Assessing the performance of asphalt mastic

by DSR fatigue testing M. Hospodka*, B. Hofko, R. Blab

Asphalt as a paving material is a mixture of mineral aggregates and bitumen with a defined void content.

coarse and fine aggregates, fines („Filler“)

Filler is the aggregate, which most of it passes a 0.063 mm sieve. [EN 13043]

Asphalt mastic

…coats the coarse and fine aggregates and works as an adhesive

Poor quality leads to premature deterioration by loss of aggregates at the surface and decreased fatigue life of the base layer

There is no state-of-the-art testing method available to assess the fatigue performance of asphalt mastic

[RIGDEN,1947]

Filler – Origin and normative requirements

Added Filler Filler aggregate of mineral origin, that has been produced separately for asphalt paving

Fines Particle size fraction smaller 0.063 mm of coarse and fine aggregates (over- and undersized particles)

Mixed Filler Filler aggregate of mineral origin, that has been mixed with calcium hydroxide Ca(OH)2

Filler – Origin and normative requirements

Specifications for filler according to ÖNORM B 3130:2016

Requirements to the fines of the fine and coarse aggregates (applicable when mass fraction is at least 10 %) quality requirements sufficient?

Mineralogy, aggregate geometry, chemical weathering, surface chemistry (chemical affinity) and bitumen compatibility are not addressed.

Grading of filler aggregates ÖNORM EN 933-10Methylene blue test ÖNORM EN 933-9Particle density of filler ÖNORM EN 1097-7Voids of dry compacted filler (Rigden) ÖNORM EN 1097-4Delta ring and ball test ÖNORM EN 13179-1Water susceptibility of fillers ÖNORM EN 1744-4Chemical analysis ÖNORM EN 1744-1Content of calcium carbonate in added fillers ÖNORM EN 196-21Content of calcium hydroxide in mixed fillers ÖNORM EN 459-1Bitumen number ÖNORM EN 13179-2

Development of a performance criterion for mastic based on fatigue life.

Fatigue: …is the deterioration of a material due to repeatedly applied loads

Fatigue test is already standardized for asphalt mixture e.g. 4-Point Bending Beam Test (EN 12697-24)

Challenge: Tests are time-intense and material-consuming

Wanted: Suitable testing device and setup for asphalt mastic

Scientific Approach – Dynamic Shear-Rheometer (DSR):

- Fatigue failure caused by oscillatory shear stress

- Device is highly available due to the use for binder testing

- Only a small sample volume is needed for DSR tests

- Please note: The device has to meet minimum requirements in terms of electrical torque.

[ASPHALT INSTITUTE, Lexington 1994]

Dynamic Shear-Rheometer (DSR), Results:

- Complex shear modulus |G*| Ratio of peak stress to the peak strain in harmonic sinusoidal oscillation

- Phase lag δ Phase difference (time lag) between stress and strain to characterize a material regarding to

elasticity (e.g. rubber band)

[TechGalerie GmbH, Düsseldorf 2016] [Lemerg.com, Uploaded by A_Arina, 2015]

and viscosity (e.g. play doh).

Sample Preparation & Testing Parameter

Bitumen: 70/100 rheologically simple material

Sample type: Mixing ration of mass fraction bitumen:filler = 1:1.5

Manually mixing of preheated bitumen and filler with a stirring rod

DSR testing parameter: Plate-Plate testing system with Ø 8 mm

Sample height: 3 mm Cooling capacity

Test temperature: 10 °C Creep (deformation)

Test frequency: 30 Hz Test duration

Test mode: Controlled stress (CSS)

[SCHRAMM, Karlsruhe 1995]

First attempt, first failure

– Failure at the lower interface / bottom plate

– Partially adhesion / cohesion failure at the upper interface / top plate

– Aim: pure cohesion failure within the mastic specimen

Cylindrical specimen shape is not suitable for fatigue testing!

Lower interface / bottom plate Upper interface / top plate

Solution: Sample geometry with predetermined point of failure

Step-by-step sample preparation in the DSR

Step 1: Apply 8 mm DSR bottom plate

Step-by-step sample preparation in the DSR

Step 2: Apply silicone mold on DSR plate

Silicone mold

Rubber band

Step-by-step sample preparation in the DSR

Step 3: Fill mastic sample into the mold

melted mastic sample (180 °C)

Step-by-step sample preparation in the DSR

Step 4: Lift DSR to testing gap (3 mm)

Remove excess mastic

Step-by-step sample preparation in the DSR

Step 5: Remove the silicone mold

Results – Fatigue curve – Crack propagation till fatigue failure

Sample Failure

Time for adaption

Mastic sample after successful fatigue test

- Cohesion failure at the predetermined point of failure

Results – Comparing two different asphalt mastic mixtures

300

350

400

450

500

550

600

1.000 10.000 100.000 1.000.000

Sch

ub

span

nu

ng

[kP

a]

Lastwechsel [-]

Wöhlerkurven zweier Mastixproben

Einzelversuche A Mittelwert A Einzelversuche B Mittelwert B

y = 3728x-0,2

R² = 0,99y = 6384x-0,225

R² = 0,997

Mastixprobe A Mastixprobe B

767,000 275,000

Mastic mixture B is capable of 2.8-times the number of load

cycles till failure comparing to mastic

mixture A

Summary & Conclusions

High performance demands to asphalt pavements need high quality components

– Bitumen

– Aggregates (Filler / Fines!)

Assessment of the fatigue performance of asphalt mastic by DSR

Correlation analysis of various filler parameters and results of fatigue tests:

– No significant impact of filler mineralogy

– Significant impact of filler grading curve (d60, <6.3, <63 µm)

– Impact of filler morphology highly likely

[dpa/Picture Alliance, Germany 2013]

Questions?

M. Hospodka*, B. Hofko, R. Blab

Assessing the performance of asphalt mastic by dynamic shear rheometer 1

fatigue testing 2

Markus Hospodka1, Bernhard Hofko1, Ronald Blab1 3

（1 Vienna University of Technology，Institute of Transportation, Gusshausstrasse 28/E230-3, 4

Austria, markus.hospodka@tuwien.ac.at, bernhard.hofko@tuwien.ac.at, 5

ronald.blab@tuwien.ac.at ) 6

ABSTRACT 7

Early failure of asphalt pavements is a common issue all around the world. Damages are 8

caused by various reasons like binder or aggregate quality, an inadequate mix design or improper 9

handling in the construction process. The effects of binder, aggregates and mix design have been 10

widely studied and state-of-the-art testing methods are available for both, hot-mix asphalt 11

(HMA) and for each component. An important part in HMA belongs to the asphalt mastic, where 12

no standardized method is available to allow a quality control. Asphalt mastic is the mix of 13

bitumen with aggregates smaller than 63 (125) µm and covers the coarse aggregates as the actual 14

binding component in the mix. This research aims at developing a testing method for asphalt 15

mastic based on fatigue tests. The dynamic shear rheometer (DSR) was found as a suitable 16

device for this purpose. The DSR fatigue test consists of the 8 mm parallel-plate geometry 17

widely used for binder performance grading with a sample height of 3 mm. Instead of a 18

cylindrical specimen shape, a hyperboloid of one sheet is applied. This shape predetermines the 19

point of failure and prevents from adhesion/interface failures between the mastic specimen and 20

the upper or lower DSR stainless steel plate. The specimens are prepared directly in the DSR 21

employing a silicone mould to ensure an exact specimen shape. This test can be applied to all 22

DSR devices without costly changes or additional equipment as long as sufficient cooling 23

capacity and torque can be provided from the DSR. 24

Keywords: mastic, filler, fines, fatigue, DSR 25

1. INTRODUCTION 26

In recent years, damage has increasingly occurred in the case of bituminous top layers, 27

such as the loss of aggregates and decreased fatigue life of the base layers in various parts of 28

Central and Western Europe. These damages can not be attributed either to an unusual climate or 29

a high traffic load. A possible cause of damage is a lack of serviceability of the asphalt mastic 30

(bitumen + mineral fine fraction), like bad adhesion to the aggregates or overall strength. This 31

can be adressed to poor quality of one of the two components, bitumen and added filler or fines. 32

Preliminary work on this subject suggests that the mineralogical composition of the added filler 33

or fines affects the quality of the mastic and thus the adhesion to the coarse aggragates and 34

strength. It can be assumed that a lack of quality of the added filler or fines is responsible for 35

premature damage. Possibly, the use of non-quality-assured fines from the fine and coarse 36

aggregates instead of added filler or mixed filler can be accounted to these damages. This study 37

aims to develop a suitable test to assess the quality of the mastics with regard to durability 38

(fatigue life). 39

The European Standard EN 12697-24 [1] defines fatigue as „...the reduction of strength of 40

material under repeated loading when compared to the strength of a single load“. Fatigue is the 41

2

progressive and localized structural damage that occurs when a material is subjected to cyclic 1

loading below the stress limit untill the state of serviceability limit or total failure is reached. 2

This effect also occurs in asphalt pavements and affects all asphalt layers. Due to heavy 3

goods vehicle traffic, microcracks occur in the order of fractions of a millimeter. These loads 4

cause tensile stress in the base layer and thus microcracks are formed. In the case of repeated 5

loads, these cracks are propagated upwards and are linked to macrocracks, which are visible. If 6

these cracks finally pass through to the top layer, a cracking pattern typical of fatigue damage 7

occurs. Another form of fatigue damage affects the asphalt surface. The coarse aggregates are 8

glued together by asphalt mastic and under repeated stresses, poor mastic leads to loss of 9

aggregates on the surface. While there are standardized testing methods available for bitumen, 10

aggregates and hot-mix asphalt (HMA), there is no testing method available to assess the 11

durability of asphalt mastic. Thus, several researchers already published their work to account 12

the fatigue performance of asphalt mastic or mortar (bitumen + filler + sand) in different testing 13

setups and maschines, like DSR [2], Annular Shear Rheometer (ASR) [3-6], Dynamic 14

Mechanical Analysis (DMA) [7-9] and Tension Compression Tests [10, 11]. 15

2. APPROACH 16

The 4-Point Bending-Beam Test (4PBB) according to EN 12697-24 [1] is one of the 17

standardized fatigue tests for HMA. Since the aim of this paper is the development of a fatigue 18

test for asphalt mastic, the 4PBB test is described in more detail for a better understanding of 19

fatigue. In the 4PBB test, a prismatic beam is supported at 4 points and is dynamically loaded at 20

the two inner bearings. The sample is pulled upwards and pushed downwards until a defined 21

deformation/strain is reached in the center of the beam. The force required for the predetermined 22

strain is measured and the dynamic modulus is calculated continuosly. Therefore, the 4PBB test 23

is a strain-controlled test. Due to the increasing number of load cycles during the test progress, 24

the beam becomes weaker and the dynamic modulus is getting lower. When the dynamic 25

modulus reaches half of its initial value at the beginning of the 4PBB test the fatigue criterion is 26

met. Tests have to be carried out at 3 different strain levels with at least 6 samples each level to 27

account for the limited statistic certainty. This results in at least 18 samples that have to be 28

tested. Including the mixing of HMA, sample fabrication and preparation for testing, it takes 29

about 3 weeks to complete all necessary steps for one single HMA. Thus, the 4PBB test is a time 30

consuming test and it is also not applicable to the testing of mastic or mortar. 31

This paper aims to find a fatigue testing method suitable for asphalt mastic applicable on a 32

device that is already available in most of the commercial laboratories of pavement engineering. 33

The dynamic shear rheometer (DSR) was found as a suitable device with good availability due to 34

the fact that it is used for binder grading. 35

3. DEVELOPMENT OF THE FATIGUE TEST 36

3.1. DSR loading mode & fatigue failure criterion 37 DSR are capable of applying oscillatory stress- and strain-controlled loads on small 38

specimens. Deciding between stress- and strain-controlled loading modes, the stress-controlled 39

mode is in favour because it is more comparable to what happens on site. Thinking about a heavy 40

goods vehicle driving on an asphalt pavement, the induced stress is unchanged and the strain is a 41

function of the stiffness of the material. Under strain-controlled loading mode stiffer materials 42

are subjected to higher stress levels than materials with lower stiffness. Changing the loading 43

3

mode also requires changing the fatigue criterion. While 50% of the initial modulus is used as a 1

criterion for strain-controlled tests, the true failure of the specimen is applied to stress-controlled 2

tests. Figure 1 shows a typical fatigue curve under stress-controlled loading mode obtained by 3

DSR within this paper. 4

5

0

10

20

30

40

50

0

20

40

60

80

100

0 100,000 200,000 300,000 400,000 500,000 600,000

Ph

ase

lag δ

[ ]

Dy

nam

ic M

od

ulu

s |G

*| [M

Pa]

Load cycles N [-]

Dynamic modulus |G*| Phase lag δAdaption phase

Point of failure

6 FIGURE 1 left: Fatigue curve under stress-controlled loading mode 7

right: fatigued specimen 8 9

A fatigue curve obtained by stress-controlled testing consists of four typical phases: 10

Phase (1) is the adaption phase dominated by thixotropy [12] and little heating of the specimen 11

caused be energy dissipation (both casues a reversible loss of stiffness). Phase (2) consist of a 12

continuous propagation of microcracks. In Phase (3) these microcracks are linked to macrocracks 13

until in Phase (4) these macrocracks are linked to a yield line. Finally, the specimen breaks due 14

to fatigue failure. The failure is reflected in both, dynamic modulus and phase lag. In this study, 15

the point of failure is determined by the sudden drop of the phase lag as shown in Figure 1. 16

17

3.3. Preparation of asphalt mastic samples 18 Depending on grading curve, air void and binder content, HMA is categorized into asphalt 19

concrete (AC), stone mastic asphalt (SMA), mastic asphalt (MA) and porous asphalt (PA). The 20

ratio of binder to filler/fines is different within these mix designs. A mixing ratio of 1 part binder 21

to 1.5 parts of filler/fines by weight (m/m) is chosen to obtain a ratio almost similar to the ratio 22

used in AC. In preliminary tests mixing ratios of up to 1:2.5 have been tested where rations from 23

1:2.0 show a decreasing repeatability. This can either be accounted to bad mixing quality or the 24

grain shape. The higer the filler/fines content the higher is the interaction level between each 25

single grain. The higher the interaction level the more prone is a poured DSR specimen to the 26

exact position of each grain. Both, the filler or fines and the binder are heated up to 180°C in a 27

thermal chamber. The next step is to pour the binder into the metal can with the filler/fines inside 28

and it is then steered with a metal or glas rod until a homgenious asphalt mastic is obtained. A 29

temperature controlled heat gun is used to maintain the temperature during mixing. The mastic 30

samples are immediately stored in a fridge at 5°C to avoid settling of particles after mixing. Two 31

different fillers (limestone and quarzite) have been tested in this study. Both of them with a 32

maximum grain size of 125 µm. It is important to keep a certain maxiumum grain size when 33

comparing fillers or fines because of the sensitivity to fatigue tests. As a binder a 70/100 paving 34

grade bitumen with a PG 58-28 is used. It is known from literature that the stiffness of a binder 35

4

has an impact to the fatigue life of the mastic. This behaviour is not accounted in this paper, but 1

is part of the ongoing project. 2

3

3.3. Test parameters & sample geometry 4 While a temperature sweep is applied during binder grading the fatigue test is carried out at 5

one single temperature only. The temperature is selected according to the creep stiffness of the 6

mastic. The higher the creep stiffness is, the higher the applicable temperature can be chosen for 7

the fatigue test. It is necessary to maintain a certain minimum stiffness at test temperature to 8

avoid creep deformation. In this study and in accordance to other researchers a test temperature 9

of 10°C is selected [12-32]. Fatigue performance testing requires a high number of load cycles 10

until fatigue failure. It is recommended to select a high frequency to conduct these tests in a 11

limited time to to be economical. For that reason a testing frequency of 30 Hz is selected in this 12

study. With 30 Hz it is possible to perform 108,000 load cycles within one hour. It is possible to 13

choose even higher frequencies as far as an appropriate DSR is available. However, higher 14

frequencies cause higher dissipated energy and thus, more friction heating to the mastic sample. 15

There are two different parallel-plate testing geometries used for binder grading, PP08 and 16

PP25. The numbers determine the diameter of the specimen and are applied to the upper (PP25) 17

and lower temperature testing range (PP08) according to AASHTO M 320 [33]. Due to the fact 18

that bitumen is a highly temperature-dependent material and DSR are limited in its applicable 19

torgue, PP08 has to be used for fatigue testing. In preliminary tests the standard specimen 20

geometry shape (diameter 8 mm, height 2 mm) used for binder grading have also been used for 21

fatigue tests. After extensive pretests, it can be concluded that a cylindric specimen shape (even 22

with a height of 3 mm) is not suitable for fatigue testing due to the fracture behaviour. In all 23

these tests performed, the specimens failed either as pure interfacial/adhesion failure (picture 1 in 24

Figure 2) or in a combination of adhesion and cohesion failure (picture 2 in Figure 2). However, 25

several researches published work employing a cylindric specimen shape for fatigue testing of 26

pure binder or mastic [12, 13, 15-19, 21-23, 25-30, 32, 34, 35]. Some researches came to the 27

same conclusion regarding the disadvantageous use of a cylindric specimen shape for fatigue 28

testing. 29

30

31 FIGURE 2 Failure types, hyperboloid shape, FE model, silicone mould, alt. solution 32

33 A solution has been found in a redesigned specimen geometry (picture 3 in Figure 2). This 34

geometry is based on the PP08 testing geometry with a specimen height of 3 mm and a 35

predetermined point of failure in the middle of the height. Higher specimens are not 36

recommended due to the limited cooling capacity of the DSR (thermal gradient). The 37

predetermination is realised by circular necking of the original diameter of 8 mm down to 6 mm. 38

A small platform of 0.3 mm at both ends of the hyperboloid avoids unfavourable stress 39

concentrations in the edges. In literature, researches solved that issue with other specimen shapes 40

like dog-bone or even larger cylinders with clamps or terminals at both ends [14, 20, 24, 27]. 41

These specimen shapes requires a DSR with a temperature-controlled chamber instead of a 42

5

temperature-controlled hood. This leads to expensive, additional equipment not common or even 1

not available for standard DSR setups employed for binder grading. 2

Picture 4 in Figure 2 also shows the shear stress distribution in the hyperboloid where the 3

stress goes from higher (red) to lower levels (blue), calculated by finite element analysis 4

software Abaqus. With this geometry it is possible to obtain true cohesion failure within the 5

mastic specimen. 6

7

3.4. Specimen preparation in the DSR 8 The specimen preparation has to be carried out directly in the DSR because it is not 9

possible to trim a hyperbolid shape by hand. A reusable silicone mould is employed to ensure an 10

accurate specimen. The silicone mould is made out of two-component silicone and is 11

temperature-resistant of up to 180°C (picture 5 in Figure 2). Thus, it is possible to pour the 12

molten mastic sample directly into the silicone mould loaded in the DSR. The loading of molten 13

samples guarantees an ideal bonding between mastic and the smooth stainless steel surfaces of 14

the DSR. It is optional to remove excess mastic at the top of the mould because it has no impact 15

to the fatigue test. The silicone mould is removed after a cooling period of at least 10 minutes 16

resulting in an accurate specimen shape (picture 6 in Figure 2). 17

4. RESULTS 18

4.1. Hyperboloid – A challenging specimen shape 19 It must be noted that the dynamic modulus calculated by the DSR software (Anton Paar 20

RheoCompass) is not the true dynamic modulus of the mastic sample being measured. This issue 21

is related to the hyperboloid specimen shape. Parallel-plate tests are usually run with cylindric 22

specimen shapes and therefore, all the calculations within the DSR software are based on a 23

cylindric shape with a diameter of 8 mm and a sample height of 3 mm. Equalling Ø6 and Ø8 mm 24

gives a correction factor of 3.2. Hence, a cylindric specimen with Ø8 mm is expected to be 3.2 25

times higher in dynamic modulus than a cylindric specimen with Ø6 mm. Oscillatory shear tests 26

on a bitumen 70/100 with the same conditions as applied for the fatigue tests (10°C, 30 Hz 27

sample height of 3 mm) on both, Ø8 mm cylindric shape and hyperboloid resulted in a correction 28

factor of 2.4. This means that the dynamic modulus of the Ø8 mm cylindric specimen is not as 29

high as expected from calculations or vice versa, the hyperboloid is stiffer than expected. It is 30

highly likely that the contributing diameter of the hyperboloid is higher than 6 mm because of 31

the circular necking. Probably, this correction factor is varying depending on the grading curve 32

and grain shape of the filler/fines as well as the grading or (polymer-) modification of the binder. 33

This issue will be looked into in future work. 34

35

4.2. Repeatability of the developed fatigue test 36 Tests are required to have a good repeatability and comparability to guarantee a wide 37

spread application. Figure 8 shows the repeatability of 10 fatigue tests at a shear stress level of 38

400 kPa of both, dynamic modulus at the beginning of each fatigue test and the fatigue strength 39

expressed as the number of load cycles until failure. The dynamic modulus is obtained after 40

10 seconds, which is equal to 300 load cycles. While EN 14770 [36] for standard DSR tests 41

presets the comparability of the dynamic modulus to 10 %, there is no value given for the 42

repeatability. It has to be accounted that this comparability is based on a round-robin test with a 43

cylindric specimen shape. Because there is no repeatability given, the mean ±5 % is shown in 44

Figure 3. The 50% (median) and 95 % confidence interval of the standard error (SEM) is 45

6

calculated to look into the quality of the obtained fatigue strength. With a probability of 95 % the 1

true fatigue strength is between 188,000 and 212,000 load cycles. This is about ±6 % of the 2

mean. It can be seen that both, dynamic modulus and fatigue strength show a good repeatability. 3

This proves that the hyperboloid specimen shape can be prepared very accuratly and the entire 4

DSR setup is suitable for fatigue testing. 5

6

4.1. Comparison of mastic samples 7 For the determination of the fatigue strength of a mastic sample and for the comparsion to 8

other samples it is necessary to fit a logarithmic stress-cycle (S-N) curve (Wöhler curve) into the 9

fatigue test results obtained of at least three different shear stress levels. Figure 4 shows S-N 10

curves of two different mastic samples. Both curves are fitted to the mean values of three single 11

fatigue tests at four different shear stress levels, respectively. Both S-N curves show an excellent 12

coefficient of determination of R²=0.99. 13

14

100

106

112

118

124

150,000 180,000 210,000 240,000

Dy

na

mic

Mo

du

lus

|G*

| t=10s [M

Pa

]

Load cycles N [-]

Single tests Mean Load cycles Mean Dynamic modulus

Comparability

EN 14770

95 % confidence interval

of the standard error

(SEM)

y = -86.23ln(x) + 1377

R² = 0.9928

y = -97.15ln(x) + 1607.1

R² = 0.9923

300

350

400

450

500

550

600

1,000 10,000 100,000 1,000,000

Sh

ear

stre

ss [

kP

a]

Load cycles N [-]

Single tests A Mean A Single tests B Mean B Log. (Mean B)

Mastic sample A

(quarzite)

Mastic sample B

(limestone)

15 FIGURE 3 left: Repeatability test with hyperboloid shape at 10°C, 400 kPa 16

right: Stress-cycle curves of two different mastic samples 17

5. CONCLUSIONS & OUTLOOK 18

5.1. Conclusions 19 This study shows that a standard DSR equipped with components used for binder grading 20

(temperature-controlled hood, PP08 geometry) is capable of fatigue testing of asphalt mastic. 21

Therefore, no costly changes are necessary. The developed hyperbolid specimen shape with 22

predetermined point of failure gives a good repeatability. This makes it possible to compare the 23

fatigue durability of different mastic samples with S-N curves at different shear stress levels. 24

25

5.2. Outlook 26 This paper is limited to one binder and two different fillers. Further tests with different 27

binders and filler/fines are still ongoing. Another aim of this project is to determine the impact of 28

different temperatures on the fatigue strength of mastic. As soon as the fatigue strength of several 29

fillers/fines is found, a correlation analysis with 4PBB results will be carried out. The actual state 30

of the developed fatigue test does no allow any kind of weathering. Because fillers/fines can 31

have a high water susceptibility, it is important to find a suitable solution for takeing the water 32

susceptibitily in the fatigue test into account in the future. 33

34

7

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