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Using the surface free energy (SFE) method to investigate the effects of additives on moisture susceptibility of asphalt mixtures
International Journal of Adhesion and Adhesives 95 (2019) 102437
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Using the surface free energy (SFE) method to investigate the effects of additives on moisture susceptibility of asphalt mixtures
T
Zhang Deruna,∗, Rong Luob a b
Texas A&M Engineering Experiment Station (TEES), Texas A&M University System, 7607 Eastmark Drive, College Station, TX 77840, USA School of Transportation, Wuhan University of Technology, 1178 Heping Avenue, Wuhan, Hubei Province 430063, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Additives Asphalt mixtures Surface free energy (SFE) Adhesion Moisture susceptibility
Conventional methods for evaluating the effects of additives on moisture damage resistance of asphalt mixtures are either empirical or unable to quantify the contributions of material component properties to the overall mixture performance. To overcome these drawbacks, this study proposes a surface free energy (SFE) method to investigate the effects of various additives on moisture susceptibility of asphalt mixtures. One neat asphalt and one acidic gravel aggregate are selected as the test materials along with 6 commonly-used additives: a warm mix asphalt additive, two nano-materials, a hydrated lime, a Portland cement and a non-amine liquid asphalt antistripping agent. All the additives are blended with the neat asphalt to fabricate modified asphalt binders. The SFE components of these modified asphalt binders and the gravel aggregates are measured with the Wilhelmy plate method and the vapor adsorption method, respectively. An energy ratio, defined as the ratio of adhesion of asphalt-aggregate to that of asphalt-aggregate-water, is calculated and used to rank the asphalt mixtures that consist of gravel and asphalt binders modified with different additives in terms of their moisture susceptibility performance. In order to validate the proposed SFE method, two mixture moisture susceptibility tests, the modified boiling water test and the indirect tensile strength test, are conducted on loose and compacted asphalt mixtures, respectively. A consistent moisture susceptibility ranking is obtained from the SFE method and the mixture moisture susceptibility tests, which validates the SFE method proposed in this paper can be used to accurately quantify the effects of additives on the moisture susceptibility of asphalt mixtures.
1. Introduction Due to heavier traffic loading, higher traffic volume and severer environmental conditions, asphalt pavements constructed from the traditional asphalt materials are more and more vulnerable to premature distresses, such as rutting, fatigue cracking, shoving and raveling, etc. These premature distresses have detrimental effects on the pavement performance, which further result in a lower pavement serviceability and a higher maintenance (or rehabilitation) cost. In order to meet the practical demands in constructing asphalt pavements with high performance as well as low construction cost, there is an increasing trend in modifying asphalt with various additives. For instance, to reduce the susceptibility of asphalt mixtures to moisture damage, Xiao and Amirkhanian developed different types of liquid antistripped agent to strengthen the bonding between asphalt and aggregates [1]. For the purpose of improving rutting resistance and extending fatigue life of asphalt mixtures, Shafabakhsh and Ani attempted to modify asphalt with nano-sized materials (TiO2 and SiO2) due to
∗
their fairly high surface area and small size [2]. To decrease energy consumption and fuel emissions for the production of hot mix asphalt (HMA), wax mix asphalt (WMA) additive was used to fabricate the WMA asphalt mixture. This new type of additive has been verified to significantly lower asphalt viscosity, which thus effectively reduces the energy consumption [3]. Since all these additives are proven to be capable of enhancing one or multiple functional performance of asphalt pavements, studies of their effects on asphalt binders or asphalt mixtures have attracted an upward interest, among which the evaluation of additive effects on moisture-induce damage resistance of asphalt mixtures is a very importance aspect. Moisture-induce damage of asphalt mixtures is defined as a loss of bond between asphalt-aggregate or within asphalt cement attributable to the action of moisture. To characterize the effects of additives on mixture moisture susceptibility, various mechanical tests have been adopted so far. For instance, Kim et al. conducted the boiling water test and the pull-off tensile strength test on asphalt mixtures modified by anti-stripping additives [4]. Shu et al. used the standard tensile strength
Corresponding author. E-mail addresses: [email protected] (D. Zhang), [email protected] (R. Luo).
https://doi.org/10.1016/j.ijadhadh.2019.102437
Available online 18 September 2019 0143-7496/ Published by Elsevier Ltd.
International Journal of Adhesion and Adhesives 95 (2019) 102437
D. Zhang and R. Luo
ratio (TSR) test, the Superpave indirect tension (IDT) test and the Hamburg Wheel Tracking test on WMA containing recycled asphalt pavement (RAP) [5]. Kim et al. performed asphalt pavement analyzer (APA) test and semi-circular bend (SCB) fracture test to examine two widely used WMA approaches [6]. Overall, all these conventional tests emphasize comparing the properties change of asphalt mixtures before and after moisture condition, in terms of the adhesion loss in loose mixtures or the strength loss in compacted mixtures. They are relatively simple with only some know-how required. However, they are unable to address any mechanisms governing the moisture damage caused within asphalt mixtures. Moreover, they focus on providing an overall performance evaluation for the asphalt mixtures rather than correlating the mixture performance to the material properties, making it difficult to quantify the contribution of each component to the moisture damage. In contrast to the conventional methods, the surface free energy (SFE) method, as a newly emerging technology, has an ability to quantify the adhesion between asphalt-aggregate from the perspective of thermodynamics [7]. The surface free energy (SFE), by definition, is the revisable work done to create a unit area of new surface, which has a unit of mJ/m2. According to the Good-van Oss-Chaudhury (GvOC) theory [8], the SFE of a material is generally divided into two components: Lifshitz nonpolar component (γ LW ) and Lewis polar component (γ AB ). The Lewis polar component further includes the Lewis polar-acid component (γ + ) and the Lewis polar-base component (γ −). In this regard, the total SFE can be expressed in the form of SFE components as follows:
γ = γ LW + γ AB = γ LW + 2 γ +γ −
Fig. 1. Adhesive bond energy of asphalt mixture under dry and wet conditions.
(1)
Conventionally, the SFE components of asphalt binder and aggregate can be measured through the contact angle approach and the spreading pressure approach, respectively [9,10]. Accordingly, the GvOC models applied to these two approaches are expressed in Eq. (2) and Eq. (3), respectively.
γA+γL− +
(1 + cos θ) γL = 2( γALW γLLW +
2γL + πe(SL) = 2( γSLW γLLW +
γS+γL− +
γA−γL+ )
γS−γL+ )
(2)
signifying a better resistance of an asphalt mixture to moisture damage. Since the SFE method suffices to provide a direct evaluation of adhesion of asphalt mixtures in absence and presence of water from SFE properties of each material component, it has been widely used to optimize the asphalt-aggregate combination as well as to determine the optimum content of additive blended with the asphalt binder or aggregates [12].
(3)
2. Motivation and objectives
where γL is the total SFE of test liquid; θ is the contact angle between asphalt-test liquid; πe(SL) is the spreading pressure between aggregatetest liquid; and subscript “S”, “W”, “A” refer to as aggregate, water and asphalt binder, respectively. With the measured asphalt and aggregate SFE components, the adhesion of asphalt-aggregate under dry and wet conditions (as shown in Fig. 1(a)(b)) can be easily calculated from the SFE component interactions, which are expressed in Eq. (4) and Eq. (5), respectively. Based on the calculated adhesive values, an energy ratio (ER ) has been put forward to quantify the moisture susceptibility of asphalt mixtures, which is defined as the ratio of adhesion of asphalt-aggregate to that of asphalt-aggregate-water, as shown in Eq. (6) [11]. a ΔGSA = 2( γALW γSLW +
a ΔGSWA
ER =
γA+γS− +
γA−γS+ )
(4)
LW LW LW + + γALW γW − γSLW γALW − γW + γW ⎡ γSLW γW ⎤ ⎢ ⎥ − − − ⎥ = −2 ⎢ ( γS + γA − γW ) ⎢ ⎥ − + + + + − − + ⎢+ γW ( γS + γA − γW ) − γS γA − γS γA ⎥ ⎣ ⎦
(5)
a ΔGSA a ΔGSWA
(6)
a ΔGSA ,
In the context of using the SFE method to evaluate the moisture susceptibility of asphalt mixtures, the first step is to measure the SFE components of asphalt binder and aggregate. As shown in Eq. (2) and Eq. (3), normally, three test liquids, forming a liquid set, along with their measured contact angle values (spreading pressure values) on the same asphalt (aggregate), can be used to determine the three asphalt (aggregate) SFE components. In the aggregate SFE measurements, a fixed set of three liquids (distilled water, methyl propyl ketone (MPK) and toluene) are usually selected. With respect to the asphalt SFE measurements, five liquids (i.e. distilled water (W), formamide (F), glycerol (G), ethylene glycol (E) and diiodomethane (D)) are commonly used to design different liquid sets to determine the asphalt SFE components. The determined asphalt SFE components, however, have been reported to depend strongly on the liquid set selected [13], which further result in the discrepant values of ER for the same asphalt mixtures. Thus, the current SFE method is unable to evaluate the mixture moisture susceptibility accurately due to its weakness in obtaining consistent energy ratio. To overcome this drawback, the authors have proposed a novel method to ensure the consistent energy ratio in previous studies [14,15]. In this method, two liquid sets (“WFED” and “WFGED”) are first screened out as the optimum liquids sets for the determination of asphalt SFE components using the GvOC model. The two adhesion values and the energy ratio are then calculated based on the Owens-Wendt (OW) model [16]. This method has been successfully used to evaluate the moisture susceptibility of asphalt mixtures and asphalt mastics. However, its feasibility was verified with limited types of asphalt binder, in particularity with only two modified asphalt binders [14]. More types of modified asphalt
a ΔGSWA
where are the adhesion of asphalt mixtures under dry and wet conditions, respectively. In theory, a higher value of ER corresponds to a higher adhesion between asphalt-aggregate and a lower energy release of the mixture system due to the replacement of asphalt by water on aggregates, 2
International Journal of Adhesion and Adhesives 95 (2019) 102437
D. Zhang and R. Luo
binder are still required to fully evaluate its feasibility. Considering that different types of additive can be used to fabricate various types of modified asphalt binder, this study will attempt to use the proposed SFE method to investigate the effects of additives on the moisture susceptibility of asphalt mixtures. Six different types of additive will be selected as the representative additive materials, all of which are prevalent in improving one or multiple performance of asphalt mixtures. In summary, this study aims at assessing the effects of additive on the moisture susceptibility of asphalt mixtures with a proposed SFE method, which includes:
Table 2 Chemical compositions of the gravel aggregate.
(1) Measure the SFE components of the selected asphalt with and without the additives as well as the SFE components of the selected aggregates; (2) Quantify the effects of additives on the mixture moisture susceptibility based on the adhesion values and the energy ratio determined from the proposed SFE method; (3) Evaluate the feasibility of the proposed SFE method through the laboratory mixture moisture susceptibility test results.
Chemical composition of aggregates
Value
Sodium oxide, Na2O (%) Silicon dioxide, SiO2 (%) Aluminium oxide, Al2O3 (%) Ferric oxide, Fe2O3 (%) Sulphur trioxide, SO3 (%) Calcium oxide, CaO (%) Magnesium oxide, MgO (%) Insoluble residue (%)
2.077 87.495 6.096 1.621 0.061 0.276 0.032 0.882
Table 3 Properties and percentage of each type of additive. Additive
Percentage
SW
0.5%
SiO2
5%
Property value: Neutral • PH point: 285 °C • Flash 99.9% • Purity: surface area: 240 m /g • Specific grain size: 20 nm • Average 2.40 g/cm • Density: 99.9% • Purity: surface area: 40 m /g • Specific grain size: 25 nm • Average 1.15 g/cm • Density: 95.0% • Purity: 2.21 g/cm • Density: 2.94 g/cm • Density: 0.92 g/ml • Density: • Water content (105 °C, 1 h): 0.02% 2
3. Test materials
3
ZnO
4%
2
3.1. Asphalt and aggregates
3
A neat #70 asphalt (graded based on the penetration value) and a gravel aggregate are selected as the test materials in this study. Asphalt mixtures prepared with such two materials are moisture susceptible, which will facilitate the comparison of different additives in term of their effectiveness in improving moisture damage resistance of asphalt mixtures. The properties of the #70 asphalt and the chemical compositions of the gravel aggregate are summarized in Table 1 and Table 2, respectively.
4.1.1. Wilhelmy plate test The Wilhelmy plate test is used to measure the SFE components for each type of asphalt binder with the aid of a high-precision tensiometer (KRÜSS GmbH, Hamburg, Germany). The testing principle of this method is to measure the dynamic contact angle between the asphalt binder and the test liquid based on a kinetic force balance. As shown in Fig. 2, the microbalance of the equipment constantly measures the force applied to the asphalt-coated plate when the plate is advancing into the liquid at a constant rate of 3 mm/min. The test temperature is controlled at 20 ± 0.5 °C by the thermostat and monitored by the temperature sensor. Then the contact angle between the asphalt and the test liquid can be calculated as:
Table 1 Conventional properties of the selected asphalt.
N70* Specification
65 60–80
47.3 ≥46.0
Ductility (cm) 5 cm/min, 15 °C
88 ≥20
150 ≥100
3
4.1. Surface free energy (SFE) measurement tests
In order to obtain a homogenous modified asphalt, a high-speed
5 cm/min, 10 °C
5% 0.4%
4. Laboratory testing
3.3. Preparation of asphalt binder modified with additives
Softening point (°C)
PC NA
shearing method is utilized to add the additive into the neat asphalt with the aid of a high-shear mixer (JRJ 300-S, Shanghai, China). First, the neat asphalt is heated at 150 °C for 30 min in the mixer. Then a certain amount of additive that satisfies the requirement shown in Table 3 is added into the mixer and blended with the asphalt at a shear rate of 4000 rmp for 10 min. Subsequent to the completion of the mixing process, the asphalt binder modified with each type of additive is finally fabricated. In this study, the modified asphalt binders are labeled by the source followed the type of the additive. For instance, the modified asphalt of “N70 + SW” refers to as the neat #70 asphalt modified with 0.5% of Sasobit® wax. For the purpose of comparison, the neat #70 asphalt binder is used as the control asphalt in this study.
A total of 6 different types of additives are selected as the additive materials, including a WMA additive (Sasobit® wax (SW)) (AkzoNobel, China), two nano-materials (nano SiO2 and nano ZnO) (Jingbei Company, China), a hydrated lime (HL) (Xingyinhe Chemical Company, China), a Portland cement (PC) (Wuhan Wuganghuaxin Cement Co., Ltd., China), and a non-amine liquid asphalt anti-stripping agent (NA) (Chongqing Haimu Traffic Technology Co., Ltd., China). All these additives have previously been used as the modifiers to enhance the performance of asphalt mixtures, of which properties are summarized in Table 3. To facilitate the evaluation of the effects of each additive on the moisture susceptibility of asphalt mixtures, all the selected additives are mixed with the neat #70 asphalt binder to fabricate the modified asphalt binders rather than being mixed with the asphalt mixtures. As recommended by the manufacture and reported in literature, percentage of each type of additive by weight of bitumen is also included in Table 3.
Penetration (100 g, 5s, 25 °C)
5%
3
3.2. Additives
Asphalt
HL
ΔF + abhρL g ⎤ θ = arccos ⎡ ⎢ 2(a + b) γ ⎥ L ⎦ ⎣
(7)
where θ is the dynamic contact angle between the asphalt binder and the test liquid; a, b are the width and thickness of the asphalt-coated plate; ρL is the liquid density; γL is the liquid surface tension; g is the
Note: N70* stands for the selected neat #70 asphalt. 3
International Journal of Adhesion and Adhesives 95 (2019) 102437
D. Zhang and R. Luo
Fig. 2. Dynamic contact angle measured from Wilhelmy plate test.
acceleration of gravity; and ΔF is the force measured by the microbalance at an immersion height of h . Based on the measured contact angle and the known SFE components of the test liquids, the GvOC model is used to determine the SFE components of asphalt binder. More details about this method can be referred to elsewhere [10]. As stated previously, in order to compute the three SFE components (i.e. γALW , γA+, γA−) of asphalt binder, at least three test liquids with the measured contact angles are required to establish an equation set based on Eq. (2). In this study, the five commonly-used test liquids (W, F, G, E and D) are selected as the test probes. Their SFE components at 20 °C are listed in Table 4. The contact angle (θ ) between each liquid and each asphalt binder is measured three times and the average of the three contact angle values is calculated as the final contact angle value, which is summarized in Table 5. In order to evaluate the test repeatability, the standard deviation (σ) of these three measured contact angle values for each asphalt-liquid combination is also computed. The small σ value (maximum is only 3.22°) shown in Table 5 indicates that the Wilhelmy plate test conducted in this study has a fairly good repeatability.
πe(SL) =
W F G E D
γL−
γLAB
γL
21.8 39.0 34.0 29.0 50.8
25.5 2.28 3.92 1.92 0
25.5 39.6 57.4 47.0 0
51.0 19.0 30.0 19.0 0
72.8 58.0 64.0 48.0 50.8
n dp p
(8)
kD
(9)
where nD, B, kD are the model coefficients of the DA model. Prior to determining the spreading pressure, another important parameter, the specific surface area of the aggregates must be calculated, which is accomplished through the modified Brunauer-EmmettTeller (M-BET) model as follows [19]:
p (c′ − 1) p 1 = + n (k 0 p0 − p) c′n m′ k 0 c′n m′ p0
(10)
where k 0 is a constant; p0 is the saturated vapor pressure; c′ is a constant related to the adsorption heat; and n m′ is the monolayer capacity of the vapor molecules adsorbed on the aggregates. In the M-BET model, the constant k 0 is first determined based on a linear fit of the plot of p /[n (k 0 p0 − p)] against p / p0 in the entire relative pressure range. Assuming that the vapor molecules adsorbed on the aggregates are hexagonal closest packing, the specific surface area used for the spreading pressure calculation can be then obtained as: 2
A=
I ′ (k 0 − 1) 1.091NA M ⎞3 ⋅⎛⎜ − ⎟ M (k 0 K ′ + I ′) ⎝ NA ρL ⎠ k 0 (K ′ + I ′)(k 0 K ′ + I ′)
(11)
where NA is the Avogadro constant; K ′ and I ′ are the slope and intercept of the linearly fitting line on the plot of p /[n (k 0 p0 − p)] against p / p0 , respectively. Combing Eqs. (8), (9) and Eq. (11), spreading pressure between the aggregate and the probe liquid can be calculated. Then substituting the calculated spreading pressure into the GvOC equation (Eq. (3)) [9], the three SFE components of the aggregate can be finally determined. As mentioned previously, distilled water, MPK, and toluene are commonly selected as the three probe vapors in vapor adsorption test. Table 6 presents the SFE components of the latter two liquids, whereas those of the distilled water have been given in Table 4. The vapor adsorption isotherms of these three probe vapors with respect to the gravel aggregates are exhibited in Fig. 4, which are the average values from the three replicates.
SFE Components ( × 10−3 J/m2)
γL+
p0
p ⎧ ⎫ n = nD exp −B ⎡ln ⎛⎜ 0 ⎞⎟ ⎤ ⎢ ⎥ ⎬ ⎨ p ⎝ ⎠ ⎣ ⎦ ⎭ ⎩
Table 4 SFE components of the probe liquids at 20 °C [17].
γLLW
∫0
where R is the universal gas constant; T is the test temperature; M is the molar mass of the probe vapor; A is the specific surface area of the aggregates; n is the total capacity per unit adsorbent mass of the adsorbed probe vapor molecules; and p is the vapor pressure. For aggregate, its vapor adsorption isotherm is modeled using the Dubinin-Astakhov (DA) equation, which is [9]:
4.1.2. Vapor adsorption test The vapor adsorption test is employed to measure the SFE components of the gravel aggregates, which is completed through the Gravimetric Sorption Analyzer (IsoSORP® STATIC, RUBOTHERM GMBH, Bochum, Germany). As illustrated in Fig. 3, in this test the dry aggregate samples stored in a container are placed in a seal test chamber under vacuum condition. Then a probe vapor is released into the chamber and stabilized at 10 different vapor pressures at the test temperature of 20 °C. After that, the saturated amount of vapor adsorbed on the aggregates corresponding to each vapor pressure is measured by the magnetic suspension balance and then used to construct a vapor adsorption isotherm. Based on this measured vapor adsorption isotherm, the spreading pressure of the aggregate-vapor πe(SL) can be calculated using the Gibbs adsorption equation [18]:
Test Liquid
RT MA
4
International Journal of Adhesion and Adhesives 95 (2019) 102437
D. Zhang and R. Luo
Table 5 Wilhelmy plate test results of asphalt-liquid. Asphalt Binder
N70 N70 N70 N70 N70 N70 N70
+ + + + + +
W (°)
SW SiO2 ZnO HL PC NA
F (°)
G (°)
E (°)
D (°)
θ
σ
θ
σ
θ
σ
θ
σ
θ
σ
103.49 96.59 104.52 100.12 93.79 101.58 88.00
0.38 1.28 0.09 0.39 0.91 0.18 0.13
85.55 87.37 86.07 85.05 80.65 86.17 71.25
0.01 0.01 0.54 0.04 0.30 1.29 0.21
89.51 89.21 90.30 88.94 88.28 90.19 81.44
0.12 2.34 0.21 0.02 0.24 0.11 2.60
74.83 74.74 74.93 74.55 74.30 76.94 64.80
0.03 0.35 0.68 0.13 0.74 0.91 0.53
81.00 77.71 81.62 85.17 93.09 80.64 90.23
2.98 3.22 0.57 1.80 1.34 0.16 0.11
4.2. Moisture susceptibility test
Table 6 SFE components of the probe vapors at 20 °C [15].
4.2.1. Modified boiling water test The boiling water test in accordance with ASTM D3625 is employed to evaluate the moisture susceptibility of loose asphalt mixtures that consists of gravel aggregates and different types of asphalt binder [20]. In this test, the asphalt-coated aggregate mixture is conditioned in the boiling water and then a visual observation is conducted to evaluate the loss of adhesion in the loose asphalt mixture due to the boiling water action. To further quantify this adhesion loss, an image analysis method is adopted to modify the boiling water test [14]. More specifically, the asphalt-coated area and the total area of 10 selected asphalt-coated gravel particles are measured through an image processing software. Then the percentage P of the remaining asphalt are calculated using Eq. (12). A larger value of P indicates less asphalt is stripped from the aggregate, which therefore corresponds to a low moisture sensitivity of an asphalt mixture.
Test Liquid
MPK Toluene
SFE Components ( × 10−3 J/m2)
γLLW
γL+
γL−
γLAB
γL
21.7 28.3
0 0
19.2 2.7
0 0
21.7 28.3
20
P=
∑ j = 1 Aj′ 20
∑ j = 1 Aj
× 100% (12)
where Aj and Aj′ are the total surface area and the asphalt-coated area of the jth picture of the asphalt-coated aggregate particle, respectively. 4.2.2. Indirect tensile strength (ITS) test The indirect tensile strength (ITS) test, following the guideline specified in ASTM D4867 [21], is employed to evaluate the moisture susceptibility of the compacted asphalt mixture in this study. A type of AC-13 (nominal aggregate size is 13 mm) is selected as the aggregate gradation. The optimum asphalt content for each gravel-modified asphalt mixture is determined through the Marshall design method in accordance with the ASTM D6927 [22]. Controlling the binder content as the optimum one is to minimize the impacts of structures and
Fig. 4. Vapor adsorption isotherms of three probe vapors.
volumetrics on the performance of asphalt mixtures so as to maximize those of the material properties, which facilitates the study of the additive effects on the mixture moisture susceptibility. Given the specific aggregate gradation and the optimum asphalt content, a set of compacted asphalt mixture specimens with 7 ± 0.5% air voids are then
Fig. 3. Schematic illustration of the vapor adsorption test. 5
International Journal of Adhesion and Adhesives 95 (2019) 102437
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Fig. 5. Tensile strength of asphalt mixture measured from the ITS test.
fabricated, which are divided into two subsets of at least three specimens each. One subset is unconditioned and used as the control set, while the other subset is partially saturated with water to a saturation of 55–88% and then undergoes a freeze-thaw cycle. After that, a tensile splitting tester is used to measure the tensile strength for each subset of the asphalt mixture samples, as shown in Fig. 5. The tensile strength ratio (TSR) is finally calculated and used to evaluate the moisture susceptibility of the compacted asphalt mixture, which is given by:
TSR =
Stm × 100% Std
(13)
where Stm and Std are the average tensile strength of the moistureconditioned subset and that of the dry subset, respectively. 5. Results and discussion 5.1. Surface free energy (SFE) determination based on the proposed method
Fig. 6. Variations of SFE of N70 with the liquid set.
5.1.1. Surface free energy (SFE) of asphalt binders As reported in the authors' previous paper [14], the SFE components of asphalt binder determined from the GvOC model are dependent strongly on the liquid set selected from the five commonly-used probe liquids (W, F, G, E and D). Different liquid sets result in discrepant SFE values for the same asphalt binder. This problem also arises in this study when adopting different liquid sets to determine the SFE components for the same investigated asphalt binder. Fig. 6 gives an example to show the variations of the SFE of N70 with the liquid set. This kind of dependency will further result in the significant variation of the energy ratio that is calculated based on the asphalt SFE components. As mentioned previously, to ensure the consistent energy ratio, a proposed method developed by the authors is adopted in this study [14]. In that method, “WFED” and “WFGED” are first selected as the optimum liquid sets. The contact angle between each liquid and the asphalt binder is then measured through the Wilhelmy plate method. Eq. (2) is finally used to establish an overdetermined equation set to determine the SFE of asphalt binder. In order to reduce the computational efforts, only “WFGED” is selected in this study as the optimum liquid set when calculating the SFE components for all the 7 types of asphalt binder. The calculation results are summarized in Table 7. Apparently, compared to the nonpolar component, the polar components are relatively small for all the investigated asphalt binders, which is plausible because asphalt is intrinsically composed of high molecular
Table 7 Determined SFE of the investigated asphalt binders. Asphalt Binder
N70 N70 N70 N70 N70 N70 N70
+ + + + + +
SW SiO2 ZnO HL PC NA
SFE Components ( × 10−3 J/m2)
Ratio
γALW
γA+
γA−
γAAB
γA
γA+/ γA−
16.75 18.22 16.44 14.76 11.56 16.95 12.95
0.79 0.20 0.83 1.01 1.63 0.48 2.72
0.71 3.89 0.53 1.88 5.29 1.57 6.10
1.49 1.75 1.32 2.76 5.87 1.74 8.14
18.24 19.98 17.77 17.52 17.43 18.69 21.09
1.11 0.05 1.57 0.54 0.31 0.31 0.45
weight hydrocarbons, thus exhibiting little polar activity [23]. The ratio of the polar-acid component to the polar-base component is also included into Table 7, which will be used for the analysis of the effects of additives on the moisture susceptibility of the asphalt mixture in the following sections.
5.1.2. Surface free energy (SFE) of aggregates In order to determine the SFE components of the selected gravel aggregates, the DA model (Eq. (9)) is first utilized to model the 6
International Journal of Adhesion and Adhesives 95 (2019) 102437
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Table 9 Determined SFE components of gravel aggregates. Aggregate
Gravel
SFE Components ( × 10−3 J/m2)
γSLW
γS+
γS−
γSAB
γS
79.45
10.71
305.39
114.38
193.83
5.2. Energy ratio calculation In order to determine the energy ratio, two types of adhesive bond energy shown in Eq. (6) have to be calculated. As stated before, the SFE components of asphalt binder calculated from the GvOC model possesses the dependency of the liquid set selected, which further results in the dependency of the adhesive bond energy on the liquid set, making it impossible to obtain the consistent energy ratio. To overcome this liquid set dependency, an approach put forward by the authors in a previous study is employed to calculate the adhesive bond energies based on the Owens-Wendt (OW) model [16]:
Fig. 7. Modeling of adsorption isotherms using the DA model.
1
1
1
1
AB AB LW LW + − + − a LW LW ΔGSA( p) = 2 ⎡ (γA γS ) 2 + (γA γS ) 2 ⎤ = 2 ⎡ (γA γS ) 2 + 2(γA γA γS γS ) 4 ⎤ ⎦ ⎣ ⎣ ⎦ (14)
1
1
1
LW LW LW LW LW a LW LW ΔGSWA( p) = 2 ⎡ (γS γW ) 2 + (γA γW ) 2 − (γA γS ) 2 − 2γA ⎤ ⎣ ⎦ 1
1
1
1
+ − + − γW ) 4 − (γA+γA−γW γW ) 4 ⎤ − 4 ⎡ (γA+γA−γS+γS−) 4 + (γA+γA−) 2 − (γS+γS−γW ⎣ ⎦
a a where ΔGSA( p) , ΔGSWA(p) are the adhesive bond energy of asphalt-aggregate subjected to dry and wet conditions, respectively, both of which are calculated using the proposed method. It is to be noted that the SFE components of asphalt binder shown in Eqs. (14) and (15) are determined using the liquid set of “WFGED” as the optimum liquid set. Based on the calculated adhesive bond energies, the energy ratio (ER (p) ) in the proposed SFE method can be then determined through:
Fig. 8. Linear fitting results based on the M-BET model. Table 8 Determined specific surface area and spreading pressure of gravel. Parameter
2
A (m /g) πe(SL) ( × 10−3 J/m2)
Probe Vapor Distilled Water
MPK
Toluene
2.2895 147.18
1.0107 68.62
0.4578 48.99
(15)
ER (p) =
measured adsorption isotherms of the three probe vapors shown in Fig. 4. In the meantime, the M-BET model (Eq. (10)) is used to linearly fit the plot of p /[n (k 0 p0 − p)] versus p / p0 in the full range of relative vapor pressure. The fitting results of these two models are separately shown in Fig. 7 and Fig. 8, both of which have an R2 value great than 0.99, demonstrating the excellent fitting performance of the DA model and the M-BET model. It can also be seen from Fig. 7 that the saturated adsorption amount of vapor on the gravel aggregates is ranked as: distilled water > MPK > toluene, which is consistent with those observed from other aggregate resources [19]. Compared to toluene, MPK is observed to have a lower value of total SFE (or nonpolar SFE component) but a higher saturated adsorption amount on gravel aggregates. The possible reason accounting for this observation is that the interaction between aggregate and the test liquid vapor is polarity-dominant because the polar base component (γL−) of MPK is significantly larger than that of toluene. Based on all these fitting results, Eqs. (8)–(11) are used to calculate the specific surface area and the spreading pressure of gravel with each probe vapor. The calculation results are summarized in Table 8. Finally, the SFE components of the gravel aggregates are determined by substituting the calculated spreading pressure of the three probe vapors into Eq. (3). The determined SFE components of gravel are summarized in Table 9. It is obvious that the polar component of gravel aggregates is greater that its nonpolar component, which substantiates that the polar interaction is predominant between the gravel aggregates and the probe liquid vapors.
a ΔGSA( p) a ΔGSWA( p)
(16)
The SFE components of the asphalt binders measured from the Wilhelmy plate test (Table 7), along with those of the gravel aggregates measured from the vapor adsorption test (Table 9), are combined together to calculate the adhesive bond energy of each asphalt-gravel combination under both dry and wet conditions using Eqs. (14) and (15). Then the energy ratio (ER (p) ) of each asphalt-aggregate combination can be determined through Eq. (16). All the calculation results are summarized in Table 10. To examine the applicability of the traditional GvOC method, the adhesive bond energies and the energy ratio (ER ) calculated from the traditional SFE method based on Eqs. (4)–(6) are also included in Table 10. As mentioned previously, a higher ratio of ER indicates the lower moisture susceptibility of an asphalt mixture. Thus, when using the energy ratio as an index to evaluate the moisture susceptibility, the different asphalt-gravel combinations can be ranked based on the proposed method and the traditional method. However, as shown in Table 10, these two methods produce distinct rankings in this study. More specifically, the ranking derived from the proposed method is: N70+SiO2 > N70 > N70+PC > N70+ZnO > N70+SW > N70+HL> N70 + NA; while the ranking from the traditional method is: N70 + SW > N70 + PC > N70 > N70 + SiO2 > N70 + ZnO > N70 + HL > N70 + NA. To further validate the proposed method, the SFE method results are compared with the mixture moisture susceptibility test results, which is detailed in the next section. 7
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Table 10 Calculation results of the adhesive bond energies and the energy ratio. Gravel asphalt mixture
N70 N70 N70 N70 N70 N70 N70
+ + + + + +
SW SiO2 ZnO HL PC NA
Adhesive Bond Energy ( × 10−3 J/m2)
Energy Ratio
a ΔGSA( p)
a ΔGSA
a ΔGSWA( p)
a ΔGSWA
ER (p)
ER
99.11 104.42 96.87 104.01 112.43 101.60 125.19
109.54 104.63 108.89 112.59 120.29 105.81 137.96
−46.96 −44.74 −47.80 −45.97 −44.31 −46.07 −39.56
−93.34 −106.84 −92.71 −94.46 −94.76 −99.47 −84.42
2.1106 2.3338 2.0267 2.2624 2.5373 2.2057 3.1648
1.1735 0.9793 1.1745 1.1919 1.2693 1.0638 1.6341
Table 11 Test results of the boiling water test and ITS test. Gravel asphalt mixture
N70 N70 N70 N70 N70 N70 N70
+ + + + + +
SW SiO2 ZnO HL PC NA
Boiling Water Test
Indirect Tensile Strength Test
∑ Aj′ (cm2)
∑ Aj (cm2)
P (%)
Stm (MPa)
Std (MPa)
TSR (%)
20.1811 8.2137 23.8158 10.0249 4.9691 12.7900 3.5913
81.1753 74.9943 74.0596 82.0736 72.0374 72.8072 73.5284
75.14 89.05 67.84 87.79 93.10 82.43 95.12
0.700 0.821 0.537 0.978 0.943 0.781 0.716
0.973 0.977 0.761 1.183 1.091 0.991 0.804
72.01 84.04 70.63 82.66 86.45 78.80 89.12
five additives helps mitigate the susceptibility of asphalt mixture to moisture to a certain degree. Among them, the non-amine liquid asphalt anti-stripping agent (NA) is the most effective one, which leads to the remarkably higher improvement in comparison with the other additives. A possible reason is that the chemical composition of this additive mainly consists of the hydrophobic hydrocarbon groups and the phosphorous hydroxyl group, both of which work together to form a strong chemical bond between asphalt-aggregate. As shown in Fig. 10, when this kind of additive is added into asphalt and contacted with gravel aggregate, its hydrocarbon “tails” molecules will progressively anchor into the asphalt, while its phosphorous hydroxyl group (positive sites) will bind strongly to the silanol groups (negative sites) on the aggregate surface. Accordingly, the polar interactions between asphaltaggregate are significantly strengthened, which can be inferred from the increase of Lewis polar component (γAAB ) of asphalt binder (from 1.49 to 8.14 J/m2). These stronger polar interactions are deemed to intensify the chemical bonding at the interface of asphalt-aggregate, thus enhancing the moisture damage resistance of the gravel asphalt mixtures. In addition, nano SiO2 is found to be the only additive lowering the moisture damage resistance of the gravel asphalt mixture. To gain better understanding of the mechanism of effects of this additive on the moisture susceptibility, Fig. 11 exhibits the adhesive bond energies of gravel in combination with each type of modified asphalt binder in the presence and absence of water. It is clear that nano SiO2 is the only a additive decreasing the dry adhesive bond energy ( ΔGSA ) of the N70 control asphalt mixture while increasing the wet adhesive bond energy a ( ΔGSWA ). One possible explanation is that the neat #70 asphalt binder is somewhat acidic in nature, whose acid component is 0.79 mJ/m2 and the base component is 0.71 mJ/m2. Normally, a good adhesion is difficult to be obtained when such an acidic asphalt is coated onto the acidic gravel aggregate, as reported by Ghabchi et al. [24]. As shown in Table 7, the acidic nano SiO2 is found to strengthen the acidity of the N70 asphalt based on the facts that it increases the acid component of the N70 asphalt by 5% and decreases its base component by 26%, resulting in an increased ratio of γA+/ γA− from 1.11 to 1.57. It thus lowers the adhesion between asphalt-aggregate. Except for this nano SiO2, all the other additives are found to lower the acidity of the N70 asphalt with the decreased ratios of γA+/ γA− when using the control asphalt as
5.3. Comparison with moisture susceptibility test results The boiling water test results as well as the ITS test results for all the gravel asphalt mixtures are listed in Table 11 and presented in a bar graph of Fig. 9. To gain a visual comparison, the energy ration results of ER(p) and ER shown in Table 10 are also included in this bar graph. Theoretically, higher values of TSR and P indicate the less loss of adhesion in asphalt mixtures in presence of water and therefore the lower moisture susceptibility of the asphalt mixtures, suggesting a higher energy ratio. It can be easily identified from Fig. 9 that the rankings of both TSR and P are in agreement with the ER(p) ranking rather than the ER ranking. As such, in contrast to the traditional GvOC method, the proposed SFE method shows capability to accurately characterize the effects of additive on the moisture susceptibility of asphalt mixtures. 5.4. Analysis of additive effects on mixture moisture susceptibility As observed from Fig. 9, different additives provide the distinct effectiveness in enhancing the moisture damage resistance of asphalt mixtures is distinct. Except for nano SiO2, the addition of the remaining
Fig. 9. Test results of TSR, P and ER for each gravel-asphalt combination. 8
International Journal of Adhesion and Adhesives 95 (2019) 102437
D. Zhang and R. Luo
Fig. 10. Anti-stripping additive act as bridge between asphalt-gravel aggregate.
structure and functional groups of each modified asphalt binder are currently being investigated through the scanning electron microscopy (SEM) test and the Fourier-transform infrared spectroscopy (FTIR) test, respectively. Acknowledgements The authors acknowledge the financial support of the Ministry of Transport of China (Project No. 2014318J22120). Special thanks are to the 1000-Youth Elite Program of China for the start-up funds for purchasing the laboratory equipment that is crucial to this research. References [1] Xiao F, Amirkhanian SN. Effects of liquid antistrip additives on rheology and moisture susceptibility of water bearing warm mixtures. Constr Build Mater 2010;24(9):1649–55. [2] Shafabakhsh GH, Ani OJ. Experimental investigation of effect of Nano TiO2/SiO2 modified bitumen on the rutting and fatigue performance of asphalt mixtures containing steel slag aggregates. Constr Build Mater 2015;98:692–702. [3] Oliveira JRM, Silva HMRD, Abreu LPF, Fernandes SRM. Use of a warm mix asphalt additive to reduce the production temperatures and to improve the performance of asphalt rubber mixtures. J Clean Prod 2013;41:15–22. [4] Kim YR, Pinto I, Park SW. Experimental evaluation of anti-stripping additives in bituminous mixtures through multiple scale laboratory test results. Constr Build Mater 2012;29:386–93. [5] Shu X, Huang B, Shrum ED, Jia X. Laboratory evaluation of moisture susceptibility of foamed warm mix asphalt containing high percentages of RAP. Constr Build Mater 2012;35:125–30. [6] Kim YR, Zhang J, Ban H. Moisture damage characterization of warm-mix asphalt mixtures based on laboratory-field evaluation. Constr Build Mater 2012;31:204–21. [7] Hamedi GH, Tahami SA. The effect of using anti-stripping additives on moisture damage of hot mix asphalt. Int J Adhesion Adhes 2018;81:90–7. [8] Oss CJV, Chaudhury MK, Good RJ. Interfacial Lifshitz-van der Waals and polar interactions in macroscopic systems. Chem Rev 1988;88(6):927–41. [9] Zhang D, Luo R. Modeling of adsorption isotherms of probe vapors on aggregates for accurate determination of aggregate surface energy components. Constr Build Mater 2017;134:374–87. [10] Luo R, Zhang D, Zeng Z, Lytton RL. Effect of surface tension on the measurement of surface energy components of asphalt binders using the Wilhelmy Plate Method. Constr Build Mater 2015;98:900–9. [11] Xu W, Luo R, Zhang K, et al. Experimental investigation on preparation and performance of clear asphalt. Int J Pavement Eng 2019;19(5):416–21. [12] Arabani M, Hamedi GH. Using the surface free energy method to evaluate the effects of liquid antistrip additives on moisture sensitivity in hot mix asphalt. Int J Pavement Eng 2013;15(1):66–78. [13] Volpe CD, Siboni S. Acid-case surface free energies of solids and the definition of scales in the Good-van Oss-Chaudhury Theory. J Adhes Sci Technol 2000;14(2):235–72. [14] Zhang D, Liu H. A proposed approach for determining consistent energy parameters based on the surface free energy theory. J Mater Civ Eng 2018;30(11):04018287. [15] Zhang D, Luo R, Zeng Z. Characterization of surface free energy of mineral filler by spreading pressure approach. Constr Build Mater 2019;218:126–34. [16] Owens DK, Wendt RC. Estimation of the surface free energy of polymers. J Appl Polym Sci 1969;13(8):1741–7. [17] Oss CJV. Interfacial forces in aqueous media. second ed. New York, USA: CRC Press; 2006. [18] Jura G, Harkins WD. Surface of solids. XI. Determination of the decrease (π) of free surface energy of a solid by an adsorbed film. J Am Chem Soc 1944;66(8):1356–62. [19] Zhang D, Luo R. Modifying the BET model for accurately determining specific surface area and surface energy components of aggregates. Constr Build Mater 2018;175:653–63. [20] ASTM. Standard practice for effect of water on bituminous-coated aggregate using boiling water. West Conshohocken, PA: ASTM D3625; 2012. [21] ASTM. Standard test method for effect of moisture on asphalt concrete paving mixtures. West Conshohocken, PA: ASTM D4867; 2014. [22] ASTM. Standard test method for Marshall stability and flow of asphalt mixtures. West Conshohocken, PA: ASTM D6927; 2015. [23] Hamedi GH, Nejad FM, Oveisi K. Estimating the moisture damage of asphalt mixture modified with nano zinc oxide. Mater Struct 2016;49(4):1165–74. [24] Ghabchi R, Singh D, Zaman M, Tian Q. Mechanistic evaluation of the effect of WMA additives on wettability and moisture susceptibility properties of asphalt mixes. J Test Eval 2013;41(6):933–42.
Fig. 11. Adhesive bond energy of asphalt-grave under dry and wet conditions.
reference, which further leads to a higher dry adhesive bond energy and a lower wet adhesive bond energy for the gravel asphalt mixtures and therefore a greater energy ratio and the better resistance to moisture damage. Thus, from the perspective of SFE theory, it may be concluded that decreasing the ratio of γA+/ γA− of asphalt binder is beneficial to the moisture damage resistance of asphalt mixtures containing a given type of aggregates. 6. Conclusions and on-going work This paper presents the use of a proposed surface free energy (SFE) method to investigate the effects of six commonly-used additives (Sasobit® wax (SW), nano SiO2, nano ZnO, hydrate lime (HL), Portland cement (PC), and a non-amine liquid asphalt anti-stripping agent (NA)) on the moisture susceptibility of asphalt mixtures. The main conclusions include: (1) The proposed SFE method involves using the optimum liquid set (“WFGED” or “WFED”) to calculate the asphalt SFE components based on the GvOC model (Eq. (2)) as well as utilizing the OW model to determine the adhesive bond energies (Eqs. (14) and (15)) and the energy ratio (Eq. (16)) of the asphalt mixtures; (2) Ranking of the moisture damage resistance of asphalt mixtures measured from the mixture moisture susceptibility tests is consistent with that of the energy ratio results determined from the proposed SFE method. This validates that the proposed SFE method can be used to accurately quantify the effects of additives on the moisture susceptibility of asphalt mixtures; (3) From the perspective of SFE theory, lowering the ratio of γA+/ γA− of asphalt binder may be helpful towards the enhancement of the moisture damage resistance of asphalt mixtures containing a given type of aggregates. To gain a better understanding of the mechanisms of the additive effects on the mixture moisture damage resistance, the changes of
9
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International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Using the surface free energy (SFE) method to investigate the effects of additives on moisture susceptibility of asphalt mixtures
T
Zhang Deruna,∗, Rong Luob a b
Texas A&M Engineering Experiment Station (TEES), Texas A&M University System, 7607 Eastmark Drive, College Station, TX 77840, USA School of Transportation, Wuhan University of Technology, 1178 Heping Avenue, Wuhan, Hubei Province 430063, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Additives Asphalt mixtures Surface free energy (SFE) Adhesion Moisture susceptibility
Conventional methods for evaluating the effects of additives on moisture damage resistance of asphalt mixtures are either empirical or unable to quantify the contributions of material component properties to the overall mixture performance. To overcome these drawbacks, this study proposes a surface free energy (SFE) method to investigate the effects of various additives on moisture susceptibility of asphalt mixtures. One neat asphalt and one acidic gravel aggregate are selected as the test materials along with 6 commonly-used additives: a warm mix asphalt additive, two nano-materials, a hydrated lime, a Portland cement and a non-amine liquid asphalt antistripping agent. All the additives are blended with the neat asphalt to fabricate modified asphalt binders. The SFE components of these modified asphalt binders and the gravel aggregates are measured with the Wilhelmy plate method and the vapor adsorption method, respectively. An energy ratio, defined as the ratio of adhesion of asphalt-aggregate to that of asphalt-aggregate-water, is calculated and used to rank the asphalt mixtures that consist of gravel and asphalt binders modified with different additives in terms of their moisture susceptibility performance. In order to validate the proposed SFE method, two mixture moisture susceptibility tests, the modified boiling water test and the indirect tensile strength test, are conducted on loose and compacted asphalt mixtures, respectively. A consistent moisture susceptibility ranking is obtained from the SFE method and the mixture moisture susceptibility tests, which validates the SFE method proposed in this paper can be used to accurately quantify the effects of additives on the moisture susceptibility of asphalt mixtures.
1. Introduction Due to heavier traffic loading, higher traffic volume and severer environmental conditions, asphalt pavements constructed from the traditional asphalt materials are more and more vulnerable to premature distresses, such as rutting, fatigue cracking, shoving and raveling, etc. These premature distresses have detrimental effects on the pavement performance, which further result in a lower pavement serviceability and a higher maintenance (or rehabilitation) cost. In order to meet the practical demands in constructing asphalt pavements with high performance as well as low construction cost, there is an increasing trend in modifying asphalt with various additives. For instance, to reduce the susceptibility of asphalt mixtures to moisture damage, Xiao and Amirkhanian developed different types of liquid antistripped agent to strengthen the bonding between asphalt and aggregates [1]. For the purpose of improving rutting resistance and extending fatigue life of asphalt mixtures, Shafabakhsh and Ani attempted to modify asphalt with nano-sized materials (TiO2 and SiO2) due to
∗
their fairly high surface area and small size [2]. To decrease energy consumption and fuel emissions for the production of hot mix asphalt (HMA), wax mix asphalt (WMA) additive was used to fabricate the WMA asphalt mixture. This new type of additive has been verified to significantly lower asphalt viscosity, which thus effectively reduces the energy consumption [3]. Since all these additives are proven to be capable of enhancing one or multiple functional performance of asphalt pavements, studies of their effects on asphalt binders or asphalt mixtures have attracted an upward interest, among which the evaluation of additive effects on moisture-induce damage resistance of asphalt mixtures is a very importance aspect. Moisture-induce damage of asphalt mixtures is defined as a loss of bond between asphalt-aggregate or within asphalt cement attributable to the action of moisture. To characterize the effects of additives on mixture moisture susceptibility, various mechanical tests have been adopted so far. For instance, Kim et al. conducted the boiling water test and the pull-off tensile strength test on asphalt mixtures modified by anti-stripping additives [4]. Shu et al. used the standard tensile strength
Corresponding author. E-mail addresses: [email protected] (D. Zhang), [email protected] (R. Luo).
https://doi.org/10.1016/j.ijadhadh.2019.102437
Available online 18 September 2019 0143-7496/ Published by Elsevier Ltd.
International Journal of Adhesion and Adhesives 95 (2019) 102437
D. Zhang and R. Luo
ratio (TSR) test, the Superpave indirect tension (IDT) test and the Hamburg Wheel Tracking test on WMA containing recycled asphalt pavement (RAP) [5]. Kim et al. performed asphalt pavement analyzer (APA) test and semi-circular bend (SCB) fracture test to examine two widely used WMA approaches [6]. Overall, all these conventional tests emphasize comparing the properties change of asphalt mixtures before and after moisture condition, in terms of the adhesion loss in loose mixtures or the strength loss in compacted mixtures. They are relatively simple with only some know-how required. However, they are unable to address any mechanisms governing the moisture damage caused within asphalt mixtures. Moreover, they focus on providing an overall performance evaluation for the asphalt mixtures rather than correlating the mixture performance to the material properties, making it difficult to quantify the contribution of each component to the moisture damage. In contrast to the conventional methods, the surface free energy (SFE) method, as a newly emerging technology, has an ability to quantify the adhesion between asphalt-aggregate from the perspective of thermodynamics [7]. The surface free energy (SFE), by definition, is the revisable work done to create a unit area of new surface, which has a unit of mJ/m2. According to the Good-van Oss-Chaudhury (GvOC) theory [8], the SFE of a material is generally divided into two components: Lifshitz nonpolar component (γ LW ) and Lewis polar component (γ AB ). The Lewis polar component further includes the Lewis polar-acid component (γ + ) and the Lewis polar-base component (γ −). In this regard, the total SFE can be expressed in the form of SFE components as follows:
γ = γ LW + γ AB = γ LW + 2 γ +γ −
Fig. 1. Adhesive bond energy of asphalt mixture under dry and wet conditions.
(1)
Conventionally, the SFE components of asphalt binder and aggregate can be measured through the contact angle approach and the spreading pressure approach, respectively [9,10]. Accordingly, the GvOC models applied to these two approaches are expressed in Eq. (2) and Eq. (3), respectively.
γA+γL− +
(1 + cos θ) γL = 2( γALW γLLW +
2γL + πe(SL) = 2( γSLW γLLW +
γS+γL− +
γA−γL+ )
γS−γL+ )
(2)
signifying a better resistance of an asphalt mixture to moisture damage. Since the SFE method suffices to provide a direct evaluation of adhesion of asphalt mixtures in absence and presence of water from SFE properties of each material component, it has been widely used to optimize the asphalt-aggregate combination as well as to determine the optimum content of additive blended with the asphalt binder or aggregates [12].
(3)
2. Motivation and objectives
where γL is the total SFE of test liquid; θ is the contact angle between asphalt-test liquid; πe(SL) is the spreading pressure between aggregatetest liquid; and subscript “S”, “W”, “A” refer to as aggregate, water and asphalt binder, respectively. With the measured asphalt and aggregate SFE components, the adhesion of asphalt-aggregate under dry and wet conditions (as shown in Fig. 1(a)(b)) can be easily calculated from the SFE component interactions, which are expressed in Eq. (4) and Eq. (5), respectively. Based on the calculated adhesive values, an energy ratio (ER ) has been put forward to quantify the moisture susceptibility of asphalt mixtures, which is defined as the ratio of adhesion of asphalt-aggregate to that of asphalt-aggregate-water, as shown in Eq. (6) [11]. a ΔGSA = 2( γALW γSLW +
a ΔGSWA
ER =
γA+γS− +
γA−γS+ )
(4)
LW LW LW + + γALW γW − γSLW γALW − γW + γW ⎡ γSLW γW ⎤ ⎢ ⎥ − − − ⎥ = −2 ⎢ ( γS + γA − γW ) ⎢ ⎥ − + + + + − − + ⎢+ γW ( γS + γA − γW ) − γS γA − γS γA ⎥ ⎣ ⎦
(5)
a ΔGSA a ΔGSWA
(6)
a ΔGSA ,
In the context of using the SFE method to evaluate the moisture susceptibility of asphalt mixtures, the first step is to measure the SFE components of asphalt binder and aggregate. As shown in Eq. (2) and Eq. (3), normally, three test liquids, forming a liquid set, along with their measured contact angle values (spreading pressure values) on the same asphalt (aggregate), can be used to determine the three asphalt (aggregate) SFE components. In the aggregate SFE measurements, a fixed set of three liquids (distilled water, methyl propyl ketone (MPK) and toluene) are usually selected. With respect to the asphalt SFE measurements, five liquids (i.e. distilled water (W), formamide (F), glycerol (G), ethylene glycol (E) and diiodomethane (D)) are commonly used to design different liquid sets to determine the asphalt SFE components. The determined asphalt SFE components, however, have been reported to depend strongly on the liquid set selected [13], which further result in the discrepant values of ER for the same asphalt mixtures. Thus, the current SFE method is unable to evaluate the mixture moisture susceptibility accurately due to its weakness in obtaining consistent energy ratio. To overcome this drawback, the authors have proposed a novel method to ensure the consistent energy ratio in previous studies [14,15]. In this method, two liquid sets (“WFED” and “WFGED”) are first screened out as the optimum liquids sets for the determination of asphalt SFE components using the GvOC model. The two adhesion values and the energy ratio are then calculated based on the Owens-Wendt (OW) model [16]. This method has been successfully used to evaluate the moisture susceptibility of asphalt mixtures and asphalt mastics. However, its feasibility was verified with limited types of asphalt binder, in particularity with only two modified asphalt binders [14]. More types of modified asphalt
a ΔGSWA
where are the adhesion of asphalt mixtures under dry and wet conditions, respectively. In theory, a higher value of ER corresponds to a higher adhesion between asphalt-aggregate and a lower energy release of the mixture system due to the replacement of asphalt by water on aggregates, 2
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binder are still required to fully evaluate its feasibility. Considering that different types of additive can be used to fabricate various types of modified asphalt binder, this study will attempt to use the proposed SFE method to investigate the effects of additives on the moisture susceptibility of asphalt mixtures. Six different types of additive will be selected as the representative additive materials, all of which are prevalent in improving one or multiple performance of asphalt mixtures. In summary, this study aims at assessing the effects of additive on the moisture susceptibility of asphalt mixtures with a proposed SFE method, which includes:
Table 2 Chemical compositions of the gravel aggregate.
(1) Measure the SFE components of the selected asphalt with and without the additives as well as the SFE components of the selected aggregates; (2) Quantify the effects of additives on the mixture moisture susceptibility based on the adhesion values and the energy ratio determined from the proposed SFE method; (3) Evaluate the feasibility of the proposed SFE method through the laboratory mixture moisture susceptibility test results.
Chemical composition of aggregates
Value
Sodium oxide, Na2O (%) Silicon dioxide, SiO2 (%) Aluminium oxide, Al2O3 (%) Ferric oxide, Fe2O3 (%) Sulphur trioxide, SO3 (%) Calcium oxide, CaO (%) Magnesium oxide, MgO (%) Insoluble residue (%)
2.077 87.495 6.096 1.621 0.061 0.276 0.032 0.882
Table 3 Properties and percentage of each type of additive. Additive
Percentage
SW
0.5%
SiO2
5%
Property value: Neutral • PH point: 285 °C • Flash 99.9% • Purity: surface area: 240 m /g • Specific grain size: 20 nm • Average 2.40 g/cm • Density: 99.9% • Purity: surface area: 40 m /g • Specific grain size: 25 nm • Average 1.15 g/cm • Density: 95.0% • Purity: 2.21 g/cm • Density: 2.94 g/cm • Density: 0.92 g/ml • Density: • Water content (105 °C, 1 h): 0.02% 2
3. Test materials
3
ZnO
4%
2
3.1. Asphalt and aggregates
3
A neat #70 asphalt (graded based on the penetration value) and a gravel aggregate are selected as the test materials in this study. Asphalt mixtures prepared with such two materials are moisture susceptible, which will facilitate the comparison of different additives in term of their effectiveness in improving moisture damage resistance of asphalt mixtures. The properties of the #70 asphalt and the chemical compositions of the gravel aggregate are summarized in Table 1 and Table 2, respectively.
4.1.1. Wilhelmy plate test The Wilhelmy plate test is used to measure the SFE components for each type of asphalt binder with the aid of a high-precision tensiometer (KRÜSS GmbH, Hamburg, Germany). The testing principle of this method is to measure the dynamic contact angle between the asphalt binder and the test liquid based on a kinetic force balance. As shown in Fig. 2, the microbalance of the equipment constantly measures the force applied to the asphalt-coated plate when the plate is advancing into the liquid at a constant rate of 3 mm/min. The test temperature is controlled at 20 ± 0.5 °C by the thermostat and monitored by the temperature sensor. Then the contact angle between the asphalt and the test liquid can be calculated as:
Table 1 Conventional properties of the selected asphalt.
N70* Specification
65 60–80
47.3 ≥46.0
Ductility (cm) 5 cm/min, 15 °C
88 ≥20
150 ≥100
3
4.1. Surface free energy (SFE) measurement tests
In order to obtain a homogenous modified asphalt, a high-speed
5 cm/min, 10 °C
5% 0.4%
4. Laboratory testing
3.3. Preparation of asphalt binder modified with additives
Softening point (°C)
PC NA
shearing method is utilized to add the additive into the neat asphalt with the aid of a high-shear mixer (JRJ 300-S, Shanghai, China). First, the neat asphalt is heated at 150 °C for 30 min in the mixer. Then a certain amount of additive that satisfies the requirement shown in Table 3 is added into the mixer and blended with the asphalt at a shear rate of 4000 rmp for 10 min. Subsequent to the completion of the mixing process, the asphalt binder modified with each type of additive is finally fabricated. In this study, the modified asphalt binders are labeled by the source followed the type of the additive. For instance, the modified asphalt of “N70 + SW” refers to as the neat #70 asphalt modified with 0.5% of Sasobit® wax. For the purpose of comparison, the neat #70 asphalt binder is used as the control asphalt in this study.
A total of 6 different types of additives are selected as the additive materials, including a WMA additive (Sasobit® wax (SW)) (AkzoNobel, China), two nano-materials (nano SiO2 and nano ZnO) (Jingbei Company, China), a hydrated lime (HL) (Xingyinhe Chemical Company, China), a Portland cement (PC) (Wuhan Wuganghuaxin Cement Co., Ltd., China), and a non-amine liquid asphalt anti-stripping agent (NA) (Chongqing Haimu Traffic Technology Co., Ltd., China). All these additives have previously been used as the modifiers to enhance the performance of asphalt mixtures, of which properties are summarized in Table 3. To facilitate the evaluation of the effects of each additive on the moisture susceptibility of asphalt mixtures, all the selected additives are mixed with the neat #70 asphalt binder to fabricate the modified asphalt binders rather than being mixed with the asphalt mixtures. As recommended by the manufacture and reported in literature, percentage of each type of additive by weight of bitumen is also included in Table 3.
Penetration (100 g, 5s, 25 °C)
5%
3
3.2. Additives
Asphalt
HL
ΔF + abhρL g ⎤ θ = arccos ⎡ ⎢ 2(a + b) γ ⎥ L ⎦ ⎣
(7)
where θ is the dynamic contact angle between the asphalt binder and the test liquid; a, b are the width and thickness of the asphalt-coated plate; ρL is the liquid density; γL is the liquid surface tension; g is the
Note: N70* stands for the selected neat #70 asphalt. 3
International Journal of Adhesion and Adhesives 95 (2019) 102437
D. Zhang and R. Luo
Fig. 2. Dynamic contact angle measured from Wilhelmy plate test.
acceleration of gravity; and ΔF is the force measured by the microbalance at an immersion height of h . Based on the measured contact angle and the known SFE components of the test liquids, the GvOC model is used to determine the SFE components of asphalt binder. More details about this method can be referred to elsewhere [10]. As stated previously, in order to compute the three SFE components (i.e. γALW , γA+, γA−) of asphalt binder, at least three test liquids with the measured contact angles are required to establish an equation set based on Eq. (2). In this study, the five commonly-used test liquids (W, F, G, E and D) are selected as the test probes. Their SFE components at 20 °C are listed in Table 4. The contact angle (θ ) between each liquid and each asphalt binder is measured three times and the average of the three contact angle values is calculated as the final contact angle value, which is summarized in Table 5. In order to evaluate the test repeatability, the standard deviation (σ) of these three measured contact angle values for each asphalt-liquid combination is also computed. The small σ value (maximum is only 3.22°) shown in Table 5 indicates that the Wilhelmy plate test conducted in this study has a fairly good repeatability.
πe(SL) =
W F G E D
γL−
γLAB
γL
21.8 39.0 34.0 29.0 50.8
25.5 2.28 3.92 1.92 0
25.5 39.6 57.4 47.0 0
51.0 19.0 30.0 19.0 0
72.8 58.0 64.0 48.0 50.8
n dp p
(8)
kD
(9)
where nD, B, kD are the model coefficients of the DA model. Prior to determining the spreading pressure, another important parameter, the specific surface area of the aggregates must be calculated, which is accomplished through the modified Brunauer-EmmettTeller (M-BET) model as follows [19]:
p (c′ − 1) p 1 = + n (k 0 p0 − p) c′n m′ k 0 c′n m′ p0
(10)
where k 0 is a constant; p0 is the saturated vapor pressure; c′ is a constant related to the adsorption heat; and n m′ is the monolayer capacity of the vapor molecules adsorbed on the aggregates. In the M-BET model, the constant k 0 is first determined based on a linear fit of the plot of p /[n (k 0 p0 − p)] against p / p0 in the entire relative pressure range. Assuming that the vapor molecules adsorbed on the aggregates are hexagonal closest packing, the specific surface area used for the spreading pressure calculation can be then obtained as: 2
A=
I ′ (k 0 − 1) 1.091NA M ⎞3 ⋅⎛⎜ − ⎟ M (k 0 K ′ + I ′) ⎝ NA ρL ⎠ k 0 (K ′ + I ′)(k 0 K ′ + I ′)
(11)
where NA is the Avogadro constant; K ′ and I ′ are the slope and intercept of the linearly fitting line on the plot of p /[n (k 0 p0 − p)] against p / p0 , respectively. Combing Eqs. (8), (9) and Eq. (11), spreading pressure between the aggregate and the probe liquid can be calculated. Then substituting the calculated spreading pressure into the GvOC equation (Eq. (3)) [9], the three SFE components of the aggregate can be finally determined. As mentioned previously, distilled water, MPK, and toluene are commonly selected as the three probe vapors in vapor adsorption test. Table 6 presents the SFE components of the latter two liquids, whereas those of the distilled water have been given in Table 4. The vapor adsorption isotherms of these three probe vapors with respect to the gravel aggregates are exhibited in Fig. 4, which are the average values from the three replicates.
SFE Components ( × 10−3 J/m2)
γL+
p0
p ⎧ ⎫ n = nD exp −B ⎡ln ⎛⎜ 0 ⎞⎟ ⎤ ⎢ ⎥ ⎬ ⎨ p ⎝ ⎠ ⎣ ⎦ ⎭ ⎩
Table 4 SFE components of the probe liquids at 20 °C [17].
γLLW
∫0
where R is the universal gas constant; T is the test temperature; M is the molar mass of the probe vapor; A is the specific surface area of the aggregates; n is the total capacity per unit adsorbent mass of the adsorbed probe vapor molecules; and p is the vapor pressure. For aggregate, its vapor adsorption isotherm is modeled using the Dubinin-Astakhov (DA) equation, which is [9]:
4.1.2. Vapor adsorption test The vapor adsorption test is employed to measure the SFE components of the gravel aggregates, which is completed through the Gravimetric Sorption Analyzer (IsoSORP® STATIC, RUBOTHERM GMBH, Bochum, Germany). As illustrated in Fig. 3, in this test the dry aggregate samples stored in a container are placed in a seal test chamber under vacuum condition. Then a probe vapor is released into the chamber and stabilized at 10 different vapor pressures at the test temperature of 20 °C. After that, the saturated amount of vapor adsorbed on the aggregates corresponding to each vapor pressure is measured by the magnetic suspension balance and then used to construct a vapor adsorption isotherm. Based on this measured vapor adsorption isotherm, the spreading pressure of the aggregate-vapor πe(SL) can be calculated using the Gibbs adsorption equation [18]:
Test Liquid
RT MA
4
International Journal of Adhesion and Adhesives 95 (2019) 102437
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Table 5 Wilhelmy plate test results of asphalt-liquid. Asphalt Binder
N70 N70 N70 N70 N70 N70 N70
+ + + + + +
W (°)
SW SiO2 ZnO HL PC NA
F (°)
G (°)
E (°)
D (°)
θ
σ
θ
σ
θ
σ
θ
σ
θ
σ
103.49 96.59 104.52 100.12 93.79 101.58 88.00
0.38 1.28 0.09 0.39 0.91 0.18 0.13
85.55 87.37 86.07 85.05 80.65 86.17 71.25
0.01 0.01 0.54 0.04 0.30 1.29 0.21
89.51 89.21 90.30 88.94 88.28 90.19 81.44
0.12 2.34 0.21 0.02 0.24 0.11 2.60
74.83 74.74 74.93 74.55 74.30 76.94 64.80
0.03 0.35 0.68 0.13 0.74 0.91 0.53
81.00 77.71 81.62 85.17 93.09 80.64 90.23
2.98 3.22 0.57 1.80 1.34 0.16 0.11
4.2. Moisture susceptibility test
Table 6 SFE components of the probe vapors at 20 °C [15].
4.2.1. Modified boiling water test The boiling water test in accordance with ASTM D3625 is employed to evaluate the moisture susceptibility of loose asphalt mixtures that consists of gravel aggregates and different types of asphalt binder [20]. In this test, the asphalt-coated aggregate mixture is conditioned in the boiling water and then a visual observation is conducted to evaluate the loss of adhesion in the loose asphalt mixture due to the boiling water action. To further quantify this adhesion loss, an image analysis method is adopted to modify the boiling water test [14]. More specifically, the asphalt-coated area and the total area of 10 selected asphalt-coated gravel particles are measured through an image processing software. Then the percentage P of the remaining asphalt are calculated using Eq. (12). A larger value of P indicates less asphalt is stripped from the aggregate, which therefore corresponds to a low moisture sensitivity of an asphalt mixture.
Test Liquid
MPK Toluene
SFE Components ( × 10−3 J/m2)
γLLW
γL+
γL−
γLAB
γL
21.7 28.3
0 0
19.2 2.7
0 0
21.7 28.3
20
P=
∑ j = 1 Aj′ 20
∑ j = 1 Aj
× 100% (12)
where Aj and Aj′ are the total surface area and the asphalt-coated area of the jth picture of the asphalt-coated aggregate particle, respectively. 4.2.2. Indirect tensile strength (ITS) test The indirect tensile strength (ITS) test, following the guideline specified in ASTM D4867 [21], is employed to evaluate the moisture susceptibility of the compacted asphalt mixture in this study. A type of AC-13 (nominal aggregate size is 13 mm) is selected as the aggregate gradation. The optimum asphalt content for each gravel-modified asphalt mixture is determined through the Marshall design method in accordance with the ASTM D6927 [22]. Controlling the binder content as the optimum one is to minimize the impacts of structures and
Fig. 4. Vapor adsorption isotherms of three probe vapors.
volumetrics on the performance of asphalt mixtures so as to maximize those of the material properties, which facilitates the study of the additive effects on the mixture moisture susceptibility. Given the specific aggregate gradation and the optimum asphalt content, a set of compacted asphalt mixture specimens with 7 ± 0.5% air voids are then
Fig. 3. Schematic illustration of the vapor adsorption test. 5
International Journal of Adhesion and Adhesives 95 (2019) 102437
D. Zhang and R. Luo
Fig. 5. Tensile strength of asphalt mixture measured from the ITS test.
fabricated, which are divided into two subsets of at least three specimens each. One subset is unconditioned and used as the control set, while the other subset is partially saturated with water to a saturation of 55–88% and then undergoes a freeze-thaw cycle. After that, a tensile splitting tester is used to measure the tensile strength for each subset of the asphalt mixture samples, as shown in Fig. 5. The tensile strength ratio (TSR) is finally calculated and used to evaluate the moisture susceptibility of the compacted asphalt mixture, which is given by:
TSR =
Stm × 100% Std
(13)
where Stm and Std are the average tensile strength of the moistureconditioned subset and that of the dry subset, respectively. 5. Results and discussion 5.1. Surface free energy (SFE) determination based on the proposed method
Fig. 6. Variations of SFE of N70 with the liquid set.
5.1.1. Surface free energy (SFE) of asphalt binders As reported in the authors' previous paper [14], the SFE components of asphalt binder determined from the GvOC model are dependent strongly on the liquid set selected from the five commonly-used probe liquids (W, F, G, E and D). Different liquid sets result in discrepant SFE values for the same asphalt binder. This problem also arises in this study when adopting different liquid sets to determine the SFE components for the same investigated asphalt binder. Fig. 6 gives an example to show the variations of the SFE of N70 with the liquid set. This kind of dependency will further result in the significant variation of the energy ratio that is calculated based on the asphalt SFE components. As mentioned previously, to ensure the consistent energy ratio, a proposed method developed by the authors is adopted in this study [14]. In that method, “WFED” and “WFGED” are first selected as the optimum liquid sets. The contact angle between each liquid and the asphalt binder is then measured through the Wilhelmy plate method. Eq. (2) is finally used to establish an overdetermined equation set to determine the SFE of asphalt binder. In order to reduce the computational efforts, only “WFGED” is selected in this study as the optimum liquid set when calculating the SFE components for all the 7 types of asphalt binder. The calculation results are summarized in Table 7. Apparently, compared to the nonpolar component, the polar components are relatively small for all the investigated asphalt binders, which is plausible because asphalt is intrinsically composed of high molecular
Table 7 Determined SFE of the investigated asphalt binders. Asphalt Binder
N70 N70 N70 N70 N70 N70 N70
+ + + + + +
SW SiO2 ZnO HL PC NA
SFE Components ( × 10−3 J/m2)
Ratio
γALW
γA+
γA−
γAAB
γA
γA+/ γA−
16.75 18.22 16.44 14.76 11.56 16.95 12.95
0.79 0.20 0.83 1.01 1.63 0.48 2.72
0.71 3.89 0.53 1.88 5.29 1.57 6.10
1.49 1.75 1.32 2.76 5.87 1.74 8.14
18.24 19.98 17.77 17.52 17.43 18.69 21.09
1.11 0.05 1.57 0.54 0.31 0.31 0.45
weight hydrocarbons, thus exhibiting little polar activity [23]. The ratio of the polar-acid component to the polar-base component is also included into Table 7, which will be used for the analysis of the effects of additives on the moisture susceptibility of the asphalt mixture in the following sections.
5.1.2. Surface free energy (SFE) of aggregates In order to determine the SFE components of the selected gravel aggregates, the DA model (Eq. (9)) is first utilized to model the 6
International Journal of Adhesion and Adhesives 95 (2019) 102437
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Table 9 Determined SFE components of gravel aggregates. Aggregate
Gravel
SFE Components ( × 10−3 J/m2)
γSLW
γS+
γS−
γSAB
γS
79.45
10.71
305.39
114.38
193.83
5.2. Energy ratio calculation In order to determine the energy ratio, two types of adhesive bond energy shown in Eq. (6) have to be calculated. As stated before, the SFE components of asphalt binder calculated from the GvOC model possesses the dependency of the liquid set selected, which further results in the dependency of the adhesive bond energy on the liquid set, making it impossible to obtain the consistent energy ratio. To overcome this liquid set dependency, an approach put forward by the authors in a previous study is employed to calculate the adhesive bond energies based on the Owens-Wendt (OW) model [16]:
Fig. 7. Modeling of adsorption isotherms using the DA model.
1
1
1
1
AB AB LW LW + − + − a LW LW ΔGSA( p) = 2 ⎡ (γA γS ) 2 + (γA γS ) 2 ⎤ = 2 ⎡ (γA γS ) 2 + 2(γA γA γS γS ) 4 ⎤ ⎦ ⎣ ⎣ ⎦ (14)
1
1
1
LW LW LW LW LW a LW LW ΔGSWA( p) = 2 ⎡ (γS γW ) 2 + (γA γW ) 2 − (γA γS ) 2 − 2γA ⎤ ⎣ ⎦ 1
1
1
1
+ − + − γW ) 4 − (γA+γA−γW γW ) 4 ⎤ − 4 ⎡ (γA+γA−γS+γS−) 4 + (γA+γA−) 2 − (γS+γS−γW ⎣ ⎦
a a where ΔGSA( p) , ΔGSWA(p) are the adhesive bond energy of asphalt-aggregate subjected to dry and wet conditions, respectively, both of which are calculated using the proposed method. It is to be noted that the SFE components of asphalt binder shown in Eqs. (14) and (15) are determined using the liquid set of “WFGED” as the optimum liquid set. Based on the calculated adhesive bond energies, the energy ratio (ER (p) ) in the proposed SFE method can be then determined through:
Fig. 8. Linear fitting results based on the M-BET model. Table 8 Determined specific surface area and spreading pressure of gravel. Parameter
2
A (m /g) πe(SL) ( × 10−3 J/m2)
Probe Vapor Distilled Water
MPK
Toluene
2.2895 147.18
1.0107 68.62
0.4578 48.99
(15)
ER (p) =
measured adsorption isotherms of the three probe vapors shown in Fig. 4. In the meantime, the M-BET model (Eq. (10)) is used to linearly fit the plot of p /[n (k 0 p0 − p)] versus p / p0 in the full range of relative vapor pressure. The fitting results of these two models are separately shown in Fig. 7 and Fig. 8, both of which have an R2 value great than 0.99, demonstrating the excellent fitting performance of the DA model and the M-BET model. It can also be seen from Fig. 7 that the saturated adsorption amount of vapor on the gravel aggregates is ranked as: distilled water > MPK > toluene, which is consistent with those observed from other aggregate resources [19]. Compared to toluene, MPK is observed to have a lower value of total SFE (or nonpolar SFE component) but a higher saturated adsorption amount on gravel aggregates. The possible reason accounting for this observation is that the interaction between aggregate and the test liquid vapor is polarity-dominant because the polar base component (γL−) of MPK is significantly larger than that of toluene. Based on all these fitting results, Eqs. (8)–(11) are used to calculate the specific surface area and the spreading pressure of gravel with each probe vapor. The calculation results are summarized in Table 8. Finally, the SFE components of the gravel aggregates are determined by substituting the calculated spreading pressure of the three probe vapors into Eq. (3). The determined SFE components of gravel are summarized in Table 9. It is obvious that the polar component of gravel aggregates is greater that its nonpolar component, which substantiates that the polar interaction is predominant between the gravel aggregates and the probe liquid vapors.
a ΔGSA( p) a ΔGSWA( p)
(16)
The SFE components of the asphalt binders measured from the Wilhelmy plate test (Table 7), along with those of the gravel aggregates measured from the vapor adsorption test (Table 9), are combined together to calculate the adhesive bond energy of each asphalt-gravel combination under both dry and wet conditions using Eqs. (14) and (15). Then the energy ratio (ER (p) ) of each asphalt-aggregate combination can be determined through Eq. (16). All the calculation results are summarized in Table 10. To examine the applicability of the traditional GvOC method, the adhesive bond energies and the energy ratio (ER ) calculated from the traditional SFE method based on Eqs. (4)–(6) are also included in Table 10. As mentioned previously, a higher ratio of ER indicates the lower moisture susceptibility of an asphalt mixture. Thus, when using the energy ratio as an index to evaluate the moisture susceptibility, the different asphalt-gravel combinations can be ranked based on the proposed method and the traditional method. However, as shown in Table 10, these two methods produce distinct rankings in this study. More specifically, the ranking derived from the proposed method is: N70+SiO2 > N70 > N70+PC > N70+ZnO > N70+SW > N70+HL> N70 + NA; while the ranking from the traditional method is: N70 + SW > N70 + PC > N70 > N70 + SiO2 > N70 + ZnO > N70 + HL > N70 + NA. To further validate the proposed method, the SFE method results are compared with the mixture moisture susceptibility test results, which is detailed in the next section. 7
International Journal of Adhesion and Adhesives 95 (2019) 102437
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Table 10 Calculation results of the adhesive bond energies and the energy ratio. Gravel asphalt mixture
N70 N70 N70 N70 N70 N70 N70
+ + + + + +
SW SiO2 ZnO HL PC NA
Adhesive Bond Energy ( × 10−3 J/m2)
Energy Ratio
a ΔGSA( p)
a ΔGSA
a ΔGSWA( p)
a ΔGSWA
ER (p)
ER
99.11 104.42 96.87 104.01 112.43 101.60 125.19
109.54 104.63 108.89 112.59 120.29 105.81 137.96
−46.96 −44.74 −47.80 −45.97 −44.31 −46.07 −39.56
−93.34 −106.84 −92.71 −94.46 −94.76 −99.47 −84.42
2.1106 2.3338 2.0267 2.2624 2.5373 2.2057 3.1648
1.1735 0.9793 1.1745 1.1919 1.2693 1.0638 1.6341
Table 11 Test results of the boiling water test and ITS test. Gravel asphalt mixture
N70 N70 N70 N70 N70 N70 N70
+ + + + + +
SW SiO2 ZnO HL PC NA
Boiling Water Test
Indirect Tensile Strength Test
∑ Aj′ (cm2)
∑ Aj (cm2)
P (%)
Stm (MPa)
Std (MPa)
TSR (%)
20.1811 8.2137 23.8158 10.0249 4.9691 12.7900 3.5913
81.1753 74.9943 74.0596 82.0736 72.0374 72.8072 73.5284
75.14 89.05 67.84 87.79 93.10 82.43 95.12
0.700 0.821 0.537 0.978 0.943 0.781 0.716
0.973 0.977 0.761 1.183 1.091 0.991 0.804
72.01 84.04 70.63 82.66 86.45 78.80 89.12
five additives helps mitigate the susceptibility of asphalt mixture to moisture to a certain degree. Among them, the non-amine liquid asphalt anti-stripping agent (NA) is the most effective one, which leads to the remarkably higher improvement in comparison with the other additives. A possible reason is that the chemical composition of this additive mainly consists of the hydrophobic hydrocarbon groups and the phosphorous hydroxyl group, both of which work together to form a strong chemical bond between asphalt-aggregate. As shown in Fig. 10, when this kind of additive is added into asphalt and contacted with gravel aggregate, its hydrocarbon “tails” molecules will progressively anchor into the asphalt, while its phosphorous hydroxyl group (positive sites) will bind strongly to the silanol groups (negative sites) on the aggregate surface. Accordingly, the polar interactions between asphaltaggregate are significantly strengthened, which can be inferred from the increase of Lewis polar component (γAAB ) of asphalt binder (from 1.49 to 8.14 J/m2). These stronger polar interactions are deemed to intensify the chemical bonding at the interface of asphalt-aggregate, thus enhancing the moisture damage resistance of the gravel asphalt mixtures. In addition, nano SiO2 is found to be the only additive lowering the moisture damage resistance of the gravel asphalt mixture. To gain better understanding of the mechanism of effects of this additive on the moisture susceptibility, Fig. 11 exhibits the adhesive bond energies of gravel in combination with each type of modified asphalt binder in the presence and absence of water. It is clear that nano SiO2 is the only a additive decreasing the dry adhesive bond energy ( ΔGSA ) of the N70 control asphalt mixture while increasing the wet adhesive bond energy a ( ΔGSWA ). One possible explanation is that the neat #70 asphalt binder is somewhat acidic in nature, whose acid component is 0.79 mJ/m2 and the base component is 0.71 mJ/m2. Normally, a good adhesion is difficult to be obtained when such an acidic asphalt is coated onto the acidic gravel aggregate, as reported by Ghabchi et al. [24]. As shown in Table 7, the acidic nano SiO2 is found to strengthen the acidity of the N70 asphalt based on the facts that it increases the acid component of the N70 asphalt by 5% and decreases its base component by 26%, resulting in an increased ratio of γA+/ γA− from 1.11 to 1.57. It thus lowers the adhesion between asphalt-aggregate. Except for this nano SiO2, all the other additives are found to lower the acidity of the N70 asphalt with the decreased ratios of γA+/ γA− when using the control asphalt as
5.3. Comparison with moisture susceptibility test results The boiling water test results as well as the ITS test results for all the gravel asphalt mixtures are listed in Table 11 and presented in a bar graph of Fig. 9. To gain a visual comparison, the energy ration results of ER(p) and ER shown in Table 10 are also included in this bar graph. Theoretically, higher values of TSR and P indicate the less loss of adhesion in asphalt mixtures in presence of water and therefore the lower moisture susceptibility of the asphalt mixtures, suggesting a higher energy ratio. It can be easily identified from Fig. 9 that the rankings of both TSR and P are in agreement with the ER(p) ranking rather than the ER ranking. As such, in contrast to the traditional GvOC method, the proposed SFE method shows capability to accurately characterize the effects of additive on the moisture susceptibility of asphalt mixtures. 5.4. Analysis of additive effects on mixture moisture susceptibility As observed from Fig. 9, different additives provide the distinct effectiveness in enhancing the moisture damage resistance of asphalt mixtures is distinct. Except for nano SiO2, the addition of the remaining
Fig. 9. Test results of TSR, P and ER for each gravel-asphalt combination. 8
International Journal of Adhesion and Adhesives 95 (2019) 102437
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Fig. 10. Anti-stripping additive act as bridge between asphalt-gravel aggregate.
structure and functional groups of each modified asphalt binder are currently being investigated through the scanning electron microscopy (SEM) test and the Fourier-transform infrared spectroscopy (FTIR) test, respectively. Acknowledgements The authors acknowledge the financial support of the Ministry of Transport of China (Project No. 2014318J22120). Special thanks are to the 1000-Youth Elite Program of China for the start-up funds for purchasing the laboratory equipment that is crucial to this research. References [1] Xiao F, Amirkhanian SN. Effects of liquid antistrip additives on rheology and moisture susceptibility of water bearing warm mixtures. Constr Build Mater 2010;24(9):1649–55. [2] Shafabakhsh GH, Ani OJ. Experimental investigation of effect of Nano TiO2/SiO2 modified bitumen on the rutting and fatigue performance of asphalt mixtures containing steel slag aggregates. Constr Build Mater 2015;98:692–702. [3] Oliveira JRM, Silva HMRD, Abreu LPF, Fernandes SRM. Use of a warm mix asphalt additive to reduce the production temperatures and to improve the performance of asphalt rubber mixtures. J Clean Prod 2013;41:15–22. [4] Kim YR, Pinto I, Park SW. Experimental evaluation of anti-stripping additives in bituminous mixtures through multiple scale laboratory test results. Constr Build Mater 2012;29:386–93. [5] Shu X, Huang B, Shrum ED, Jia X. Laboratory evaluation of moisture susceptibility of foamed warm mix asphalt containing high percentages of RAP. Constr Build Mater 2012;35:125–30. [6] Kim YR, Zhang J, Ban H. Moisture damage characterization of warm-mix asphalt mixtures based on laboratory-field evaluation. Constr Build Mater 2012;31:204–21. [7] Hamedi GH, Tahami SA. The effect of using anti-stripping additives on moisture damage of hot mix asphalt. Int J Adhesion Adhes 2018;81:90–7. [8] Oss CJV, Chaudhury MK, Good RJ. Interfacial Lifshitz-van der Waals and polar interactions in macroscopic systems. Chem Rev 1988;88(6):927–41. [9] Zhang D, Luo R. Modeling of adsorption isotherms of probe vapors on aggregates for accurate determination of aggregate surface energy components. Constr Build Mater 2017;134:374–87. [10] Luo R, Zhang D, Zeng Z, Lytton RL. Effect of surface tension on the measurement of surface energy components of asphalt binders using the Wilhelmy Plate Method. Constr Build Mater 2015;98:900–9. [11] Xu W, Luo R, Zhang K, et al. Experimental investigation on preparation and performance of clear asphalt. Int J Pavement Eng 2019;19(5):416–21. [12] Arabani M, Hamedi GH. Using the surface free energy method to evaluate the effects of liquid antistrip additives on moisture sensitivity in hot mix asphalt. Int J Pavement Eng 2013;15(1):66–78. [13] Volpe CD, Siboni S. Acid-case surface free energies of solids and the definition of scales in the Good-van Oss-Chaudhury Theory. J Adhes Sci Technol 2000;14(2):235–72. [14] Zhang D, Liu H. A proposed approach for determining consistent energy parameters based on the surface free energy theory. J Mater Civ Eng 2018;30(11):04018287. [15] Zhang D, Luo R, Zeng Z. Characterization of surface free energy of mineral filler by spreading pressure approach. Constr Build Mater 2019;218:126–34. [16] Owens DK, Wendt RC. Estimation of the surface free energy of polymers. J Appl Polym Sci 1969;13(8):1741–7. [17] Oss CJV. Interfacial forces in aqueous media. second ed. New York, USA: CRC Press; 2006. [18] Jura G, Harkins WD. Surface of solids. XI. Determination of the decrease (π) of free surface energy of a solid by an adsorbed film. J Am Chem Soc 1944;66(8):1356–62. [19] Zhang D, Luo R. Modifying the BET model for accurately determining specific surface area and surface energy components of aggregates. Constr Build Mater 2018;175:653–63. [20] ASTM. Standard practice for effect of water on bituminous-coated aggregate using boiling water. West Conshohocken, PA: ASTM D3625; 2012. [21] ASTM. Standard test method for effect of moisture on asphalt concrete paving mixtures. West Conshohocken, PA: ASTM D4867; 2014. [22] ASTM. Standard test method for Marshall stability and flow of asphalt mixtures. West Conshohocken, PA: ASTM D6927; 2015. [23] Hamedi GH, Nejad FM, Oveisi K. Estimating the moisture damage of asphalt mixture modified with nano zinc oxide. Mater Struct 2016;49(4):1165–74. [24] Ghabchi R, Singh D, Zaman M, Tian Q. Mechanistic evaluation of the effect of WMA additives on wettability and moisture susceptibility properties of asphalt mixes. J Test Eval 2013;41(6):933–42.
Fig. 11. Adhesive bond energy of asphalt-grave under dry and wet conditions.
reference, which further leads to a higher dry adhesive bond energy and a lower wet adhesive bond energy for the gravel asphalt mixtures and therefore a greater energy ratio and the better resistance to moisture damage. Thus, from the perspective of SFE theory, it may be concluded that decreasing the ratio of γA+/ γA− of asphalt binder is beneficial to the moisture damage resistance of asphalt mixtures containing a given type of aggregates. 6. Conclusions and on-going work This paper presents the use of a proposed surface free energy (SFE) method to investigate the effects of six commonly-used additives (Sasobit® wax (SW), nano SiO2, nano ZnO, hydrate lime (HL), Portland cement (PC), and a non-amine liquid asphalt anti-stripping agent (NA)) on the moisture susceptibility of asphalt mixtures. The main conclusions include: (1) The proposed SFE method involves using the optimum liquid set (“WFGED” or “WFED”) to calculate the asphalt SFE components based on the GvOC model (Eq. (2)) as well as utilizing the OW model to determine the adhesive bond energies (Eqs. (14) and (15)) and the energy ratio (Eq. (16)) of the asphalt mixtures; (2) Ranking of the moisture damage resistance of asphalt mixtures measured from the mixture moisture susceptibility tests is consistent with that of the energy ratio results determined from the proposed SFE method. This validates that the proposed SFE method can be used to accurately quantify the effects of additives on the moisture susceptibility of asphalt mixtures; (3) From the perspective of SFE theory, lowering the ratio of γA+/ γA− of asphalt binder may be helpful towards the enhancement of the moisture damage resistance of asphalt mixtures containing a given type of aggregates. To gain a better understanding of the mechanisms of the additive effects on the mixture moisture damage resistance, the changes of
9