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Road Materials and Pavement Design

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An overview of the use of nanoclay modifiedbitumen in asphalt mixtures for enhanced flexiblepavement performances

Fernando C. G. Martinho & José Paulo S. Farinha

To cite this article: Fernando C. G. Martinho & José Paulo S. Farinha (2017): An overviewof the use of nanoclay modified bitumen in asphalt mixtures for enhanced flexible pavementperformances, Road Materials and Pavement Design, DOI: 10.1080/14680629.2017.1408482

To link to this article: https://doi.org/10.1080/14680629.2017.1408482

Published online: 01 Dec 2017.

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Road Materials and Pavement Design, 2017https://doi.org/10.1080/14680629.2017.1408482

STATE OF THE ART

An overview of the use of nanoclay modified bitumen in asphalt mixturesfor enhanced flexible pavement performances

Fernando C. G. Martinho a∗ and José Paulo S. Farinha b

aDepartment of Chemical Engineering, Instituto Superior Técnico – University of Lisbon, Lisboa, Portugal;bCQFM, Institute of Nanoscience and Nanotechnology, Instituto Superior Técnico – University of Lisbon,Lisboa, Portugal

(Received 5 May 2017; accepted 10 November 2017 )

Bitumens are complex materials, produced mainly from crude oil as a by-product of petroleumrefineries, but may also be obtained from bio-oils or solid hydrocarbons. Bitumen specifica-tions have become stringent, mainly because they are used as binders in asphalt mixtures thathave to offer good resistance to weather and traffic erosion. There are many different types ofadditives that can be used to improve bitumen properties and, consequently, asphalt mixturesperformance. Nanoclays are one of the newest additives being used in paving grade bitumensto increase their in-service performance. This paper covers the main aspects of asphalt mix-tures with nanoclays, including constituent materials, mix design and mechanical performanceissues, as well as technical specifications.

Keywords: asphalt mixtures; bitumen; fatigue life; nanoclay; permanent deformation;stiffness

Highlights• Nanoclays can be used to enhance the mechanical properties of bitumens and asphalt

mixtures at a reasonable cost.• The mixing process of nanoclays with bitumen is generally easy, before its mixture in

asphalt concrete.• Hot and warm mix asphalts (HMA and WMA) with nanoclays can use a mix design similar

to those of traditional mixtures.• Nanoclays mixed in different types of bitumens (with or without polymers) offer good

mechanical performance.• Since long-term performance is still unknown, no life-cycle cost analysis is available.

1. IntroductionThe main aim of this review is to cover the use of nanoclays in hot mix asphalt (HMA) and inwarm mix asphalt (WMA), focusing especially in the laboratory mix design and the mechanicalperformance of those materials. HMA are mixtures of several ingredients, including binders,fillers, aggregates and other additives (Figure 1). WMA differ from HMA in that the former areproduced and laid down at temperatures 20–40°C lower.

*Corresponding author. Email: fernando.martinho@tecnico.ulisboa.pt

© 2017 Informa UK Limited, trading as Taylor & Francis Group

2 F.C.G. Martinho and J.P.S. Farinha

Figure 1. Asphalt mixtures (HMA or WMA) and its components.

Over the last two decades, the immense industry of production and lay-down of asphalt mix-tures (Asphalt Institute & Eurobitume, 2015) have developed and applied many innovations,in order to meet economic and environmental objectives (Capitão, Picado-Santos, & Martinho,2012). The industry has been focused specially on improving the in-service mechanical per-formance of the asphalt mixtures. In addition, other aspects, such as the energy consumptionreduction and the minimisation of the environmental impacts (during the asphalt mix produc-tion and its transport and application), are receiving greater attention. To this end, new binders(including bitumens, as called in Europe or asphalts in North America) have been developed.

The most common base bitumens are complex mixtures of aliphatic, aromatic and naphthenichydrocarbons. The chemical components of these bitumens are usually grouped into two cate-gories: asphaltenes and maltens (Jahromi & Khodaii, 2009). Asphaltenes contain high-molecularweight phenols and heterocyclic compounds, while maltens (the de-asphalted fraction of bitu-men) are divided into saturates (the fraction eluted with n-pentane), aromatics (the fraction elutedwith benzene) and resins (those adsorbed on clays or eluted from silica gel by polar eluents)(Garcia, 2014).

Many additives have been developed to minimise certain pathologies which take place duringthe in-life service of asphalt mixtures, such as, for example, cracking, permanent deformationand erosion (Zhu, Birgisson, & Kringos, 2014). Some of them allow the adjustment of variousproperties of the binders, such as their softening temperature and viscosity. For example, thebinder/aggregate adhesivity may be chemically improved by adding surfactants. Such additivesalso enable to the reduction of the mixing temperature and the compaction in the layers of flexiblepavements (Abdullah, Zamhari, Hainin, Oluwasola, Hassan, et al., 2016; Abdullah, Zamhari,Hainin, Oluwasola, Yusoff, et al., 2016; Capitão et al., 2012).

As pointed out by Zhu et al. (2014), although these modifying agents improve many bitu-men properties, they also introduce some drawbacks, such as stability problems in the storage ofblends, a lower resistance to ageing and higher cost. Nevertheless, the main advantages of mod-ified bitumen justify their use in asphalt mixtures, especially, when higher gas barrier properties,enhanced mechanical behaviour and better thermal properties are achieved.

One of the most recent strategies to improve the performance of asphalt concrete mixturesconsists in adding nanomaterials to the binders. In particular, it has been described that some

Road Materials and Pavement Design 3

properties of the binders can be strongly improved by mixing small amounts of nanoclays, dis-persed at nanometer level (Galooyak, Dabir, Nazarbeygi, & Moeini, 2010; Jahromi & Khodaii,2009; Liu, van de Ven, Wub, Yu, & Molenaar, 2011).

Table 1 summarises the testing procedures mentioned in the next sections and in the literature,the applicable European Norms and their corresponding ASTM and AASHTO standards.

1.1. BitumensThe world use of bitumen in 2015 was approximately 87 million tons per year (Asphalt Institute& Eurobitume, 2015). The paving industry represents the main area of all applications (with 85%of the total). The size of this market has stimulated the scientific community to better understandthe composition and the rheological behaviour of the different kinds of binders that can be usedin asphalt mixtures.

Bitumens (or asphalts) are complex materials, produced mainly from crude oil (as a by-productof oil refineries). They can also have other origins (Garcia, 2014), such as biomass pyrolysis (bio-oil and bio-char) or solid hydrocarbons (Gilsonite). The complexity of these materials is linkedto the fact that bitumens are composed of numerous hydrocarbons. Depending on the chemicalspecies resulting from its constitution and preparation process (saturate, cyclic or aromatic), thesematerials are classified as paraffinic, naphthenic or aromatic (Guern, Chailleux, Farcas, Dreessen,& Mabille, 2010).

Bitumens have not only very different chemical compositions, but also a wide range of molec-ular weights. These molecular weights were pointed by Jahromi and Khodaii (2009) as being inthe range of 1000–100,000 for asphaltenes, 500–50,000 for resins, 300–20,000 for aromatics and300–1500 for saturates. However, other authors argue that those weights are not so elevated, con-sidering values of up to 2000 g/mol (Redelius, 2009), 3500 g/mol (Paliukaite, Vaitkus, & Zofka,2014) or 7000 g/mol (Weigel & Stephan, 2017), such as summarised in Table 2.

Tendentially, binders with greater amounts of resins and asphaltenes have a greater stiffnessand tensile strength (TS), when compared to those with more saturates and aromatics. Theseresults suggest that it is fundamental to know and compare the chemical composition of bindersin many cases, especially for those which include recycled or aged binders or binders withrejuvenators (Sultana & Bhasin, 2014).

Redelius and Soenen (2015) classified asphaltenes by their solubility. Asphaltenes contain themolecules with the strongest dispersive interactions in bitumen, as well as the strongest π–π

(pi–pi) interactions.The chemical composition of bitumens also shows that up to 10 wt.% comprises sulphur, nitro-

gen and oxygen, while the remaining portion includes essentially carbon and hydrogen (Polacco,Filippi, Merusi, & Stastna, 2015).

1.2. Nanomaterials and nanoclaysAccording to the European Union Recommendation 2011/696/EU, nanomaterials are materialswhich contain particles, “in an unbound state or as an aggregate or as an agglomerate and where,for 50% or more of the particles in the number size distribution, one or more external dimensionsis in the size range 1 nm–100 nm” (or from 10−9 up to 10−7 m).

Some nanomaterials have been used to develop a generation of high-performance materi-als with impact in many sectors, including the paving of transport infrastructures (Jamshidi,Hasan, Yao, You, & Hamzah, 2015). One of the most promising materials for construction ispolymer matrix nanocomposites (PMCs), thanks to their remarkable mechanical, thermal anddurability properties. Polymer-layered silicate nanocomposites, in particular, have been studied

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Table 1. Some of the European Norms (EN) used and corresponding ASTM and AASHTO standards.

Materials PropertiesMain test meth-ods/procedures Test ref. Parameters EN standard ASTM standard

AASHTOstandard

Binders Penetration grade Penetration PG Pen EN 1426 D 5 T 49Softening point Ring and ball temperature SP R&B EN 1427 D 36 T 53Viscosity Dynamic viscosity RV η EN 13302 D 2196/D 4402 T 316/TP 48Permanent deformation/

stiffness/ low-temperaturecracking

Dynamic shear rheometer DSR G*, δ, RJ EN 14770 D 7175 T 315/TP 5-97Bending beam rheometer BBR S(t) EN 14771 D 6648 T 313

Compliance and recovery Multiple stress creep andrecovery

MSCR J nr, R, J nr,diff EN 16659 D 7405 T 350/TP 70

Elastic recovery ER RE EN 13398 D 6084/D 113 T 51Short-term ageing Rolling thin-film oven test RTFOT – EN 12607-1 D 2872 T 240Long-term ageing Pressure ageing vessel PAV – EN 14769 D 6521 R 28

Asphaltmixtures

Stability and flow Marshall test MT MS, F EN 12697-34 D 6927//D 1559 T 245Water sensitivity Indirect tensile strength test ITS ITSR EN 12697-23 D 6931/D 4867 T 283Low-temperature cracking Tensile stress restrained

specimen test/uniaxialtensile stress test/relaxationtest

TSRST/UTST/RT Tr.temp, Fr.temp. EN 12697-46 TP 10

Stiffness Flexural loading/tension-compression/diametricloading

e.g. 4PB E*, δ EN 12697-26 D 4123 T 342Resistance to fatigue ε6 EN 12697-24 D 7460 T 321

Resistance to permanentdeformation

Wheel-tracking test/Hamburgwheel-track

WTT/HWT RD, PRD, WTS EN 12697-22 T 324

Cyclic compression test CCT fc EN 12697-25

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Table 2. Bitumen: characterisation of its fractions.

Content inbitumen(wt.%)

Molecularweight(g/mol)

Density @20°C

(g/cm3)

Solubilityparameter(MPa0.5) H/C ratio

Bitumen compounds ColorPhysical

state [1]∗ [2] [1] [2] [1] [1] [1]

MaltenesOils

Saturates (paraffins) White Liquid 05–15 05–20 ≈ 600 470–880 ≈ 0.90 15.0–17.0 ≈ 2Aromatics (or naphthene

aromatics)Dark brown Liquid 30–45 30–65 ≈ 800 (avg.) 570–980 ≈ 1.00 17.0–18.5

Resins (or polar aromatics) Dark brown Solid or semi-solid 20–35 30–45 300–2000 780–1400 ≈ 1.07 18.5–20.0Asphaltenes (insolubles in

hexane and heptane)Black Solid 05–25 05–25 800–3500 400–7000 ≈ 1.15 17.6–21.7 0.98–1.56

Notes: [1] As per Paliukaite et al. (2014); [2] as per Weigel and Stephan (2017). ∗Recommendation to have a good quality bitumen.

6 F.C.G. Martinho and J.P.S. Farinha

Figure 2. Montmorillonite structure (A) and its surface treatment (B) (Jahromi & Khodaii, 2009).Reproduced with permission of Elsevier.

since the 1990s (Silvestre, Silvestre, & Brito, 2015). However, though some authors expect nan-oclay composites to replace mineral fillers in asphalt mixtures (Iskender, 2016), their use hasbeen hindered by the fact that these clays are hydrophilic, thus difficult to use as fillers in mostpolymer matrices.

Natural nanoclays are one of the most common, safe (if properly handled), inexpensive andsustainable layered silicates, with montmorillonite (MMT) being the most used (Bonati et al.,2013). Other natural clays, like hectorite, saponite (Silvestre et al., 2015) and kaolinite (Zhuet al., 2014), have also been extensively studied for use in composites.

Cloisite, an organically modified montmorillonite (OMMT), is a quaternary ammonium salt-modified natural MMT with a 2:1 layer structure and high surface charge (Farias et al., 2016;Walters et al., 2014). The other well-known nanoclay, kaolinite, has a 1:1 layer structure andvery low surface charge. MMT has a 2:1 layered structure with two silica tetrahedron layerssandwiching an alumina octahedron layer (Bonati et al., 2013) (Figure 2).

In order to increase its specific surface area, MMT is generally subject to a surface treatment toseparate the individual clay sheets, known as exfoliation (Figure 2(B)). The exfoliation of MMTis determined by its capacity to exchange ions. In the case of MMT, the cation exchange capacity(CEC) ranges from 80 to 120 meq/100 g (milli-equivalents per 100 g), while in kaolinite thesevalues are between 3 and 5 (Jahromi & Khodaii, 2009).

Considering the capacity of the silicate particles to disperse into individual layers, nanoclaynanocomposites can be organised into three groups: flocculated (immiscible), intercalated andexfoliated (Silvestre et al., 2015). The characteristics of each of these groups are illustrated inFigure 3, which includes examples of typical wide-angle X-ray scattering (WAXS) spectra andtransmission electron microscopy (TEM) images.

In the exfoliated nanocomposites the individual layers of clay are dispersed in a continuouspolymer matrix and are spaced apart at a distance dependent on the clay loading (Silvestre et al.,2015). Intercalated nanocomposites are systems where the polymer has penetrated the galleriesbetween silicate sheets, but has not completely delaminated them. In these nanocomposites thereis a significant interaction between silicate layers, but the structure is not wholly exfoliated.Flocculated (immiscible) nanocomposites have predominantly the same structure as an inter-calated nanocomposite, but some silicate layers are flocculated due to hydroxylated edge–edgeinteraction.

In exfoliated clays the separation of clay sheets results in a nanoclay with an extensive activesurface area (in the range 700–800 m2/g). This helps create a better intensive interaction betweenthe nanoclay and the bitumen matrix (Jahromi & Khodaii, 2009), and thus enhance the behaviourof asphalt mixtures.

OMMT is typically prepared by cation exchange between Na-MMT galleries and a surfactant,such as cetyltrimethyl ammonium bromide (CTAB) (Haerudin et al., 2010). Recent works have

Road Materials and Pavement Design 7

Figure 3. Different states of dispersion of organoclays in polymers with WAXS (middle) and TEM (top)results (Paul & Robeson, 2008). Reproduced with permission of Elsevier.

shown the synergic effects obtained by mixing alkaline fillers with layered silicates. In particular,OMMT allows the reduction of the total filler amount, which represents an economical advantage(Bonati et al., 2013), when compared to other additives that do not lead to any reduction in thisfraction.

Mohammadiroudbari, Tavakoli, Aghjeh, and Rahi (2016) studied the effect of nanoclays onthe morphology of polyethylene-modified bitumen and the results showed that the content ofthe polymeric phase, the functionality of the polyethylene and the process used for mixing thecomponents has a strong influence on the state of nanoclay intercalation or exfoliation.

Nanoclays are already used in many nanocomposites in the building industry, namely in poly-meric coatings and foams (Silvestre et al., 2015) and in the production of asphalt mixtures forflexible pavements (Abdullah, Hainin, Yusoff, Zamhari, & Hassan, 2016; Abdullah, Zamhari,Hainin, Oluwasola, Hassan, et al., 2016; Abdullah, Zamhari, Hainin, Oluwasola, Yusoff, et al.,2016; Ameri, Vamegh, Imaninasab, & Rooholaminib, 2016; Ashish et al., 2016; Babagoli,Mohammadi, & Ameri, 2017; Blom, De Kinder, Meeusen, & Van den Bergh, 2017; Ezzat, El-Badawy, Gabr, Zaki, & Breakah, 2016; Farias et al., 2016; Galooyak et al., 2010; Iskender, 2016;Jahromi & Khodaii, 2009; Jamshidi et al., 2015; Liu et al., 2011; Silvestre et al., 2015).

2. Nanoclay-modified bitumens2.1. Modified bitumens and nanoclaysBitumen modifications help achieve the desired properties required in the asphalt pavementsand have been commonly performed with the addition of thermoplastic or elastomeric poly-mers. Different types of polymers are used in polymer-modified bitumens (PMB) to produceenhanced asphalt mixtures, increasing the mechanical performance and durability of pavement,

8 F.C.G. Martinho and J.P.S. Farinha

and reducing its maintenance operations (thus, offsetting the higher initial investment) (Polaccoet al., 2015).

Technical developments in the field of bitumen polymer modification during the last fourdecades include the application of several plastomers, such as polyethylene (PE), polypropy-lene (PP), poly(ethylene-r-vinyl acetate) (EVA) and poly(ethylene-co-butyl-acrylate) (EBA), andthermoplastic elastomers, such as poly(styrene-b-butadiene-b-styrene) block copolymer (SBS),poly(styrene-b-isoprene-b-styrene) (SIS) and poly(styrene-b-ethylene-butene-b-styrene) (SEBS)(Ameri, Mohammadi, Vamegh, & Molayem, 2017; Aziz, Rahman, Hainin, & Bakar, 2015;Babagoli et al., 2017; Li, Wu, Liu, Cao, & Amirkhanian, 2017; Zhu et al., 2014).

Different solutions have also been studied in order to minimise the drawbacks of many polymermodifiers, among which, saturation, the need for functionalisation (including the application ofreactive polymers) and the use of extra additives (sulphur and antioxidants). In contrast, there areonly a few studies of bitumen blends with innovative modifiers (Munera & Ossa, 2014). Amongthese, clay minerals are especially interesting to use with PMB (Zhu et al., 2014), due to theirlower cost, hydrophobicity and the ability to be dispersed at the nanoscale.

Regarding the costs, Hossain et al., cited by Ashish et al. (2016) and by Li, Xiao et al. (2017),reported that two nanoclay-modified bitumens (NMB) were “22 and 33%” less expensive thanthe similar PMB. In another study, Farias et al. (2016) were more expressive and pointed toa reduction of “approximately 36%” when clay can replace polymer. However, based on ourexperience, this is an excessive percentage which may be connected with the low MMT unitprice considered.

Dispersed hydrophobic clay minerals can have an intercalated or exfoliated structure (Jasso,Bakos, Stastna, & Zanzotto, 2012), with the latter being more efficient for use with PMB. Amongother benefits, the addition of exfoliated hydrophobic clays can increase the storage stability,viscosity, stiffness and rutting resistance of PMBs (Zhu et al., 2014).

However, Zhu et al. (2014) noted that completely exfoliated hydrophobic clay minerals struc-tures in PMB are difficult to achieve, and therefore, only partial improvements in the propertiesat low temperatures are obtained, namely ductility. These factors may limit the use of hydropho-bic clay minerals in PMB. Furthermore, Yao et al. (2013) showed that depending on how themodification of the asphalt binders is preformed, the nano- or micro-materials can suffer bothchemical reactions and physical dispersion when mixed in bitumen.

Although bitumens modified with a physical mixture of polymer and clay may be non-homogeneous, the use of the clay can still have a positive effect on the material. Jasso et al.(2012) tested a PMB containing 3 wt.% of OMMT and the results showed an internal networkstronger than the equivalent PMB without the clay. One of the most important advantages ofusing OMMT is enhancing the storage stability of PMBs (which depends on the bitumen, poly-mer and modification process) (Polacco et al., 2015). Galooyak et al. (2010) found that by addinga suitable amount of exfoliated nanoclay to SBS-modified bitumen, not only the performance ofPMBs was improved, but also the storage stability. As well as, the use of nanoclays may allow areduction in the consumption of polymer, as shown by Farias et al. (2016) who compared a PMB(4% of SBS) with an SBS/nanoclays bitumen.

The addition of clay as a third constituent into PMB with linear and branch SBS block copoly-mer was investigated by Golestani, Nejad, and Galooyak (2012). In their study the samples wereprocessed by melt blending (MB) with different amounts of OMMT and the results showedimprovements in the physical properties, rheological behaviour and the storage stability of thePMB. However, these authors also observed that the high-temperature storage stability decreasedwith the total amount of OMMT added. The same conclusion was presented by Ilyin, Arin-ina, Mamulat, Malkin, and Kulichikhin (2014) when meta-kaolin was used. Ortega, Navarro,García-Morales, and McNally (2015) also noticed a degradation in the visco-elastic behaviour in

Road Materials and Pavement Design 9

some conditions, although the addition of 10 wt.% nanoclay improved the bitumen rheologicalresponse.

In another study, Jahromi and Khodaii (2009) proposed an approach where bitumen was mod-ified with only a small amount of nanoclay, and its physical properties were effectively improvedwhen the clay was spread at the nanoscopic scale. They mention that those particles, whencompared with Cloisite particles, were more curly and smaller.

Although the preparation of PMB using traditional methods has been amply reported, theinfluence of some parameters, such as temperature, shearing speed and processing time, has notbeen generally investigated in detail. Fang, Yu, Liu, and Li (2013) focused on the processes fordispersing a large amount of nanoparticles into bitumen and enhancing the compatibility of thedifferent phases. Iskender (2016) states that nanoclay materials can replace mineral fillers, but theuse of clay minerals in large amounts may spoil the elastic properties of PMB, such as observedby Golestani et al. (2012). Nazzal, Kaya, Gunay, and Ahmedzade (2012) also observed that theaddition of nanoclays in the asphalt binder amplifies the adhesive strengths, although there is aslight decline in cohesive forces.

El-Shafie, Ibrahim, and El Rahman (2012) show that the physical and mechanical prop-erties of binders are frankly improved by an efficient dispersion of the clay, due to thelarge surface area and the stiffening effect of the nanoclay (by forming bond chains withinthe binder), and increase the softening temperature (up to 12°C relative to the base asphaltbinders). Jahromi and Khodaii (2009) also demonstrated that a small percentage of nanoclaymixed in the bitumen causes significant changes in rheological parameters, decreasing duc-tility and penetration, as well as the enhancement of the resistance to ageing and softeningpoint.

Golestani et al. (2012) through the addition of OMMT to PMBs showed that the softeningpoint and the viscosity increased, while the penetration decreased, with important effects on elas-tic recovery and ductility. El-Shafie et al. (2012) used nanoclays that were organically modified,obtained from macroclay, to conclude that the kinematic viscosity value, at 150°C, increases145% when compared with the control binder. Ziari, Babagoli, and Akbari (2015) claim that theuse of Bentonite (BT) for the modification of bitumen can increase the viscosity. Walters, Fini,and Abu-Lebdeh (2014) also confirm that the viscosity of a bio-modified binder with bio-charand nanoclay was found to be clearly higher than without the nanoclay.

Different aspects of the use of nanocomposites in asphalt binders were also studied by Yaoet al. (2013), who confirmed that the addition of selected nano- or micro-materials enhancesanti-oxidation effects on the modified asphalt binder, and by Li, Wu et al. (2017), who used anOMMT to prove that its addition can minimise the volatile organic compound (VOC) emissionof bitumen (due to the fact that the interlayer space of the nanoclays was filled by the lightcomponents of bitumen).

In Table 3 and Figure 4 we show some information on recently reported experiments regardingdifferent types of bitumen (all, except two, with a penetration grade over 50 dmm), nanoclaycontent (from 2 to 30 wt.% of bitumen), methods of the preparation and blending (mainly themelt-intercalated (MI) process and shear mixing procedure) and the kinds of structures obtained(mainly intercalated).

The information in Table 3 shows that several authors also used blends of polymers (such asSBS, EVA and SEBS) and nanoclays. In one of those cases (Galooyak et al., 2010), the authorsprepared an OMMT/SBS blend by using the MI technique and obtained an exfoliated structure,which promote the homogenous distribution of OMMT in PMB. However, Zhu et al. (2014) statethat hydrophobic clay minerals are hard to be perfectly exfoliated.

There are great differences in the mixing parameters (temperature, speed and duration) used(Table 3 and Figure 5), probably due to differences in bitumen chemistry and in their penetration

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Table 3. Nanoclay-modified binders and asphalt mixtures evaluated – some recent experience.

Nanoclays identification Blending of nanoclays with bitumenEffects

reported on

Years Reference Materials Abbrev.Commercial

nameStructure

typeBitumen

type

Nanoclaycontent (wt.

% ofbitumen)

Nonoclayspreparation or

blendingmethods

Mix temp.(°C)

Mixspeed(rpm)

Duration(min) Binder

Asphaltmixture

2017 [1] Org. mod. mont. OMMT I/E 67 dmm 2%, 5% and 10% MB 160 2500 120 P[2] Montmorillonite

+ SBSMMT + fibers

+ SBS60/70 2–5% MI 160 5000 60 P P

[3] Org. mod. mont. OMMT Bentonite I/E 35/50 3% and 5% MB 145 4500 ± 100 P P[4] Organophilic

nanoclayPG 58-22 3% MB 5000 P P

2016 [5] Montmorillonite+ fibers

MMT + fibers 60/70 2%, 4% and 6% PM 155 5000 120 P P

[6] Org. mod. mont. OMMT Cloisite PG 58-22 2% and 6% Ultrasonic mixer@65 W

150 20 P P

[7] Org. mod. mont. OMMT Cloisite I/E 86 dmm 2%, 4% and 6% MB 155 ± 5 4000 120 P[8] Org. mod.

mont. + SBSOMMT + SBS 50/70 4% Dispersed in

toluene withSBS

160 2000 120 P

2015 [9] Montmorillonite MMT PG64-28 2% and 4% Dispersed inorganicsolvent

160 2500 180 P P/N

[10] Nanoclay 60/70 2% and 4% PM 150 1000–2000 60–120 P[11] Org. mod.

mont. + SBSOMMT + SBS Cloisite E PG 58-10 1.5%. (SBS/

OMMT = 100/25)MB 180 4000 45 P P

[12] Montmorillonite MMT 70/100 3% SM + SP 150 1550 90 P[13] Nanoclay PG 58-22 3% and 6% SM + SP 150 1550 90 P

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[14] Nanoclay Bentonite 60/70 10%, 15%, 20%, 25%and 30%

MB 140 4000 15 P P

[15] Nanomernanoclays

PG58-34 2% and 4% MB 130 4000 120 P

[16] Org. mod.mont. + MDI

OMMT+ MDI

Cloisite I 160/220 10% Dispersed intoluene withMDI

150

800–1000and

18,000–20,000 20 P

[17] Org. mod. mont. OMMT 60/80 1%, 3% and 5% PM 145–155 4000 60 P2014 [18] Org. mod.

mont. + SEBSOMMT

+ SEBSCloisite E 44 dmm 3% to 6% (100/10-

100/30 SEBS-C15Aratios)

MB 180 1200 30 P

[19] Montmorillonite MMT E PG 64 3–5% MB 160 ± 5 6000 25 P

[20] Nanoclay+ Bio- char

Cloisite +Bio- char PG 64-22 2% and 4% MB P

2013 [21] Nanomernanoclays

PG 58-34 2% and 4% MB 130 4000 120 P

[22] Org. mod. mont. OMMT E 80/100 3.0%, 5.0% and 9.0% PM 150 2000 60 P[23] Org. mod. mont. OMMT Cloisite 50/70 3.33% MB 160 ± 5 4000 30 P P

2012 [24]

Org. mod.mont. +

PP + SBSOMMT +SBS + PP 60/70 PP = 2–7%;

SBS = 1–5%;Nanoclay = 1–3%

MB 150 3000 30

[25] Org. mod.mont. +surfact.

OMMT SPREA MISR I 60/70 2%, 4%,6% and 8%

MB 160 120 P

[26] Org. mod. mont. OMMT Cloisite 200/300 SBS/ OMMT =3%/ (1–2%)

MB > 180 P

[27] Org. mod.mont. + SBS

OMMT + SBS Cloisite I 85/100 SBS/ OMMT =100/ 12.5 to 100/ 50%

MB 180 ± 5 4000 45 P

[28] Org. mod.mont. + SBS

OMMT + SBS I 60/80 4% (SBS) + 3%(OMMT)

MB 180 2000 30 P

(Continued).

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Table 3. Continued.

Nanoclays identification Blending of nanoclays with bitumenEffects

reported on

Years Reference Materials Abbrev.Commercial

nameStructure

typeBitumen

type

Nanoclaycontent (wt.

% ofbitumen)

Nonoclayspreparation or

blendingmethods

Mix temp.(°C)

Mixspeed(rpm)

Duration(min) Binder

Asphaltmixture

2011 [29] Nanoclay PG 64-28 2% and 4% Dispersed inorganicsolvent

160 2500 180 P

[30] Org. mod. mont. OMMT I 70/100 4% Treated withcationexchangereaction

P

2010 [31] Org. mod.mont. + EVA

OMMT + EVA Cloisite/Dellite I 50/70 2% and 4% PM/Mba 180 4000 60 P

[32] Organic. mod.bentonite

OBT Bentonite E 60/70 1%, 2%, 4%, 5% and6%

SM + SP 160 4000 60 P

[33] Org. mod.mont. + SBS

MMT/ OMMT+ SBS

Nanofil

E(OMMT),I (MMT) 85/100 MI 180 4000 120 P

2009 [34] Org. mod. mont. OMMT Cloisite/Nanofil I 60/70 2%, 4% and 7% MI 150 5500 30 P

Notes: [1] As per Vargas, Moreno, Montiel, Manero, and Vázquez (2017); [2] as per Babagoli et al. (2017); [3] as per Blom, De Kinder, Meeusen and Van den Bergh (2017); [4] asper Melo and Trichês (2017); [5] as per Taherkhani (2016); [6] as per Ameri et al. (2016); [7] as per Ashish et al. (2016); [8] as per Farias et al. (2016); [9] as per Khadrah, Elshrief,and Sabry (2015); [10] as per Omar et al. (2015); [11] as per Golestani, Nama, Nejad, and Fallah (2015); [12] as per Santagata, Baglieri, Tsantilis, and Chiappinelli (2015); [13] as perSantagata, Baglieri, Tsantilis, Chiappinelli, and Aimonetto (2015); [14] as per Ziari et al. (2015); [15] as per Jamshidi et al. (2015); [16] as per Ortega et al. (2015); [17] as per Pei et al.(2015); [18] as per Zapién-Castillo et al. (2015); [19] as per Abdullah, Zamhari, Buhari, Nayan, and Hainin (2014); [20] as per Walters et al. (2014); [21] as per Yao et al. (2013); [22]as per Mahdi, Muniandy, Yunus, Hasham, and Aburkaba (2013); [23] as per Bonati et al. (2013); [24] as per Yazdani and Pourjafar (2012); [25] as per El-Shafie et al. (2012); [26] asper Jasso et al. (2012); [27] as per Golestani et al. (2012); [28] as per Zhang, Yu, and Wu (2012); [29] as per Goh, Akin, You, and Shi (2011); [30] as per Liu et al. (2011); [31] as perSureshkumar et al. (2010); [32] as per Zare-Shahabadi, Shokuhfar, and Ebrahimi-Nejad (2010); [33] as per Galooyak et al. (2010); [34] as per Jahromi and Khodaii (2009).Structure types: E: exfoliated; I: intercalated. Effects on binder/asph. mix: P: positive; N: negative. Nanoclays preparation or blending methods: MB: melt blending; SM: simple shearmixing; SP: sonication phase; MI: melt-intercalated process; PM: physical mixing; Mba: Master batch.

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Figure 4. Nanoclay contents in recent literature.

Figure 5. Blending of nanoclays with bitumen: temperatures, speeds and durations used in recentliterature.

grades, that lead to significant variations on the final properties of NMB. This represents a seriousobstacle when looking for direct correlations among the reported results.

2.2. Asphalt mixtures with nanoclay-modified bitumenA recent justification for the use of higher performance binders in enhanced asphalt mixtures (asis the case of NMB) is related with the bitumen consumption reduction during the infrastructurelifecycle. This fact also contributes to decrease fossil fuel consumption and global emissions,as supported by the Paris Agreement (signed during the Conference of Paris, held in December2015).

Cheng, Shen, and Xiao (2011) found that WMA which incorporated Sasobit (a synthetic waxobtained by the Fischer–Tropsch process from syngas) and a nano-size hydrated lime had animportant increment in the indirect tensile strength (ITS), stiffness and flow number (both in dryand wet conditions), when compared to samples that include micro-size hydrated lime. The use ofadditives in WMA, for example, chemical surfactants or organic waxes (such as Sasobit), allowtheir handling (mixing, spreading and compaction) at lower temperatures (Capitão et al., 2012),facilitating the incorporation of nanoclays (Abdullah, Hainin, et al., 2016; Abdullah, Zamhari,Hainin, Oluwasola, Hassan, et al., 2016; Abdullah, Zamhari, Hainin, Oluwasola, Yusoff, et al.,2016; Cheng et al., 2011).

Yao et al. (2013) found that asphalt binders which incorporate non-modified nanoclay andpolymer-modified nanoclay (PMN) have a lower deflection and a higher stiffness than samplesprepared with only asphalt binder, resulting in a better permanent deformation resistance.

Polacco et al. (2015) confirmed that organo-modified clays are typically more compatiblewith bitumen than with the polymers. This impacts the results obtained in different preparationmethods. In the physical mix method, only a small portion of the clay acts as a surfactant inimproving the bitumen/polymer contact, whereas in the master-batch method the polymer/claypremix allows a better interaction between the additives, resulting in a more durable binding.

14 F.C.G. Martinho and J.P.S. Farinha

Those authors also stated that one of the main drawbacks of bitumen/polymer/clay system isrelated with the binder viscosity gain attributable to the clay. Consequently, higher temperaturesmay be required during the handling of the asphalt mixtures, in contrast with the actual tendencyto privilege the use of WMA (Abdullah, Hainin, et al., 2016; Abdullah, Zamhari, Hainin, Oluwa-sola, Hassan, et al., 2016; Abdullah, Zamhari, Hainin, Oluwasola, Yusoff, et al., 2016; Capitãoet al., 2012; Cheng et al., 2011).

Abdullah, Zamhari, Hainin, Oluwasola, Hassan, et al. (2016) mixed a 80/100 penetration gradebitumen with a MMT clay surface modified with two different chemical compounds: dimethyldialkyl amine (35–45 wt.%) and octadecylamine (35–45 wt.%) plus aminopropyl-triethoxysilane(0.5–5.0 wt.%). They concluded that the nanoclay which includes the first compound increasessignificantly the viscosity, while the latter compound reduces the viscosity of the base bitumenand offers the same performance as a chemical WMA-modified asphalt binder.

A different method, plasma modification, was also used by Karahancer et al. (2014) to enhancethe performance of HMA. They modified the surface of limestone mineral filler by plasma pro-cessing, using three different components: hexamethyldisiloxane (HMDSO), methylmethacrylate(MMA) and silicon tetrachloride (SiCl4), concluding that plasma modification is a sustainabletechnique to get better asphalt mixtures.

Finally, Zapién-Castillo, Rivera-Armenta, Chávez-Cinco, Salazar-Cruz, & Mendoza-Martínez(2015) used mixtures of MMT nanoclay with a block SEBS copolymer (in the range from 3% to6% of SEBS/nanoclay modifier) to study the phase segregation problem, achieving a better fieldbehaviour of the asphalt mixture.

3. Design and characterisation of asphalt mixtures with nanoclays3.1. IntroductionSome asphalt mixtures design methods are empirical and based on volumetric properties. Mean-while, advanced systems consider not only these properties, but also the mechanical performanceof binders and mixtures. A wide characterisation of all materials (aggregates, filler, bitumen andadditives) used to produce asphalt mixtures (HMA or WMA) must be made and correlated withthe performance required for pavements where they will be applied.

In Table 4 we show a brief overview of tests performed by different authors for thecharacterisation of binders and modified binders.

In the next subsections are presented and analysed some considerations on the design andcharacterisation of modified binders and asphalt mixtures, including NMB.

3.2. Design and characterisation of modified bindersThe development of specifications for binders and mixtures tests goes back to the early decadesof the twentieth century and was based on indirect measurements using classical tests, such aspenetration or ring and ball tests. These tests are still used, but do not provide a direct quan-tification of the additive segregation in the samples tested (Polacco et al., 2015). For example,Munera and Ossa (2014) studied multicomponent (MC) bitumen blends and characterised themby penetration and softening point, but added rheological tests to evaluate the response of eachproperty in function of the amount of polymer used. However, Jamshidi et al. (2015) claimthat some traditional rheological properties, such as viscosity and the plastic component ofthe complex modulus (G* sinδ), should be used to study the consequences of nanomaterialsincorporation.

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Table 4. Characterisation of binders – some tests used.

Test ref. Test/procedure Usefulness Advantages Drawbacks Some references

PG Penetration grade Used to determine the penetrationgrade of binders at 25°C

Inexpensive test with a simplepreparation. Bitumengrade in terms of itshardness. Good measureof predicting ruttingresistance

The amount of bitumen usedmay not represent well therest of the tank. Resultsare very sensitive to thetest conditions and bitumenspecimen preparation.

Ezzat et al. (2016), Guernet al. (2010), Munera andOssa (2014), Li, Wu et al.(2017), Iskender (2016),Abdullah, Zamhari, Hainin,Oluwasola, Hassan, et al.(2016), Jahromi andKhodaii (2009)

SP Softening point Used to determine the softeningpoint of binders (ring andball apparatus) = temperatureat which the binder hasa penetration grade of≈ 800 dmm

Inexpensive test with a simplepreparation. It permits theclassification of bitumensand it gives an indicationof its tendency to flowat elevated temperatureswhen it is in service

The amount of binder usedmay not represent wellthe rest of the tank. Itis not suitable for PMB,in particular for highlymodified PMB.

Ezzat et al. (2016), Guernet al. (2010), Munera andOssa (2014), Li, Wu et al.(2017), Iskender (2016),Abdullah, Zamhari, Hainin,Oluwasola, Hassan, et al.(2016), Jahromi andKhodaii (2009)

RV Rotational viscosity Used to determine the viscosityof binders at severaltemperatures. Can measureviscosities of opaque, settlingor non-Newtonian fluids

Quick test with a simplepreparation which requiresa small amount of binder. Itis sufficient to represent theworkability of the binder

The small amount of binderused may not representwell the rest of the tank.Binders with a highconcentration of additivesmay have variations inresults.

You et al. (2011), Ezzatet al. (2016), Jamshidiet al. (2015), Li, Wu et al.(2017), Babagoli et al.(2017)

DSR Dynamic shearrheometer

Used to describe the linearvisco-elastic properties ofbitumen over a range offrequencies and temperatures.It gives the complex shearmodulus, IG*I, the phaseangle, δ, and its components

Simple preparation. Itallows to inferredsome parameters likedeformation [G*/sin(δ)]and fatigue [G* × sin(δ)]behaviours

The precision of this test hasnot yet been established

You et al. (2011), Ezzat et al.(2016), Jamshidi et al.(2015), Santagata, Baglieri,Tsantilis, and Chiappinelli(2015), Yao et al. (2013),Mollenhauer, Tušar, andEberl (2016), Li et al.(2017), Zare-Shahabadiet al. (2010), Batista et al.(2016)

(Continued).

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Table 4. Continued.

Test ref. Test/procedure Usefulness Advantages Drawbacks Some references

BBR Bending beamrheometer

Used to test asphalt binders atlow temperatures in order todetermine their susceptibilityto thermal cracking

Largely software controlled.Good reproducibility ingeneral

Available experience mainlyin USA and Canada. Someuncertainties in modifiedbitumen

Mollenhauer et al. (2016), Li,Wu et al. (2017), Weigeland Stephan (2017),Abdullah, Zamhari, Hainin,Oluwasola, Hassan, et al.(2016), Pei et al. (2015),Zare-Shahabadi et al.(2010)

RTFOT Rolling thin-filmoven tests

Permits the artificially short-termageing of binders and thedetermination of its resistanceto hardening under theinfluence of air and heat

Can simulate the bindersshort-term ageing whichoccurs during mixingand lay-down of asphaltmixtures

Modified binders may havedifferent ageing patterns, soRTFO may not representwell the plant and fieldageing

You et al. (2011), Ezzat et al.(2016), Abdullah, Hainin,et al. (2016), Vargaset al. (2017), Aziz et al.(2015), Santagata, Baglieri,Tsantilis, Chiappinelli, andAimonetto (2015)

PAV Pressure ageingvessel

Binders are exposed to heatand pressure to simulate anartificially long-term ageingover a 7- to 10-year period.Simulate binder ageing duringthe asphalt mixture service life

The use of high pressureincreases the diffusion ofoxygen into the bindersample, which limits theloss of volatiles compoundswhile ageing the sample

Modified binders may havedifferent ageing patterns.Consequently, PAV maynot represent the field long-term ageing. Discussionabout recommendedtemperatures is still open

Guern et al. (2010), Abdullah,Hainin, et al. (2016), Zhao,Huang, Ye, Shu, and Jia(2014), Aziz et al. (2015)

IatroscanTLC-FID

Iatroscanchromatography

Used to determine the bitumenchemical fractions, groupedinto four classes (SARAfractions)

Offers good accuracy andprecision, in addition tolow solvent consumptionand quick analysis. Organicsubstances, which showno UV-absorption andno fluorescence, can beanalysed

Reproducibility needs tobe improved. Variableresponse factors for thepolar fractions

Guern et al. (2010), Ortegaet al. (2015), Paliukaiteet al. (2014), Ortega,Navarro, García-Morales,and McNally (2017),Polacco et al. (2015)

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FTIR Fourier-transforminfraredspectroscopy

The chemical bonding andmolecular-level interactionsof binders can be analysedthrough the band positions.Extensively used in manyindustries

Provides the IR absorbancespectrum of a solid,liquid or gas. It permitsquantitative and qualitativeanalysis of samples. Itallows studying ageingeffects through monitoringvariations in the spectra

Reduced sensitivity due tofluorescence of certainelements. It has a highinitial cost which permitsonly a spot test

Guern et al. (2010), Ortegaet al. (2015), Zhang et al.(2012), Yao et al. (2013),Abdullah, Hainin, et al.(2016), Polacco et al.(2015), Karahancer et al.(2014), Li, Wu et al.(2017), Babagoli et al.(2017)

DSC Differentialscanningcalorimetry

Used to observe glass transition,fusion and crystallisationevents and temperatures. Canalso be used to study oxidationand other chemical reactions.It is possible to make kineticstudies

Versatile tool which canbe operated over a widerange of pressures andtemperatures. Providesinformation on theenergetic changesassociated with chemicalreactions

Scanning rate and delaytimes. Low accuracyin results. Difficultto interpret. Requirescalibration in temperaturerange

Guern et al. (2010), Liu,Shaopeng, van de Ven,Molenaar, and Besamusca(2010), Abdullah, Hainin,et al. (2016), Polacco et al.(2015), Sureshkumar et al.(2010).

HS-SEC Size-exclusionchromatographyunder high-speedconditions

Used to separate and quantifydifferent materials in solutionbased on their molecular size

Can be applied to largemolecules or macromolec-ular complexes. Goodseparation of large fromsmall molecules with a lowvolume of eluate

In some conditions, the bandsof eluting molecules maybe broadened or onlyapproximate measurementscan be obtained

Guern et al. (2010), Weigeland Stephan (2017)

XRD X-ray diffraction Used to examine the morpho-logical structures (crystallinecompounds) and chemi-cal reactions of binders.Quantitative analysis

Powerful and rapid test. Min-imal sample preparation.Can be used to determinethe degree of exfoliationof a nanomaterial. Mea-surements with variabletemperature/pressure

Requires samples with a highdegree of homogeneity.Some uncertainties inthe results interpretation.Inadequate for amorphousmaterials. Light elementsare harder to detect

Ortega et al. (2015), Bonatiet al. (2013), Walterset al. (2014), Zhang et al.(2012), Liu et al. (2010),Golestani et al. (2012),Vargas et al. (2017),Melo and Trichês (2017),Polacco et al. (2015),Li, Wu et al. (2017),Iskender (2016), Pei et al.(2015), Sureshkumar et al.(2010), Zare-Shahabadiet al. (2010), Jahromi andKhodaii (2009)

(Continued).

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Table 4. Continued.

Test ref. Test/procedure Usefulness Advantages Drawbacks Some references

AFM Atomic forcemicroscopy

Used to measure the attractiveor repulsive force betweena sharp AFM tip and thesample surface. Used to testthe binder-aggregate adhesion

No previous preparationof samples. Very highresolution. Imaging almostall types of surfaces. Cananalyse the dispersiondegree of nano-modifiedbitumen

Slow scan time. Single scanimage size

Ortega et al. (2015), Zhanget al. (2012), Polacco et al.(2015), Li, Wu et al. (2017)

SEM Scanning electronmicroscopy

Offers imaging characterisation,crystal structure informationand chemical composition.Used to observe themorphologies of neat bitumen,nanoclays and modifiedbinders

High resolution. Easy tooperate. Permits 3Dimages. Samples with anythickness. Wide range ofmagnifications

Big size and expensiveequipment. Requiresvacuum and pre-preparation of smallsamples (conductive andsolid). Lower resolutionthan TEM. It is not ideal toanalyse bitumen samples

Polacco et al. (2015),Karahancer et al. (2014),Li, Wu et al. (2017),Babagoli et al. (2017)

TEM Transmissionelectronmicroscopy

Can be used to characterise thenanoscale structure behavioursof nano-modified binders andtheir fracture morphologies

Very high resolution, betterthan SEM. Offer thepossibility to investigatedifferent materials.Quantitative identificationof structural defects

Big size and expensiveequipment. Requirespre-preparation of smallsamples (conductiveand solid). Destructivetechnique. It is not ideal toanalyse bitumen samples

Vargas et al. (2017), Paul andRobeson (2008), Muneraand Ossa (2014), Pei et al.(2015)

Road Materials and Pavement Design 19

You et al. (2011) used rotational viscosity (RV), dynamic shear rheometry (DSR), rollingthin-film oven tests (RTFOT) and a direct tensile test (DTT) to show that two different nanoclaysenhanced the binder complex modulus (G*) and its viscosity. Ezzat et al. (2016) also used an RVand a DSR to demonstrate that the addition of 3 wt.% of nanoclay can enhance the performanceof the asphalt binder. However, when bitumen is blended with polymers, the DSR results mayhave poor correlation with the permanent deformation resistance (Batista et al., 2016).

Rheological measurements (flow curves and temperature sweeps in oscillatory shear), X-raydiffraction (XRD), atomic force microscopy (AFM) and FTIR were used by Ortega et al. (2015)to analyse the individual and joint effects of two modifiers (an OMMT, Cloisite 20A®, and a poly-meric MDI, methylene diphenyl diisocyanate). Bonati et al. (2013) also used XRD techniques tomeasure the distance between layers and this information gives indications for the calculation ofthe extent and level of exfoliation of the nanoclay in the binder. In addition, Walters et al. (2014)and Vargas et al. (2017) used XRD measurements of nanoclay-modified samples to show alsothe intercalation and exfoliation of nanoclays in bitumens. The latter researchers also used TEMto evaluate the extent of intercalation and dispersion of MMT in the binder.

Regarding the specimen preparation for tests, Zhang et al. (2012) claimed that MB is a deter-minant in achieving a good division of the nanoclay in MMT/SBS-modified bitumen. Theycharacterised the microstructures of this blending by XRD, FTIR and AFM. Those researchersalso investigated the effect of MMT on ultraviolet (UV) ageing of the PMB with SBS, and usedAFM to confirm that a network configuration was formed in this blend, having obtained anintercalated structure.

Galooyak et al. (2010) prepared SBS and OMMT/SBS-modified bitumen by a melt intercala-tion process and concluded, through XRD analysis, that an exfoliated structure was obtained.

Zhang et al. (2012) concluded that, in the case PMB with SBS, the viscosity ageing index(VAI) and the softening point (R&B) increment decrease when a sodium montmorillonite (Na+-MMT) is added and can be further reduced if mixed with another montmorillonite (OMMT). TheVAI is calculated in function of the viscosity values before and after UV ageing, obtained underthe European standard EN 13302. R&B represents the temperature at which material under stan-dardised test conditions attains a specific consistency, determined under the European standardEN 1427.

Guern et al. (2010) tried to understand, through Iatroscan chromatography, how the ageing ofthe chemical species is processed for different bitumens, confirming that the observed increasein resin or asphaltenes content is due to the aromatics or polar resin association, respectively.

Sultana and Bhasin (2014) investigated the effect of chemical composition on TS and rheologyof asphalt binders, testing two different binders that were separated in the four SARA (saturated,asphaltenes, resins and aromatics) fractions, based on their polarity. The results evidenced anincrease not only in the TS but also in the stiffness of the binders. Jamshidi et al. (2015) useda Brookfield RV, at elevated temperatures (when the modified asphalt is in the viscous phase),to prove that the activation energy of the binders were lower than when using a DSR at anintermediate temperatures (when the binders are in a visco-elastic phase). They claimed thatthe energy required by the modified asphalt binders (MAB) depends on the type and contentof nanomaterials used, in addition to the physical phase of the sample (which is a function ofthe temperature). Santagata, Baglieri, Tsantilis, and Chiappinelli (2015) used a DSR to analysethe base bitumen and the additive used, having concluded that the MMT nanoclays can affectthe fatigue strength and the recovery of bituminous binders. Yao et al. (2013) used FTIR spec-troscopy to show the influence of nano- or micro-materials in asphalt binders, and explained thatwhen they are exposed to sunlight and heat, its oxidation can be decreased.

Jasso et al. (2012) studied the impact of the addition of MMT and OMMT on the high-temperature properties of conventional asphalt modified by linear SBS block copolymer. They

20 F.C.G. Martinho and J.P.S. Farinha

were able to prove the linear visco-elastic (LVE) properties of PMBs through the use ofoscillatory shear flows.

Liu et al. (2010) used the X-ray photoelectron spectroscopy to characterise the composition ofthe organic surfactant cations in the nanoclay in order to evaluate two bitumen nanoclay modi-fiers (both organic surfactant-modified MMT). They also used differential scanning calorimetryand thermogravimetric analysis (DSC-TG) to obtain the thermal stability of the OMMTs andconfirmed that the surfactants in the two nanoclays have different thermal behaviour at temper-atures below 200°C, which means that no problems are expected in the use of this modifiedbitumen (used at temperatures below 180°C).

Golestani et al. (2012) used XRD to show that the linear SBS-nanocomposite-modifiedbitumens may form an exfoliated configuration. However, modified bitumen with branch SBS-nanocomposite (BSN) may form a different structure (intercalated). The authors also showedthat nanoclays can improve the rheological behaviours of those binders as well as their physicalproperties and the storage stability. The same conclusion was drawn by Farias et al. (2016) withthe use of Cloisite.

Ashish et al. (2016) used a linear amplitude sweep (LAS) test to confirm that an increase in theCloisite 30B® content originates a growth in the number of the load cycle to failure. They werealso able to prove that the LVE limit of NMB follows the tendency for growing binder stiffnessas the addition of nanoclay increases.

Abdullah, Hainin, et al. (2016) studied short- and long-term artificial ageing of an NMB.They characterised the morphological and chemical changes using field-emission scanningelectron microscopy (FE-SEM) and FTIR, and conducted rheological evaluations using DSC.Among other conclusions, they reported that modified binders give an important contribution foradhesion between aggregates, which induces a better moisture resistance.

Yazdani and Pourjafar (2012) applied an orthogonal array (a fractional factorial design whichminimises the number of test runs when used to estimate how multiple process variables influ-ence, simultaneously, the performance) of the Taguchi method to optimise a modified bitumencomposition. They prepared binder samples with 4.5% of bitumen 60/70 content and tested threecontrol factors (a polypropylene plastomer, an SBS elastomer and an MMT nanoclay) and fourlevels of concentration contents (from 1% up to 7%). After some software calculations, theyfound that SBS with 3%, PP with 5% and nanoclay with 1.5% were the optimised concentrations,having noted that a small amount of nanoclay caused enormous changes in results.

Many other tests and methods can be used to study the binder composition and its properties.For example, Guern et al. (2010) used n-heptane precipitation, Iatroscan chromatography (whichcombines thin-layer liquid chromatography on silica gel, TLC, with flame ionisation detection),Fourier-transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC), toobtain information on bitumen chemistry (related with oxidisation of aromatics or resins, ageing,chemical organisation, etc.). They also used size-exclusion chromatography under high-speedconditions (HS-SEC), which yields information relative to asphaltene associations, allowing thesimulation of several years of road ageing through 25 h at 100°C and 2.1 MPa, in a pressureageing vessel (PAV) test (respecting, in Europe, the Standard EN 14769).

Regarding the study of low-temperature attributes of binders, it is also recommended thatRTFOT (short-term ageing) and PAV (long-term ageing) procedures are applied before testingthe blends (Mollenhauer et al., 2016).

Usually, when a polymer is also added to the blend, a solution intercalation method is usedfirst. The clay is premixed with the melted polymer in order to get an intercalation of polymer(between the clay layers) (Farias et al., 2016). Then, this polymer/nanoclay composite is blendedwith bitumen and, finally, this NMB is ready to be mixed with the aggregates and filler to produceasphalt mixtures.

Road Materials and Pavement Design 21

Other procedures have been developed to improve the final quality of the blends, namely insitu polymerisation, solvent blending and dry blending methods. Meanwhile, more investigationsare still needed mainly on dispersion conditions and their evaluation (Li, Wu et al., 2017).

Regarding common mixing conditions, it seems that there are no consensus opinions sinceup to now different methods, temperatures, speed and durations were used to produce nanoclay-modified binders and asphalt mixtures. For example, in the cases mentioned in Table 3 authorsused sample preparations with temperatures ranging from 130°C to 180°C, mixing speeds from1550 to 5000 rpm (omitting one outlier) and durations ranging from 15 to 180 min.

3.3. Design and characterisation of asphalt mixturesRecent design methods to prepare HMA or WMA mixtures contemplate different stages inits development: selection of materials (granulometric curve of the aggregates mixture, binderand mixture types and additives); selection of binder content and calculation of the volumet-ric properties of the mixture; evaluation of parameters such as coating compactability, watersensitivity and workability; and mechanical performance of the final mixture (Capitão et al.,2012).

The use of nanoclays may create supplementary design problems, introducing some difficultiesto disperse them homogeneously in the bitumen. Therefore, the design of such asphalt mixturesshould take into account the characteristics of all materials and the industrial process followed inthe bitumen plant (bitumen supplier) or in the asphalt mixing plant (under the responsibility of thecontractor). Recommendations regarding mix design and asphalt properties can be extracted fromthe available literature, covered in the following sections. The huge differences and uncertaintyin the literature, regarding both the used materials and the test procedures, currently preclude anyconclusion on the main trends of asphalt mixtures containing nanoclays.

The performance of plasma-modified mineral fillers in HMA might also be estimated by deter-mining the Marshall stability (MS) and the ITS (Karahancer et al., 2014). MS is the maximumresistance to deformation of a moulded asphalt specimen, determined in Europe by the stan-dard EN 12697-34. ITS test yields the maximum tensile stress applied on a cylindrical specimenloaded diametrically until it breaks at the specified test temperature and speed of displacementof the compression testing machine (in Europe, using the standard EN 12697-23).

Ziari et al. (2015) evaluated the fatigue and rutting resistance of HMA mixtures prepared byBT-modified bitumen. They modified bitumen with 10–30 wt.% of BT and the results showedthat its physical properties were improved. The same conclusion was taken by Crucho, Neves,Capitão, Picado, and Garcia (2016) when they used only 4 wt.% of BT.

Other results showed an increase in shear strength and rutting resistance and the fatigue lifewas longer than in conventional HMAs. Walters et al. (2014) studied a naturally occurring nan-oclay (an OMMT, Cloisite 30B®), blended at 2 and 4 wt.%, with and without a bio-binder (5 wt.%of dry mass). The bio-binder was obtained from swine manure through the use of a thermochem-ical conversion process. This process was then followed by a filtration to produce the bio-char.The bio-char was ground to nanoscale and mixed with a virgin asphalt binder at 2, 5 and 10 wt.%.Ashish et al. (2016) also used Cloisite 30B® in percentages up to 6 wt.% to show that the fatiguelife of the mixtures was improved, but only for strain levels that do not exceed 1%. Another strat-egy was followed by Ortega et al. (2015) by mixing pure bitumen with another OMMT (Cloisite20A®) and a polymeric MDI.

Iskender (2016) evaluated the mechanical properties of asphalt mixtures with an NMB usinga modified Lottman test (MLT) and repeated creep tests (RCT) with Nottingham asphalt tester(NAT). He concluded that a 2% nanoclay content optimises the resistance to rutting, strippingand cracking.

22 F.C.G. Martinho and J.P.S. Farinha

To investigate the effects of nano-modification of the asphalt binder (with OMMT) and theinfluence of the mineral filler composition and the aggregate gradation on the fire-resistancebehaviour of different asphalt mixtures, Bonati et al. (2013) used a cone calorimeter test (CCT)with a heat radiation of 70 kW/m2. They concluded that the mineral filler composition has astrong influence in the fire reaction and the alkaline filler amount can be reduced if it is coupledwith OMMT.

4. Mechanical performance evaluation of binders and asphalt mixtures with nanoclays4.1. Binder properties4.1.1. IntroductionThe literature shows that nanoclays can improve the behaviour of binders used in asphalt mix-tures. In fact, recent reports in the literature confirm that nanoclays can be successfully used asmodifiers to change different physical and mechanical properties of asphalt binders, improvingthe elastic and visco-elastic behaviour of PMBs (Abdullah, Hainin, et al., 2016; Ameri et al.,2016; El-Shafie et al., 2012; Farias et al., 2016; Golestani et al., 2015; Iskender, 2016; Silvestreet al., 2015; Walters et al., 2014; You et al., 2011).

In the following subsections the overall performance trends of bitumens blended with differenttypes of nanoclays are discussed.

4.1.2. Binder stiffnessIn the case of nanoclays, their addition to bitumens can increase the binder stiffness, G*. Forsome authors, this improvement is due to an increase in the concentration of polar fractions inthe binder (Sultana & Bhasin, 2014). This seems to depend on the polarity of the polar fractionand on the frequency of loading (or load time). Some processes (e.g. oxidation or air blowing)may have more impact on the stiffness at high temperature/low rate of loading than at low temper-atures/higher rates of loading. Similarly, other processes such as the addition of softer materialsor rejuvenators (which restore ductility and adhesion properties of the binders) may result in adecrease in stiffness along the frequency spectrum.

From DSR results, Yao et al. (2013) concluded that the addition of four different modifiers(nanomer I.44P, carbon microfiber, non-modified nanoclay and PMN) can all increase the com-plex shear modulus of a MAB, especially at high and intermediate temperatures. However, atlow temperatures, the recovery ability of these MAB may be reduced (due to its higher stiffness).Golestani et al. (2015) also found enhanced stiffness modulus in modified bitumens, especiallyat higher temperatures.

Other authors confirmed that nanoclay modifications can not only help increase G*, but alsodecrease phase angle (δ), when compared to unmodified bitumen (Abdullah, Hainin, et al., 2016;Abdullah, Zamhari, Hainin, Oluwasola, Hassan, et al., 2016; Farias et al., 2016; Golestani et al.,2015; Jahromi & Khodaii, 2009; Melo & Trichês, 2017; Pei et al., 2015; Vargas et al., 2017; Youet al., 2011; Zapién-Castillo et al., 2015; Zare-Shahabadi et al., 2010). This means that a lowerplastic component is present in G*. In this respect, Jahromi and Khodaii (2009) confirmed, foran NMB, that G* increases by decreasing temperature and/or increasing frequency of loading,whereas δ increases as temperature increases and/or frequency of loading decreases.

4.1.3. Resistance to fatigue crackingSome authors argue that it is always necessary to check the fatigue performance, because thetotal complex shear modulus, G* sinδ [fatigue cracking index used by Superpave™ specification(Zhao et al., 2014)], resulting from DSR tests, may or may not be suitable for other materials

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than bitumen (Aziz et al., 2015). Superpave™ is an American design method for pavements toreplace the Marshall, Hveem or Durier design methods. Moreover, the addition of OMMT canchange the fatigue properties of bitumen, due to the type of surfactant used, which determineshow the interfacial interaction between bitumen and MMT occurs (Liu et al., 2011).

Santagata, Baglieri, Tsantilis, and Chiappinelli (2015) also concluded that nano-sized addi-tives, such as MMT nanoclays, can influence the fatigue and recovery behaviour of binders. Theyverified that the fatigue response of base bitumens was always improved by this kind of modi-fication. However, the efficiency of the nanoparticles depends greatly on the physico-chemicalproperties of the base components, which greatly influence the morphological configuration thatthe additives take then on the bituminous blend.

4.1.4. Resistance to extreme temperaturesThe susceptibility to high and low temperatures and/or the low-temperature cracking resistanceof NMB have been studied by different authors (Bonati et al., 2013; Ezzat et al., 2016; Munera& Ossa, 2014; Ortega et al., 2015; Pei et al., 2015; You et al., 2011; Zare-Shahabadi et al.,2010; Zare-Shahabadi et al., 2015). In all cases, the nanoclays were found to enhance the asphaltbinders resistance to extreme temperatures, at least, in hot climatic conditions.

Jahromi and Khodaii (2009) confirmed that the temperature susceptibility (defined as the rateat which the consistency of binders changes with a change in temperature, usually characterisedby the penetration index, PI, calculated as defined in Europe by the standard EN 12591) is lowerfor NMB than for unmodified binders. However, Zapién-Castillo et al. (2015) warn that evenwhen the PI points to reducing the bright and rigid appearance of bitumen at low temperatures(which means a lower sensitivity to low temperatures), the results are inconclusive and, therefore,further studies should be carried out.

You et al. (2011) tested two different surfactant-modified nanoclays (intercalated MMTnanoclays), blended in bitumen (at 2 and 4 wt.% nanoclay by weight of bitumen) at high tem-perature. Through direct tension tests they concluded that a 2 wt.% modified bitumen had betterlow-temperature cracking resistance.

Nanoclays can also be used in polymer-based flame retardants (FR) for improving asphalt fireresistance. Bonati et al. (2013) showed that, due to a migration mechanism (which leads theclay platelets to the top of the burning sample, when the FR fillers are mixed with OMMT), theperformance of the protective layer of coal improves, resulting in a clear decrease of the smokeand heat released.

Ortega et al. (2015, 2017) studied the impact on rheological properties of a combination ofMDI (a blend of methylene diphenyl diisocyanate and its higher homologues) with nanoclays.They addressed the improvement in thermoreological behaviour including thermal stability ofthe different bituminous products, particularly those used for roofing and road applications. Theseauthors claimed that the addition of 2 wt.% MDI and subsequent curing at 150°C, enhanced thecomposite properties, due to the chemical interactions between the MDI and the bitumen/claycomposite.

Crucho et al. (2016) studied the influence of several nanomaterials, one being a nanoclay (BT).They found that an NMB (which included 4 wt.% BT) increased penetration (16%) and viscosity(6%), but the softening point remains without changes. These facts confirm the tendency forhigher resistance at high temperatures.

4.1.5. Ageing resistanceUsually, the short-term ageing process of the asphalt binders is simulated using RTFOT (respect-ing the European standard EN 12607-1) and the long-term ageing process is replicated with PAV

24 F.C.G. Martinho and J.P.S. Farinha

(European standard EN 14769) (Hill & Jennings, 2011; Yao et al., 2013; Zhao et al., 2014).However, Aziz et al. (2015) have pointed out that some alternative binders (for example, madewith recycled products, such as waste cooking oil, plastic and tire rubber) may have significantlydifferent ageing characteristics, suggesting that RTFO and PAV tests may not represent well themixing plant and field ageing.

Walters et al. (2014) reported that the addition of nanoparticles to bio-MAB was very effectiveto improve its ageing resistance. Guern et al. (2010) studied the impact of ageing on chemicalspecies (saturate, cyclic or aromatic) and changes in the chemical organisation, depending on thetype of bitumen. They concluded that the agglomeration during its ageing can be influenced bythe lack of crystallised fractions.

Liu et al. (2011) showed that mastics with OMMT revealed better ageing resistance than con-ventional base bitumens due to its barrier properties and Li, Wu et al. (2017) proved that thiseffect was also due to its inhibition to VOCs emission. Furthermore, Galooyak et al. (2010)observed that layered silicates can prevent oxygen diffusion into PMB, and thus the OMMT canimprove the ageing resistance of PMB. Finally, Zhang et al. (2012) proved that the addition ofMMT clay can successfully prevent changes in the micro-morphology of binders and Li, Xiao,Amirkhanian, You, and Huang (2017) also summarised in its review that the oxidation process(which leads to ageing) can be retarded by nanoclays.

4.2. Asphalt mixture properties4.2.1. IntroductionUsually, the performance of asphalt mixtures is studied by taking into account their mainmechanical properties (namely, fatigue life, stiffness, resistance to permanent deformation, low-temperature cracking and ageing). The incorporation of nanomaterials has a strong influenceon the structures of the blends and, consequently, on its rheological behaviour, even leadingto WMA showing properties comparable to HMA (Abdullah, Hainin, et al., 2016; Abdul-lah, Zamhari, Hainin, Oluwasola, Hassan, et al., 2016; Abdullah, Zamhari, Hainin, Oluwasola,Yusoff, et al., 2016; Jamshidi et al., 2015).

The mechanical performance of all types of asphalt mixtures, including those containingnanoclays, varies with the type of mixture as well as its composition. Fracture and strengthcharacteristics, which are critical to pavement performance, are determined by aggregate min-eralogical composition (Cui, Blackman, Kinloch, & Taylor, 2014; Zhang, Apeagyei, Airey, &Grenfell, 2015), gradation (Aziz et al., 2015), bitumen content (Capitão et al., 2012) and adhesionbinder/aggregates (Cui et al., 2014).

Since the specific conditions used in tests (such as load characteristics and temperatures)can significantly influence their results, in the following paragraphs are discussed the overallperformance trends of mixtures produced with different types of nanoclays.

4.2.2. MS, TS, water sensitivity and binder/aggregate interactionThe Marshall stability MS, obtained in Marshall tests, was found to be higher in asphalt mixturesprepared with NMB than in control mixtures, produced with bitumen or PMB (Babagoli et al.,2017; Karahancer et al., 2014; Li, Wu et al., 2017; Taherkhani, 2016). However, Ziari et al.(2015) confirmed this tendency only while the percentage of BT was lower than 20% (contentby weight of bitumen), a value from which the MS started to decrease. In these tests, the flow ofthe mixtures did not follow any specific tendency.

Goh et al. (2011) studied the effects on the moisture sensitivity of asphalt mixtures producedwith nanoclays and carbon micro-fibers, in terms of TS ratio, calculated as the ratio between

Road Materials and Pavement Design 25

the TS (indirect, by diametral compression) of wet specimens (conditioned in water) and that ofdry specimens. The results indicated that, for all mixtures which included nanoclay and carbonmicro-fibers, the TS ratio values are higher than those recommended (typically defined with aminimum value of 0.9). In addition, it seems that the incorporation of 1.5% of nanoclay intomixtures increased the TS. Consequently, in regions with severe winters, these mixtures will beable to better withstand the effects of materials applied to promote the thaw. An increase in thepolar fractions of the binders also leads to an increase in its TS (Sultana & Bhasin, 2014).

Regarding water sensitivity, or moisture susceptibility, it is well accepted that the aggregatemineralogical composition plays an important rule on aggregate–bitumen bonds. For example,Zhang et al. (2015) tested, through three different methods, four aggregate types (two granitesand two limestones) and two types of bitumen (40/60 and 70/100 pen), concluding that the aggre-gates effect was greater than the bitumen effect. In addition, Ashish et al. (2016) observed that asandstone aggregate provided the best interaction with bitumen and it was followed by granite,limestone and finally basalt aggregates.

Other authors also have demonstrated that polymer- and nanocomposite-modified asphaltsare more resistant to moisture damage than conventional asphalt mixtures mainly due to thereduction of air voids (Ameri et al., 2016; Ashish et al., 2016; Golestani et al., 2015; Iskender,2016; Karahancer et al., 2014; Taherkhani, 2016). Li, Xiao et al. (2017) and Melo and Trichês(2017) also explained that this higher moisture resistance is due to a lower surface tension and abetter chemical affinity binder-aggregate. However, Ashish et al. (2016) noted that the additionof nanoclay increases the adhesion aggregate-binder mainly in dry conditions.

4.2.3. Asphalt mixture stiffnessIn general, the results indicate that the binder’s visco-elastic properties of low shear viscosity(LSV), complex modulus and phase angle, have no impact on the air voids content in the asphaltmixture, but their influence in the value of the resilient modulus of elasticity at 20°C is significant(for asphalt mixtures compacted at 125°C) (Nicholls, Valentin, Benešová, & Eberl, 2016).

The addition of NMB to asphalt mixtures can increase the HMA (or WMA) complex stiffnessmodulus (E*), determined in Europe by the standard EN 12697-26.

Several authors confirmed this fact and also a reduction in the phase angle (δ), which meansa lower plastic component in E*, when compared to asphalt mixtures with an unmodified binder(Abdullah, Hainin, et al., 2016; Abdullah, Zamhari, Hainin, Oluwasola, Hassan, et al., 2016;Farias et al., 2016; Golestani et al., 2015; Melo & Trichês, 2017).

However, Abdullah, Zamhari, Hainin, Oluwasola, Yusoff, et al. (2016) proved that, for WMAwith NMB, the stiffness at 25°C had lower values when compared with HMA (in the sameconditions).

4.2.4. Resistance to fatigue crackingMost studies report an improvement in the resistance to fatigue of asphalt mixtures modified withnanoclays, which leads to a higher fatigue life (determined, in Europe, by the standard EN 12697-24) and longer durability than conventional HMA or WMA. This was confirmed by Ziari et al.(2015) when they tested a BT-modified bitumen. These authors proved that mixtures containing10% and 15% BT have longer fatigue life than the control mixture; however, they also statedthat the modification of the base bitumen with higher percentages of BT did not enhance thefatigue life of the mixtures. Meanwhile, Melo and Trichês (2017) tested a conventional binderwith 3 wt.% (by weight of binder) of an organophilic nanoclay and concluded that the asphaltmixture produced with this blend presented greater resistance to fatigue (@ 15°C and10 Hz).

26 F.C.G. Martinho and J.P.S. Farinha

Ashish et al. (2016) concluded that the use of Cloisite 30B®, in percentages up to 6 wt.% andfor strain levels that do not exceed 1%, enhances the mixture fatigue life. This may be related tocluster formation in the asphalt blends, which reduce the propensity for cracking.

Through indirect tensile fatigue tests, Abdullah, Zamhari, Hainin, Oluwasola, Yusoff, et al.(2016) also verified that, for WMA and HMA mixtures, the fatigue life was similar, but that forWMA with nanoclay (compacted at 135°C) it increases considerably.

4.2.5. Resistance to permanent deformation (rutting)The resistance to permanent deformation (determined, in Europe, under the standard EN12697-22) is one of the most important parameters to achieve stable and long-lasting pave-ments, especially when used in the base or binder course layers. Based on rheological results,Golestani et al. (2012) concluded that the highest rutting resistance in their asphalt mixtureswas obtained when an SBS polymer was combined with OMMT in the proportion of 100/25(SBS/OMMT).

In general, bitumen modification with nanoclays improves rutting resistance, especially atelevated temperatures (Abdullah, Hainin, et al., 2016; Abdullah, Zamhari, Hainin, Oluwasola,Hassan, et al., 2016; Ameri et al., 2016; Ameri et al., 2017; Ezzat et al., 2016; Farias et al.,2016; Golestani et al., 2015; Melo & Trichês, 2017; Vargas et al., 2017; Yao et al., 2013). Ziariet al. (2015) also stated that the addition of BT improves not only the physical properties ofbitumen, but can also increase the rutting resistance and the shear strength of HMA. They useda dynamic creep test (DCT) to prove that the addition of BT could considerably reduce thepermanent deformation which leads to a better rutting resistance of asphalt mixtures at hightemperatures. The wheel-tracking tests (WTT) showed that mixtures containing BT gave a betterrutting resistance (thus reducing its permanent deformation). Ameri et al. (2016) reported thesame conclusion when they tested PMB (where the polymer was an SBS) with and withoutnanoclay.

Jamshidi et al. (2015) suggested characterising the influence of nanomaterials on asphaltbinders using a non-dimensional Superpave™ rutting factor gradient analysis (∇NSRP) (Hill &Jennings, 2011; Zhao et al., 2014), to compare the resistance to permanent deformation of bitumi-nous mixtures with and without modified binder. In this respect, they also found that the ∇NSRPgradient is influenced by the type and content of the nanomaterial and by the temperatures usedin tests.

Iskender (2016) also concluded that binders with (2–5 wt.%) nanoclays have a higher rut-ting resistance (in average 50% larger than the control mixtures), using an MLT water damageconditioning method, which includes hot water conditioning (respecting the AASHTO T283standard).

4.2.6. Low-temperature crackingThe low-temperature behaviour of asphalt mixtures is evaluated through the low-temperaturecracking resistance. This important parameter can be determined by different test procedures,being the tensile stress restrained specimen test (TSRST), as defined in EN 12697-46, the onewhich has the largest number of results available (Mollenhauer et al., 2016).

The addition of NMB significantly improved the low-temperature cracking resistance ofasphalt mixtures (Li, Wu et al., 2017; Mollenhauer et al., 2016). A similar conclusion was alsoreached by other authors, for example, Iskender (2016) and Galooyak et al. (2010). However, itwas reported by Abdullah, Hainin, et al. (2016) that the usual increase in stiffness of NMBs maylead to a reduction in resistance to low-temperature cracking.

Road Materials and Pavement Design 27

5. ConclusionsNanoclays have a high potential for application in some types of asphalt mixtures. These addi-tives can improve the binder stiffness and, consequently, the asphalt mix stiffness, fatigue life,permanent deformation resistance of asphalt mixtures and ageing resistance of binders at anacceptable cost. OMMT, in particular, is a promising alternative to improve the lifetime of asphaltpavements. However, before its widespread use some issues still need further investigation,namely the efficient dispersion of the nanoparticles.

Since different sources of binders can be used in the same type of asphalt mixture, the resultsobtained with its modification can have many variations. For this reason, it is important to assess,for example, the individual properties of each bitumen fraction, the differences in those proper-ties for each crude source and the performance of the final blends, at different rates of loadingand temperatures. It is also desirable that the introduction of nanoclays in binders confirms thatpossible storage stability problems will be minimised and its use in asphalt mixtures consider itslong-term performance and recyclability in future.

Regarding the design of new mixtures, further work is needed to find the best procedures toevaluate the uniformity of nanoparticles dispersion. The validation of results by studying a widerarray of base materials and their most adequate combinations must be also developed, as well asthe influence of the mixing methods used and its correlation with temperature, speed and durationadopted. In this context, from a statistical point of view, the most frequent values mentioned inthe reviewed literature were the following: 3 wt.% of bitumen for nanoclay content, 160°C forthe temperature used, 4000 rpm for the mixing speed chosen and 120 min for the mixing processduration.

An important and actual driving factor for the use of bitumens modified with nanoclays isrelated to the contribution that this additive can give to sustainability and reduction of globalgreenhouse gas emissions (due to performance improvement and reduction of bitumen and fuelconsumption), as recommended in the Paris Agreement (December 2015).

At this point it is premature to conclude the life-cycle cost and the cost–benefit analysisof NMB on an industrial scale since no data on their long-term performance are yet avail-able. Nanoclay-modified binders have a promising future in the pavement industry, especiallyif these materials are combined with WMA approaches, to take the best advantages of bothtechnologies.

ORCIDFernando Martinho http://orcid.org/0000-0002-6645-5062José Paulo S. Farinha http://orcid.org/0000-0002-6394-5031

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