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Application of a strain sweep test to assess fatigue behavior of asphalt binders
Construction and Building Materials 36 (2012) 906–912
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Application of a strain sweep test to assess fatigue behavior of asphalt binders R. Botella ⇑, F.E. Pérez-Jiménez, R. Miró Technical University of Catalonia, Transportation and Land Infrastructure Department, C/Jordi Girona 1-3, B1 215, 08034 Barcelona, Spain
h i g h l i g h t s " We developed a new procedure to characterize asphalt binders fatigue behavior. " It has a simple test geometry and it is easy to perform. " We name it EBADE test. " It showed a good repeatability. " It showed adequate sensitivity to frequency and binder type.
a r t i c l e
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Article history: Received 24 February 2011 Received in revised form 21 May 2012 Accepted 4 June 2012 Available online 24 July 2012 Keywords: Asphalt binder Fatigue Strain sweep test Uniaxial test Tension–compression test
a b s t r a c t Fatigue cracking is the most common failure mechanism of bituminous pavements. Asphalt binders gives cohesion to the mixture, and are therefore particularly prone to fatigue. The Road Research Laboratory of the Technical University of Catalonia is developing a uniaxial tension–compression test to characterize the fatigue behavior of asphalt binders by performing fatigue strain sweep tests. This paper describes the efforts to calibrate the test procedure by evaluating its repeatability and sensitivity to test temperature/frequency and binder type. The test procedure presented a reasonably good repeatability in all parameters computed, and was sensitive to both the test frequency and the binder type. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Fatigue cracking is the most common failure mechanism of bituminous pavements. It is almost impossible to prevent a pavement from cracking after a long period of service. When designing, asphalt pavements service life is determined in terms of resistance to fatigue cracking by evaluating asphalt mixtures under fatigue conditions. Fatigue properties of mixtures are strongly related to those of binders [1–6]. However, aggregates also play an important role in this matter. Gradation and aggregate properties affect mixture fatigue behavior as well; in particular, the smallest particles (typically smaller than 0.063 mm) blend with the asphalt binder, modifying its rheological properties [7,8]. Several authors have extensively studied the fatigue behavior of asphalt binders. The most common device to evaluate the rheological properties of binders is the Dynamic Shear Rheometer. This equipment has been used to characterize fatigue behavior of asphalt binders by applying strain-controlled tests and evaluating the number of cycles to ⇑ Corresponding author. E-mail address: [email protected] (R. Botella). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.06.059
failure [9–13]. These tests have also been carried out under stress-controlled conditions [13]. Focusing on strain-controlled fatigue tests, most follow the same basic procedure. A constant displacement is applied, strain is computed taking into account the geometrical shape of the specimen tested, and the number of cycles to failure is determined using a failure criterion. The most commonly used criterion is based on the value of the stiffness modulus calculated in every cycle of the test. The initial modulus is set as a reference and the test stops when the value of the modulus is lower than a certain percentage of the initial value (typically 50%). Each test performed represents a dot in the double logarithmic graph strain amplitude versus number of cycles to failure. By carrying out several tests and plotting the data in the former representation, it is possible to obtain the so-called fatigue law of the material by fitting the following equation.
e ¼ a Nb ;
ð1Þ
where e is the strain applied, N the number of cycles to failure, and a and b are the parameters to adjust. The main disadvantage of this procedure is that several tests must be performed at different strain amplitudes each, and
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therefore at different strains. Because of the time needed to perform a single test, long time periods are required to obtain the fatigue characterization of an asphalt binder. The Road Research Laboratory of the Technical University of Catalonia is developing a new experimental procedure to evaluate the fatigue properties of asphalt binders, using a cyclic uniaxial tension–compression test, i.e., the EBADE procedure (standing for Ensayo de BArrido de DEformaciones which means strain sweep test in Spanish). Instead of applying a constant displacement, the whole test is divided in different steps. At each step, the specimen undergoes a number of cycles at a given constant displacement amplitude; after that the tests stops for 1 min and starts again at a high displacement amplitude. In a single test, low and high strain levels are tested, applying different displacements velocities to the material. The main goal of this research is to relate the damage evolution obtained in each strain step with the binder fatigue behavior at the given strain, and therefore be able to sketch the fatigue law of this binder using a strain sweep test. This paper describes the first stage of this research project, in which the test procedure repeatability and sensitivity to different test configurations and variables are evaluated. 2. State of the practice 2.1. Dynamic Shear Rheometer The Dynamic Shear Rheometer has been used to obtain the rheological properties of asphalt binders and characterize their fatigue behavior since early and late 90s, respectively [9,14,11,12]. During the development of the Strategic Highway Research Program (SHRP) [16,17] the dynamic shear modulus of asphalt binders became of interest due to the tendency of aggregates to transmit shear loads to the binder between them under dynamic loading from traffic [15]. Using the DSR, Fig. 1, the dynamic shear properties of the binder are obtained, and based on them, a fatigue parameter is defined, jG⁄j sind, where G⁄ is the shear complex modulus and d is the phase angle or the delay between the signals imposed and received. However, various researchers have found this parameter unable to indicate resistance to fatigue damage [18,19]. During the development of the NCHRP Project 9–10 (Superpave Protocols for Modified Asphalt Binders), the time sweep test was introduced [20]. It consists of performing cyclic constant strain tests until complete failure of the specimen. Researchers found that data from these tests correlated to the mixture fatigue characteristics much better than the specification parameter jG⁄j sind. 2.2. Linear Amplitude Sweep test The Linear Amplitude Sweep (LAS) test has been recently described [15] as an accelerated method to evaluate fatigue behavior
Fig. 1. Schematic of the Dynamic Shear Rheometer [15].
Fig. 2. Linear Amplitude Sweep test strain input signal [15].
of asphalt binders using the DSR in strain-controlled mode. The test temperature and frequency are the same as in time sweep tests, i.e., 10 °C and 10 Hz. The test procedure consists of applying an initial 100 cycles at 0.1% strain to determine the undamaged linear viscoelastic properties of the binder. Each subsequent load step consists of 100 cycles at a rate of increase of 1% applied strain per step for 20 steps, beginning at 1% and ending at 20% applied strain, Fig. 2. 2.3. Annular Shear Rheometer Rompu et al. [21] used the annular shear rheometer to evaluate the fatigue behavior of asphalt binders in cyclic shear tests, Fig. 3. This equipment covers a wider range of frequencies/temperatures than the DSR and provides more homogeneous test conditions [22]. The tests were carried out applying a sinusoidal strain signal and measuring the shear stress in the moving part of the test equipment. During the tests, several pauses were made every certain number of cycles to apply small strains at different frequencies and thus obtain the value of the linear complex modulus. In addition, the equipment performed complex modulus measurements using ultrasonic waves. The test temperature and frequency were kept constant at 10 °C and 10 Hz, respectively, for all fatigue tests. 2.4. Tension–compression test Due to the viscoelastic properties of asphalt binders, shear tests are easy to implement. However, shear stresses are not the most
Axial sinusoidal loadings Bitumen or mastic
Fig. 3. Schematics of the Annular Shear Rheometer [21].
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Axial sinusoidal loadings
Diabolo sample
The specimens were cylinders of 20 mm of diameter and around 40 mm in height. The asphalt binder was heated to 120 °C in the oven and subsequently poured into the molds. Specimens were left to cool at room temperature for 2 h more and then put in the fridge at 10 °C for at least 24 h. Once the specimens were removed from the mold, they were glued to a servohydraulic press in order to perform the tests. The EBADE test procedure consists of applying a sinusoidal displacement signal (tension–compression) to the specimen at a frequency of 10 Hz. 60 lm of displacement amplitude are applied for a specified number of cycles. Then, the test stops for 1 min and starts again with an amplitude 60 lm higher (120 lm total amplitude) for the same number of cycles, and so on. The test ends with total failure of the specimen. In all studies presented in this paper, the strain step duration was fixed at 3000 cycles. Fig. 7 shows an example of the displacement input signal. Several parameters can be computed during the test. The most important are maximum stress, rmax, complex modulus, jE⁄j, and dissipated energy density, ED, during each cycle. Due to the delay between stress and strain an ellipse is formed in the stress vs. strain plot. Using the maximum stress and strain it is possible to obtain the complex modulus in the following equation:
jE j ¼
Fig. 4. Schematics of the diabolo-shaped specimen tension–compression fatigue test [6].
important mode of loading of pavement asphalt mixtures. In fact, tension and compression stresses are responsible for most of the damage to these materials. Chailleux et al. [6] have recently developed a new uniaxial tension–compression fatigue test, Fig. 4. A diabolo-shaped specimen is glued to the test equipment using a cyanoacrylate adhesive. In order to avoid an adhesive failure, the test geometry was designed to concentrate the stresses on the ‘‘shoulders’’ of the diabolo. The test procedure is a time sweep and consists of applying a sinusoidal vertical displacement signal to the specimen while recording the resultant load. The fatigue test starts with a series of small strain cycles employed to obtain the linear complex modulus within the linear viscoelastic domain. After that, a higher constant strain is applied until the specimen fails. The test temperature and frequency are kept constant at 10 °C and 10 Hz, respectively. 2.5. EBADE procedure The EBADE procedure consists of a tension–compression cyclic test that performs strain sweeps in cylindrical asphalt binder specimens. It combines the set up of the tension–compression test developed by Chailleux et al. [6] with an input signal similar to that of the LAS test, introduced in Section 2.2. As stated before, tension– compression tests represent the most important form of damage to bituminous pavements better than shear tests, and the strain sweep tests have been proved to be a fine method to characterize linear, non-linear and fatigue behavior of asphalt binders. Thus, the EBADE procedure combines both features to characterize fatigue behavior of bituminous binders in a quick and easy manner. This paper presents the efforts to calibrate and validate the procedure.
rmax : emax
ð2Þ
As can be seen in Fig. 8, as long as the test data describe an ellipse in the tension– compression plot, the ratio between rmax and emax is very close to the semimajor axis slope. Furthermore, the dissipated energy density is proportional to the area of the ellipse in the tension–compression graph. To compute this area from the test data, the Gauss Determinant Formula was used in the following equation:
S¼
1 jðx1 y2 þ . . . þ xn y1 Þ ðy1 x2 þ . . . þ yn x1 Þj; 2
where S is the area inside the data points in [x] [y] units and xi, yi are the coordinates of the n data points sorted clockwise or counterclockwise. The list below enumerates the three studies conducted to properly calibrate the EBADE procedure. 1. Repeatability. 2. Frequency sensitivity. 3. Binder type sensitivity.
4. Results 4.1. Repeatability In order to determine the repeatability of the test procedure seven tests were carried out under the same conditions and using the same binder, B 60/70. The results obtained in each test are plotted together in Figs. 9 and 10. Specifically, Fig. 9 shows that the parameter with the highest variability is the complex modulus measured in the two first strain steps. As the test progresses, the complex moduli measured in the tests become more similar and at higher strains some overlap. The dissipated energy density was the parameter with the least variation between tests. The difference between tests 1, 2 and 6 was close to zero, and the average difference for all the tests was below 6%. The shape of the dissipated energy density curve makes
3. Materials and testing methods All the specimens were fabricated using a cylindrical mold, Fig. 5, and four different binders, B 60/70, B 40/50, B 13/22,1 and a polymer modified binder, BM3c (see Fig. 6. 1 The two numbers represent the penetration grade range at 25 °C in dmm for each binder. Therefore, lower grades result in harder binders.
ð3Þ
Fig. 5. Mold used to fabricate the cylindrical specimens.
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160
Complex modulus (MPa)
140 120 100 80 60 40 20 0
0
5000 10000 15000 20000 25000 30000 35000 40000 45000
Fig. 6. EBADE test set up.
Cycles Test 1
Displacement input signal
Test 2
Test 3
Test 6
Test 7
Fig. 9. Complex modulus versus number of cycles during the EBADE tests.
150 7000
90 60 30 0 -30 -60 -90 -120 -150
Time Fig. 7. Example of the displacement input signal in a two cycle per step EBADE test.
Dissipated energy density (J/m3)
Displacement (µm)
120
6000 5000 4000 3000 2000 1000 0
0
5000 10000 15000 20000 25000 30000 35000 40000 45000
Cycles
Approximation to the complex modulus using the test data
Test 1
Test 2
Test 3
Test 6
Test 7
Fig. 10. Dissipated energy density versus number of cycles during the EBADE tests.
Tension
Table 1 Numerical values for the parameters computed in the EBADE tests.
Compression
Test
rmax (MPa)
eF (%)
E⁄ 1st (MPa)
E⁄ 2nd (MPa)
ED
1 2 3 6 7 Average CV (%)
0.32 0.32 0.37 0.31 0.33 0.33 6.5
0.90 0.74 0.74 0.97 0.90 0.85 10.8
111 88 124 97 112 106 12.0
95 93 123 86 107 101 12.8
6061 5424 6218 6258 6345 6061 5.5
max
(J/m3)
Fig. 8. Computation of the complex modulus using the ellipse shaped test data.
it possible to define a reliable failure criterion due to its sudden fall when failure takes place. When the dissipated energy density reaches the maximum value, failure occurs short after that and normally in the same strain step. All numerical values obtained for the seven tests performed are shown in Table 1. The parameters in Table 1 are: maximum stress amplitude achieved during the test, rmax, strain amplitude at which failure takes place, eF, average complex modulus during the first and second steps, E⁄ 1st and E⁄ 2nd, and maximum dissipated energy density during the test, ED max. CV is the coefficient of variation obtained by dividing the sample standard deviation by the average value. As said before, the maximum dissipated energy density is the parameter with the least variation between tests, followed by the
maximum stress achieved during the test. The failure strain was defined as the strain of the step at which the dissipated energy density reaches its maximum value. Variability in this parameter was acceptable. Finally, the average complex modulus during the first and second steps was the parameter with the highest coefficient of variation.
4.2. Frequency sensitivity After studying the repeatability of the test, the frequency sensitivity was evaluated. Viscoelastic materials fulfill the time–temperature superposition principle [23,24], which states that data obtained at different frequencies can overlap with data obtained at different temperatures using the appropriate shift factor. That
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R. Botella et al. / Construction and Building Materials 36 (2012) 906–912 Table 2 Numerical values for the parameters computed at different frequencies in the EBADE tests.
Complex modulus (MPa)
250
200
150
100
Frequency (Hz)
rmax (MPa)
eF (%)
E⁄ 1er (MPa)
ED max (J/m3)
1 5 10 15 20 25 30
0.25 0.33 0.33 0.37 0.36 0.33 0.36
1.04 0.98 0.90 0.90 0.75 0.74 0.81
55 89 112 135 105 171 194
6021 3827 6345 5318 4641 3038 4742
50
0
0
5000 10000 15000 20000 25000 30000 35000 40000 45000
Cycles 1 Hz
5 Hz
15 Hz
20 Hz
25 Hz
30 Hz
10 Hz
Fig. 11. Complex modulus versus number of cycles in the EBADE tests performed at different frequencies.
means that by analyzing the frequency sensitivity the temperature sensitivity of the test procedure can also be obtained. Seven specimens were fabricated using the same asphalt binder B 60/70 as in the repeatability study. Seven tests were performed, at seven different test frequencies: 1, 5, 10, 15, 20, 25, and 30 Hz. At frequencies over 20 Hz the test equipment was not able to control the displacement signal as precisely as at lower frequencies. Hence, differences may not only be due to frequency changes but also to small variations in the strain amplitude applied. Fig. 11 shows the evolution of the complex modulus throughout the tests for each different frequency. As can be observed, the complex modulus increases with the frequency. That was the expected result because increasing the test frequency is equivalent to lowering the test temperature, apart from a shift factor [24]. Looking at the dissipated energy density plot, Fig. 12, it is clear that higher frequencies lead to bigger drops in the energy values at each strain step. Furthermore, at higher frequencies this phenomena takes place at lower strains, i.e., dissipated energy density decrease within a strain step. That may be caused by damage or a change in materials response when exposed to a different displacement rate. Table 2 summarizes the tests results by grouping the most important parameters computed during the tests at different frequencies. It is clear that higher test frequencies result in higher complex moduli and lower failure strains. However, maximum values of dissipated energy density values are not so easy to read since various
factors influence it. It has a maximum value at 10 Hz, but higher frequencies show lower maximum DED values. That may be caused by the response of this parameter during the strain sweep test. At the same strain amplitude higher frequencies lead to higher stress– strain loops, but at the same time failure takes place at lower strain values. For instance, at very low frequencies the test can last more strain steps but the DED in each of them is lower, on the other hand at very high frequencies the DED is higher at the first strain steps but the binder can only endure a few strain steps. Clearly, the test procedure is sensitive to the test temperature and so are each of the parameters computed during testing. The most sensitive was the complex modulus, which also showed the most variation in the repeatability study. 4.3. Binder type sensitivity In order to check the test sensitivity to the asphalt binder tested, four different binders were studied: B 13/22, B 40/50, B 60/70 and a polymer modified binder BM3c. Figs. 13 and 14 illustrate the evolution of the complex modulus and the dissipated energy density with the number of cycles for the four binders. As expected, lower penetration grades led to higher complex moduli and lower failure strains, Fig. 13. B 13/22 showed the highest complex modulus and the fastest degradation rate; therefore, its failure strain was the lowest of the four binders. On the other hand, B 60/70 obtained the lowest complex modulus of the three conventional binders, but also showed a much slower degradation rate. Finally, B 40/50 exhibited an intermediate behavior. The dissipated energy density ranked the binders in the same manner as the complex modulus. In Fig. 14, it is clear that the hardest binder starts to lose dissipated energy density within the same
300
6000
Complex modulus (MPa)
Dissipated energy density (J/m3)
7000
5000 4000 3000 2000 1000 0
0
5000 10000 15000 20000 25000 30000 35000 40000 45000
250 200 150 100 50 0
0
10000
20000
Cycles 1 Hz
5 Hz
10 Hz
15 Hz
30000
40000
50000
60000
Cycles 20 Hz
25 Hz
30 Hz
Fig. 12. Dissipated energy density versus number of cycles in the EBADE tests performed at different frequencies.
13/22
60/70
40/50
BM3c
Fig. 13. Complex modulus versus number of cycles in the EBADE tests carried out to four different asphalt binders.
R. Botella et al. / Construction and Building Materials 36 (2012) 906–912
Dissipated energy density (J/m3)
7000 6000 5000 4000 3000 2000 1000 0
0
10000
20000
30000
40000
50000
60000
Cycles 13/22
40/50
60/70
BM3c
Fig. 14. Dissipated energy density versus number of cycles in the EBADE tests on the four asphalt binders.
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stress remained the same or decreased slightly, the complex modulus decreased enough to consider that the specimen had failed. One explanation for this behavior is that as the strain amplitude increases and the binder bonds are broken, the binder starts to behave as a polymer because polymer bonds are less prone to failure. As can be seen, the EBADE test shows the difference in behavior of binders under fatigue conditions, as well as the decrease of the complex modulus during the failure process. This procedure also makes it possible to obtain the energy released during the damage process and the maximum strain amplitude that the binder can undergo before breaking. Among binders with similar initial complex modulus, the one which dissipates the most energy or achieves the highest strain levels at failure has the best fatigue behavior. These three parameters could characterize fatigue behavior of asphalt binders. The Road Research Laboratory of the Technical University of Catalonia is currently applying this procedure to different asphalt binders in order to achieve validation and establish common criteria to characterize fatigue behavior of asphalt binders. 5. Conclusions
Table 3 Numerical values for the parameters computed in the EBADE tests on the four binders. Binder
rmax (MPa)
eF (%)
E⁄ 1er (MPa)
ED max (J/m3)
13/22 40/50 60/70 BM3c
0.72 0.50 0.33 0.20
0.37 0.58 0.90 >1.50
285 157 112 68
4578 5550 6345 6795
0,8 0,7
Stress (MPa)
0,6 0,5 0,4 0,3
The research presented in this paper was used to validate EBADE as a suitable test procedure for application to asphalt binders. It showed reasonably good repeatability; in particular, the maximum dissipated energy density exhibited the best repeatability, CV of C5.5%, while the initial complex modulus showed the highest variation, CV of 12%. The maximum stress during the test showed a CV of 6.5% and the failure strain of 10.8%. Moreover, EBADE sensitivity to test frequency–temperature was clear and the results and trends fitted with the behavior of asphalt binders. The trend observed in the initial complex modulus was clear and fitted with the expected behavior for this material. The maximum dissipated energy density with the frequency showed a maximum at 10 Hz. That was explained by the opposite behaviors this parameter had during strain sweep tests, it increases when the binder showed higher complex modulus (higher frequencies) but it decreased when the failure strain decreased (also higher frequencies). Finally, the EBADE test was able to distinguish between binders with different penetration grades and polymer-modified binders, the expected results being obtained. Lower penetration grades led to higher initial complex moduli.
0,2
6. Ongoing work
0,1 0
0
10000
20000
30000
40000
50000
60000
Cycles 13/22
40/50
60/70
BM3c
Fig. 15. Maximum stress versus cycles in the EBADE tests carried out to four different asphalt binders.
strain step, sooner than B 60/70 binder. Its stress–strain loops are bigger at the same strain applied but, since it fails at a much lower strain, the maximum DED value achieved during the test by the softest binder is higher. Table 3 contains the numerical values for the four binders. The behavior of the polymer-modified binder was totally different from that of the rest of binders. Its initial complex modulus was the lowest but its degradation rate was also the slowest. As the test progressed, the stress that the test equipment needed to deform the specimen decreased so slowly with the strain applied that it was impossible to break the specimen in two parts during the test, Fig. 15. However, since the strain amplitude increased and the
After validation of the EBADE test as suitable test procedure to evaluate the properties of binders, ongoing work at the Road Research Laboratory of the Technical University of Catalonia aims to relate the results from the strain sweep tests with the asphalt binder fatigue law obtained using several time sweep tests.2 To this end, several time sweep tests will be performed using the EBADE test configuration and results will be correlated with those obtained in strain sweep tests. References [1] Boussad N, DesCroix P, Dony A. Prediction of mix modulus and fatigue law from binder rheology properties. J Assoc Asphalt Paving Technol 1996;65:40. [2] Reese R. Properties of aged asphalt binder related to asphalt concrete fatigue life. J Assoc Asphalt Paving Technol 1997;66:604. [3] Soenen H, De La Roche C, Redelius P. Fatigue behaviour of bituminous materials: from binders to mixes. Int J Road Mater Pavement Des 2003;4:7–27. [4] Castro M, Sánchez JA. Estimation of asphalt concret fatigue curves – a damage theory approach. Constr Build Mater 2007;22:1232–8. 2 Time sweep tests are strain-controlled tests in which the strain applied to the specimen remains constant until specimen failure.
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[5] Reyes Ortiz OJ, Hernández Noguera JA, Pérez Jiménez FE. Relation between the fatigue behaviour of bituminous and bituminous asphalt mixture. In: Asphalt – Road For Life, Euroasphalt and Eurobitumen Congress; 2008. [6] Chailleux E, Bodin D, de La Roche C, Leguern M, Vignard N. Fatigue behaviour of bitumen in tension–compression loading mode: rheological analysis and comparison with mix fatigue. Adv Test Charact Bituminous Mater 2009. [7] Pérez Jiménez FE, Miró Recasens R, Martínez A. Effect of filler nature and content on the behaviour of bituminous mastics. Int J Road Mater Pavement Des 2008;9(Special Issue):417–31. [8] Faheem AF, Bahia HU. Modelling of asphalt mastics in terms of filler–bitumen interaction. Int J Road Mater Pavement Des 2010;11:281–303. [9] Bahia HU, Zhai H, Bonnetti K, Kose S. Non-linear viscoleastic and fatigue properties of asphalt binders. J Assoc Asphalt Paving Technol 1999;68:1. [10] Shenoy A. Fatigue testing and evaluation of asphalt binders using the Dynamic Shear Rheometer. J Test Eval 2002;30(4):303–12. [11] Wu S, Ye Q, Li N. Investigation of rheological and fatigue properties of asphalt mixtures containing polyester fibers. Constr Build Mater 2008;22:2111–5. [12] Masad E, Huang CW, Airey G, Muliana A. Nonlinear viscoelastic analysis of unaged and aged asphalt binders. Constr Build Mater 2008;22:2170–9. [13] O.J. Reyes Ortiz. Analisis del fallo a fatiga de ligantes hidrocarbonados. Parámetros que definen su comportamiento. PhD thesis, Universidad Politécnica de Cataluña, 2009. [14] Airey GD, Rahimzadeh B, Collop AC. Linear rheological behavior of bituminous paving materials. J Mater Civil Eng 2004;16(3):212–20. [15] Johnson CM. Estimating asphalt binder fatigue resistance using an accelerated test method. PhD thesis, University of Wisconsin–Madison; 2010. [16] Anderson DA, Christensen DW, Bahia HU, Dongre R, Sharma MG, Antle CE, Button J. Binder characterisation, physical properties. Technical report, SHRP-
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Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Application of a strain sweep test to assess fatigue behavior of asphalt binders R. Botella ⇑, F.E. Pérez-Jiménez, R. Miró Technical University of Catalonia, Transportation and Land Infrastructure Department, C/Jordi Girona 1-3, B1 215, 08034 Barcelona, Spain
h i g h l i g h t s " We developed a new procedure to characterize asphalt binders fatigue behavior. " It has a simple test geometry and it is easy to perform. " We name it EBADE test. " It showed a good repeatability. " It showed adequate sensitivity to frequency and binder type.
a r t i c l e
i n f o
Article history: Received 24 February 2011 Received in revised form 21 May 2012 Accepted 4 June 2012 Available online 24 July 2012 Keywords: Asphalt binder Fatigue Strain sweep test Uniaxial test Tension–compression test
a b s t r a c t Fatigue cracking is the most common failure mechanism of bituminous pavements. Asphalt binders gives cohesion to the mixture, and are therefore particularly prone to fatigue. The Road Research Laboratory of the Technical University of Catalonia is developing a uniaxial tension–compression test to characterize the fatigue behavior of asphalt binders by performing fatigue strain sweep tests. This paper describes the efforts to calibrate the test procedure by evaluating its repeatability and sensitivity to test temperature/frequency and binder type. The test procedure presented a reasonably good repeatability in all parameters computed, and was sensitive to both the test frequency and the binder type. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Fatigue cracking is the most common failure mechanism of bituminous pavements. It is almost impossible to prevent a pavement from cracking after a long period of service. When designing, asphalt pavements service life is determined in terms of resistance to fatigue cracking by evaluating asphalt mixtures under fatigue conditions. Fatigue properties of mixtures are strongly related to those of binders [1–6]. However, aggregates also play an important role in this matter. Gradation and aggregate properties affect mixture fatigue behavior as well; in particular, the smallest particles (typically smaller than 0.063 mm) blend with the asphalt binder, modifying its rheological properties [7,8]. Several authors have extensively studied the fatigue behavior of asphalt binders. The most common device to evaluate the rheological properties of binders is the Dynamic Shear Rheometer. This equipment has been used to characterize fatigue behavior of asphalt binders by applying strain-controlled tests and evaluating the number of cycles to ⇑ Corresponding author. E-mail address: [email protected] (R. Botella). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.06.059
failure [9–13]. These tests have also been carried out under stress-controlled conditions [13]. Focusing on strain-controlled fatigue tests, most follow the same basic procedure. A constant displacement is applied, strain is computed taking into account the geometrical shape of the specimen tested, and the number of cycles to failure is determined using a failure criterion. The most commonly used criterion is based on the value of the stiffness modulus calculated in every cycle of the test. The initial modulus is set as a reference and the test stops when the value of the modulus is lower than a certain percentage of the initial value (typically 50%). Each test performed represents a dot in the double logarithmic graph strain amplitude versus number of cycles to failure. By carrying out several tests and plotting the data in the former representation, it is possible to obtain the so-called fatigue law of the material by fitting the following equation.
e ¼ a Nb ;
ð1Þ
where e is the strain applied, N the number of cycles to failure, and a and b are the parameters to adjust. The main disadvantage of this procedure is that several tests must be performed at different strain amplitudes each, and
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therefore at different strains. Because of the time needed to perform a single test, long time periods are required to obtain the fatigue characterization of an asphalt binder. The Road Research Laboratory of the Technical University of Catalonia is developing a new experimental procedure to evaluate the fatigue properties of asphalt binders, using a cyclic uniaxial tension–compression test, i.e., the EBADE procedure (standing for Ensayo de BArrido de DEformaciones which means strain sweep test in Spanish). Instead of applying a constant displacement, the whole test is divided in different steps. At each step, the specimen undergoes a number of cycles at a given constant displacement amplitude; after that the tests stops for 1 min and starts again at a high displacement amplitude. In a single test, low and high strain levels are tested, applying different displacements velocities to the material. The main goal of this research is to relate the damage evolution obtained in each strain step with the binder fatigue behavior at the given strain, and therefore be able to sketch the fatigue law of this binder using a strain sweep test. This paper describes the first stage of this research project, in which the test procedure repeatability and sensitivity to different test configurations and variables are evaluated. 2. State of the practice 2.1. Dynamic Shear Rheometer The Dynamic Shear Rheometer has been used to obtain the rheological properties of asphalt binders and characterize their fatigue behavior since early and late 90s, respectively [9,14,11,12]. During the development of the Strategic Highway Research Program (SHRP) [16,17] the dynamic shear modulus of asphalt binders became of interest due to the tendency of aggregates to transmit shear loads to the binder between them under dynamic loading from traffic [15]. Using the DSR, Fig. 1, the dynamic shear properties of the binder are obtained, and based on them, a fatigue parameter is defined, jG⁄j sind, where G⁄ is the shear complex modulus and d is the phase angle or the delay between the signals imposed and received. However, various researchers have found this parameter unable to indicate resistance to fatigue damage [18,19]. During the development of the NCHRP Project 9–10 (Superpave Protocols for Modified Asphalt Binders), the time sweep test was introduced [20]. It consists of performing cyclic constant strain tests until complete failure of the specimen. Researchers found that data from these tests correlated to the mixture fatigue characteristics much better than the specification parameter jG⁄j sind. 2.2. Linear Amplitude Sweep test The Linear Amplitude Sweep (LAS) test has been recently described [15] as an accelerated method to evaluate fatigue behavior
Fig. 1. Schematic of the Dynamic Shear Rheometer [15].
Fig. 2. Linear Amplitude Sweep test strain input signal [15].
of asphalt binders using the DSR in strain-controlled mode. The test temperature and frequency are the same as in time sweep tests, i.e., 10 °C and 10 Hz. The test procedure consists of applying an initial 100 cycles at 0.1% strain to determine the undamaged linear viscoelastic properties of the binder. Each subsequent load step consists of 100 cycles at a rate of increase of 1% applied strain per step for 20 steps, beginning at 1% and ending at 20% applied strain, Fig. 2. 2.3. Annular Shear Rheometer Rompu et al. [21] used the annular shear rheometer to evaluate the fatigue behavior of asphalt binders in cyclic shear tests, Fig. 3. This equipment covers a wider range of frequencies/temperatures than the DSR and provides more homogeneous test conditions [22]. The tests were carried out applying a sinusoidal strain signal and measuring the shear stress in the moving part of the test equipment. During the tests, several pauses were made every certain number of cycles to apply small strains at different frequencies and thus obtain the value of the linear complex modulus. In addition, the equipment performed complex modulus measurements using ultrasonic waves. The test temperature and frequency were kept constant at 10 °C and 10 Hz, respectively, for all fatigue tests. 2.4. Tension–compression test Due to the viscoelastic properties of asphalt binders, shear tests are easy to implement. However, shear stresses are not the most
Axial sinusoidal loadings Bitumen or mastic
Fig. 3. Schematics of the Annular Shear Rheometer [21].
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Axial sinusoidal loadings
Diabolo sample
The specimens were cylinders of 20 mm of diameter and around 40 mm in height. The asphalt binder was heated to 120 °C in the oven and subsequently poured into the molds. Specimens were left to cool at room temperature for 2 h more and then put in the fridge at 10 °C for at least 24 h. Once the specimens were removed from the mold, they were glued to a servohydraulic press in order to perform the tests. The EBADE test procedure consists of applying a sinusoidal displacement signal (tension–compression) to the specimen at a frequency of 10 Hz. 60 lm of displacement amplitude are applied for a specified number of cycles. Then, the test stops for 1 min and starts again with an amplitude 60 lm higher (120 lm total amplitude) for the same number of cycles, and so on. The test ends with total failure of the specimen. In all studies presented in this paper, the strain step duration was fixed at 3000 cycles. Fig. 7 shows an example of the displacement input signal. Several parameters can be computed during the test. The most important are maximum stress, rmax, complex modulus, jE⁄j, and dissipated energy density, ED, during each cycle. Due to the delay between stress and strain an ellipse is formed in the stress vs. strain plot. Using the maximum stress and strain it is possible to obtain the complex modulus in the following equation:
jE j ¼
Fig. 4. Schematics of the diabolo-shaped specimen tension–compression fatigue test [6].
important mode of loading of pavement asphalt mixtures. In fact, tension and compression stresses are responsible for most of the damage to these materials. Chailleux et al. [6] have recently developed a new uniaxial tension–compression fatigue test, Fig. 4. A diabolo-shaped specimen is glued to the test equipment using a cyanoacrylate adhesive. In order to avoid an adhesive failure, the test geometry was designed to concentrate the stresses on the ‘‘shoulders’’ of the diabolo. The test procedure is a time sweep and consists of applying a sinusoidal vertical displacement signal to the specimen while recording the resultant load. The fatigue test starts with a series of small strain cycles employed to obtain the linear complex modulus within the linear viscoelastic domain. After that, a higher constant strain is applied until the specimen fails. The test temperature and frequency are kept constant at 10 °C and 10 Hz, respectively. 2.5. EBADE procedure The EBADE procedure consists of a tension–compression cyclic test that performs strain sweeps in cylindrical asphalt binder specimens. It combines the set up of the tension–compression test developed by Chailleux et al. [6] with an input signal similar to that of the LAS test, introduced in Section 2.2. As stated before, tension– compression tests represent the most important form of damage to bituminous pavements better than shear tests, and the strain sweep tests have been proved to be a fine method to characterize linear, non-linear and fatigue behavior of asphalt binders. Thus, the EBADE procedure combines both features to characterize fatigue behavior of bituminous binders in a quick and easy manner. This paper presents the efforts to calibrate and validate the procedure.
rmax : emax
ð2Þ
As can be seen in Fig. 8, as long as the test data describe an ellipse in the tension– compression plot, the ratio between rmax and emax is very close to the semimajor axis slope. Furthermore, the dissipated energy density is proportional to the area of the ellipse in the tension–compression graph. To compute this area from the test data, the Gauss Determinant Formula was used in the following equation:
S¼
1 jðx1 y2 þ . . . þ xn y1 Þ ðy1 x2 þ . . . þ yn x1 Þj; 2
where S is the area inside the data points in [x] [y] units and xi, yi are the coordinates of the n data points sorted clockwise or counterclockwise. The list below enumerates the three studies conducted to properly calibrate the EBADE procedure. 1. Repeatability. 2. Frequency sensitivity. 3. Binder type sensitivity.
4. Results 4.1. Repeatability In order to determine the repeatability of the test procedure seven tests were carried out under the same conditions and using the same binder, B 60/70. The results obtained in each test are plotted together in Figs. 9 and 10. Specifically, Fig. 9 shows that the parameter with the highest variability is the complex modulus measured in the two first strain steps. As the test progresses, the complex moduli measured in the tests become more similar and at higher strains some overlap. The dissipated energy density was the parameter with the least variation between tests. The difference between tests 1, 2 and 6 was close to zero, and the average difference for all the tests was below 6%. The shape of the dissipated energy density curve makes
3. Materials and testing methods All the specimens were fabricated using a cylindrical mold, Fig. 5, and four different binders, B 60/70, B 40/50, B 13/22,1 and a polymer modified binder, BM3c (see Fig. 6. 1 The two numbers represent the penetration grade range at 25 °C in dmm for each binder. Therefore, lower grades result in harder binders.
ð3Þ
Fig. 5. Mold used to fabricate the cylindrical specimens.
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160
Complex modulus (MPa)
140 120 100 80 60 40 20 0
0
5000 10000 15000 20000 25000 30000 35000 40000 45000
Fig. 6. EBADE test set up.
Cycles Test 1
Displacement input signal
Test 2
Test 3
Test 6
Test 7
Fig. 9. Complex modulus versus number of cycles during the EBADE tests.
150 7000
90 60 30 0 -30 -60 -90 -120 -150
Time Fig. 7. Example of the displacement input signal in a two cycle per step EBADE test.
Dissipated energy density (J/m3)
Displacement (µm)
120
6000 5000 4000 3000 2000 1000 0
0
5000 10000 15000 20000 25000 30000 35000 40000 45000
Cycles
Approximation to the complex modulus using the test data
Test 1
Test 2
Test 3
Test 6
Test 7
Fig. 10. Dissipated energy density versus number of cycles during the EBADE tests.
Tension
Table 1 Numerical values for the parameters computed in the EBADE tests.
Compression
Test
rmax (MPa)
eF (%)
E⁄ 1st (MPa)
E⁄ 2nd (MPa)
ED
1 2 3 6 7 Average CV (%)
0.32 0.32 0.37 0.31 0.33 0.33 6.5
0.90 0.74 0.74 0.97 0.90 0.85 10.8
111 88 124 97 112 106 12.0
95 93 123 86 107 101 12.8
6061 5424 6218 6258 6345 6061 5.5
max
(J/m3)
Fig. 8. Computation of the complex modulus using the ellipse shaped test data.
it possible to define a reliable failure criterion due to its sudden fall when failure takes place. When the dissipated energy density reaches the maximum value, failure occurs short after that and normally in the same strain step. All numerical values obtained for the seven tests performed are shown in Table 1. The parameters in Table 1 are: maximum stress amplitude achieved during the test, rmax, strain amplitude at which failure takes place, eF, average complex modulus during the first and second steps, E⁄ 1st and E⁄ 2nd, and maximum dissipated energy density during the test, ED max. CV is the coefficient of variation obtained by dividing the sample standard deviation by the average value. As said before, the maximum dissipated energy density is the parameter with the least variation between tests, followed by the
maximum stress achieved during the test. The failure strain was defined as the strain of the step at which the dissipated energy density reaches its maximum value. Variability in this parameter was acceptable. Finally, the average complex modulus during the first and second steps was the parameter with the highest coefficient of variation.
4.2. Frequency sensitivity After studying the repeatability of the test, the frequency sensitivity was evaluated. Viscoelastic materials fulfill the time–temperature superposition principle [23,24], which states that data obtained at different frequencies can overlap with data obtained at different temperatures using the appropriate shift factor. That
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R. Botella et al. / Construction and Building Materials 36 (2012) 906–912 Table 2 Numerical values for the parameters computed at different frequencies in the EBADE tests.
Complex modulus (MPa)
250
200
150
100
Frequency (Hz)
rmax (MPa)
eF (%)
E⁄ 1er (MPa)
ED max (J/m3)
1 5 10 15 20 25 30
0.25 0.33 0.33 0.37 0.36 0.33 0.36
1.04 0.98 0.90 0.90 0.75 0.74 0.81
55 89 112 135 105 171 194
6021 3827 6345 5318 4641 3038 4742
50
0
0
5000 10000 15000 20000 25000 30000 35000 40000 45000
Cycles 1 Hz
5 Hz
15 Hz
20 Hz
25 Hz
30 Hz
10 Hz
Fig. 11. Complex modulus versus number of cycles in the EBADE tests performed at different frequencies.
means that by analyzing the frequency sensitivity the temperature sensitivity of the test procedure can also be obtained. Seven specimens were fabricated using the same asphalt binder B 60/70 as in the repeatability study. Seven tests were performed, at seven different test frequencies: 1, 5, 10, 15, 20, 25, and 30 Hz. At frequencies over 20 Hz the test equipment was not able to control the displacement signal as precisely as at lower frequencies. Hence, differences may not only be due to frequency changes but also to small variations in the strain amplitude applied. Fig. 11 shows the evolution of the complex modulus throughout the tests for each different frequency. As can be observed, the complex modulus increases with the frequency. That was the expected result because increasing the test frequency is equivalent to lowering the test temperature, apart from a shift factor [24]. Looking at the dissipated energy density plot, Fig. 12, it is clear that higher frequencies lead to bigger drops in the energy values at each strain step. Furthermore, at higher frequencies this phenomena takes place at lower strains, i.e., dissipated energy density decrease within a strain step. That may be caused by damage or a change in materials response when exposed to a different displacement rate. Table 2 summarizes the tests results by grouping the most important parameters computed during the tests at different frequencies. It is clear that higher test frequencies result in higher complex moduli and lower failure strains. However, maximum values of dissipated energy density values are not so easy to read since various
factors influence it. It has a maximum value at 10 Hz, but higher frequencies show lower maximum DED values. That may be caused by the response of this parameter during the strain sweep test. At the same strain amplitude higher frequencies lead to higher stress– strain loops, but at the same time failure takes place at lower strain values. For instance, at very low frequencies the test can last more strain steps but the DED in each of them is lower, on the other hand at very high frequencies the DED is higher at the first strain steps but the binder can only endure a few strain steps. Clearly, the test procedure is sensitive to the test temperature and so are each of the parameters computed during testing. The most sensitive was the complex modulus, which also showed the most variation in the repeatability study. 4.3. Binder type sensitivity In order to check the test sensitivity to the asphalt binder tested, four different binders were studied: B 13/22, B 40/50, B 60/70 and a polymer modified binder BM3c. Figs. 13 and 14 illustrate the evolution of the complex modulus and the dissipated energy density with the number of cycles for the four binders. As expected, lower penetration grades led to higher complex moduli and lower failure strains, Fig. 13. B 13/22 showed the highest complex modulus and the fastest degradation rate; therefore, its failure strain was the lowest of the four binders. On the other hand, B 60/70 obtained the lowest complex modulus of the three conventional binders, but also showed a much slower degradation rate. Finally, B 40/50 exhibited an intermediate behavior. The dissipated energy density ranked the binders in the same manner as the complex modulus. In Fig. 14, it is clear that the hardest binder starts to lose dissipated energy density within the same
300
6000
Complex modulus (MPa)
Dissipated energy density (J/m3)
7000
5000 4000 3000 2000 1000 0
0
5000 10000 15000 20000 25000 30000 35000 40000 45000
250 200 150 100 50 0
0
10000
20000
Cycles 1 Hz
5 Hz
10 Hz
15 Hz
30000
40000
50000
60000
Cycles 20 Hz
25 Hz
30 Hz
Fig. 12. Dissipated energy density versus number of cycles in the EBADE tests performed at different frequencies.
13/22
60/70
40/50
BM3c
Fig. 13. Complex modulus versus number of cycles in the EBADE tests carried out to four different asphalt binders.
R. Botella et al. / Construction and Building Materials 36 (2012) 906–912
Dissipated energy density (J/m3)
7000 6000 5000 4000 3000 2000 1000 0
0
10000
20000
30000
40000
50000
60000
Cycles 13/22
40/50
60/70
BM3c
Fig. 14. Dissipated energy density versus number of cycles in the EBADE tests on the four asphalt binders.
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stress remained the same or decreased slightly, the complex modulus decreased enough to consider that the specimen had failed. One explanation for this behavior is that as the strain amplitude increases and the binder bonds are broken, the binder starts to behave as a polymer because polymer bonds are less prone to failure. As can be seen, the EBADE test shows the difference in behavior of binders under fatigue conditions, as well as the decrease of the complex modulus during the failure process. This procedure also makes it possible to obtain the energy released during the damage process and the maximum strain amplitude that the binder can undergo before breaking. Among binders with similar initial complex modulus, the one which dissipates the most energy or achieves the highest strain levels at failure has the best fatigue behavior. These three parameters could characterize fatigue behavior of asphalt binders. The Road Research Laboratory of the Technical University of Catalonia is currently applying this procedure to different asphalt binders in order to achieve validation and establish common criteria to characterize fatigue behavior of asphalt binders. 5. Conclusions
Table 3 Numerical values for the parameters computed in the EBADE tests on the four binders. Binder
rmax (MPa)
eF (%)
E⁄ 1er (MPa)
ED max (J/m3)
13/22 40/50 60/70 BM3c
0.72 0.50 0.33 0.20
0.37 0.58 0.90 >1.50
285 157 112 68
4578 5550 6345 6795
0,8 0,7
Stress (MPa)
0,6 0,5 0,4 0,3
The research presented in this paper was used to validate EBADE as a suitable test procedure for application to asphalt binders. It showed reasonably good repeatability; in particular, the maximum dissipated energy density exhibited the best repeatability, CV of C5.5%, while the initial complex modulus showed the highest variation, CV of 12%. The maximum stress during the test showed a CV of 6.5% and the failure strain of 10.8%. Moreover, EBADE sensitivity to test frequency–temperature was clear and the results and trends fitted with the behavior of asphalt binders. The trend observed in the initial complex modulus was clear and fitted with the expected behavior for this material. The maximum dissipated energy density with the frequency showed a maximum at 10 Hz. That was explained by the opposite behaviors this parameter had during strain sweep tests, it increases when the binder showed higher complex modulus (higher frequencies) but it decreased when the failure strain decreased (also higher frequencies). Finally, the EBADE test was able to distinguish between binders with different penetration grades and polymer-modified binders, the expected results being obtained. Lower penetration grades led to higher initial complex moduli.
0,2
6. Ongoing work
0,1 0
0
10000
20000
30000
40000
50000
60000
Cycles 13/22
40/50
60/70
BM3c
Fig. 15. Maximum stress versus cycles in the EBADE tests carried out to four different asphalt binders.
strain step, sooner than B 60/70 binder. Its stress–strain loops are bigger at the same strain applied but, since it fails at a much lower strain, the maximum DED value achieved during the test by the softest binder is higher. Table 3 contains the numerical values for the four binders. The behavior of the polymer-modified binder was totally different from that of the rest of binders. Its initial complex modulus was the lowest but its degradation rate was also the slowest. As the test progressed, the stress that the test equipment needed to deform the specimen decreased so slowly with the strain applied that it was impossible to break the specimen in two parts during the test, Fig. 15. However, since the strain amplitude increased and the
After validation of the EBADE test as suitable test procedure to evaluate the properties of binders, ongoing work at the Road Research Laboratory of the Technical University of Catalonia aims to relate the results from the strain sweep tests with the asphalt binder fatigue law obtained using several time sweep tests.2 To this end, several time sweep tests will be performed using the EBADE test configuration and results will be correlated with those obtained in strain sweep tests. References [1] Boussad N, DesCroix P, Dony A. Prediction of mix modulus and fatigue law from binder rheology properties. J Assoc Asphalt Paving Technol 1996;65:40. [2] Reese R. Properties of aged asphalt binder related to asphalt concrete fatigue life. J Assoc Asphalt Paving Technol 1997;66:604. [3] Soenen H, De La Roche C, Redelius P. Fatigue behaviour of bituminous materials: from binders to mixes. Int J Road Mater Pavement Des 2003;4:7–27. [4] Castro M, Sánchez JA. Estimation of asphalt concret fatigue curves – a damage theory approach. Constr Build Mater 2007;22:1232–8. 2 Time sweep tests are strain-controlled tests in which the strain applied to the specimen remains constant until specimen failure.
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