Limits on Adhesive Bond Energy for Improved Resistance of Hot-Mix Asphalt to Moisture Damage

PART 1

Bituminous–Aggregate Combinations to Meet Surface Requirements

3

Transportation Research Record: Journal of the Transportation Research Board,No. 1970, Transportation Research Board of the National Academies, Washington,D.C., 2006, pp. 3–13.

The loss of physical adhesion between the aggregate and the asphaltbinder is one of the important mechanisms that accelerate moisturedamage in hot-mix asphalt pavements. In this study, two parametersrelated to bond energy—adhesive bond energy between the aggregateand the asphalt and reduction of free energy when asphalt debonds fromthe aggregate surface in the presence of moisture—were quantifiedwith surface energies of both materials. Threshold values of theseparameters to identify asphalt–aggregate combinations susceptible topremature moisture damage were derived by comparison of the valuesof these parameters with observed field performance for several mixes.Results show significant differences in bond energies developed betweenvarious aggregates and a given binder. This finding illustrates theimportance of binder–aggregate compatibility and the sensitivity ofcalculated bond strength to surface energy measurements. Asphaltbinders from different sources with the same performance grade werealso found to develop different bond energies with any given aggregate.The results show that binders differ in their sensitivity to changes inaggregate source in terms of the developed bond energy. The method-ology of using surface energy and concomitant bond energy calcula-tions to assess the moisture sensitivity of asphalt concrete mixes isdiscussed as well as the advantages of using this technique comparedwith conventional mechanical tests.

Moisture damage is a major form of pavement distress resultingin high maintenance costs for state and federal highways. Severaltheories (1–7) have been proposed to explain moisture damage in hot-mix asphalt (HMA). Most of these theories attribute moisture dam-age to loss of adhesion resulting from one or more of the followingmechanisms:

• Poor mechanical interlocking,• Failure of physical adhesion between asphalt and aggregate in

the presence of moisture, and• Chemical interactions between asphalt and minerals on the

aggregate surface.

Current laboratory procedures for assessing moisture sensitivityof HMA rely on comparison of mechanical properties of uncondi-

tioned specimens and moisture-conditioned specimens. Althoughthis approach is helpful in comparative analysis of the moisture sus-ceptibility of various mixes, it does not focus on measuring the fun-damental material properties related to the mechanisms just described.As such, the results cannot be used to explain causes for poor orgood performance and do not provide feedback into the process ofredesigning better-performing mixes. It is therefore necessary tosupplement the mechanical properties normally measured with fun-damental properties that affect physical adhesion between asphaltand aggregate and the propensity to lose this bond in the presenceof water.

Recent studies at Texas A&M University have shown that acomprehensive characterization of moisture damage should in-clude measurements of fracture, healing, and viscoelastic proper-ties. These mechanical properties are influenced by the binder–aggregate bond energy and other physical characteristics of themix such as volumetrics, asphalt film thickness, air void dis-tribution, aggregate gradation, and aggregate shape characteris-tics (8–11). A comprehensive mechanistic-based approach hasbeen developed to account for the interactions of these factors ininfluencing moisture damage (8, 9). This approach substantiatesthe need to account for binder–aggregate bond energy in charac-terizing HMA healing and fracture properties both in a dry stateand in the presence of moisture. This approach, of course, alsoleads to a clearer understanding of the ability of a specific com-bination of asphalt binder and aggregate (a specific HMA) toresist moisture damage.

The strength of physical adhesion between the asphalt and theaggregate in a dry condition is quantified in terms of the adhesivebond energy. A high magnitude of adhesive bond energy is desir-able for HMA to perform as a durable composite. Displacement ofasphalt from the asphalt–aggregate interface by water is associatedwith an overall reduction in free energy of the system, making thisprocess thermodynamically favorable (12). A greater magnitudeof reduction in free energy of the system would mean greater propen-sity for water to debond asphalt from the aggregate surface andvice versa. Therefore, a low magnitude of reduction in free energyis desirable to resist debonding in the presence of water. On thebasis of this explanation, an asphalt–aggregate combination witha high adhesive bond energy (in the dry state) and a low magni-tude of reduction in free energy in the presence of moisture shouldhave a reduced potential to debond and therefore will possess agreater resistance to moisture damage. Hereafter in this discus-sion, the term “bond energy parameters” will be used to refer toboth adhesive bond energy between aggregate and asphalt in thedry condition and change in free energy of the system when water

Limits on Adhesive Bond Energy forImproved Resistance of Hot-Mix Asphalt to Moisture Damage

Amit Bhasin, Eyad Masad, Dallas Little, and Robert Lytton

Texas Transportation Institute, Department of Civil Engineering, Texas A&M University, 3135 TAMU, College Station, TX 77843-3135.

displaces asphalt from the asphalt–aggregate interface. The bondenergy parameters are calculated with surface energies of the asphalt,aggregate, and water and are used to predict moisture sensitivityof asphalt concrete mixes.

In the first part of this paper, a simplified description of the prin-ciples employed in measuring surface energies of aggregates andasphalts and calculating the bond energy parameters is presented.More details on the fundamentals and experimental measurementsof asphalt binder and aggregate surface energy can be found in studiesby Little, Lytton, and their coworkers (13–15). In the second partresults on the relationship between bond energy and moisture sen-sitivity of several field mixes are discussed. The results show thatbond energy parameters can be used to identify effectively mixturesthat are prone to moisture damage. A threshold value for an index thatcombines the dry and wet bond energy parameters was also derivedfrom these comparisons. The threshold value is recommended as ascreening tool to select optimum asphalt–aggregate combinationsthat are the least susceptible to moisture damage on the basis of theirfundamental surface properties.

4 Transportation Research Record 1970

SCOPE AND OBJECTIVES

The main objective of this study was to recommend parameters basedon aggregate–binder adhesive bond energy that can be used to iden-tify asphalt mixtures that are susceptible to moisture damage. Thisobjective is achieved through the following steps:

• Measurement of the surface energies of a wide range of asphaltbinders and aggregates,

• Calculation of bond energy parameters for various asphalt–aggregate combinations from their individual surface energycomponents,

• Comparison of bond energy parameters with the resistance ofasphalt mixtures to moisture damage, and

• Investigation of the influence of aggregate types and bindersources on bond energy parameters and moisture damage.

The experimental and analytical steps used to measure surfaceenergies of asphalt and aggregate, calculate bond energy parameters,and predict moisture sensitivity of mixes are shown in Figure 1.

Wilhelmy Plate Test: Asphalt slides tested with

probe liquids

Output: Contact angles of probe

liquids with asphalt

Analysis: Compute work of adhesion with different probe liquids

Result: Three surface energy

components of asphalt

Adsorption Test: Aggregate samples tested

with probe vapors

Output: Spreading pressures of

probe vapors and specific surface areas of aggregates

Analysis: Compute work of adhesion with different probe vapors

Result: Three surface energy

components of aggregate

Performance Related Parameters: 1. Dry adhesive bond energy 2. Reduction of free energy in

presence of water

Moisture Sensitivity of Mixes: Field performance / mechanical

testing

Experimental

Analytical

FIGURE 1 Steps to determine moisture sensitivity of mixes with surfaceenergy measurements.

BACKGROUND ON SURFACE ENERGY AND BOND ENERGY PARAMETERS

Surface Energy

Surface energy (γ) is defined as the amount of external work done ona material to create a new unit surface area in a vacuum. Typically,surface energy is presented in units of ergs per square centimeter.One of the popular theories to explain surface energy characteristicsis the Good–van Oss–Chaudhury (GVOC) (16) theory, according towhich the total surface energy of any material is divided into threecomponents on the basis of the type of molecular forces acting on thesurface. These three components are the nonpolar component, alsoreferred to as the Lifhsitz–van der Waals (LW) component, γLW; theLewis acid component, γ+; and the Lewis base component, γ−. Totalsurface energy of the material, γ total, is obtained by combining thesethree components as follows:

It is rarely feasible to measure surface energies of solids directly.A more practical method to estimate surface energy of a solid is tomeasure its work of adhesion with probe liquids or vapors. A liquidor vapor can be used as a probe if the surface energy componentsof the liquid are known, it is homogenous and pure, and it does notchemically interact (react or dissolve) with the solid surface that isbeing measured.

Materials such as asphalts and polymers have low-energy surfaces;that is, the total surface energy of the solid is less than the total surfaceenergy of the probe liquid. When a liquid comes into contact with alow-energy surface—for example, when an asphalt slide is immersedin a probe liquid—it forms a finite contact angle measured at theboundary of the liquid meniscus and the solid. The contact angle, θ,of a probe liquid on the asphalt surface can be measured experi-mentally and is related to the surface energy components of thesematerials.

Materials such as aggregates have high-energy surfaces; that is,the total surface energy of the solid is much higher than the totalenergy of the probe liquid. When a probe liquid comes into contactwith a clean high-energy surface, it spreads over the surface of thesolid. In fact, whenever a clean high-energy surface is exposed tovapors of the probe liquid, the molecules of the probe vapor areadsorbed on the solid surface, reducing its free energy. This reduc-tion in free energy of the solid is the spreading pressure, π, and canbe measured experimentally from adsorption experiments. The massof the probe vapor physically adsorbed on the aggregate surface andthe equilibrium spreading pressure, πe, depend on the surface energiesof these materials.

The work of adhesion between a solid with unknown surfaceenergy, X, and a probe vapor, P, is calculated from the experimentallymeasurable parameters: contact angle, θ, and equilibrium spreadingpressure, πe. The work of adhesion is also related to the three sur-face energy components of these two materials by using the GVOCtheory as shown in Equation 2. For a low-energy surface such asasphalt, the equilibrium spreading pressure, πe, is negligible and isset to 0, and only the contact angle, θ, is determined experimentally.Similarly, for a high-energy surface such as aggregate, the contactangle, θ, is set to 0 and only the equilibrium spreading pressure, πe,is determined experimentally. Since the left-hand side of Equation 2,

γ γ γ γtotal LW= + + −2 1( )

Bhasin, Masad, Little, and Lytton 5

relating the Young–Dupré equation and the GVOC equation, containsthree unknowns (square roots of the surface energy components ofasphalt or aggregate), it is necessary to determine the work of adhe-sion with at least three probe liquids or vapors in order to generate aset of three equations that can be solved simultaneously and determinethe unknown surface energy components of the solid.

Experimental Methods to Determine SurfaceEnergies of Asphalt and Aggregate

In this study, surface energy components of asphalt binders were cal-culated by measuring contact angles with various probe liquids inthe Wilhelmy plate method. In this method the contact angle is cal-culated from simple force equilibrium considerations (Equation 3)of a glass slide thinly coated with the asphalt film and immersed ina probe liquid.

where

Pt = perimeter of asphalt slide,γ p

Total = total surface energy of probe liquid,θ = contact angle between bitumen and liquid,

Vim = volume of slide immersed in liquid,ρL = liquid density,

ρair = air density, andg = local acceleration due to gravity.

The contact angle is measured in a dynamic mode by continuouslyimmersing about 5 mm of the asphalt slide into the liquid at a constantrate. A DCA 315 microbalance with WinDCA software from ThermoCahn Instruments was used to perform the test, acquire force data,and calculate contact angles.

A schematic of the test setup is shown in Figure 2. Three replicateslides of the asphalt were tested with each of the three probe liquids(distilled water, glycerol, and methylene iodide). Coefficients ofvariation of measured contact angles among replicates were typicallywithin 3% to 4%. The measured contact angles from the three liquidswere used in Equation 2 to calculate the surface energy components ofthe asphalt. This test is fast (about 15 min per slide), easy to perform,and requires a low capital outlay to purchase and set up the equipment.More details of experimental and analytical procedures related to theWilhelmy plate method may be found elsewhere (14).

Surface energy components of aggregates were calculated by mea-suring spreading pressures with three probe vapors (n-hexane, methylpropyl ketone, and water) with the universal sorption device (USD)from Rubotherm, Inc. The USD records the mass of the vapor adsorbedon the aggregate surface at different partial vapor pressures of the probevapor. The relationship between mass of the vapor adsorbed and par-tial vapor pressure of the probe vapor is referred to as the adsorption

cos ( )θρ ρ

γ=

+ −( )ΔF V g

PL

t P

im air

Total3

2 2 2 1γ γ γ γ γ γ π γ θX P X P X P e PLW LW total+ + = + +(+ − − + cos )) ( )2

work of adhesion, GVOC theory:

three unknown terms for surface energy of solid

work of adhesion, Young–Dupré equation:

experimentally measured/known terms

isotherm. Spreading pressure of the probe vapor on the aggregatesurface is obtained from the adsorption isotherm as follows:

where

R = universal gas constant,T = test temperature,n = mass of vapor adsorbed per unit mass of aggregate at vapor

pressure p, andA = aggregate-specific surface area.

A custom-built manifold and software were used to control the test,acquire adsorption data, and report the spreading pressures. Aggre-gates passing the No. 4 sieve and retained on the No. 8 sieve were usedfor testing with the USD. The specific surface area of the aggregatewas also calculated with the Brunauer, Emmett, and Teller (BET)equations from the adsorption results of the USD. More details onexperimental and analytical procedures related to the USD methodmay be found in the paper by Bhasin and Little (15).

Bond Energy Parameters and Moisture Damage

Figure 3a shows a single-phase homogenous system. On the basisof the definition of surface energy, the external work required tofracture this material and create two new surfaces of unit area isequal to two times the total surface energy of the material (2γ), which

πe

pRT

A

n

pdp

n= ∫04( )


is referred to as the cohesive bond energy of the material. Figure 3bshows a two-phase system with materials A and S, representing asphaltand aggregate, respectively. When external work is applied to causean adhesive failure at the asphalt–aggregate interface, a unit surfacearea of each A and S is created and the asphalt–aggregate interfaceAS is lost. From the definition of surface energy, the energy requiredto create a unit area of A is γA and a unit area of S is γS. Also, sincethe original asphalt–aggregate interface is lost, the interfacial sur-face energy, γAS, is the work done by the system favoring the fractureprocess. Addition of these energy terms results in the total adhesivebond energy as follows:

The total adhesive bond energy can also be expressed as thework of adhesion between the two materials as shown previously inEquation 2:

Therefore, the dry adhesive bond energy, ΔGAS, between the asphaltand the aggregate can be calculated when all three surface energycomponents of both the asphalt and the aggregate are known. The lasttwo terms in Equation 6 represent the acid–base bond energy, ΔGAB

AS.A high magnitude of adhesive bond energy is desirable for mixes sinceit indicates stronger bonding between the asphalt and the aggregate.

Equation 7 is developed by combining Equations 5 and 6; itexpresses the interfacial surface energy between the asphalt and aggre-gate, γAS, in terms of their individual surface energy components. Theinterfacial surface energy is necessary in order to explain the reductionin free energy of the system when water displaces asphalt fromthe asphalt–aggregate interface.

Figure 3c shows a three-phase system of asphalt, aggregate, andwater before and after water displaces asphalt from the aggregate–asphalt interface. Subscripts S, A, and W are used to represent theaggregate, asphalt, and water phases, respectively. Equation 8 presentsthe net work done to displace a unit area of the asphalt–aggregateinterface (AS) by water and create a unit area of the asphalt–waterinterface (AW ) and the aggregate–water interface (SW ) in terms oftheir respective interfacial surface energies.

The interfacial surface energy of any two phases is calculated fromtheir respective surface energy components by using Equation 7. Thevalue of ΔGWAS is typically negative. This characteristic indicates thatdebonding of the asphalt–aggregate interface by water is associatedwith an overall reduction in free energy of the system and thereforeis thermodynamically favorable. A larger magnitude of ΔGWAS impliesa greater reduction in free energy of the system and a greater potentialfor water to displace asphalt from the aggregate–asphalt interface.Therefore, a low magnitude of reduction in free energy of the systemis desirable and indicates better resistance to debonding in the presenceof water. The negative value of work required to cause debonding,

ΔGWAS AW SW AS= + −γ γ γ ( )8

γ γ γ γ γ γ γ γ γAS A S A S A S A S= + − − −+ − − +2 2 2 7LW LW ( )

ΔGAS A S A S A S= + ++ − − +2 2 2 6γ γ γ γ γ γLW LW ( )

ΔGAS A S AS= + −γ γ γ ( )5

FIGURE 2 Schematic representation of Wilhelmy plate method.

ΔGWAS, can also be interpreted as the work done by the system favor-ing debonding, and therefore in the presence of water, less externalwork is required to cause the same amount of damage.

The dry adhesive bond energy between asphalt and aggregate,ΔGAS, and the reduction in free energy due to debonding of asphalt–aggregate interface by water, ΔGWAS, are the two key bond energyparameters that can be used to assess moisture sensitivity of anyasphalt–aggregate combination in an HMA mixture.


EXPERIMENTS AND RESULTS

Materials, Testing, and Results

In all, 16 field mixes from Texas, Ohio, Kansas, and Nevada wereincluded in this study. The moisture sensitivity was classified as goodor poor for eight of these mixes on the basis of qualitative inspec-tion of pavements and field cores as well as mechanical testing of

Work done for each side of new unit area created = γCohesive bond energy = Total work done = 2γ

Work done to create new unit area of A = γA

Work done to create new unit area of S = γS

Work due to loss of interface AS = - γAS

Adhesive bond energy = Total work done = γA + γS - γAS

Interface AB

Material A

Material S

SW

AW

Asphalt

Water(W)

Aggregate

SA

Aggregate

Asphalt

Water(W) Debonding

SA = Removal of Aggregate-Asphalt Interface; Work done = -γSA

SW = Formation of Aggregate-Water Interface; Work done = γSW

AW = Formation of Asphalt-Water Interface; Work done = γAW

Total Work Done = γAW + γSW - γSA

(a)

(b)

(c)

FIGURE 3 Work done for (a) cohesive failure in single-phase system, (b) adhesivefailure in two-phase system, and (c) displacement in three-phase system.

laboratory-prepared specimens (8, 17 ). Moisture sensitivity of theremaining eight mixes was not established.

Table 1 shows the 16 mix designs along with their rated resistanceto moisture damage, constituent binder grades, and aggregate types.Similar binder grades or aggregate types with suffixes A, B, and soforth, indicate that these were obtained from different sources. The16 mix designs were composed of 14 different asphalt binders and10 different aggregates. Some mixes had a combination of morethan one aggregate type. Surface energies for the 14 asphalt bindersand 10 aggregates were determined with the Wilhelmy plate methodand the USD, as described previously. Tables 2 and 3 show the surfaceenergy components for the selected asphalt binders and aggregates,respectively.


Surface energy values were used to compute the two bond energyparameters. Since surface energy components of asphalts and aggre-gates are measured individually, the bond energy parameters can becalculated for all possible combinations of asphalt and aggregateand not just the 16 selected mixes. This computation allows engineersto examine the influence of different combinations of aggregate andbinders on moisture damage and to select the optimum combinations.For mixes that contain more than one type of aggregate, the bondenergy parameters were calculated for each asphalt–aggregate pair.It is reasonable to consider that the asphalt–aggregate combinationthat provides the poorest or most critical values of these parameterswill govern the mixture performance. Table 4 gives the values of thetwo bond energy parameters.

Asphalt binders have low surface energy values as compared withthose of aggregates. The LW component of asphalt binders varies themost among different asphalt binders, whereas the LW componentsof different aggregate types do not differ significantly. However,

TABLE 1 Summary of Mix Designs

Mix Design No. Performance Asphalt Grade Aggregate Types

1

2

3

4

5

6

7

8

9

10

11

12

1314

15

16

Good

Poor

Good

Good

Good

Good

Poor

Poor

Not Rated

PG 76-22 A

PG 76-22 B

PG 76-22 C

PG 76-22 D

PG 64-22 A

PG 64-28 A

PG 64-22 B

PG 64-22 C

PG 64-22 D

PG 64-22 E

PG 64-22 F

PG 64-22 G

PG 64-22 H

PG 64-22 I

Granite A

Quartzite A

Gravel A

Sandstone A

Limestone B

Kansas aggregates (KAG)

Combination of differentaggregate sources

Nevada aggregates (NAG)Combination of different

aggregate sources

Limestone A

Sandstone B

Gravel B

TABLE 2 Surface Energy Components of Selected Asphalt Binders

Surface Energy (ergs/cm2)

Asphalt Binder LW Acid Base Total

PG 64-22 A 29.9 0.00 1.00 29.9

PG 64-22 B 23.4 1.01 0.00 23.4

PG 64-22 C 23.9 0.35 0.00 23.9

PG 64-22 D 16.9 0.75 0.55 18.2

PG 64-22 E 28.7 0.66 0.00 28.7

PG 64-22 F 30.5 1.14 0.00 30.5

PG 64-22 G 24.5 0.85 0.00 24.5

PG 64-22 H 27.7 0.10 0.58 28.2

PG 64-22 I 27.2 1.04 0.00 27.2

PG 64-28 A 17.9 0.13 2.88 19.1

PG 76-22 A 14.7 1.33 1.79 17.8

PG 76-22 B 24.2 0.07 1.31 24.7

PG 76-22 C 12.1 1.13 2.84 15.7

PG 76-22 D 21.8 0.63 0.65 23.1

TABLE 3 Surface Energy Components of Selected Aggregates

Surface Energy (ergs/cm2)

Aggregate LW Acid Base Total

Granite A 56.3 43.5 782.7 425.2

Gravel A 59.5 1.2 286.0 96.5

Gravel B 63.5 7.7 546.3 193.2

Limestone A 59.9 18.8 561.1 265.4

Limestone B 58.0 1.8 401.1 111.1

Quartzite A 60.9 8.9 545.0 200.1

Sandstone A 62.5 2.0 222.6 105.0

Sandstone B 64.0 8.5 316.9 167.8

KAG 60.9 7.5 1085.4 240.8

NAG 59.6 8.3 458.3 182.8

there is a significant difference in the base component of aggregatesurface energy, which is an order of magnitude higher than the othersurface energy components of an aggregate. Surface energy com-ponents in Tables 2 and 3, when used in Equations 6 and 7, showthat the most significant contribution to adhesive bond strength isfrom two sources: (a) the product of the LW components of asphaltand aggregate and (b) the product of the acid component of asphaltand the base component of aggregate. Therefore, although the acidcomponent of the asphalt is small, because of the high magnitude ofthe base component of aggregate, it acts as a scale factor in the over-all contribution to dry adhesive bond strength. The contribution ofthe base component of asphalt and the acid component of aggregateis small.

Relationship of Bond Energy Parameters to Moisture Damage

To use a single value to gauge resistance to moisture damage, thetwo bond energy parameters were combined as a dimensionless energyratio, shown in Equation 9. As discussed earlier, the acid and base


components of asphalt and aggregate, respectively, are importantcontributors to the work of adhesion. The acid–base contribution ofthe bond energy parameters was calculated from Equations 6, 7, and8 by subtracting the LW component of the materials from theseequations. The corresponding ratio of these energy terms is referredto as the acid–base energy ratio, denoted by superscript AB, and isalso shown in Equation 9:

A high magnitude of dry adhesive bond strength and a low mag-nitude of reduction in free energy when water displaces asphaltfrom an aggregate surface are desirable in order to promote HMAresistance to moisture-induced damage. Therefore, a relatively highvalue of the energy ratio, R, for an asphalt–aggregate pair suggestsbetter resistance to moisture-induced damage. This ratio is usedhere to quantify conveniently the effects of the two bond energyparameters into a single value. The energy ratio was calculated forall possible asphalt–aggregate combinations. Figures 4 and 5 show

RG

GR

G

GAS

WAS

AB ASAB

WASAB

Total = =ΔΔ

ΔΔ

( )9

TABLE 4 Bond Energy Parameters for All Asphalt–Aggregate Pairs

Granite A Gravel A Gravel B Limestone A Limestone B Quartzite A Sandstone A Sandstone B KAG NAG

Dry adhesive bond strength (ergs/cm2)

PG 64-22 A 95 87 93* 93 86* 91 89 93 91 90

PG 64-22 B 129 109 124 123 114 122 107 113 142* 118

PG 64-22 C 106 95 106 104 98 104 95 99 115* 101

PG 64-22 D 120 94 110 111 99 109 93 101 125* 105

PG 64-22 E 126 110 123 121 114 122 109 115 137* 118

PG 64-22 F 143 121 138 136 127 136 119 126 157 131*

PG 64-22 G 126 108 122 120 112 120 106 112 138 116*

PG 64-22 H 107 94 103 103 95 102 95 100 107 99*

PG 64-22 I 135 115 131 129 120 129 113 120 149 124*

PG 64-28 A 106 81 93* 97 83* 93 82 90 99 90

PG 76-22 A 140* 101 122 126 108 122 99 110 143 116

PG 76-22 B 103* 87 97 98 88 96 89 94 100 93

PG 76-22 C 134 93* 115 119* 100 114* 92* 103* 134 109

PG 76-22 D 125 101 116 117 105 115 100* 108* 130 111

Change in free energy by wetting (ergs/cm2)

PG 64-22 A −239 −83 −161* −177 −116* −163 −65 −106 −258 −143

PG 64-22 B −200 −55 −124 −142 −82 −126 −42 −80 −201* −109

PG 64-22 C −219 −64 −139 −158 −95 −141 −50 −91 −224* −122

PG 64-22 D −208 −69 −137 −153 −97 −139 −55 −92 −217* −122

PG 64-22 E −206 −56 −128 −146 −85 −130 −42 −82 −208* −112

PG 64-22 F −193 −49 −117 −136 −76 −119 −36 −74 −193 −103*

PG 64-22 G −203 −56 −126 −145 −84 −128 −43 −82 −205 −111*

PG 64-22 H −227 −75 −150 −166 −106 −151 −58 −98 −240 −132*

PG 64-22 I −197 −52 −121 −140 −80 −123 −39 −77 −198 −106*

PG 64-28 A −228 −87 −160* −173 −118* −160 −71 −108 −249 −142

PG 76-22 A −195* −68 −131 −145 −94 −132 −55 −89 −205 −116

PG 76-22 B −230* −81 −156 −171 −113 −158 −65 −104 −248 −138

PG 76-22 C −200 −75* −139 −151* −101 −139* −62* −95* −214 −123

PG 76-22 D −208 −67 −136 −152 −96 −138 −53* −90* −218 −121

NAG = aggregate used in certain test sections in Nevada; KAG = aggregate used in certain test sections in Kansas.*Indicates actual mix design.


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

1 2 3 4 5 6 7 8

Mix Number

Ene

rgy

Rat

io, R

Tot

al

Good

Poor

Limestone

Gravel

FIGURE 4 Mixture performance and total energy ratio.

Good

Poor

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1 2 3 4 5 6 7 8

Mix Number

Ene

rgy

Rat

io, R

AB

FIGURE 5 Mixture performance and acid–base energy ratio.


The bond energy parameters and energy ratios are dependent on thesurface energies of both asphalt and aggregate. The coefficients ofvariation (CVs) in the bond energy parameters for each of the bindersand aggregates were calculated and reported in the last column androw, respectively, of Tables 5 and 6. Both binders and aggregatesvary significantly in their sensitivity to changes in aggregate sourcesin terms of the adhesive bond energy.

Results also show that asphalt binders with the same performancegrade (PG) from different sources have different surface energy com-ponents (Table 2) resulting in significantly different bond energyratios with the same aggregate (Tables 5 and 6). For example, thetotal energy ratio parameter varies from 0.74 to 1.66 when aggre-gate Limestone B is combined with PG 64-22 grade asphalts fromnine different sources. Similarly, the energy ratios vary considerablywhen a given aggregate type is combined with different asphaltbinders.

CONCLUSIONS

The following conclusions can be drawn from this study:

• Surface energy measurements and concomitant bond energycalculations can be used as an effective tool to identify asphalt–aggregate pairs that are susceptible to premature moisture-induceddamage.

• An energy ratio parameter was introduced to assess moisture sen-sitivity of mixes with different asphalts and aggregates. On the basisof the mechanism of displacement of asphalt by water at the asphalt–

the relationship between energy ratio and moisture sensitivity ofmixes based on the eight mix designs with known performance.The plots confirm the hypothesis that mixtures with poor perfor-mance have lower values of the total and the acid–base energyratios. Mixes with poor resistance to moisture damage had a totalenergy ratio of less than about 0.8 and an acid–base ratio of lessthan approximately 0.17. The distinction between mixes performingwell and poorly on the basis of the acid–base energy ratio was moreevident compared with the total energy ratio. Mixes 4 through 8contained two aggregates. The energy parameter was calculatedfor each asphalt–aggregate pair for these mixes. This approach helpsin determining if a specific aggregate type was more critical ininfluencing mixture performance.

Influence of Aggregate Type and Binder Sourceon Energy Parameters

To investigate the influence of aggregate type and binder source onmoisture sensitivity of mixes, the information from Figures 4 and 5was used to classify moisture sensitivity of mixes into four groups(A, B, C, and D) based on the values of the energy parameter. A totalof 140 asphalt concrete mixes is theoretically possible by the com-bination of any one of the 14 asphalt binders with any one of the10 aggregates. Tables 5 and 6 present the total and the acid–baseenergy ratios for all 140 combinations of asphalt and aggregate,respectively. Each combination is shaded as shown in the legend toidentify its expected moisture sensitivity. The 21 combinations ofasphalt and aggregate that make up the selected 16 mix designs fromTable 1 are identified with an asterisk.

TABLE 5 Total Energy Ratio and Performance Level for All Asphalt–Aggregate Pairs

Granite Gravel Gravel Limestone Limestone Quartzite Sandstone Sandstone BinderA A B A B A A B KAG NAG CV%

PG 64-22 A 0.40 1.05 0.58* 0.53 0.74* 0.56 1.37 0.88 0.35 0.63 45.62

PG 64-22 B 0.65 1.98 1.00 0.86 1.38 0.97 2.54 1.41 0.71* 1.08 42.88

PG 64-22 C 0.49 1.48 0.76 0.66 1.04 0.74 1.90 1.10 0.51* 0.82 42.02

PG 64-22 D 0.58 1.38 0.80 0.72 1.03 0.78 1.69 1.10 0.58* 0.86 33.45

PG 64-22 E 0.61 1.96 0.97 0.83 1.34 0.94 2.57 1.40 0.66* 1.05 44.95

PG 64-22 F 0.74 2.47 1.18 1.00 1.66 1.14 3.29 1.71 0.81 1.27* 51.26

PG 64-22 G 0.62 1.92 0.97 0.83 1.34 0.94 2.47 1.37 0.68 1.04* 46.89

PG 64-22 H 0.47 1.26 0.69 0.62 0.90 0.67 1.63 1.02 0.45 0.75* 43.03

PG 64-22 I 0.69 2.20 1.08 0.92 1.51 1.05 2.87 1.55 0.75 1.17* 48.82

PG 64-28 B 0.46 0.93 0.59* 0.56 0.70* 0.58 1.16 0.83 0.40 0.64 34.74

PG 76-22 A 0.72* 1.49 0.93 0.87 1.15 0.92 1.79 1.24 0.70 1.00 28.68

PG 76-22 B 0.45* 1.07 0.62 0.57 0.78 0.61 1.37 0.91 0.40 0.68 35.86

PG 76-22 C 0.67 1.24* 0.83 0.79* 0.99 0.82* 1.48* 1.09* 0.62 0.89 17.18

PG 76-22 D 0.60 1.50 0.85 0.77 1.10 0.83 1.88* 1.19* 0.60 0.92 30.67

Aggregate 17.23 28.77 17.46 19.37 21.15 21.68 31.45 22.28 27.04 18.22CV%

CV% = coefficient of variation in the total energy ratio

Greater than 1.5 A (good)

0.75 to 1.5 B

0.5 to 0.75 C

Less than 0.5 D (poor)

* Actual mix design

aggregate interface, it is hypothesized that a high value of thisparameter is desirable for resistance of the mix to moisture damage.This hypothesis was confirmed by the performance of eight mixdesigns.

• On the basis of comparison of moisture sensitivity of mixes andcalculated energy ratio parameters of their constituent asphalt andaggregates, a threshold value for the energy ratio was recommendedto identify asphalt–aggregate combinations that are susceptible tomoisture damage.

• Surface energy characteristics of the same PG binder can varysignificantly, depending on the source of the binder. Similarly, surfaceenergy characteristics of generically similar aggregates can vary sig-nificantly, depending on the mineral composition and geologicalorigin of the aggregate.

• The portion of the bond energy that results from the interactionof the acid component of asphalt and the base component of aggregatecontributes the most to the total adhesive bond strength of the mix.

• The moisture resistance of an asphalt–aggregate combina-tion depends on surface energy characteristics of both the asphaltand the aggregate. The analysis technique presented can be usedto (a) select compatible aggregate–binder combinations that wouldimprove HMA resistance to moisture damage, (b) explain causesfor poor or good adhesion based on surface characteristics of aggre-gates and binders, and (c) supplement the current mechanical testsfor measuring moisture susceptibility with fundamental materialproperties.

• The bond energy parameters play an important role in determin-ing the resistance of an HMA mixture to moisture damage. However,a comprehensive characterization of HMA moisture susceptibility and


performance should include the mix fracture, healing, and viscoelasticproperties.

A detailed and comprehensive study to correlate bond-energy-related parameters to moisture sensitivity of HMA is being conducted.Future findings are expected to lead to further refinement of theselected parameters and the threshold values presented in this paper.

ACKNOWLEDGMENTS

The authors acknowledge the Texas Department of Transportation andthe National Cooperative Highway Research Program for their fund-ing and Corey Zollinger for his help in executing some experimentaltasks in this research.

REFERENCES

1. Ishai, I., and J. Craus. Effect of Filler on Aggregate-Bitumen AdhesionProperties in Bituminous Mixtures. Proc., Association of Asphalt PavingTechnologists, Vol. 46, 1976.

2. Scott, J. A. N. Adhesion and Disbanding Mechanisms of Asphalt Usedin Highway Construction and Maintenance. Proc., Association of AsphaltPaving Technologists, Vol. 47, 1977, p. 19.

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TABLE 6 Acid–Base Energy Ratio and Performance Level for All Asphalt–Aggregate Pairs

Granite Gravel Gravel Limestone Limestone Quartzite Sandstone Sandstone BinderA A B A B A A B KAG NAG CV%

PG 64-22 A 0.05 0.02 0.03* 0.05 0.02* 0.04 0.04 0.05 0.02 0.04 45.62

PG 64-22 B 0.28 0.61 0.38 0.33 0.48 0.37 0.69 0.44 0.33* 0.39 42.88

PG 64-22 C 0.15 0.30 0.20 0.18 0.25 0.19 0.34 0.23 0.17* 0.20 42.02

PG 64-22 D 0.28 0.47 0.33 0.32 0.39 0.33 0.54 0.40 0.29* 0.35 33.45

PG 64-22 E 0.22 0.45 0.29 0.26 0.36 0.28 0.52 0.33 0.25* 0.30 44.95

PG 64-22 F 0.30 0.66 0.41 0.36 0.52 0.40 0.76 0.48 0.35 0.42* 51.26

PG 64-22 G 0.25 0.54 0.34 0.30 0.43 0.33 0.62 0.39 0.29 0.35* 46.89

PG 64-22 H 0.12 0.16 0.13 0.13 0.14 0.13 0.19 0.16 0.10 0.13* 43.03

PG 64-22 I 0.28 0.62 0.38 0.34 0.49 0.38 0.71 0.45 0.33 0.40* 48.82

PG 64-28 B 0.19 0.19 0.17* 0.19 0.16* 0.17 0.23 0.21 0.13 0.18 34.74

PG 76-22 A 0.43* 0.67 0.49 0.47 0.56 0.49 0.76 0.59 0.42 0.51 28.68

PG 76-22 B 0.13* 0.14 0.12 0.13 0.12 0.12 0.16 0.15 0.09 0.13 35.86

PG 76-22 C 0.42 0.58* 0.45 0.45* 0.50 0.45* 0.67* 0.55* 0.38 0.48 17.18

PG 76-22 D 0.26 0.42 0.30 0.29 0.35 0.30 0.49* 0.36* 0.26 0.32 30.67

Aggregate 17.23 28.77 17.46 19.37 21.15 21.68 31.45 22.28 27.04 18.22CV%

CV% = coefficient of variation in the acid-base energy ratio

Greater than 0.33 A (good)

0.17 to 0.33 B

0.11 to 0.17 C

Less than 0.11 D (poor)

* Actual mix design

5. Curtis, C. W., R. L. Terrel, L. M. Perry, S. Al-Swalilmi, and C. J. Braanan.Importance of Asphalt-Aggregate Interactions in Adhesion. Proc., Asso-ciation of Asphalt Paving Technologists, Vol. 60, 1991, p. 476.

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7. Little, D. N., D. R. Jones, and S. Logaraj. Chemical and MechanicalMechanisms of Moisture Damage in Hot Mix. Presented at NationalSeminar on Moisture Sensitivity, San Diego, Calif., 2002.

8. Lytton, R., E. Masad, E. Zollinger, R. Bulut, and D. Little. Measure-ments of Surface Energy and Its Relationship to Moisture Damage.Report 0-4524-1. Texas Department of Transportation, Austin, 2005.

9. Song, I., D. N. Little, E. Masad, and R. L. Lytton. Comprehensive Eval-uation of Damage in Asphalt Mastics Using X-Ray CT, ContinuumMechanics, and Micromechanics. Proc., Association of Asphalt PavingTechnologists, Vol. 74E, 2005.

10. Cheng, D. Surface Free Energy of Asphalt-Aggregate Systems and Per-formance Analysis of Asphalt Concrete Based on Surface Free Energy.Ph.D. dissertation. Texas A&M University, College Station, 2002.

11. Kim, Y. R., and D. N. Little. Development of Specification Type Tests toAssess Damage and Healing Properties of Bitumens and Mastics. ReportFHWA/473630. FHWA, U.S. Department of Transportation, 2003.


12. Majidzadra, K., and F. N. Brovold. Special Report 98: State of the Art:Effect of Water on Bitumen-Aggregate Mixtures. HRB, National ResearchCouncil, Washington, D.C., 1968.

13. Hefer, A. W., D. N. Little, and R. L. Lytton. A Synthesis of Theories andMechanisms of Bitumen-Aggregate Adhesion Including Recent Advancesin Quantifying the Effects of Water. Proc., Association of AsphaltPaving Technologists, Vol. 74E, 2005.

14. Hefer, A. W., A. Bhasin, and D. N. Little. Bitumen Surface Energy Char-acterization Using a Contact Angle Approach. Journal of Materials inCivil Engineering, ASCE, forthcoming.

15. Bhasin, A., and D. N. Little. Characterization of Aggregate SurfaceEnergy Using the Universal Sorption Device. Journal of Materials inCivil Engineering, ASCE, forthcoming.

16. Van Oss, C. J., R. J. Good, and M. K. Chaudhury. Additive and Non-additive Surface Tension Components and Interpretation of ContactAngles. Langmuir, Vol. 4, 1988, pp. 884–891.

17. Zollinger, C. J. Application of Surface Energy Measurements to EvaluateMoisture Susceptibility of Asphalt and Aggregates. Master’s thesis. TexasA&M University, College Station, 2005.

The Characteristics of Bituminous–Aggregate Combinations to Meet SurfaceRequirements Committee sponsored publication of this paper.

Documents

Limits on Adhesive Bond Energy for Improved Resistance of Hot-Mix Asphalt to Moisture Damage