50923638 Advances in Bituminous Road Construction by Prof Prithvi Singh Kandhal

8/6/2019 50923638 Advances in Bituminous Road Construction by Prof Prithvi Singh Kandhal

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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 grading system,

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:


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1. Tests and specifications are intended for bitumen "binders," whichinclude both modified and unmodified bitumens.

2. The physical properties measured by Superpave bitumen tests aredirectly related to field performance by engineering principles rather than

just the experience.

3. A long-term bitumen aging test, which simulates aging of bitumensduring 5-10 years in service, was developed and included for the first

time.

4. Tests and specifications are designed to eliminate or minimize threespecific types of asphalt pavement 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 temperatureexperienced at the project site 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 at the lowest

pavement temperature during winter at the project site.


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Figure 1: Superpave performance grade bitumen testing is conductedover 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 PG82. 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 of 3oC. A PG 70-4 bitumen will be specified for paving that

project.


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Aggregate

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

fabricated easily 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, which normally

has rounded particles.

Figure 2: Schematic

of equipment fortesting fine aggregate

angularity (FAA)


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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

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


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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

particle orientation. 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.

Fig. 4a Stone matrix asphaltcross-section

Fig. 4b Conventional hot mix asphaltcross-section


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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, cellulose fiber is added to

the mix to minimize drain down of bitumen in trucks during transportation of

the mix from plant 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.

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 the following 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.


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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.


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Warm mix asphalt offers the following significant advantages:

Energy savings. The most obvious benefit of WMA is the reduction infuel consumption. Fuel is 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 nonvisible) from the burning 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 odourboth at the plant and the 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 bindertakes place when it is mixed 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.


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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 rightcontaining warm mix asphalt at 250 F (121 C). (Photo courtesy:

Matthew Corrigan, FHWA)

Extended paving season. By producing WMA at normal HMAtemperatures, it may be possible 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 compared to 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 maybe used to improve the 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.


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Increased amount of RAP. Research has shown that the percentage ofreclaimed asphalt pavement (RAP) can be increased in WMA compared to

HMA during hot recycling.

Generation of carbon credits for India. Developing countries likeIndia 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 in

coating 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.


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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:

1. When properly used, recycling can result in substantial savings over theuse 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 theneed for new materials. This translates to substantial savings in

aggregate resources and demand for asphalt binder (bitumen),especially during supply interruptions. Even though there may be an

abundant 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 newmaterials in quality. Hot mix 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 thesurface course.

4. Recycling can maintain pavement geometrics as well as pavementthickness. The existing pavement 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 toconventional construction techniques. This factor is of significant

importance during an energy crisis like the one experienced during the1972 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


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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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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.

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 not

sufficient 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.


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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 inIndia for Paving Bitumen. 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 Asphalt Materials, Mixture Design and Construction.' NAPAEducation Foundation, Lanham, Maryland, Second Edition, 1996.

Kandhal, P.S. and F. Parker. 'Aggregate Tests Related to AsphaltConcrete Performance in Pavements.' Transportation Research Board,National Cooperative Highway Research Program Report 405, 1998.

Kandhal, P.S. 'Aggregate Tests for Hot Mix Asphalt:' State of thePractice. Transportation Research Board Circular No. 479, December,1997.

Kandhal, P.S. Design, Construction, and Maintenance of Open-GradedAsphalt Friction Courses. 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 QualityImprovement Publication QIP-122 (Revised Edition), March 2002.

Kandhal, P.S. Warm Mix Asphalt Technologies: An Overview. Journal ofthe Indian Roads Congress, Volume 71-2, 2010.

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

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


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About the Author

Prof. Prithvi Singh Kandhal is 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 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 membersfrom all continents in the world)

Chairman, American Society for Testing and Materials (ASTM)International Committee on Road Paving Standards (responsible for over

200 highway standards used worldwide)

Chairman, Transportation Research Board Committee on Asphalt Roads,U.S. National Academy 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.

Courtesy : NBMCW March 2011 Issue

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