agpt-t274-16 charact of flex stiffness and fatigue

January 2016 | page 1

AUSTROADS TEST METHOD AGPT/T274

Characterisation of Flexural Stiffness and Fatigue Performance of Bituminous Mixes

1. Preface

This asphalt test method was prepared by the Pavement Structures Working Group and the Asphalt Research Working Group on behalf of Austroads. Representatives of Austroads, ARRB Group and the Australian Asphalt Pavement Association have been involved in the development and review of this test method.

2. Foreword

This test method should be read in conjunction with the European Standards EN 12697-24:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 24: resistance to fatigue and EN 12697-26:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 26: stiffness, published by the European Committee for Standardization. This Austroads method provides instructions on conducting tests in accordance with these European Standards, while complying with Austroads specimen preparation methodology and test conditions.

3. Scope

The test method specifies procedures for the characterisation of the stiffness and fatigue behaviour of bituminous mixtures using a four-point bending test configuration. The test procedure to characterise the complex modulus is contained in Section 10 of this Austroads test method. The procedure to determine the fatigue performance is described Section 11. Section 12 contains a method to develop a complex modulus master curve from modulus results obtained using the method in Section 10. The tests are performed on compacted bituminous material under a sinusoidal displacement-controlled loading on prismatic specimens.

The modulus and fatigue characterisation tests can be run independently, or can be run consecutively on the same specimen, with the fatigue test following the modulus test.

Contents

1. Preface ........................................................... 1 2. Foreword ........................................................ 1 3. Scope ............................................................. 1 4. Safety Disclaimer ............................................ 2 5. Further Development ...................................... 2 6. References ..................................................... 2 7. Equipment ...................................................... 3

7.1 Basic Features ...................................... 3 7.2 Thermostatic Chamber .......................... 4 7.3 Checking of the Testing Equipment ....... 4

8. Specimen Preparation .................................... 4

8.1 Specimen Storage Prior to Test ............. 4 8.2 Measurement of Specimen Dimensions 4 8.3 Measurement of Density and Air Voids .. 5

9. Test Procedures ............................................. 5 10. Stiffness Test .................................................. 5

10.1 Definitions, Terms and Symbols ............ 5 10.2 Testing Principle .................................... 5 10.3 Equipment ............................................. 5 10.4 Specimen Preparation ........................... 5 10.5 Test Conditions ..................................... 6

10.5.1 Test Temperatures .................... 6 10.5.2 Loading Frequencies ................ 6

10.6 Testing Procedure ................................. 6 10.7 Expression of Results ............................ 6 10.8 Test Report ........................................... 7

11. Fatigue Resistance Test ................................. 7

11.1 Definitions, Terms and Symbols ............ 7 11.2 Testing Principle .................................... 7 11.3 Equipment ............................................. 7 11.4 Specimen Preparation ........................... 7 11.5 Test Conditions ..................................... 8 11.6 Testing Procedure ................................. 8 11.7 Expression of Results ............................ 8 11.8 Test Report ........................................... 8

12. Derivation of the Master Curve ....................... 9

12.1 Principle ................................................ 9 12.2 Experimental Data ............................... 10 12.3 Test Report ......................................... 12

Austroads Test Method AGPT/T274 Characterisation of Flexural Stiffness and Fatigue Performance of Bituminous Mixes


The procedure is used:

(a) to determine the modulus of the material at different temperatures and frequencies and to develop flexural modulus master curves

(b) to rank bituminous mixtures on the basis of stiffness and fatigue

(c) as a guide to relative performance in the pavement

(d) to obtain data for estimating the structural behaviour of the road

(e) to judge test data according to specifications for bituminous mixtures.

Because this test method does not impose a particular type of testing device, the precise choice of the test conditions depends on the possibilities and the working range of the device used. For the choice of specific test conditions, the requirements of the product standards for bituminous mixtures need to be respected. The applicability of this Austroads test method is described in the product standards for bituminous mixtures.

4. Safety Disclaimer

Warning: the use of this Austroads test method may involve hazardous materials, operations and equipment. The method does not purport to address all of the safety problems associated with its use. It is the responsibility of the user to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

5. Further Development

Revision of the accuracy and number of replicates. Derivation of an uncertainty of measurement statement.

6. References

The following documents are referred to in this method:

Austroads AGPT/T220 Preparation of asphalt slabs suitable for laboratory characterisation

AGPT04/07 Guide to pavement technology part 4: asphalt European standards EN 12697-24:2012 Bituminous mixtures: test methods for hot mix asphalt: part 24: resistance to fatigue

EN 12697-26:2012 Bituminous mixtures: test methods for hot mix asphalt: part 26: stiffness Standards Australia AS 1545:1976 Methods for the calibration and grading of extensometers AS 2193: 2005 Calibration and classification of force-measuring systems

AS 2891.1: 2008 Methods of sampling and testing asphalt: sampling of asphalt (superseded) AS 2891.2.1:2014 Methods of sampling and testing asphalt: sample preparation: mixing, quartering

and conditioning of asphalt in the laboratory

AS 2891.7.1:2015 Methods of sampling and testing asphalt: determination of maximum density of asphalt: water displacement method

AS 2891.8:2014 Methods of sampling and testing asphalt: voids and density relationships for compacted asphalt mixes

AS 2891.9.2:2014 Methods of sampling and testing asphalt: determination of bulk density of compacted asphalt: presaturation method



7. Equipment

As described in Section D.2 of EN 12697-24:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 24: resistance to fatigue, except as detailed in this Austroads test method.

7.1 Basic Features

The specimen shall be subjected to four-point bending. The test configuration is shown schematically in Figure 7.1. Free rotation and longitudinal translation shall be provided at the outer clamps, which provide the support points, as well as at the inner clamps, where a sinusoidal load is applied using a suitable mechanism. The effective length, L, is the centre to centre distance between the support points. For the customary Austroads test configuration L = 355.5 mm. The centre to centre distance between each of the clamps shall be equal to one-third of L.

The frequency, f, of the sinusoidal load shall have a range of 0.1 to at least 10 Hz, and preferably up to 60 Hz for modulus testing, with an accuracy of ± 0.1 Hz. The load cell shall have a range of at least ± 4500 N with a resolution of 2.2 N or better and an accuracy of ± 20 N. The displacement transducer should have a measuring range of ± 1.0 mm with a resolution of 0.25 µm or better and an accuracy of ± 1 µm.

The resonance frequency of the load cell, the vertical displacement transducer and the coupled moving mass should be at least 10 times as high as the test frequency.

Figure 7.1: Basic principles of four-point bending testing

The measurement of the force applied by the two middle clamps shall take place at the midpoint of effective length L.

The measurement of the deflection shall take place at the top or bottom surface of the beam specimen. Deflection shall be measured at the midpoint of L and at the midpoint of the width of the beam. To enable verification of the required pure bending of the specimen, the displacement of the two inner clamps shall also be measured.



7.2 Thermostatic Chamber

The thermostatic chamber shall be capable of maintaining the test temperature within ± 0.5°C (throughout the duration of the test). The thermostatic chamber shall have sufficient internal space to accommodate the loading frame, at least two test specimens in addition to the specimen to be tested, a dummy temperature specimen, adjustment of the loading frame and the displacement measuring devices, and the placing of specimens into, and removal from, the loading frame.

7.3 Checking of the Testing Equipment

Equipment shall be checked in accordance with Section 8 of EN 12697-24:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 24: Resistance to fatigue.

The assumption of pure bending in the specimen shall be verified in accordance with Section D.2.2 of EN 12697-24:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 24: Resistance to fatigue.

8. Specimen Preparation

As described in Section D.3 of EN 12697-24:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 24: resistance to fatigue, except as detailed in this Austroads test method.

Specimens shall be prepared from asphalt slabs, which were mixed either in the plant or in the laboratory, conditioned, and compacted in the laboratory in accordance with AGPT/T220, or which were obtained from a pavement. Slabs shall be cut to produce beam specimens of rectangular cross-section, transverse to the direction of compaction, with dimensions 390 ± 5 mm in length, 50 ± 5 mm in depth and 63.5 ± 5 mm in width.

The specimens shall have all cut faces and shall not have imperfections likely to adversely affect the performance of the test or the quality of the data captured during the test. If any such imperfections are apparent prior to testing, or become apparent during the test or upon examination of the data captured, the test result shall be rejected.

Care is to be exercised at all times to ensure that the specimens remain flat prior to testing.

8.1 Specimen Storage Prior to Test

Specimens and the slabs from which they are to be prepared shall be stored in an environment where the temperature does not exceed 30 °C. Wherever practicable, tests should be performed within 30 days of the date of compaction for laboratory-prepared slabs or the date of removing slabs from field pavements. During this storage period the specimens should be placed on a flat, stiff surface.

8.2 Measurement of Specimen Dimensions

Measure the beam specimen width and depth at five locations along the beam as follows:

(a) at points within 20 mm of each end

(b) within 10 mm of the centre of the beam

(c) within 10 mm of points located 90 mm in either direction from the centre of the beam.

The mean of the five measurements for each dimension shall be reported to the nearest 0.1 mm. Reject the beam if any one of the five measurements, for either width or depth, differs by more than 1.5 mm from the respective mean value.



8.3 Measurement of Density and Air Voids

Because asphalt properties are heavily dependent on the level of compaction (specifically, per cent air voids), testing shall be carried out on each replicate specimen to determine per cent air voids.

Air voids shall be determined as follows:

(a) Determine the maximum density of the asphalt mix in accordance with AS 2891.7.1 for each mix type. Determine the bulk density of each beam specimen in accordance with AS 2891.9.2. Calculate the air voids in the beams in accordance with AS 2891.8.

(b) Austroads fatigue tests are typically performed on beam specimens with an air void content of 5.0% ± 0.5%. For some mix types, or design situations, a different air void content may be specified as appropriate.

(c) The bulk density shall be determined either before or after the fatigue test. However, if the bulk density is determined before testing, then the specimens shall be dried to constant mass at ambient temperature prior to placing in the loading frame.

(d) The maximum density of the asphalt from field pavements, if determined using the beam specimens, shall be determined upon completion of the fatigue test.

9. Test Procedures

The test procedure to characterise the stiffness of asphalt is contained in Section 10 of this Austroads test method. The procedure to determine the fatigue performance is described Section 11. Section 12 contains a method to develop a complex modulus master curve from modulus results obtained using the method in Section 10.

The modulus and fatigue characterisation tests can be run independently, or can be run consecutively with the fatigue test following the modulus test.

10. Stiffness Test

10.1 Definitions, Terms and Symbols

Definitions, Terms and Symbols shall be as specified in Section 3 of EN 12697-26:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 26: stiffness.

10.2 Testing Principle

As described in Sections 4 and B.1 of EN 12697-26:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 26: stiffness.

10.3 Equipment

As described in Section 7 of this Austroads test method.

10.4 Specimen Preparation


Stiffness characterisation tests shall be conducted on a minimum of four specimens.

Specimens may be, and generally should be, re-used for consecutive tests at different temperatures.



Provided that the stiffness test is performed at a strain level ≤ 50 micro-strain and to a maximum of 100 load cycles, per test condition, specimens may be re-used for fatigue testing upon completion of the stiffness test.

10.5 Test Conditions

10.5.1 Test Temperatures

The temperature in the thermostatic chamber, in the vicinity of the specimen, shall be equal to the specified temperature to ± 0.5 °C. For each test temperature, the specimen shall be placed in the thermostatic chamber for at least four hours before testing.

The difference between two test conditions should not exceed 10 °C. A typical set of temperatures could be 0 °C, 10 °C, 20 °C and 30 °C. Test temperatures above 30 °C should be used with care especially for possible problems of non-linearity and also for possible creep of the specimens. When choosing the temperature sequence for testing, take note that in most thermostatic chambers heating specimens to a desired test temperature will be achieved faster than cooling them.

10.5.2 Loading Frequencies

The range of frequencies is equipment dependent, with hydraulic equipment typically capable of higher frequencies than pneumatic equipment.

Most equipment is able to cover a range between 0.1 Hz and 20 Hz. However, it is preferable to make it as wide as possible in order to allow a logarithmic presentation of the isotherms. A typical set of frequencies could be 0.1 Hz, 0.2 Hz, 0.5 Hz, 1 Hz, 2 Hz, 5 Hz, 10 Hz, 20 Hz, 30 Hz and again at 1 Hz. This last measurement is to check that the specimen has not been damaged during the loading at the various frequencies. If the difference between stiffness of the specimen at the original and last measurements at identical frequency and at the same temperature is greater than 3%, it can be concluded that the specimen is damaged and, therefore, cannot be used for further testing (e.g. at different temperatures). Be aware that resonance phenomena may occur especially at high frequencies.

Be aware that heat may accumulate in the specimen. This problem is especially dominant at prolonged measurements and/or higher frequencies.

The waveform should be sinusoidal and harmonic. Any distortion is the sign of an abnormal set-up or of a resonance phenomenon that can disturb the measurement.

10.6 Testing Procedure

The test procedure shall be as described in Section B.4 of EN 12697-26:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 26: stiffness (four point bending test in sinusoidal loading mode).

10.7 Expression of Results

Results shall be expressed as described in Section 9 of EN 12697-26:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 26: stiffness.



10.8 Test Report

The test report shall include the following information:

(a) General:

1. reference to this test method, including version date 2. age of the specimens at the time of testing (in days).

(b) Information on specimens:

1. sample number 2. type and origin of asphalt mix 3. method of manufacture or sampling (e.g. plant mixed or laboratory mixed) 4. air void content of beam specimens.

(c) Information on the test and results:

1. temperature at which the test was carried out 2. test frequencies 3. strain level in micro-strain 4. for each specimen and each combination of test temperature and frequency: complex modulus and

phase angle, or real and imaginary components of the complex modulus (E1 and E2). (d) Optional information on a set of tests:

1. plots of data and graphs 2. master curve developed from test data in accordance with the procedure in Section 12 of this

Austroads test method 3. results of equipment check as per Section 7.3 of this Austroads test method.

11. Fatigue Resistance Test

11.1 Definitions, Terms and Symbols

Definitions, Terms and Symbols shall be as specified in Sections 3.1 and 3.5 of EN 12697-24:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 24: Resistance to fatigue.

11.2 Testing Principle

As described in Sections 7.4 and D.1 of EN 12697-24:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 24: Resistance to fatigue.

11.3 Equipment


11.4 Specimen Preparation




11.5 Test Conditions

One fatigue line result comprises measurements on not less than 18 individual specimens. The test shall be undertaken at not less than three loading levels (e.g. three strain levels in constant deflection mode). The levels for the chosen loading mode shall be chosen in such a way that the fatigue lives are within the range 104 to 2 × 106 cycles. The load conditions shall be selected so that the number of cycles to failure exceeds 106 for at least 20% of tests.

The Austroads asphalt fatigue resistance test is typically performed at a temperature of 20 °C and a loading frequency of 10 Hz.

Be aware that heat may accumulate in the specimen. This problem is especially dominant at prolonged measurements and/or higher frequencies.

11.6 Testing Procedure

The test procedure shall be as described in Section D.4 of EN 12697-24:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 24: Resistance to fatigue (four point bending test in sinusoidal loading mode).

11.7 Expression of Results

Results shall be expressed as described in Section D.5 of EN 12697-24:2012, Bituminous mixtures – Test methods for hot mix asphalt – Part 24: Resistance to fatigue.

11.8 Test Report


(a) General:

1. reference to this Austroads test method, including version date 2. age of the specimens at the time of testing (in days).

(b) Information on specimens:

1. sample number 2. type and origin of asphalt mix 3. method of manufacture or sampling (e.g. plant mixed or laboratory mixed) 4. air void content of beam specimens.

(c) Information on the test and results:

1. temperature at which the test was carried out 2. test frequency 3. strain level in micro-strain 4. chosen failure criterion (e.g. 50% stiffness reduction) 5. initial stiffness of the specimen measured at the 50th load cycle 6. number of cycles to failure for each specimen 7. comments on incidents during test.

(d) Optional information on a set of tests:

1. results of equipment check as per Section 7.3 of this Austroads test method 2. average number of cycles and the standard deviation obtained for each level of the chosen loading

mode 3. representation of the fatigue line and regression parameters A0 and A1 4. initial strain corresponding with a fatigue life of 106 cycles for the chosen failure criteria and set of

test conditions.



12. Derivation of the Master Curve

12.1 Principle

To develop modulus relationships that are valid at any combination of loading speed and temperature, tests are run at different load frequencies and temperatures in accordance with the procedure in Section 10 of this Austroads test method. The results are shifted to fit a continuous function at a selected reference temperature, this is the so-called modulus master curve. An example of a master curve constructed using flexural modulus testing on an AC14 asphalt mix with Class 320 bitumen is shown in Figure 12.1. Once the master curve is developed, the modulus at any point of interest can be calculated; the process is typically automated through the use of software. In the example of Figure 12.1, and using the model form of the master curve (Equation 1), it can be determined that the modulus at 25 °C and 10 Hz is equal to the modulus at the reference temperature (in this case 20 °C) and 1.58 Hz, resulting in a value of 4908 MPa.

Figure 12.1: E* Master curve from four point bending tests



12.2 Experimental Data

To construct the master curve shown in Figure 12.1, tests were run at 0 °C, 10 °C, 20 °C and 30 °C. A frequency sweep was performed at each of the temperatures using load frequencies of 0.1 Hz, 0.5 Hz, 1 Hz, 5 Hz, 10 Hz, 20 Hz. The results are shown in Table 12.1.

Table 12.1: Example of complex modulus results

Temperature (°C)

Frequency (Hz)

Complex modulus for replicate specimens (MPa)

Statistics

7B2 14B3 15B1 27B3 Mean (MPa)

STDEV (MPa)

CoV (%)

0

0.1 13 837 12 821 12 689 16 403 13 938 1 722 12.4

0.5 16 099 14 867 15 184 19 126 16 319 1 943 11.9

1 16 740 15 540 16 046 19 935 17 065 1 975 11.6

5 18 501 16 967 17 909 22 267 18 911 2 325 12.3

10 19 137 17 639 18 727 23 130 19 658 2 399 12.2

20 19 837 18 247 19 477 23 803 20 341 2 406 11.8

10

0.1 6 875 7 135 7 291 8 169 7 368 561 7.6

0.5 9 044 9 214 9 378 10 386 9 506 603 6.3

1 10 168 10 236 10 444 11 467 10 579 604 5.7

5 12 513 12 574 12 787 13 965 12 960 680 5.2

10 13 469 13 487 13 721 14 893 13 893 677 4.9

20 14 271 14 296 14 509 15 844 14 730 750 5.1

20

0.1 1 791 2 055 2 348 2 357 2 138 270 12.6

0.5 3 275 3 225 3 688 3 601 3 447 231 6.7

1 4 163 4 134 4 520 4 612 4 357 244 5.6

5 6 304 6 107 6 674 6 740 6 456 302 4.7

10 7 235 7 106 7 753 7 785 7 470 350 4.7

20 8 146 7 916 8 695 8 725 8 371 403 4.8

30

0.1 595 571 551 689 602 61 10.1

0.5 992 896 863 1 029 945 78 8.3

1 1 303 1 169 1 136 1 333 1 235 97 7.9

5 2 601 2 303 2 287 2 648 2 460 191 7.8

10 3 404 2 993 2 959 3 437 3 198 257 8.0

20 4 139 3 618 3 607 4 120 3 871 299 7.7

Various functions can be used to construct a master curve from such data. The model form recommended in this Austroads test method is known as the sigmoidal model. The sigmoidal function is shown in Equation 1.

𝑙𝑙𝑙𝑙𝑙𝑙10|𝐸𝐸∗| = 𝛿𝛿 +𝛼𝛼

1 + 𝑒𝑒𝛽𝛽+𝛾𝛾 𝑙𝑙𝑙𝑙𝑙𝑙10 𝑓𝑓𝑟𝑟 1

where

𝑓𝑓r = reduced frequency

δ,α,β, γ = fitting parameters



The results of the complex modulus tests at different temperatures as shown in Table 12.1 are shifted with respect to time of loading until a single smooth curve emerges, by means of the reduced frequency parameter (fr). The reduced frequency is defined in Equation 2 as the actual loading frequency in the test multiplied by the time-temperature shift factor, aT.

𝑓𝑓𝑟𝑟 = 𝑎𝑎𝑇𝑇 × 𝑓𝑓 2

where

𝑓𝑓 = frequency (Hz)

𝑎𝑎𝑇𝑇 = shift factor as a function of temperature (°C)

𝑇𝑇 = temperature (°C)

The shift function is fitted using a second-order polynomial equation (Equation 3).

log10( a𝑇𝑇) = a(T − T𝑟𝑟𝑟𝑟𝑓𝑓)2 + b(T − T𝑟𝑟𝑟𝑟𝑓𝑓) 3

where

a, b = fitting parameters

𝑇𝑇𝑟𝑟𝑟𝑟𝑓𝑓 = Reference temperature

Microsoft Excel solver can be used to simultaneously determine the optimum values for the fitting parameters a, b, α, β, γ, δ, in Equation 1 and Equation 2, against a set of experimental values of E* determined in accordance with Section 10. Fit optimisation is performed by maximising the coefficient of determination (R2) of the fit. To ensure consistency of curve fitting, use of the seed values provided in Table 12.2 is recommended.

Table 12.2: Recommended seed values for master curve fitting

δ α β γ a b

2.7 1.6 –1.0 –0.73 3E-04 –0.1

The frequency shift factor as a function of temperature is used to construct the temperature shift factor in Figure 12.2. In the fitting process, a reference temperature is selected, for which fr is set equal to f. The reference temperature (Tref) used for the fit in this case is 20 °C. This translates into a shift factor of 1 at 20 °C, which plots as 0 in the figure (log10(1) = 0).



Figure 12.2: Example temperature shift factor

The master curve parameters providing the best fit to the data in Table 12.1 are shown in Table 12.3. With these parameters entered into Equation 1 and Equation 2, the modulus of the asphalt material can now be calculated for any combination of loading frequency and temperature, and to plot the master curve as shown in Figure 12.1.

Table 12.3: Master curve fitting parameters

δ α β γ a b R2 Tref (°C)

1.147 3.255 –1.176 –0.4729 6.735E-04 –0.1618 0.998 20

12.3 Test Report


(a) all information required as per Section 10.8

(b) numerical values of the master curve fitting parameters in four significant figures.



Amendment Record

Amendment No. Clauses amended Action Date

1 10.5.1, 11.8, 12.1, 12.2 Substitution 12/01/2016

2 Table 12.1, 12.2 and 12.3 Format 18/01/2016

Key

Format Change in format

Substitution Old clause removed and replaced with new clause

New Insertion of new clause

Removed Old clauses removed

agpt-t274-16 charact of flex stiffness and fatigue

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