Investigating the effects of using nanomaterials on

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Investigating the effects of using nanomaterials on moisture damage of HMA a

a

Gholam Hossein Hamedi , Fereidoon Moghadas Nejad & Khosro a

Oveisi a

Department of Civil & Environmental Engineering, Amirkabir University of Technology, Tehran, Iran Published online: 18 Mar 2015.

To cite this article: Gholam Hossein Hamedi, Fereidoon Moghadas Nejad & Khosro Oveisi (2015): Investigating the effects of using nanomaterials on moisture damage of HMA, Road Materials and Pavement Design, DOI: 10.1080/14680629.2015.1020850 To link to this article: http://dx.doi.org/10.1080/14680629.2015.1020850

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Road Materials and Pavement Design, 2015 http://dx.doi.org/10.1080/14680629.2015.1020850

Investigating the effects of using nanomaterials on moisture damage of HMA Gholam Hossein Hamedi, Fereidoon Moghadas Nejad ∗ and Khosro Oveisi Department of Civil & Environmental Engineering, Amirkabir University of Technology, Tehran, Iran

Downloaded by [Dokuz Eylul University ] at 23:44 18 March 2015

(Received 3 November 2014; accepted 15 February 2015 )

In the past few years, several studies have been conducted focusing on the use of nanomaterials as additives in asphalt mixtures. Nevertheless, the effects of these additives on the moisture susceptibility of asphalt mixtures have not been studied. In this study, the effects of using nano-CaCO3 as an antistrip additive on moisture susceptibility of asphalt mixtures have been assessed using the surface free energy (SFE) method and modified Lottman test. Hot mix asphalt (HMA) samples were made with asphalt binders containing 0, 2, 4 and 6% of nano-CaCO3 and two sources of aggregate, namely limestone and granite. One, three and five freeze–thaw cycles were applied to HMA samples in a modified Lottman test to explore the effect of nanomaterials, clearly. The results of the modified Lottman test show that the wet/dry ratio values of indirect tensile strength (ITS) for mixtures containing nanomaterials were higher than those of control samples. In addition, the results of the SFE method indicate that nano-CaCO3 increases the wettability of the asphalt binder on the aggregate and promotes the adhesion between the asphalt binder and aggregate. Adding nanomaterials leads to the decrease of the acid component of SFE and increases the basic component of SFE of the asphalt binder that leads to an increase of adhesion between the asphalt binder and sensitive aggregate against moisture damage. Keywords: hot mix asphalt; moisture damage; nanomaterials; calcium carbonate; surface free energy; modified Lottman test

1. Introduction Durability is one of the most important properties of an asphalt mixture. A key factor affecting the durability of asphalt pavements is moisture damage (Grenfell et al., 2014). Excessive moistureinduced damage of asphalt pavement layers is a major concern for road construction industries in rainy climatic areas (Ahmad, Yusoff, Hainin, Rahman, & Hossain, 2014). Moisture damage is the degradation of the mechanical properties of the material attributable to the presence of moisture in its microstructure (Kumar & Anand, 2012). A common manifestation of moisture-induced damage in asphalt mixtures is the loss of adhesion at the aggregate–asphalt mastic interface and/or cohesion within the bulk mastic (Apeagyei, Grenfell, & Airey, 2014; Moghadas Nejad, Arabani, Hamedi, & Azarhoosh, 2013). Moisture damage of asphalt pavements can lead to serious distress, reduced performance, and increased maintenance of asphalt pavements (Kennedy, Roberts, & Lee, 1983; Xiao, Jordan, & Amirkhanian, 2009). To address the problem of moisture damage, many agencies have

*Corresponding author. Email: [email protected]

© 2015 Taylor & Francis

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G.H. Hamedi et al.


utilised specific antistrip additives in an attempt to increase adhesion at the aggregate–asphalt binder interface (Sebaaly, 2007). One appropriate approach of using antistrip additives is to cover the aggregate surface with a suitable agent to reverse the predominant electrical charges at the surface and decrease the surface energy of the aggregate. Another approach is promoting the adhesion and cohesion properties of the asphalt binder using liquid antistrip additives. Liquid antistrip additives are chemical surfactants that decrease the aggregate’s surface tension and thus promote better surface coverage (Sebaaly, 2007). Most chemical additives used in asphalt mixes are composed of amines, a basic compound derived from ammonia. Amines include long hydrocarbon chains that are able to efficiently wet the aggregate surface because of their strong affinity to the silica compounds of the aggregates (Mercado, 2007). Although using the mentioned antistrip additives has many advantages, there are several problems in using them. The use of hydrate lime has some significant disadvantages: • • • • •

Additional handling of the aggregate; Additional space for both lime-treated and untreated stockpiles; Potential threats to the safety and health of workers exposed to the lime dust; Lime can be washed from the aggregate during marination; Adding dry lime to the asphalt binder and storing the lime-modified binder prior to mixing with the aggregate (Little, Epps, & Sebaaly, 2001); and • Addition of hydrated lime to asphalt binders reduced the ageing propensity of asphalt by adsorbing naturally occurring oxidation catalysts or promoters from the asphalt binders (Petersen, 2009; Souliman, Hajj, & Sebaaly, 2014).

The use of liquid antistrip additives has some significant disadvantages: • No significant variation in the moisture damage strength of the various mixtures with liquid antistrip additives in the field (Tohme, Sebaali, Hajj, & Johnston, 2004; Sebaaly, Little, Hajj, & Bhasin, 2007); and • Negative effect of liquid antistrip additives on rutting, ageing, and fatigue cracking of asphalt mixtures (Sebaaly et al., 2007; Thomas, McKay, & Branthaver, 2006; Won & Ho, 1994). As mentioned, despite the advantages and popularity of hydrated lime and liquid antistrip additives, these materials suffer some drawbacks. In the present research, it has been attempted to evaluate the effects of nanomaterials, as a new antistrip additive, on moisture damage of hot mix asphalt (HMA).

1.1. Literature review Due to the new potential uses of particles on a nanometer scale, nanotechnology has recently attracted considerable scientific interest. Nanotechnology includes the techniques of manipulating the structure at the nanometer scale to develop a new generation of tailored, multifunctional cementation of composites with superior mechanical performance and durability potentially having a range of novel properties such as low electrical resistivity, self-sensing capabilities, self-cleaning, self-healing, high ductility, and self-control of cracks. To enable pavements to accommodate increasing traffic intensity and axle loads in varying climate environments, high-quality bitumen is required. Special binders are also needed for other applications, such as bridges and airport runways. These examples suggest the necessity of


Road Materials and Pavement Design

3

asphalt binders with suitable modifiers (Orange, Martin, Menapace, Hemsley, & Baumgardner, 2004). In spite of the fact that there are many researches in the field of using antistrip agents in HMA, there are few studies that have evaluated the effect of nanoparticles on moisture damage of HMA. Rajasekar, Heinrich, Das, and Das (2009) used styrene butadiene rubber (SBR)-nanoclay as a modifier of asphalt binders. The results of their study indicated that using SBR-nanoclay leads to an increase in indirect tensile strength (ITS). Moghadas Nejad, Azarhoosh, Hamedi, and Azarhoosh (2012) investigated the effects of nano-coating on the moisture damage of asphalt mixtures. The results of their study indicated that aggregate coating with a suitable agent increases the ratio of wet/dry values of ITS and indirect tensile fatigue in treated samples compared to the control mix. Khodaii, Khailfeh, Dehand, and Hamedi (2013) assessed the effect of a liquid antistrip agent, namely Zycosoil. The results of this research showed that Zycosoil decreases the difference in the free energy of the adhesion of the aggregate– asphalt binder between dry and wet conditions and this difference equals the amount of energy released when stripping occurs. Coating of the aggregate surface with Zycosoil decreases this difference, and subsequently causes the mixture to be more resistant to moisture damage. 1.2. The statement and objectives of the present study One of the main distresses of HMA is moisture damage. A method for decreasing this type of distress is using antistrip additives. In this paper, the effect of nanoparticles as antistrip agents on the moisture damage of HMA was evaluated. In this study, two types of aggregates with different sensitivities were evaluated against moisture damage (limestone and granite aggregates) and the asphalt binder with 60–70 penetration grade and nano-CaCO3 in three different percentages by the weight of the asphalt binder (2, 4 and 6). The surface free energy (SFE) method and modified Lottman test (ASHTO T283) were used to evaluate the effects of the modified asphalt binder by nano-CaCO3 on the moisture damage of the asphalt mixture. The specific objectives of this study are to: • • • •

determine the SFE components of the asphalt binder with and without nano-CaCO3 ; investigate the effects of nano-CaCO3 on moisture damage of HMA with laboratory tests; compare the results of laboratory tests and the thermodynamic method; and select the aggregate, asphalt binder, and antistrip additive systems that are more resistant to moisture damage in HMA.

2. SFE theory With information about characteristics of cohesion and adhesion of an asphalt mixture, moisture susceptibility, self-healing, and fatigue cracking of that mixture can be studied (Cheng, 2002). According to thermodynamic theory, change in cohesion and adhesion of SFE is related to fracture in the asphalt binder or fracture in the asphalt binder–aggregate contact surface. Therefore, to calculate adhesion and cohesion, components of SFE of an asphalt binder should be determined. SFE of a solid (or liquid) material is defined as work required for increasing a surface unit under vacuum conditions. Therefore, free energy of adhesion means the required energy for creation of two surfaces of two contacting materials. Cheng (2002) tried to develop SFE of the aggregate–asphalt binder in the presence or absence of water (wet and dry conditions) with the theory of Good and Van Oss (1971).

4

G.H. Hamedi et al. By applying Equation (1), SFE of total aggregate and asphalt binder can be determined. = LW + AB

(1)


where is the total free energy; LW is the non-polar or Lifshitz-van der Waals component of SFE, and AB is the polar component or acid–base component of SFE of asphalt binder or aggregate. The acid–base component of SFE can be obtained from the following equation with an acid– base component: √ (2) AB = 2 + − In this equation, + is the acid component and − is the base component of SFE. SFE of cohesion (Gci ) is formed based on a fracture surface unit which occurs in a material. In this case, two new surface units are created and the relationship between this concept and SFE, which has been defined before, is clearly evident. This relationship is shown in Equation (3): Gci = 2ic

(3)

SFE of adhesion (Ga ) of the asphalt binder–aggregate system is the energy required for a crack with unit surface in the asphalt binder–aggregate contact surface under vacuum condition which can be written according to the following equation: Gai

=

GaLW i

+

GaAB i

=2

Slw Llw

+ − − + S L + S L +

(4)

where Gai is the SFE of adhesion, GaLW is the non-polar component of SFE of adhesion, i is the polar component of SFE of adhesion, and LLW , L+ , and L− are the non-polar , GaAB i acid, and base components of SFE of the asphalt binder, respectively, and SLW , S+ , and S− are the non-polar, acid, and base components of SFE of the aggregate, respectively. It is necessary to mention SFE of adhesion in the presence of water because moisture damage occurs when water is present in the asphalt binder–aggregate system. Equation (5) expresses SFE of adhesion for two materials (asphalt binder and aggregate) in the presence of a third material (water). ⎡

GaLSW = +13 + 23 − 12

⎢ ⎢ ⎢ ⎢ ⎢ ⎢ =⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎣

⎤ + − LW LW + 4 W W − 2 L W ⎥ ⎥ ⎥ + − − LW ⎥ ⎥ − 2 W L − 2 L+ W − 2 SLW W ⎥ ⎥ ⎥. + − − − 2 W S − 2 S+ W + 2 LLW SLW ⎥ ⎥ ⎥ ⎥ ⎦ + 2 L+ S− + 2 S− S+ LW ) (2W

(5) + − LW , W , and W are non-polar, acid, and base components of SFE of water. In this equation, W If SFE of adhesion is positive, these two materials have adhesion to each other and the larger the value, the more force is required for removal of adhesion between them and cracking. On the contrary, when SFE of adhesion is negative (usually among three contacting materials and when the third material enters the system of two raw materials), it causes fracture of the first material to the second material.


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Table 1. Mixture descriptions. Mix (#) 1 2 3 4 5 6 7 8

Aggregate type

Asphalt binder type

Nano-CaCO3 content (%)

Granite Granite Granite Granite Limestone Limestone Limestone Limestone

AC 60–70 AC 60–70 AC 60–70 AC 60–70 AC 60–70 AC 60–70 AC 60–70 AC 60–70

0 2 4 6 0 2 4 6


Table 2. Properties of asphalt binders used in this research.

Asphalt binder Base asphalt binder Modified asphalt binders 2% nano-CaCO3 4% nano-CaCO3 6% nano-CaCO3

Viscosity (mPas)

Penetration grade (mm/10)

Softening point (°C)

150°C

135°C

115°C

Ductility (cm)

Flash point (°C)

69

47

0.776

0.289

0.156

112

313

63 61 58

50 52 54

0.921 0.957 0.982

0.327 0.349 0.367

0.176 0.203 0.217

122 128 131

329 340 347

3. Materials This research includes one base asphalt binder, asphalt binder of 60–70 penetration grade, two types of aggregate, limestone and granite, and nano-CaCO3 in three different percentages. Eight different mixtures were made with these compositions of aggregates, asphalt binders, and nanoCaCO3 . Table 1 shows a summary of mixture components. 3.1. Asphalt binder To characterise the properties of the four types of asphalt binders, conventional test methods were applied. The engineering properties of the asphalt binders are presented in Table 2. In this experimental study, an asphalt binder of 60–70 penetration grade from Isfahan mineral oil refinery was used as the base asphalt binder. There are three modified asphalt binders achieved by adding nanoparticles to the base asphalt binder. 3.2. Aggregates The chemical composition of the aggregates is listed in Table 3. The physical properties of the aggregates are given in Table 4. Table 5 shows the aggregate gradation used in this study (the mean limits of ASTM specifications for dense aggregate gradation). The nominal size of this gradation was 19.0 mm. Limestone and granite powder were used respectively as filler in mixtures containing limestone and granite aggregates. During this study, researchers used limestone and granite aggregates. The main cause of this selection was that limestone and granite have different mineralogy and surface structure that cause them to have different performances against moisture damage. 3.3. Nano-calcium carbonate Nano-calcium carbonate is a nanomaterial with CaCO3 molecular formula and is assigned discrete CAS registry numbers 471-34-1. Under special surface treatment, nano-calcium carbonate

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Table 3. Chemical composition of the two types of aggregates. Properties

Limestone

Granite

8.6 4.8 19 1.3 0.7 1.8 54.7

7.1 68.1 16.2 14.8 1.4 0.8 2.4

pH Silicon dioxide, SiO2 (%) R2 O3 (Al2 O3 + Fe2 O3 ) (%) Aluminum oxide, Al2 O3 (%) Ferric oxide, Fe2 O3 (%) Magnesium oxide, MgO (%) Calcium oxide, CaO (%)


Table 4. Physical properties of the two types of aggregates. Test

Standard

Specific gravity (coarse aggregate) Bulk SSD Apparent

ASTM C 127

Specific gravity (fine aggregate) Bulk SSD Apparent

ASTM C 128

Specific gravity (filler) Los Angeles abrasion (%) Flat and elongated particles (%) Sodium sulphate soundness (%) Fine aggregate angularity

ASTM D854 ASTM C 131 ASTM D 4791 ASTM C 88 ASTM C 1252

Limestone

Granite

Specification limit

2.612 2.644 2.663

2.652 2.664 2.686

– – –

2.619 2.638 2.659

2.652 2.656 2.681

– – –

2.649 27 8.4 3.9 44.2

2.659 21 5.9 2.3 51.8

– Max 45 Max 10 Max 10–20 Min 40

Table 5. Gradation of the aggregates used in the study. Sieve (mm)

19

12.5

4.75

2.36

0.3

0.075

Lower–upper limits Passing (%)

100 100

90–100 95

44–74 59

28–58 43

5–21 13

2–10 6

has many advantages such as uniform particle size distribution, higher specific surface area, and lower absorption of water (Henan Kingway Chemicals Co., 2014). It has been observed in previous studies that the dosage of nanomaterial additives is normally between 1% and 8% by the weight of asphalt binders. The percentages used in this study were 2, 4, and 6% by the weight of the asphalt binder. Nano-CaCO3 used in the present study was manufactured by Henan Kingway Chemicals Co., Ltd. (China). Physical properties of nanoCaCO3 are given in Table 6.

4. Experimental design and procedure The experimental flow chart of this study is shown in Figure 1. At least three separate samples were produced for each aggregate blend and asphalt binder to determine the reproducibility of the results.


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Table 6. Physical properties of nano-calcium carbonate (Henan Kingway Chemicals Co., 2014).


Properties Crystal Structure Shape of particle Density (gr/cm3 ) Refractive index BET specific surface area (m2 /gr) Average grain size (nm) Oil absorption (gDop/100 gr) Bulk density (gr/cm3 ) Whiteness (%) pH Water (%)

Figure 1.

Nano-CaCO3 Calcite Cubic 2.5–2.6 1.5–1.7 32 ± 2 ≈ 60 ≤ 35 0.35–0.45 92 ≤ 8.0–10.5 ≤ 0.5

Flow chart of experimental design procedures.

4.1. Preparation of asphalt binders To prepare the modified asphalt binder, 2%, 4%, and 6% of nano-CaCO3 particles were added to the base asphalt binder. A ‘nanoparticle distributor’ device with high rotation rate and a temperature control chamber was used, since homogenous distribution of the nanoparticles was significantly important. The asphalt binder was first heated at about 140–150°C until it became fluid in the mixer. The asphalt binder was mixed with the nanoparticles for 4–5 min at a speed of 14,000 rpm. Then, the prepared asphalt binder was used in preparing the asphalt samples.

4.2. Mix design According to the ASTM D1559 standard, the asphalt mixtures were designed using the standard Marshall mix design procedure with 75 blows on each side of the cylindrical samples. The optimum asphalt binder contents were found to be 5.6% and 5.1%, respectively, for limestone and granite aggregates. The main reason for this event is that the surface of the limestone aggregate is porous and the greater portion of the asphalt binder is non-effective compared to the granite aggregate.

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It is noteworthy that the mix design was performed with the base asphalt binder without any additives. Furthermore, the preceding asphalt binder quantities were used in all mixtures so that the amount of asphalt binder would not confound the analysis of the test data. 4.3. The modified Lottman test (AASHTO T283) In the 1930s, the development of tests to determine the water sensitivity of asphalt mixtures began. Since then, in an attempt to identify the susceptibility of asphalt mixtures to water damage, numerous tests have been developed (Airey & Choi, 2002). A general consensus in the industry is that laboratory tests performed on compacted HMA have the potential to be better indicators of moisture sensitivity than tests on loose mixture samples (Solaimanian, Bonaquist, & Tandon, 2007). Presently, the AASHTO T283 test is used in most laboratories to assess the moisture susceptibility of HMA. This test, however, does not directly address any mechanisms that govern stripping and is only an indicator of moisture-induced damage (Zaman & O’Rear, 2006). The AASHTO T283 test had a good correlation with field performance of asphalt mixtures, which has made it the commonest test for studying the moisture susceptibility of HMA and the effects of using antistrip additives. The modified Lottman test involves loading a cylindrical specimen with vertical compressive loads. This generates a relatively uniform tensile stress along the vertical diametrical plane. Failure normally occurs in the form of splitting along this loaded plane (Aksoy, Samlioglu, ¸ Tayfur, & Özen, 2005). Six samples from each mix (dry and wet) were prepared and compacted. The compacted specimens should have air void contents in the range of 7 ± 0.5%. Half of the compacted specimens are conditioned. First, a vacuum of 13–67 kPa absolute pressure is applied to partially saturate specimens to a level between 70% and 80%. Vacuum-saturated samples are kept in a freeze cycle ( − 18°C for 16 hours) and then a thaw cycle (60°C water bath for 24 hours). After this period, the specimens are considered conditioned. The other three samples remain unconditioned. The failure load for each sample was recorded at 25 ± 0.5°C (Figure 2). ITS for each sample was calculated using the following formula: ITS =

2000P , tπ d

(6)

where ITS is the indirect tensile stress (kPa), P the failure load (N), t the sample thickness (mm), and d the sample diameter (mm). Using the following equation, the indirect tensile strength ratio (TSR) was determined: TSR = 100

ITScond ITSuncond

,

(7)

where ITScond is the average ITS of the wet specimens (kPa) and ITSuncond the average ITS of the dry specimens (kPa). In this study, to clearly investigate the effect of additives, 1, 3, and 5 freeze–thaw cycles were applied to specimens in the AASHTO T283 test. 4.4. Surface free energy measurement In the present research, using Wilhelmy Plate (WP) established by Herfer et al., the surface energy of asphalt binders was measured (Hefer, 2004; Hefer, Bhasin, & Little, 2006).



Figure 2.

9

Components of the ITS test for an HMA sample.

Based on Young-Dupre’ equation, Good and Van Oss (Van Oss, Chaudhury, & Good, 1988) suggested the following equation: LW LW 0.5 + (S+ L− )0.5 + (S− L+ )0.5 ], − GaL,S = WaL,S = GTotal L, (1 + cos θ ) = 2[(S L )

(8)

where GaL,S is the Gibbs free energy of adhesion, WaL,S is work of adhesion, θ is the contact angle of a probe liquid (L), in contact with a solid (S), and other parameters are surface energy characteristics of both the liquid and solid. Equation (8) is the fundamental equation used to calculate the SFE components of the asphalt binder by measuring contact angles. In this equation, the solid (S) is replaced by the asphalt binder under consideration and the liquid (L) is any probe liquid, in this context defined as a liquid with known SFE characteristics. If the square roots of the three unknown surface energy components of the asphalt binder are represented as x1 , x2 , and x3 , Equation (8) can be rewritten as follows: LTotal (1 + cos θ ) = 2[(SLW )0.5 ∗ x1 + (S+ )0.5 ∗ x2 + (S− )0.5 ∗ x3 ].

(9)

The measured contact angle of a probe liquid with asphalt binder and surface energy components of the probe liquid are substituted into Equation (9) to create a linear equation with unknown x1 − x3 . In 1863, Wilhelmy first proposed an indirect measurement method, whereby a plate is immersed into a liquid, to quantify the contact angle between the plate and the liquid. This is a quasi-static contact angle measurement technique, since the plate is in motion (moving at a few microns per second) throughout the process. From simple force equilibrium considerations, the difference between the weight of a plate measured in air and partially submerged in a probe liquid (F) is expressed in terms of buoyancy of the liquid, liquid surface energy, contact angle,


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G.H. Hamedi et al.

Figure 3.

ITS for mixtures made with limestone aggregate with and without nano-CaCO3 .

and geometry of the plate. Therefore, the contact angle between the liquid and the surface of the plate can be calculated from this equilibrium, as shown in Equation (10). cos θ =

F + Vim (ρL − ρair ∗ g) , Pt Ltotal

(10)

where P t is the perimeter of the asphalt binder-coated plate, Ltotal the total surface energy of the liquid, θ the dynamic contact angle between the asphalt binder and the liquid, Vim the volume immersed in the liquid, ρ L the density of the liquid, ρ air the air density, and g the local gravitational force. There are three unknowns for the asphalt binder semi-solid in Equation (4): SLW , S− , S+ . These unknowns are the three components of asphalt binder SFE: Lifshitz-van der Waals, Lewis base, and Lewis acid, respectively. To resolve these parameters, at least three solvent liquids, the surface energies of which are known, must be used to produce three simultaneous equations. Water, glycerin, and formamide were used here as liquid solvents because of their relatively large SFE, immiscibility with the asphalt binder, and differing SFE components.

5. Results and discussion 5.1. Results and discussion of the AASHTO T283 test Using the modified Lottman (AASHTO T283) test, ITS of the asphalt mixtures was determined. In Figures 3 and 4, the ITS values for asphalt mixtures are presented with and without antistrip modification in dry and wet conditions. It was observed that the ITS values of the wet mixes are lower than those for dry mixes at the end of the loading test. This was expected, since the presence of water causes a reduction in the aggregate–asphalt binder adhesion, and thus the strength of the asphalt mixture samples decreases under loading. After conditioning, mixtures with nano-CaCO3 generally exhibited less decrease than control mixtures. The results of Figures 3 and 4 show that using the nano-CaCO3 increases the ITS values of the asphalt mixture samples even in dry conditions. The main reason for this event is the fact



Figure 4.

11

ITS for mixtures made with granite aggregate with and without nano-CaCO3 .

that this additive increases the adhesion between the aggregate and the asphalt binder. Also, the stiffness of an asphalt binder can have an effect on moisture susceptibility of the asphalt mixture. Stiffer asphalts are generally harder to peel from an aggregate, or take longer to peel, and thus have more resistance to moisture damage. As shown in Table 2, using nano-CaCO3 decreases the penetration grade of the asphalt binder. This causes an increase in stiffness of the asphalt binder and thus resistance to moisture damage in asphalt mixture samples containing the modified asphalt binder. In wet conditions, the increase in the ITS values of modified samples is higher. It can be concluded that adding nano-CaCO3 to mixtures improves the adhesion and cohesion of the binder and does not allow the displacement of asphalt components from the aggregate surface easily by water. Thus, nano-CaCO3 provides more reasonable mixtures than unmodified mixtures. Higher values of TSR give better resistance to moisture damage in mixtures. The minimum permissible TSR should be 70–80%, in order to have an asphalt mixture that sufficiently resists moisture and water-related damage, otherwise known as stripping (Solaimanian, Harvey, Tahmoressi, & Tandon, 2003; Solaimanian et al., 2007). The TSR values of the mixtures are shown in Figures 5 and 6. Based on the calculation results, the TSR values of mixes with the modified asphalt binder in wet conditions are higher compared to mixes with the base asphalt binder. For all freeze–thaw cycles used in this study, the ratios of the ITS values of wet/dry mixtures containing nano-CaCO3 were higher than those of the control mixtures. Antistrip additives are known to build up layers of molecules on the surfaces. A monolayer of additives will increase the adhesion by forming a layer with the oil-soluble side facing the asphalt binder. Different percentages of additives have different improvement effectiveness in the water resistance of mixtures. The results of adding nano-CaCO3 indicate that the addition of this additive causes the TSR of specimens exposed to this condition to improve. In fact, adding 2% of the nano-CaCO3 results in a significant increase in the samples’ TSR. In addition, adding 4% and 6% of the nano-CaCO3 leads to an increase in TSR value. As can be seen, the TSR of samples decreases with an increase in the number of freeze–thaw cycles. The decrease in TSR along with the increase in freeze–thaw cycles could be attributed to the loss of adhesion of the mixture and/or cohesion of bitumen.


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Figure 5. Effects of nano-CaCO3 and freeze–thaw cycles on TSR in mixtures made with limestone aggregate.

Figure 6. Effects of nano-CaCO3 and freeze–thaw cycles on TSR in mixtures made with granite aggregate.

Figures 5 and 6 show that in different freeze–thaw cycles, the samples containing granite aggregate have a weaker performance against moisture compared to samples with limestone. This is similar to the results of Peltonen research (1992). Peltonen calculated the correlations between the polarities of the silicates and adhesion. The increase in the silica dioxide content causes an increase in the polarity of the stone surface and decrease in the adhesion. In the mixtures containing granite aggregate, using nano-CaCO3 is more effective in increasing the TSR values. 5.2. Results and discussion of the SFE measurement The bond strength between asphalt and aggregate plays a fundamental role in evaluating the moisture sensitivity of HMA mixtures (Canestrari, Cardone, Graziani, Santagata, & Bahia, 2010). Using the dynamic WP method, SFE components of the asphalt binder were determined. Here, water, glycerin, and formamide were used as liquid solvents because of their relatively large SFE, immiscibility with the asphalt binder, and differing SFE components. The surface energy components of the three liquids are listed in Table 7.


13

Table 7. Surface free energies of solvent liquids (erg/cm2 ). Solvent liquids Water Glycerol Formamide

Ltotal

LLW

L+

L−

LAB

72.6 62.8 58

21.6 34 39

25.5 3.92 2.28

25.5 57.4 39.6

51 28.8 19

Table 8. SFE components of asphalt binders.


Asphalt binder types Contact angle (°) with water Contact angle (°) with glycerol Contact angle (°) with formamide Total SFE, (ergs/cm2 ) Lifitz–van der Waal component, LW (ergs/cm2 ) Acid–Base Component, AB (ergs/cm2 ) Acidic component, + (ergs/cm2 ) Basic component, − (ergs/cm2 )

AC

AC + 2% nano-CaCO3

AC + 4% nano-CaCO3

AC + 6% nano-CaCO3

127.14 116.85 114.18 15.89 13.69 2.20 2.58 0.47

126.13 116.25 112.97 19.26 17.26 2.00 1.73 0.58

124.88 115.13 111.51 21.97 19.94 2.03 1.45 0.71

124.56 114.96 111.24 22.76 20.73 2.03 1.32 0.78

An asphalt binder is generally hydrophobic in nature and aggregates are mostly hydrophilic. Therefore, it is difficult to wet hydrophilic aggregates with a hydrophobic asphalt binder (Hamedi & Moghadas Nejad, 2014; Wasiuddin, Zaman, & O’Rear, 2008). Wetting is the ability of a liquid to preserve contact with a solid surface, resulting from intermolecular interactions when the two are brought together. Although wetting is different from adhesion, according to the ideas of some researchers, a prerequisite for good adhesion is wetting (Vijayaram, Sulaiman, Hamouda, & Ahmad, 2006). The smaller the contact angles, the higher the wetting tendency. Table 8 shows that addition of nano-CaCO3 slightly reduces the contact angle of water, glycerol, and formamide in modified asphalt binders, which causes the increase of the wettability of the asphalt binder over the aggregate. Table 8 shows the total SFE and its components for the asphalt binder with and without nanomaterials obtained in this study. A general trend is that the total SFE increases with an increase in nano-CaCO3 percent. The total SFE of AC 60–70 increases from 15.89 erg/cm2 to 19.26 erg/cm2 , 21.97 erg/cm2 , and 22.76 erg/cm2 , respectively, for 2%, 4%, and 6% nano-CaCO3 . On comparison, the results show that adding an antistrip additive increases the non-polar SFE component of the asphalt binder. It is clear from the results presented in 8 that increasing the percentage of nano-CaCO3 causes the non-polar SFE to increase. An asphalt binder is acidic in nature. In the case of an acidic aggregate and acidic asphalt binder, the surface chemistry of Lewis acids and bases does not favour adhesion, and a good bond between the acidic aggregate and the acidic asphalt binder is difficult to obtain (Tarrer & Wagh, 1991). Table 8 shows that adding nano-CaCO3 causes a decrease in the acid component of SFE and an increase in the basic component of SFE of the asphalt binder, which causes an increase in adhesion between the asphalt binder and the acidic aggregate such as granite, which is prone to moisture damage. The main cause for this event is that nano-CaCO3 is made up of precipitated calcium carbonate. Precipitated calcium carbonate is an organic compound that has generally basic properties.

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Table 9. SFE components of granite aggregate (erg/cm2 ). Surface free energy components (ergs/cm2 ) Aggregate type

Basic

Acidic

Acid–Base

Lifitz–van der Waal

Total SFE

Granite Limestone

582.12 524.97

46.12 38.83

327.70 285.55

212.73 229.26

540.43 514.81


Table 10. Components of SFE of adhesion.

Mixture 1 2 3 4 5 6 7 8

Asphalt binder

Asphalt Asphalt binder in binder– Asphalt– Water– the presence Aggregate aggregate water aggregate of water

Asphalt binder Granite Asphalt binder with 2% nano-CaCO3 Asphalt binder with 4% nano-CaCO4 Asphalt binder with 6% nano-CaCO5 Asphalt binder Limestone Asphalt binder with 2% nano-CaCO3 Asphalt binder with 4% nano-CaCO4 Asphalt binder with 6% nano-CaCO5

194.75 195.00 199.81 200.25 194.19 195.57 200.91 201.53

57.54 59.59 62.18 62.84 57.54 59.59 62.18 62.84

447.83 − 167.22 − 165.42 − 151.15 435.08 − 153.90 − 151.15 − 151.19

− 165.42

− 153.22

Using a Universal Sorption Device (USD), the aggregate surface energy was quantified in this study. Total SFE and its components of the asphalt binder obtained with USD are listed in Table 9. When components of SFE of the asphalt binder and the aggregate are clear, one can obtain SFE of the asphalt binder–aggregate under dry and wet conditions with Equations (4) and (5). The results of free energy of adhesion are given in Table 10. Free energy of adhesion is an important parameter in the determination of moisture susceptibility of the asphalt mixtures. Free energy of adhesion means that if two materials contact each other, adhesion force will be created between two materials. The larger the number, the more the adhesion power will be. Free energy of adhesion between two materials in the presence of the third material may be negative. In this case, when the third material enters the effect place of the first two materials, adhesion between these two materials will be removed and it will be placed between two materials. The use of nanoparticles improved adhesion between the asphalt binder and the aggregate compared with the control samples.

5.3. Results of the scanning electron microscopy test In this regard, in order to investigate the appearance of nanoparticles and their mixing method inside a ‘nanoparticle distributor’ device, scanning electron microscopy (SEM) was used. This image was obtained from scanning the sample by concentrated loads of the electron. As can be seen in Figure 7, the image was taken with magnification of 50,000x and voltage of 30 kw. The appearance of these nanoparticles is nearly spherical and the following image demonstrates the validity of the size and appearance of these nanoparticles.



Figure 7.

15

SEM of nano-CaCO3 particles mixed in the asphalt binder.

6. Conclusion Moisture damage of HMA pavements is not a distress by itself, but represents a conditioning process after which several distresses may occur individually or simultaneously. In the present research, the effects of nanomaterials on moisture damage of HMA have been investigated by mechanical tests and identifying the mechanisms that affect the adhesive bond between the aggregates and the asphalt binders and the cohesion free energy of the aggregates and the asphalt binders. In fact, the aim of this study was to achieve reduction in the HMA moisture damage. The following conclusions can be drawn from the present study: • The ratios of the ITS values of wet/dry mixtures containing nano-CaCO3 were higher than those of mixtures containing control mixtures for two types of aggregates used in the present study. Therefore, a nano-additive makes the mixtures more resistant to moisture damage. • In the mixtures containing granite aggregate, using nano-CaCO3 was more effective. • The effects of nano-CaCO3 in the promotion of TSR values were more evident in higher freeze–thaw cycles. • Nano-CaCO3 causes the decrease in contact angle between modified asphalt binders and solvent liquid. This event causes an improvement in the wettability of the asphalt binder over the aggregate. • Nano-additives cause an increase in the base component and reduction in asphalt binder SFE acid components. This leads to an increase in adhesion between the acidic aggregate that is more sensitive to moisture damage and the asphalt binder. • The results of this investigation indicate that nanomaterials can be used as antistrip additives to decrease the moisture damage of HMA. The results of the modified Lottman test validated the results of the SFE method.

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Disclosure statement No potential conflict of interest was reported by the authors.


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