Construction and Building Materials 49 (2013) 611–617

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Construction and Building Materials

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Laboratory evaluation of warm mix asphalt mixtures containing electricarc furnace (EAF) steel slag

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.08.034

⇑ Corresponding author at: Department of Civil Engineering, Iran University ofScience and Technology (IUST), Narmak, Tehran, Iran. Tel.: +98 (21) 77240399; fax:+98 (21) 77240398.

E-mail address: ameri@iust.ac.ir (M. Ameri).

Mahmoud Ameri a,b,⇑, Saeid Hesami c, Hadi Goli a

a Department of Civil Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iranb Chief Executive Officer of Center of Excellence of PMS, Safety and Transportation, IUST, Narmak, Tehran, Iranc Department of Civil Engineering, Noushirvani Institute of Technology, Babol, Iran

h i g h l i g h t s

� Steel slag aggregates improved the Marshall stability of hot mix asphalt mixtures.� Moisture susceptibility of warm mixtures containing steel slag was improved.� Warm mix asphalt mixtures containing steel slag aggregates showed higher resilient modulus.� Steel slag aggregates improved resistance to permanent deformation in warm mixtures.

a r t i c l e i n f o

Article history:Received 17 June 2013Received in revised form 13 August 2013Accepted 26 August 2013

Keywords:BitumenSteel slagWarm mix asphaltHot mix asphaltSasobit

a b s t r a c t

This paper presents the results of a laboratory study which was aimed to verify possibility of using elec-tric arc furnace (EAF) steel slag (SS) in substitution of natural limestone (LS) aggregates in warm mixasphalt (WMA) mixture. The SS was used as fine and coarse portion of aggregate gradation in HMA mix-tures, and as coarse portion of aggregate gradation in WMA mixture. The surface texture of both LS and SSaggregates was observed through scanning electron microscope (SEM). The results illustrated that theporosity and the roughness of the SS aggregate are higher than that of the LS aggregate. Moreover, lab-oratory tests results indicated that use of coarse SS aggregate in WMA mixture enhances Marshall stabil-ity, resilient modulus, tensile strength, resistance to moisture damage and resistance to permanentdeformation of the mixture. Hence use of coarse electric arc furnace steel slag aggregate in productionof WMA mixture is recommended.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years the concept of ‘‘comprehensive development’’has evolved to the more developed idea of ‘‘sustainable compre-hensive development’’. The concept of sustainable developmenttries to make equilibrium among various fundamental factorsincluding environment, economy, security, and social issues, whichmust be considered for the conservation of the nature as inheri-tance for future generations [1–3].

Transportation system has a decisive role in comprehensivesustainable development of a country. Nowadays an efficienttransportation system has vital importance for economic develop-ment. As a constituent element of the national infrastructure, thetransportation system satisfies human and economic needs interms of freight transportation and passenger carriage. Actually

the system is the basic element of supply chain of all industrieswhich is used by all economic and non-productive activity sectorswithout any exception [4–6].

Road pavement can be considered to be one of the fundamentalelements of the transportation system. Significant amounts of bitu-men and natural aggregates are used in the construction of asphaltconcrete pavements. Considering the compatibility of asphalt con-crete pavements construction with the principles of sustainabledevelopment, researchers are seeking for new ways to reduce con-struction cost, and lower emissions.

The economic costs and adverse environmental effects ofasphalt concrete pavement construction generate from two impor-tant sources; first and the most important is the heat sourcesrequired to process the paving materials, and the second is theingredients cost (bitumen and aggregates) [7].

The energy consumption and the release of pollutant gasses isthe consequence of drying and heating mineral aggregate and bitu-men at temperatures above 140 �C [8,9]. Therefore, scientists havedeveloped a number of new technologies for asphalt pavement

612 M. Ameri et al. / Construction and Building Materials 49 (2013) 611–617

materials generally referred to as warm mix asphalt (WMA), whichproduce asphalt mixture in the temperature range of 100–140 �C,without compromising the workability and mechanical perfor-mance attained [8,9]. There is also numbers of benefits in warm-mix asphalt production including: Environmental benefits (Lowerplant emissions and fumes), Economic benefits (Reduced energyconsumption and financial cost), Paving benefits (Improved work-ability and compaction efficiency, provision of longer haul dis-tances, and quicker turnover to traffic due to shorter coolingtime, and Production benefits (Increased Recycled Asphalt Pave-ment (RAP) content and location of plant site in urban areas)[10–15].

On the other hand to satisfy the need for natural stone materi-als, which is needed for construction, reconstruction, and repairs ofroad pavements, a wide range of industrial waste as secondary re-sources have been utilized.

Among wastes used as road building material, steel slag (SS) isone of the most popular one, due to certain environmental savingproperties including: preservation of natural ecosystem by meansof reducing the amount of dumped wastes, and allowing reducingthe consumption of natural aggregate in asphalt concrete pave-ments production every year [16–21].

Due to economic and environmental advantages, WMA hasbeen used to produce different kinds of asphalt mixtures, such aspolymer-modified asphalt, sulfur modified asphalt, stone matrixasphalt (SMA), dense graded asphalt, crumb rubber modified(CRM) asphalt, and in some cases with different aggregates suchas asphalt mixture containing recycled asphalt pavement (RAP)[8]. Therefore different WMA mixtures have different performancecharacteristics depending on the material used in them.

As a result, production of asphalt mixture which contains SS asan aggregate by means of WMA technologies is a more efficient ap-proach that includes WMA technology application as well as indus-trial waste substitution benefits simultaneously.

It is reminded that application of SS in the mixture can be rec-ommended if its use does not have significant negative effect onthe performance of the produced asphalt concrete pavement, eventhough having grate cost and emission reduction cover the minorreduction in the performance characteristics. The objectives of thisresearch study are to conduct laboratory tests on WMA mixturescontaining SS aggregates and to evaluate its performance for pro-ducing a WMA mixture that is more environmental friendly.

Table 2Properties of asphalt binder.

2. Materials and experimental procedures

The testing program for this study was divided into three main phases. In thefirst phase the four different aggregate gradations that are shown in Table 1, wasselected. Then mixtures of HMA concrete were prepared in accordance with theMarshall method (ASTM D1559) to obtain optimum bitumen content (O.B.C) foreach of the selected gradations, based upon the maximum bulk specific gravity,maximum Marshall stability and 4% air void [22]. In the second phase Marshall,ITS and dynamic creep tests were conducted on each of the HMA specimens withthe (O.B.C) as controlling mixtures. In the third phase, the WMA specimens wereprepared and tested.

Test Standard test Unit Result

Specific gravity (25 �C) ASTM D70 gr/cm3 1.03Flash point (Cleveland) ASTM D92 �C 308Penetration (25 �C) ASTM D5 �C 62Ductility (25 �C) ASTM D113 cm 100

2.1. Materials

For both HMA and WMA mixtures a 60/70 Penetration grade bitumen obtainedfrom Pasargard Oil Company was used. The properties of the bitumen are presentedin Table 2. The crushed limestone (LS) aggregates used in this study for both HMA

Table 1Gradations of aggregates for each type of mixture.

Type of mixture Aggregate Composition

A Steel slag as fine and coarse aggregateB Steel slag as coarse aggregate + limestone as fine aggregateC Limestone as fine and coarse aggregateD Steel slag as fine aggregate + limestone as coarse aggregate

and WMA mixtures were collected from Asbcharan quarry located in eastern part ofTehran province, city of Rudehen. The gradation of the aggregates used in this re-search study is shown in Fig. 1. The steel slag used was electric arc furnace steel slagwhich was obtained from Ahvaz steel manufacturing facility. The physical proper-ties of the LS and SS aggregate are shown in Table 3.

Chemical properties of the aggregate have an important role in their behavior ofthe mixtures. For this reason, the chemical properties of both LS and SS were ana-lyzed with XRF (X ray fluorescence) and are presented in Table 4.

To have an explicit image from surface texture of the aggregates used in thisstudy their surface texture was observed using scanning electron microscope(SEM). The SEM consists of an electron optical column which generates and focusesan electron beam over the specimen surface. The beam impinges on the specimenand produces signals which can be detected as backscattered electrons (BE or BSE),secondary electrons (SE) and X-rays. These electrons are measured by electrondetectors and converted into images [17]. The captured SEM micro graphs of LSand SS are presented in Figs. 3 and 4 respectively.

As the pictures show the morphology and texture of the two aggregates are dif-ferent. SS aggregate has higher porosity than natural LS aggregate. Porosity intui-tively increases the percent of bitumen required to produce asphalt mixture. Theimages also clarify that the surface texture of SS is rougher than that of LS whichimproves adhesion capability of SS. Adhesion between bitumen and aggregate de-pends not only upon bitumen properties but it is also depends upon aggregateproperties too.

As a common WMA additives [8,9], Sasobit was used as WMA organic bitumenmodifier. For WMA mixtures, first base bitumen was blended with Sasobit, and thenthe modified binder was used for preparation of WMA mixtures with optimumbitumen content. Properties of the Sasobit used in this study are presented inTable 5.

2.2. Sample preparation

Four types of aggregate compositions were used in this research study for prep-aration of specimens according to Table 1. For preparation of HMA specimens, theaggregates were heated in an oven for 24 h at the temperature of 165 �C, and thenthe heated bitumen was added into the aggregates. For WMA mixtures the aggre-gates were heated at 135 �C (Which is the temperature for the production ofWMA with Sasobit) and then the modified bitumen was added.

Replacing both the fine and coarse aggregate fraction of the mixtures by 100%SS results in excessive void, bulking problems, and increases the internal volumeexpansion risk and high bulk density [23]. To alleviate these problems and accord-ing to the test results conducted on HMA mixtures in phase two of this study, thetype B aggregate was selected to fabricate WMA test specimens containing SSaggregate.

2.3. Testing program

2.3.1. Marshall stability, flow and Marshall quotient testMarshall stability and flow test was conducted on specimens with various bitu-

men contents and the optimum bitumen content was determined in accordancewith ASTM D1559 and their Bulk specific gravities and air void contents were mea-sured in accordance with ASTM D2726. In addition the Marshall quotient (MQ) (theratio of stability (KN) to flow (mm)) was also calculated. Higher MQ value indicatesthat the mixture has better resistance to shear stresses and permanent deformation[24].

2.3.2. Resilient modulus testWhen the elastic theory is employed to analyze pavements, the resilient moduli

of asphalt surface, base and subgrade materials in flexible pavements are used inplace of elastic modulus. Resilient modulus is the ratio of the repeated axial

Softening point ASTM D36 �C 49Kinematic viscosity @ 120 �C ASTM D2170 mm2/s 810Kinematic viscosity @ 135 �C ASTM D2170 mm2/s 420Kinematic viscosity @ 150 �C ASTM D2170 mm2/s 232Penetration index (PI)a – – �1.12Penetration viscosity number (PVN)b – – �0.56

a PI = [1952 – 500 log (Pen25) – 20SP]/[50 log (Pen25) – SP – 120].b PVN = [�6.387 + 1.195 log (Pen25) + 1.5 log (Visco135)]/[0.79511 – 0.1858 log

(Pen25)].

0

20

40

60

80

100

0 0.075 0.3 2.36 4.75 12.5 19

Lower rangeUpper range

Sieve size /mm

Perc

ent p

assi

ng %

Fig. 1. Gradation of designated aggregate.

Table 3Physical properties of aggregates.

Test Standard Limestone Steel slag

Los angeles abrasion (%) ASTM C-535 22.6 14.2Specific gravity (coarse agg.) ASTM C-127 2.49 3.01Specific gravity (fine agg.) ASTM C-128 2.48 3.06

M. Ameri et al. / Construction and Building Materials 49 (2013) 611–617 613

deviator stress to the recoverable axial strain [25]. In this research study the resil-ient modulus of mixtures was determined in accordance with Australian: AS2891.13.1-1995.

For both WMA and HMA mixtures five specimens with optimum bitumen con-tent were tested at temperature of 25 �C using a haversine load pulse at 1 Hz with0.1 s loading time and 0.9 s rest period. The estimated poisons ratio was 0.35 andthe maximum applied load was 570 N with 100 repetitions for preconditioningloads.

2.3.3. Indirect tensile strength (ITS) testAn important characteristic of asphalt concrete is its resistance to tensile loads

and hence cracking [26]. Although cracking of asphalt pavements is not immedi-ately fatal, but certainly represents a type of tensile distress. The tensile strengthof the asphalt concrete is determined by indirect tensile strength (ITS) test. In thistest, a cylindrical specimen is subjected to compressive loads between two loadingstrips, loading the vertical diametrical plane and generating a relatively uniformtensile stress along the plane. The loading is continued to the failure and splittingthe specimen along the loading plane. The following equation is used to calculatethe ITS value for the specimen:

ITS ¼ 2Pmax

ptdð1Þ

where Pmax is the maximum applied load (KN), t is thickness of the specimen (mm), dis diameter of the specimen. For each type of the mix, five specimens were loaded atdeformation rate of 50 mm/min at 25 �C.

2.3.4. Resistance to moisture damage testPresence of moisture in asphalt concrete can weakens bonds between the

aggregate and bitumen leading to adhesive and also cohesive deterioration of themixture and decrease in load carrying capacity of asphalt pavement [27,28]. In thisresearch study the moisture susceptibility of asphalt concrete is evaluated by per-forming AASHTO T283 test. Six specimens for each mixture type with air void levelof 7 ± 1% were selected. Three specimens were conditioned by vacuum saturation at

Table 4Chemical composition of aggregates used in this research study.

Aggregate type Oxide content (%)

SiO2 Fe2O3 Al2O3 CaO

Limestone 17.25 0.93 2.11 43.01EAF steel slag 18.72 35.16 2.75 25.58

55–80% saturation level, followed by freeze–thaw cycle (16 h at �18 �C and subse-quent soaking in water bath at 60 �C for 24 h). The other three specimens were re-mained dry and were tested without moisture conditioning.

The tensile strength ratio is the tensile strength of the conditioned specimens tothe tensile strength of the specimens in ordinary dry condition and may be ex-pressed as follows:

TSR ¼WetITSDryITS

ð2Þ

where WetITS is the average ITS value for conditioned specimens and DryITS is theaverage ITS value for unconditioned specimens. A TSR value of 0.7 is typically spec-ified as a minimum acceptable value and mixtures with TSR values more than thatare relatively resisted to moisture damage [29].

2.3.5. Dynamic creep testRutting in asphalt concrete pavements under traffic loading occurs predomi-

nantly at elevated temperatures. Rutting in the asphalt surface layer is more sensi-tive to resistance of the layer to shape distortion due to plastic flow than it is to thelayer resistance to densification or volume change [30]. Based on the consider-ations, the dynamic creep test in this research study was conducted in accordancewith US.NCHRP 9-19 [31] Superpave W2 to evaluate the propensity for rutting inthe mixtures. The test was conducted at 50 �C. The stress of 450 Kpa was used asdeviator stress with 0.1 s loading and 0.9 s rest time for each loading pulse cycle.A typical creep test curve drawn for a sample in this research study is shown inFig. 2. As the figure illustrates, the curve can be divided into three distinct regions.In the primary stage of permanent deformation (up to around 1500 cycle), adecreasing rate of plastic deformations, which is predominantly associated withhigh initial level of rutting and volumetric change can be observed. In the secondarystage (nearly from 1500 to 7000 cycle), a constant rate of plastic deformation andrutting change is exhibited that is also associated with volumetric changes. How-ever, shear deformations increase at increasing rate in this stage. In tertiary stage(after 7000 cycle), high level of rutting predominantly associated with plastic(shear) deformations under no volume change is observed. Flow number, whichis defined as the number of cycle in which permanent strain in the mixture dramat-ically increases (the transition point from secondary to tertiary stage) [31], wasused as criterion to compare the mixtures potential to rutting.

3. Results and discussions

3.1. Marshall stability, flow and Marshall quotient test

Table 6 presents the Marshall test properties including (O.B.C)of all HMA mixtures tested in this research study. The values areobtained from replicate of three test specimens. It is observed thataddition of SS increases the optimum bitumen content of the mix-tures. This is attributed to the porous surface texture of SS, whichwas clearly demonstrated by the SEM images captured from bothlimestone and SS surface texture shown in Figs. 3 and 4. The(O.B.C) for type D mixture is higher than that for type B mixtures.This is due to higher porous specific surface area steel slag used asfine portion of the aggregates in type D mixtures.

Bulk specific gravity and Marshall stability and flow results forHMA specimens are also presented in Table 6. The results in Table6 show that mechanical and also physical properties of the mix-tures produced with SS are higher. Predominately this is due tothe higher bulk specific gravity of SS.

Based upon the results presented in Table 6, Marshall stabilityand flow of the HMA mixture increase simultaneously if the coarseportion of the aggregates is replaced with SS leading to larger valuefor MQ of the mix. In general, SS aggregates have higher angularity,angle of internal friction, and higher bulk specific gravity relativeto other natural stones [20]. This higher internal angle of frictionleads to better aggregate interlock and makes SS a suitable aggre-

MgO Na2O TiO2 K2O MnO

0.75 0.08 0.107 0.66 0.0467.50 0.29 1.595 0.13 0.304

Acc

umul

ated

Str

ain

Cycle

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2000 4000 6000 8000 10000 12000 14000

Fig. 2. Typical creep test curve.

Table 5Sasobit properties.

Characteristics Standard Description

Congealing point ASTM D938 106 �CPenetration @ 25 �C ASTM D1321 <1 d mmPenetration @ 65 �C ASTM D1321 6 d mmAppearance – Prills (diameter = 1 mm)

Fig. 3. SEM micrographs of limestone. Fig. 4. SEM micrographs of steel slag.

614 M. Ameri et al. / Construction and Building Materials 49 (2013) 611–617

gate for improving the rutting resistance of pavement layers. Theseunique properties of SS have attributed to larger value of MQparameter indicating higher resistance to shear stresses and per-manent deformations.

3.2. Resilient modulus (MR)

The test results for resilient modulus values of four types ofHMA and two types of WMA mixtures are presented in Fig. 5.The results indicate that in general, the WMA mixtures have higherresilient modulus values than HMA mixtures. By comparing theWC type specimens with C type ones that both used the same typeand grading of aggregates, the WC specimens have higher resilientmodulus. The Sasobit modified binder is stiffer and has better ten-sile strength [32]. Thus, the improvement in resilient modulus ofWMA mixtures is attributed to the stiffer Sasobit modified binder.The same results were obtained comparing WB and B specimens.Among WMA specimens, WB type specimens that contain SS havelower resilient modulus values than WC type specimens. The SSused in this study has lower CaO/SiO2 value than limestone, whichresults lower aggregate-binder adhesion. Thus, the decrease inresilient modulus values resulted by addition of SS to WMA mix-tures is related to the chemical composition of SS and limestone

used in this research. But it should be noted that, despite havinglower resilient modulus than WC type specimens, the WB typespecimens have higher resilient modulus than all types of HMAspecimens.

3.3. Indirect tensile strength (ITS)

The average tensile strength values for all six types of HMA andWMA mixtures are represented in Fig. 6. As the results indicate,among HMA specimens, the type C ones, which contain only lime-stone aggregates, have higher tensile strength values than others.This is due to the better adhesion of limestone aggregates to binderthan SS, which generates from chemical composition of the aggre-gates (higher CaO/SiO2 value for limestone).

Table 6Marshall tests results.

Mixture type Bulk specific gravity Air void content (%) O.B.C (%) Marshall stability (KN) Flow (mm) MQ (KN/mm) VMA (%)

A 2.77 4.81 5.2 11.33 3.8 2.98 12.95B 2.54 4.51 4.57 10.81 3.3 3.28 10.73C 2.35 4.47 4.34 9.06 2.9 3.12 10.17D 2.56 4.12 4.71 8.83 3.1 2.85 11.49

33073110 3061 2933

4316

3653

0500

100015002000250030003500400045005000

A B C D WC WBSpecimen

Res

ilien

t M

odul

us (

MP

a)

Fig. 5. Resilient modulus test result.

789 795891

657

989893

708 723825

620

935879

0

200

400

600

800

1000

1200Unconditioned

Conditioned

A B C D WC WB

Indi

rect

Ten

sile

Str

engt

h (K

Pa)

Specimen

Fig. 6. ITS values for dry and conditioned specimens.

M. Ameri et al. / Construction and Building Materials 49 (2013) 611–617 615

In general, the WMA mixtures have better tensile strength thanHMA mixtures. The higher tensile strength values of WMA mix-tures are attributed to better tensile performance of Sasobit mod-ified binder.

3.4. Resistance to moisture damage

The tensile strength values for the six types of mixtures are de-picted in Fig. 6 for both dry and conditioned specimens. Basedupon the results presented in Fig. 6, the dry and conditioned tensilestrength for both WB and WC type mixtures are higher than that ofHMA mixtures. Fig. 7 shows the results of TSR values for six typesof HMA and WMA specimens in this study. The moisture suscepti-bility of WMA mixtures is an important issue considering theirlower mixing and compaction temperature. Based upon the TSRvalues obtained, the WMA mixtures have better moisture damage

resistance than HMA mixtures. Among WMA mixtures the WBtype specimens containing SS as coarse aggregate have higherTSR values than WC type ones. This is attributed to the higher bitu-men content and better coating of aggregates by bitumen. The typeD specimens have higher TSR values than other three types of HMAspecimens, which is attributed to the low air void content and highoptimum bitumen content value for this type of specimens. Thelower air void content in type D specimens decreases the waterpenetration into the mixture. At the same time the higher bitumencontent results better coating of aggregates in this type ofmixtures.

3.5. Permanent deformation (dynamic creep)

The dynamic creep test results are presented in Fig. 8. Aggregateinterlock and bitumen stability are two important factors affecting

0.8970.909

0.926

0.944 0.945

0.984

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

A B C D WC WB

TSR

Val

ue

Specimen

Fig. 7. TSR values.

535

6500

3946

940

12286

14879

0

2000

4000

6000

8000

10000

12000

14000

16000

A B C D WC WBSpecimen

Flo

w N

umbe

r

Fig. 8. Dynamic creep test results.

616 M. Ameri et al. / Construction and Building Materials 49 (2013) 611–617

the creep performance of asphalt concrete. Modification of binderwith Sasobit stiffens the modified binder. Moreover, addition ofSS to the lithic skeleton of the mixtures increases the aggregateinterlock and improves the resistance of the mixture to verticalloading permanent deformations. The dynamic creep test resultsin Fig. 8 indicates that in general, the WMA mixtures have signifi-cantly better resistance to permanent deformations than HMAmixtures which is attributed to stiffer Sasobit modified binder usedin WMA mixtures.

The simultaneous affection of stiffer modified binder and betteraggregate interlock in WB type mixtures resulted significantenhancement in resistance to permanent deformation. Thus, thistype of SS warm mix asphalt mixture is a good choice for applica-tion as a rutting resistant asphalt mixture.

4. Conclusion

On the basis of the limited tests results obtained in this researchstudy, the following conclusions are drawn:

� Based on the Marshall test results replacing the coarse por-tion of the aggregate in HMA mixture with the coarse por-tion of the SS aggregate leads to a concurrent increase inthe Marshall stability and flow with an improve MQparameter.

� Based on other tests results of this research study theadvantageous of using coarse SS aggregate in productionof WMA mixture are:

(a) Better adhesive properties, better resistance to moisturedamage and hence a more durable mixture.

(b) Better aggregate interlocking, higher cohesive property andhence better resistance to permanent deformation.

(c) Higher MQ parameter and hence more flexible and energyabsorbing mixture.

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