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International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 10, October 2017, pp. 435–453, Article ID: IJCIET_08_10_045

Available online at http://http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=8&IType=10

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication Scopus Indexed

OPTIMUM BITUMEN CONTENT FOR

BITUMINIOUS CONCRETE – AN

ALTERNATIVE APPROACH FOR ESTIMATION

R. P. Panda

Research Scholar, SOA University, Bhubaneswar, Odisha, India

S. S. Das*

Department of Civil Engineering, VSSUT, Burla, Odisha, India

P. K. Sahoo

Dean ITER, SOA University, Bhubaneswar, Odisha, India

ABSTRACT

The major parameter in design of hot mix asphalt (HMA) widely used in flexible

pavements as base course and surfacing course all over the world, is to find out the

optimum bitumen content (OBC) required in achieving the desired objective. Bitumen

is a costly material and needs to be designed very carefully to achieve the desired

objectives, namely structural strength, skidding resistance, resistance to oxidation,

impermeability of HMA. The traditional method of mix design is a lengthy

cumbersome process. In this paper an alternative method is suggested to find out the

OBC from the physical properties of aggregate used in a particular type of HMA

(Bituminous Concrete) with a particular grade of bitumen (Viscosity Grade - 30). In

this method, bitumen need for different purposes of HMA have been calculated and

added together for calculating the OBC. Forty-five samples collected both from State

Highways and National Highways, geographically spread in different parts of Odisha,

India are analysed. OBC calculated by alternative method developed from the simple

physical property of the aggregate used such as gradation, elongation and flakiness

and water absorption/ porosity for a particular type of HMA and specific grade of

bitumen. The results of alternative method are comparable with the actual

consumption of the bitumen obtained from extraction test.

Keywords: Hot Mix Asphalt, Optimum Bitumen Content, Bituminous Concrete, Mix

Design.

Cite this Article: R. P. Panda, S. S. Das and P. K. Sahoo, Optimum Bitumen Content

for Bituminious Concrete – An Alternative Approach for Estimation, International

Journal of Civil Engineering and Technology, 8(10), 2017, pp. 435–453

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=10

R. P. Panda, S. S. Das and P. K. Sahoo


1. INTRODUCTION

Basic objectives of hot mix asphalt (HMA) design is to develop a non-decaying material,

blending asphalt and aggregates economically with sufficient stability and durability under

service condition. A good HMA mix is the balance of all the six important desirable

properties such as stability, durability, impermeability, workability, flexibility and fatigue

resistance [1, 2]. The primary role of aggregate is to provide structural strength in terms of

stability, durability and fatigue resistance, whereas the role of bitumen is to provide the

bonding, impermeability, flexibility and workability in the HMA. The HMA is designed

worldwide using Marshall mix design, Hveem mix design and Superpave gyratory mix design

methods [3]. In India, the mix design method adopted is stipulated in specification of Road

and Bridge works (Fifth Revision) published by Indian Road Congress (IRC), New Delhi,

2013, which is based on the Marshall method, as described in Asphalt Institute Manual

Series-2 (MS-2) [4]. However, all the mix design methods have common criterion such as to

find out just and sufficient asphalt to ensure a durable pavement, stable under the designed

traffic load, having optimum air voids (upper limit to prevent excessive environmental

damage and lower limit to allow room for initial densification due to traffic, temperature

variation etc.) and desired workability.

Marshall mix design method used for determining the optimum bitumen content (OBC), is

a very cumbersome process. The design is a six step process as follows

i. specimen preparation (having different trail blends)

ii. determination of different physical properties of each specimen for making the graph

stated in step “iv” below

iii. determination of Marshall stability and flow of each specimen

iv. preparation of different graphical plots primarily to know bitumen content as stated in

step “v” below

v. determination of bitumen content corresponding to maximum stability, maximum bulk

specific gravity of mix and median of designed air void percent

vi. Average of bitumen contents as obtained for the three properties stated in step “v”.

The average as obtained in step “vi” is the OBC for that specific type of aggregate and

bitumen in that HMA type.

The process is not only time consuming but also require a laboratory with trained

technical work force. The developing countries like India have deficient in sophisticated

laboratory as well as technical manpower in remote areas where the development of road

network, using locally available materials is essential for comprehensive growth of the

country. Therefore, the design of a HMA mix becomes a difficult task for field engineers

engaged in construction of bituminous pavement in remote areas. To overcome this difficulty,

an alternative approach for calculating the OBC from the physical properties of the material

used in HMA is presented in the paper. OBC has been calculated theoretically in this

approach for different samples of material used in highway construction in different parts of

Odisha, India and compared with the actual consumptions in the field obtained from the

bitumen extraction test.

2. HMA DESIGN AN OVERVIEW

HMA is a heterogeneous material of aggregate, mineral filler, bitumen, additives and air

voids. The bitumen is the costliest materials used in HMA and therefore, its optimal judicious

use is a prerequisite from economical point of view. The optimum bitumen requirement in

HMA is the sum total of following components viz. (i) to coat the aggregate, (ii) to float/coat

the dust particle, (iii) to be absorbed inside the aggregate (filling voids within the aggregate)

Optimum Bitumen Content for Bituminious Concrete – An Alternative Approach for Estimation


and (iv) for filling part of the air void for improving impermeability and leaving some air void

for stability and temperature variation. To overcome the difficulty for designing the HMA a

systematic overview of works by researchers for ascertaining the various mix design variables

correlating to the physical properties of materials used are enumerated below.

The first a mix design method for calculating the OBC for California Department of

Transport was develop in 1930’s by Francis Hveem. Contemporarily, Bruce Marshall in

1930’s developed a mix design method for calculating the OBC for Mississippi Highway

Department with a cumbersome process having some lengthy steps requiring skilled

technicians and sophisticated laboratories.

Kumar (1976) [5] studied the effect of air void, film thickness and permeability on

hardening of HMA. He has observed that, the factor of film thickness to permeability as the

best indicator to predict the hardening resistance in open graded (single sized) HMA, whereas

permeability alone is the best indicator to predict the hardening resistance in close graded

(graded sized) HMA. He has further observed that there exist a linear relationship between log

of permeability and percentage air void in HMA.

Kandhal and Khatri (1992) [6] found a relation between asphalt absorption in HMA with

the physical properties of both the aggregate and bitumen. Concluded asphalt absorption is

effected inversely with viscosity at mixing temperature and also effected by pore size.

Kandhal and Chakraborty (1996) [7] studied the effect of film thickness on ageing

behavior through the strategic highway research program short term oven aging (SHRP

STOA) and long term oven aging (SHRP LTOA) and measured in terms of resilient modulus,

tensile property and recovered binder property in HMA, observed that accelerated aging

would occur if the film thickness is less than 9-10 micron with a compaction to 8% air void.

Bruce et al. (2000) [8] studied the effect of VMA on different types of HMA and the

VMA collapse; conclude that the VMA collapse caused by a combination of two elements,

generation of fines and asphalt absorption. The study further stressed the importance of

correct assessment asphalt absorption, as incorrect asphalt absorption value lead to incorrect

computation of air void, VMA or voids filled with asphalt which has multifarious effect on

durability, stability, raveling, cracking, stripping, early hardening, construction problems

(segregation & tenderness) etc.

Chen et al. (2005) [9] studied the effect of aggregate characteristics such as elongation,

flatness and other shape indices on the performance of the HMA. Evaluated the particle index

(PI) purely on the basis of aggregates geometric characteristics such as shape and surface

texture with simple tests and formula. Four different types of aggregate shapes i.e. cubical,

rod, disk and blade shapes considered in the study and ascertained that the change in rotation

angle of coarse aggregate found to be correlated well with the internal resistance of HMA.

Concluded that best rutting resistance shown by the cubical aggregate whereas flaky and/or

elongated aggregate have lower compatibility and higher breakage.

Christensen and Bonaquist (2006) [10] stressed the need for modified volumetric

requirements such as air void, VMA, voids filled with aggregate (VFA) etc in Superpave mix

design. Established that, increasing maximum VMA from 1.5% to 2% above prescribed

minimum, increase VMA in the rage of 0.5% to 1% and/or design air void content changes

from 4% to 3% to 5% range. The study also conclude that, establishing maximum VMA

values with elimination of VFA requirements make the Superpave system simpler and more

direct and reduce the chance of poor rut resistance design. Maintaining design air void at 4%

and increasing VMA will improve fatigue resistance as it increase the volume of effective

bitumen content (VBE). Further, increasing both specific surface area and minimum VMA

will increase the rut resistance but decreases the permeability and thus need to be judiciously

designed for overall performance of HMA. To reach the target air void level, change in design



compaction level along with amount of compaction energy required in the field occurs along

with change in air void.

Sridhar et al. (2007) [11] observed that the Hugo hammer with indentation face simulate

better field conditions compared to the Marshall hammer. Hugo hammer compacting the

maximum density gradation satisfies all the mix criteria except the minimum film thickness

requirement of 8 to 6 microns. Accordingly recommended maximum density grading to be

considered as the best grading for Hugo hammer compaction, which simulate the field

conditions by increasing the binder content fulfilling the durability and film thickness criteria

of dense bituminous macadam (DBM) type HMA mix.

Rao et al. (2007) [12] assessed that the existing 75 blow Marshall compaction is

inadequate to represent the in-situ densities of the HMA attained in field under heavy axle

load and ascertained that the HMA is more susceptible to failure due to one or more of the

following combination(s), (i) Inadequate initial compaction, making the HMA vulnerable to

high secondary compaction under traffic, (ii) Relative high bitumen content that allows lower

air void (below 3%) leading to rutting under high temperature during summer, and (iii)

Relatively low bitumen content and high air voids, leading to top-down cracking, raveling and

stripping (making the HMA less durable).

Heitzman, Michael (2007) [1] studied the film thickness component model with specific

focus on rutting and fatigue. The study further ascertained that film thickness value

determines the thickness of effective asphalt binder coating on each particle in the mix.

Although film thickness can only be computed (not measured) ensure about adequate asphalt

binder in HMA to achieve a desired level of stability.

Hamzah et al. (2010) [13] ascertained that the density, stability and air void of HMA mix

incorporating the geometrically cubical aggregates were significantly higher than other type

of aggregates, found that the bitumen content required for all geometrically cubical aggregates

is lower than the normal aggregate mixtures. This is due to the fact that more bitumen is

required to coat the flat and elongated aggregates.

Hafeez and Kamal (2010) [14] studied the effect of ratio of mineral filler to asphalt on the

performance of the HMA and concluded that the distribution and size of filler also affected

the void content of the HMA. Established, HMA designed with lower asphalt content has an

optimum range of mineral filler to asphalt ratio.

Hmoud (2011) [15] studied the relation of VMA and film thickness on the performance of

HMAs. The VMA depends on the maximum size of the aggregate and observed that the film

thickness has very much importance on the performance of HMA, as low bitumen content

increases the stiffness of pavement and high bitumen content increases the skidding problem.

Naidu and Adiseshu (2013) [16] studied the effect of particle shape and size of coarse

aggregate on the engineering properties of HMA and concluded that the particle index of

coarse aggregate significantly affects the engineering properties of the HMA. The particle

shape determines how the aggregate packed into a dense configuration and develop the

internal resistance in HMA.

Liao et al. (2013) [17] ascertained that both mineral filler and bitumen greatly influence

the mechanical properties of the HMA such as moisture damage, stiffness, oxidation, rutting,

cracking behavior, workability and compaction.

Ahmed and Attia (2013) [18] established that rutting resistance in HMA affected by mix

gradation and type of aggregate and Marshal properties. Coarser gradation had highest

resistance, while open gradation has lowest resistance. As far as Marshall property only

Marshall flow had the highest linear correlation with rutting (coefficient of determination R2



of 0.74) and Marshall stability had the lowest linear correlation with rutting (coefficient of

determination R2 of 0.21).

Ramli et al. (2013) [19] ascertained that the fine aggregate with higher values of aggregate

angularity in HMA improves the rutting resistance.

Afaf (2014) [20] established coarse aggregate gradation has (i) reasonably well from the

consideration of Marshall tests properties, (ii) better stiffness and deformation of HMA at

higher temperature, and (iii) best performance against flow. As regards the type of aggregates

(Lime Stone, Dolomite & Basalt), the highest value of stability in case of lime stone

aggregate and lowest value in case of basalt aggregate observed.

It is evident from the above works, that the HMA should be designed to be durable for

different types of externally applied load and environment conditions. The choice of HMA

mix depends on the environmental condition and type of load. Each type of mix has ranges of

different materials i.e. (i) quantity of aggregate with gradation, (ii) quantity of mineral filler,

(iii) quantity of bitumen required depending on the grade of bitumen used, (iv) quantity of

additives (lime or cement in Bituminous Concrete) required, and (v) volume of air void. This

has been studied in the restricted zones of Superpave aggregate gradation and tabulated in the

standards and specification of the different countries. Further, it indicates that the bitumen

content, type and gradation; quality of aggregate and quantity of mineral filler mostly

influence stability, durability, impermeability, workability, flexibility and fatigue resistance of

the HMA in addition to quantity of additives and volume of air voids. Accordingly, it is

important to consider the following points to have a structurally effective and economically

sound HMA.

a) Sound/ Balanced size aggregate having strength and appropriate shape (non-

hydrophobic nature)

b) OBC, which is sufficient enough to have right bonding, having optimum void for

balanced strength & impermeability to suit with the type of HMA

From the overview, it has been ascertained that bitumen needs to be optimally used to

have better and effective HMA. Bitumen is basically required for (a) to sufficiently coat the

aggregate, (b) to float/ coat the dust particle, (c) absorbed inside the aggregate i.e. void filling

with in the aggregate, (d) fill part of the voids for improving impermeability leaving some

void for air for stability and temperature variation. To have better understanding the pictorial

representation (not to scale) is shown in Figure 1.

Figure 1 Pictorial representation of HMA



In this paper, an alternative method of calculating the OBC has been attempted instead of

cumbersome mix design procedures for HMA type Bituminous Concrete (BC) using grade of

bitumen as Viscosity Grade (VG) - 30. Initially the surface area of the aggregates such as

coarse aggregate, fine aggregate and dust has been calculated from the gradation and

elongation and flakiness of the aggregate used in HMA. The surface areas have been

multiplied with the film thickness to calculate the bitumen required for coating the aggregate

and dust. Further, the bitumen require to be absorbed within the pores has been asses from the

water absorption test of the aggregates. In a nutshell, the OBC is calculated as a function of

simple physical properties of the aggregate used, such as gradation, water absorption and

derives predictive relationship relating the density of the HMA, voids in mineral aggregate

(VMA), air voids and texture of HMA.

3. DESIGN PROCEDURE OF HMA

The design procedure with background of the proposed alternative method is elaborated.

3.1. Objective

The OBC required in a HMA is calculated based on the physical properties of the aggregate

used after knowing the type of HMA and grade of Bitumen. This process of calculating OBC

has further been repeated for other samples and compared with the actual bitumen content

derived by the bitumen extraction test. The samples tested in this study have been collected

from the field which are undergoing the actual field condition and performing well.

3.2. Codal provisions

“Bituminous Concrete” (BC) is selected for the study of HMA widely used for surface course

in heavily trafficked Indian roads. The specification requirement governed for execution of

BC in India is as per clause No. 507 of Indian Road Congress (IRC), 2013 “Specification for

Road and Bridge Works (Fifth Revision)”. Before testing the samples some of the important

codal provisions, which are required to be met by the researcher while selecting specimen are

(i) minimum percent voids in mineral aggregate (VMA) as specified in Table 500-12 of the

ibid Specification/ Standard [4] shown in Table 1, (ii) physical requirements for coarse

aggregate for bituminous concrete as specified in Table 500-16 of the ibid Specification/

Standard [4] shown in Table 2, (iii) composition of bituminous concrete (BC) pavement layer

as specified in Table 500-17 of the ibid Specification/ Standard [4] shown in Table 3 and (iv)

variations in plant mix from the job mix formula as specified in Table 500-18 of the ibid

Specification/ Standard [4] shown in Table 4.

Table 1 Codal Provision for Minimum Percent of VMA

Nominal Maximum

Particle Size (mm)

Minimum VMA Percent Related to Design Percentage Air Voids

3.0 4.0 5.0

26.5 11.0 12.0 13.0

37.5 10.0 11.0 12.0

Table 2 Codal Provision for Physical Requirements for Coarse Aggregate for BC

Property Test Specification Method of Test

Cleanliness (Dust) Grain Size

Analysis

Max 5% passing

0.075 mm sieve IS:2386 Part I

Particle shape Combined

Flakiness and Max 35% IS:2386 Part I



Elongation Indices

Strength

Los Angeles

Abrasion Value or

Aggregate Impact

Value

Max 30%

Max 24%

IS:2386 Part IV

Durability

Soundness either:

sodium Sulphate

of Magnesium

sulphate

Max 12%

Max 18%

IS:2386 Part V

Polishing Polished Stone

Value Min 55 BS:812-114

Water Absorption Water Absorption Max 2% IS:2386 Part III

Stripping

Coating and

Stripping of

Bitumen

Aggregate Mix

Minimum retained

coating 95% IS:6241

Water sensitivity Retained Tensile

Strength Min 80% AASHTO 283

Table 3 Codal Provision for Composition of BC Pavement Layer

Grading 1 2

Nominal Aggregate Size 19 mm 13.2 mm

Layer Thickness 50 mm 30-40 mm

IS Sieve (mm) Cumulative % by weight of total aggregate passing

45

37.5

26.5 100

19 90-100 100

13.2 59-79 90-100

9.5 52-72 70-88

4.75 35-55 53-71

2.36 28-44 42-58

1.18 20-34 34-48

0.6 15-27 26-38

0.3 10-20 18-28

0.15 5-13 12-20

0.075 2-8 4-10

Bitumen contents % by mass of

total mix Min 5.2* Min 5.4*

Corresponding to Specific gravity of Aggregate being 2.7 (in case of more than 2.7, this

can be reduced). Further, in regions having highest mean air temperature is 30 degree

centigrade or less and lowest daily temperature is – 10 degree centigrade or lower, this can be

increased by 0.5 percent.



Table 4 Codal Provision for Permissible Variations in Plant Mix from the Job Mix Formula

Description Permissible Variation

Aggregate Passing 19 mm sieve or larger + / – 7%

Aggregate passing 13.2 mm, 9.5mm + / – 6%

Aggregate passing 4.75mm + / – 5%

Aggregate passing 2.36 mm, 1.18mm, 0.6mm + / – 4%

Aggregate passing 0.3mm, 0.15mm + / – 3%

Aggregate passing 0.075mm + / – 1.5%

Binder content + / – 0.3%

Mixing temperature + / – 10 degree Centigrade

3.3. Bitumen requirement for HMA

To overcome difficulties in calculating the bitumen requirement in a compacted HMA, a

mathematical model has been developed by using simple physical property of the aggregate

such as gradation, elongation and flakiness and water absorption/ porosity. Here the surface

area of coarse aggregate, fine aggregate and dust particle are initially calculated from simple

gradation, elongation and flakiness index on each sample specimen of compacted HMA [21].

Subsequently, the theoretical requirement of bitumen are calculated for different categories

viz. (i) coat the aggregate, (ii) float/ coat the dust, (iii) void filling inside the aggregate, and

(iv) filling part of the voids for impermeability leaving some void for air for stability &

temperature variation. Thereafter, all four components are added to get the total theoretical

requirement of bitumen for particular HMA which has been compared with the actual

consumption of field obtained from bitumen extraction tests of specimen.

Proposed Empirical Formula:

VFAFDUSTATotal TBCTBCTBCTBCTBC +++= (1)

Where,

TBCTOTAL = Theoretical Bitumen Consumption in one Cubic metre of HMA

TBCA = Theoretical Bitumen Consumption to coat the Aggregate in one Cubic metre of

HMA

TBCDUST = Theoretical Bitumen Consumption to coat the Dust in one Cubic metre of

HMA

TBCAF = Theoretical Bitumen Consumption filled inside the pores within the aggregate

in one Cubic metre of HMA

TBCVF = Theoretical Bitumen Consumption to fill the part of void in one Cubic metre of

HMA

Aggregate in HMA blend is considered as both coarse aggregate and fine aggregate. In a

HMA blend all particles above 4.75 mm size called as course aggregate, particles Between

4.75 mm to 75 micron size as fine aggregate, all particles below 75 micron size as dust.

essFilmThicknSATBC AA *= (2)

Where, SAA = Surface Area of the Aggregate in one Cubic metre of HMA

FACAA SASASA += (3)

Where,

SACA = Surface Area of the Coarse Aggregate in one Cubic metre of HMA Blend

SAFA = Surface Area of the Fine Aggregate in one Cubic metre of HMA Blend



Similarly,essFilmThicknSATBC DUSTDUST *=

(4)

Where,

SADUST = Surface Area of the Dust in one Cubic metre of HMA

Similarly, tehinAggregacentagewitFillingPerbtionWaterAbsorTBCAF *= (5)

Similarly, oidcentageofVFillingPerdBalanceVoiTBCVF *= (6)

)( DUSTA TBCTBCedBalanceVoi +−= (7)

Where, e = VMA in the HMA

3.3.1. Bitumen required to coat the aggregate

Bitumen required to coat the aggregate should be sufficient for having better cohesion in

HMA. Several studies have been carried out to understand the film thickness required for

better bonding [1,5,11,11,22] showed its dependence on type, grade and temperature. But

mostly film thickness varies between 7 to10 microns for better cohesion. The research has

also established that the cohesion increases up to certain film thickness and decreases

thereafter due to loss of inter particle friction [11,15,22]. Study has been carried out to

calculate the surface area of the irregular surface such as aggregate using multiple silhouettes,

in which a machine vision system has been considered for calculating the volume as well as

surface area of the object [23]. Determining surface area of the total aggregate blend by

current procedure [21] requires only the gradation expressed as total percent (by weight)

passing through each sieve. Each percent passing value represents all the particles smaller

than that sieve. Therefore, the surface area values are not a direct expression of total surface

area for aggregate particles on a specific sieve and do not account for differences in aggregate

particle specific gravity [1]. However, one can calculate the surface area of an aggregate to

certain degree of accuracy knowing the sieve size in which it has passes and the sieve size in

which it has retained along with its flakiness & elongation property. The closer the sieve size

the better is the result. Here, an aggregate has been assumed as having shape of cylinder with

its diameter as mean of the upper limit and lower limit in which the sieve passes and retained

respectively. The length of the cylinder has been considered as equal to its diameter as

considered above in addition to the effect of elongation and flakiness of the aggregate as

shown below in Figure 2.

Figure 2 Assumed shape of the aggregate



The calculated area is suitably modified/ enhanced for roughness and texture. The area has

been calculated separately for coarse aggregate (above 4.75 mm size) and fine aggregate

(between 4.75 mm to 75 micron size) for better results (Panda et al. 2016) [21]. Now, the

bitumen requirement to coat the aggregate can be calculated by multiplying the surface area

of aggregate and film thickness. The film thickness requirement basically depends on the

grade of bitumen and the type of HMA. In the present study the HMA type considered is BC

and the grade of bitumen used is VG-30. Here the film thickness derived computationally for

best fit is 7 micron. In case of the other type of HMA and/ or grade of bitumen the film

thickness can be changed to some other value e.g. this can be reduced if HMA type

considered is DBM or if grade of bitumen used is modified bitumen.

3.3.2. Bitumen required to float/ coat the dust

Bitumen is required to float/ coat the dust particle to have better impermeability in HMA. In

the past experiment revealed that the air void increases with increase in ratio of mineral filler

to asphalt ratio [14]. The mineral filler content greatly influences the performance of the

heavily trafficked highways and optimum mineral filler and bitumen content is required for

particularly in coarse graded HMA designed at low asphalt content [14]. The dust particle i.e.

the particle passing 75 micron is considered to be coated as the specific gravity of dust is

higher than that of bitumen. As per in line of course aggregate the surface area of dust also

can also be calculate to certain degree of accuracy. The dusts are assumed to be spherical with

its diameter 40 micron as shown below in Figure 3.

Figure 3 Assumed shape of the dust

The area so calculated is suitably modified/ enhanced for roughness and texture. Now, the

bitumen require to coat the dust can be calculated by multiplying the surface area of dust and

film thickness.

3.3.3. Void filling inside the aggregate

Bitumen being a viscous material has a tendency to fill the gaps / part of the gaps exists inside

the aggregate. Although this depends on the grade of bitumen, temperature and size of voids,

it mostly varies between 10-25%. The lesser is the value if water absorption value is low,

indirectly because of thinner size of voids. Now, the bitumen requirement for absorption with

in the aggregate can be indirectly calculated to certain degree of accuracy after knowing the

water absorption value of Aggregate.



3.3.4. Filling part of the voids for impermeability leaving some void for air for stability &

temperature variation

Bitumen is a visco-elastic material. It shows the properties of viscous material at liquid state

and the properties of elastic material at solid state. This has caused due to variation in

pavement temperature. Therefore, the selection of grade of bitumen is most vital to counteract

environmental effects. For this reason, the extra bitumen is required for filling part of the

voids for impermeability, leaving some void for air for stability & temperature variation. The

study shows that the void required for air is 4% for better stability and temperature variation.

Now, the bitumen requirement for filling part of the voids for impermeability; leaving some

void for air for stability and temperature variation can be indirectly calculated to certain

degree of accuracy after knowing the balance void and deduction the voids for air. Thereafter,

the bitumen component of each four category can be added to know the OBC in a HMA.

3.4. Calculation of theoretical bitumen consumption of the sample using present

method

To calculation of theoretical bitumen requirement of the sample, following tests has to be

carried out on sample specimen. The gradation test of the aggregates is required to calculate

the surface area of (a) course aggregate, (b) fine aggregate and (c) dust particle. The

elongation & flakiness test is required to fine tune the surface area of both the coarse

aggregate and fine aggregates. The water absorption value of the aggregate is required

primarily to calculate the bitumen required to fill inside the aggregates. The specific gravity of

aggregates, bitumen and HMA are required to calculate VMA. On a specimen first all the

tests as mentioned are conducted, thereafter the theoretical value of bitumen consumption has

been calculation as below. The sample specimen “1” is obtained from 40 mm thick BC,

executed on a National Highways, in the Kalahandi district of Odisha conforming to Ministry

of Road Transport and Highways (MoRT&H) specification Clause No. 507 (Specification for

Road & Bridges Works (Fifth Revision) published by IRC, 2013). The average temperature in

the region is 28 degree centigrade. The lowest temperature is 5 degree centigrade and

maximum temperature is 50 degree centigrade. The bitumen used is straight run bitumen

conforming to VG-30. The test results of the sample specimen describe below with its grain

size distribution in Table 5.

Table 5 Grain size Distribution of the Sample Specimen

Max IS sieve Size (mm) Min IS sieve Size (mm) % of content

26.5 19.0 0

19.0 13.2 0

13.2 9.5 18.7

9.5 4.75 30.7

4.75 2.36 9.3

2.36 1.18 10.5

1.18 0.6 4

0.6 0.3 9.1

0.3 0.15 6.3

0.15 0.075 7.45

0.075 Below 3.95



Specific gravity of stone used 2.74

Specific gravity of bitumen used 1.02

Specific gravity of compacted BC mix 2.44

Flakiness and elongation 13%

Water absorption 0.82%

Bitumen content obtained from extraction test 5.22%

3.4.1. Assumption/ consideration

The following assumptions are made for calculating the some of the basic parameters used in

the theoretical calculation.

Air Void assumed as 4% only to obtain the theoretical VMA as 15.07%

The film thickness derived computationally for best fit is 7 micron (as the sample selected

is having HMA type BC and bitumen grade is straight run bitumen of grade VG-30, this can

be suitably altered for other type of HMA and grade of bitumen e.g. this can be reduced if

HMA type considered is DBM or if grade of bitumen used is modified bitumen)

Assume 50% of air void left out after coating will be filled with extra bitumen leaving rest

50% for air void.

Assume average size of dust particle as diameter of 40 mm and spherical in shape in line

with earlier study of Panda et al. (2016) [21].

Assume 20% of water absorption value as the volume of the bitumen filled inside the pore

in line with earlier study of Kandhal and Khatri (1992) [6].

Calculate Void in Mineral Aggregate (theoretically)

Let Void in Mineral Aggregate = e

Assume Void for air = 4%

Bulk specific gravity of Stone = 2.74 (Assume also for dust)

Bulk specific gravity of Bitumen = 1.02

Bulk density of BC after compaction correction = 2.44

(1-e) * 2.74 + (e-0.04)* 1.02 = 2.44

After solving e = 15.0698%

The area of the aggregates has been calculated based on the model developed by Panda et

al. (2016) [21]. As per the model the area of different type of aggregates in the first sample

are as under:

Total Area of Coarse Aggregate = 1728.80 square metre

Total Area of Fine Aggregate = 4002.22 square metre

Total Area of Dust = 8051.38 square metre

Film Thickness (derived computationally for best fit) = 7 micron

Volume of Bitumen for Coating Coarse Aggregate = 1728.80 × 7 * 10-6 × 100% =

1.21016%

Volume of Bitumen for Coating Fine Aggregate = 4002.22 × 7 * 10-6 × 100% =

2.801554%

Volume of Bitumen for Coating Dust = 8051.38 × 7 * 10-6 × 100% =

5.635968%

Total void left for Bitumen & Air void = e- 1.21016%- 2.801554%- 5.422118% =

5.635968%



Filled by Bitumen (50%) = 2.711059%

Void Left with Air (50%) = 2.711059%

Volume of Bitumen required for filling voids within the aggregate = 20% of water

absorption = 0.164%

Total volume of Bitumen = 1.21016% +2.801554% +5.635968% +2.711059% +0.164%

= 12.522741%

Weight of Bitumen = 12.522741%* 1.02* 1000 Kg = 127.732 Kg

Weight of Aggregate = (1-e)* 2.74* 1000 Kg = 2327.088 Kg

Total Weight of Mix = 127.732 + 2327.088 = 2454.82 Kg

Bitumen content by weight (%) = 127.732 / 2454.82 * 100 = 5.2033% (This is nearer to

the actual bitumen consumption value of 5.22%)

4. MODEL JUSTIFICATION

To justify the above model, 45 samples obtained from both state highways and national

highways comprising three districts viz. Angul, Kalahandi and Ganjam widely distributed

geographically in state of Odisha, India has been tested for different physical properties to

calculate the theoretical bitumen requirement in the sample of HMA. This has been compared

with the actual consumption of bitumen. All the samples have been collected from the field

which are subject to actual field conditions and are performing well.

The average actual consumption of the bitumen in these 45 samples worked out to be

5.541% by weight of HMA. After considering the film thickness as 7 micron in the average

theoretical consumption is worked out to be 5.548%. However, by changing the film

thickness as 8 micron, the average theoretical consumption is worked out to be 5.866%.

Similarly, by changing the film thickness as 6 micron, the average theoretical consumption is

worked out to be 5.228%. Therefore, we considered the film thickness requirement as 7

micron, which was derived computationally for best fit. This has also been tested for variance

analysis such as chi- square test and observed that the results show a very high confidence

level. The data on actual bitumen consumption, theoretical consumption/ requirement

assuming film thickness as 7 micron is tabulated at Table 6 and shown in a pictorial form

Figure 4.

Table 6 Sample-wise Actual Consumption and Theoretical Requirement Bitumen by %weight of

HMA

Sample No.

Actual

Consumption of

Bitumen (A)

Theoretical

Requirement of

Bitumen with 7

micron film

thickness (T)

(T-A) (T-A)/ A

Square of

(T-A)/ A

(1) (2) (3) (4) (5) (6)

1 5.22% 5.20% -0.000167 -0.31968% 0.000000533

2 5.10% 5.18% 0.000802 1.57340% 0.000012626

3 5.20% 5.11% -0.000891 -1.71376% 0.000015272

4 5.00% 5.12% 0.001220 2.43946% 0.000029755

5 5.10% 5.14% 0.000438 0.85980% 0.000003770

6 5.40% 5.31% -0.000908 -1.68195% 0.000015276

7 5.10% 5.07% -0.000347 -0.67963% 0.000002356

8 5.10% 5.24% 0.001444 2.83129% 0.000040883



9 5.30% 5.36% 0.000591 1.11483% 0.000006587

10 5.65% 5.79% 0.001353 2.39427% 0.000032389

11 5.10% 5.24% 0.001365 2.67685% 0.000036544

12 5.80% 5.54% -0.002640 -4.55156% 0.000120157

13 5.62% 5.70% 0.000762 1.35624% 0.000010341

14 5.61% 5.54% -0.000686 -1.22353% 0.000008398

15 5.60% 5.66% 0.000618 1.10353% 0.000006820

16 6.14% 5.87% -0.002655 -4.32457% 0.000114830

17 6.00% 5.78% -0.002196 -3.65976% 0.000080363

18 5.20% 5.45% 0.002453 4.71764% 0.000115732

19 5.62% 5.64% 0.000213 0.37874% 0.000000806

20 5.62% 5.66% 0.000398 0.70790% 0.000002817

21 5.67% 5.87% 0.002025 3.56967% 0.000072276

22 5.55% 5.78% 0.002304 4.15161% 0.000095659

23 5.64% 5.70% 0.000585 1.03802% 0.000006075

24 5.20% 5.45% 0.002453 4.71764% 0.000115732

25 5.50% 5.44% -0.000605 -1.09954% 0.000006649

26 5.61% 5.65% 0.000451 0.80495% 0.000003632

27 5.61% 5.60% -0.000116 -0.20708% 0.000000240

28 5.71% 5.69% -0.000284 -0.49641% 0.000001408

29 5.68% 5.65% -0.000296 -0.52096% 0.000001540

30 5.68% 5.55% -0.001335 -2.34892% 0.000031350

31 5.63% 5.72% 0.000917 1.62836% 0.000014933

32 5.71% 5.59% -0.001223 -2.13953% 0.000026156

33 5.20% 5.27% 0.000690 1.32696% 0.000009156

34 5.09% 5.30% 0.002132 4.19183% 0.000089351

35 5.80% 5.96% 0.001557 2.68510% 0.000041817

36 5.70% 5.66% -0.000406 -0.71212% 0.000002891

37 5.86% 5.74% -0.001224 -2.08890% 0.000025570

38 5.73% 5.53% -0.002024 -3.53279% 0.000071514

39 5.69% 5.48% -0.002085 -3.66394% 0.000076385

40 5.82% 5.58% -0.002389 -4.10401% 0.000098026

41 5.80% 5.73% -0.000682 -1.17505% 0.000008008

42 6.00% 5.78% -0.002195 -3.65801% 0.000080286

43 5.20% 5.45% 0.002452 4.71546% 0.000115625

44 5.80% 5.79% -0.000145 -0.25040% 0.000000364

45 6.00% 6.13% 0.001328 -0.31968% 0.000029393

Sum 249.36% 249.67% 2.21335% 0.001680292

Average 5.54136% 5.54815% 0.04919%



Figure 4 Actual bitumen Content vs. Theoretical requirement

4.1 Results and discussions

The average theoretical consumption/ requirement is worked out to be 5.54815% by weight of

HMA after assuming the film thickness requirement as 7 micron, which is almost same as the

average actual consumption i.e. 5.54136%. This has further deliberated for variance analysis.

This has been tested for variance analysis such as Chi- Square test and observed that the

results show a very high confidence level. The Variance analysis is reported in column 5 and

6 of Table 6.

A further breakup in volumetric terms regarding the usage of bitumen in the BC the

theoretical consumption/ requirement under each category viz. (i) coat the aggregate, (ii)

float/ coat the Dust, (iii) void filling inside the aggregate, and (iv) filling part of the voids for

impermeability leaving some void for air for stability & temperature variation has been

tabulated in Table 7 and shown in a pictorial form Figure 5.

Table 7 Breakup of Bitumen Usage in BC in volumetric term

Samp

le No.

Actual

Consumptio

n of

Bitumen in

weight

Theoretical

Requireme

nt of

Bitumen in

weight

assuming

film

thickness as

7 micron

Theoretical

Requireme

nt of

Bitumen in

volumetric

term

assuming

film

thickness as

7 micron

(in %)

Volume

to Coat

the

Aggrega

te (in %)

Volume to

Float/ Coat

the Dust

(in %)

Volum

e to

Void

filling

inside

the

Aggreg

ate

(in %)

Volume to

Fill part of

the voids for

impermeabilit

y leaving

some void for

air for

stability &

temperature

variation

(in %)

(1) (2) (3) (4) (5) (6) (7) (8)

1 5.22% 5.20% 12.523 4.012 5.636 0.164 2.711

2 5.10% 5.18% 12.464 3.823 5.707 0.164 2.770

3 5.20% 5.11% 12.288 3.757 5.422 0.164 2.945

4 5.00% 5.12% 12.316 3.813 5.422 0.164 2.917

5 5.10% 5.14% 12.372 3.639 5.707 0.164 2.862



6 5.40% 5.31% 12.792 3.765 6.421 0.164 2.442

7 5.10% 5.07% 12.173 3.598 5.351 0.164 3.061

8 5.10% 5.24% 12.627 3.821 6.035 0.164 2.607

9 5.30% 5.36% 12.919 4.019 6.421 0.164 2.315

10 5.65% 5.79% 14.009 3.613 9.132 0.102 1.162

11 5.10% 5.24% 12.607 4.038 5.779 0.164 2.627

12 5.80% 5.54% 13.370 4.214 7.049 0.204 1.903

13 5.62% 5.70% 13.786 4.252 7.962 0.144 1.428

14 5.61% 5.54% 13.384 3.848 7.562 0.144 1.830

15 5.60% 5.66% 13.692 4.094 7.933 0.144 1.521

16 6.14% 5.87% 14.239 4.359 8.761 0.144 0.975

17 6.00% 5.78% 13.997 4.018 8.618 0.144 1.217

18 5.20% 5.45% 13.139 3.785 7.134 0.144 2.075

19 5.62% 5.64% 13.645 4.256 7.676 0.144 1.569

20 5.62% 5.66% 13.692 4.094 7.933 0.144 1.521

21 5.67% 5.87% 14.239 4.359 8.761 0.144 0.975

22 5.55% 5.78% 13.997 4.018 8.618 0.144 1.217

23 5.64% 5.70% 13.781 4.172 8.033 0.144 1.432

24 5.20% 5.45% 13.139 3.785 7.134 0.144 2.075

25 5.50% 5.44% 13.124 4.155 6.735 0.144 2.090

26 5.61% 5.65% 13.662 4.043 7.876 0.168 1.575

27 5.61% 5.60% 13.525 3.939 7.705 0.168 1.713

28 5.71% 5.69% 13.754 3.926 8.176 0.168 1.484

29 5.68% 5.65% 13.650 3.891 8.005 0.168 1.587

30 5.68% 5.55% 13.402 4.707 6.692 0.168 1.835

31 5.63% 5.72% 13.851 4.135 8.161 0.168 1.387

32 5.71% 5.59% 13.513 4.058 7.562 0.168 1.725

33 5.20% 5.27% 12.690 3.981 5.993 0.168 2.548

34 5.09% 5.30% 12.764 4.001 6.121 0.168 2.474

35 5.80% 5.96% 14.448 3.801 9.874 0.076 0.698

36 5.70% 5.66% 13.686 3.790 8.104 0.204 1.588

37 5.86% 5.74% 13.887 4.063 8.233 0.204 1.387

38 5.73% 5.53% 13.349 4.043 7.177 0.204 1.925

39 5.69% 5.48% 13.231 4.022 6.963 0.204 2.043

40 5.82% 5.58% 13.486 4.032 7.462 0.204 1.788

41 5.80% 5.73% 13.872 4.690 7.705 0.140 1.338

42 6.00% 5.78% 13.997 3.918 8.846 0.080 1.153

43 5.20% 5.45% 13.138 4.198 6.849 0.080 2.011

44 5.80% 5.79% 14.010 3.944 8.846 0.080 1.140

45 6.00% 6.13% 14.906 4.023 10.559 0.080 0.244

Sum 249.36% 249.67% 603.136 180.511 333.849 6.886 81.891

Avera

ge 5.54136% 5.54815%

13.4030320

1

4.011351

4

7.41886812

7

0.15302

2 1.81979



Figure 5 Different component of Percentage of Bitumen required in volumetric term

After analysing the above table and figure the following can be ascertained to a certain

degree of confidence.

i. In real term, the film thickness is much more than the assumed 7 micron due to

following:

• Uniform coating has been considered on each aggregate particle means the

distance between two consecutive aggregate particles inside the HMA is

theoretically two times the coating.

• Even after coating all the aggregate particles there are surplus bitumen for filling

part of the voids in the range of 1.81979% in volume (if divided with aggregate

surface area this will be of the range of 1.1 micron)

ii. The dust requires a more bitumen consumption/ requirement as compared to combined

coarse and fine aggregates. Thus a proper grading of aggregate has to be considered

for the HMA having balanced dust particle to optimize the bitumen consumption/

requirement. This will in turn reduce the cost drastically.

iii. In most of the cases the air void left in VMA is much less than the Codal provision of

VMA requirement. This may be due to VMA loss during/ after construction. As such a

rethinking is required to modify the Codal provision or use lesser bitumen to attain the

Codal provision. This is in turn reduces the bitumen consumption/ requirement and

reduces the cost drastically.

5. CONCLUSIONS

The alternative method developed in the present paper for calculating the OBC of BC from

the simple physical properties of the aggregate using gradation, water absorption/ porosity and

grade of bitumen only. The model is justified using 45 field samples obtained from different

region of Odisha, India. The comparison between actual consumption of bitumen and

theoretical consumption show a variation from -4.552% to 4.718% by volume. But the

average value shows a negligible 0.049% variation. Calculating OBC by this method

presented in the paper may avoid the lengthy cumbersome process of mix design of HMA by

standard mix design methods requiring skilled technician and sophisticated laboratories. This

method of mix design being derived from the simple physical properties of the aggregates

used in HMA can also be used in remote areas requiring highway development using locally

available aggregates (knowing the grade of bitumen used). With further study on different



grade of bitumen and HMA, this may be used as an alternative to standard mix design

methods of bituminous mix. Further research necessitate in this area to reduce the bitumen

requirement of BC as there is enormous room for the reduction and changing the codal

provisions and change of gradation of aggregate to reduce the cost of construction drastically.

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