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Advances in Bituminous Road Construction

Prof. Prithvi Singh Kandhal, Associate Director (Emeritus), National Center for Asphalt Technology

(NCAT), Auburn University, Alabama, U.S.A.

Introduction

An ambitious road construction plan is underway in India, which primarily involves bituminous

pavements. At the present time, Ministry of Road Transport & Highways (MORTH) Specification for

Road and Bridge Works, 2001 Edition is used for construction of all roads including national highways.

Advances in bituminous construction technologies are made in the world almost every year. This paper

describes such advances in terms of materials, mix design, special bituminous mixes, and recycling.

There is a need to incorporate these advances in MORTH specifications which are about 10 years old,

to keep abreast of latest technologies.


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Materials

Paving Bitumen

Prior to July 2006, penetration graded road paving bitumen such as 60/70 was used in India. This

grading system was based on empirical penetration test, which is conducted at 25oC. The penetration

test measures the consistency of bitumen. Although two 60/70 penetration bitumen samples from

different refineries may have similar consistency (stiffness) at 25oC, but one may be softer than the

other when tested at 60oC, which is close to the highest pavement temperature on a hot summer day.

Bitumen which is very soft at high temperature is undesirable because it can cause rutting in

bituminous pavement under heavy loads. Therefore, the Bureau of Indian Standards (BIS) adopted a

viscosity grading system for paving bitumen in July 2006 by issuing standard IS:73:2006. This system

is based on viscosity testing at 60oC. Penetration graded bitumen 60/70 was deleted and substituted

with viscosity graded VG-30. Similarly, penetration graded bitumen 80/100 was deleted and

substituted with viscosity graded VG-10.

Although the preceding advancement has been made, there is a need to advance further by adopting

performance graded (PG) bitumen, especially for national highways. The viscosity grading system

gave excellent performance results in the US for over 20 years. However, the viscosity gradingsystem, although more rational than the penetration grading system, was still based on experience. A

50-million dollar, 5-year Strategic Highway Research Program (SHRP) was undertaken from 1987 to

1992 to develop a performance based grading system for bitumen, which was based on engineering

principles to address common asphalt pavement distress problems. The so-called Superpave (acronym

for Superior Performing Pavements) performance grading system includes new bitumen tests and

specifications with the following salient features:

1. Tests and specifications are intended for bitumen "binders," which include both modified andunmodified bitumens.

2. The physical properties measured by Superpave bitumen tests are directly related to fieldperformance by engineering principles rather than just the experience.

3. A long-term bitumen aging test, which simulates aging of bitumens during 5-10 years inservice, was developed and included for the first time.

4. Tests and specifications are designed to eliminate or minimize three specific types of asphaltpavement distresses: rutting, fatigue cracking, and thermal cracking. Rutting typically occurs

at high temperature, fatigue cracking at intermediate temperature, and thermal cracking at

low temperatures.

5. As shown in Figure 1, the entire range of pavement temperature experienced at the projectsite is considered. New testing equipments were developed/adopted for testing bitumens for

this purpose. A rotational viscometer is used to measure the bitumen viscosity at 135oC. A

dynamic shear rheometer is used to measure the viscoelastic properties of the bitumen at two

temperatures: high temperature corresponding to the maximum 7-day pavement temperature

during summer at the project site, and intermediate temperature corresponding to the

average annual temperature of the pavement at the project site. A bending beam rheometer

and a direct tension tester are used to measure the rheological properties of the bitumen atthe lowest pavement temperature during winter at the project site.


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Figure 1: Superpave performance grade bitumen testing is conducted over the

entire range of temperature experienced at the project site

The Superpave performance grade (PG) bitumen is based on climate. For example, PG 64-22 bitumen

is suitable for a project location, where the average 7-day maximum pavement temperature is as

much as 64oC, and the minimum pavement temperature is 22oC.

The high temperature grades are PG 52, PG 58, PG 64, PG 70, PG 76, and PG 82. The low

temperature grades are 4, 10, -16, -22, -28, -34 and so forth. Both high and low temperature

grades are in increments of 6 Celsius degrees.

Example: A project location in Rajasthan has a maximum record 7-day pavement temperature of 70oC

in summer and a minimum record pavement temperature of3oC. A PG 70-4 bitumen will be specified

for paving that project.


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Aggregate

Figure 2: Schematic of

equipment for testing fine

aggregate angularity

(FAA)

Many advances have been made in characterization of coarse aggregate, fine aggregate and mineral

filler used in bituminous construction. However, one simple test for characterizing particle shape &

surface texture of fine aggregate (sand) passing 4.75 mm sieve can be implemented easily in India. It

is desirable to have angular fine aggregate particles in mix so as to resist rutting in bituminous

pavements. Whereas angularity of coarse aggregate (retained on 4.75 mm sieve) can be evaluated by

naked eye, it is not easy to do so in case of fine aggregate particles. The schematic of the test

equipment for measuring fine aggregate angularity (FAA) is shown in Figure 2. It can be fabricatedeasily in India. FAA test procedure has been adopted as standard AASHTO Test 304.

A calibrated cylindrical measure is filled with fine aggregate of prescribed grading by allowing the

sample to flow through a funnel from a fixed height into the cylindrical measure. The fine aggregate is

struck off at the rim, and its mass is determined by weighing. Uncompacted void content in the fine

aggregate is calculated as the difference between the volume of the cylindrical measure and the

absolute bulk volume of the fine aggregate collected in the measure. Bulk volume of the fine

aggregate is calculated from its mass and its bulk dry specific gravity.

This test is based on the concept that round particles pack closer than angular particles and therefore

produce lower uncompacted void content, that is, lower FAA value. A FAA value of 45 or more is

desirable to ensure that the fine aggregate is angular and does not contain any natural sand, whichnormally has rounded particles.

Mix Design

Marshall Mix design is currently used in India for designing bituminous mixes. In any mix design, it is

desirable to compact laboratory specimens to a density which is expected to be achieved in the

bituminous course after 2-3 years of densification under traffic. For designing bituminous mixes for

heavy traffic, 75 blows each are applied with a Marshall impact hammer on both sides of the


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specimen. This laboratory compaction level worked well in the past. However, it was observed in the

US during the 1980s that the field density of in-service bituminous pavements was significantly higher

than the laboratory design density obtained with 75 blows. This was attributed to increased truck tyre

pressures and new tyre designs with stiffer side walls. Therefore, 75-blow compaction level appeared

inadequate. Increasing the number of blows was not desirable because it merely caused degradation

(breakage) of aggregate particles in the specimen.

Figure 3: Schematic of Superpave gyratory compactor

During SHRP, a new Superpave mix design method was developed in the US. A Superpave gyratory

compactor (SGC) was developed which compacted the laboratory specimen with gyratory action (see

schematic of SGC in Figure 3) rather than impact compaction as is done with Marshall hammer.

Gyratory compaction also simulates field compaction with rollers in terms of aggregate particleorientation. Depending on the traffic level in ESALs (equivalent single axle loads) expected on the

highway, desired compaction level can be obtained in SGC by varying the number of gyrations without

causing any significant degradation of aggregate in the mix.

Another advantage of SGC is that a densification curve (number of gyrations versus compacted

density of specimen) is obtained during the compaction process. At least three different gradations of

the proposed mix are evaluated in the Superpave mix design to select the gradation which has the

strongest aggregate skeleton.

Special Bituminous Mixes

Stone Matrix Asphalt

Stone matrix asphalt (SMA) was developed in Germany in the mid 1960 and it has been used very

successfully by many countries including US as a highly rut-resistant bituminous course, both for

binder (intermediate) and wearing course for heavy traffic roads. SMA is tough, stable, rut-resistant

mix that relies on stone-on-stone contact to provide strength and a rich mortar binder to provide

durability.

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Fig. 4a Stone matrix asphalt cross-section

Figure 4 provides a cross-sectional representation of the difference between SMA and dense-graded

conventional bituminous mix. In a conventional dense graded mix, coarse aggregate particles

(retained on 4.75 mm sieve) are separated by fine aggregate matrix, which primarily carries the load.

In SMA, coarse aggregate particles have stone-on-stone contact forming a stone skeleton which

carries the load.

Since SMA mix has relatively higher bitumen content, cellulosefiber

Fig. 4b Conventional hot mix asphalt cross-section

is added to the mix to minimize drain down of bitumen in trucks during transportation of the mix fromplant to project site. Although the cost of SMA is typically about 25-30% higher than the cost of dense

graded bituminous mix, it is still economical considering life cycle costs.

SMA has been widely used in the US since 1991 for heavy-traffic roads. It must also be used in India

for heavy corridors especially when overloading is also common. Indian Roads Congress (IRC) has

recently published a tentative specification for SMA (IRC:SP:79-2008), which was drafted by the

author to facilitate its use in India. A manual containing detailed guidelines for designing and

constructing SMA mixtures was developed by the author in the US for practicing engineers.
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Open Graded Asphalt Friction Course

Open graded asphalt friction course (OGFC) is an open graded hot mix asphalt mixture with

interconnected voids that provide improved surface drainage during rainfall. The rainwater drains

vertically through the OGFC to an impermeable underlying bituminous layer and then laterally to the

day lighted (exposed) edge of the OGFC onwards to shoulder. In addition to minimizing hydroplaning

potential during rainfall and providing improved friction values on wet pavements, the OGFC offers thefollowing advantages compared to other dense graded surfaces: (a) reduced vehicle splash and spray

behind vehicles, (b) reduced tyre-pavement noise, (c) enhanced visibility of pavement markings, and

(d) reduced night time surface glare in wet weather.

Numerous states in the US currently using OGFC have experienced excellent performance in terms of

safety (improved wet pavement surface friction) and durability. This has been accomplished by one or

more of the following: use of polymer modified asphalt binders, relatively higher bitumen content (by

using cellulose fibers), and/or relatively open gradations.

Figure 5 shows an interstate highway in the US, where OGFC was used in the lanes on the right side

and dense graded bituminous mix was used in the lanes on the left side. Note the dramatic difference:

there is no standing water and absence of splash/spray on the lanes on the right side during rain.

A manual giving detailed guidelines on design, construction and maintenance of OGFC was developed

by the author for use by practicing engineers in the US. Due to economic considerations, OGFC should

be used in India selectively in regions with heavy rainfall and stretches of roads prone to accidents

resulting from skidding on wet pavement.

Warm Mix Asphalt

Figure 5: Lanes on the right have OGFC and lanes on the left have dense graded

hot mix asphalt on a interstate in the US. Note absence of standing water and

splash/spray on the lanes on the right side.

Warm mix asphalt (WMA) is a fast emerging new technology which has a potential of revolutionizing

the production of asphalt mixtures. WMA technology allows the mixing, lay down, and compaction of

asphalt mixes at significantly lower temperatures compared to hot mix asphalt (HMA). The technology


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can reduce production temperatures by as much as 30%. Asphalt mixes are generally produced at

150C or greater temperatures depending mainly on the type of binder used. WMA mixes can be

produced at temperatures of about 120C or lower.

The development of WMA was initiated in Europe in the late 1990s primarily in response to the need

for greenhouse gas reduction under the Kyoto Protocol. WMA technologies such as Aspha-min, WAM

Foam, and Sasobit were developed during that time. New WMA technologies such as Evotherm,Rediset WMX, REVIX, LEA (Low Energy Asphalt) and Double Barrel Green were later developed within

the US.

Warm mix asphalt offers the following significant advantages:

Energy savings. The most obvious benefit of WMA is the reduction in fuel consumption. Fuelis used to dry and heat the aggregate. Studies have shown that lower plant mix temperatures

associated with WMA can lead to as much as 30 percent reduction in energy consumption.

Decreased emissions.WMA produces emissions (both visible and non visible) from theburning of fossil fuels at a significantly reduced level compared to HMA (Figure 6). This would

permit asphalt plants to be located in and around non-attainment areas such as large

metropolitan areas that have air quality restrictions.

Decreased fumes and odour.WMA produces lower fumes and odour both at the plant andthe paving site compared to HMA. This would also result in improved working conditions at

both places.

Decreased binder aging.Short-term aging of liquid asphalt binder takes place when it ismixed with hot aggregate in pug mill or mixing drum. This aging is caused by the loss of

lighter oils from the liquid asphalt binders during mixing at high temperatures. It is believed

that the short-term aging of the binder will be reduced significantly because the loss of lighter

oils will be less at relatively lower mixing temperatures. This may enhance asphalt pavement

durability.

Figure 6: Emission can be seen from the hot mix asphalt at 320 F (160 C) in

left truck. No emission is visible from the truck in right containing warm mix

asphalt at 250 F (121 C). (Photo courtesy: Matthew Corrigan, FHWA)


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Extended paving season.By producing WMA at normal HMA temperatures, it may bepossible to extend the paving season into the colder months of the year or in places located on

high altitudes since the WMA additives or processes act as a compaction aid. Further by

narrowing the difference between compaction temperature and ambient air temperature the

rate of cooling is decreased. WMA may also be transported over longer distances as comparedto HMA with reduced loss of mix temperature in the hauling units. This advantage should

facilitate the Indian Border Roads Organization (BRO) in constructing asphalt roads in high

altitude and/or remote areas far away from hot mix plants.

Compaction aid for stiffer mixes.WMA additives and processes may be used to improvethe compactibility of stiff mixes when mix is produced closer to typical HMA production

temperatures. Smaller reductions in temperature may also be possible. There is extensive

experience with the use of certain types of WMA with SMA in Europe.

Increased amount of RAP.Research has shown that the percentage of reclaimed asphaltpavement (RAP) can be increased in WMA compared to HMA during hot recycling.

Generation of carbon credits for India.Developing countries like India can earn CERs(Certified Emission Reductions) or popularly known as carbon credits under the Kyoto Protocol

if technologies such as WMA are introduced and implemented.

At the present time a mix is considered warm mix in the US if the mix produced at the plant has

temperature exceeding 100C but significantly below that of a normal hot mix. WMA has a wide range

of production temperatures ranging from slightly over 100C to about 20 to 30C below typical HMA

temperatures. WMA technologies are also applicable to mixes made with polymer modified asphalt

binders.

WMA technologies can be classified broadly as (a) those that use water, (b) those that use some type

of organic additive or wax, and (c) those that use chemical additives or surfactants.

Technologies which introduce small amounts of water to hot asphalt binder, take advantage of the

phenomenon: when water turns into steam at atmospheric pressure it expands in volume by a factor

of 1,673. This causes tremendous increase in the volume of asphalt binder which not only helps incoating the aggregate easily but also lowers the mix apparent viscosity. Processes to introduce water

into the asphalt binder consist of foaming nozzles, use of hydrophilic material such as zeolite or use of

damp aggregate. Asphalt binder temperature typically is the same as that used for hot mix asphalt.

Technologies that use organic additives or waxes lower the asphalt binder viscosity above their

respective melting points. It should be ensured that their melting points are above the in-service

pavement temperatures during hot summers so that permanent deformation or rutting does not

become a problem.

Technologies that use some chemical additive and /or surfactants produce a variety of different

mechanisms to coat the aggregate at lower temperatures.

It appears WMA technology is about to take off in India. There is a need to incorporate WMA

specifications in MORTH specifications.

Recycling of Bituminous Pavements

Recycling of existing asphalt pavement materials to produce new pavement materials results in

considerable savings of material, money, and energy. The specific benefits of recycling can be

summarized as follows:


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1. When properly used, recycling can result in substantial savings over the use of new materials.Also, the cost of haulage can be avoided if recycling is performed in place. The need for

economic consideration is felt now more than ever, because of tightening budgets and ever

increasing cost of materials.

2. Recycling can help in conservation of natural resources by reducing the need for newmaterials. This translates to substantial savings in aggregate resources and demand for

asphalt binder (bitumen), especially during supply interruptions. Even though there may be anabundant supply of aggregates, the distribution of these sources does not always coincide with

the location of need.

3. Recycled materials have proven to be equal or even better than new materials in quality. Hotmix asphalt (HMA) overlay on recycled base is expected to perform better than an HMA

overlay on the existing surface, even though they have the same thickness, because the

former can substantially reduce the potential of reflective cracking through the surface course.

4. Recycling can maintain pavement geometrics as well as pavement thickness. The existingpavement structure can be strengthened by recycling without adding substantial overlays. In

some cases, the traffic disruption is lesser than that for other rehabilitation techniques.

5. Recycling can save considerable amount of energy compared to conventional constructiontechniques. This factor is of significant importance during an energy crisis like the one

experienced during the 1972 Arab oil embargo.

Over the years, recycling has become one of the most attractive pavement rehabilitation alternatives.

With the continuous accumulation of performance data, field and laboratory evaluations of recycled

mixes, and with the simultaneous development of realistic performance oriented guidelines it is

expected that recycling will continue to be the most attractive rehabilitation technique.

Different recycling methods are now available to address specific pavement distress and structural

needs. A brief description of these recycling methods follows.

The Asphalt Recycling and Reclaiming Association define five different types of recycling methods: (1)

Cold Planing; (2) Hot Recycling; (3) Hot In Place Recycling; (4) Cold In-Place Recycling; and (5) Full

Depth Reclamation.

Cold planing is described as an automatic method of removing asphalt pavement to a desired depth

and restoration of the surface to a desired grade and slope and free of humps, ruts and other

distresses. This method can be used for the roughening or texturing of a pavement to improve

frictional resistance. Cold planing is performed with a self propelled rotary drum cold planing machine

with the reclaimed asphalt pavement (RAP) transferred to trucks for removal from the job site. The

resulting pavement can be used immediately by regular traffic and overlaid at some future time or left

as a textured surface.

Hot recycling or hot mix recycling is the process in which reclaimed asphalt pavement (RAP) material

is combined with new materials, sometimes along with a recycling agent, to produce hot mix asphalt

(HMA) mixtures. Both batch and drum type hot mix plants are used to produce recycled mix. The RAP

material can be obtained by milling or ripping and crushing operation. RAP at ambient temperature

when introduced in weigh hopper of the batch plant (Figure 7) or drum of the drum plant is heated by

superheated virgin aggregate. If the amount of RAP exceeds 15-20 percent, a softer asphalt binder is

used to rejuvenate the aged asphalt binder in the RAP. The mix placement and compaction equipment

and procedures are the same as for regular HMA. Typical RAP to new aggregate ratio varies from

10:90 to 30:70 with a maximum of 50:50 (drum plant). The advantages of hot mix recycling include

significant structural improvement, equal or better performance compared to conventional HMA, and

capability to correct most surface defects, deformation, and cracking.


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Figure 7: Hot mix asphalt recycling in a batch plant

Hot in place recycling (HIR) consists of a method in which the existing pavement is heated and

softened, and then scarified or hot rotary mixed to a specified depth. New HMA or recycling agent may

be added to the RAP material during the recycling process. HIR can be performed either as a single

pass or a multiple pass operation. In single pass operation, the restored RAP material is combined

with new material. In multiple pass operation, the restored RAP material is recompacted first, and a

new wearing surface is applied later. The depth of treatment varies between 20 to 40 mm (3/4 in to

1 in). The Asphalt Recycling and Reclaiming Association (ARRA) has identified three HIR processes;

(a) surface recycling, (b) repaving, and (c) remixing. In a surface recycling operation the existing

asphalt surface is heated and scarified to a specified depth. The scarified material is combined with

aggregate and/or recycling agent. The mix is then compacted. A new overlay may or may not be

placed on the recycled mix. In the second type of HIR method, repaving, the surface recycling methodis combined with a simultaneous overlay of new hot mix asphalt (HMA). Both the scarified mix and the

new HMA are rolled at the same time.

In the case of remixing, the scarified RAP material is mixed with virgin HMA in a pug mill, and the

recycled mix is laid down as a single mix. The advantages of hot in place recycling are that surface

cracks can be eliminated, ruts and shoves and bumps can be corrected, aged asphalt binder is

rejuvenated, aggregate gradation and asphalt content can be modified, traffic interruption is minimal,

and hauling costs are minimized.

In cold in place recycling (CIR), the existing pavement material is reused without the application of

heat. Except for any recycling agent, no transportation of materials is usually required, and, therefore,

haulage cost is very low. Normally, an asphalt emulsion is added as a recycling agent. The process

includes pulverizing the existing pavement, sizing of the RAP, application of recycling agent,

placement, and compaction. The use of a recycling train, which consists of pulverizing, screening,

crushing, and mixing units, is quite common. The processed material is deposited in a windrow from

the mixing device, where it is picked up, placed, and compacted with conventional hot mix asphalt lay

down and rolling equipment. The depth of treatment is typically from 75 to 100 mm (3 to 4 in).

The advantages of cold in place recycling include significant structural improvement, treatment of

most pavement distress, improvement of ride quality, minimum hauling and air quality problems, and

capability of pavement widening.


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Full depth reclamation has been defined as a recycling method where all of the asphalt pavement

section and a predetermined amount of underlying material are treated to produce a stabilized base

course. It is basically a cold mix recycling process in which different types of additives such as asphalt

emulsions and chemical agents such as calcium chloride, Portland cement, fly ash, and lime, are

added to obtain ail improved base. The four main steps in this process are pulverization, introduction

of additive, compaction, and application of a surface or a wearing course. If the in place material is notsufficient to provide the desired depth of the treated base, new materials may be imported and

included in the processing. This method of recycling is normally performed to a depth of 100 mm to

305 mm (4 to 12 in). The advantages of full depth reclamation are that most pavement distresses are

treated, hauling costs are minimized, significant structural improvements can be made (especially in

base), material disposal problems are eliminated, and ride quality is improved.

Summary

This paper describes recent advances in bituminous road construction in terms of materials, mix

design, special bituminous mixes, and recycling. These include performance grading system for paving

bitumen; measuring particle shape of fine aggregate; Superpave mix design; stone matrix asphalt

(SMA); open graded asphalt friction course (OGFC); warm mix asphalt (WMA); and four types of

asphalt pavement recycling.

References

Kandhal, P.S. An Overview of the Viscosity Grading System Adopted in India for PavingBitumen. Indian Highways, Volume 34, No. 4, April 2007.

Roberts, F.L., P.S. Kandhal, E.R. Brown, D.Y. Lee, and T.W. Kennedy. 'Hot Mix AsphaltMaterials, Mixture Design and Construction.' NAPA Education Foundation, Lanham, Maryland,

Second Edition, 1996.

Kandhal, P.S. and F. Parker. 'Aggregate Tests Related to Asphalt Concrete Performance inPavements.' Transportation Research Board, National Cooperative Highway Research Program

Report 405, 1998.

Kandhal, P.S. 'Aggregate Tests for Hot Mix Asphalt:' State of the Practice. TransportationResearch Board Circular No. 479, December, 1997.

Kandhal, P.S. Design, Construction, and Maintenance of Open-Graded Asphalt FrictionCourses. National Asphalt Pavement Association Information Series 115, May 2002.

Kandhal, P.S. Designing and Constructing Stone Matrix Asphalt Mixtures State-of-the-Practice.National Asphalt Pavement Association Quality Improvement Publication QIP-122 (Revised

Edition), March 2002.

Kandhal, P.S. Warm Mix Asphalt Technologies: An Overview. Journal of the Indian RoadsCongress, Volume 71-2, 2010.

Kandhal, P.S. Recycling of Asphalt Pavements: An Overview. Association of Asphalt PavingTechnologists, Asphalt Paving Technology, Vol. 66, 1997.

Kandhal, P.S. and R.B. Mallick. Pavement Recycling Guidelines for State and LocalGovernments. Federal Highway Administration Publication No. FHWA-SA-98-042, December,1997.

About the Author

Prof. Prithvi Singh Kandhalis Associate Director (Emeritus) at the National Center for Asphalt

Technology (NCAT) based at Auburn University, Alabama, U.S.A. NCAT is the largest asphalt

(bitumen) road technology center in the world. Prior to joining NCAT in 1988, Kandhal served as Chief

Asphalt Engineer of the Pennsylvania Department of Transportation for 17 years. He is the first person


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born outside North America, who has held the following three very prestigious positions in the asphalt

technology area:

President, Association of Asphalt Paving Technologists (with members from all continents inthe world)

Chairman, American Society for Testing and Materials (ASTM) International Committee onRoad Paving Standards (responsible for over 200 highway standards used worldwide)

Chairman, Transportation Research Board Committee on Asphalt Roads, U.S. NationalAcademy of Sciences

Prof. Kandhal has published over 120 technical papers and has co-authored the first ever textbook on

asphalt road technology, which is used by more than 25 universities in the U.S.

NBMCW March 2011

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