HOW TO SELECT BITUMEN FOR INDIAN HIGHWAYS?

Nivitha M. R., Reashma P. S. and Murali Krishnan J. #

Department of Civil Engineering, Indian Institute of Technology Madras

Chennai 600036, India#Corresponding Author E-mail: jmk@iitm.ac.in

INTRODUCTION

Bitumen, the binder used in bituminous pavement construction is a highly complex

hydrocarbon mixture. The raw material for bitumen is the crude oil and the physical

chemistry of bitumen depends to a large extent on the properties of crude oil. It is well

known that the quality and composition of crude oil varies depending on a multitude of

factors. For instance, depositional environments of source rock which generate oils,

thermal maturity, biodegradation and migration are some of the geochemical factors

which influence the quality of crude oil drilled from the earth.

Oil companies purchase crude oils based on different factors and these include the API

gravity, the price of crude oil per barrel and demand and supply along with the refining

capabilities of the associated refineries. Since the physico-chemical properties of the

crude oil keeps changing depending on the quality of the source rock and thermal

maturity, refineries use advanced technologies so that the maximization of the production

of lighter ends can be achieved. The properties of the vacuum residue also changes based

on the properties and production process of crude oil and this result in the variability of

the quality of the bitumen produced. It is hence challenging for a highway engineer to

write clear technical specification for bitumen since this requires understanding of the

variability of the raw material (crude source) and the associated production processes

(Reashma et al., 2012). This note will discuss the issues related to the choice of binder.

This is part of the research and development work currently being carried out at Indian

Institute of Technology Madras. This note will place before the highway engineers the

list of tasks to be completed in terms of data collection and analysis so that a rational

choice of binder can be made for highway construction.

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Bituminous pavements consists of at least two layers of bituminous materials laid on

three to four granular layers and the entire structure rests on the prepared subgrade. Since

the bituminous layers are in direct contact with the traffic loads, considerable stresses and

strains are induced in these layers. As bituminous mixtures consist of aggregate particles

(rigid inert particles) of various sizes and percentages bound with the bituminous binder,

the mechanical response (stress-strain-time relationship) of the mixture is completely

dictated by the mechanical response of the binder. As bitumen exhibits viscoelastic

behaviour and its response is dependent on load time and rate (frequency), load

magnitude (amplitude) and temperature, the bituminous mixture exhibits similar response

characteristics. Hence, if one can write clear technical specification for bitumen, it is

expected that it will result in a bituminous pavement with the required performance

characteristics, all other conditions considered constant.

The distresses in a bituminous pavement can be broadly classified into rutting, fatigue

cracking, low temperature cracking and moisture induced damage. Rutting is ascribed

due to two reasons: densification of the bituminous mixture and shear flow of the

densified mixture. Rutting occurs at an accelerated pace when the temperature of the

pavement is high. As the temperature increases, the binder in the bituminous mixture

softens considerably leading to readjustment of the aggregate matrix during load

application. If the binder is stiffer at the highest temperature experienced by the

pavement, the structural readjustment cannot take place at an increased rate and hence the

deterioration of the pavement through rutting occurs at a reduced rate. As the temperature

of the pavement reduces, the binder becomes stiffer and hence the bituminous mixture

exhibits mechanical response similar to a “brittle elastic material”. Due to this brittleness,

most of the bituminous layers exhibit fatigue cracking. To alleviate such failures, it is

required that the binder exhibits response similar to a “fluid” even at such low

temperature. In such case, the bituminous mixtures will not become unduly brittle and

hence one can prevent the failure of the material due to fatigue cracking. As one can see,

contradictory requirements are expected from a binder; the binder should be “stiff” at

high temperature and “soft” at lower temperature. It now requires to be seen how to test

this “stiff” and “soft” behaviour and one also needs to understand the magnitude and

influence of “high” and “low” temperatures.

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Due to the rate and temperature dependent response of the material in real life conditions,

it is required that any laboratory characterization for the binder should be carried out at

similar loading and temperature conditions. As the speed and design traffic volume varies

for different classes of highways, the testing of the bituminous binder should also be

carried out for different amplitudes and frequencies. Since the temperature of the

environment changes gradually, the mechanical response of the bituminous mixtures also

exhibit such gradual change. Hence, the binder should not only be tested for extreme

temperatures but also characterized for the complete pavement temperature range.

Before the advent of Superpave and the wide usage of dynamic shear rheometer for

binder testing, characterization of the binder over a range of temperature and analysis of

the same was carried out by different methods. One of the most important procedures

developed was due to Heukelom (1969). He devised a chart for plotting the results of the

standard laboratory tests on bitumen against temperature to distinguish different types of

bitumen. This chart was used to estimate their performance requirement, check the

consistency of the test data and aid in the usefulness of Van der Poel’s nomograph.

Probably his chart was one of the earliest to quantify the influence of temperature and

binder property on all facets of pavement construction and service. Figure 1 shows how

the BTDC chart is conceptually related to pavement performance.

Figure 1: Conceptual use of BTDC for windows in specifications (van de Ven et al., 2004)

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To summarize, the following are some of the important issues related to the choice of

binder and its relationship to pavement performance:

1. Thermal cracking, fatigue cracking and rutting in bituminous layers are

influenced by the binder approximately to an extent of 80, 60 and 40% (Kennedy

et al., 1994). Considering the influence of the type of binder on the common

modes of distresses and the effect of temperature and traffic on the performance

of the binder, one can conclude that temperature and traffic are two main factors

influencing the performance of the pavement at any location.

2. A complete investigation of the behaviour of binder under different loading

conditions (frequency) and temperature is necessary and this will be helpful in

achieving the prediction of the performance of the binder in the field. Hence a

fundamental rheological characterization is needed to understand the viscoelastic

behavior of bitumen and this can be carried out by dynamic mechanical analysis

over a wide range of frequency and temperature.

3. Hence, the pavement engineer should first know the complete temperature

range of the pavement in the location where the highway has to be built. The

traffic has considerable influence on the expected pavement performance and this

is the second important data which is needed. Knowing the pavement temperature

range and the expected traffic, the capability of the binder to provide the

expected pavement performance should be characterized.

In the following all these issues are discussed.

HOW TO ESTIMATE DESIGN AIR AND PAVEMENT TEMPERATURE?

Currently in India, extensive studies pertaining to temperature and traffic for any location

and the associated rheological characterization of binder specific to that temperature is

not available. The temperature for any location is accounted in terms of the average air

temperature in the design code for flexible pavements, IRC-37 and the regions are

divided into hot, cold and moderate without specifying the temperature range for these

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classifications. Traffic is generally not classified except for one special case which

specifies a binder grade for high traffic in any climatic condition.

The factors influencing the pavement temperature are air temperature and latitude

(Rumney and Jimenez, 1971). In this study currently being carried out at IIT Madras, a

large database of air temperature was collected. Thirty seven locations were selected such

that they represent all the geographical areas across India. Daily maximum and minimum

air temperatures were collected for all these locations for a period of 30 years, from 1970

to 2000, from the archived database available with the Indian Meteorological

Department, Pune (IMD, 2011). Artificial Neural Networks (ANN) was chosen as the

tool for forecasting air temperature as it is considered to be effective in pattern

recognition and long term forecasting. The model was also validated for Chennai with the

daily air temperature maximum and minimum data obtained from a public domain. The

seven-day average maximum and one day minimum air temperatures were calculated and

considered as the design air temperatures. The seven day average maximum air

temperature is calculated by taking a seven day moving average of the air temperature for

the design period and the maximum temperature of this is considered. The lowest air

temperature for the entire design period is taken as the one day minimum air temperature.

The next step is to calculate the pavement temperature. Since India does not have a

database of field pavement temperature systematically collected, models developed as

part of the Long Term Pavement Performance (LTPP) program conducted in the USA

was used. Eleven sites from LTPP were chosen such that the latitudes of these locations

were similar to that of India. About 172 data points were extracted and a regression

equation was fit to this data (Nivitha and Krishnan, 2012). From this data set, the

pavement temperature data was calculated and contour maps were drawn. Figure 2 shows

the maximum pavement temperature contour for India. It can be deduced that the

pavement temperatures in India is the highest in regions of central Rajasthan, some

portions of Haryana, Uttar Pradesh, Madhya Pradesh, Bihar, Jharkhand and Chhattisgarh

approaching 70 oC. These regions are critical during winter also as the temperature

reaches close to sub-zero in some of these locations. The low temperature map is not

shown here for want of space.

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Figure 2: Maximum pavement temperature contour for India (Nivitha and Krishnan, 2012)

HOW TO CHARACTERIZE THE RHEOLOGICAL BEHAVIOR OF BINDER

FOR THE COMPLETE PAVEMENT TEMPERATURE RANGE?

The change of any predefined binder property as a function of temperature is defined as

temperature susceptibility. It is essential to quantify the variation of binder property in

relation to the temperature changes. From the earlier section, it is seen that the pavement

temperature in India varies from 20 to 70 oC (intermediate to high temperature). Hence it

is essential to study the rheological behaviour of bitumen produced in India over this

temperature range for a wide range of frequency.

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As a part of the ongoing research project, “Up-gradation of IS 73:2006 Indian Standard

Paving Bitumen Specification”1, 300 samples were collected from various refineries and

construction sites across India. To characterize the binder over the specified temperature

range, the binders were subjected to three different sets of testing - conventional testing,

performance grade testing and finally dynamic mechanical testing.

In conventional testing, all the collected samples were tested for their physical properties

as specified by IS73, 2006. Of the 300 samples, 100 samples were tested for their

performance grade properties as specified by ASTM D6373 (2007). To understand the

rheological properties, the samples were subjected to dynamic mechanical testing. The

dynamic mechanical tests were conducted using the Dynamic Shear Rheometer (DSR)

with 25 mm diameter cone and plate geometry with a gap setting of 0.047 mm. All the

tests were conducted in the linear viscoelastic regime in oscillatory domain. The dynamic

mechanical testing was performed to capture the rheological behaviour of bitumen by

considering the effect of both temperature and loading rate over a temperature range of

25 -75 oC.

If the response of the material should be quantified for a wide range of temperature and

frequency, one plots the master curve of the test data by appealing to the Time

Temperature Superposition Principle (TTSP). Use of TTSP allows one to combine the

effect of time and temperature. For constructing a master curve, it is necessary that such

curves are constructed in the temperature regime in which bitumen does not exhibit any

internal structural changes. To determine such changes, Black diagrams (dynamic

modulus Vs the phase lag) are normally plotted. Figure 3 shows a sample Black diagram

and figure 4 shows a sample master curve for a bitumen sample at unaged and short-term

aged condition. The obtained master curve gives the binder response for a frequency

range of more than 7 decades.

1 This study was funded by Bureau of Indian Standards (BIS). The work is completed and the report is submitted to BIS for approval.

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Figure 3: Black diagram Figure 4: Master curve for a sample both at unaged and short term aged conditions

From the master curve it is now possible to find out how the samples can behave even

though they are classified under the same grade (viscosity grading system). In the study

conducted at IIT Madras, such analysis was carried out for more than 100 samples and

the samples were grouped into two categories based on their temperature susceptibility.

The properties of binders classified as “better temperature susceptible” materials do not

vary drastically with temperature and hence exhibit better performance compared to the

“poor temperature susceptible” materials over a wide range of temperature.

RECOMMENDATIONS

Considerable understanding was developed at IIT Madras when this preliminary work

was carried out and the following are some of the recommendations arising from this

study:

1. Collection of pavement temperature data systematically and continuously for

select locations across the country.

2. Collection of axle load and traffic data systematically and continuously for select

locations across the country.

3. Development of binder database with as much information as possible (refinery,

production process, crude source(s), blend proportion, chemical composition, and

rheological properties at different aging conditions).

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Acknowledgements:

The authors thank the Bureau of Indian Standards for funding the research project that

enabled an extensive collection and testing of bitumen samples from various refineries

and construction sites across India, Indian Meteorological Department for providing air

temperature data and Chennai Petroleum Corporation Limited for facilitating the inter-

laboratory testing.

References:

ASTM D 6373(2007), Standard Specification for Performance Graded Asphalt Binder, American Society for Testing and Materials, Annual Book of ASTM Standards, 4.03, Section 4, Philadelphia, USA.

Elseifi, M. A., Al-Qadi, I. L., Flinstch, G. W., & Masson, J.-F. (2002). Viscoelastic Modeling of Straight Run and Modified Binders Using the Matching Function Approach. International Journal of Pavement Engineering, 3, 53-61

IMD (2011). Indian Meteorological Department. Pune, India, 2011

IS 73: 2006, Paving Bitumen – Specification, Third Revision, Bureau of Indian Standards, New Delhi, July 2006.

Kennedy, T., Huber, G. A., Harrigan, E. T., Cominsky, R. J., Hughes, C. S., Von Quintus, H. L. & Moulthrop, J. S.(1994). SHRP-A-410: Superior Performing Asphalt Pavements (Superpave): The Product of the SHRP Asphalt Research Program. Strategic Highway Research Program.

Heukelom.W (1969). A bitumen test data chart for showing the effect of temperature on the mechanical behavior of asphaltic bitumens. Journal of the Institution of Petroleum Technologists, 55, 404-417.

van de Ven, M. F. C., Jenkins. & Bahia, H. U. (2004). Concepts used for development of bitumen specifications, Proceedings of the 8th Conference on Asphalt Pavements for Southern Africa (CAPSA'04), 12 – 16 September 2004, Sun City, South Africa

Nivitha, M.R. & Krishnan, J.M. (2012). Binder selection methodology for bituminous pavements. Building and environment, Under review.

Reashma, P.S., Nivitha, M.R., Krishnan, J.M. & Veeraragavan, A. (2012). Statistical Analysis of variation of bitumen property. Journal of the Institution of Engineers (India): Series A, Under review.

Rumney, T. and Jimenez, R. (1971). Pavement temperatures in the southwest. Highway Research Record, 361, 1-19.

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