THE INFLUENCE OF CRUMB RUBBER MODIFIER (CRM) ON THE ...€¦ · asphalt by blending asphalt cement with crumb rubber (0, 5, 10, and 15 % by asphalt weight) using the wet method. Next

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International Journal of Civil Engineering and Technology (IJCIET)

Volume 9, Issue 10, October 2018, pp. 201–212, Article ID: IJCIET_09_10_021

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=10

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

©IAEME Publication Scopus Indexed

THE INFLUENCE OF CRUMB RUBBER

MODIFIER (CRM) ON THE PROPERTIES OF

ASPHALT CONCRETE MIXTURES

Esraa T. Al-Azawee

Assistant Lecturer, Civil Engineering Department,

AL-Mansour University College, Baghdad, Iraq

Zaynab I. Qasim

Assist. Professor, Civil Engineering Department,

University of Technology, Baghdad, Iraq

ABSTRACT

The use of rubber crumb as additives has been reported to improve the service life

of pavements, require less maintenance, and drive comfort. The aim of this paper is to

summarize the results of the effect of CRM on the basic properties of hot asphalt

mixtures with different CRM percentages. During the study, four different percentages

of CRM were used. The experimental work consisted of the preparation of rubberized

asphalt by blending asphalt cement with crumb rubber (0, 5, 10, and 15 % by asphalt

weight) using the wet method. Next is the preparation of Marshall samples to

determine the optimum asphalt percentage using the Marshall method. The laboratory

tests include Marshall stability and flow, and ultrasonic test. The results from this

study showed that the use of rubberized asphalt binder in mixes increased the

optimum asphalt content and enhanced the volumetric properties of asphalt mixtures

in terms of the Marshall stability and Marshall flow. However, the use of 10 % CRM

showed the best effect on the Marshall stability of the mixture. The experimental

outcome showed that the incorporation of CRM as additives in mixtures significantly

influenced the evaluated properties of the mixtures.

Key words: Asphalt mixture; Crumb rubber modifier (CRM); Marshall stability;

ultrasonic test.

Cite this Article: Esraa T. Al-Azawee and Zaynab I. Qasim, The Influence of Crumb

Rubber Modifier (CRM) on the Properties of Asphalt Concrete Mixtures,

International Journal of Civil Engineering and Technology (IJCIET) 9(10), 2018, pp.

201–212.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=10

1. INTRODUCTION

Innovative and environmentally friendly ideas on how to re-use industrial and domestic waste

products have been developed due to the increasing environmental concerns associated with

Esraa T. Al-Azawee and Zaynab I. Qasim


their presence in the environment. Their increased presence in the environment is a serious

source of concern due to their associated pollution tendencies. They have been used as cost-

effective and environmentally friendly materials in modification processes to reduce damages

to pavements. The rapid increase in the total number of automobiles globally has contributed

greatly to the increase in the generation of waste tires which has become a major

environmental problem. The incorporation of rubber crumbs derived from waste tires into

asphalt mixtures have been considered as a sustainable construction approach and has been

widely investigated as enhancers of modified asphalt mixtures properties [1-6]. As stated by

many researchers, the physical properties of the control asphalt can be enhanced by using

different industrial waste additives such as fly ash and phospho-gypsum [13,14].Rubber

crumbs incorporation into bitumen binder has been reported to improve the physical

properties of the modified binder as evidenced by the reduced penetration tendency and

binder durability [1]. Rubber crumbs have been shown through various experiments to

improve the physical characteristics of modified asphalt mixtures [2-4].

2. BACKGROUND AND SIGNIFICANCE OF WORK

Annually, more than 1.4 billion tires are sold worldwide, and this number of tires will

eventually become waste tires or all within the category of End of life tires (ELTs) with time

[7]. In Europe, the number of ELTs is bound to increase as the number of vehicles in that

region continues to increase. The large volume and durability of these tires make them more

problematic to manage. Premature pavement failures and accumulation of ELTs are both

interconnected and dependent of each other due to the high axle loading and the enormous

increase in traffic density, respectively. Rubber crumbs were first used as asphalt pavements

additives in the past 170 years ago, with the first experiment that involved the addition of

natural rubber into bitumen being reported in the 1840s [8]. These studies attempted to

demonstrate the flexibility of rubber in the pavement industry. Nowadays, different quality-

related problems are addressed with the use of rubberized bitumen materials which are

produced through wet processes. The effectiveness of this strategy is demonstrated by the

stability of roads that were constructed in the last three decades [9]. Rubber crumbs can be

introduced into asphalt mixtures through either wet or dry processes [5]. The wet process

involves the addition of rubber crumbs into hot asphalt and allowing the asphalt to react with

the rubber. During this process, the major event is the swelling of the rubber. However, in the

dry process, the rubber crumbs are first mixed with a hot aggregate before being added to the

bitumen. The resistance of an asphalt mixture to high temperate deformation and low-

temperature cracking can be improved through the addition of rubber crumbs via a dry

process [6]. Only the specified volumetric properties of an asphalt mixture can be obtained

through the wet process [3]. This work mainly aims at investigating the influence of rubber

crumbs addition into asphalt mixtures via the wet process.

3. RESEARCH METHODOLOGY

The testing program consists of physical tests that include penetration, specific gravity,

ductility, and softening point for asphalt binder. The mechanical tests include the Marshall

test and the ultrasonic test. The testing phase consisted of the preparing the asphalt mixture

samples, as well as the laboratory investigations before and after adding CRM to the mixtures.

3.1. Material

Locally available materials were utilized in this research, including the asphalt binder, mineral

filler, crushed aggregate, and CRM additive. This work employed an asphalt cement grade of

40-50 which was obtained from Al-Doura Refinery located in the south-west of Baghdad. The

The Influence of Crumb Rubber Modifier (CRM) on the Properties of Asphalt Concrete Mixtures


coarse and fine aggregates (Figure 1) used were sourced from AL- Nibaie quarry. The filler

used is a non- plastic material passing sieve No.200 (0.075mm). The filler is usually used to

fill the voids and enhance the property of the mixtures. Limestone dust was used as a mineral

filler during the study; it was sourced from the lime factory of Karbala’ governorate (Figure

2-a). The rubber crumbs used as an additive in this study were sourced from tire factories in

AL-Najaf governorate Figure 2-b. the rubber crumbs are black granules which were produced

from recycled tires. They have a specific gravity of 1.13. There are several types of

compounds in tires and the component that has the most effect on the physical properties of

modified asphalt rubber (AR) is the hydrocarbon content of the rubber; however, additional

effects can also come from the natural rubber content [10]. The rubber crumbs were sieved to

the desired sizes by passing the shredded material through No. 8 (2.36 mm) and No.50 (0. 3

mm) sieves.

Figure 1 Course and fine aggregate used in study

Figure 2 (a): Limestone mineral filler, (b) Crumb rubber used in this Study

3.2. Preparation of Modified Asphalt Cement

A wet process was followed during the preparation of the modified asphalt in this study. At

first, the asphalt cement subjected to heating (at 150 ˚C) before being mixed with different

percentages (5, 10, and 15 % by asphalt weight) of rubber crumbs. This blending was done

using a laboratory mixer (Figure 3) at a blending speed of 1300 rpm and at 170 °C.

Figure 3 Blending apparatus for mixing the rubberized asphalt



3.3. Preparation of Asphalt Mixtures

The Marshall mix design method was deployed for both the unmodified and rubber crumb

modified asphalt mixtures in this study. This Marshal mix design method is commonly used

in Iraq to design asphalt mixtures. This study involved several laboratory examination stages;

the first stage involved the selection of the aggregates, comprising of determining their

physical properties and composite grades that will meet the requirements for asphalt mixtures.

This stage followed the specification provided by the General Standards of the SCRB (R/9,

2003) [11]. The second stage involved the evaluation of the asphalt for both unmodified and

modified asphalt mixtures; the optimum asphalt content for each asphalt mixture was also

determined in this stage. The final stage is the verification of the volumetric parameters. At

this stage, three samples were prepared for each mixture. The rubber crumbs were added to

the asphalt mixtures using a wet process. The rubber crumbs were added at different

percentages of 5, 10, and 15 % by asphalt mixture weight. The asphalt was heated to about

150 °C prior to the addition of the rubber crumbs. This was aimed at the production of a

kinematic viscosity of 170 ± 20 centistokes. The temperature of the mixture was maintained at

the range of I35 – 150 °C. The mixture was manually mixed until homogeneity was achieved.

Having homogenized, the asphalt cement was added to the heated aggregate at the desired

amount and thoroughly mixed manually for about 2 minutes until the aggregates were fully

coated with asphalt.

4. RESULTS AND DISCUSSION

4.1. Aggregates

Table 1 showed the results of the tests on the general properties of the prepared aggregate in

this study while Table 2 and Figure 4 showed the result of the sieve analysis of the tested

aggregates and Figure 4. The combined aggregate gradation for the asphalt mixture is also

shown in Table 2. These results were determined following the required specifications and

compared with the specifications of SCRB (R/9, 2003) [11].

Table 1 Physical characteristics of the fine and Coarse Aggregates

Laboratory Test ASTM Designation Test Results

Coarse Aggregate

BSG C-127 2.62

ASG C-127 2.68

WA % C-127 0.46

Fine Aggregate

BSG C-128 2.64

ASG C-128 2.71

WA % C-128 0.72

Note: BSG = Bulk specific gravity, WA = Water absorption, ASG = Bulk specific gravity

Table 2 Selected aggregate gradation

Sieve

Size 3/4" 1/2" 3/8" No.4 No.8 No.50 No.200

%

Passing 100 95 83 58 36 12 5

SCRB 100 90-100 76-90 44-74 28-58 5-21 4-10



Figure 4 Selected aggregate gradation and specification limits

4.2. Asphalt Binder

Tables 3 and 4 showed the properties of modified and unmodified asphalt. The results showed

that a higher rubber crumbs percentage strongly affected (reduced) the penetration and

ductility of the resulting asphalt while lower percentages increased the softening point of the

asphalt. All the observed physical properties of the produced asphalts were within standard

requirements. The higher softening point and lower penetration of the modified asphalt

showed that the additive caused an increased stiffness of the resulting asphalts, but with

reduced flexibility. The lower ductility showed poor adhesive properties of the asphalts. The

decreased ductility can be due to the fact that the modified asphalt was manually blended, and

this could have an effect on the bitumen-crumb rubber interaction. Note that the blending

phase is a critical step towards ensuring mixture homogeneity which defines the

characteristics of the asphalt mixture.

Table 3 Physical properties of asphalt cement 40/50 penetration

Asphalt Property Units ASTM

designation

Test Results SCRB

specification

Penetration at 25 ˚C,

100 gm, 5sec

0.1 mm D5 46 4050

Flash Point, ˚C D92 256 >232

Ductility at 25 ˚C,

5cm/min

cm. D113 136 >100

Softening Point ˚C D36 52 -

Specific gravity - D70 1.03 ˃1.0

Table 4 Properties of crumb rubber modified asphalt

Asphalt Property Units ASTM

Designation

Test Results Standard Req.

(modified

asphalt) Crumb rubber

5% 10% 15%

Penetration at 25 ˚C,

100 gm, 5sec

0.1

mm

D5 40 32 27 Min.20

Flash Point, ˚C D92 265 270 277 >232

Ductility at 25 ˚C,

5cm/min

cm D113 125 97 65 >100

Softening Point ˚C D36 57 62 70 -

Specific gravity - D70 1.03 1.03 1.03 ˃1.0



The results showed that the degree of penetration of the modified bitumen was

significantly decreased with the addition of CRM. The addition of CRM increased the

viscosity of the bitumen because of the increased rubber mass. The penetration degree was

also decreased by 13, 30, and 40 % with the addition of 5, 10, and 15 % rubber crumbs,

respectively (Figure 5-a). The softening point was noted to increase with an increasing CRM

percentage due to the increased hardening of the bitumen (increased by 10, 19, 35 % with an

increasing CRM percentage of 5, 10, 15 % respectively). This can be related to the differences

in the rubber viscosity from that of bitumen (Figure 5-b). Similarly, the ductility value of the

modified bitumen was decreased compared to that of the control. This was due to the oil ratio

in the bitumen that was absorbed by the rubber particles which decreased gradually with

increased CRM percentage. The observed percentage decrease was 8, 29, and 52 % with the

addition of 5, 10, and 15 % CRM, respectively.

Figure 5 Physical properties of CRM

4.3. Mineral Filler

A summary of the physical properties of the limestone dust used in this study is presented in

Table 5. Table 5 Summary of limestones’ physical properties

4.4 Crumb Rubber

The materials specification and physical properties of CRM based on the recommendation

of Company for Tire Industry in AL-Najef City - Engineering Office -Technology

Department are presented in Table 6.

Table 6 CRMs’ physical properties and material specifications

Properties Test results

Percentage passing No.200 (0.075 mm ) 96 %

Plasticity index N.P.

Specific gravity 2.71

Property or Characteristic Unit Requirement or value

Specific Gravity - 1.13

Density grn/m3 1.320

Young's Modulus (E) MPa 2600 - 2900

Tensile Strength MPa 40 - 70

Elongation at Break % 25 - 50

Melting Point % 200

Rubber Hydrocarbon % 48 min

Carbon Black % 25 - 35

Acetone Extract % 10 - 20

Ash at 550 % 8.0 min

Metal Content % 0.03 max



5. RESULTS AND DATA ANALYSES

5.1. Optimum Asphalt Content

The optimum asphalt percentage of the unmodified and modified mixtures was determined

using a Marshall test method. The test was conducted using five binder percentages (4, 4.5, 5,

5.5 and 6 %) and three samples for each binder percentage. The sample preparation procedure

is shown in Figure 6 while the relationship between the asphalt content and the Marshall

properties of the control mixture is shown in Figure 7. The optimum asphalt content for the

selected gradation and control asphalt binder was 4.8 %.

Figure 6 Sample preparation procedure

The optimum asphalt percentage for each mixture is presented in Table 6. Th results

showed that the optimum asphalt content was increased with an increased rubber modifier

content. The higher optimum asphalt content of the mixtures is because of the thicker film of

the rubber crumb modified asphalt cement which coats the aggregates in the presence of the

rubber particles. The Marshall properties were also determined at the optimum asphalt

composition for the unmodified and modified mixtures as reported in Table 6.

Table 7 Optimum asphalt content of the asphalt mixtures

Marshal properties Mix type

Control mix 5% rubber

modified mix

10% rubber

modified mix

15% rubber

modified mix

Stability ,kN 10 10.2 10.3 10.7

Flow , mm 3.2 2.9 2.8 2.5

Bulk density ,

gm/cm3

2.38 2.38 2.372 2.366

Air void , % 3 3.4 3.8 4.1

Void filled

with asphalt , %

78 78 77 76

Void on mineral

aggregate , %

12.7 12.8 13.1 13.7

Optimum asphalt

content, %

4.8 4.9 5.1 5.4



Figure 7 Result of Marshall test for the unmodified mixture

Figure 8 showed the Marshall stability values for the unmodified and modified mixtures

against their asphalt content. The results indicate that the stability values for different

mixtures followed a typical trend as a function of their asphalt content. The values were noted

to increase with an increasing asphalt content until a maximum value where stability tends to

decrease was reached. It also indicates that the Marshall stability for the modified mixtures

increased as the percentage of rubber content increased. The stability was also found to

increase as a function of the percentage of the added rubber crumb. The stability increased to

a certain value before starting to decrease; the highest stability valued achieved was 10.5 kN

from the mixture modified with 10 % CRM.

Figure 9 Marshall flow with asphalt

content for different mixtures

Figure 8 Marshall stability with different

asphalt content for mixtures



Figure 9 presented the Marshall flow values with respect to the asphalt content for

different mixtures used. The flow values for the control and various modified mixtures were

found to increase with the asphalt content; this is commonly observed in asphalt mixtures

prepared with different asphalt contents. Figure 9 also showed a general increase in the

Marshall flow as the asphalt content increases. It was also noticed that the modified mixtures

gave lower Marshall flow values. The flow decreased as the rubber content was increased; the

maximum decrease was observed in mixtures modified with 15% CRM although this is still

within the standard specification limit (SCRB, 2003) [11].

5.2. Volumetric Properties of Asphalt Mixtures

The relationship between the air voids percentage in the total mixture and the asphalt content

in different mixtures is shown in Figure 10. The results showed the air void percentage to

decrease as the asphalt content was increased this is a common observation in asphalt

mixtures. The air void percentage of the modified mixture was also observed to be higher than

that of the control mixtures at different asphalt contents. The percentage of air voids filled

with asphalt against the asphalt content for different mixtures was shown in Figure 11. The

result indicates a similar VFA percentage trend as expected of asphalt mixtures where VFA

percentages tend to increase with the asphalt content. The %VFA of the modified mixtures

was also observed to be higher than those of the control mixture; the %VFA also increased

proportionally with the percentage rubber content.

The percentage of air voids in the mineral aggregate against the asphalt content for

different mixtures are presented in Figure 12. The results showed that the percentage of VMA

showed the typical trend expected from any asphalt mixture; the percentage VMA was found

to decrease as the asphalt content was increased; this decrease in percentage VMA persisted

until the minimum value was reached before the percentage VMA began to increase. It was

also observed that, for the same asphalt content, the percentage VMA of the modified

mixtures was higher than those of the control mixtures.

Figure 11 Air voids filled with asphalt

percent with asphalt content

Figure 10 Air void percent with asphalt

content for different mixtures



Figure 12 Percentage air voids in the mineral aggregate with asphalt content

5.3. Ultrasonic Test

The ultrasonic device is used to measure the time it takes seismic wave pulses generated by a

built-in pulse generator to travel through a specimen. The generator transforms an electrical

pulse into mechanical vibration through a transducer. A receiver which is connected to an

internal clock records the arrival time of the seismic wave. This internal clock can

automatically measure and display the travel time of the waves. The resilience modulus of the

HMA specimens is determined from the travel time and specimens’ density. A major

advantage of this test is its non-destructiveness. Additionally, both laboratory-prepared

specimens and field cores can be used in this experiment.

The prepared specimens (as earlier described) were used for the ultrasonic tests. The

specimens’ elastic modulus was determined with the aid of an ultrasonic device made up of a

timing circuit and a pulse generator. The device was coupled with piezoelectric transmitting

and receiving transducers. A dominant energy frequency of 54 kHz was imparted to the

specimen. The time it takes a wave to travel through the specimen was digitally displayed by

the timing circuit. A maximum specimen-transducers contact was ensured by using special

detachable epoxy caps on the transducers. The transducer that receives the impulse is coupled

to an internal clock for sensing the propagating waves. The internal clock detects and displays

the travel time tv. This recorded time is used to determine the constrained modulus Mv

according to the specifications of ASTM (C 597 – 02).

Mv = ρVp² = ρ (L/tv) ² (1)

where: Mv is the constrained modulus, ρ is the density (g/mm3), Vρ is the velocity of the

compression wave (mm/ms), L is the average specimens’ length (mm), tv is the travel time

(ms).

In a simpler form,

Mv

(2)

where:

M is the specimens’ mass (g), d is the specimens’ average diameter (mm). Young´s

modulus Ev can be calculated as follows:

Ev = Mv [

(3)

The determination of the Poisson´s ratio υ is based on experience; it is generally taken to

be in the range of 0.3 - 0.4 for asphaltic materials [12].

The results of Young's modulus E at 0, 5, 10 and 15 % CRM percentage showed that E

increased with the CRM content of the asphalt. Figure 13 showed that E was gradually



increasing with an increased rubber crumb content in the asphalt due to the increasing

elasticity factor, i.e, E values increased by 10, 5, and 4 % when 5, 10, and 15 % of CRM was

added, respectively.

Figure 13 Modulus of Elasticity E with rubber crumb content

The elasticity coefficient of the asphalt mixture can be increased by the addition of the

rubber crumbs to the original mixture since the elasticity factor is increased by increasing the

amount of CRM in the asphaltic mixture. This increase in elastic factor is because the CR

contains rubber materials.

6. CONCLUSIONS

Based on the experiments carried out in this research, it can be concluded that the physical

properties of rubberized asphalts which contain different percentages of rubber crumbs and

asphalt cement were influenced by the number of rubber crumbs in the mixture. The Marshall

stability of the modified mixtures increased with increasing rubber crumbs percentage. The

Marshall stability value of the control and modified mixtures at the optimum asphalt

percentage satisfied the engineering properties required by the SCRB/2003 specification for

asphalt mixtures used in the construction of surface course. The Marshall flow for the

modified mixtures showed an inverse relationship with the Marshall stability as the flow

values for various mixture was within the range 24 mm An increase in the percentage of

CRM in the modified mixtures decreased the bulk density of the mixtures. The air voids

percentage increased with increasing percentage of CRM; the control and all the modified

mixtures were within the specification range of 35%. The percentage of voids filled with

asphalt (VFA) inversely related with the percentage air void. The control and all the modified

mixtures showed a VFA percentage of 7085 percentage which satisfied the required

specification. The percentage of voids in the mineral aggregate (VMA) increased with an

increasing percentage of CRM content; the control and all the modified mixtures achieved the

minimum value of 12 %. The results of Modulus of Elasticity E was gradually increasing with

an increased rubber crumb content in the asphalt due to the increasing elasticity factor.

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Documents

THE INFLUENCE OF CRUMB RUBBER MODIFIER (CRM) ON THE ...€¦ · asphalt by blending asphalt cement with crumb rubber (0, 5, 10, and 15 % by asphalt weight) using the wet method. Next