A Critical Review of Bituminous Paving Mixes Used in India.pdf

1. INTRODUCTION

The bituminous paving mixes as specified in MORTH“Specifications for Road and Bridge Works”, FourthRevision, 200128 are commonly used in India. Some ofthese mixes have evolved since 1960s, an era when thepresent day hot mix asphalt plants were not common andmixes were produced with small portable mixing plantswith limited aggregate heating, blending and mixingcapabilities. The proliferation of bituminous paving mixesas specified in the MORTH publication basically manifestthe constraints of non-availability of modern hot-mix plantbesides likely cost reduction of lean bituminous mixes.Today, the scenario has substantially changed. There is agrowing concern to construct long lasting pavements withminimum maintenance. It is accordingly, felt that only thosespecifications for bituminous mixes be allowed, which aredurable, irrespective of their location (NH or SH or RuralRoad) because durability and long term performance iscentral to all of them.

A critical review of commonly used bituminous

paving mixes in India has been accordingly attemptedkeeping in perspective the following: (a) fundamentalsof mix selection based on their intended function andlocation 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 beupdated based on proven field performance of similarmixes in the test tracks and in regions of developedcountries like USA with climate reasonably close tothat of India. This review is also expected to be helpfulto the contractors who are currently designing theirown projects (including flexible mix selection) underthe Public-Private-Partnership (PPP) projects. Thefuture trend of mix design improvements should finallyaim to achieve long-lasting perpetual pavements.

2. FUNDAMENTALS OF MIX SELECTION BASEDON THEIR INTENDED FUNCTION AND LOCATION

WITHIN PAVEMENT STRUCTURE

Bituminous mixes are used in a flexible pavement toserve 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 differentcourses within the flexible pavement.

There is a proliferation of bituminous paving mixes in India. MORTH Specifications broadly provides 4 mixes for basecourses, 6 mixes for binder courses and 4 mixes for wearing courses. Further two grading, each of BM, DBM, SDBC and BCare specified in the MORTH specifications. Too many options for a specific bituminous course have created confusion in mixselection and are mainly responsible to a considerable extent for the poor performance of flexible pavements in India. Acase has been made out on technical grounds to have only 5 dense graded mixes of different nominal maximum aggregatesize (NMAS) in the specifications, as is the case in most developed countries of the world. The following 5 dense gradedmixes 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)

114 KANDHAL, SINHA & VEERARAGAVAN ON

� 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 flexiblepavement in a developed country like USA. Thestructural bituminous courses can consist of bituminousbinder course and bituminous surface or wearing courseas shown in Fig. 1(a). Providing structural strength isthe primary purpose of most bituminous mixes exceptthose used in very thin surfacing. The objective is todisperse appropriately the dynamic and static effectsof traffic wheel loads to the underlying pavement layerssuch as bituminous/crushed stone base course. For low-volume roads only a granular base and a bituminouswearing course may suffice based on structuralrequirements. Normally, lower layers of base coursesincluding those of bituminous base courses (as providedin developed countries) should have desired stiffnesscharacteristics to act as good foundation, which shouldbe effective in dispersing the traffic loads to the lowerlayers. Upper layers of bituminous binder course havethe requirement of being effective in re-bounding againstthe dynamic effect of traffic load. In other words, thetop layers of bituminous binder courses should haveadequate stiffness to resist rutting coupled with theflexibility to be effective in re-bounding. The flexibilitycharacteristics should, therefore, increase when goingfrom bottom to upwards layer.

From the perusal of the current literature andpractices, it is observed that the preceding requirementscan be fulfilled by using continuously dense-gradedbituminous mixes with nominal maximum aggregate size(NMAS) decreasing from base course through bindercourse to surface or wearing course30. The nominalmaximum aggregate size is defined as one sieve largerthan the first sieve to retain more than ten per cent ofcombined aggregate31.

Base course mixes, which use relatively larger sizeaggregate, are not only stiff/stable but also are economicalbecause they use relatively lower bitumen contents andneed less breaking and crushing energy/effort. Surface orwearing course mixes with smaller aggregate on the otherhand have relatively higher bitumen contents, which notonly impart high flexibility but also increase their durability.The binder (intermediate) course mix serves as a transitionbetween the base course and wearing course. Severalstudies3,30,34 have shown that permanent deformation(rutting) within flexible pavement is usually confined to thetop 100 to 150 mm of the pavement. This means both thebinder and wearing course mixes should be designed to beresistant to rutting. That is why in extreme cases of heavytraffic loads and high tyre pressures, it is considered prudentto use Stone Matrix Asphalt (SMA) mix in which due tostone-on-stone contact the load is carried directly by thecoarse aggregate skeleton. An idealized cross-section fora heavy-duty flexible pavement is shown in Fig.1(b).Recently, the Indian Roads Congress (IRC) has adopted atentative SMA specification, (IRC SP 79:2008)which couldbe used under such circumstances. Kandhal13 has publisheda manual on design and construction of SMA mixes, whichis 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

HIGHLIGHTS OF THE 178TH COUNCIL MEETING 115A CRITICAL REVIEW OF BITUMINOUS PAVING MIXES USED IN INDIA

2.2. Facilitate Subsurface Drainage

Typically, granular sub-base in a flexible pavement isintended to provide subsurface drainage. However, in manysituations where granular sub-bases contain high percentagesof fines (less than 75 micron size material) such layers arefound to be not very effective. In developed countries likeUSA, Permeable Asphalt Treated Base (PATB) has beenused extensively on major highways to provide positivesubsurface drainage. PATB basically does not constitute aconventional base course. It is considered as a separatecourse exclusively for subsurface drainage. From costconsideration, PATB is not recommended for mosthighways 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 drainageproblem situations. The lift or layer thickness of the PATBgenerally ranges from 75 to 100 mm.

PATB is provided between the granular sub-base(GSB) and the bituminous base course. Figures 2(a) and2(b) shows two typical provisions of PATB. The conceptis to provide a two-layer drainage system. It is on thepresumption that water accumulated in the sub-basealways seeks least path of flow and some part of it flowsinto the PATB rather than traveling altogether a longdistance to the edge of the sub-base. Water collected inthe PATB is then drained out in two ways. The PATBcan either be connected to a subsurface pavement edgedrain as shown in Fig. 2(a) or it can be extended all theway to the edge of the embankment or “daylighted” asshown in Fig. 2(b). This two-layer subsurface drainagesystem is very effective in quickly removing water, whichmay enter the pavement by any manner.

It is of paramount importance to ensure that thePATB layer is not blocked in any way otherwise it willbe saturated with water and thus create a “bath tub”scenario in the pavement. This phenomenon will not onlyinduce stripping21,22,24 within the PATB (due to pore waterpressure buildup under traffic) but also in the bituminouscourses overlying it. Premature failures of pavementsfrom this phenomenon have been experienced inCalifornia. In India, open-graded permeable BituminousMacadam (BM) is used as binder/base course inpavements without any positive outlet for water, whichcan result in such failures. This will be discussed laterwhen the BM mix is reviewed.

2.3. Provide Surface Friction

Mixes like bituminous concrete, semi-bituminousconcrete, besides premix carpet, mix-seal surfacing iscommonly provided as per MORTH Specification aswearing course. Some bituminous wearing course mixesare designed to provide high surface friction especiallywhen the pavement is wet. Open-Graded FrictionCourses (OGFC) usually 20-25 mm thick (Figure 1b)are designed as an open graded mix with interconnectedvoids that provide drainage during heavy rainfall. OGFCreportedly 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 underneaththe OGFC is very dense, impermeable, and highlyresistant to stripping. The design, construction, andmaintenance of OGFC is fully developed and welldocumented by Kandhal in manual and papers12,18,27.

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

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


The OGFC has been known to induce stripping in theunderlying bituminous layer15,22. OGFC basically is aspecification for cold country having skid problem. Itinvolves additional cost and hence may be used only onhighways, which are accident-prone during rains. Thiscourse should be constructed with proper anti strippingagents like hydrated lime.

If an existing pavement with OGFC needs to beoverlaid, it is necessary to first remove (mill off) theOGFC and also examine the underlying layer for potentialmoisture damage (stripping). Open graded wearingcourses like premix carpet, surface dressing and to someextent semi-dense bituminous concrete falls under theabove category. In case of overlay these wearing courselayer should be totally removed, which is often not donein India. Experience in the US has shown that if anexisting OGFC layer is not removed it will trap waterand cause premature moisture damage by stripping,particularly when the underlying layer is a dense mix.Photo 1 and 2 exhibits such failures in Australia andOklahoma (USA)15.

3. PERPETUAL PAVEMENTS – AN EMERGINGCONCEPT

The concept of Perpetual Pavement35 was firstlaunched by the Asphalt Pavement Alliance (APA) in ajoint promotional effort with Asphalt Institute, NationalAsphalt Pavement Association, and the State AsphaltPavement Associations of USA in 2003-04. Fig. 3 depictsthe perpetual design concept.

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

Photo 1. Premature OGFC related distress on ahighway west of Sydney, Australia (Ref. 15)

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

Source : US Department of Transportation FHWA

From the perusal of the Fig. 3, it will be observedthat the bituminous portion is divided into three zonesand bitumen mixes to be adopted for these three zonesare characteristically different. The first zone is wearingcourse 40-75 mm thick. It could be high quality HMA orOGFC. The succeeding zone/layer is high compressionzone with high modulus rut resistant mix 100-200 mmthick. The third layer is to cater maximum tensile strainand should be able to resist flexible fatigue. The designof the bituminous layers is done on mechanistic principlesby keeping the strain within each layer less thanendurance limit. Thus no damage accumulation takesplace in any layer and the pavement layer constructed,normally do not need any replacement/rehabilitation. Itis only the top layer which is required to be replaced incase of any renewal/strengthening.

Figure 4 shows how high asphalt content improvesfatigue resistance. This proves why internationalspecifications are opting for dense graded mixes withmore bitumen content rather than open graded mixeswith less bitumen. This emerging concept has alreadybeen followed, at least in principle, at many locations inUSA. For example, on I-710 (the Long Beach Freeway)in Los-Angeles County a mix comprising of 25 mm


OGFC + 75 mm dense-graded HMA + 150 mm asphalthas been used on the design concept of perpetualpavement (Monismith and Long, 1999). The perpetualpavements last long, provided they are built on a solidfoundation. It is observed that rutting on such roads builton sub-grade with the CBR greater than 5 per centoriginates mostly in the HMA layer. This suggests that asub-grade with a CBR greater than 5 per cent (M

R

greater than 50 MPa) is considered adequate. Properconstruction techniques and quality control are essentialfor perpetual pavements like any other pavement.

The concept of perpetual pavements is brieflyintroduced to emphasize the current trend ofdevelopment in respect of specifications of bituminousmixes. This also highlights the required change inmindset of Indian engineers to review the existingspecifications of bituminous mixes from the point ofview of long-lasting pavements rather than on theconsideration of cost and conveniences. The subsequentchapter describes the current specifications followedin many developed countries. It is felt that even thesespecifications in those countries might go furtherchanges consistent with the perpetual pavementconcepts or similar concepts emerging in future. Thepoint of concern is that long-lasting pavements goingto be future concern in India. The advantages of suchspecifications lie not only in long life but also in thereduced cost of travel with better serviceabilityconditions. Figure 7. shows one such comparison.

Fig. 4. Improve fatigue resistance with high asphalt content mixes

Source: Idaho Project Development Conference 2007

Fig. 5 shows how the design concepts of theperpetual pavements minimizes the tensile strain withpavement thickness. Figure 6. shows how the designconcept of perpetual pavements deals with hightemperature encountered during summer in case of mostof 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 GradeSource: Idaho Project Development Conference 2007

Fig. 7. Pavement serviceability comparisonSource: Idaho Project Development Conference 2007

4. EXPERIENCE IN DEVELOPED COUNTRIES (USA)

Most specifications for dense graded bituminousmixes in the USA and other countries are intendedfor four pavement courses, namely, base course,

Years


binder course, and two surface (wearing) courses (onecoarse and one fine). Table 1 shows four suchMarshall mixes used by the Georgia Department ofTransportation (GDOT) in the recent past, beforechanging to Superpave designed mixes6. The tableshows nominal maximum aggregate size (NMAS),minimum and maximum layer (lift) thickness, andgradations of the four mixes. Unlike other highwayagencies, a narrow range of material passing the 2.36mm sieve is specified by GDOT for developing thejob mix formula. The second sieve size has beenchanged from 85-100 to 90-100 per cent in this tableto suit the specified NMAS of the mix.

GDOT has held the general reputation ofconstructing one of the best and most durable flexiblepavements in the US for several years. This time periodencompasses the use of both Marshall and Superpavemixes in Georgia. Eleven projects consisting of Marshallmixes were evaluated in Georgia recently35. Theaverage rut depth after about 6 years of heavy trafficwas determined to be 1.5 mm only. Georgia’sexperience should be of interest to India becauseGeorgia being in the southeastern US has climate similarto north India. Air temperature up to 44.50 C has been

recorded in Georgia. That is why in the past, Georgiaused AC-30 viscosity grade bitumen, which is equivalentto VG-30 grade (50-70 penetration) used in India.Georgia’s experience is, therefore, quite relevant inreviewing and revising Indian dense graded mixspecifications in terms of NMAS, gradation and layer(lift) thickness.

As shown in Table 1, the base course consists of a25 mm NMAS mix; the binder course consists of a 19mm NMAS mix; and the surface (or wearing) courseconsists of 12.5 mm or 9.5 mm NMAS mix. The 12.5mm wearing course mix is used for heavy traffic roads,whereas the 9.5 mm wearing course mix is used for lowto medium traffic roads, in urban areas, and in thin (25mm) applications. Any of the four dense graded mixescan be used for leveling or profile corrective course(PCC) depending upon the required thickness. Onlydense graded mixes are used in the US in PCC30.

The Georgia DOT and some other state DOTs inthe US attempted to use 37.5 mm NMAS mix for basecourse, which was only marginally more stable than the25 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 CourseLift 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. GRADATION OF GEORGIA DOT MARSHALL MIXES (REF. 6)


to segregation resulting in honeycombing(Photo 3).

� The 37.5 mm NMAS mix becomes permeableat relatively lower air void content levels ascompared to the 25 mm NMAS mix, therebyincreasing the potential for water/moisturerelated damage. Fig. 8 clearly shows the effectof NMAS on field permeability2,26. As theNMAS increases, the permeability alsoincreases multifold at a given void level. Forexample, at an in-place air void content of 6percent, the following permeability values weremeasured for each NMAS.

9.5 mm NMAS 6x10-5 cm/sec




The preceding data26 clearly shows larger 37.5mm NMAS will be highly permeable since thepermeability increases multifold from oneNMAS to the next higher one.

� Modified Marshall method developed byKandhal20,23,25 and referred to in AsphaltInstitute MS-21, which uses 6-inch diametermould needs to be used for designing and testing37.5 mm mix.

Photo 3. Segregation of 37.5 mm NMAS mix resulting in honeycombing

Some engineers in India including 2nd author believethat the use of 37.5 mm mix should be considered forbase course in India because of overloading problem. Itis felt that this gets well validated by the observationmade before that in case of heavy traffic, stone masticasphalt is the correct specification as per internationalpractice. The axle loads in India are quite heavy andfurther 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 providingDBM with NMAS of 37.5 mm be kept underconsideration pending further research.

5. REVIEW OF FLEXIBLE PAVING MIXES USEDIN INDIA

For the purpose of this paper a detailed review ofthe following bituminous paving mixes specified in theMORTH 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 anyquality control on its volumetrics or strength (stability).The primary problem with the BM mix is that being veryopen graded, it is highly permeable and therefore willtrap 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)


not common. At that time, hot mixing was done in smallportable plants or concrete mixers in which much fineaggregate could not be used due to limitations of theavailable heating and mixing equipment. Now, good hotmix plants are normally available.

Fig. 9, 10 & 11 shows a typical cross-section offlexible pavement as being used in India. Fig. 9 does nothave a BM layer and DBM is resting directly on WMM.However, Figs. 10 and 11 show cross-sections whereBM has been used as a base, binder or profile correctivecourse (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 BMversus DBM. Should BM be deleted and DBM usedinstead in all cases? To answer that question BM andDBM should be compared both from the engineeringaspect (primary) and economical aspect (secondary).This has been done considering the following factors:

5.1.1. Permeability : It has been acknowledged inmany IRC and MORTH publications that BM is a muchmore open mix compared to the DBM. The MORTHManual for Construction and Supervision of BituminousWorks29 states on page 52, “Because of the open-gradedaggregate matrix, the voids content (in the BM) can beas high as 20-25 per cent.” Some researchers havereported air voids of about 10 per cent in lab compactedBM specimens. This range of 10-25 per cent air voidscan occur because BM has two gradings and withineach grading BM can be relatively coarse or fineconsidering the combination of lower and upper valuesfor each sieve. Table 2 gives air voids and permeabilitydata obtained recently on the BM mix8. In this case fourBM gradations were used: Grading 1 (both coarse andfine) and Grading 2 (both coarse and fine). The testdata was obtained on 150-mm diameter specimenscompacted with 75 blows (equivalent of 50 blows on100-mm diameter specimens). The air void contentranges from 8.3 to 15.4 per cent. The test data onGradings 1 and 2 are comparable because both gradingshave about the same amount of material passing 4.75mm sieve. Photos 4 and 5 show the open texture of BMspecimens Grading 1 and Grading 2, respectively. Whenthese specimens were placed under a water tap, thewater readily passed through indicating very highpermeability.

Even if the scenario of about 10 per cent air voidsin the BM in the lab is considered, the voids in the fieldcan 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, densegraded bituminous mixes become permeable when airvoids are more than 7-8 per cent. BM type open gradedmix, 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 cannotbe any argument about the fact that the BM is a highlypermeable 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


The first author has investigated and reported15,24

many real-life field case histories of prematurepavement failures from across the world. In a majorityof cases, bituminous layers, which trapped water, werethe real culprits. There was stripping of bitumen in thepermeable layer as well as in the adjacent layersoverlying or underlying it due to traffic action. Photos6 and 7 show a failure in Oklahoma, US, which wasinvestigated by the first author15. On this project anopen type binder course was used. It was saturatedwith water since there were no subsurface edge drainsat the edge of the pavement. This led to stripping in thebinder course under traffic. Note that majority of thepotholes appeared near the pavement edge wherewater 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 usedfor long term pavement performance.

5.1.2. Structural strength: Many highwayagencies across the world give structural value to aBM type mix (used for drainage) of 50 per cent ofdense graded DBM type mix. IRC Publications 37and 81 on flexible pavement design state that 7 mmof DBM is equal to 10 mm of BM. In either case, theDBM is far superior to the BM in terms of structuralstrength and fatigue life. Some engineers aresuggesting using polymer-modified bitumen (PMB) inthe BM to increase its structural strength. If that isthe objective, why not simply use the stiffer DBM inthe first place. First of all, hardly any agency in theworld 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 controlcriteria at the design or mixing stage, is not justified.Therefore, DBM is by far superior to the BM in termsof structural strength, mix design criteria, and mixproduction control.

TABLE 2. AIR VOIDS AND PERMEABILITY TEST DATA FOR FLEXIBLE MACADAM (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 orwhatever. If this fundamental requirement isdisregarded, the potential for premature pavementdistress is increased. A permeable layer always attractsand traps water, moisture or moisture vapour. Watercan come from the top, from the sides, or from thenon-bituminous courses underneath22. If there is apremix carpet (which is highly permeable) right overthe BM, rainwater will have direct access to the BMand can cause havoc.


5.1.3. Use as a PCC : It has been surmised thatBM is a good material for profile corrective course (PCC)because it resists reflection cracking. No other country isusing a permeable, water-trapping type mix for PCC. Onlydense graded mixes such as DBM or BC are used fortransverse or longitudinal profile correction in othercountries30 in courses called leveling courses or wedgecourses, which are same as India’s PCC. The reasonsfor 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 almostzero in a wedge type PCC.

It is normally argued/believed that BM has a betterresistance to reflection cracking and accordingly, manypavement designers introduced a layer of BM betweenDBM and WMM in India. It does not appear that thisconclusion is based on any research. Assuming thatBM has a better resistance to reflection cracking, it isstill a water-trapping permeable mix and as discussedearlier, the potential for failure by far exceeds theperceived advantage of resisting reflection cracking. Itis better to seal the cracks or to remove the top cracksby milling/scraping or prevent their upward movementby 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 isquite often made on the premise that BM is cheaperthan DBM and, therefore, it is suitable for use indeveloping country like India. That is not correct. Thefollowing cost analysis of BM versus DBM wasconducted at IIT, Madras in response to the revised draftBM specification published in Indian Highways forcomments5.

According to the current prevailing schedule of ratesin his area, the cost of BM layer is Rs. 3,465/cu.m andthe cost of DBM is Rs. 4,193/cu.m. Consider that100 mm of BM is required for a pavement and it is equalto 50 mm of DBM as per equivalency used in someother countries. The cost of a 2-lane highway (10 mwide including shoulders) with BM will cost Rs. 34.65lakhs/2-lane-km and the cost of the same highway withDBM will cost Rs. 20.97 lakhs/2-lane-km. Now, that isa 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 ofDBM as per IRC guidelines. In that case, the cost of highwaywith BM is Rs. 34.65 lakhs/2-lane-km and the cost of highwaywith DBM is Rs. 29.35 lakhs/2-lane-km. Thus using DBMin 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 alsobeen done based on the current 2007 schedule of rates ofBM and DBM obtained from the Rajasthan PWD Circlein Jaipur. The cost of BM in place is Rs 1,404 per ton andthe cost of DBM in place is Rs. 1,588 per ton. Consideringthat 100 mm of BM is equal to 70 mm of DBM as perIRC guidelines, the actual cost of DBM in place comes

Photo 6. Road failure in Oklahoma, US resultingfrom saturated binder course (Ref. 15)

Photo 7. Close up of stripped binder course inOklahoma, US (Ref. 15)


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

5.1.5. Traffic conditions: According to somehighway engineers, BM is intended for low-traffic roadsonly, although it is being used extensively on nationalhighways and state highways. However, the fact remainsthat any layer, which traps water, should not be used whetherit is a low-volume or high volume road. The concept ofperpetual pavement or long term performing pavements isrelevant even for less traffic roads. The need today is toconstruct pavements needing less maintenance avoidingfrequent overlays, besides providing a good riding surfacefor many years. The design concept should, therefore, besame for both heavy traffic and low traffic. In any case,the thickness of different bituminous layers will be differentdepending upon the traffic intensity. As discussed earlier,water is the enemy of the road. The only tangible argumentagainst the use of DBM could be non availability of properbatch mix plants in required number gradually to produceDBM on rural roads/state highways. It is felt that this shouldbe enforced at least to tone up the quality level of ourpavements.

5.1.6. General statements : General statementsare 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 notenvisage in concept of long-term performing pavements.The normal life of pavement in India is between 2 to 4years compared to 8 to 10 years in other countries. Thedeveloped countries are talking of perpetual/long-termpavements capable of performing for 50 years or more.This may look strange but our vision should accordinglyextend to give precedence to durability over deceptive costsaving. The concept of sound economics/engineeringsuggest that we should accept changing the permeablemixes 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 ofMORTH specifications: Grading 1 has a NMAS of37.5 mm and Grading 2 has a NMAS of 25 mm.

Table 3 gives the existing MORTH composition ofDBM Gradings 1 and 2. The specified percentage offine aggregate is the same in both gradings (28-42 per

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

Grading 1 2Lift Thickness 80-100 mm 50-75 mm

Nominal Aggregate Size 40 mm 25 mmSieve, mm Per cent Passing

45 10037.5 95-100 10026.5 63-93 90-10019 - 71-95

13.2 55-75 56-809.5 - -4.75 38-54 38-542.36 28-42 28-421.18 - -0.6 - -0.3 7-21 7-210.15 - -0.075 2-8 2-8

Bitumen Content, % Min. 4.0 Min. 4.5


cent), the main difference is just some large sizeaggregate particles (25-45 mm size) are contained inGrading 1. It was discussed earlier that the use of largestone mix (NMAS of 37.5 mm or larger) has severaldisadvantages such as segregation (Photo 3) and highpermeability (Fig. 8). Although these disadvantagesoutweigh the marginal gain in stability over a 25 mmNMAS mix, some engineers would like to use it in Indiadue to overloading problem. Therefore, DBM Grading 1has been retained. Since Grading 1 is highly permeable,it should be sealed before rainy season otherwise waterwill penetrate and damage the underlying WMM course.This neglect is commonly observed during constructionof our roads. Overall, DBM Grading 2 with 25 mmNMAS is best suited for a base course similar to GDOTbase course specification in Table 1.

The MORTH specification mentions a lift thicknessof 50-75 mm for Grading 2. It needs to be debatedwhether it should be 50-100 mm as practiced in manycountries including the US30.

Table 4. makes a comparison of existing DBMGrading 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 inIndia. As mentioned earlier, Georgia has hot climatesimilar to north India and has one of the best flexibleroads in the US. It is encouraging to note that theexisting DBM Grading 2 is reasonably similar toGDOT Base Course. Therefore, the proposed DBMBase Course Grading 2 has been kept the same asexisting DBM Grading 2 except for the percentageof fines (material passing the 0.075 mm sieve). Atleast 4 per cent fines are needed in the job mix formula(JMF) to impart some stiffness to the bitumen-finesmortar. Eight per cent fines are considered tooexcessive. It should be noted that the range of 4 to 7per cent is intended for the JMF design gradation.Normal variation during production will be allowed. Itis also recommended to specify and use 25 mm sievein lieu of 26.5 mm sieve and 12.5 mm sieve in lieu of13.2 mm sieve so that Indian standards generallyconform to world standards. It applies to allgradations. The lift thickness for DBM Base Coursewith NMAS of 25 mm has been revised to75-100 mm. Similar changes have been made toproposed DBM Grading 1 with NMAS of 37.5 mmas reported later in Table 7.

TABLE 4. COMPARISON OF GRADATIONS: EXISTING MORTH DBM GRADING 2, GDOT BASE COURSE AND 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


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 isprovided as wearing course. Semi-dense mixes whichare neither dense graded nor open graded contain theso-called “pessimum” voids when constructed. Terreland Shute32 advanced the concept of “pessimum” voidconcept for stripping. Figure 12 shows the generalrelationship between air voids and relative strength ofbituminous mixes following water conditioning. Theamount of strength loss depends upon the amount andnature of voids. As shown in the figure, at less than 4per cent air voids, the mix is virtually impermeable towater, so it is essentially unaffected. Unfortunately,region B to C of Fig. 12 is where mix is semi-dense. Asthe voids increase to D and beyond, the mix strengthbecomes less affected by water because the mix is nowfree draining like an ATPB. The region B to C can becalled “pessimum” void content because it representsopposite of optimum. The objective is to stay out of the“pessimum” void range. A “semi-dense” mix, which hasa potential for having “pessimum” voids in it, is likely totrap water and causing stripping.

by MORTH for a thin layer of 25 mm. If this is theconsideration then there is no reason as to why a densegraded 9.5 mm NMAS BC cannot be used like in othercountries in lieu of SDBC Grading 2 to provide a thinlayer of 25 mm. It should also be noted that BC is only 3per cent more expensive than the SDBC as is evidentfrom the following prices obtained from the 2007 Scheduleof Rates of Rajasthan PWD, Jaipur Circle:

Item 16.7.6 Semi-Dense Rs. 1,756 per tonBituminous 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) havebeen specified in Section 509 of the MORTHSpecifications (2001). According to MORTH, the BCcan be used for wearing and profile corrective courses.Grading 1 has a NMAS of 19 mm and Grading 2 has aNMAS of 13 mm.

As discussed earlier, a DBM base course was alreadyselected. Now, there is a need to select a binder courseand two wearing (surface) courses (one coarse for heavytraffic and one fine for light to medium traffic, urban areas,and thin application). BC Grading 1 with a NMAS of19 mm is suitable for a binder course. Its gradation needsto be revised to make it more rut resistant. BC Grading 2with a NMAS of 13 mm is suitable for a wearing courseon heavy-traffic roads. Its gradation also needs to berevised to make it more rut resistant.

There is a need to add a third BC gradation with aNMAS of 9.5 mm, which can be used for light to mediumtraffic, urban areas, and in thin (25 mm) applications.Right now, BC Grading 2 has been specified for a layerthickness of 30-45 mm. As mentioned earlier, this hasforced pavement engineers to select SDBC grading 2 ifthey wanted a thin (25 mm) application. Now that theSDBC has been eliminated, the new BC Grading with9.5 mm NMAS should replace it. This is suggested onthe premise that 25 mm wearing course is required to beprovided for less trafficked roads for cost saving.

Sinha et.al32 has studied the problem of rutting on aheavy trafficked NH 32 and have found deep ruttingwithin a year or so of construction. DBM mix was foundto have been affected with high inside pavementtemperature (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 thickerwearing courses of 40 mm should be provided on all roadsto ensure against the penetration of water from top andthereby 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


softening point of bitumen used. The pavement thus getsa kneading effect by heavy trucks moving with slowspeed/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 of150 mm thick DBM. Rut depths between 75 to 100 mmwithin 2 to 3 years of opening to traffic is quite commonin India. This needs to be researched and improved. Ahigh modulus rut resistant mix is needed to avoid ruttingproblem. The concept of perpetual pavement brieflymentioned before suggest that there is also a need toimprove the three gradations of the BC to make themmore rut resistant. This can be achieved by takingadvantage of successful field experience in an area oron a test track, which is located in a region anywhere inthe world with hot climatic conditions similar to India inthe 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 isgenerally argued that experience from other countriescannot be emulated. Indigenous research is alwayswelcome but in the absence of same we cannot continuewith poor specifications. Granite is granite or limestoneis 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. Trafficis also computed in ESALs both in Georgia and India.Therefore, there is no reason as to why experience withbituminous mixes cannot generally be used in India withsome adjustments at least to start with.

Another source of excellent experience withperformance of bituminous mixes in hot climate similarto north India is the 2.7 km long oval test track operatedby the National Center for Asphalt Technology (NCAT),Auburn University, Alabama since year 2000. This testtrack 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 (suchas granite, limestone, quartzite, gravel and slag), type ofgradation (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 applytraffic loading of 10 million ESALs to the track within a2-year cycle. This amount of traffic is equivalent to 10-year traffic on a typical interstate (national) highway inthe 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 truckswith loaded trailers. The test track is located in Alabama(latitude of 32.6 degrees) in southeastern US with climatesimilar to north India. Average maximum pavementsurface temperature of 61.40 C has been recorded onthe track. The test track has primarily used a 19 mmNMAS binder course and a 12.5 mm NMAS wearingcourse. Most of the test sections have performedextremely well with average rut depth of 3 mm andmaximum rut depth of 6 mm and no significant cracking.It should be mentioned that maximum acceptable rutdepth of a pavement is considered to be about12.5 mmduring its service life. Therefore, guidance can be takenfrom gradations of binder course and surface (wearing)course actually used on the test track with excellentperformance 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


Table 5 has been prepared to compare the existingMORTH BC Grading 1 of 19 mm NMAS mix (intendednow for use as a binder course) with the gradation specifiedby GDOT and the actual gradation range used on NCATtest track with excellent performance results. The last columnin this table has been formulated from the comparison as arecommended 19 mm NMAS BC binder course gradationfor India. Only the values for 19 mm, 13.2 mm, 9.5 mm and0.075 mm have been revised based on excellent performanceexperience in Georgia and on NCAT test track. Remainingvalues in the existing MORTH specifications for other sievesare reasonably close to Georgia and test track and, therefore,have been kept the same. The reasons for changing thevalues for 0.075 mm sieve have been discussed earlier in thepaper. The second largest sieve size (19 mm in this case) ofa dense graded mix should be 90-100 per cent material passingrather than 79-100 per cent. The reasons for the suggestedchanges are as follows:

� A few large size, isolated aggregate particleshardly increase the mix stability and canunnecessarily cause segregation problems.Photo 10 shows the presence of such scattered

particles in a compacted mat on a nationalhighway 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 andwill also have different minimum VMArequirements19. That is why; the Superpave mixdesign has control points of 90 and 100 per centfor 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 incompacted mat.

Grading Existing MORTH BC GDOT Binder NCAT Test Track Proposed BCGrading 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 mmAggregate 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


Table 6 has been prepared to compare the existing13 mm NMAS MORTH BC Grading 2 (intended nowfor use as a coarse surface or wearing course) with thegradation specified by GDOT and the gradation rangeused on NCAT test track with excellent performanceresults. The gradation range for the wearing course usedon the test track encompasses gradations below theSuperpave restricted zone (BRZ), through the restrictedzone (TRZ), and above the restricted zone (ARZ). Allthese gradations performed really well on the track. Therestricted zone specified earlier in Superpave throughwhich no gradation was permitted to pass, was alreadydeleted 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 lastcolumn of Table 6 has been formulated from thecomparison. Most of the values except for 9.5 mm and4.75 mm sieves have been revised. Reasons for revisingthe values for the second largest sieve 13.2 mm(or 12.5 mm) and the last 0.075 mm sieve were givenearlier. The values for sieves 2.36 mm down to 0.15 mm

have been generally lowered because it is believed thatthe present MORTH gradation has excessive amountsof fine aggregate (compared to GDOT and NCAT testtrack), which can increase the potential for rutting.

Table 7 shows the recommended final gradations of5 dense graded bituminous mixes for India: 37.5 mmNMAS DBM base course, 25 mm NMAS DBM basecourse, 19 mm NMAS BC binder course, 12.5 mmNMAS BC wearing course Grading 1, and a new9.5 mm NMAS BC wearing course Grading 2. The finer9.5 mm NMAS BC wearing course Grading 2 has beenadapted from the GDOT specification for Marshall mixes(Table 1), which has an excellent performance historyfor application on low to medium traffic roads, in urbanareas, and in thin (25 mm) applications. Values for threesieve sizes 1.18 mm, 0.6 mm and 0.15 mm have beeninterpolated. It is recommended to use Table 7 to revisethe gradations of MORTH DBM mix and BC mixes. Allthese 5 mixes are adequate for constructing conventionaldense graded bituminous pavements in India, both fornew construction and overlays. They are also suitablefor 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 90-100

9.5 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


earlier, it is not necessary to use all three courses (base,binder, and wearing) in a new flexible pavement unlessthe traffic is very high. For example, the followingcombinations can be used depending upon the totalthickness of the bituminous course(s) required as perstructural design based on IRC:37.

Note: Wearing course grading 1 is recommended forheavy traffic roads. Wearing course grading 2 isrecommended for light to medium traffic roads, in urbanareas, and for thin (25 mm) applications.

� WMM + DBM Base Course + BC Bindercourse + BC Wearing Course

� WMM + BC Binder Course + BC WearingCourse

� WMM + BC Wearing Course only

It has been surmised by some that a BC wearingcourse is too stiff and will crack if placed directly overWMM. This is not correct because the BC wearingcourse 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 includingAustralia and South Africa. Similarly, a bituminous overlayrequired for strengthening flexible pavement can consistof the following depending upon the required thicknessas 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 roadwith severe overloading problem, it is recommended tomodify the BC wearing course and BC binder course(that is, the top 100 mm of the pavement only, which islikely to rut) as follows:

� Ensure to use viscosity graded VG-30 gradebitumen as per latest IS73:20064, which issignificantly more rut resistant than the old 60/70 penetration bitumen.

� Use polymer modified bitumen (PMB)� Use Stone Matrix Asphalt (SMA) as per IRC

Specifications approved recently.

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

Grading Proposed DBM Proposed DBM Proposed BC Proposed BC Proposed BCBase 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 mmAggregate 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 4-7

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


6. CONCLUSIONS

1. For ensuring long-term performing pavementsfocus should shift to dense graded bituminousmixes rather than open graded lean bituminousmixes. The future trend is towards introducinghigh modulus rut resistant mix material to takecare of problems of rutting in bituminous layers.Rutting is broadly due to inadequacies in our mixdesign rather than granular bases or sub-grades.

2. Presently there is a proliferation of bituminouspaving mixes in India. Many of them are opengraded lean bituminous mixes. Too many optionsfor a specific bituminous course have createdconfusion in mix selection. Many pavementdesign engineers do not understand the individualcharacteristics of these mixes and, therefore,select them based on their specified layerthickness and/or cost.

3. Most countries of the world generally do not havemore than four dense graded mixes.Notwithstanding other lean mixes like BUSG, BPMthere are 8 mixes like BM (2 gradings), DBM (2gradings), SDBC (2 gradings), and BC (2 gradings).The study recommends in total five numbers ofbituminous mixes in lieu of current eight specifiedin MORT&H specifications as per details below.

� 37.5 mm NMAS DBM Base CourseGrading 1

� 25 mm NMAS DBM Base CourseGrading 2

� 19 mm NMAS BC Binder Course

� 12.5 mm NMAS BC Wearing CourseGrading 1

� 9.5 mm NMAS BC Wearing CourseGrading 2

4. Paper recommends phasing out of lean mixesas they are mainly responsible for poorperformance of bituminous pavements. It isrecommended not to use them on primarynetwork like NH or even State Highways.Specifications should restrict the mixes to ensurebetter quality roads.

5. Some new specifications like those similar toopen graded mixes such as permeable asphalttreated base (PATB) and OGFC may be

introduced to complete the specifications. BothPATB and OGFC need to be used in India onlyin exceptional circumstances due to theiradditional costs. These may be used whenspecifically required.

6. Bituminous Macadam is a highly permeable mixand promotes rutting. Use of BituminousMacadam a very popular mix at present maybe deleted and substituted with DBM becauseit is finally cost effective and better performing.Similarly, use of Semi-dense BituminousConcrete is also not considered to be allowed inthe specifications. It suffers from “pessimum”voids, which have potential to trap waterresulting in moisture damage. It should besubstituted by Bituminous Concrete as it is betterperforming and cost effective.

7. Use of thick wearing/surface course of 40 mm isrecommended in a tropical country like India toseal the pavement top from the ingress of waterand moisture. To take care of possible use ofwearing course of 25 mm on cost considerationas was the case in SDBC a 9.5 mm NMAS BCwearing course may be introduced in thespecifications after removing SDBC.

8. The Profile Corrective Course (PCC) shouldbe only in DBM or BC. The current use of BMas PCC should be discouraged and not allowed.

7. RECOMMENDATIONS

A. A suitable research study to study theperformance of specifications of bituminousmixes suggested above be carried out on thepattern of National Center for AsphaltTechnology (NCAT), Auburn University,Alabama since year 2000 by constructing a testtrack. Pending the above research output thesuggested change in the specifications may beincorporated and their performance in field beevaluated with respect to older specifications.

B. The research scheme suggested above shouldattempt new mixes which comprises of highmodulus rut resistant material to tackle theproblem of rutting on the lines of design conceptof perpetual pavements.

C. Ongoing debate for the use of larger Nominal


Maximum Aggregate Size (NMAS) in DBMetc. should be settled by conducting indigenousresearch and the existing grading should bemodified suitably. Pending the above researchoutput suggested grading be incorporated in thespecifications.

ACKNOWLEDGEMENT

The opinions expressed in this paper are those ofthe authors only.

REFERENCES

1. Asphalt Institute. Mix Design Methods for AsphaltConcrete. MS-2, Sixth Edition, 1997.

2. Brown, E.R. et al. Relationship of Air Voids, LiftThicknesses, and Permeability of Hot Mix AsphaltPavements, TRB, NCHRP Report 531, 2004.

3. Brown, E.R. and S. A. Cross. A Study of In-PlaceRutting of Asphalt Pavements. Asphalt PavingTechnology, Vol. 58, 1989.

4. Bureau of Indian Standards. Paving Bitumen –Specification (Third Revision) IS 73:2006, July 2006.

5. Comments on IRC Draft Flexible MacadamSpecifications published in Indian Highways, Vol.35, No. 8, August 2007.

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A CRITICAL REVIEW OF BITUMINOUS PAVING MIXES USED IN INDIA

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