Measuring Asphalt-Aggregate Bond Strength Under Different Conditions

Measuring Asphalt-Aggregate Bond Strength Under Different

Conditions

H. Bahia, R. Moraes & R. Velásquez Department of Civil and Environmental Engineering, University of Wisconsin-Madison, United States

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ABSTRACT: Moisture damage in pavements can be defined as the loss of stiffness and strength

in the asphalt mixture due to a combination of mechanical loading and moisture intrusion.

There are three mechanisms by which moisture degrades mixture performance: loss of

cohesion within the asphalt, adhesive failure between aggregate and asphalt, and degradation

of the aggregate. The aggregate mineralogy, texture and porosity play a major role in the bond

strength development. Furthermore, asphalt composition affects the bond strength since

variation in concentrations of asphaltenes and maltenes can create different adhesion with the

mineral surface. The purpose of this paper is to investigate the mechanisms by which moisture

can affect the bond between asphalt and aggregates. The loss of bond strength due to moisture

conditioning was evaluated by means of the newly developed Bitumen Bond Strength Test

(BBS). An experimental matrix, which included different binders, modifications, and

aggregate types, to account for different chemical and physical conditions in the aggregate-

asphalt interface, was completed in this study. The results indicate that the bond strength of

asphalt-aggregate systems is highly dependent on modification techniques and moisture

exposure time. Polymers are found to improve the adhesion between the asphalt and aggregate

as well as the cohesion within the binder.

KEY WORDS: Moisture Damage, Stripping, Cohesion, Adhesion, Asphalt Composition,

Bitumen Bond Strength Test (BBS).

1. INTRODUCTION

In asphalt mixtures, moisture damage is defined as the loss of stiffness and strength due to

moisture exposure under mechanical loading. Moisture damage reduces asphalt pavement

integrity by accelerating distresses such as bleeding, cracking, rutting and raveling (Hicks et

al., 2003). It is recognized that resistance of asphalt pavements to distresses depends on the

mechanics of the bonding at the aggregate-binder interface, which could be highly affected by

moisture conditions.

There are three mechanisms by which moisture degrades an asphalt mixture: (a) loss of

cohesion within the asphalt mastic, (b) failure of the adhesive bond between aggregate and

asphalt (i.e., stripping), and (c) degradation of the aggregate (Copeland et al., 2007).

The loss of cohesion occurs when water interacts with the asphalt binder resulting in a

reduction in material integrity (Hicks et al., 2003). The loss of adhesion happens when water

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penetrates the interface between asphalt binder and aggregate. If the bond between the asphalt

binder and the aggregate surface is sufficient, failure will occur within the asphalt binder.

However, if poor bond exists, failure will happen between the asphalt and the aggregate

interface (Kanitpong and Bahia, 2003). Stripping occurs when failure of the interface between

the asphalt and the aggregate happens due to the presence of moisture. Water can cause

stripping in different ways, such as spontaneous emulsification, displacement, detachment,

pore pressure, hydraulic scouring, and osmosis (Aksoy et al., 2005).

In this paper, the reduction in the asphalt-aggregate bond strength due to moisture after

different conditioning times was experimentally investigated by means of the Bitumen Bond

Strength (BBS). Different base binders, modifications, and aggregate types were used to

account for a broad range of chemical and physical conditions of the asphalt-aggregate

interface.

2. LITERATURE REVIEW

2.1. Asphalt-Aggregate Adhesion Mechanisms

Most likely a combination of mechanisms occurs synergistically to produce adhesion. The

three main mechanisms related to adhesion between asphalt binder and aggregate are physical,

chemical, and mechanical interactions (Bhasin, 2006). The theories that fundamentally

explain the adhesive bond between asphalt binder and aggregates are: mechanical theory,

chemical theory, weak boundary theory, and thermodynamic theory (Kanitpong and Bahia,

2003; Terrel and Shute, 1989). The mechanical theory indicates that bonding of aggregate-

binder is affected by physical properties of the aggregate such as porosity, texture, and surface

area. The chemical theory suggests that adhesion depends on the pH and the functional groups

of both the asphalt binder and aggregate. The weak boundary theory suggests that rupture

always occurs at the weakest link of the asphalt-aggregate interface. Finally, the

thermodynamic theory studies the attraction between aggregate-asphalt-water due to the

difference in surface tension. These theories and associated mechanisms are not exclusively

independent and many researchers agree that a combination of mechanisms could take place

and result in weakening the asphalt mixture. What has been identified as a major challenge is

a system that can effectively measure bond strength and evaluate effect of moisture exposure

on its level.

2.2. Factors Influencing Adhesive Bond Between Asphalt and Aggregate

2.2.1. Effect of Asphalt Binder Characteristics

Previous research has identified viscosity, chemical composition, film thickness and surface

energy of asphalt binder as major factors affecting the adhesion of aggregate-asphalt systems.

(Kanitpong and Bahia, 2003; Bahia et al., 2007).

It has been reported that asphalt binder with higher viscosity has higher resistance to

displacement by water than those with lower viscosity (Thelen, 1958). Also, higher

concentration of polar compounds in high viscosity binders leads to better wetting within the

asphalt-aggregate interface.

The chemical characteristics of asphalt vary significantly with the crude oil source used in

its production. Petersen et al. (1982) ranked various asphalt functional groups and identified

that asphalts containing compounds such as carboxylic acids and sulfoxides have higher water

absorption. However, asphalts with these compounds are easily removed from the aggregate

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surface by water. He observed that asphalt binders containing ketones and nitrogen are the

least susceptibility to moisture damage.

In terms of geometry, samples with thicker asphalt film tend to have cohesive failure after

moisture conditioning. On the other hand, specimens with thinner asphalt film have adhesive

failure (Kanitpong and Bahia, 2003).

With respect to surface energy, according to the thermodynamic theory of asphalt-

aggregate adhesion, low values of this fundamental property for the asphalt is preferable to

provide better wetting.

2.2.2. Effect of Aggregate Characteristics

The characteristics of the aggregate play a major role in ensuring good adhesion. Size and

shape of aggregate, pore volume and size, surface area, chemical constituents at the surface,

acidity and alkalinity, adsorption size surface density, and surface charge or polarity are some

of the widely cited characteristics in the literature.

Aggregates are commonly classified as either hydrophilic or hydrophobic with regard to

their affinity to water (Bhasin, 2006; Kanitpong and Bahia, 2003; Tarrar and Wagh, 1992).

Hydrophilic aggregates are considered to be acidic due to their high content of silica. They

have better affinity for water than asphalt binder. Hydrophobic aggregates, on the other hand,

are considered to be chemically basic, with low silica content. They tend to have greater

affinity for asphalt than water. In general, hydrophobic aggregate have higher resistance to

stripping (i.e., adhesive failure) than hydrophilic aggregates. For example, limestone is

classified as hydrophobic aggregate and granite is considered as hydrophilic. It is important to

note that the level of basic or acidic condition of the limestone and granite aggregates may

vary according to their chemical composition.

Physical characteristics of aggregate surface such as roughness, porosity, dust coating and

surface moisture also affect adhesion in asphalt-aggregate systems. For example , rough

surfaces and therefore larger contact area are preferred for better adhesive bond. Furthermore,

some porosity is desirable to provide mechanical interlock. Aggregates that have large pores

on their exposed surfaces, such as limestone, appear to show stronger bonds with asphalt than

aggregates that have smaller or fewer pores on the surface (e.g., granite) (Kanitpong and

Bahia, 2003; Tarrer and Wagh, 1992). Moisture and dust can significantly reduce the bond

strength of aggregate-asphalt systems. Dust has the tendency to trap air and cause improper

bond. When moisture is present in the pores of the aggregate surface, asphalt is prevented

from contacting aggregate surface for good bonding.

4. MATERIALS AND TESTING PROCEDURE

4.1 Materials

Two types of aggregates which are known to have different moisture sensitivity were selected:

limestone and granite. Two commonly used asphalt binders were selected in this study: Flint

Hills (FH) PG 64-22 and CRM PG 58-28. Also, four modified asphalt binders were prepared:

FH64-22+1%PPA (modified with 1% by weight of polyphosphoric acid), FH64-

22+0.7%Elvaloy (modified with 0.7% by weight of Elvaloy), CRM58-28+1%PPA (modified

with 1% by weight of PPA) and CRM58-28+2%LSBS (modified with 2% by weight of Linear

Styrene Butadiene Styrene).

For conditioning media, tap water is used to investigate the effects of conditioning media

on the adhesion between asphalt and aggregate.

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4.2. Bitumen Bond Strength Test (BBS)

The challenge to quantitatively evaluate the adhesive bond between asphalt and aggregate is

to identify a test which is simple, quick and repeatable for evaluating adhesion properties of

asphalt-aggregate systems. Furthermore, no method is included in the Superpave binder

specifications to evaluate adhesive characteristics of asphalt binders (Copeland et al., 1998).

Youtcheff and Aurilio (1997) used the Pneumatic Adhesion Tensile Testing Instrument

(PATTI), originally developed for the coating industry, to measure the moisture susceptibility

of asphalt binders. In this study, the Bitumen Bond Strength Test (BBS), which is a

significantly modified version of the original PATTI (Meng, 2010), was used to evaluate the

asphalt-aggregate bond strength.

The main components of the BBS equipment are: pressure hose, portable pneumatic

adhesion tester, piston, reaction plate and metal pull-out stub (Figure 1). Before running a test,

the piston is placed over the pull-out stub and the reaction plate screwed on it. Then,

compressed air is introduced through the pressure hose to the piston. An upward pulling force

on the specimen is applied by the pull-out stub. During the test, failure occurs when the

applied pressure exceeds the cohesive strength of the asphalt binder or the adhesive strength

of the binder-aggregate interface. The pressure at failure is recorded and the pull-off tensile

strength (POTS) is calculated by:

psA

CAgBPPOTS

(1)

where,

Ag = contact area of gasket with reaction plate (mm2)

BP = burst pressure (kPa)

Aps = area of pull stub (mm2)

C = piston constant

The pull-out stub has a rough surface that can prevent asphalt debonding from the stub

surface by providing mechanical interlock and larger contact area between the asphalt binder

and stub. The pull-out stub in the BBS test has a diameter of 20 mm with a surrounding edge,

used to control film thickness. The stub edge has a thickness of 800 μm.

4.2.1. Aggregate Sample Preparation

Aggregate plates were cut with similar thickness and parallel top and bottom surfaces. After

cutting and lapping, aggregates plates are immersed in distilled water in an ultrasonic cleaner

for 60 minutes at 60°C to remove any residue from the cutting process and neutralize the

surface of aggregate to its original condition. It should be mentioned that the lapping is done

to provide a control on the roughness of the surface.

4.2.2. Asphalt Sample Preparation

The aggregate surface and pull-out stubs are degreased with acetone to remove moisture and

dust which could affect adhesion. After cleaning with acetone, the pull-out stubs and the

aggregate plates are heated in the oven at 65°C for a minimum of 30 minutes to remove

absorbed water on the aggregate surface and provide a better bond between the asphalt binder

and the aggregate. The asphalt binders are heated in oven at 150°C. The stubs are removed

from the oven and an asphalt binder sample is placed immediately on the surface of the stub

for approximately 10 seconds. Then, the aggregate plate is removed from the oven and the

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stub with the asphalt sample is pressed into the aggregate surface firmly until the stub reaches

the surface and no excess of asphalt binder is observed to be flowing. The stubs need to be

pushed down as straight as possible and twisting needs to be avoided to reduce the formation

of trap air bubbles inside the sample and to minimize stresses.

Before testing, dry samples are left at room temperature for 24 hours. For wet conditioning,

samples are first left at room temperature for 1 hour to allow for the aggregate-binder-stub

system to reach a stable temperature. Then, samples are submerged into a water tank at 40°C

for the specified conditioning time. After conditioning time is completed, samples are kept at

room temperature for 1 hour before testing.

Pullout Stub

Pressure Ring

Pressure Plate

Pullout Stub

Asphalt Binder

Ring Support

Substrate

Figure 1: Bitumen Bond Strength Test (BBS)

4.2.3 Testing Procedure

The BBS testing procedure can be summarized with the following steps:

Before testing, air supply and pressure hose connection should be checked.

Set the rate of loading to 100 psi/s. Measure sample temperature using a thermometer

before starting the test.

Place circular spacer under the piston to make sure that the pull-off system is straight

and that eccentricity of the stub is minimized.

Carefully place the piston around the stubs and resting on the spacers not to disturb the

stub or to induce unnecessary strain in the sample. Screw the reaction plate into the

stub until the pressure plate just touches the piston.

Apply pressure at specified rate.

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After testing, the maximum pull-off tension is recorded and the failure type is

observed. If more than 50% of the aggregate surface is exposed, then failure is

considered to be adhesive; otherwise, it is a cohesive failure.

5. ANALYSIS OF RESULTS

5.1. Effect of Conditioning Time

In this study, samples were conditioned in tap water for 0, 6, 24, 48, and 96 hours. The effect

of conditioning time on the pull-off strength of the asphalt-aggregate systems tested can be

observed in Figure 2.

Figure 2. Influence of conditioning time on the pull-off tensile strength (POTS) for different

asphalt-aggregate systems.

The average pull-off strength was calculated from four replicates. The conditioning of

specimens in water caused a significant reduction in the pull-off strength and a change in the

failure mode from cohesive to adhesive type (see Table 1), regardless of the selected asphalt

binder or aggregate. The change in failure mode is expected since water penetrates through

the aggregate, which is a porous material, and hence weakens the bond at the interface

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(Kanitpong and Bahia, 2003). The longer the conditioning time in water, the weaker the

interface bond and the lower the pull-off strength value observed.

5.2. Effect of Asphalt Modification

The effects of modification of asphalt are clearly indicated by the BBS testing results. For

example, Figure 2 shows that the modified FH64-22 binders have higher dry average pull-off

tensile strength in comparison to the neat binder for both granite and limestone aggregates.

The asphalts modified with PPA show less susceptibility to moisture conditioning in

comparison to neat asphalts. Note that the effect of PPA is better in granite than in limestone

aggregates.

Asphalt binder modified with elvaloy also show moisture resistance improvements for the

granite case compared to the neat asphalt. However, for the limestone case, no significant

difference between FH64-22 neat and FH64-22+Elvaloy were observed.

Failure mechanisms are also affected by modification type. Table 1 indicates that failure

type (i.e., cohesive and adhesive failure) changes according to modification, aggregate type

and conditioning time. Note that all unconditioned (i.e. dry) samples showed cohesive failure

(i.e., failure within asphalt). On the other hand, adhesive failure (i.e., between aggregate and

binder) was observed for some conditioned specimens.

The results show that the failure type after 6 hours of conditioning time for the FH 64-22

asphalt changes from adhesive to cohesive when PPA is used as modification. These

observations indicate that PPA improves the bond of the interface between the asphalt and

granite. All samples containing PPA have cohesive failure, which indicates that the bond at

the aggregate-binder interface is greater than the cohesive strength of the binder at the

specified testing conditions.

5.3. Effect of Aggregate Type

The nature and chemical characteristics of aggregates greatly affect bond strength and failure

mechanisms of asphalt-aggregate systems as indicated by Table 1. On both limestone and

granite surfaces the failure mode changed after moisture exposure, showing that the nature of

the aggregate greatly affects adhesion.

It can be seen that for all limestone samples, the failure type was cohesive, which indicates

that the adhesive bond in the asphalt-aggregate interface is larger than the cohesive strength of

the binders. Also, Figure 2 indicates that limestone aggregates have higher adhesive bond to

asphalt than granite aggregates, and thus more resistance to adhesive failure.

The pull-off tensile strength obtained from BBS tests performed is highly influenced by the

cleanness of the surface of the aggregate plate. Inconsistent and unexpected results for some

of the samples conditioned at 48 and 96 hours were obtained when the aggregate plate used

was different than the plate used for the 0, 6, and 24 hours tests. It appears that slight changes

of the aggregate surface can greatly affect the magnitude of the pull-off tensile strength.

Therefore, it is always important to perform moisture susceptibility experiments using the

aggregates from the same source and to be consistent in sample preparation.

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Table 1. Influence of conditioning time, modification, and aggregate type in the failure mode.

Asphalt Binder

Type

*CT

(hr)

Failure Type

Granite Limestone

FH64-22 neat

Dry Cohesion Cohesion

6 Adhesion Cohesion

24 Adhesion Cohesion

48 50%A -50%C 50%A -50%C

96 Cohesion Cohesion

FH64-22+Elvaloy


6 Cohesion Cohesion


48 Cohesion 50%A -50%C


FH6422+1%PPA


6 Cohesion Cohesion




CRM 58-28 neat


6 Cohesion Cohesion



96 Adhesion Adhesion

CRM 58-28+2%SBS


6 Adhesion Cohesion


48 50%A -50%C Adhesion

96 Adhesion Adhesion

CRM58-28+1%PPA


6 Cohesion Cohesion




*Conditioning Time (CT)

6. CONCLUSIONS

This paper shows promising results regarding characterization of asphalt-aggregate bond

under different conditions by means of a simple to perform test. The results and analysis lead

to the following conclusions:

The Bitumen Bond Strength (BBS) test can effective measure the effects of

conditioning time and modification on the bond strength of asphalt-aggregate systems.

The pull-off tensile strength decreases when samples are conditioned in water,

regardless of the selected asphalt binder or aggregate type. Bond measurements for the

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dry samples have lower coefficient of variation than for the samples tested after water

conditioning.

Conditioning of specimens in water causes not only loss of pull-off tensile strength but

also a change in the failure mechanism. In absence of water, failure usually happens

within the asphalt (i.e. cohesive failure). After water conditioning, the failure changes

from total cohesive to adhesive failure.

It is observed that the bonding between asphalt and aggregate under wet conditions is

highly dependent on binder modification type and conditioning time.

Polymers are found to improve the adhesion between the asphalt and aggregate as well

as the cohesion within the binder.

Polyphosphoric Acid (PPA) significantly improves the moisture resistance of asphalt-

aggregate systems tested in this study. The effect is especially noticed for granite or

acidic aggregates. All samples containing PPA have a cohesive failure, which

indicates that the bond at the aggregate-binder interface is greater than the cohesive

strength of the binder.

.

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Measuring Asphalt-Aggregate Bond Strength Under Different Conditions

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