shows the effect

of NMAS on field permeability2,26. As the

NMAS increases, the permeability also

increases multifold at a given void level. For

example, at an in-place air void content of 6

percent, the following permeability values were

measured for each NMAS.

9.5 mm NMAS 6x10-5 cm/sec




The preceding data26 clearly shows larger 37.5

mm NMAS will be highly permeable since the

permeability increases multifold from one

NMAS to the next higher one.

Modified Marshall method developed byKandhal20,23,25 and referred to in Asphalt

Institute MS-21, which uses 6-inch diameter

mould needs to be used for designing and testing

37.5 mm mix.

Photo 3. Segregation of 37.5 mm NMAS mix resulting inhoneycombing

Some engineers in India including 2nd author believe

that the use of 37.5 mm mix should be considered for

base course in India because of overloading problem. It

is felt that this gets well validated by the observation

made before that in case of heavy traffic, stone mastic

asphalt is the correct specification as per international

practice. The axle loads in India are quite heavy and

further the speed is low with many stop/start condition,

the rutting of bituminous mixes like DBM in India is quitecommon32 . It is, therefore, felt that issue of providing

DBM with NMAS of 37.5 mm be kept under

consideration pending further research.

5. REVIEW OF FLEXIBLE PAVING MIXES USED

IN INDIA

For the purpose of this paper a detailed review of

the following bituminous paving mixes specified in the

MORTH Specifications (2001) is undertaken.

a. Bituminous Macadam (BM)

b. Dense Bituminous Macadam (DBM)

c. Semi-Dense Bituminous Concrete (SDBC)

d. Bituminous Concrete (BC)

5.1. Bituminous Macadam (BM)

Bituminous Macadam (BM) is an open graded,

permeable, and recipe type mix produced without any

quality control on its volumetrics or strength (stability).

The primary problem with the BM mix is that being very

open graded, it is highly permeable and therefore will

trap moisture or water. BM and SDBC were developedseveral years ago, when conventional hot mix plants were

Fig. 8. Effect of nominal maximum aggregate size (NMAS)on permeability of in-place pavement (Ref. 26)


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not common. At that time, hot mixing was done in small

portable plants or concrete mixers in which much fine

aggregate could not be used due to limitations of theavailable heating and mixing equipment. Now, good hot

mix plants are normally available.

Fig. 9, 10 & 11 shows a typical cross-section of

flexible pavement as being used in India. Fig. 9 does not

have a BM layer and DBM is resting directly on WMM.

However, Figs. 10 and 11 show cross-sections where

BM has been used as a base, binder or profile corrective

course (PCC) with no outlet for water thus creating a

bath tub situation within the pavement.

Fig. 11. Flexible pavement with BM as a base/binder course

Fig. 10. Flexible pavement with BM as a base course or PCC

Fig. 9. Typical cross-section of flexible pavement in India

The fundamental question thus boils down to BM

versus DBM. Should BM be deleted and DBM used

instead in all cases? To answer that question BM andDBM should be compared both from the engineering

aspect (primary) and economical aspect (secondary).

This has been done considering the following factors:

5.1.1. Permeability : It has been acknowledged in

many IRC and MORTH publications that BM is a much

more open mix compared to the DBM. The MORTH

Manual for Construction and Supervision of Bituminous

Works29 states on page 52, Because of the open-graded

aggregate matrix, the voids content (in the BM) can be

as high as 20-25 per cent. Some researchers have

reported air voids of about 10 per cent in lab compactedBM specimens. This range of 10-25 per cent air voids

can occur because BM has two gradings and within

each grading BM can be relatively coarse or fine

considering the combination of lower and upper values

for each sieve. Table 2 gives air voids and permeability

data obtained recently on the BM mix8. In this case four

BM gradations were used: Grading 1 (both coarse and

fine) and Grading 2 (both coarse and fine). The test

data was obtained on 150-mm diameter specimens

compacted with 75 blows (equivalent of 50 blows on

100-mm diameter specimens). The air void contentranges from 8.3 to 15.4 per cent. The test data on

Gradings 1 and 2 are comparable because both gradings

have about the same amount of material passing 4.75

mm sieve. Photos 4 and 5 show the open texture of BM

specimens Grading 1 and Grading 2, respectively. When

these specimens were placed under a water tap, the

water readily passed through indicating very high

permeability.

Even if the scenario of about 10 per cent air voids

in the BM in the lab is considered, the voids in the field

can be as much as 15 per cent (at least 95 per centcompaction of the lab density is usually required).

According to numerous studies all over the world, dense

graded bituminous mixes become permeable when air

voids are more than 7-8 per cent. BM type open graded

mix, which has a large number of interconnected voids,

becomes permeable at relatively lower air voids, i.e.

for air voids more than 5-7 per cent . So there cannot

be any argument about the fact that the BM is a highly

permeable mix compared to the DBM. It has been said,

three things are important in highway construction

drainage, drainage, and drainage. No permeable asphaltlayer is desirable within the pavement structure (unless


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The first author has investigated and reported15,24

many real-life field case histories of premature

pavement failures from across the world. In a majority

of cases, bituminous layers, which trapped water, were

the real culprits. There was stripping of bitumen in thepermeable layer as well as in the adjacent layers

overlying or underlying it due to traffic action. Photos

6 and 7 show a failure in Oklahoma, US, which was

investigated by the first author15. On this project an

open type binder course was used. It was saturated

with water since there were no subsurface edge drains

at the edge of the pavement. This led to stripping in the

binder course under traffic. Note that majority of the

potholes appeared near the pavement edge where

water accumulated and did not have any positive outlet.

Potholes are often found more in number in the rightlane, which carried heavy truck traffic. It is, therefore,

felt that BM layer without an outlet should not be used

for long term pavement performance.

5.1.2. Structural strength: Many highway

agencies across the world give structural value to a

BM type mix (used for drainage) of 50 per cent of

dense graded DBM type mix. IRC Publications 37

and 81 on flexible pavement design state that 7 mm

of DBM is equal to 10 mm of BM. In either case, the

DBM is far superior to the BM in terms of structural

strength and fatigue life. Some engineers are

suggesting using polymer-modified bitumen (PMB) in

the BM to increase its structural strength. If that is

the objective, why not simply use the stiffer DBM in

the first place. First of all, hardly any agency in the

world uses PMB in a base course mix. Moreover,

using PMB in an un-designed, recipe type BM mix,

which unlike the DBM has hardly any quality control

criteria at the design or mixing stage, is not justified.

Therefore, DBM is by far superior to the BM in terms

of structural strength, mix design criteria, and mix

production control.

TABLE 2. AIR VOIDSAND PERMEABILITY TEST DATAFOR FLEXIBLEMACADAM (REF. 8)

Mix Type Bitumen Content, % Air Voids, % Permeability, cm/sec

BM Grading 1 (Coarse) 3.25 13.6 3.4

BM Grading 1 (Fine) 3.25 8.9 0.4

BM Grading 2 (Coarse) 3.4 15.4 3.6

BM Grading 2 (Fine) 3.4 8.3 0.6

Photo 5. Open surface texture of BM Grading 2

Photo 4. Open surface texture of BM Grading 1

it is specifically for drainage with proper outlets)

whether it is a PCC, base course, binder course or

whatever. If this fundamental requirement is

disregarded, the potential for premature pavement

distress is increased. A permeable layer always attracts

and traps water, moisture or moisture vapour. Water

can come from the top, from the sides, or from the

non-bituminous courses underneath22. If there is a

premix carpet (which is highly permeable) right over

the BM, rainwater will have direct access to the BMand can cause havoc.


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5.1.3. Use as a PCC : It has been surmised that

BM is a good material for profile corrective course (PCC)

because it resists reflection cracking. No other country is

using a permeable, water-trapping type mix for PCC. Only

dense graded mixes such as DBM or BC are used for

transverse or longitudinal profile correction in other

countries30 in courses called leveling courses or wedge

courses, which are same as Indias PCC. The reasons

for using dense mixes are: to stay away from water-

trapping permeable mixes and also to facilitate easy

feathering of the mix from a specified depth to almost

zero in a wedge type PCC.

It is normally argued/believed that BM has a betterresistance to reflection cracking and accordingly, many

pavement designers introduced a layer of BM between

DBM and WMM in India. It does not appear that this

conclusion is based on any research. Assuming that

BM has a better resistance to reflection cracking, it is

still a water-trapping permeable mix and as discussed

earlier, the potential for failure by far exceeds the

perceived advantage of resisting reflection cracking. It

is better to seal the cracks or to remove the top cracks

by milling/scraping or prevent their upward movement

by use of geo-synthetics, rather than over trust the BMto prevent reflection cracking at the cost of stripping.

5.1.4. Cost considerations : The use of BM is

quite often made on the premise that BM is cheaper

than DBM and, therefore, it is suitable for use in

developing country like India. That is not correct. The

following cost analysis of BM versus DBM was

conducted at IIT, Madras in response to the revised draft

BM specification published in Indian Highways for

comments5.

According to the current prevailing schedule of ratesin his area, the cost of BM layer is Rs. 3,465/cu.m and

the cost of DBM is Rs. 4,193/cu.m. Consider that

100 mm of BM is required for a pavement and it is equal

to 50 mm of DBM as per equivalency used in some

other countries. The cost of a 2-lane highway (10 m

wide including shoulders) with BM will cost Rs. 34.65

lakhs/2-lane-km and the cost of the same highway with

DBM will cost Rs. 20.97 lakhs/2-lane-km. Now, that is

a saving of Rs. 13.7 lakhs per km or 39.5 per cent.

Then, assume that 100 mm of BM is equal to 70 mm of

DBM as per IRC guidelines. In that case, the cost of highwaywith BM is Rs. 34.65 lakhs/2-lane-km and the cost of highway

with DBM is Rs. 29.35 lakhs/2-lane-km. Thus using DBM

in lieu of BM will reduce the cost by Rs. 5.3 lakhs per km,

which amounts to a saving of about 15 per cent.

Comparative cost analysis of BM and DBM has also

been done based on the current 2007 schedule of rates of

BM and DBM obtained from the Rajasthan PWD Circle

in Jaipur. The cost of BM in place is Rs 1,404 per ton and

the cost of DBM in place is Rs. 1,588 per ton. Considering

that 100 mm of BM is equal to 70 mm of DBM as per

IRC guidelines, the actual cost of DBM in place comes

Photo 6. Road failure in Oklahoma, US resulting

from saturated binder course (Ref. 15)

Photo 7. Close up of stripped binder course in

Oklahoma, US (Ref. 15)


11/20


out to be Rs. 1,112 per ton. That is a saving of Rs. 292 per

ton or 21 per cent, when DBM is used in lieu of BM.

The preceding cost analyses have clearly establishedthat DBM is much cheaper than the BM on equivalency

cost basis.

5.1.5. Traffic conditions: According to some

highway engineers, BM is intended for low-traffic roads

only, although it is being used extensively on national

highways and state highways. However, the fact remains

that any layer, which traps water, should not be used whether

it is a low-volume or high volume road. The concept of

perpetual pavement or long term performing pavements is

relevant even for less traffic roads. The need today is to

construct pavements needing less maintenance avoiding

frequent overlays, besides providing a good riding surface

for many years. The design concept should, therefore, be

same for both heavy traffic and low traffic. In any case,

the thickness of different bituminous layers will be different

depending upon the traffic intensity. As discussed earlier,

water is the enemy of the road. The only tangible argument

against the use of DBM could be non availability of proper

batch mix plants in required number gradually to produce

DBM on rural roads/state highways. It is felt that this should

be enforced at least to tone up the quality level of our

pavements.

5.1.6. General statements : General statements

are sometimes made like (a) BM is a popular mix or (b)

BM has been widely used with success throughout thecountry. The concept of success is myopic and it does not

envisage in concept of long-term performing pavements.

The normal life of pavement in India is between 2 to 4

years compared to 8 to 10 years in other countries. The

developed countries are talking of perpetual/long-term

pavements capable of performing for 50 years or more.

This may look strange but our vision should accordingly

extend to give precedence to durability over deceptive cost

saving. The concept of sound economics/engineering

suggest that we should accept changing the permeable

mixes by dense and relatively less permeable mixes to givelong life to our bituminous pavements.

5.2. Dense Bituminous Macadam (DBM)

At the present time the dense flexible macadam (DBM)

is specified for use as a base course and/or binder course.

Two gradations of the DBM are specified in Section 507 of

MORTH specifications: Grading 1 has a NMAS of

37.5 mm and Grading 2 has a NMAS of 25 mm.

Table 3 gives the existing MORTH composition of

DBM Gradings 1 and 2. The specified percentage of

fine aggregate is the same in both gradings (28-42 per

TABLE 3. EXISTING MORTH GRADATIONS FOR DENSE BITUMEN MACADAM (DBM) (REF. 28)

Grading 1 2

Lift Thickness 80-100 mm 50-75 mm

Nominal Aggregate Size 40 mm 25 mm

Sieve, mm Per cent Passing

45 100

37.5 95-100 100

26.5 63-93 90-10019 - 71-95

13.2 55-75 56-80

9.5 - -

4.75 38-54 38-54

2.36 28-42 28-42

1.18 - -

0.6 - -

0.3 7-21 7-21

0.15 - -

0.075 2-8 2-8Bitumen Content, % Min. 4.0 Min. 4.5


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cent), the main difference is just some large size

aggregate particles (25-45 mm size) are contained in

Grading 1. It was discussed earlier that the use of largestone mix (NMAS of 37.5 mm or larger) has several

disadvantages such as segregation (Photo 3) and high

permeability (Fig. 8). Although these disadvantages

outweigh the marginal gain in stability over a 25 mm

NMAS mix, some engineers would like to use it in India

due to overloading problem. Therefore, DBM Grading 1

has been retained. Since Grading 1 is highly permeable,

it should be sealed before rainy season otherwise water

will penetrate and damage the underlying WMM course.

This neglect is commonly observed during construction

of our roads. Overall, DBM Grading 2 with 25 mm

NMAS is best suited for a base course similar to GDOT

base course specification in Table 1.

The MORTH specification mentions a lift thickness

of 50-75 mm for Grading 2. It needs to be debated

whether it should be 50-100 mm as practiced in many

countries including the US30.

Table 4. makes a comparison of existing DBM

Grading 2 and GDOT gradation for base course

(extracted from Table 1) and proposes a new gradation

for 25 mm NMAS DBM Base Course to be used in

India. As mentioned earlier, Georgia has hot climate

similar to north India and has one of the best flexibleroads in the US. It is encouraging to note that the

existing DBM Grading 2 is reasonably similar to

GDOT Base Course. Therefore, the proposed DBM

Base Course Grading 2 has been kept the same as

existing DBM Grading 2 except for the percentage

of fines (material passing the 0.075 mm sieve). At

least 4 per cent fines are needed in the job mix formula

(JMF) to impart some stiffness to the bitumen-fines

mortar. Eight per cent fines are considered too

excessive. It should be noted that the range of 4 to 7

per cent is intended for the JMF design gradation.

Normal variation during production will be allowed. It

is also recommended to specify and use 25 mm sieve

in lieu of 26.5 mm sieve and 12.5 mm sieve in lieu of

13.2 mm sieve so that Indian standards generally

conform to world standards. It applies to all

gradations. The lift thickness for DBM Base Course

with NMAS of 25 mm has been revised to

75-100 mm. Similar changes have been made to

proposed DBM Grading 1 with NMAS of 37.5 mm

as reported later in Table 7.

TABLE 4. COMPARISONOF GRADATIONS: EXISTING MORTH DBM GRADING 2, GDOT BASE COURSEAND PROPOSED DBM BASE COURSE

Grading Existing DBM GDOT Base Course Proposed DBM Base

Grading 2 Course Grading 2

Lift Thickness 50-75 mm 75-125 mm 75-100 mm

Nominal Aggregate Size 25 mm 25 mm 25 mm

Per cent Passing

37.5 100 100 100

26.5 90-100 90-100 90-100

19 71-95 - 71-95

13.2 56-80 60-80 56-80

9.5 - - -

4.75 38-54 - 38-54

2.36 28-42 32-40 28-42

1.18 - - -

0.6 - - -

0.3 7-21 11-19 7-21

0.15 - - -

0.075 2-8 4-7 4-7

Bitumen Content, % Min. 4.5 4.0-5.5 4.0-5.5


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5.3. Semi-Dense Bituminous Concrete (SDBC)

There is no engineering logic in using a semi-dense

mix when only dense, continuously graded mixes aretechnically desirable. In most developed countries30

either dense mixes (HMA) are provided or OGFC is

provided as wearing course. Semi-dense mixes which

are neither dense graded nor open graded contain the

so-called pessimum voids when constructed. Terrel

and Shute32 advanced the concept of pessimum void

concept for stripping. Figure 12 shows the general

relationship between air voids and relative strength of

bituminous mixes following water conditioning. The

amount of strength loss depends upon the amount and

nature of voids. As shown in the figure, at less than 4per cent air voids, the mix is virtually impermeable to

water, so it is essentially unaffected. Unfortunately,

region B to C of Fig. 12 is where mix is semi-dense. As

the voids increase to D and beyond, the mix strength

becomes less affected by water because the mix is now

free draining like an ATPB. The region B to C can be

called pessimum void content because it represents

opposite of optimum. The objective is to stay out of the

pessimum void range. A semi-dense mix, which has

a potential for having pessimum voids in it, is likely to

trap water and causing stripping.

by MORTH for a thin layer of 25 mm. If this is the

consideration then there is no reason as to why a dense

graded 9.5 mm NMAS BC cannot be used like in othercountries in lieu of SDBC Grading 2 to provide a thin

layer of 25 mm. It should also be noted that BC is only 3

per cent more expensive than the SDBC as is evident

from the following prices obtained from the 2007 Schedule

of Rates of Rajasthan PWD, Jaipur Circle:

Item 16.7.6 Semi-Dense Rs. 1,756 per ton

Bituminous Concrete (SDBC)

Grading 2

Item 16.8.6 Bituminous Concrete Rs. 1,812 per ton

(BC) Grading 2

5.4. Bituminous Concrete (BC)

Two gradings of the Bituminous Concrete (BC) have

been specified in Section 509 of the MORTH

Specifications (2001). According to MORTH, the BC

can be used for wearing and profile corrective courses.

Grading 1 has a NMAS of 19 mm and Grading 2 has a

NMAS of 13 mm.

As discussed earlier, a DBM base course was already

selected. Now, there is a need to select a binder course

and two wearing (surface) courses (one coarse for heavy

traffic and one fine for light to medium traffic, urban areas,

and thin application). BC Grading 1 with a NMAS of

19 mm is suitable for a binder course. Its gradation needs

to be revised to make it more rut resistant. BC Grading 2

with a NMAS of 13 mm is suitable for a wearing course

on heavy-traffic roads. Its gradation also needs to be

revised to make it more rut resistant.

There is a need to add a third BC gradation with a

NMAS of 9.5 mm, which can be used for light to medium

traffic, urban areas, and in thin (25 mm) applications.

Right now, BC Grading 2 has been specified for a layer

thickness of 30-45 mm. As mentioned earlier, this has

forced pavement engineers to select SDBC grading 2 if

they wanted a thin (25 mm) application. Now that the

SDBC has been eliminated, the new BC Grading with

9.5 mm NMAS should replace it. This is suggested on

the premise that 25 mm wearing course is required to be

provided for less trafficked roads for cost saving.

Sinha et.al32 has studied the problem of rutting on a

heavy trafficked NH 32 and have found deep rutting

within a year or so of construction. DBM mix was found

to have been affected with high inside pavement

temperature (DBM layer) during summer, exceeding the

Fig. 12. Pessimum voids in semi-dense mixes (Ref. 32)

It is felt that in a tropical country like India thicker

wearing courses of 40 mm should be provided on all roads

to ensure against the penetration of water from top and

thereby to prevent crack initiation from top. Unfortunately,

the use of SDBC has been advertently promoted to someextent because only SDBC Grading 2 has been specified


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softening point of bitumen used. The pavement thus gets

a kneading effect by heavy trucks moving with slow

speed/stop-start condition. The quality of bitumen needsto be toned up to take care of such problem of rutting.

The rutting was observed almost the entire depth of

150 mm thick DBM. Rut depths between 75 to 100 mm

within 2 to 3 years of opening to traffic is quite common

in India. This needs to be researched and improved. A

high modulus rut resistant mix is needed to avoid rutting

problem. The concept of perpetual pavement briefly

mentioned before suggest that there is also a need to

improve the three gradations of the BC to make them

more rut resistant. This can be achieved by taking

advantage of successful field experience in an area or

on a test track, which is located in a region anywhere in

the world with hot climatic conditions similar to India in

the absence of any significant research in India.

One can emulate the gradations of the base course,

binder course and surface course used in Georgia. It is

generally argued that experience from other countries

cannot be emulated. Indigenous research is always

welcome but in the absence of same we cannot continue

with poor specifications. Granite is granite or limestone

is limestone, whether it is in Georgia or India. Similarly,

AC-30 bitumen used in Georgia in the past is similar toVG-30 bitumen (50-70 penetration) used in India. Traffic

is also computed in ESALs both in Georgia and India.

Therefore, there is no reason as to why experience with

bituminous mixes cannot generally be used in India with

some adjustments at least to start with.

Another source of excellent experience with

performance of bituminous mixes in hot climate similar

to north India is the 2.7 km long oval test track operated

by the National Center for Asphalt Technology (NCAT),

Auburn University, Alabama since year 2000. This test

track has 46 asphalt test sections, which comprise ofdifferent binder course and wearing course mixes.

Variables include type of mix (such as Marshall,

Superpave, SMA and OGFC), type of aggregate (such

as granite, limestone, quartzite, gravel and slag), type of

gradation (such as coarse graded and fine graded)11,16,

and type of bitumen (such as unmodified and modified).

Real trucks with loaded trailers are used to apply

traffic loading of 10 million ESALs to the track within a

2-year cycle. This amount of traffic is equivalent to 10-

year traffic on a typical interstate (national) highway in

the US. Performance of the asphalt test sections in termsof rut resistance and cracking is monitored periodically.

Photos 8 and 9 show the NCAT Test Track and trucks

with loaded trailers. The test track is located in Alabama

(latitude of 32.6 degrees) in southeastern US with climatesimilar to north India. Average maximum pavement

surface temperature of 61.40 C has been recorded on

the track. The test track has primarily used a 19 mm

NMAS binder course and a 12.5 mm NMAS wearing

course. Most of the test sections have performed

extremely well with average rut depth of 3 mm and

maximum rut depth of 6 mm and no significant cracking.

It should be mentioned that maximum acceptable rut

depth of a pavement is considered to be about12.5 mm

during its service life. Therefore, guidance can be taken

from gradations of binder course and surface (wearing)

course actually used on the test track with excellent

performance under hot climatic conditions.

Photo 9. Trucks with loaded trailers applying traffic load toNCAT Test Track

Photo 8. NCAT Test Track in Auburn University, Alabama


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Table 5 has been prepared to compare the existing

MORTH BC Grading 1 of 19 mm NMAS mix (intended

now for use as a binder course) with the gradation specifiedby GDOT and the actual gradation range used on NCAT

test track with excellent performance results. The last column

in this table has been formulated from the comparison as a

recommended 19 mm NMAS BC binder course gradation

for India. Only the values for 19 mm, 13.2 mm, 9.5 mm and

0.075 mm have been revised based on excellent performance

experience in Georgia and on NCAT test track. Remaining

values in the existing MORTH specifications for other sieves

are reasonably close to Georgia and test track and, therefore,

have been kept the same. The reasons for changing the

values for 0.075 mm sieve have been discussed earlier in the

paper. The second largest sieve size (19 mm in this case) of

a dense graded mix should be 90-100 per cent material passing

rather than 79-100 per cent. The reasons for the suggested

changes are as follows:

A few large size, isolated aggregate particles

hardly increase the mix stability and can

unnecessarily cause segregation problems.

Photo 10 shows the presence of such scattered

particles in a compacted mat on a national

highway in India.

With 79-100 per cent passing the second sieve,the same mix can have different NMAS values,

which will create confusion in mix selection and

will also have different minimum VMA

requirements19. That is why; the Superpave mix

design has control points of 90 and 100 per cent

for the second largest sieve size.

TABLE 5. COMPARISON OF GRADATIONS: EXISTING MORTH BC GRADING 1, GDOT BINDER COURSE, NCAT TRACK BINDER COURSE

AND PROPOSED BC BINDER COURSE

Photo 10. Large size, isolated aggregate particles in

compacted mat.

Grading Existing MORTH BC GDOT Binder NCAT Test Track Proposed BC

Grading 1 Course Binder Course Binder Course

Lift Thickness 50-65 mm 45-75 mm - 50-75 mm

Nominal 19 mm 19 mm 19 mm 19 mm

Aggregate Size


26.5 100 100 100 100

19 79-100 90-100 97-100 90-100

13.2 59-79 60-89 66-86 66-86

9.5 52-72 55-75 48-80 55-75

4.75 35-55 - 32-53 35-55

2.36 28-44 30-36 24-38 28-44

1.18 20-34 - 20-30 20-34

0.6 15-27 - 16-24 15-27

0.3 10-20 11-19 11-15 10-20

0.15 5-13 - 7-12 5-13

0.075 2-8 4-7 4-8 4-8

Bitumen Content 5.0-6.0 4.0-5.5 4.0-5.0 4.0-5.5


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Table 6 has been prepared to compare the existing

13 mm NMAS MORTH BC Grading 2 (intended now

for use as a coarse surface or wearing course) with thegradation specified by GDOT and the gradation range

used on NCAT test track with excellent performance

results. The gradation range for the wearing course used

on the test track encompasses gradations below the

Superpave restricted zone (BRZ), through the restricted

zone (TRZ), and above the restricted zone (ARZ). All

these gradations performed really well on the track. The

restricted zone specified earlier in Superpave through

which no gradation was permitted to pass, was already

deleted from the Superpave mix design based on a half-

million dollar research study conducted by Kandhal andCooley10,14,17.

Again, the proposed gradation for India in the last

column of Table 6 has been formulated from the

comparison. Most of the values except for 9.5 mm and

4.75 mm sieves have been revised. Reasons for revising

the values for the second largest sieve 13.2 mm

(or 12.5 mm) and the last 0.075 mm sieve were given

earlier. The values for sieves 2.36 mm down to 0.15 mm

have been generally lowered because it is believed that

the present MORTH gradation has excessive amounts

of fine aggregate (compared to GDOT and NCAT testtrack), which can increase the potential for rutting.

Table 7 shows the recommended final gradations of

5 dense graded bituminous mixes for India: 37.5 mm

NMAS DBM base course, 25 mm NMAS DBM base

course, 19 mm NMAS BC binder course, 12.5 mm

NMAS BC wearing course Grading 1, and a new

9.5 mm NMAS BC wearing course Grading 2. The finer

9.5 mm NMAS BC wearing course Grading 2 has been

adapted from the GDOT specification for Marshall mixes

(Table 1), which has an excellent performance history

for application on low to medium traffic roads, in urban

areas, and in thin (25 mm) applications. Values for three

sieve sizes 1.18 mm, 0.6 mm and 0.15 mm have been

interpolated. It is recommended to use Table 7 to revise

the gradations of MORTH DBM mix and BC mixes. All

these 5 mixes are adequate for constructing conventional

dense graded bituminous pavements in India, both for

new construction and overlays. They are also suitable

for PCC based on the required thickness. As mentioned

TABLE 6. COMPARISON OF GRADATIONS: EXISTING MORTH BC GRADING 2, GDOT SURFACE COURSE, NCAT TRACK SURFACE COURSE

AND PROPOSED 12.5 MM BC SURFACE COURSE

Grading BC Grading 2 GDOT Wearing NCAT Test Track Proposed BC

Course Wearing Course Wearing Course

Lift Thickness 30-45 mm 35-60 mm - 30-50 mm

Nominal 13 mm 12.5 mm 12.5 mm 12.5 mm

Aggregate Size


19 100 100 100 100

13.2 79-100 90-100 94-99 90-1009.5 70-88 70-85 73-92 70-88

4.75 53-71 - 51-73 53-71

2.36 42-58 44-48 34-54 38-54

1.18 34-48 - 22-38 24-38

0.6 26-38 - 17-29 17-29

0.3 18-28 10-25 12-19 12-22

0.15 12-20 - 7-11 7-15

0.075 4-10 4-7 4-8 4-7

Bitumen Content 5.0-7.0 5.0-7.0 4.3-7.8 5.0-7.0


17/20


earlier, it is not necessary to use all three courses (base,

binder, and wearing) in a new flexible pavement unless

the traffic is very high. For example, the following

combinations can be used depending upon the total

thickness of the bituminous course(s) required as per

structural design based on IRC:37.

Note: Wearing course grading 1 is recommended for

heavy traffic roads. Wearing course grading 2 is

recommended for light to medium traffic roads, in urban

areas, and for thin (25 mm) applications.

WMM + DBM Base Course + BC Binder

course + BC Wearing Course

WMM + BC Binder Course + BC Wearing

Course

WMM + BC Wearing Course only

It has been surmised by some that a BC wearing

course is too stiff and will crack if placed directly over

WMM. This is not correct because the BC wearing

course has relatively lower stiffness due to its lowerNMAS (12.5 or 9.5 mm) and high bitumen content. This

combination is being used in other countries including

Australia and South Africa. Similarly, a bituminous overlay

required for strengthening flexible pavement can consist

of the following depending upon the required thickness

as per IRC:81:

BC Binder Course + BC Wearing Course

BC Wearing Course only

Unlike most developed countries, overloading is amajor concern in India. On very heavily trafficked road

with severe overloading problem, it is recommended to

modify the BC wearing course and BC binder course

(that is, the top 100 mm of the pavement only, which is

likely to rut) as follows:

Ensure to use viscosity graded VG-30 grade

bitumen as per latest IS73:20064, which is

significantly more rut resistant than the old 60/

70 penetration bitumen.

Use polymer modified bitumen (PMB)

Use Stone Matrix Asphalt (SMA) as per IRCSpecifications approved recently.

TABLE 7. PROPOSED FINAL FIVE DENSE GRADED BITUMINOUS MIXES FOR INDIA

Grading Proposed DBM Proposed DBM Proposed BC Proposed BC Proposed BC

Base Course Base Course Binder Course Wearing Course Wearing Course

Grading 1 Grading 2 Grading 1 Grading 2

Lift Thickness 75-100 mm 75-100 mm 50-75 mm 30-50 mm 25-40 mm

Nominal 37.5 mm 25 mm 19 mm 12.5 mm 9.5 mm

Aggregate Size


45 100 - - - -

37.5 90-100 100 - - -

26.5 63-93 90-100 100 - -

19 - 71-95 90-100 100 -13.2 55-75 56-80 66-86 90-100 100

9.5 - - 55-75 70-88 90-100

4.75 38-54 38-54 35-55 53-71 55-75

2.36 28-42 28-42 28-44 38-54 40-55

1.18 - - 20-34 24-38 29-44

0.6 - - 15-27 17-29 21-33

0.3 7-21 7-21 10-20 12-22 14-25

0.15 - - 5-13 7-15 7-15

0.075 4-7 4-7 4-7 4-7 4-7Bitumen Content 4.0-5.5 4.0-5.5 4.0-5.5 5.0-7.0 5.2-7.5


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6. CONCLUSIONS

1. For ensuring long-term performing pavements

focus should shift to dense graded bituminousmixes rather than open graded lean bituminous

mixes. The future trend is towards introducing

high modulus rut resistant mix material to take

care of problems of rutting in bituminous layers.

Rutting is broadly due to inadequacies in our mix

design rather than granular bases or sub-grades.

2. Presently there is a proliferation of bituminous

paving mixes in India. Many of them are open

graded lean bituminous mixes. Too many options

for a specific bituminous course have created

confusion in mix selection. Many pavementdesign engineers do not understand the individual

characteristics of these mixes and, therefore,

select them based on their specified layer

thickness and/or cost.

3. Most countries of the world generally do not have

more than four dense graded mixes.

Notwithstanding other lean mixes like BUSG, BPM

there are 8 mixes like BM (2 gradings), DBM (2

gradings), SDBC (2 gradings), and BC (2 gradings).

The study recommends in total five numbers of

bituminous mixes in lieu of current eight specifiedin MORT&H specifications as per details below.

37.5 mm NMAS DBM Base Course

Grading 1

25 mm NMAS DBM Base Course

Grading 2

19 mm NMAS BC Binder Course

12.5 mm NMAS BC Wearing Course

Grading 1

9.5 mm NMAS BC Wearing Course

Grading 2

4. Paper recommends phasing out of lean mixes

as they are mainly responsible for poor

performance of bituminous pavements. It is

recommended not to use them on primary

network like NH or even State Highways.

Specifications should restrict the mixes to ensure

better quality roads.

5. Some new specifications like those similar to

open graded mixes such as permeable asphalttreated base (PATB) and OGFC may be

introduced to complete the specifications. Both

PATB and OGFC need to be used in India only

in exceptional circumstances due to theiradditional costs. These may be used when

specifically required.

6. Bituminous Macadam is a highly permeable mix

and promotes rutting. Use of Bituminous

Macadam a very popular mix at present may

be deleted and substituted with DBM because

it is finally cost effective and better performing.

Similarly, use of Semi-dense Bituminous

Concrete is also not considered to be allowed in

the specifications. It suffers from pessimum

voids, which have potential to trap waterresulting in moisture damage. It should be

substituted by Bituminous Concrete as it is better

performing and cost effective.

7. Use of thick wearing/surface course of 40 mm is

recommended in a tropical country like India to

seal the pavement top from the ingress of water

and moisture. To take care of possible use of

wearing course of 25 mm on cost consideration

as was the case in SDBC a 9.5 mm NMAS BC

wearing course may be introduced in the

specifications after removing SDBC.

8. The Profile Corrective Course (PCC) should

be only in DBM or BC. The current use of BM

as PCC should be discouraged and not allowed.

7. RECOMMENDATIONS

A. A suitable research study to study the

performance of specifications of bituminous

mixes suggested above be carried out on the

pattern of National Center for Asphalt

Technology (NCAT), Auburn University,Alabama since year 2000 by constructing a test

track. Pending the above research output the

suggested change in the specifications may be

incorporated and their performance in field be

evaluated with respect to older specifications.

B. The research scheme suggested above should

attempt new mixes which comprises of high

modulus rut resistant material to tackle the

problem of rutting on the lines of design concept

of perpetual pavements.

C. Ongoing debate for the use of larger Nominal


19/20


Maximum Aggregate Size (NMAS) in DBM

etc. should be settled by conducting indigenous

research and the existing grading should bemodified suitably. Pending the above research

output suggested grading be incorporated in the

specifications.

ACKNOWLEDGEMENT

The opinions expressed in this paper are those of

the authors only.

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19. Kandhal, P.S., K.Y. Foo, and R.B. Mallick. Critical

Review of VMA Requirements in Superpave.

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Research Record 1609, 1998.

20. Kandhal, P.S. and T. Wu. Precision of Marshall

Stability and Flow Test Using 6-inch (152.4-mm)

Diameter Specimens. ASTM. Journal of Testing

and Evaluation, Vol. 24, No. 1, January, 1996.

21. Kandhal, P.S. Field and Laboratory Evaluation of

Stripping in Asphalt Pavements: State of the Art

Report. Transportation Research Board,

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22. Kandhal, P.S. Moisture Susceptibility of HMA

Mixes: Identification of Problem and Recommended

Solutions. National Asphalt Pavement Association,

Quality Improvement Publication (QIP) No. 119,

December 1992.

23. Kandhal, P.S. Large Stone Asphalt Mixes: Design

and Construction. Proceedings, Association ofAsphalt Paving Technologists, Vol. 59, 1990.


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24. Kandhal, P.S., C.W. Lubold, and F.L. Roberts. Water

Damage to Asphalt Overlays: Case Histories.

Proceedings, Association of Asphalt PavingTechnologists, Vol. 58, l989.

25. Kandhal, P.S. Changes in Mix Design to Improve

Performance: Selected State Experiences.

Proceedings, Association of Asphalt Paving

Technologists,.Vol. 57, l988.

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Permeability of Superpave Designed Pavements.

National Center for Asphalt Technology, Report 03-

02, June 2003.

27. Mallick, R.B., P.S. Kandhal, L.A. Cooley Jr., andD. Watson. Design, Construction, and Performance

of New-Generation Open-Graded Friction Courses.

Asphalt Paving Technology, Volume 69, 2000.

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Specifications for Road and Bridge Works, Section

500, Fourth Revision, 2001, Indian Roads Congress,

New Delhi.

29. Ministry of Road Transport & Highways. Manual

for Construction and Supervision of FlexibleWorks.

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132 KANDHAL, SINHA & VEERARAGAVANONA CRITICAL REVIEWOF BITUMINOUS PAVING MIXESUSEDIN INDIA

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Pavement Mix Type Selection Guide. Information

Series 128, 2001.

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Mixture Design and Construction. NAPA Education

Foundation, Lanham, Maryland, Second Edition,

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32. Sinha V.K., H.N. Singh and Saurav Shekhar Rutting

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33. Sinha V.K. Perpetual Pavements An EmergingConcept Editorial, Indian Highways, Indian Roads

Congress, August 2008 issue.

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Water Sensitivity. SHRP Report SHRP-A/IR-89-

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Ndesign Compaction Levels for Georgia. Paper

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Transportation Research Board, Washington, DC,

January 2008.

a critical review of bituminous paving mixes used in india

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