Adhesive and Cohesive Properties of Asphalt-Aggregate

Adhesive and Cohesive Properties ofAsphalt-Aggregate Systems

Subjected to Moisture Damage

Francesco Canestrari [email protected] Cardone [email protected] Graziani [email protected] A. Santagata [email protected] Politecnica delle Marche, Ancona, 60100, Italy

Hussain U. Bahia [email protected] of Wisconsin-Madison, Madison, WI 53706, USA

AbstractThe bond strength between asphalt and aggregate plays a fundamental role in evaluating themoisture sensitivity of HMA Mixtures. In this study the effect of water on adhesive and cohe-sive properties of asphalt-aggregate systems was investigated using a modified version of thePATTI. The device was used to measure the pull-off strength on different asphalt-aggregatecombinations and to evaluate the influence of water immersion at two different temperatures.In particular, six asphalt binders were employed in combination with two aggregate types, hav-ing different asphalt affinity. The effect of the aggregate surface temperature during specimenpreparation was also tested. In the first phase of the study the within-laboratory repeatability ofthe test procedure was investigated. The results showed the PATTI test is able to evaluate withgood precision the pull-off strength and that its repeatability depends on the failure type (ad-hesive or cohesive). In the second phase of the study a full factorial experiment was employedto verify the reliability of the test for routine use in determining the adhesive and cohesiveproperties of asphalt-aggregate combinations and the effects of moisture damage. The resultsshowed that, in the dry condition, the test was able to measure the internal cohesion of the as-phalt binders. The results also showed the effects of water damage on the pull-off strength andthe decisive role of asphalt-aggregate affinity was clearly highlighted. Using wet conditioningof the PATTI samples it was proven that water affects the adhesive bond between asphalt andaggregate much more than the asphalt cohesion. Moreover, the results indicate that aggregatetemperature during sample preparation has only a limited effect on the adhesive strength

KeywordsAsphalt Binder-Aggregate Interaction, Moisture Sensitivity, Pull-off Test, Adhesion, Cohesion.

1 Introduction

Moisture damage is a major cause of premature failure in asphalt concrete pavement as it accel-erates or causes some typical pavement distresses such as bleeding, rutting, cracking, ravellingand potholes (Kiggundu and Roberts, 1988; Terrel and Al-Swailmi, 1994).

Moisture-induced damage consists of a dislodging process of the aggregate from the HMApavement under the action of traffic loading (Hicks, 1991; Kandhal, 1994; Kiggundu andRoberts, 1988; Tunnicliff and Root, 1984). This degradation process, commonly known asstripping, can be regarded as a combination of adhesive and cohesive failures (Fromm, 1974;Kiggundu and Roberts, 1988; Stuart, 1990; Taylor and Khosla, 1983; Terrel and Shute, 1989).

Road Materials and Pavement Design x(x): xxx-xxx

Canestrari F, Cardone F, Graziani A, Santagata FA, Bahia HU

Adhesive failure is characterized by the separation of the asphalt coating from the aggre-gate. It is caused by the action of water at the asphalt-aggregate interface that weakens theadhesive bond between asphalt and the aggregate surface (Hicks, 1991). Different theories aschemical reaction, molecular orientation, mechanical adhesion and surface energy are used toexplain this adhesion bond (Hefer et al., 2005).

Cohesive failure is due to the separation of molecules within the asphalt film. In this case,the water the cohesive bonds within the asphalt binder itself, for example through an emulsi-fication process. A cohesive failure mechanism can also lead to an adhesive failure when theemulsification effects reach the aggregate surface (Fromm, 1974).

The methods for measuring moisture sensitivity of HMA are currently based on tests per-formed on asphalt concrete mixtures. Some of these involve visual inspection (qualitative de-terminations) of aggregate coating degradation on loose mixtures after water immersion (e.g.EN 12697-11). Other methods are based on the comparison of mechanical properties measuredon unconditioned and conditioned compacted samples or through tests that consider the com-bined interaction of loads, temperature and moisture (Kim and Coree, 2006; Lottman, 1982;Solaimanian et al., 2003; Stuart, 1990; Terrel and Al-Swailmi, 1994; Gubler et al., 2005).

These well recognized methods are likely to measure bulk properties of the mixture ratherthan specific changes in the adhesion and cohesion properties of the asphalt-aggregate systemdue to the water action. Therefore, it is not surprising that they cannot be considered sufficientpredictors of HMA stripping.

A better understanding of the moisture sensitivity of HMA could be achieved with a quanti-tative measurement of the influence of water on both adhesion and cohesion, performed directlyon the asphalt-aggregate system (Youtcheff and Aurilio, 1997; Huang et al., 2002; Kanitpongand Bahia, 2003).

This type of measures can be carried out with a simple pull-off test, using a specific devicedeveloped for the coating industry. In particular the Pneumatic Adhesion Tensile Testing In-strument (PATTI), has been proposed by several Authors to evaluate the adhesive and cohesiveproperties of asphalt-aggregate combinations in the presence of water (Youtcheff and Aurilio,1997; Kanitpong and Bahia, 2003, 2005; Copeland et al., 2007; Santagata et al., 2009).

1.1 Research objectivesThe experimental study presented in this paper was focused on the evaluation of the PATTI de-vice as a routine test for determining the adhesive and cohesive properties of asphalt-aggregatecombinations and the effects of moisture damage on them. The pull-off strength and its reduc-tion, produced by the immersion in water, were used to assess the effects of moisture damage,with reference to the failure mechanism (adhesive or cohesive).

The experimental investigation was organized in two phases and a statistical analysis wasperformed in order to obtain a first estimation of the test repeatability and to evaluate the signif-icance of each test factor on the pull-off strength.

2 The testing device

The Pneumatic Adhesion Tensile Testing Instrument (PATTI) was initially developed by theNational Institute for Standardization and Testing (NIST), to investigate the adhesiveness (orbond) between a paint, coating or general adhesive and a rigid substrate (Fig. 1a).

The PATTI measures the pull-off strength of the coating-substrate system through the ap-plication of a tensile force ASTM D4541 (2005). Its use for the measurement of the adhesiveand cohesive properties of asphalt-aggregate combinations is quite straightforward.

A small asphalt sample is applied on a proper metal pull-stub that is pressed immediatelyonto a prepared aggregate surface to establish a good asphalt-aggregate bond. A self-aligning

2 Road Materials and Pavement Design Volume x, Number x

Moisture Damage of Asphalt-Aggregate Systems

(a) (b)

Figure 1: PATTI device (a) and its schematic drawing (b)

Figure 2: Modified pull-stub

piston and a reaction plate are screwed to the pull-stub and a pulling force is exerted using apneumatic system (Fig. 1b). The pulling force is increased and failure occurs when the adhesivestrength of the asphalt-aggregate system or the cohesive strength of the asphalt is reached. Thecontrol module records the failure force, which is converted into pull-off tensile strength (kPa)as a function of the bonding surface area.

This test configuration, with minor modifications, was used in several studies (Youtcheffand Aurilio, 1997; Kanitpong and Bahia, 2003, 2005). It was pointed out that, at the highaggregate temperature (i.e. 135 ◦C and 90 ◦C) adopted during sample preparation, the hotasphalt flow out as the pull-stub was pressed on the aggregate surface. This resulted in a decreaseof the adhesion area between the asphalt and the steel surface of the pull-stub. As a consequencethe failure often occurred at the asphalt-steel interface and was not representative of the realadhesion and/or cohesion strength of the asphalt-aggregate system.

In this study a modified version of the PATTI set up was used (Santagata et al., 2009). Thehead of the pull-stub was improved with a 200 µε thick perimetrical edge. This provided thenecessary lateral confinement for the asphalt binder during the specimen preparation. Moreover,eight cuts made along the edge allowed the excess asphalt binder to flow out as the stub waspressed on the aggregate surface. An excellent control of the asphalt film thickness was obtainedso guaranteeing a complete adhesion between the asphalt and metal pull-stub. Figure 2 showsthe details of the modified pull-stub and the specimen set up.

With these improvements the PATTI results became more reliable and the device a practicaltool to evaluate the adhesion/cohesion properties of asphalt-aggregate system and the effects ofmoisture damage (Santagata et al., 2009).

Road Materials and Pavement Design Volume x, Number x 3


3 Experimental program

3.1 MaterialsTwo plain asphalt binders and four polymer-modified binders were used in this study. The plainbinders had different penetration grade, 50/70 pen and 70/100 pen respectively. The modi-fied binders were obtained by modifying base binders, CRM and FH, containing two differentasphaltene contents: 9.0 % and 16.25 % respectively. Each binder was modified with 0.7 %Elvaloy polymer and with 2.0 % SBS linear polymer.

Two aggregate types with different binder affinity were selected as substrate. The first wasa fairly compact limestone which was known to have good asphalt affinity and the second was aporphyry aggregate

3.2 Specimen preparation and conditioningThe asphalt binder was heated to 135 ◦C to reach the required viscosity and perfectly adhere tothe metal pull-stub which was immediately pressed onto the aggregate surface (Kanitpong andBahia, 2003; Santagata et al., 2009).

The aggregate specimens were prepared by saw cutting in small plates, approximately5 mm thick. Before pressing the pull-stub, the surface of the aggregate plates were cleaned anddried by heating for 12 hours using an oven. Prior to testing the specimens were conditioned for24 hours in different environments. All the pull-off tests were performed at 25 C.

3.3 Testing planThe precision of the test procedure was initially investigated through the evaluation of thewithin-laboratory repeatability (ISO 1994). Four samples were prepared, employing two modi-fied asphalt binders, CRM-Elvaloy and FH-SBS, in combination with the two selected aggregatetypes (limestone and porphyry). Moreover, the aggregate surface was heated at two differenttemperatures, 90 ◦C and 135 ◦C. Fifteen replicates were prepared for each test sample, resultingin a total of 60 specimens. The pull-off tests were carried out after conditioning by immersionin distilled water at 25 ◦C.

In the second phase, the reliability of the test procedure for routine use was investigated.A full factorial experiment was designed to include twelve asphalt-aggregate combinations (sixbinders and two aggregates), two aggregate surface temperatures (90 ◦C and 135 ◦C) and threeconditioning environments:

1. in air, at 25 (dry condition);

2. immersion in distilled water at 25 ◦C for 24 hours;

3. immersion in distilled water at 40 ◦C for 24 hours.

Five replicates were prepared for each of the 72 test combinations, resulting in testing atotal of 360 specimens.

4 Analysis of results

4.1 Failure typesThe failure mechanism was visually investigated after each test and recorded as one of threemajor failure types. If the aggregate surface remained completely coated by the binder (Fig. 3a)it was assumed that failure occurred inside the asphalt film and was thus denoted as purely co-hesive (C). In this case asphalt-aggregate adhesion strength exceeded the binder pulling strengthor internal cohesion. If the aggregate surface remained clean after the test (Fig. 3b) it was as-sumed that failure occurred at the asphalt-aggregate interface and was denoted as adhesive (A).



(a) Cohesive (b) Adhesive (c) Cohesive/Adhesive

Figure 3: Type of failures from pull-off tests

Table 1: Results of phase I

Binder Aggregate Temperature Failure Type Pull-off StrengthC A C/A Average Std.Dev CoV

◦C n. n. n. kPa kPa %

CRM–Elvaloy Limestone 135 15 0 0 1754 211 12CRM–Elvaloy Porphyry 135 15 0 0 1003 170 17

FH–SBS Limestone 90 5 0 10 2740 216 8FH–SBS Porphyry 90 0 15 0 544 80 15

When the aggregate surface remained partially coated with asphalt (Fig.3c), the failure couldnot be defined as (purely) cohesive or (purely) adhesive. A third type was therefore identified ascohesive-adhesive for this hybrid failure type (C/A).

4.2 Phase I Testing4.2.1 Test resultsThe results of the first phase are summarized in Tab. 1. The observed failure type is reportedalong with the average, standard deviation and coefficient of variation of the pull-off strength.

For the CRM-Elvaloy binder a cohesive failure was observed in all specimens, while theFH-SBS binder yielded an aggregate-dependent failure. In particular a purely adhesive failurewas observed with the Porphyry aggregate, while with the Limestone aggregate purely cohesiveand hybrid cohesive-adhesive failures were observed.

In terms of pull-off strength the four samples show a wide range of values, from 544 kPato 2740 kPa, and had coefficients of variation that ranged between 8% and 17%.

4.2.2 Statistical analysisA statistical analysis was performed on the data obtained in phase I to get a first estimation ofthe precision of the test method.

All the tests were performed by the same operator, with the same apparatus, in similarlaboratory conditions and within the shortest practical period of time, hence a within-laboratoryrepeatability could be estimated. Commonly the normal probability function is used to describethe distribution of independent random measurements obtained under repeatability conditions.The hypothesis of normality was verified using the Shapiro-Wilk test (Wadsworth, 1990).



Table 2: Shapiro-Wilk test results

Binder Aggregate Temperature S2 b2 Wcalc Wtab◦C kPa2 kPa2

CRM–Elvaloy Limestone 135 621631 609700 0.9808 0.8810CRM–Elvaloy Porphyry 135 403328 388960 0.9644 0.8810

FH–SBS Limestone 90 651732 611846 0.9388 0.8810FH–SBS Porphyry 90 90051 76789 0.8527 0.8810

For this test, the elements of each sample were initially ordered, from the smallest to thelargest (x1, . . . ,xn) and the quantities S2 and b were calculated as follows:

S2 = ∑n

x2i −

1n

(∑n

x1

)2

(1)

b =k

∑i=1

an−i+1 (xn−i+1− xi) (2)

where k = n/2 if n is even, or k = (n−1)/2 if n is odd, and the an−i+1 are tabled in functionof n (Wadsworth, 1990). Then, the test statistic W was calculated as follows:

Wcalc =b2

S2 (3)

Finally, the Wcalc value was compared with a theoretical Wtab determined as a function ofthe sample size (n) and for a 5% significance level. When the calculated value Wcalc is greaterthan Wtab the hypothesis of normality is accepted.

The results of the Shapiro-Wilk test are summarized in Table 2. The Wcalc is greater thanthe tabled Wtab for 3 of the 4 test conditions. Only the sample obtained with the FH-SBS asphaltin combination with porphyry at 90 ◦C, did not comply with the normal distribution. In thiscase, where only purely adhesive failure were observed, the distribution was in fact negativelyskewed, presenting a longer tail on the left side (lower values of the pull-off strength).

4.2.3 Repeatability standard deviationThe standard deviation values reported in Tab. 1 can be considered as a first estimation of themodified PATTI repeatability. The low value obtained for purely adhesive failures (80 KPa),suggests that a different repeatability should be expected for this type of failure.

This hypothesis was verified by comparing each couple of sample variances with an F-test.The test is based on verifying the hypothesis that the variances of two normal populations σ2

xand σ2

y , are equal between them.Let x1, . . . ,xn and y1, . . . ,yn be the random samples of n and m observations from these

populations, respectively. The Null Hypothesis (H0) to be verified is:

H0 : σ2x = σ

2y (4)

If the sample variances S2x and S2

y are considered, then test statistic F can be calculated asfollows:



Table 3: Hypothesis test on variance results

Binder type Test conditions Hypothesis Test Binder type Test conditions Hypothesis Test

CRM-Elvaloy Limestone - T=135 °C CRM-Elvaloy Limestone - T=135 °C

CRM-Elvaloy Porphyry - T=135 °C FH-SBS Porphyry - T=90 °C

CRM-Elvaloy Porphyry - T=135 °C CRM-Elvaloy Limestone - T=135 °C

FH-SBS Porphyry - T=90 °C FH-SBS Limestone - T=90 °C

CRM-Elvaloy Porphyry - T=135 °C FH-SBS Porphyry - T=90 °C

FH-SBS Limestone - T=90 °C FH-SBS Limestone - T=90 °C

H 0 rejected

H 0 accepted

H 0 rejectedH 0 accepted

H 0 accepted

H 0 rejected

F =S2

x

S2y

(5)

Following the Fischer distribution, with n− 1 and m− 1 degrees of freedom, the NullHypothesis H0 is rejected if:

S2x

S2y> Fα/2,n−1,m−1 or if (6)

S2x

S2y< F1−α/2,n−1,m−1 (7)

where Fα/2,n−1,m−1 and F1−α/2,n−1,m−1 are the upper and lower 100α/2 percentage points ofthe Fischer distribution, and α is the significance level (5 %). The F-test results are summarizedin Tab. 3.

Based on the variance estimated for adhesive failures (FH-SBS asphalt in combination withporphyry at 90 ◦C), the Null Hypothesis is always rejected. This confirms that the repeatabilitymeasured for adhesive failures is significantly different (lower) from the repeatability measuredfor cohesive or hybrid failures. Moreover, this result can be considered a strong statistical con-clusion for the chosen α value (Montgomery and Runger, 2003).

On the other hand, comparing the repeatability variances for cohesive and hybrid failuresthe Null Hypothesis is accepted. This means that these variances cannot be considered statis-tically different and a combined repeatability standard deviation can be computed for purelycohesive or hybrid failures:

sr = 200 kPa (8)

This value is considered a first estimation, exclusively when purely cohesive or hybridfailures are observed. The corresponding repeatability limit can be calculated as:

r = 1.96σ√

2 (9)

where 1.96 is the value of the standard normal distribution (Z) corresponding to a 95 %probability level, and σ is the population standard deviation. Since σ is unknown, the samplestandard deviation sr estimated above is used. Hence, for purely cohesive or hybrid failures weobtain:

r = 553 kPa (10)



Similarly, for adhesive failures:

r = 229 kPa (11)

4.3 Phase II testing4.3.1 Evaluation of the general test resultsThe results of the second phase, grouped for each asphalt binder, are summarized in Tab. 4. Theaverage and standard deviation of five pull-off strength measurements are reported, along withthe predominant failure type. The same test results were plotted in Figure 4, highlighting theaggregate type.

Some general observations could be made on the pull-off values:

– purely adhesive strength varied between 500 kPa and 1100 kPa;

– all purely adhesive failures were observed with porphyry aggregates;

– purely cohesive strength varied between 1200 kPa and 3700 kPa;

– the aggregate type did not seem to influence purely cohesive strength;

– all dry conditioned specimen yielded cohesive failures;

– for hybrid failures, limestone aggregates generally yielded higher strength than porphyryaggregates.

The PATTI results confirm the key role of asphalt-aggregate affinity and the better adhesionprovided by limestone aggregates. Moreover, a clear separation, at approximately 1100 kPa,appeared to exist between purely adhesive and purely cohesive strength. This could be identifiedas the upper limit of the adhesive strength as far as porphyry aggregate are considered.

The data precision, in terms of repeatability, was comparable with the values estimatedin phase I. Only 5 out of 56 samples characterized by a cohesive or hybrid failure showed astandard deviation that was significantly higher than 200 kPa. The pull-off values characterizedby adhesive failures had a generally higher variability.

In Fig.5 the test results were divided for each asphalt, highlighting the other test variables.This graphical representation is more suitable for a clear analysis of the data and for highlightingthe effects of moisture damage.

4.3.2 Pull-off strength after dry conditioningIn the pull-off tests performed after dry conditioning (24 hours in air at 25 ◦C) all the specimensshowed cohesive failures, regardless of asphalt-aggregate combination and aggregate surfacetemperature. Therefore, without the influence of water, the asphalt-aggregate adhesive bond isfound always higher than the asphalt film inner cohesion. As a consequence the dry pull-offstrength is not found to be significantly affected by the aggregate surface temperature or theaggregate type.

A one-way ANOVA at 95 % confidence level was used to verify this hypothesis. Thisprocedure is a generalization of the t-test when more than two population means have to becompared. In this case, for each asphalt binder, the four pull-off strengths measured after thedry condition were compared. Although each population should be verified to be normallydistributed when ANOVA is used, it is not practical to test this assumption with only five ob-servations. The assumption that the data are normally distributed is reasonable since ANOVA isnot strongly affected by small departures from normality (Ryan, 2007). Moreover, the results ofPhase I support the assumption of a normal distribution.



Table 4: Pull-off strength results for the investigated asphalt binders

Average (kPa)

Std.Dev. (kPa) Failure Average

(kPa)Std.Dev.

(kPa) Failure Average (kPa)

Std.Dev. (kPa) Failure

Limestone T=135°C 2693 328 C 2628 153 C 1708 459 C/A

Limestone T=90°C 3010 141 C 2076 152 C/A 1864 156 C/A

Porphyry T=135°C 2746 194 C 1340 63 C/A 1019 247 A

Porphyry T=90°C 2695 164 C 889 188 A 747 131 A

Aggregate Type and

Temperature

Asphalt 70/100

No immersion Conditioning @25°C Conditioning @40°C

Average (kPa)


(kPa)Std.Dev.





Porphyry T=135°C 3172 528 C 1994 396 C/A 658 103 C/A

Porphyry T=90°C 3456 114 C 950 70 A 716 54 C/A

Aggregate Type and

Temperature

Asphalt 50/70


Average (kPa)


(kPa)Std.Dev.




Limestone T=90°C 1813 116 C 1767 106 C 1496 211 C

Porphyry T=135°C 1957 198 C 1185 224 C 915 201 C/A

Porphyry T=90°C 1837 181 C 752 23 A 798 87 A

Aggregate Type and

Temperature

Asphalt CRM 0.7% Elvaloy




Table 4 (continued) Pull-off strength results for the investigated asphalt binders

Average (kPa)


(kPa)Std.Dev.





Porphyry T=135°C 2425 183 C 1022 125 A 546 60 A

Porphyry T=90°C 2009 148 C 576 94 A 476 107 A

Aggregate Type and

Temperature

Asphalt CRM 2% SBS Linear


Average (kPa)


(kPa)Std.Dev.



Limestone T=135°C 2936 128 C 3244 266 C 2253 126 C

Limestone T=90°C 2745 108 C 2491 154 C/A 1941 36 C

Porphyry T=135°C 3160 184 C 1213 249 C/A 1373 220 C

Porphyry T=90°C 2905 213 C 816 63 A 882 69 A

Aggregate Type and

Temperature

Asphalt FH 0.7% Elvaloy


Average (kPa)


(kPa)Std.Dev.





Porphyry T=135°C 3332 96 C 982 92 A 679 174 A

Porphyry T=90°C 3678 118 C 569 117 A 542 51 A

Aggregate Type and

Temperature

Asphalt FH 2% SBS Linear




0

100

200

300

400

500

600

700

800

0 500 1000 1500 2000 2500 3000 3500 4000

Test Level - Average Pull-Off Strength (kPa)

Stan

dard

Dev

iatio

n (k

Pa)

Adhesive Failure - Porphyry Cohesive Failure - Limestone

Cohesive Failure - Porphyry Hybrid Failure - Limestone

Hybrid Faiiure - Porphyry Repeatability - Phase I

Figure 4: Pull-off strength results for all binder illustrating the failure type

The first step in the analysis was to test the assumption of equality of variances, and thishas been performed using Bartletts test. This gave a positive result for all the asphalt binders.The results of ANOVA are summarized in Tab. 5. For each binder and for the two experimentalfactors (aggregate type and surface temperature), the test outcome is shown along with the rele-vant p-value. In nine out of twelve cases the p-value considerably exceed the significance levelα = 0.05 (i.e. p > α = 0.05). This confirmed that in these cases the pull-off strength measuredby the PATTI device after dry conditioning was independent from both the aggregate surfacetemperature and aggregate type and therefore could be properly considered a measure of thebinder internal cohesion.

The dry pull-off strength was used to rank the six binders considered in the study. The aver-age values were reported in Table 6 along with the coefficient of variation. Table 7 summarizesthe results of pairwise comparisons between these values performed using the t-test. The results

Table 5: Results of statistical analysis to evaluate the aggregate surface and aggregate type onpull-off strength

Significant? p-value Significant? p-value

50/70 YES 0.0444 NO 0.6529

70/100 NO 0.6759 NO 0.0844

CRM-Elvaloy NO 0.2701 NO 0.3929

FH-Elvaloy YES 0.0077 YES 0.0185

CRM-SBS NO 0.7618 NO 0.5136

FH-SBS NO 0.1309 NO 0.6684

Aggregate Temperature Aggregate typeBinder type



0

1000

2000

3000

4000

No conditioning Conditioning@25°C

Conditioning@40°C


Conditioning@40°C

Conditioning

Pull-

off s

tren

gth

(kPa

)

Porphyry 135°C Porphyry 90°C Limestone 135°C Limestone 90°C

Asphalt 70/100 Asphalt 50/70

Hybrid or cohesive failure

Adhesive failure

(a)

0

1000

2000

3000

4000


Conditioning@40°C


Conditioning@40°C

Conditioning

Pull-

off s

tren

gth

(kPa

)


Asphalt CRM Elvaloy Asphalt CRM-SBS


Adhesive failure

(b)

Asphalt FH Elvaloy Asphalt FH- SBS

0

1000

2000

3000

4000


Conditioning@40°C


Conditioning@40°C

Conditioning

Pull-

off s

tren

gth

(kPa

)



Adhesive failure

(c)

Figure 5: Pull-off strength results for each binder illustrating the moisture conditioning effects



Table 6: Binder ranking in terms of dry pull-off strength at 25 ◦C

1 2 3 4 5 6

Binder FH-SBS 50/70 FH-Elvaloy 70/100 CRM-SBS CRM-Elvaloy

Average (kPa) 3498 3360 2936 2716 2329 1866

CoV (%) 5.3 10.8 7.3 8.5 16.6 8.4

Table 7: t-test results to evaluate the composition and modification type effect on the pull-offstrength

Significant? p-value Significant? p-valueCRM-Elvaloy FH-Elvaloy YES < 0.0001 CRM-Elvaloy

CRM-SBS YES < 0.0001

CRM-SBS FH-SBS YES < 0.0001 FH-Elvaloy

F-SBS YES < 0.0001

50/70 pen 70/100 pen YES < 0.0001

Pairs of bindersChemical Composition

Binder typeModification Type

confirmed that the PATTI device was effective in detecting the effect of both composition andmodification agent.

4.3.3 Effect of water immersion on pull-off strengthThe effect of water on the adhesive and cohesive properties of asphalt-aggregate combinationswas evaluated performing PATTI tests after 24 hours of water immersion at 25 ◦C and 40 ◦C.

A decrease of the pull-off strength was generally measured. In many cases a change in thefailure type was also observed as hybrid or purely adhesive failures. The aggregate type provedto have a major role in this phenomena.

With the porphyry aggregate, an adhesive failure and a significant drop of pull-off resis-tance is measured for almost all test conditions, and in particular after immersion at 40 ◦C. Thismeans that moisture, penetrating through the pores of the aggregate, and from the sides of theasphalt film, reached the asphalt layer interface decreasing the adhesion strength to a level belowthe cohesive strength of the asphalt.

A purely adhesive failure was not observed with the limestone aggregate, indicating itsbetter binder affinity. When purely cohesive failures are observed, a small reduction in pull-offstrength is measured, as compared to the corresponding dry condition, whereas higher reduc-tions in strength are measured for hybrid failures. This could mean that, when a hybrid failureoccurs, it begins with an adhesion loss at a fairly weaker point of the asphalt-aggregate interface.Then, as a consequence of the reduced bonding area, a greater tensile stress is imposed to theasphalt film that comes to a cohesive failure.

4.3.4 Effect of aggregate surface temperatureVarying the temperature of the aggregate surface between 90 ◦C and 135 ◦Cwas expected toinfluence the adhesion between asphalt and aggregate, and hence the moisture susceptibility.Higher surface temperature should result in higher adhesion values since the lower asphaltviscosity allow better bonding, which should lead to lower sensitivity to water damage. Thishypothesis was tested by performing pairwise comparisons with the t-test. For specimens con-ditioned with immersion at 25 ◦C, the test confirmed the effect of the aggregate surface tem-



Table 8: Influence of surface temperature on pull-off strength after immersion at 25 ◦C


CRM-Elvaloy YES 0.003 CRM-Elvaloy NO 0.1333

FH-Elvaloy YES 0.008 FH-Elvaloy YES 0.0003

CRM-SBS YES 0.0002 CRM-SBS NO 0.1729

FH-SBS YES 0.0003 FH-SBS YES 0.0089

50/70 YES 0.0002 50/70 NO 0.1218

70/100 YES 0.001 70/100 YES 0.0004

Binder typeSurface temperature


Porphyry Aggregate Limestone Aggregate

Table 9: Influence of surface temperature on pull-off strength after immersion at 40 ◦C


CRM-Elvaloy NO 0.2673 CRM-Elvaloy YES 0.0008

FH-Elvaloy YES 0.0014 FH-Elvaloy YES 0.0007

CRM-SBS NO 0.2386 CRM-SBS NO 0.7466

FH-SBS NO 0.1297 FH-SBS NO 0.5525

50/70 NO 0.3508 50/70 NO 0.3964

70/100 NO 0.2215 70/100 NO 0.4923

Porphyry Aggregate Limestone Aggregate



perature only for porphyry aggregates (Tab. 8), where adhesive failures prevail. For limestoneaggregates, however, the better adhesive bond with asphalt yielded purely cohesive or hybridfailures and therefore the surface temperature had minor effects.

For specimens conditioned with immersion at 40 ◦C, the t-test revealed that the surfacetemperature effect were negligible for both aggregate types (Tab. 9). In particular for porphyrysamples, where purely adhesive failures were observed, the effect was so severe that the positiveeffect of an higher surface temperature was lost.

4.3.5 Effect of water temperatureThe water temperature during the conditioning phase, 25 ◦C or 40 ◦C, was an important param-eter to control the moisture damage. The importance of the aggregate type was again empha-sized. With porphyry aggregates, immersion at 25 ◦C yielded a significant reduction of pull-offstrength as the failure mechanism changed from cohesive to adhesive. With limestone aggregate,this effect was less important.

The immersion at 25 ◦C of the PATTI specimen had a small effect on the asphalt internalcohesion. This is clear from analyzing the pull-off strength loss, in comparison with the dryconditioning, measured on limestone samples (Tab. 10). A t-test also showed that for two binders(FH-Elvaloy and CRM-SBS) this reduction was not even significant.



Table 10: Effect of moisture on the pull-off strength after immersion at 25 ◦C

Significant? p-value Pull-off loss

CRM-Elvaloy YES 0.035 8%

FH-Elvaloy NO 0.820 0%

CRM-SBS NO 0.354 3%

FH-SBS YES < 0.0001 19%

50/70 YES < 0.0001 35%

70/100 YES 0.0015 18%

Limestone Aggregate

Water conditioning@25°CBinder type

Table 11: Loss in pull-off strength due to the immersion in water at 40 ◦C

Limestone Porphyry Limestone Porphyry

CRM-Elvaloy 36% 55% FH-SBS 28% 83%

FH-Elvaloy 26% 63% 50/70 69% 79%

CRM-SBS 34% 77% 70/100 37% 68%

Binder typeLoss in pull-off strength (%)

Binder typeLoss in pull-off strength (%)

On the other hand immersion at 25 ◦C had a clear effect on adhesion, at least for theporphyry aggregates. The adhesive strength dropped below the internal cohesion of the asphaltand a significant reduction of pull-off strength is measured with the change in failure mechanism:from cohesive to adhesive.

The same phenomena were observed comparing the PATTI results after immersion at40 ◦C, with the dry conditioning results (Tab. 11): water acted mainly on the asphalt-aggregateadhesive bond. For the porphyry aggregate, this effect was remarkable and adhesive failureswere always observed as the adhesion strength dropped below the internal cohesion of the as-phalt film. With limestone aggregates, the loss in pull-off strength was less severe. In fact,because of the better asphalt affinity, hybrid failures were prevalent.

4.3.6 Evaluation of the asphalt bindersThe results of the PATTI tests showed that a comparison between asphalt binders is possibleonly if taking into account the aggregate type. In other words, the asphalt-aggregate interactionsneed to be considered. As shown in Table 11, the loss in pull-off strength was lower for bothmodified binders (SBS and Elvaloy) than the un-modified asphalts for the Limestone aggre-gates. For the Porphyry aggregates, on the other hand, the FH-SBS binder showed higher loss(more water damage) than the unmodified asphalts. It is also worth noting that the CRM basedmodified binders (CRM-Elvaloy and CRM-SBS) show higher resistance to cohesion loss (withLimestone) than the FH-Based modified binders. This trend suggests that there are positive ef-fects of higher asphaltene content on cohesion, while the modification type effects on cohesionare somewhat negligible. For the porphyry aggregate, where adhesive failures prevailed, a dif-ferent effect of modification is observed since in this case better results (less loss in strength)were obtained with the Elvaloy modification.



5 Conclusions

The experimental study presented in this paper was focused on the evaluation of the modifiedPATTI device as a routine test for determining the stripping potential of asphalt-aggregate com-binations. Six asphalt binders and two aggregate types were considered in the study.

The precision of the test procedure was investigated. A statistical analysis showed that thewithin-laboratory repeatability is highly dependent on the failure mechanism.. In the case ofcohesive and hybrid failures a repeatability standard deviation of 200 kPa was estimated.

The research study also investigated the pull-off strength and its reduction produced bywater immersion in order to assess the effects of moisture damage. All the tests performedafter dry conditioning showed cohesive failures, regardless of asphalt-aggregate combination oraggregate surface temperature. A statistical analysis confirmed that, in this case, could the pull-off strength could be considered a measure of the binder internal cohesion. Tests performed afterwater immersion showed that the asphalt-aggregate affinity controls the transition from cohesiveto adhesive failures. With porphyry aggregates adhesive failures were observed for almost alltest conditions, together with a significant drop of strength. With limestone aggregate purelyadhesive failures were never observed, indicating a better binder affinity.

In general the modified binders used in this study showed a lower water sensitivity, in termsof cohesion loss. A better asphalt-aggregate affinity was obtained with the Elvaloy modificationcompared to the SBS modification. Moreover, a higher asphaltene content of base binders re-sulted in a higher cohesive and adhesive strength.

Considering the overall results of this study, the proposed modified PATTI test is found tobe a repeatable, reliable and practical method to investigate the adhesion and cohesion propertiesof asphalt-aggregate bonding. Moreover, its ability to discriminate between different asphalt-aggregate systems in terms of moisture sensitivity could prove to be a suitable method for de-signing asphalt mixtures with high moisture resistance.

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Adhesive and Cohesive Properties of Asphalt-Aggregate