Reference data set for fatigue and stiffness properties by 4-point bending tests

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1 2 nd Workshop on Four Point Bending, Pais (ed.), University of Minho. ISBN Reference data set for fatigue and stiffness properties by 4-point bending tests E. Hauser Institute for Road Construction & Maintenance, Vienna, Austria D.V. Vliet Rijkswaterstaat Dienst Verkeer en Scheepvaart, Delft, Holland ABSTRACT: This paper presents an interlaboratory test campaign organized by Ministerie van Verkeer en Waterstaat Rijkswaterstaat, Dienst Wegen Waterbouwkunde (DWW) to conduct fatigue and stiffness tests according to EN 12697, part 24 and 26, respectively, on two asphalt concrete mixtures with the four point bending beam test. In the framework of this interlaboratory test of four laboratories the fatigue and stiffness behaviour of two common Dutch asphalt concrete mixtures AC 22 base 70/100, mixture A and B, was determined by means of the 4-point bending beam tests in order to assess the precision (repeatability and reproducibility) of the test method. In addition the established values were cross-checked in respect to their reproducibility by tests carried out at a foreign laboratory. For that purpose each of the four participating Dutch laboratories produced several additional AC beams for testing at the 5 th laboratory. In addition, specimens of both mixtures A and B were also produced at that laboratory with the same constituents and mix design. Before starting the 4-point bending tests the equipment had been calibrated by means of a special control sample (Delrin). Each beam testing started with a frequency sweep test for the determination of the stiffness. Thereby, the dynamic modulus E* and the phase lag ϕ had been considered at 8 Hz and 20 C according to EN 13108, part 20. The fatigue testing conditions specified were sinusoidal excitation at 30 Hz and 20 C using controlled strain mode. Thereby the failure criterion was the number of load applications when the mix stiffness has decreased to half of its initial value. The outcome of this interlaboratory test was a good correlation in the calibration tests, whereas the clueing of the clamps accords a specific importance in dependence of using bitumen or a two-component adhesive. In the course of the stiffness and fatigue trials the manufacturing of asphalt slabs reflect the outcomes of the reproducibility of the testing results. Thus, it is important to give a close attention concerning the equal distribution of the asphalt mixture particularly with regard to the homogeneity of the asphalt slabs. 1 INTRODUCTION Fatigue is an important material property of bituminous mixes, which effects a reduction of asphalt behaviour used for flexible road pavements. Therefore, the stiffness and fatigue characteristics serve as input parameters for pavement design in order to obtain the layer thicknesses of the road construction. Within the framework of the European standards several test procedures are described to assess stiffness and fatigue behaviour in laboratory testing. The work described above was contracted by Ministerie van Verkeer en Waterstaat, Rijkswaterstaat, Dienst Weg- en Waterbouwkunde (DWW) to conduct fatigue and stiffness tests according to EN part 24 and part 26, respectively, on two asphalt concrete mixtures with the 4-point bending beam test. The tests were carried out at four asphalt laboratories with the aim to give a valuable guidance with respect to set up a wide proficiency testing circus on 4- point bending tests and to reduce the variances between the bending beam devices. These laboratories manufactured test beams to perform the 4-point bending test for determination the stiffness and fatigue behaviour on these two common mixes. Furthermore, each laboratory manufactured seven more test beams for each mix in order to make a cross-checking 93

2 over the four laboratories with respect to the execution of the 4-point bending tests. Moreover, specimens of both mixtures A and B were produced at that foreign laboratory with the same constituents and mix design due to approach the influence of specimen preparation. 2 TEST METHODS 2.1 Testing equipments The 4-point bending beam tests were performed with servo-hydraulic machines by four participating laboratories, whereas one participant carried out the tests with a servo-pneumatic testing machine. Table 1 gives an overview of the testing equipment for the several laboratories and some characteristics of the devices. Table 1. Overview of the 4-point bending devices Participating laboratories Testing machine Dynamic axial load [kn] Frequencies [Hz] Lab 1 Servo-hydraulic ± 10 0,1 up to 100 Lab 2 Servo-hydraulic ± 10 0,1 up to 100 Lab 3 Servo-hydraulic ± 10 0,1 up to 100 Lab 4 Servo-pneumatic ± 10 0,1 up to 40 Lab 5 Servo-hydraulic ± 50 0,1 up to Test procedure and testing program The test procedure performed in this interlaboratory test is the strain controlled 4-point bending beam device. Thereby, the prismatic shaped asphalt specimen is loaded with sinusoidal cyclic strain amplitude at a constant temperature. For this purpose, the specimen is glued with aluminium clamps either by means of bitumen 20/30 (lab 1 till lab 4) or a two-component adhesive (lab 5). After mounting it in the 4-point bending beam device at +20 C, the sample is tempered for one hour before starting the testing. The deformation of the specimen is measured at the bottom surface of the sample between the two inner clamps. Furthermore, the approached force is constituted by a hydraulic valve and measured in a load cell. In order to qualify the long-term behaviour of asphalt materials the classical fatigue criterion had been consulted, where the dynamic modulus was decreased to half of its initial value. Following testing program was carried out by the participating laboratories: Conventional asphalt tests: - Determination of the dimensions of prismatic specimens according to EN Determination of the maximum density according to EN Determination of the bulk density of prismatic specimens according to EN Determination of the void content of prismatic specimens according to EN Performance-based test methods: - Specimen preparation by roller compaction according to EN Calibration test on Delrin beam Test frequencies: 8 and 30 Hz Test temperature: +20 C Strain amplitude: 100 μm/m - Stiffness tests according to EN Test frequency: 8 Hz Test temperature: +20 C Strain amplitude: 50 μm/m 94

3 - Fatigue tests according to EN Test frequency: 30 Hz Test temperature: +20 C Strain amplitudes: the three levels for the chosen deflection mode had been chosen in such a way that the fatigue lives are within the range 10 4 to 2x10 6 cycles. 2.3 Calibration of the 4-point bending beam devices In the framework of this interlaboratory test campaign the 4-point bending beam device has been checked by means of stiffness tests, which were performed on a calibration beam made of the material polyoxymethylene (POM) Aceton (Delrin) at 20 C. Thereby, the clamps are glued on the sample with a thin film of bitumen 20/30 (Figure 1). Figure 1. Preparation of the Delrin beam for calibration 3 MATERIALS AND SPECIMEN PREPARATION 3.1 Tested materials Within the test campaign two types of asphalt concrete mixes had been pulled up to perform the 4-point bending beam tests. In the following, the compositions of the asphalt materials are described in Table 2 and Table 3. Table 2. Composition of AC 22 base 70/100 50%PR Mixture A Component Material Mass-% Bitumen B70/100 2,2 Filler Wigras 40K Produktiestof 1,1 1,0 Aggregate Grofzand 0/2 [0-2] 13,7 Ned. Steenslag 8/11 < 2 mm 0,2 Ned. Steenslag 8/11 C5,6 2 mm 0,3 Ned. Steenslag 8/11 C8 5,6 1,9 Ned. Steenslag 8/11 C11,2 C8 10,5 Ned. Steenslag 11/16 C16 C11,2 11,5 Ned. Steenslag 16/22 C22,4 C16 7,2 Ned. Steenslag 16/22 C31,5 C22,4 0,6 Reclaimed asphalt Asfaltgranulaat Breekasfalt < 5,6 32,9 Asfaltgranulaat Breekasfalt 5,6 /16 14,2 Asfaltgranulaat Breekasfalt > 16 2,9 95

4 Table 3. Composition of AC 22 base 70/100 50%PR Mixture B Component Material Mass-% Bitumen B70/100 1,9 Filler MinCom AF 40 Produktiestof 1,1 1,4 Aggregate Rivierzand 0/2 [0-2] 20,8 Ned. Steenslag 8/11 < 2 mm 0,3 Ned. Steenslag 8/11 C5,6 2 mm 1,1 Ned. Steenslag 8/11 C8 5,6 2,3 Ned. Steenslag 8/11 C11,2 C8 7,2 Ned. Steenslag 11/16 C16 C11,2 5,2 Ned. Steenslag 16/22 C22,4 C16 7,7 Ned. Steenslag 16/22 C31,5 C22,4 0,9 Reclaimed asphalt Asfaltgranulaat Breekasfalt < 5,6 27,2 Asfaltgranulaat Breekasfalt 5,6 /16 21,9 Asfaltgranulaat Breekasfalt > 16 1,1 3.2 Manufacturing of the test beams Mixing process The participating laboratories produced their asphalt mixtures by means of different types of mixers. In Table 4 the various mixing types and its volume capacity are listed. Table 4. Mixing process Mixer Type Volume capacity [litre] Lab 1 Planetary mixer 30 Lab 2 Planetary mixer 40 Lab 3 Planetary mixer 50 Lab 4 Planetary mixer 10 Lab 5 Opposite rotation pug mill 30 Within the mixing process, the mineral aggregate was heated at 165 C in the dryer for five hours and afterwards it was filled into the pug mill. The aggregates were mixed for five minutes to guarantee homogeneity. Subsequently the tempered 160 C hot bitumen was added. The asphalt was mixed again for five minutes before tempered recycled asphalt was added Compaction After that the mix was removed from the mixer and slabs were manufactured by means of various compaction methods. All laboratories compaction method is common that they use roller compactors in order to manufacture asphalt slabs. In Table 5 the different types of compaction methods are listed. Furthermore the participating laboratories had to use as less as possible maximum of three charges to fill the slabs in order to guarantee homogeneous specimens. Table 5. Types of compaction methods Laboratory Charges Type of compaction method Lab 1 2 Compacted with a steel roller compactor Lab 2 3 Compacted with a steel roller compactor Lab 3 2 Compacted with a hammer followed by a steel roller compactor Lab 4 2 Compacted with a steel roller compactor Lab 5 1 Segment roller compactor - Controlled strain mode 96

5 3.2.3 Specimen preparation and storage After cooling the asphalt slabs were stored at room temperature for one week before they were cut into prismatic specimens (50 x 50 x 450 mm³ = width x height x length) by means of an asphalt saw. Afterwards all beams were stored in a conditioning cabinet at a temperature of 10 C before the 4-point bending tests. 4 RESULTS 4.1 Specimen properties In the course of the representational project, the four laboratories manufactured a number of extra beams for the cross-checking at the 5 th laboratory. Figure 2 shows two specimens of mixture B, which have been manufactured by different laboratories. Hence, the uniformly apportionment of the ready-mix is not given on the upper shown specimen (specimen A) because the bigger sizes of the grain are accumulated on the left side of the specimen, whereas sample B is approximately uniformly distributed with regard to the several fractions. A B Figure 2. Two specimens of mixture B manufactured by different laboratories Figure 3 illustrates the mean value and the standard deviation of the bulk density of the delivered specimens from all laboratories. According to EN , annex B the standard deviation on the mean value of the bulk density shall be less than 1%. As shown in the diagrams both values, namely the average value as well as the limit values, are presented. Figure 3. Bulk density of the specimens (Mixture A and mixture B) 4.2 Calibration test results In the framework of this project a round robin test was performed for measurements of stiffness on Delrin beams in order to investigate the differences between the laboratories. Each laboratory 97

6 used a specific denoted and one laboratory - lab 3 - measured all specimens. To correct for possible different levels between the specimens the results of the different laboratories were compared with lab 3. Figure 4. Differences to the dynamic stiffness modulus E* in [MPa] and the phase angle ϕ in [ ], reference laboratory lab 3 Figure 4 illustrates the differences to the dynamic stiffness modulus E* with respect to the reference laboratory lab 3. Thereby, the results deviate with a maximum of 5%. An important exception on this conclusion is the results for laboratory 5 when a two-component adhesive was applied to glue the clamps on the Delrin beam. Thereby, the results are about 1000 MPa above all other laboratories, which mean that the dynamic stiffness modulus is about 25% higher than those tests, where the clamps are fixed by means of bitumen 20/30. This result is also reflected in the data of the phase angle, for which the value obtained was negative. As a consequence of these testing results, the European Standard should address the method to use clamps on the beams when performing the 4-point bending tests. 4.3 Stiffness test results In the following chapter the analyzed parameter is the dynamic stiffness modulus E*, which is obtained at the temperature of 20 C and the testing frequency of 8 Hz. Thereby, two approaches are chosen: first the data are analysed only from the measurements at the reference laboratory (lab 5), where the samples are prepared by the different laboratories (lab 1 till lab 4) secondly the data are analysed from the measurements of the laboratories own fabricated samples In the course of the statistical analyses, for both mixtures (mixture A and mixture B) the variation in measurements is split up in variances due to the following sources: variation between laboratories: in the second type of analysis this source of variation is due to differences in fabrication and measurement among the laboratories; in the first type of analysis only differences in fabrication are involved variation between slabs within laboratory: in both types of analysis this variation is due to systematic differences between the slabs from which the samples are obtained In the following tables (table 6 and table 7) the standard deviation for the dynamic stiffness modulus E* and its percentages of the mean are listed in dependence of the chosen approaches. 98

7 Table 6. Standard deviation σ for the dynamic stiffness modulus E* (at 20 C and 8 Hz) and percentages of the mean Samples from 4 laboratories, measurements at the reference laboratory (lab 5) Source of variation Mixture σ [MPa] Percentages [%] Average value x [MPa] between laboratories 67 1 A between slabs between laboratories B between slabs Table 7. Standard deviation σ for the dynamic stiffness modulus E* (at 20 C and 8 Hz) and percentages of the mean Samples and measurements from 4 laboratories Source of variation Mixture σ [MPa] Percentages [%] Average value x [MPa] between laboratories A between slabs between laboratories B between slabs In the Tables 6 and 7 the average values of the dynamic stiffness modulus E* are given. Thereby the mean values measured at the reference laboratory (lab5) are generally higher than those measured at the different laboratories (lab 1 till lab 4), which results from the different type of fixing the clamps. Thereby, the participants 1 till 4 glued their steel clamps by means of a thin film of bitumen (20/30), which is based on the testing temperature of 20 C, whereas laboratory lab 5 employ a two-component adhesive for fixing their clamps on the sample. Furthermore, it can be concluded from the stiffness testing results that the scattering in the dynamic stiffness modulus is comparatively low. The type 1 analysis shows in Table 6 that there are marginal standard deviations concerning the different types of manufacturing the specimens. In contrast to that Table 7 illustrates the differences in the testing system of each laboratory. Thereby, the higher standard deviations in comparison to the type 1 analysis (Table 6) result especially therein that participant 4 carry out their tests by means of a pneumatic system. Furthermore, for mixture A the produced samples of laboratory 1 have been included in the statistical analyses even though the density of the samples for this participant was not within the specification limits. 4.4 Fatigue test results Conventional fatigue criterion, fatigue lines The fatigue relationship, which describes the damage of bituminous mixtures, is presented in Equation 1. N fat = k1 1 ε k 2 (1) where N fat = number of load cycles till fatigue [-]; ε = strain amplitude [m/m]; and k 1, k 2 = parameters for describing the fatigue function. Thereby, the fatigue failure criterion is defined as the number of load applications when the dynamic modulus E* has decreased to half of its initial value. On the basis of the fatigue testing results, the fatigue line can be drawn by making a linear regression between the natural logarithms of N fat and the natural logarithms of the initial strain amplitude at the 100 th load cycle as presented in Equation 2. ln ( ) 1 k k ln N = ln + 2 (2) fat 1 ε 99

8 4.4.2 Each data to reference laboratory The following fatigue data are analyzed only from the measurements at the reference laboratory, where the samples are prepared by the different laboratories (type 1 analysis). Figure 5. Fatigue test results in comparison to the reference laboratory Mixture A Table 8. Mixture A - Parameters k 1 and k 2 ; coefficient of determination R 2 and the strain amplitude ε 6 at 10 6 load cycle Laboratory k 1 k 2 R 2 ε 6 [-] [-] [%] [μm/m] Lab 1 4,353E-11 3, Reference lab (Specimen from lab 1) 3,069E-24 7, Lab 2 7,015E-18 5, Reference lab (Specimen from lab 2) 5,033E-17 5, Lab 3 1,125E-18 5, Reference lab (Specimen from lab 3) 1,011E-17 5, Lab 4 1,422E-23 7, Reference lab (Specimen from lab 4) 9,324E-10 3, Table 8 shows the results of the fatigue tests the parameters k 1 and k 2, respectively and the strain amplitude ε 6 at the 10 6 load cycle. Thereby, the participants 2 and 3 correspond in a good way with those results tested with their own produced samples by the reference laboratory. As mentioned in the chapter before, for mixture A the densities of the specimens produced by participant 1 are not within the specification limits, so they have to be excluded in the statistical analysis. Furthermore, the differences of the results regarding participant 4 are based on the variable testing system. Thereby, laboratory 4 carry out their fatigue tests by means of a pneumatic system, whereas the reference laboratory performs their trials by using a hydraulic system. Additionally, the amount of data has an effect of the testing results. The participants 1 till 4 have tested 18 samples, whereas the reference laboratory applies its results on simply 6 tested specimens, thus the outliers make an important impact on the parameters. 100

9 Figure 6. Fatigue test results in comparison to the reference laboratory Mixture B Table 9. Mixture B - Parameters k 1 and k 2 ; coefficient of determination R 2 and the strain amplitude ε 6 at 10 6 load cycle Laboratory k 1 k 2 R 2 ε 6 [-] [-] [%] [μm/m] Lab 1 7,593E-15 5, Reference lab (Specimen from lab 1) 1,056E-08 3, Lab 2 1,854E-16 5, Reference lab (Specimen from lab 2) 5,880E-06 2, Lab 3 6,650E-11 4, Reference lab (Specimen from lab 3) 1,070E-23 7, Lab 4 1,044E-17 5, Reference lab (Specimen from lab 4) 7,221E-09 3, Table 9 illustrates the results of the fatigue tests for mixture B. Thereby, the outcome of the fatigue trials behaves similar to the stiffness results Datas from the reference laboratory The fatigue data are analyzed from the measurements from the laboratories own fabricated samples. Figure 7 shows the fatigue lines. 101

10 ln N 16,0 15,0 14,0 13,0 12,0 11,0 Lab 1 Lab 2 Lab 3 Lab 4 Lab 5 y = 3,490x -18,366 R² = 0,60 7 y = 2,787x -12,045 R² = 0,173 y = 7,426x -52,892 R² = 0,721 y = 3,525x -18,746 R² = 0,339 y = 7,408x -52,942 R² = 0,840 ln N 16,0 15,0 14,0 13,0 12,0 11,0 Lab 1 Lab 2 Lab 3 Lab 4 Lab 5 y = 7,401x -54,155 R² = 0,725 y = 5,557x -37,527 R² = 0,874 y = 5,685x -39,133 R² = 0,851 y = 3,692x -20,795 R² = 0,788 y = 6,377x -45,365 R² = 0,545 10,0 8,8 8,9 9,0 9,1 9,2 9,3 ln (1/ ε) 9,4 10,0 8,8 9,0 9,2 9,4 9,6 9,8 ln (1/ ε) Figure 7. Fatigue lines according to EN (Mixture A and mixture B) Table 10. Overview of the fatigue testing results (Mixture A and mixture B) Mixture Laboratory k 1 [-] k 2 [-] R 2 [%] ε 6 [μm/m] Lab 1 3, , Lab 2 5, , A Lab 3 1, , Lab 4 9, , Lab 5 1, , Lab 1 1, , Lab 2 5, , B Lab 3 1, , Lab 4 7, , Lab 5 1, , Figure 6 shows the fatigue lines for mixture A (left) and mixture B (right). The parameters k 1 and k 2, respectively and the strain amplitude ε 6 at the 10 6 load cycle are illustrated on Table 10. For mixture A, the results of participant 1 vary immense because of the fluctuations of the samples density. Furthermore, the third strain amplitude couldn t be tested due to the in homogeneities of the produced specimens. For mixture B, the testing parameters of the participants 1, 2 and 4 deviate immense from the results of the participants 3 and 5. On the basis of visual expertise, the samples of participant 2 have inhomogeneous arrangement of the mixture through the length of the specimen as shown in Figure 2. 5 CONCLUSION OF THE WORK The term fatigue describes the continuously degradation of its structure and the failure of a material due to repeatable loading. The resistance to fatigue is one of the proper behaviours, which determines the durability and consequently the useful life of asphalt mixtures used for road pavements. Within this interlaboratory project on the performance of 4-point bending beam testing in terms of stiffness and fatigue behaviour of asphalt mixtures the following cognitions have been resulted: - Calibration of the 4-point bending beam device The differences of the dynamic stiffness modulus are small for the participating laboratories. Though, there should be a declaration in carrying out calibration on 4-point bending beam tests in the European Standard. This includes not only the gluing of the clamps due to the application of bitumen or two-component adhesive. Furthermore this contains also the force of the torque moment. 102

11 - Specimen preparation homogeneity In the course of the manufacturing of asphalt slabs it is important to give a close attention concerning the equal distribution of the asphalt mixture particularly with regard to the homogeneity of the asphalt slabs. Hence, the accurate initial weight, the specimen preparation and the very same procedure by carrying out the tests will result in repeatable outcomes. - Specimen dimension In the framework of this interlaboratory project the dimensions of the prismatic specimens have been 50 x 50 x 450 mm³ = width B x height H x length L tot. With regard to the European Standard EN 12697, part 24 the recommended width B and the height H should be at least three times the maximum grain size in the tested material. Moreover, the effective length L should not be less than six times of the width B or the height H. As an output of this project the prismatic samples should have the recommended dimension according to the European Standard with regard to the inhomogeneity of the samples. - Stiffness test results The average mean values measured at the reference laboratory (lab5) are generally higher than those measured at the different laboratories (lab 1 till lab 4), which results from the different type of fixing the clamps. Furthermore, the accuracy of the measurements is good, except for mixture A concerning the data measured from the laboratories own fabricated samples. - Fatigue test results The mean values measured at the reference laboratory (lab 5) are generally lower than those measured at the different laboratories (participant 1 till participant 4), which follows from the different type of gluing the clamps on the samples. Thereby, participant 5 uses a twocomponent adhesive, which is more rigid than the bitumen 20/30. - Effects to the pavement design Determination of the layer thickness In order to obtain bituminous layer thickness, appropriate laboratory tests are required to obtain stiffness and fatigue behaviour of asphalt mixtures. Therefore, the determination of key material parameters for determination of the dynamic stiffness modulus and the fatigue characteristics, respectively are necessary for input data of the pavement design and furthermore, the evaluation of confidence intervals is recommended. Within the scope of this interlaboratory campaign the first steps has been taken in this direction. 6 REFERENCES EN , Bituminous mixtures Test methods for hot mix asphalt Part 24: Resistance to fatigue. EN , Bituminous mixtures Test methods for hot mix asphalt Part 26: Stiffness. Di Benedetto, H. et al Fatigue of bituminous mixtures. In RILEM TC 182-PEB Performance testing and evaluation of bituminous material Hauser, E Determination of stiffness & fatigue properties of Dutch STAB 2007 mixes by 4-point bending tests. Vienna. Monismith, C. et al Stiffness of Asphalt-Aggregate Mixes. In Strategic Highway Research Program - SHRP-A-388. University of California, Berkeley. Monismith, C. et al Fatigue Response of Asphalt-Aggregate Mixes. In Strategic Highway Research Program SHRP-A-404. University of California, Berkeley. Poot, M. R. et al STAB reference dataset for fatigue and dynamic stiffness properties. Delft. 103