Use of Surface Free Energy method to evaluate the moisture susceptibility of sulfur extended asphalts modified with antistripping agents

Construction and Building Materials 98 (2015) 456–464

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Use of Surface Free Energy method to evaluate the moisture susceptibility of sulfur extended asphalts modified with antistripping agents G.H. Shafabakhsh a, M. Faramarzi b,⇑, M. Sadeghnejad c a b c

Dept. of Civil Engineering, Semnan University, Semnan, Iran Dept. of Civil Engineering, University of Guilan, Rasht, Iran Young Researchers and Elite Club, Safadasht Branch, Islamic Azad University, Tehran, Iran

h i g h l i g h t s
Replacing of bitumen with sulfur (Googas) increased moisture susceptibility of asphalt mixtures.
Addition of antistripping agent (NZ) improved the resistance of SEA against moisture damages.
SFE test results were compatible with the classic mechanical test results.
Limestone aggregate led more resistant mixtures against moist condition compared to the granite aggregate.

a r t i c l e

i n f o

Article history: Received 19 May 2015 Received in revised form 19 August 2015 Accepted 21 August 2015

Keywords: SEA Sulfur Antistripping agent Moisture susceptibility Surface Free Energy

a b s t r a c t Aggregate–asphalt binder adhesion bond is a vital factor that affects the resistance of asphalt mixtures against moisture damages. Since, moisture damages are the main defects of sulfur extended asphalts (SEAs), this study aims to improve it by reinforcing the adhesion between asphalt binder and aggregate. An antistripping additive named nanotechnology Zycotherm (NZ) was used to achieve this goal. Surface Free Energy (SFE) method was applied to examine the effectiveness of NZ additive in improving the moisture susceptibility of SEA. The research team utilized two common mechanical tests (indirect tensile strength, and dynamic modulus) to evaluate the validation of SFE method. All samples were constructed with two different aggregates, limestone and granite. In the SFE method, the measured surface energy components of constitutive materials were used to calculate the bond strength between them in dry and wet conditions. The findings showed that adding NZ was a successful technique to compensate the deteriorated adhesion due to using sulfur. Also it was demonstrated that SFE test results were so compatible with the common mechanical tests in predicting moisture damages. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Considerable growth in bitumen cost during the last four decades has motivated the researchers to look for new ways to reduce the consumption of bitumen. One of the most welcomed alternatives was replacing the bitumen to some extent by other materials such as sulfur. At the beginning, using untreated liquid sulfur to modify HMAs led to some constructional problems like emission of hazardous gases, such as, H2 S: While applying liquid sulfur associated with some constructional problems, it had such benefits as decreasing the production cost and improving the mechanical properties of asphalt mixtures, such as stiffness [1]. So, there was ⇑ Corresponding author. E-mail address: [email protected] (M. Faramarzi). http://dx.doi.org/10.1016/j.conbuildmat.2015.08.123 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

a desire to enjoy the advantageous properties of sulfur, alongside eliminating its implementation problems. Eventually, a kind of dust-free sulfur pellet was developed by treating the raw sulfur with the polymeric base additives. In addition, the new physical form and chemical compound of treated sulfur made it more convenient and safe to be transported from the producer factory to the construction site. Conducted changes in the new version of sulfur product made it appropriate to be used in asphalt mixture as a bitumen extender, while, the previous problems about the emission of annoying odors and hazardous gases such as H2 S, was solved to a high extent [2]. Utilizing the new sulfur pellets reduced the mixing and compacting temperatures to about 135 °C and 90 °C respectively. Because of the changed mixing operation temperature, the new asphalt mixture goes in warm mix asphalt category. Since the mixing temperature of SEA is about 30 °C lower

G.H. Shafabakhsh et al. / Construction and Building Materials 98 (2015) 456–464

than HMAs, replacing this new asphalt mixture with the conventional HMAs leads to a high saving in production expenses, as less fuel is consumed. It is worth mentioning that temperature increment becomes more energy consuming progressively at higher temperatures, so, the 30 °C reduction in the mixing temperature could be so valuable [3]. In spite of the improvements in the mechanical properties of SEAs, there are also some negative consequences too. SEAs are more vulnerable against fatigue phenomenon and wet condition compared to the conventional asphalt mixtures [4]. Extending bitumen with sulfur increases the hardness of asphalt binder, consequently, the cohesion of asphalt binder as well as the adhesion between aggregates and binder decrease [5].There are various methods to improve these weakened properties and reduce moisture sensitivity in asphaltic mixtures. One of the main methods is the addition of antistripping agents (ASAs) which leads to a reinforced bond between asphalt binder and aggregates. In this study, a new generation of modified sulfur pellets (Googas) and ASA (nanotechnology Zycotherm) are used in the production of warm-mix asphalt (WMA) samples. 1.1. SFE method The main reason of SEA weakness against moisture conditions is decreased adhesion and cohesion properties of asphalt mixture ingredients. Nowadays, it is still common to use the mechanical tests in dry and wet conditions to evaluate the asphaltic mixture resistance against moisture damages. These kind of mechanical tests provide a general comprehension about the asphalt mixture behavior in moist conditions, in which, all other effective parameters, in addition to adhesion bond parameter, influence the results. Although the classic tests give a comprehensive understanding about the asphalts’ behavior, they also have some deficiencies:
Low compatibility with the real loading and pavement conditions.
Inability to consider the constitutive material properties alongside with environmental destructive mechanisms.
Being time consuming. In order to eliminate these deficiencies, researchers tried to figure out a method capable of characterizing the properties of asphalt mixture’s ingredients in wet condition. The new method was supposed to be able to measure the adhesion bond between aggregates and asphalt binder as well as the cohesion of binder, because they are the most determinant parameters in moisture susceptibility of asphalt mixtures. SFE parameters of asphalt binder and aggregate are significant characteristics that could be used to investigate the moisture susceptibility of asphalt mixture. In the SFE method, it is possible to measure the adhesion bond between asphalt binder and aggregates in quantitative form; also this new method helps to determine the tendency of mixture for replacing the asphalt binder with water. This replacement is the mechanism in which stripping occurs in asphalt mixtures. The relative calculations would be done according to the basic thermodynamic laws. The Obtained results provide complete information about the material properties along with the adhesion bond strength between aggregates and binders, therefore, this method would be so helpful to select the appropriate materials and their combinations in mixing design of asphaltic mixtures; so, the selected mixture would be more resistant against moisture damages [6]. 1.2. The aggregate surface area (%) in contact with water Cheng et al. [7] utilized the theory of ‘‘nonlinear Viscoelastic behavior” for substances with diffused damage. This theory which

457

was developed by Schapery et al. [8], was used by Cheng et al. to explain the function of loaded asphalt concrete in either controlled stress or controlled strain mode. Cheng et al. applied the cyclic loading mechanism tests which were compatible with the Schapery’s ‘‘distributed damages theory” criteria. According to this new method, cyclic loading tests could be used to predict the moisture damages by measuring the aggregate surface area (P%) in contact with water in asphalt mixtures. As shown in Eq. (1) the wet to dry ratio of asphalt dynamic modulus can be calculated if the adhesion bond values between binder and aggregates are obtained for the asphalt mixture in dry and moist conditions [9].

Ewet ½DG12  ð1  PÞ þ DG123  P ¼ Edry DG12

ð1Þ

In which, DG12 is the adhesion bond energy between aggregates and asphalt binder; DG123 is the adhesion bond energy between aggregates and asphalt binder in a moist condition; P is aggregate surface area in contact with water in asphalt mixtures (%); and E is complex modulus in dry or wet condition. If the dynamic modulus parameter is rewritten in the term of
 ratio between stress and strain re the following relationship will be obtained when the test is conducted in constant stress mode.


r  edry ½DG12  ð1  PÞ þ DG123  P Ewet ¼
erwet ¼ ¼  Edry ewet DG12 e dry

ð2Þ

In which, edry is the applied strain in the asphalt mixture specimen under dry condition, and ewet is the applied strain in the asphalt mixture specimen under wet condition. All the variables in this equation are known from SFE and dynamic modulus tests, so the P parameter (the aggregate surface area (%) in contact with water) is the only unknown and could be calculated from this equation.

1.3. The statement and objectives of the present study In this experimental study, the moisture susceptibility of SEA is evaluated by SFE method and the results are compared with the indirect tensile strength (ITS) and the dynamic modulus (DM) tests for validation. Wet to dry ratios of ITS and DM tests are used to investigate the moisture susceptibility of different samples by mechanical methods, and eventually, these ratios are compared with the SFE test results. Another applied criterion for examining the moisture susceptibility and compatibility of constitutive materials is a combined method (SFE and DM) which determines the aggregate surface area (%) in contact with water per cycle of DM test (P). The main goals of this research are:
Evaluating the SEA moisture susceptibility and the effect of using antistripping additive (NZ) on this phenomenon.
Comparing SFE test results with classic mechanical tests like ITS and DM to determine moisture susceptibility of asphaltic mixtures.
Utilizing the SFE method to determine the best mix design in the production of studied SEA. A new generation of modified sulfur mix additive (Googas) and ASA (nanotechnology Zycotherm) are used in the production of warm-mix asphalt (WMA) samples. Tests in dry and wet conditions are performed on different combinations of additives (sulfur and ASA) and aggregates.

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1.4. Literature review

Table 2 Gradation of aggregates used in the present study.

Estakhri et al. [10] performed a comprehensive investigation on WMA, including mix design, experimental characterization, and in place tests. It was found that binder–aggregate adhesion bond was weakened in WMA mixtures, in comparison with the conventional HMA mixture. This result was obtained by doing SFE test on the WMA samples produced with different combination of constitutive materials (aggregate, binder, and WMA additive). Arabani et al. [11] evaluated the effect of using different WMA additives (Sasobit and asphamin) as well as antistripping agent on the moisture susceptibility of WMA mixture applying SFE method. It was found that the used WMA additives increased the acidity and decreased the base component of modified bitumen. Since the used aggregate had acidic property, it weakened the adhesion bond and consequently made a negative effect on moisture damage resistance. On the other hand, adding the antistripping agent (Zycosoil) increased the adhesion energy in wet condition and made WMA mixture more resistant against moisture damages. Ghabchi et.al [12] used another WMA additive named Evotherm and applied the SFE method to evaluate the moisture susceptibility of different samples in this study. Measured SFE parameters and works of adhesion demonstrated improved resistance of modified-WMA mixtures against moisture damages. Nejad et.al [13] modified two types of aggregates by hydrated lime treatment and investigated the moisture susceptibility of constructed HMAs by SFE test. It was concluded that hydrated lime-modified samples enjoyed lower acidity and higher base component. Also, the dry/wet ratio of free energy (compatibility ratio) reduced considerably for hydrated lime modified samples. All of these changes implied a more resistant mixture against moisture damages. In another study by Arabani et al. [14] the moisture susceptibility of liquid antistripping additive (LAA) – modified HMA was evaluated by SFE method and validated by a mechanical test (dynamic modulus test). It was found that the total SFE of LAA-modified bitumen increased due to LAA modification. Increase in total SFE led to weakening of adhesion bond which was compatible with obtained results by the performed dynamic modulus test. 2. Experimental procedure 2.1. Used Materials 2.1.1. Asphalt binder and aggregate In this experimental study, the Initial unmodified bitumen was produced by Tehran oil refinery and was in the range of 60/70-penetration grade. The physical properties of this bitumen are shown in Table 1. Selected aggregates exhibited appropriate mechanical properties like high strength and enough toughness and hardness. Also, Crushed aggregates were used to make higher stability. The common application of modified asphalt mixtures is in the Topeka and binder layers of pavement, therefore, the grading of aggregates were done according to standard grading of these layers. Table 2 shows the selected grading according to the AASHTO standard [15] which categorizes in type 4 scale of this standard (aggregate grading for Topeka and binder layers). Because of high dependency of SFE test on aggregates chemical compositions, they are presented for both of aggregates in Table 3. 2.1.2. Additives Googas is produced by compounding sulfur, plasticizer and other additives, which increase the range of melting, freezing and evaporation points of sulfur. It is a polymeric sulfur product, which has been synthesized in the R&D unit of the ZENIT Company in granular shape (solid pellet).Sulfur pellets could be substituted for a main part of bitumen in SEAs (about 50%) and modify the mechanical proper-

Sieve (mm)

19

11.5

4.75

2.36

0.3

0.075

Lower–upper limits Passing (%)

100 100

90–100 92

44–74 53

28–58 55

5–11 12

2–10 6

Table 3 Chemical composition of used Aggregates measured by electron microprobe analyzer. Properties

Limestone

Granite

PH SiO2 (%) R2 O3 (%) Al2 O3 (%) Fe2 O3 (%) MgO (%) CaO (%)

8.6 4.1 18.2 1.2 0.4 11.7 51.1

7.2 69.9 16.1 14.9 1.6 0.9 2.2

Table 4 Physical properties of Googas. Test title

Result

PH Physical shape Color Odor Water solubility Relative specific weight to water Average size of pellets Viscosity

7.6 Granule Gray Odorous Non soluble 1.89 g/cm3

Melting point

2 mm Different viscosities in different temperatures Minimum 90 °C

Table 5 Physical properties of zycotherm. Properties

Result

Form Color Flash point Explosion hazard Density Freezing point Solubility PH value Viscosity

Liquid Pale yellow >80 °C (176 °F) Not known 1.01 g/ml 5 °C (35 °F) Miscible with water 10% solution in water neutral or slightly acidic 100–500 CPS

ties of asphalt mixture [3]. Using Googas decreases energy consumption via decreasing the temperature needed for mixing and compaction operations. As this material is cheaper than bitumen, it can decrease the costs of asphalt production and energy consumption. Physical properties of Googas material are shown in Table 4. Nanotechnology Zycotherm (NZ) is a WMA additive produced by Zydex Company, Gujarat, India. This is an odor free, chemical warm mix additive that leads to the production of asphalt mixtures with lower mix and compaction temperatures as well as reducing the moisture susceptibility of asphaltic pavements as an antistripping agent. Physical properties of Zycotherm additive are shown in Table 5.

2.2. Mix design methodology Mix design of different samples was done applying the common Marshall Mix design method, in accordance with the ASTM D1559 [16]. Four parameters of marshal test (stability, flow, density and void) were utilized to achieve the optimum content of asphalt binders. The optimum bitumen contents of limestone

Table 1 Physical properties of used asphalt binder. Properties

Purity grade

Flash point

Softening point

Penetration Grade at 25 °C

Ductility at 25 °C

Viscosity

Density

Loss of heating

Unit Value

% 99

°C 262

°C 54

mm/10 67

cm 104

mPa s 349

– 1.03

% 0.05

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G.H. Shafabakhsh et al. / Construction and Building Materials 98 (2015) 456–464 aggregate-mixture were found to be 5.5% and 4.3% for conventional and SEA mixtures, respectively. On the other hand, for granite aggregate-mixture, this content determined 5% and 4%. The research team used 35% Googas by the weight of bitumen for constructing sulfur contained samples, which was the optimum amount obtained by doing ITF (Indirect Tensile Fatigue) and LWT (Loaded Wheel tracking) tests. To prevent the influence of different binder contents on the results of performed tests, all the samples (with different combination of additives) were produced with the same binder content for each aggregate type. More details about mixing operation and the way it has been done are outside the scope of this article and could be found in another article written by the authors [17]. 2.3. Dynamic modulus test In this experimental study, the dynamic modulus test was applied to calculate an important parameter (P) in the prediction of moisture damages; also it was considered as a validating test using another achieved parameter (K). The first parameter is ‘‘aggregate surface area (%) in contact with water” (P), and the second parameter is ‘‘dry/wet ratio of dynamic modulus values” (K). Six specimens were prepared for every sample, including three for the unconditioned and three for the conditioned modes. The unconditioned specimens were not under moisture conditioning before test but the conditioned specimens were treated by the following stages in accordance with the AASHTO T283 [18]:
Immersing in water (up to 55–80% saturation Level).
freezing at 18 °C for 16 h.
Placing in a water bath at 60 °C for 24 h.

rmax emax

ð3Þ

In which rmax ¼ 200 MPa; and emax is the maximum recorded strain for each cycle. Also, the wet/dry ratio of the dynamic modulus could be calculated by the following equation for different cycles:

K¼

Ewet Edry

ð4Þ

In which, the more the K parameter, the more resistant asphalt mixture against moisture damages. 2.4. Surface Free Energy method While, mechanical tests are used for estimation of moisture damages, the results of these tests are not very precise and that is because of the difference between the real field conditions and the laboratorial conditions. So, the simulation with the mechanical test is not as exact as necessary, that is why the researchers looked for a new method to be more fundamental and precise. The initial studies investigated the possible methods for measuring aggregate and bitumen surface energy [6,7]. If thermodynamic law is used to evaluate the changes in free energy of adhesion (between aggregate and binder) and cohesion (in binder), it will be understood that, adhesion energy is changed due to the emergence of debonding between aggregates and binder and generation of micro cracks in binder, respectively. In every asphalt mixture, it is essential to measure the SFE components of asphalt binder and aggregates to calculate the adhesion energy between constitutive materials and cohesion within the asphalt binder. Obtaining the SFE components of asphalt binder and aggregates will be helpful in estimation of healing, moisture susceptibility and fatigue phenomena [7]. Introduced equations by Good et al. [20] were applied by Cheng et al. [7] to evaluate adhesion work between asphalt binder and aggregates in dry and moist conditions. For each aggregate and asphalt binder, the total SFE could be obtained by the following equation:

C ¼ CLW þ CAB

ð5Þ

where C is total SFE parameter; CLW is the Lifshitz–van der Waals parameter; and CAB is acid–base parameter. The acid–base parameter can be rewritten in a new form which is constituted of separate acid and base components (Eq. (6))

0:5

ð6Þ

þ



In which C is the acid component of SFE; and C is the base component of SFE. The SFE of cohesion (DGci ) is the necessary energy to break a material apart in a unit area under the vacuum condition [7]. The DGci parameter could be obtained by the following equation:

DGci ¼ 2Ci

ð7Þ a

On the other hand, the SFE of adhesion (DG ) is defined as the necessary energy to separate two bonded material in a unit area of interconnection point, under the vacuum condition [7]. The (DGa ) parameter could be calculated by the following equation:

DGai ¼ DGiaLW þ DGiaAB ¼ 2

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

LW CLW 2 C1 þ

qffiffiffiffiffiffiffiffiffiffiffiffiffi

Cþ2 C1 þ

qffiffiffiffiffiffiffiffiffiffiffiffiffi

C2 Cþ1

ð8Þ

In which, DGai is the SFE of adhesion; DGai LW is the LW parameter of adhesion;

þ DGiaAB is the acid–base parameter of adhesion; CLW , C 1 1 , and C1 are the SFE param-

 eters of binder; and CLW , Cþ 2 2 , and C2 are the SFE parameters of aggregate. It is necessary to calculate the SFE of adhesion (between binder and aggregate) in presence of water to evaluate the asphalt mixture behavior in moist condition. The following equation could be used to calculate the SFE of adhesion between two connected materials, when a third material exists in the system. [7]:

DGabs 123 ¼ c13 þ c23  c12

The output of dynamic modulus test could be used to predict the asphalt pavement behavior in the either forms of elastic or linear Viscoelastic [19]. The main output of dynamic modulus test is the complex modulus (E ) which could be described as a parameter that correlates stress to strain for a linear Viscoelastic substance that is under sinusoidal loading. In this test, uniaxial compressive load is exerted to the cylindrical specimen in cyclic sinusoidal mode. The E parameter could be calculated by dividing the maximum applied stress to the maximum recorded strain in each cycle of loading (Eq. (3)) which would be named dynamic modulus if the absolute values are considered. The cyclic loading was performed in a frequency of 1 Hz and magnitude of 200 MPa while the temperature was set at 25 °C. Since this test was performed in the constant stress mode, the dynamic modulus parameter could be calculated by measuring the maximum recorded strain during each cycle.

E ¼

CAB ¼ 2ðCþ C Þ

ð9Þ

In this equation, indexes 1, 2, 3 could be replaced by aggregate, asphalt binder and water respectively. In the Eq. (9), cij is the adhesion bond between materials i and j and could be obtained using SFE values in the following equations: AB cij ¼ cLW ij þ cij

cLW ij ¼

qffiffiffiffiffiffiffiffi

ð10Þ

 cLW i

cAB ij ¼ 2

qffiffiffiffiffiffi

cþi 

qffiffiffiffiffiffiffiffi2

cLW i

ð11Þ

qffiffiffiffiffiffi pffiffiffiffiffiffi pffiffiffiffiffiffi cþi ð ci  cj Þ

ð12Þ

As this system (aggregate, asphalt binder and water) is an unstable one, aggregates tend to separate from asphalt binder and make a bond with water. This process releases energy; as a result, the value of energy of adhesion will be negative. The more the value is negative, the more the system is unstable [20]. Dry/wet ratio of adhesion energy is a capable factor to evaluate resistance of any mixture against moisture damages. This factor is called compatibility ratio (CF) and could be obtained by applying Eqs. (8) and (9), as it is shown in the following equation:

CF ¼

DGai

ð13Þ

DGabs 123

2.4.1. Surface Free Energy measurement USD (Universal Sorption Device) and WP (Wilhelmy Plate) tests are the most common methods for measuring SFE parameters of aggregate and bitumen; these tests were introduced for the first time by Bhasin et al. [21] and Hefer et al. [22] respectively. Since it is impossible to measure the SFE parameters of a solid surface directly, it should be done by the help of an intermediate material which is called probe. For this purpose, the work of adhesion between the probe substance (known SFE components) and solid surface (unknown SFE components) should be measured. Since aggregates enjoy high SFE values, adsorption method could be an appropriate procedure to determine the work of adhesion between aggregates and probe agent. This procedure is based on the correlation between the vapor pressure and magnitude of absorbed vapor by the aggregates surface. In order to calculate the SFE parameters of each aggregate by Eq. (14), three different probe materials are necessary, since there is a linear relationship between the SFE components of aggregate and the adhesion work. As a result, a ‘‘three equations and three unknowns” would be obtained which should be solved to obtain three SFE components of each aggregate.

W aS;V ¼ pe þ 2Ctotal ¼ 2 V

h











LW CLW þ CþS Cl þ CS Cþl S Cl

i

where, S and V refer to the aggregates and vapor probe respectively,

ð14Þ WaS;V is

work of

is probe’s total surface energy; and pe is the equilibrium spreading adhesion; Ctotal V pressure of the probe over the stones surface. In USD method, the gas-adsorption parameters of probes should be applied to calculate the SFE parameters of each aggregate. The USD apparatus is constituted of a balance system, temperature controller, computer, vacuum apparatus and its regulator, pressure gauge, vacuum dissector, and a probe container. The aggregates were dried and sieved (between 4.75 mm and 2.36 mm). Stones sizes were checked by the stone holder applied in the USD form of aluminum mesh. Nearly 40 g of stones, remained on the 2.36-mm sieve,

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was dust removed with neat water. The stones were then put in a washing procedure to be completely prepared for the test. The washing procedure was constituted of four stages. These stages consisted of washing the stones with: (1) distilled water, (2) methanol, (3) hexane, and (4) methanol again. The washing procedure was followed by four-hour oven drying. Then the aggregates were hold awhile in the room temperature, and then transferred to the aluminum container of USD test machine. Three chosen probe materials in this study are n-hexane, MPK, and water, which are Nonpolar, monopolar and bipolar materials respectively and their SFE characteristics are measured before (Table 6). The pressure in which these probe materials are spread over the stones could be calculated by the following equation:

pe ¼

RT MA

Z

pn

0

n dp p

ð15Þ

In which, R is the universal gas coefficient; T is the testing temperature; M is the probe material molecular weight; n is mass of probe material that penetrates in stone’s unit mass (at probe pressure p); and A is the stone’s specific surface area. The following equation should be used to calculate the stones specific surface area. This equation is called BET equation.

 A¼

 nm  N0 a M

CLW

CA

CB

CAB

Ctotal

Water Formamide Glycerol

21.8 39 34

25.5 2.28 3.92

25.5 39.6 57.4

51 19 30

72.8 58 64

 þ Lifshitz–van der Waals (CLW s Þ, Lewis base (Cs Þ, and Lewis acid (Cs Þ:are three unknown parameters in Eq. (19) which could be solved if three known probe materials be used. There were three reasons for choosing those three materials as probes. (1) Having various SFE characteristics, (2) having high values of SFE components, (3) asphalt binder would not be dissolved in them. More discussion about the SFE method is out of scope of this study and could be found in other papers [20,21].

3. Results

1 SþI

ð17Þ

Using the Young-Dupre equation, a relationship was made between the Gibbs



adhesion bond DGaL;S , the work of adhesion WaL;S , the contact angle (h) related to a probe material (L), which is in touch with a solid (S), and the SFE components of solid and liquid materials (Eq. (18)).

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi LW DGaL;S ¼ W aL;S ¼ Ctotal ð1 þ cos hÞ ¼ 2 CLW þ CþS Cl þ CS Cþl L S Cl

ð18Þ

Eq. (18) is applied to obtain the SFE parameters of asphalt binder by measurement of contact angles. Variables of this equation consist of S (solid) and L (liquid), which, solid represents the asphalt binder and liquid should be replaced by the probe materials that have characterized SFE components (Table 7). By replacing the square root of unknown SFE parameters of asphalt binder with x1 , x2 , and x3 , the Eq. (18) could be rewritten as the following equation :



qffiffiffiffiffiffiffiffiffi

qffiffiffiffiffiffiffi

qffiffiffiffiffiffiffi

Ctotal ð1 þ cos hÞ ¼ 2 x1 : CLW þ x2 Cl þ x3 Cþl L l

ð19Þ

The SFE components of probe material and the obtained value of contact angle from the Wilhelmy plate test should be placed in Eq. (18) to achieve a linear equation with three unknowns (x1 , x2 , and x3 Þ. The resulted equation will be in the same form of the Eq. (14) which was used to calculate the SFE components of aggregates. If the force equilibrium be made between the ordinary weight of plate (dry weighting) and the weight of plate in the form of being half submerged in probe substance, DF would be a parameter in this equilibrium that is related to the surface energy of liquid, dimension of plate and contact angle. Contact angle between plate and probe liquid could be calculated by the following equation which is obtained from the aforementioned force equilibrium.

Cosh ¼

Absorbate

ð16Þ

In which, N0 is number of Avogadro; nm is stone monolayer capacity; and a is the projected area of one molecule. Monolayer capacity is described as the quantity of molecules needed for covering one layer of stone surface. This parameter could be determined by obtaining the gradient (S) and the cross point (I) of the best fit line between p=nðp0  pÞ and p=p0 values (obtained by least square method), in which, p = partial pressure of the probe vapor, p0 = saturation pressure of the probe vapor, n = mass of absorbed vapor to the unit area of stones. The best fit line is accurate for the partial pressure in a range between 0 and 0.35, because the BET relationship could be used for this range of values.

nm ¼

Table 7 Surface Free Energies of solvent liquids (erg/cm2 ).

DF þ V im ðqL  qair  gÞ

ð20Þ

Pt Ctotal L

In which, P t = plate’s perimeter covered by asphalt binder; C = total SFE of the probe; h = plate-probe contact angle; V im = volume of immersed part of plate; qL = probe material density; qair = density of air, and eventually g = gravitational force.

3.1. Surface Free Energy test 3.1.1. Asphalt binders In this study the WP method was applied to measure the SFE parameters of different asphalt binder samples and the obtained results are shown in the Table 8. A significant point about the SFE of binders is that the components which are related to the polarity are so effective in making a binder more talented to form a bond with aggregates [23]. This phenomenon originates in the high polarity of aggregates. But opposite to such a desire, the bitumen enjoys a high value of Lifshitz –van der Waals (19.28 (erg/cm2 )) that is the Nonpolar component of SFE, while has a low value of polar (0.89 (erg/cm2 )). Low polarity property of asphalt binder makes it more susceptible in presence of water, as the water is inherently a very polar material. The SFE’s acid–base parameter of bitumen is a low value (0.89) which originates in the low values of acid (1.23) and base (0.16) components. As it is intelligible by the acid and base component values, bitumen is an acidic material which is in accordance with the former presumes. As presented in Table 8, the acid/base ratio for all samples is lower than the neat AC bitumen, especially, when this is modified with the antistripping additive (NZ). For this sample the ratio reduced from 7.68 for AC to 1.46 for NZ-modified binder and led to a stronger bond between aggregates and binder. Totally, it is hard to form an adhesion bond between asphalt binder and aggregates as they are both acidic materials, so it could be very advantageous if the acid component of binder be reduced and, on the other hand, the base and the Lifshitz–van der Waals components be increased [24]. Antistripping additive materials positively influence on the formation of bonds between aggregate and binder by a mechanism like that. As it is presented in Table 8, the Lifshitz–van

Table 8 Surface Free Energy components of asphalt binder with sulfur and antistripping additives. Asphalt – binder

AC 60– 70

AC with NZ

AC with Googas

AC with NZ and Googas

Contact angle (°) with water Contact angle (°) with glycerol Contact angle (°) with formamide Total surface free energy, C(erg/cm2 ) Lifshitz–van der Waals component,

76.88 85 78 20.17 19.28

87.02 74.76 66.54 28.03 25.75

69.63 84.78 82.5 11.68 10.49

76.58 82.5 79.07 18.93 17.88

total L

Table 6 Surface Free Energies of solvent for aggregates (erg/cm2 ). Absorbate Water n-Hexane Methyl propyl keton

CLW 21.8 18.4 24.7

CA 25.5 0 0

CB 25.5 0 19.6

CLW (erg/cm2 ) CAB 51 0 0

C

total

72.8 18.4 24.7

Acid–base component, CAB (erg/cm2 ) Acidic component, Cþ (erg/cm2 ) Basic component, C (erg/cm2 ) Ratio between acid-to-base

0.89

2.28

1.19

1.05

1.23 0.16 7.68

1.38 0.94 1.46

1.48 0.24 6.16

1.39 0.2 6.95

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G.H. Shafabakhsh et al. / Construction and Building Materials 98 (2015) 456–464 Table 9 Surface Free Energy components of aggregates (erg/cm2 ). Type of aggregate

(1) Acid component (Cþ )

(2) Base component (C )

(3)Acid–base

(4) Lifshitz –van der Waals

(5)Total surface free

component (CAB )

component (CLW )

energy (Ctotal )

(6)ratio between acid-to-base

Limestone Granite

6.89 0.34

237.09 478

80.83 25.5

50.15 57.3

130.98 82.8

0.29 0.0007

Table 10 Free energy of adhesion (erg/cm2 ). (1) Types of aggregate

(2) Type of asphalt mixture

(3) Asphalt binder – aggregate

(4) Wateraggregate

(5) Asphalt binder-aggregate in presence of water

(6) CR

Limestone Limestone Limestone Limestone Granite Granite Granite Granite

CHMA AWMA SWMA ASWMA CHMA AWMA SWMA ASWMA

98.44 113.14 85.91 98.54 115.44 129.32 102.8 116.09

248.15

60.35 58.45 64.12 59.91 92.59 91.5 96.46 91.6

1.63 1.93 1.33 1.64 1.25 1.41 1.07 1.26

297.38

der Waals component and acid/base ratio have changed pleasantly as a result of using antistripping additive (NZ). As it was aforementioned, asphalt binders have low polarity while aggregates enjoys polar surface. So, when the binder and aggregates are initially blended, the main agent of bond formation is the Lifshitz–van der Waals component [25]. It could be seen in Table 8 that the Nonpolar component (CLW ) of neat and sulfur-modified binders increased by addition of NZ as an antistripping agent. So, utilizing NZ improved the bonding agents and could be a successful technique to reduce the moisture damages by increasing the adhesion bond. 3.1.2. Aggregates As it was aforementioned, almost all aggregates have polar surface. It was proved in an investigation carried out by Peltonen et al. [26] that there is a direct relationship between the polarity of silicates and adhesion bond, so that, growth in silica dioxide percentage in aggregate makes its surface more polar and consequently weakens the adhesion property. The USD method was applied in this study to measure the SFE values of aggregates. Table 9 shows different SFE components for both of analyzed aggregates (limestone and granite). The total SFE of aggregates are presented in column number 8 and shows a greater value for limestone in comparison with granite. The acid–base components of SFE for aggregates are shown in column number 3. It shows that this component is greater for limestone (80.83 erg/cm2 ) compared to granite (25.5 erg/cm2 ). Column number 4 presents the Lifshitz– van der Waals that is a nonpolar component and shows lower values for the limestone (50.15 erg/cm2 ) in comparison with the granite aggregate (57.3 erg/cm2 ). Considering the effects of higher

CLW and lower CAB (higher polarity) in improvement of bond between binder and aggregates, the carried out comparisons demonstrate a weaker adhesion for the limestone compared to the granite aggregates. 3.1.3. Adhesion bonds When the SFE parameters of asphalt binders and aggregates are measured, it is possible to calculate the adhesion bond between binders and aggregates in dry and moist conditions by Eqs. (8) and (9) respectively. As it is aforementioned, the positive value of ‘‘free energy of adhesion” means that constitutive materials tend to stay bonded, and the more the SFE values, the more strong bonds; while the negative values of ‘‘free energy of adhesion” is

equivalent with an unstable system and a tendency in the materials to be separated from each other. Since, the asphalt binder has lower values of SFE compared to the water, water is more able to wet aggregate surface in competition with asphalt binder. It means that binder separates from aggregate surface and water will be replaced, so, the more stable water-aggregate system will be formed [27]. As it is shown in Table 10 and was discussed previously in the last section, granite made a more powerful bond with different binders in absence of water, compared to the limestone, while limestone made more resistant mixtures due to higher ‘‘free energy of adhesion” values in moist condition. It could be justified by considering the free energy of adhesion in the water-granite system (297.38 erg/cm2 ) which enjoys a greater magnitude in comparison with the water-limestone system (248.15 erg/cm2 ). It implies that granite is more hydrophobic in comparison with limestone. As it could be comprehended from the Table 10, the sulfur extended warm mix asphalt (SWMA) showed ‘‘free energy of adhesion” (85.91 and 102.8 erg/cm2 for limestone and granite respectively) lower than the control mix (98.44 and 115.44 erg/cm2 ) while adding NZ (ASWMA) compensate this deterioration almost to the same extent (98.54 and 116.09 erg/cm2 ). As it is shown in the Table10, NZ-modified binder led to the highest surface energy of adhesion for both of limestone and granite aggregates (113.14 and 129.32 respectively); because NZ is an organosilane compound and can form silanols (Si–OH) groups. As silanols are so reactive, a siloxane bonds (Si–O–Si) can be formed between NZ and inorganic surfaces such as sand and gravel surfaces which constitute from silanol groups. Such an interaction between NZmodified binders and aggregates led to a reinforced bond, and improved the adhesion bond that will make a more resistance asphaltic mixture against moisture damage. In contradiction with the dry condition in which granite showed better adhesion, as it is shown in the Table 10, the bond between aggregates and binders are more unstable in presence of water for the granite compared to the limestone. 3.2. Selection of aggregate It is preferable for a mixture to enjoy high adhesion energy between the aggregate and the binder in dry condition and low released energy in moist condition. To consider these two parameters, Dry/Wet ratio of adhesion energy is a capable factor to evaluate resistance of any mixture against moisture damages. This factor is called compatibility ratio (CR). Since, in the moist condition energy is released from the system, its value (DGabs 123 Þis negative, so in calculation of compatibility ratio the absolute value would be used. Application of this ratio for evaluating mixtures durability is so advantageous in mix designing, because by measuring the SFE components of different binders and aggregates, it is possible to calculate their various combinations and recognize the best compatible combinations to be utilized [20]. In order to do such a calculation for 8 different prepared asphalt samples, SFE characteristics of 4 different asphalt binders as well as two used aggregates were applied from Tables 8 and 9 respectively. As it is presented in Table 10, the values in column number 3, 4, 5 and 6 are calculated using Eq. (8), Eq. (8), Eq. (9) and Eq. (13)

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Fig. 1. Aggregate surface area in contact with water (%) via number of cycles (Limestone).

respectively. Compatibility ratio (CR) is the major parameter of this table that is calculated using values of column number 3 and number 5. Its values for different samples is the most reliable and complete criteria to evaluate the durability of any asphaltic sample by SFE method. On the basis of calculated ratios, all of the samples had higher ratio with limestone in comparison with granite. Consequently limestone made a better compatibility and more powerful bond with binders; so it could be concluded that limestone made a more resistant mixture against wet condition and should be chosen as the selected aggregate in mix design. As shown in the Table 10, NZ-modified benders (AWMA samples) resulted the greatest ratios for the both kinds of aggregate, therefore, could make the most compatible mixture. On the other hand, adding NZ to the sulfur-modified binder (ASWMA samples) changed back the deteriorated ratio to the almost same level of unmodified mixtures (CHMA samples). So, addition of NZ made the SEA mixtures more resistant against moisture damages, as it was the main goal of using NZ in this study.

Fig. 2. Aggregate surface area in contact with water (%) via number of cycles (Granite).

3.3. Relationship between surface energy and dynamic modulus methods Fig. 3. TSR values vs. mixture type.

The percentage of aggregate area in contact with water (P) via number of cycles in DM test is illustrated by the diagrams in Figs. 1 and 2. Considering this parameter, limestone resulted lower stripping because of better adhesive between aggregates and binders. As it is comprehensible by the AWMA sample diagram in the both of figures, this sample led the best application against stripping phenomenon (the least index P) followed by ASWMA, CHMA and SWMA samples. So, extending 0.35 of bitumen by sulfur pellets made it more vulnerable against stripping, although addition of NZ reached the index P to the values near to the CHMA sample, especially when limestone was used. Using sulfur made considerably more increase in the index P for the granite aggregates compared to the limestone. It is because of better compatibility of sulfur (Googas) with lime-base (limestone) than silicate-base (granite) materials to form bond with their aggregates. Obtained results by calculating P factor proved that using antistripping agent (NZ) was a successful method to reduce the moisture sensitivity of SEAs, especially when the granitic aggregate was applied. Such improvement happens, because, NZ is constituted of silanols (Si-OH) groups and siloxane bonds (Si–O–Si) forms between NZ and inorganic surface of aggregate which are made of silanol groups.

3.4. Mechanical moisture susceptibility tests Moisture susceptibility of asphalt mixtures could be predicted by SFE method and validated by mechanical tests [19], such as dynamic modulus (DM) and indirect tensile strength (ITS) tests. These two tests were carried out on different samples with and without sulfur and antistripping additive modification. Each sample was prepared in two categories of unconditioned and wet conditioned. As it was expected, wet conditioning led to lower ITS and dynamic modulus values compared to the unconditioned specimens. Such a lower strength was due to the weakened adhesion bond between binders and aggregates under the influence of wet conditioning and interaction between water and binder. In order to evaluate and compare the moisture susceptibility of different samples, wet/dry ratio was calculated for the both of DM and ITS values. In the Fig. 3, the tensile strength ratio (TSR) of all samples produced with two types of aggregates (limestone and granite) is illustrated. It is Comprehensible from this figure that mixtures made by limestone enjoyed lower susceptibility against wet condition due to having higher TSR values. This result is so closed to one

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463

Fig. 4. Dynamic modulus-wet/dry ratios via number of cycles (limestone).

Fig. 5. Dynamic modulus-wet/dry Ratios via number of cycles (granite).

that was obtained by the SFE method in the Section 3.1.3; Also, as it is shown in the Fig. 3, this ratio improved by addition of NZ as antistripping additive in ASWMA mixtures especially for limestone-mixture. In order to analyze and compare the effect of moist condition on the DM parameter of different samples (modified with sulfur and antistripping additive), the wet/dry (K) ratios of DM test values are illustrated In the Figs. 4 and 5. Higher values of K in these two figures equivalents to the better resistance against moisture susceptibly. If Figs. 4 and 5 be compared with each other, it could be understood that limestone-mixtures led to a higher K ratio rather than granite-mixtures. Also SWMA showed lower K ratio in comparison with CHMA, so had higher sensitivity against moist conditions. Almost all of the common aggregate types which could be used in asphalt mixtures are acidic. Bitumen that is the other ingredient of asphaltic mixtures is acidic too.

Therefore, in the presence of water, aggregates tend to make a bond with water instead of bitumen. As it was aforementioned, sulfur is a relatively acidic material that increases the acidity of asphalt binder and consequently makes the SEAs more susceptible than HMAs. While NZ as antistripping additive is a polar material which could make the binders more polar and leads formation of more powerful bond (siloxane bond) between bituminous binders and aggregates surface constituted of silanol groups. After breaking the alkoxy group, the chemical reaction between RSi and SiOH leads to the formation of Si–O–Si siloxane bond which is the most powerful bond that would be made. As a result, a bond will be formed between the pendant alkyl group and siloxane (chemically created compound from the silanol). The beneficial point of new formed alkyl group is its compatibility with bitumen ingredients like Maltene which could be advantageous in wetting process

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and consequently lower air space in asphalt mixtures; this improvement finally leads to reduced interfacial surface tension and better resistance against stripping phenomenon. As a result, addition of NZ as antistripping agent improves the K value that demonstrates the lower sensitivity of NZ-modified mixtures with and without sulfur modification. As illustrated in Figs. 4 and 5, The K value diagrams of NZ-modified SEA is close to the conventional mixture diagram. It demonstrates that adding NZ was a successful technique to improve deteriorated resistance of SEA against moisture susceptibility. All of these results by dynamic modulus and ITS tests were compatible with the obtained results by SFE method to a high extent. 4. Conclusion In this study NZ as antistripping agent was used to improve the moisture susceptibility of SEA. To evaluate the effectiveness of this modification, SFE which is relatively a new method was applied, while research team benefited two other mechanical tests (ITS and dynamic modulus tests) to investigate the validation of this new method. All the samples were constructed with two different aggregates (Limestone and Granite), therefore the effects of aggregate properties such as acidity level was considered and evaluated. The following conclusions can be drawn from the present study:
Since the sulfur is an acidic base material it increased the acid component of asphalt binder, on the other hand, sulfur decreased Lifshitz–van der Waals component of binder, so the bond between the sulfur modified binder and acidic aggregates weakened.
Addition of anti stripping agent (NZ) reduced the acid-to-base ratio in the binders and consequently made it more compatible with the acidic aggregates.
The adhesion energy between the Asphalt binder and the granite aggregates was greater in comparison with limestone in dry condition, but limestone showed lower free energy of adhesion in contact with water which made limestone-mixtures more resistant at wet condition compared with the granite-mixtures.
Calculated compatibility ratios (CR) demonstrated higher values for limestone-mixtures in comparison with granite-mixtures. It shows that limestone is more resistant aggregate against moisture damages and could be chosen as the selected aggregate.
NZ as antistripping agent improved the adhesion between asphalt binder and aggregates by increasing and decreasing of adhesion energy, in dry and wet conditions respectively.
According to the SFE test results such as compatibility ratio and also performed mechanical test, the best performance of asphalt mixtures against moisture damages acquired for AWMA, CHMA, ASWMA and SWMA respectively.
The percentage of aggregate in contact with the water (P) in different cycles of loading was lower for limestone compared to the granite. Moreover, sulfur extended mixtures (SWMAs) showed considerably higher P, while NZ modification (ASWMAs) improved this value to a magnitude near to the unmodified asphalt mixture.
The wet/dry ratios in dynamic modulus and ITS tests increased by addition of NZ as antistripping agent, with (ASWMA) and without (AWMA) sulfur modification. It proved that adding NZ was a successful technique to reduce the moisture susceptibility of asphalt mixtures.
Mechanical tests which were applied to validate the SFE method showed satisfying results that makes this method more reliable to be used as a practical way of moisture susceptibly evaluation.

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