Assessing asphalt mixture moisture susceptibility through intrinsic adhesion, bitumen stripping and mechanical damage

Durability is one of the most important properties of an asphalt mixture. A key factor affecting the durability of asphalt pavements is moisture damage. Moisture damage is generally considered to be the result of two main mechanisms; the loss of adhesion between bitumen and the aggregate and the loss of cohesion within the mixture. Conventional test methods for evaluating moisture damage include tests conducted on loose bitumen-coated aggregates and those conducted on compacted asphalt mixtures. This paper looks at results from the rolling bottle and the saturated ageing tensile stiffness (SATS) tests in an attempt to better understand the underlying processes and mechanisms of moisture damage with the help of surface energy measurements on the constituent bitumen and aggregates. Combinations of materials were assessed using both the rolling bottle and SATS tests. The surface energy properties of the binders were measured using a dynamic contact angle analyser and those of the aggregates using a dynamic vapour sorption device. From these surface energy measurements, it was possible to predict the relative performance of both the simple rolling bottle test and the more complicated SATS test. Mineralogical composition of the aggregates determined using a mineral liberation analyser was used to explain the differences in performance of the mixtures considered.


Introduction
The majority of the roads throughout the world are constructed by using asphalt mixtures. However, like any other man-made structure, asphalt pavements deteriorate with the passage of time. Moisture damage is considered to be one of the major causes of distress in asphalt pavements with almost 2.5 billion pounds being spent annually by local authorities in England and Wales for road maintenance and rehabilitation works (Annual Local Authority Road Maintenance (ALARM) Survey, 2006). Although not all damage is caused directly by moisture, its presence increases the extent and severity of already existing distresses like cracking, potholes and rutting (Miller & Bellinger, 2003). The presence of moisture results in the degradation of the mechanical properties of the asphalt mixture, i.e. loss of stiffness and mechanical strength, which ultimately leads to the failure of the road structure. Moisture damage thus has a great economic impact as it causes 2. Materials 2.1. Constitutive materials Four aggregates commonly used in UK asphalt mixtures were chosen for the study. The aggregates (two limestones and two granites) were selected based on their difference in mineralogy and the fact that they exhibit different moisture damage performance (Airey, Masad, Bhasin, Caro, & Little, 2007). The mineralogy of the different aggregates was studied using a mineral liberation analyser (MLA) in order to understand their constituent mineral phases to help with the overall analysis of results.
MLA comprises a procedure used to identify the mineral phases present in aggregates by combining a large specimen chamber automated scanning electron microscope (SEM) and multiple Energy Dispersive X-ray detectors with automated quantitative mineralogy software. The software controls the SEM hardware to quantitatively analyse mineral and material samples. Automated stage control and image acquisition allows for rapid and systematic back scattered electron (BSE) imaging and subsequent X-ray analysis of thousands of mineral grains and particles. Automatic recalibration ensures consistent results. An FEI Quanta 600 SEM with MLA capability was used for the mineral phase determination (see Figure 1). Aggregate samples were prepared by casting aggregates in resin, followed by polishing of the surface. The samples were then carbon coated to make them electron conductive and scanned in BSE mode with electron dispersive X-ray analysis being carried out in an array of spots across the particles. The resultant spectra were then used to determine mineral phases at specific points in the microstructure which allowed mineralogical maps to be generated for each of the aggregate types.
The MLA results for all the aggregates used in this study (Limestone A, B and Granite A, B) are presented in Table 1. The results show that the aggregates have significantly different mineralogical make-up with Limestone A being made up of predominantly (about 97%) calcite. Granite A, on the other hand, is made up of a number of different mineral phases with the predominant phase being quartz, but with significant quantities of albite and K_feldspar. It is believed that the large proportion of the quartz phase has the potential to lead to deleterious moisture properties, due to the poor adhesion between quartz and bitumen. However, there is also evidence that a high feldspar content can be responsible for interfacial failure between bitumen and aggregate surfaces (Horgnies, Darque-Ceretti, Fezai, & Felder, 2011). A typical MLA result shown as an image obtained from Granite A is shown in Figure 2.    In general, the limestone aggregates, being basic, are believed to perform better in practice as well as in moisture sensitivity tests, while the granite aggregates have been found to perform poorly in previous moisture sensitivity work (Grenfell, Ahmad, Airey, Collop, & Elliott, 2012).

Asphalt mixture
Twelve combinations of the four aggregates (two limestones and two granites) and three bitumens (15, 50 and 100 penetration grades) were included in the study. A standard continuously graded 0/32 mm (28 mm) dense bitumen macadam (DBM) base material was used with the four aggregate types. The grading curves for Limestone A and Granite A, based on the mixture design for the 0/32 mm DBM, are shown in Figure 3. All four asphalt mixtures (two limestones and two granites) were within the specified grading envelope.
A target binder content of 4% by total mixture mass was selected for all the asphalt mixtures and roller compacted slabs (305 mm × 305 mm × 100 mm) were manufactured and finally cored and trimmed to produce 100 mm diameter by 60 mm high specimens with a target air voids content of between 8% and 10% (typical of field cores). Only cores that achieved this target were selected for the SATS test.

Testing methodology and underlying theories 3.1. Surface free energy
The SFE of a material is defined as the energy needed to create a new unit surface area of the material in a vacuum condition. The surface energies of bitumen and aggregate or a bitumenaggregate system (asphalt mixture) are mainly comprised of an apolar (non-polar) component and an acid-base component (Fowkes, 1962;Good & van Oss, 1991;Good, 1992). Equation (1) is used to describe the total surface energy and its components: where γ is the surface energy of bitumen or aggregate (mJ/m 2 ), γ LW the Lifshitz-van der Waals component of the surface energy (mJ/m 2 ), and γ AB the acid-base component of the surface energy (mJ/m 2 ). The Lifshitz-van der Waals force contains at least three components: London dispersion forces, Debye induction forces, and Keesom orientation forces (Maugis, 1999). The acid-base interaction includes all interactions of electron donor (proton acceptor)-electron acceptor (proton donor) type bonds including hydrogen bonding. To quantitatively predict and treat the acid-base interaction, Good and van Oss (1991) postulated a resolution of the acid-base term, γ AB into a Lewis acidic surface parameter and a Lewis basic surface parameter. The relationship among the γ AB and its components is shown in Equation (2): where: γ + is the Lewis acid component of surface interaction, and γ − the Lewis base component of surface interaction.

Dynamic contact angle tests
Surface energy components of the bitumen were determined indirectly using contact angle measurements. A Cahn model dynamic contact angle (DCA) analyser ( Figure 4) was used to measure the contact angles of a set of three carefully selected probe liquids on bitumen coated glass slides under dynamic conditions. The probe liquids used included water, glycerol and diiodomethane. All the tests were conducted at room temperature (23 • C ± 2 • C) and 50% ± 5% relative humidity. During the test, a clean 40 mm × 24 mm × 0.45 mm No. 15 microscope glass slide was coated with bitumen and hung from the balance of the DCA equipment with the help of a crocodile clip ( Figure 4). A beaker containing a probe liquid was placed on a movable stage positioned under the glass slide. The bottom edge of the slide was kept parallel to the surface of the probe liquid. The bitumen-coated glass slide was then immersed up to a maximum depth of 5 mm (advancing) and then withdrawn (receding) from the liquid by moving the stage up and down, respectively, at a constant speed of 40 microns/sec while continuously recording the change in mass of the bitumen-coated slide with depth of immersion. The measured mass-depth relationships were used to estimate the force acting on the bitumen-slide while being immersed or removed from probe liquid and used subsequently to determine the contact angle between the bitumen and the probe liquid.
The contact angle (θ) values are obtained by considering the equilibrium forces acting on the bitumen-coated slide while advancing and receding from the probe liquid using Equation (3) (Bhasin, 2006): where P t is the perimeter of the bitumen coated plate, γ L the total surface energy of the probe liquid, F the difference between weight of plate in air and partially submerged in probe liquid, V im the volume of solid immersed in the liquid, ρ L the density of the liquid, ρ air the air density, and g the gravitational force.
To obtain surface energy values for the bitumen, contact angle values for at least three probe liquids are measured and applied to the Young-Dupré equation (Equation (4)) for the work of adhesion (W SL ) between the two materials. Three equations are thus produced using the known surface energy components of the three probe liquids for the determination of the three surface energy components (γ LW , γ + , γ − ) of the bitumen. The surface energy components of the three probe liquids (water, glycerol and diiodomethane) used here can be found in Grenfell et al. (in press).
where subscripts L and S represent liquid and solid, respectively, and θ is the contact angle.

Dynamic vapour sorption test
It is difficult to use the contact angle technique on high surface energy materials like aggregates (SFE values generally > 60 mJ/m 2 ) as probe liquids readily spread on high energy surfaces and it is difficult to obtain accurate contact angles. Therefore, for this study, a dynamic vapour sorption (DVS) system (DVS Advantage 2, Surface Measurement Systems, Middlesex, UK) shown in Figure 5 was used to determine sorption isotherms for the various aggregates and probe vapour combinations and the results used to determine the SFE components of the aggregate. The desired partial vapour pressures were varied from 0% to 95% with 5-10% increments (14 steps). Prior to testing, the aggregates to be tested were first washed with deionised water and then dried in an oven to constant mass (up to 16 h). Aggregate fraction passing 5 mm and retained on Figure 5. DVS device (DVS Advantage 2) for determining sorption isotherms of aggregates exposed to various probe liquids. An ultra-sensitive microbalance continuously measures weight changes of aggregate resulting from vapour adsorption.
2.36 mm was used. The upper limit on aggregate size is dictated by the material holding capacity of the sample chamber. The cleaned oven-dried aggregate samples (less than 10 g) were again pre-heated in the DVS sample chamber at a temperature of 110 • C for up to 5 h to completely dry the samples before the sorption test.
To perform the sorption test, carefully selected probe vapours (octane, ethyl acetate, and chloroform) with known SFE components (Grenfell et al., in press) were passed through the aggregate sample, under controlled temperature and partial vapour pressure conditions, with the aid of an inert carrier gas (nitrogen). The probes that were chosen for the aggregate testing had relatively low surface tension values as compared to the ones that are used for testing the bitumen to aid the ability to achieve a uniform adsorption/monolayer of the probe on the aggregate surface. Due to the surface characteristics of the aggregate, vapour probes get adsorbed on their surfaces which results in an increase in the mass of the aggregate sample that is then measured using a sensitive balance.
During the test, the aggregate material was exposed to different concentrations/vapour pressures of the probe liquids (shown in Figure 6) and the increase in mass of the aggregates because of adsorption of the probe vapours on the aggregate surface was measured. All the tests were performed at a temperature of 25 • C. The change in mass of an aggregate sample was plotted against the increasing partial vapour pressure values to generate sorption isotherms which were used to estimate specific surface area (SSA) and spreading equilibrium pressures of the aggregates.  . Typical sorption isotherm obtained for Limestone A aggregate using octane vapour as probe for partial vapour pressures (concentration) ranging from 0% to 95% with 5-10% increments (14 steps). The isotherms obtained for the various aggregates showed characteristics typical of Type II isotherms.

Specific surface area
A typical obtained adsorption isotherm is shown in Figure 7 for Limestone A aggregate with octane probe vapour for partial vapour pressures (concentrations) ranging from 0% to 95%. Similar isotherms were obtained for the other aggregates. From Figure 7, it can be seen that the plot of adsorbed mass versus partial vapour pressures for Limestone A shows characteristics typical of Type II isotherms (Erbil, 2006). This suggests that the Brunauer-Emmett-Teller (BET) theory model can be used to fit the sorption isotherms (up to 35% partial vapour pressure), using the Langmuir approach (Equation (5)) as shown in Figure 8, where a plot of P/(P 0 − P)n against P/P 0 gives a straight line from which the BET constant (c) and the specific amount of vapour adsorbed on the surface of the aggregate (n m ), can be obtained. The results were used to estimate Figure 8. BET model was used to fit the sorption isotherms up to 35% partial vapour pressure. The slope and intercept of the BET plots were used to estimate the parameters n m and c that used to calculate surface area of the aggregate.
where P is the partial vapour pressure (Pa), P 0 the saturated vapour pressure of solvent (Pa), n the specific amount adsorbed on the surface of the absorbent (mg), and c the BET constant (parameter theoretically related to the net molar enthalpy of the adsorption).
where SSA is the specific surface area of solid (m 2 ), n m the monolayer specific amount of vapour adsorbed on the aggregate surface (mg), N 0 the Avogadro's number, 6.022 × 10 23 mol −1 , M the molecular weight of the vapour (g/mol), and α the projected or cross-sectional area of the vapour single molecule (m 2 ).

Spreading pressure
In addition to estimating the specific surface as previously described, the sorption isotherms were also used to calculate the spreading pressure which is required to determine surface energy components of the aggregates. Adsorption of vapour molecules on the aggregate surface reduces its SFE, so spreading pressure, as a result of adsorption of the vapour molecules, can be expressed as follows: where π e is the spreading pressure at maximum saturated vapour pressure or equilibrium spreading pressure (mJ/m 2 ), γ S the aggregate surface energy in vacuum, and γ SV the aggregate surface energy after exposure to vapour.
Spreading pressure at maximum saturation vapour pressure, π e for each solvent, is calculated by using the following Gibbs free energy model (Equation (8)): where R is the universal gas constant (83.14 cm 3 bar/mol.K) and T the absolute temperature (K). By introducing spreading pressure, π e , in the Young-Dupre relation (Equation (4)), the following relationship is obtained: The contact angle value for high energy solids such as aggregates is zero, therefore, Equation (9) can be re-written as By substituting the above relation in Equation (4), the following equation is obtained: From Equation (11), if the spreading pressures from three different probe vapours are measured, then the three surface energy components of the aggregates (γ LW S , γ + S , γ − S ) can be determined by solving three simultaneous equations.
The surface energy properties of the bitumen and the aggregates on their own have very little significance. However, when combined thermodynamically, they are helpful for estimating the interfacial work of adhesion between the two materials, with or without the presence of moisture.

Dry and wet work of adhesion
The main objective for measuring surface energy of bitumen and aggregates is to be able to estimate the moisture sensitivity of asphalt mixtures using the principles of thermodynamics and physical adhesion. This objective was accomplished by using the surface energy properties of the aggregate and bitumen to calculate their interfacial work of adhesion (dry bond strength) and the reduction in free energy of the system (work of debonding) when water displaces bitumen from the aggregate-bitumen interface Equations (12) and (13). For an asphalt mixture to be durable and less sensitive to moisture, it is desirable that the work of adhesion between the bitumen and the aggregate be as high as possible.
In addition to the two parameters: dry bond strength and work of debonding, a third parameter, the cohesion of bitumen, can be calculated from the surface energy properties of bitumen. These three bond energy parameters (bitumen cohesion, dry bond strength, and work of debonding) can then be used to assess the moisture sensitivity of an asphalt mixture. Bitumen cohesion is the cohesive bond strength of the material and is estimated as twice the total surface energy of the material. Dry bond strength (W a BA ) is defined as given in Equation (12) as the interfacial work of adhesion between the bitumen (B) and aggregate (A). A bigger value of dry bond strength suggests greater adhesion between the two materials and hence more resistance against debonding.
Equation (13) gives the work of debonding (W a BWA ) which is considered as the reduction in bond strength of a bitumen-aggregate system when water (W ) is introduced into the system or when water displaces the bitumen from the aggregate surface. A smaller value of this parameter for a given bitumen-aggregate system is indicative of a better moisture damage performance of that system.

Bitumen-aggregate bond energy parameters
The ratio (ER 1 ) between the adhesive bond energy values in the dry condition (W a BA ) and in the presence of water (W a BWA ) can be used to predict the moisture sensitivity of asphalt mixtures. A higher value of energy ratio indicates better resistance to moisture damage for that bitumenaggregate combination. Bhasin et al. (2006) used energy ratio ER 1 to study different types of asphalt mixtures and concluded that mixtures with a ratio higher than 1.5 were more moisture resistant than the ones with ratios lower than 0.8.
Aggregates with higher surface roughness and greater surface area are supposed to bond better with bitumen by providing more bond area and better interlocking. In order to accommodate this effect, a second bond energy parameter (ER 1 *SSA) obtained by multiplying the bond energy ratio (ER 1 ) with SSA has been proposed in addition to ER 1 to predict moisture sensitivity of asphalt mixtures.
Wetting/coating of an aggregate with bitumen is not only affected by the surface properties of the two materials; the viscosity or cohesion of the bitumen itself also plays a very important role. Bitumen with lesser cohesion and greater affinity for the aggregates will have a higher wetability and will coat the aggregate surface more than bitumen having lesser wettability characteristics. However, softer bitumen having lesser cohesion may be more prone to emulsification (decrease in cohesion) in the presence of water. The effects of cohesion and wettability on moisture resistance can be accounted for by modifying the ER 1 parameter by replacing the bond strength in the dry condition (W a BA ) with a wettability relationship (W a BA − W BB ). This new moisture sensitivity assessment parameter (ER 2 ) is given by Equation (15). In order to accommodate the effects of the aggregate micro-texture on the bitumen-aggregate bond strength in the presence of moisture, the bond parameter ER 2 can be multiplied by SSA of the aggregates to obtain a fourth bond energy parameter (ER 2 *SSA).
where (W a BA ) and (W BB ) represent bitumen-aggregate dry bond strength and bitumen cohesion, respectively.
These four bitumen-aggregate bond energy parameters (ER 1 , ER 1 × SSA, ER 2 and ER 2 × SSA) were used to assess the moisture susceptibility of the asphalt mixtures. In all cases, higher energy ratios are associated with mixtures with better moisture resistance.

Rolling bottle test
The RBT was conducted in accordance with EN 12697-11 (Bituminous mixtures -Test methods for hot mix asphalt Part 11 -Determination of the affinity between aggregate and bitumen). Dustfree aggregate (passing 5 mm and retained on 2.36 mm sieve) samples weighing 170 g were dried in an oven at 105 ± 5 • C overnight to constant mass and then coated with 5.7 g of molten bitumen. Mixing of the aggregates with bitumen was conducted at 120 ± 5 • C. The aggregate-bitumen mixture was then cooled loose at room temperature. The loose mixture was stored at ambient temperature for 12 to 64 h before testing. Each of the test bottles were filled to about half their volume with deionised water and about 150 g of the loose aggregate-mixture was placed in each bottle. The whole assembly was put in the bottle roller rotating at a speed of 60 rotations per minute for 6 h.
At the end of the test, the degree of bitumen coverage of the aggregate particles were estimated by visual observation and recorded to the nearest 5%. The procedure (i.e. rotation in the bottle roller and measuring of bitumen coverage) is repeated for three more cycles (24, 48, and 72 h) with fresh water replacing the fouled water in the test bottle at the end of each cycle and the degree of bitumen coverage being measured.

SATS test
The SATS test is the first procedure of its kind that combines the ageing and water damage mechanisms to which an asphalt pavement is subjected in service within a single laboratory test. The test has been found to successfully reproduce the loss in stiffness observed with high modulus asphalt material in the field (Collop, Choi, Airey, & Elliott, 2004a), and to distinguish between poor performing material and alternative mixtures incorporating aggregates with good durability track records, when manufactured at a 4% binder content and 8% air voids (Airey, Choi, Collop, & Elliott, 2003;Choi, 2005;Choi et al., 2002;Collop, Choi, & Airey, 2004b). It has also been shown that the SATS test is able to rank asphalt mixtures in terms of moisture sensitivity, in the same order as the AASHTO T283 procedure (Anon, 2000), although the relative performance of a mixture containing a moisture sensitive aggregate was significantly lower in the SATS test (Airey, Choi, Collop, Moore, and & Elliott, 2006).
The standard SATS procedure involves conditioning of five pre-saturated specimens simultaneously in a pressure vessel under 2.1 MPa air pressure at a temperature of 85 • C for a period of 65 h (see Grenfell et al., 2012). This conditioning is followed by a cooling period of 24 h before the air pressure is released and the vessel opened to remove the specimens for stiffness testing. This procedure is applicable for asphalt mixture specimens made with 15 pen bitumen with a modified SATS protocol being used to test asphalt mixtures with softer 50 pen and 100 pen binders (Grenfell et al., 2012).

DCA tests
Results of the DCA contact angle measurements for the three bitumens and three probe liquids are presented in Figure 9. For each bitumen probe liquid combination, 5 repeats were carried out. The two extreme values were discarded and then the average of the remaining 3 was used as the contact angle value. The repeatability of the DCA test has been previously discussed by Hefer, Bhasin, and Little (2006). From Figure 9, it can be seen that water exhibited the largest contact angle values (93.0-95.1 • ) with the bitumen followed by glycerol (80.2-84.2 • ) and then diiodomethane (58.3-68.9 • ). The higher the contact angle between a solid and a liquid, the lower the potential Figure 9. Contact angle of probe liquids obtained from the DCA test. The higher the contact angle between a solid and a liquid, the lower the potential for the two to adhere to each other. for the two to adhere to each other. In general, contact angles above 70 • suggest hydrophobic tendencies, therefore, the results appear to support the hydrophobic nature of bitumen. The contact angle values were used to estimate surface energy components as previously discussed. The resulting surface energy components for the three bitumens are presented in Table 2. As expected, the softer 100 pen bitumen exhibited comparatively lower total surface energy (19.1 mJ/m 2 ) with the results for the 50 pen and 15 pen bitumen being very similar.

DVS tests
For the four aggregates, only fractions passing the 5 mm sieve and retained on the 2.36 mm sieve were tested and reported in this paper. The results were used to estimate SSA and equilibrium pressure from which the surface energy parameters were calculated. Only one run was carried out for each probe liquid-aggregate combination in this work, but the repeatability of such testing has been previously discussed by Bhasin and Little (2007).

Specific surface area
SSA obtained for the four aggregates are presented in Table 3 using octane as the probe vapour. SSA for the various aggregates showed large differences depending on aggregate type. The differences can be attributed to the different micro-structure of the aggregates. The SSA obtained for each aggregate was used in two different ways: (1) to determine the equilibrium spreading pressure and (2) to calculate the moisture compatibility ratios.

Equilibrium spreading pressure
The SSA values were used to calculate the equilibrium spreading pressures on the aggregate surfaces for all the three probes. Octane, being non-polar in nature, is supposed to give more accurate values of surface area (because non-polar substances do not have affinity for polar substances). The obtained spreading pressures are provided in Table 4.

Surface energy components
Computed surface energy components (γ LW S , γ + S , γ − S ) as well as the total surface energy (γ T S ) for the aggregates are provided in Table 5. The results show that surface energy properties vary considerably, in terms of surface energy components as well as total surface energy, amongst the different aggregates. The differences can be attributed to different elemental and mineralogical compositions of the aggregates. The test results indicate that there is not a big difference between the van der Waals components of the aggregates though Limestone A has a value (75.2 mJ/m 2 ) that is slightly higher than the SE (67.8 mJ/m 2 ) for Granite A.
Differences were seen when the total surface energy of the four aggregates (a measure of adhesive potential of a material) are compared. On the basis of total surface energy alone, and for the same bitumen, Granite A (γ T = 353.5 mJ/m 2 ) should theoretically form stronger adhesive bonds than Limestone A (γ T = 222.7 mJ/m 2 ). Note that this assertion assumes a completely dry aggregate.

Dry and wet work of adhesion
Work of adhesion results for the various aggregate-bitumen combinations are presented in Table 6. The results show both the influence of the different aggregates and bitumen on work of adhesion. Work of debonding values for the aggregate-bitumen combinations are presented in Table 7. In addition to the work of adhesion, the greater the magnitude of work of debonding when water displaces bitumen from the aggregate-bitumen interface, the greater will be the thermodynamic potential that drives the moisture damage. The results show that for a given aggregate, work of debonding increases (in magnitude) for softer bitumen compared to harder (stiffer) binders. Similarly, the effect of aggregate on work of debonding increases as one moves from limestone to granite. Table 8 shows the aggregate-bitumen bond energy parameters (ER 1 , ER 2 , ER 1 *SSA and ER 2 *SSA) for the asphalt mixtures. These four bond energy parameters can be used to predict moisture sensitivity of asphalt mixtures using threshold values defined to separate 'good' from 'poor' moisture damage performing aggregate-bitumen combinations. The threshold limits are 0.75 for ER 1 , 0.50 for ER 2 , 0.50 for ER 1 *SSA and 0.35 for ER 2 *SSA (Bhasin, 2006;Bhasin et al., 2006). The aggregate-bitumen combinations that are classified as 'poor' based on these threshold limits are shown in bold in Table 8. The results show that the ranking of the 'good' versus 'poor' moisture damage performing aggregate-bitumen combinations for ER 1 and ER 2 are quite similar; both parameters placing the same number of combinations in 'good' versus 'poor' categories. The results for the other two parameters, ER 1 *SSA and ER 2 *SSA, are also similar but the later placed slightly more mixtures in the 'poor' category. The results suggest, for the materials considered, that ER 1 and ER 2 are sensitive to binder cohesion as the softer 100 pen bitumen showed lower ratios irrespective of the aggregate type.

Aggregate-bitumen bond energy parameters related to moisture damage
Compared to the ER 1 and ER 2 parameters, the results for ER 1 *SSA and ER 2 *SSA show the significant influence of SSA on the selection of 'good' versus 'poor' moisture damage performing aggregate-bitumen combinations. Because of the apparent large influence of SSA on moisture sensitivity of asphalt mixtures shown in Table 8, the bond parameters ER 1 *SSA and ER 2 *SSA appear to be more suitable indices for determining the performance of the different aggregatebitumen combinations with a clear distinction in terms of 'good' and 'poor' aggregates. Most of the granite-bitumen combinations were identified as 'poor' aggregates which is in agreement with past field experience with these aggregates.

RBT tests
In the RBT method, the degree of bitumen coverage of the coated aggregate particles was assessed after 6, 24, 48, and 72 h of conditioning in a bottle roller. Sample results for two aggregates (Limestone A and Granite A) coated with 50 pen bitumen are presented in Figure 10. Similar results were obtained for all the aggregate-bitumen combinations. From Figure 10, it could be seen that the percentage of bitumen coverage decreased slowly with testing time for Limestone A, while on the contrary, percentages for Granite A reduced sharply during the test period. For instance, during the first 6 h, Limestone A showed only a 2% binder loss while Granite A showed about 20% loss. In addition, the percentage of binder loss for granite at 6 h is equal to that for the limestone aggregate at 72 h. The RBT results in Figure 10 ranking the limestone aggregate as better than the granite is in agreement with both the surface energy results presented in Table 8 and the observed field performance of these aggregates.

SATS tests
SATS tests were carried out using the modified SATS protocol (Grenfell et al., 2012) on both the limestone and the granite aggregates in the form of 0/32 mm dense road base mixtures using the same 50 pen bitumen. Results from the SATS tests can be seen in Figure 11, which demonstrates the superior moisture resistance of the mixture made with the limestone aggregate. It can be seen that the retained stiffness for the limestone mixture specimens is in excess of 0.8, whereas for the granite mixture, the retained stiffness ranges from 0.2 to 0.5. It is generally accepted that if Figure 10. A plot of the percentage of aggregates that remain coated with bitumen as a function of conditioning time during the RBT. The results show Limestone A is more resistant (higher per cent remaining coated) than Granite A for the mixtures considered. Figure 11. Comparison of SATS retained stiffness with retained saturation for 50 pen mixtures. SATS appears to rank limestone better (higher retain stiffness at a given saturation level) than granite in terms of moisture resistance. a retained stiffness of less than 0.6 is achieved that a mixture is considered moisture susceptible. This suggests the granite mixtures are more moisture sensitive than the limestone mixtures. The repeatability of the SATS test results is covered in Grenfell et al. (2012).

Discussion
As previously indicated, the key objective of this study was to determine if the moisture sensitivity assessment parameters for different bitumen-aggregate combinations obtained by using surface energy parameters of the individual materials can identify 'good' and 'poor' performing asphalt mixtures and to determine how the surface energy-based prediction compare with two standard asphalt mixture tests (RBT and SATS). Figure 12 shows a plot depicting the relationships between SATS retained stiffness and the bond energy parameter ER 1 compare with the RBT per cent bitumen coverage. In all cases, a higher value of the parameter suggests better resistance to moisture damage. On this basis, aggregate-bitumen combinations plotting near the upper righthand side of the plot (equivalent to higher values of ER 1 , RBT coverage and/or SATS retained stiffness) are expected to be more moisture resistant than mixtures plotting in the lower left hand side. The results show that limestone mixtures should be expected to perform better than Figure 12. Plots of SATS retained stiffness and bitumen coverage during RBT as a function of bond energy ratio, ER 1 , for 50 pen bitumen showing the influence of aggregate type on moisture sensitivity. Both SATS and RBT ranked limestone mixtures higher (higher retained stiffness and higher bitumen coverage) than granite mixtures which is in agreement with compatibility ratio obtained from surface energy measurements.
granite mixtures. The results also illustrate the close agreement between the three techniques for evaluating moisture sensitivity. The 'good' performance of most of the limestone mixtures observed in this study could be attributed to several physico-chemical and mineralogical characteristics of the aggregates used. Analysis of the sorption isotherms indicated differences in surface energy characteristics of the aggregates. The effect of physico-chemical characteristics of the aggregates is manifested in the sensitivity of the bond energy parameters presented in Table 8.
Results of mineralogical characterisation of the aggregates determined using an MLA on samples of both the limestone and the granite aggregates are presented in Table 1. The differences in moisture sensitivity seen in the aggregates can be attributed in part to the mineralogical characteristics of the aggregates.

Conclusions
This paper presents results from the RBT and SATS tests in an attempt to better understand the underlying processes and mechanisms of moisture damage with the help of surface energy measurements on the constituent materials (bitumen and aggregates) and aggregate morphology from MLA measurements. The following conclusions were reached based on the results presented in the paper.
• Surface energy parameters obtained from the DCA testing suggests cohesive strength varies with bitumen grade. Surface energy of the softest bitumen tested (100 pen) was about 60% that of the stiffer bitumens (15 pen and 50 pen). • The adhesive bond strengths for both the dry and the wet conditions were used to compute four compatibility ratios using the surface energy parameters obtained for the bitumen and aggregates. Higher magnitudes of the ratios suggest better resistance to moisture damage. The results show that for a given aggregate, moisture resistance of stiffer binders is higher than for softer binders. The results also show that for a given bitumen grade, and for the aggregates considered in this study, the limestone aggregate mixtures should exhibit higher resistance (higher ratios) to moisture damage. • The four aggregate-bitumen bond energy parameters (ER 1 , ER 2 , ER 1 *SSA and ER 2 *SSA) can be used to predict moisture sensitivity of asphalt mixtures using threshold values (0.75 for ER 1 , 0.50 for ER 2 , 0.50 for ER 1 *SSA and 0.35 for ER 2 *SSA) defined to separate 'good' from 'poor' moisture damage performing aggregate-bitumen combinations. Most of the aggregates that were identified as 'poor' aggregates in this study have also been found to perform poorly in previous studies. • Compared to the ER 1 and ER 2 parameters, the results for ER 1 *SSA and ER 2 *SSA showed the significant influence of SSA on the selection of 'good' versus 'poor' moisture damage performing aggregate-bitumen combinations. Because of the apparent large influence of SSA on moisture sensitivity of asphalt mixtures shown in this study, the bond parameters ER 1 *SSA and ER 2 *SSA appear to be more suitable indices for determining the performance of the different aggregate-bitumen combinations with a clear distinction in terms of 'good' and 'poor' aggregates. • Results from the RBT showed that the percentage of bitumen coverage (a measure of adhesiveness) varies depending on the aggregate type. About 90% of the limestone aggregate remained coated with bitumen at the end of the RBT compared with only 20% for the granite aggregate. This suggests that in the presence of moisture, limestone aggregate maintains a better adhesive bond with bitumen than granite aggregate which agrees with the surface energy ranking of the mixtures. • Moisture damage factors (moisture factors) obtained from the SATS tests for limestone aggregate asphalt mixtures were comparatively higher than that for granite mixtures. Higher moisture factors indicate better moisture resistance. Thus, the SATS test results appear to be in agreement with the RBT as well as the surface energy parameters. • Mineralogical testing of the aggregates, using MLA, showed that Limestone A and Limestone B are composed predominantly (97%) of calcite. The dominant mineral phases in Granite A consisted of quartz (33%), albite (28%), and feldspar (17%) and Granite B albite (33%), anorthite (19%) and quartz (16%). Therefore, the differences in moisture sensitivity of the mixtures observed in this study for the different aggregates can be attributed in part to aggregate mineralogy. • Both the RBT and SATS are useful in evaluating moisture damage in asphalt mixtures as the ranking obtained in these empirical tests are similar to surface energy and mineralogical characteristics of the asphalt mixtures. • Surface energy properties of the materials combined with the parameters obtained by conventional moisture sensitivity assessment techniques can also contribute towards the development of a material screening protocol for determining the best combinations of bitumen and aggregates for the local road material providing better bitumen-aggregate adhesion and less susceptibility to moisture damage/stripping.