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Fatigue characterization of binder with aging in two length scales: sand asphalt mortar and parallel plate binder film
Construction and Building Materials 237 (2020) 117588
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Fatigue characterization of binder with aging in two length scales: sand asphalt mortar and parallel plate binder film Santosh Reddy Kommidi a, Yong-Rak Kim a,⇑, Lilian Ribeiro de Rezende b a b
Department of Civil Engineering, University of Nebraska-Lincoln, Lincoln, NE, USA School of Civil and Environmental Engineering, Federal University of Goias, Goiania, Goias, Brazil
h i g h l i g h t s Fatigue characterization of binder with aging was compared at two length scales. A realistic binder film thickness was attempted using Sand Asphalt Mortar (SAM). SAM fatigue tests can sensitively capture the influence of aging on binder fatigue. SAM fatigue tests can distintively capture progressive microcracking and macrocracking phases.
a r t i c l e
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Article history: Received 21 March 2019 Received in revised form 27 October 2019 Accepted 11 November 2019
Keywords: Sand asphalt mortar Binder Fatigue Aging Linear amplitude sweep Time sweep
a b s t r a c t Fatigue-cracking resistance of asphaltic materials and flexible pavements is strongly related to the properties of the binder under aging and its interaction with aggregate particles, which are small-scale physical-mechanical characteristics of components. Currently, there are binder test methods including the AASHTO TP 101 that are widely used to evaluate fatigue characteristics of asphalt binder. Despite the scientific leap for predicting and characterizing binder fatigue behavior in pavements, there are opportunities to better mimic the true state of the binder subjected to fatigue loading in pavements. This study attempted fatigue damage characterization using sand asphalt mortar (SAM) specimens, which are anticipated to better represent the realistic geometry (such as micrometer thick film) of binder in mixtures than typical binder fatigue tests that use 2-mm thick parallel plate specimens, while testing repeatability-efficiency can still be met due to the use of standard Ottawa sands as a load carrier between binder films. Multiple fatigue-related tests (i.e., time sweep and amplitude sweep) were conducted on both 2-mm thick binder specimens and SAM specimens by varying the level of aging to investigate the effects of binder length scale and aging on fatigue cracking behavior. The laboratory procedure to fabricate SAM specimens was presented with the testing repeatability, which supports the validity of the method. It was observed from the fatigue testing results that SAM testing could capture the microcracking and macrocracking phases more distinctively and sensitively than conventional binder fatigue testing, particularly with the influence of aging. These test results and findings imply that the SAM can be considered a proper solid phase to evaluate the properties and damage characteristics of binders and mastics under various conditions. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Fatigue cracking is one of the most commonly observed distresses in asphaltic pavements. This form of cracking usually appears along the wheel path on the pavement surface, and it is mostly due to the repeated passing of heavy axle loads. The sever-
⇑ Corresponding author. E-mail addresses: [email protected] (S.R. Kommidi), yong-rak. [email protected] (Y.-R. Kim), [email protected] (L.R. de Rezende). https://doi.org/10.1016/j.conbuildmat.2019.117588 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
ity of the fatigue cracking in pavements increases as the binder ages. Several factors, such as aggregate gradation, air voids percentage and distribution, binder-aggregate interactions, variation of binder film thickness across components in asphalt concrete (AC) mixture, binder aging, etc., affect the overall AC fatigue behavior [1–4]. However, taking all these factors into consideration to understand the fatigue resistance of AC mixtures and AC pavements can be quite complex. Alternatively, researchers have conducted cyclic fatigue tests on cylindrical or beam AC samples [5–8]. More commonly, federal and state agencies usually rely on
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the results of binder tests to select appropriate fatigue damage resistant material. This is based on the widely-accepted assumption that the binder phase is critically responsible for the integrity of mixtures in pavements; it is also perhaps the weakest component that is prone to fatigue crack initiation. Traditionally, the binder characteristic that is most frequently used to evaluate fatigue was the |G*|sin(d) of the Superpave performance grade (PG) system which provides a value limit of 5,000 kPa after the laboratory long-term aging. However, several researchers have shown that the |G*|sin(d) parameter is not well correlated with field fatigue evaluation [9–14]. Bahia et al. [15] suggested that the strain level used for the PG system was relatively low when compared to the actual strain level experienced by the binder within AC mixtures. They suggested the evaluation of higher nonlinear strain levels to better assess fatigue phenomena in AC mixtures. Later, Masad and Somadevan [16] investigated the influence of localized strain distribution on the mechanical response of AC and concluded that the strain levels in the mastic and/or binder phase were quite high and most likely to be in the nonlinear region. To include the effects of high strain levels in the binder analysis, Bahia et al. [15] suggested the use of time sweep tests. The time sweep testing can simulate a more realistic fatigue process. Anderson et al. [17] conducted the time sweep tests and claimed that the soft behavior (|G*| values less than 15 MPa) of binder results in edge effects and instable flow when testing with parallel plate geometry at intermediate temperatures. They suggested that the fatigue characterization obtained by the binder time sweep tests at intermediate temperatures (e.g., 20 °C to 25 °C) would not represent true fatigue behavior. They concluded that fatigue characterization should be performed at relatively low temperatures where the binder stiffness would be large enough to reduce edge effects and to observe fatigue-associated cracking (such as micro-cracking) phenomena. Johnson et al. [18,19] proposed a much faster fatigue cracking evaluation method based on a monotonic loading procedure using the parallel plate geometry. They proposed the use of a viscoelastic continuum damage (VECD) approach to evaluate the fatigue resistance of the binders at different loadings. Hintz et al. [20] further developed the methodology to a simple strain sweep test along with a frequency sweep test, concluding that the two tests would be enough to characterize and analyze the fatigue behavior of binders using VECD theory. Later, several modifications were made to the approach by incorporating the energy-based failure criterion. Wang et al. [21,22] used maximum stored pseudo strain energy (PSE) for defining failure in linear amplitude sweep (LAS) and time sweep tests using a simplified-VECD theory for analyzing the results. A unified fatigue failure criterion was proposed by Wang et al. [21] and Safaei et al. [23] using PSE-based failure criterion to incorporate the effects of temperature, loading mode, and history. More recently, several other studies have improved understanding through experimental characterization of fatigue damage in asphaltic media [24–29]. As aforementioned, several different approaches including the AASHTO TP 101 Standard Method of Test for Estimating Fatigue Resistance of Asphalt Binders Using the Linear Amplitude Sweep have brought various advancements from the Superpave |G*|sin(d) criterion for better predicting and characterizing binder fatigue behavior in pavements. However, most of them are based on testing of binder film of usually around 2 mm, which is clearly very different from actual geometry (considered to be around 2–100 mm) [30] of binder or more specifically mastic film in AC mixtures. Furthermore, the influence of aging on binder fatigue behavior was not well addressed as a predictor of longer fatigue lives for long-term aged binders at low strain levels [31,32]. Considering the highly nonlinear and age-sensitive viscoelastic (rate-, time-, and temperature-dependent) behavior and
cracking of binders, binder fatigue testing should better mimic the true state of the binder placed in AC mixtures. Kim et al. [33–36] attempted such testing of micrometer thick film in binder samples by incorporating fatigue tests on sand asphalt mortar (SAM). They proposed a new sample fabrication methodology, where uniformly graded Ottawa sand particles were mixed with a properly estimated amount of binder to simulate arbitrary (but considered reasonable) binder film thickness. Based on their proposed test geometry (50 mm tall and 12.5 mm in diameter cylindrical rod sample) for fatigue testing using the dynamic mechanical analyzer, the fabricated SAM specimen can properly represent a micrometer scale binder film (based on the dosage of binder) with sand acting as a load bearing skeletal system. Many researchers have tried to include the effect of particles by analyzing the intermediate scale using fine aggregate matrix (FAM). However, the main advantage of the proposed SAM approach is the possibility of replicating this test in any laboratory and maintaining the same particle distribution, which in the case of FAM, it is not feasible, since each research study considers its own particles and mixing recipe. A series of previous studies showed that the SAM can successfully and sensitively evaluate material-specific fatigue damage characteristics of various binders, as demonstrated in Fig. 1(a). Also, as illustrated in Fig. 1(b), the time sweep fatigue testing of SAM samples can provide a reasonable fatigue failure point, or ‘‘transition point,” that is well matched to the peak phase angle in a strain-controlled mode [37]. Similar tests in a stresscontrolled mode were performed by Martono et al. [38]. Their analysis was based on dissipated energy ratio (DER), which indicated that the SAM samples provided a good repeatability and were free of edge effects. 2. Research objective and scope The objective of this study is to assess the fatigue characterization of asphalt binders in two different configurations—2-mm parallel plate binder film and SAM—at different levels of aging conditions. The two different configurations consequentially represent the two different length scales of the binder (i.e., 2-mm for the parallel plate binder film and 2–100 mm for the SAM). The micrometer length scale of binder in SAM would depend on the dosage of binder selected in fabricating the SAM specimens, details of which are described in Sections 3.2 and 3.3. In addition, the SAM testing method was intended to be improved from its initial development [33–37] so that testing repeatability and efficiency can be upgraded. The two different binder length scales considered in this study are expected to provide the core aspects of binder geometry on fatigue damage and binder cracking performance, particularly the influence of aging. To that end, multiple fatigue tests, including time sweep and strain sweep tests, were performed on both configurations. Mechanical responses with damage and subsequent fatigue failure were observed, and test-analysis results between the two length scales were compared. 3. Materials and sample preparation 3.1. Selection of materials A polymer modified binder (PG 64-28) was selected for this study. The binder was aged in the laboratory according to ASTM D2872 and ASTM D6521 for simulating short-term and long-term aging, respectively. The short-term aging of binder was simulated using the Rolling Thin Film Oven Test (RTFOT) where a thin film of binder is heated in the oven for 85 min at 163 °C under a continuous air flow of 4000 ml/min. The long-term aging of binder was simulated using the Pressurized Aging Vessel (PAV) experiment, in which 50 g of RTFOT-aged binder is poured into a pan and conditioned in the PAV vessel for 20 h at 2.1 MPa pressure. For preparing
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Fig. 1. SAM test results [37]: (a) strain-fatigue life of various binders; (b) a typical time sweep test result that presents progressive fatigue damage accumulation and failure.
SAM specimens, an Ottawa sand was selected. The material characteristics met the specifications of ASTM C778. The use of a standard sand (Ottawa sand in this study) for SAM fabrication is very advantageous, since it can be easily replicated in other laboratories internationally. Regarding the aging, it should be noted that only the binder was aged, and the aged binder was used to prepare SAM specimens by mixing the aged binder with the ASTM standard sand. 3.2. SAM specimen fabrication In the SAM sample preparation, a binder content of 4% by weight of total SAM mixture. It was estimated that, for a binder dosage of 4% by weight, the film thickness obtained is approximately around 5 mm using the analytical method given by Radovskiy [39]. A detailed description on determination and estimation of film thickness is described in Section 3.3. The film thickness was estimated by assuming that all the sand particles were spherically shaped and had a constant density of 2.65 g/cm3. Based on the sieve size distribution (ASTM C778) the number of particles retained in each sieve was obtained using the diameter of the particles in each respective sieve as the average of the current and the previous sieve size. As shown in Figs. 2 and 3, the SAM specimens were prepared in two stages. In the first stage (called the ‘‘mixing stage,” shown in Fig. 2) the sand particles and binder were heated separately at 135 °C for 30 min, and then thoroughly mixed at the same temperature until all sand particles were coated well with binder. The loose SAM mixture was poured into small containers, stored at 5 °C, and capped to protect it from any unnecessary aging until it was reheated to compact individual SAM testing specimens.
In a later stage (called the ‘‘compaction stage”), the loose SAM mixture was compacted to cylindrical specimens in a metallic mold. As illustrated in Fig. 3, the mold has two halves that are assembled together. Eleven grams of loose SAM and the mold were placed in the oven at 135 °C for 20 min. Based on several trial and error processes and observations by Kim et al. [33–37], the usage of 11 g of SAM mixture for compaction was chosen to make a compacted cylinder (50 mm height and 12.5 mm diameter) in a unit weight of 2.155 g/cm3, which appeared appropriate to be tested in a solid bar state and subjected to cracking due to cyclic loading. The authors are currently investigating in detail into the correct geometry size and dosage that would appropriate fatigue testing of solid bar samples. Resulting outcomes will be presented as an extension of the current work. As detailed in Fig. 3, after heating the loose SAM mixture and the mold, the SAM mixture was immediately poured into the mold and compacted using the compaction device. The compaction device allows one to apply a constant load (up to 200 lbs. force) using a side lever on the top face of the cylinder specimen. After several trial and error processes, the compaction of SAM specimens was achieved through a series of three loading rounds. In the first round, a load of 60 lbs. of force was applied for 15 s; the mold was then flipped over, and the same load was applied for another 15 s. The same procedure was followed for the second and third rounds, but the load was increased to 80 lbs. and 100 lbs. of force, respectively. This would enable the SAM specimen to be compacted evenly from both ends, which is targeted to increase testing repeatability. After the compaction process was completed, another 15 min were allowed for cooling down the mix and the mold. Then, the two mold halves were separated to take the compacted SAM specimen from the mold. The cylindrical SAM geometry can make data analysis easy by preventing complex
Fig. 2. SAM mixing process.
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Fig. 3. SAM fabrication (in particular compaction) process.
stress distributions under a torsional testing mode [33–37]. For torsional testing in the dynamic shear rheometer (DSR), two fixtures were secured to both ends of each SAM specimen using epoxy glue. The gluing was done carefully, so as not to cause any undesirable stress concentrations at either end. Moreover, during the gluing process, it was determined that the fixtures needed to be inline. To this end, a small tool was designed and fabricated to make sure the two fixtures lined up. Fig. 3 illustrates the SAM specimens with the fixtures glued, as well as the tools used to compact the specimens. The entire fabrication process (mixing, compaction, gluing, and installation) of SAM specimens was improved from its initial development [33–37].
3.3. Binder film thickness estimation in SAM Since SAM samples are used to indirectly represent a more realistic binder film thickness for analyzing the binder fatigue properties it is important to estimate the binder thickness that is being generated for a given dosage of binder when preparing the SAM samples. Two methods were incorporated for estimating the SAM binder film thickness, one using analytical formulas presented by Radovskiy [39] and the second method using image analysis of the cross-section of SAM specimens obtained using a laser scanning microscopy (LSM). LSM is a type of microscope that scans the surface of the specimen using a focused laser beam and produces considerably higher resolution images compared to conventional optical microscopes [40]. To conduct the LSM investigation, two cylindrical SAM specimens of 50 mm with different binder dosages (i.e., 6, 8, and 10%) were prepared, and smaller cylindrical pieces of 15 mm thick were carefully cut using a low speed digital diamond saw. The SAM surface was then lightly polished avoiding heat and material loss by using diamond lapping films. Then, three images were captured from each specimen. In each image, the distance between two particles filled with binder was measured at ten different locations. The film thickness was considered as a half of the distance between the two particles. As the compaction energy was fixed for all SAM specimens, the impact on film thickness variation is related only with the asphalt binder content used in SAM preparation. Table 1 presents the film thickness calculated using the analytical formulas, where the density of sand and binder was 2.650 g/cm3 and 1.000 g/cm3, respectively. Fig. 4 shows examples of images obtained from the LMS at each dosage. Table 1 also presents the range of ten measurements in each image. Both methods show that with the increase of binder content, the film thickness also
Table 1 SAM Film Thickness Estimated. Asphalt binder content (%)
Film thickness (lm) estimated Calculated using analytical formulas
Range observed by LMS images
4 6 8 10
5 9.0 12.0 15.0
N/A 10.5–28.0 22.5–70.5 43.5–97.5
increases; however, the values estimated between the two methods presented difference and generally lower than the mastic film thickness (100 lm) surrounding large aggregates reported by Elseifi et al. [30].
4. Laboratory tests and results 4.1. Frequency sweep test To characterize linear viscoelastic properties of binders in each testing scheme (i.e., 2-mm parallel plate of binder film and cylindrical SAM specimens), strain sweep tests were first performed prior to the frequency sweep test to obtain the linear limits of the samples. For binders, the strain sweep test was performed at 0 °C with a frequency of 10 Hz and at 45 °C with 0.1 Hz loading frequency. The two temperature-frequency combinations (i.e., 0 °C 10 Hz and 45 °C 0.1 Hz) represent either stiffest or most compliant testing conditions experienced by the following frequency sweep testing where loading frequencies vary from 0.1 Hz to 10 Hz at different testing temperatures. Based on the strain sweep test results, a strain amplitude of 0.01% was finally selected for binders as a level of strain that is sufficiently small to perform the frequency sweep tests for identifying the linear viscoelastic properties. Similarly, for SAM specimens, the strain sweep (LAS*) test was performed at two temperature-frequency combinations: 10 °C-10 Hz and 45 °C-0.1 Hz, since the following frequency sweep testing was conducted by varying frequencies from 0.1 Hz to 10 Hz at a range of testing temperatures from 10 °C to 45 °C. Fig. 5(a) shows the strain applied to the SAM samples during the LAS* experiment, and Fig. 5(b) shows the |G*| changes due to strain sweeping at different testing temperatures. As observed in Fig. 5 (b), the response of the SAM changes from linear to nonlinear and then to damage due to cracking as strains increase. When the SAM response is within the linear viscoelastic regime, the modulus |G*| will remain constant to follow the homogeneity concept of linearity. Based on the results a strain amplitude of 0.0001% was selected for performing the frequency sweep testing within the linear regime. After completing the frequency sweep testing, dynamic shear modulus |G*| master curves were then formed using the frequency-temperature superposition principle. Fig. 6 shows the master curves for all the samples tested (i.e., three 2-mm thick binders and three SAMs at different aging conditions: unaged, RTFOT short-term aged, and PAV long-term aged).
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Fig. 4. LSM images of SAM prepared with (a) 6%; (b) 8%; (c) 10% binder content.
Fig. 5. SAM test result: (a) strain sweep LAS*; (b) |G*| response to the LAS* at different temperature.
Obviously, aging increased the stiffness in both cases, but the behavior between the two looks somewhat different. SAM showed an asymptotic converging of stiffness at high and low loading frequencies, while parallel plate binder testing did not show such behavior at low frequencies. In addition, the slopes of stiffness over loading frequencies between the two groups are different. The slope of SAM was lower than that of the binder samples, which must be related to the contribution of sand particles in the sample. Sand particles enable the binder film to be very thin and they
induce friction and contact within the mixture, which is a typical phenomenon observed in AC mixtures. 4.2. Fatigue test (time sweep and amplitude sweep) To characterize the fatigue damage of binders in different testing schemes, two types of cyclic fatigue tests—the amplitude sweep test and the time sweep test—were conducted in this study. The test matrix is given in Table 2. It was intended to incorporate
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Fig. 6. Frequency sweep test results.
Table 2 Testing Matrix for Fatigue Characterization of Binder and SAM. Binder (2-mm Parallel Plate) Aging
Time Sweep (Controlled Strain) at 25 °C
Time Sweep (Controlled Stress) at 25 °C
Strain Sweep at three temperatures
Unaged RTFOT-aged PAV-aged
3%, 4%, 5% 4%, 5%, 6% 5%, 6%, 7%
50 kPa 75 kPa N/A
LAS (25, 20, 15 °C) LAS (25, 20, 15 °C) LAS (25, 20, 15 °C)
study. It has been applied to both the LAS test [25] and binder time sweep test [22] and was found to be a reasonable indicator of fatigue failure of binders based on the observation that C N reached its peak value at the same time when the material yielded during the LAS test. Typical results from the strain-controlled binder time sweep test (parallel plate geometry) are shown in Fig. 7. The results are from binders with different levels of aging tested at the same loading amplitude (5% strain) and testing temperature (25 °C). It can be noted that C N did not clearly show a peak value, but instead presented a transition in the slope of the C N curve when the binder was not aged or was short-term aged. Although further investigation and testing would be required to reach more definite conclusions, it is anticipated that the gradual increase in C N observed from the unaged and short-term aged binders might be due to the testing temperature of 25 °C). At this temperature, flow behavior is significant and more dominant than cracking, and the presence of polymer in the binder might resist the faster rate of damage accumulation. When the binder time sweep was conducted under the stresscontrolled mode, a different trend in |G*| and phase angle over the increased loading cycles was observed compared to the strain-controlled binder time sweep testing. In the case of the strain-controlled mode, the stress within the sample reduced as damage accumulated, implying that the sample takes less effort to deform to a particular state once the sample has experienced
SAM (50-mm tall and 12.5-mm diameter cylinder) Aging
Time Sweep (Controlled Strain) at 25 °C
Strain Sweep at 25 °C
Unaged RTFOT-aged PAV-aged
0.20%, 0.25%, 0.35% 0.25%, 0.35%, 0.45% 0.20%, 0.25%, 0.30%
LAS* (25 °C) LAS* (25 °C) LAS* (25 °C)
the effects of temperature, loading rate (i.e., frequency), loading amplitude (either stress or strain), and the influence of aging on the fatigue resistance of binders. The linear amplitude sweep (LAS) is the standardized amplitude sweep procedure (AASHTO TP 101 [41]), which is currently the most commonly used for characterizing binder fatigue. The LAS* is another form of the strain sweep test where the only difference compared to LAS is that amplitude sweep process is conducted in the logarithmic mode as shown in Fig. 5(a). The LAS* was employed for SAM specimens because the rate of increase in strains in the typical LAS protocol was not quite feasible to apply to the SAM due to the very small strains applied to the SAM. The time sweep tests were conducted with varying loading amplitudes in either strain-controlled or stress-controlled mode. The time sweep test is a conventional fatigue test performed by applying cyclic loading at constant amplitude in the DSR. It is considered somewhat impractical because it requires much time to finish and yields a relatively high variability in testing results; however, it can mimic actual fatigue loading and damage induced. To compare fatigue damage characteristics and fatigue failure (observed from different testing schemes and binders with different levels of aging), a simple indicator to represent the fatigue failure was sought. As reported in many studies [22,33,37,42–44], several fatigue failure criterion have been proposed to define a robust failure definition in binder fatigue tests. From among those, a peak in C N values, where C is the normalized dynamic modulus and N is the number of loading cycles, was selected in this
(a) C vs. N and C*N vs. N
(b) δ vs. N, and C*N vs. N Fig. 7. Strain-controlled binder time sweep test results.
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significant damage. However, in the stress-controlled mode, the sample was subjected to a constant level of stress, meaning that to maintain the same level of stress the sample must undergo larger deformation once there is significant damage. This is clearly observed in Fig. 8. There was a phase of significant decrease in |G*| values and an increase in phase angle. Once the transition point (or the peak in C N curve) occurred, there was a faster rate of drop in the |G*| value and a corresponding increase in phase angle; this indicates increased loss in structural integrity and significant deformation within the binder sample. This trend was observed in all levels of aging (i.e., unaged, RTFO, and PAV) under the stress-controlled time sweep tests. Fig. 9 shows binder LAS test results at 25 °C. In contrast to the binder time sweep test in the strain-controlled mode, the peak of C N appeared clearly. Based on the observed peaks in C N curves, fatigue lives of binders resulting from the LAS tests at three different testing temperatures were determined. An example set of data from the time sweep test conducted on a SAM specimen is shown in Fig. 10. The data is from an unaged SAM specimen subjected to a strain amplitude of 0.25% at 25 °C. During the initial stage of the test there was a rapid decrease in |G*| and an increase in phase angle. After the initial stage, one can notice in the second stage that a linear increase in phase angle and a decrease in |G*| exist. Kim et al. [37] reported that, from the observation with the X-ray computed tomography imaging, the second stage under fatigue is related to microcracking in the sample. This phase was succeeded by a transition point, beyond which there was an increased rate of damage accumulation, which is associated with macrocracking due to fatigue [37]. The macrocracking phase induced the faster rate of decrease in |G*| and a simultaneously increased rate of phase angle. At the end of the fatigue test the
(a) C vs. N and C*N vs. N
(b) δ vs. N, and C*N vs. N Fig. 8. Stress-controlled binder time sweep test results.
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(a) C vs. N and C*N vs. N
(b) δ vs. N, and C*N vs. N Fig. 9. Binder LAS test results at 25 °C.
Fig. 10. An example result of SAM time sweep testing.
sample broke apart completely, which can be witnessed by the sudden drop in the phase angle. Unlike the parallel plate binder time sweep testing under the strain-controlled mode, there was no ambiguity in defining fatigue failure from the strain-controlled SAM time sweep testing. A similar trend was also observed in RTFO- and PAV-aged SAM specimens, as shown in Fig. 11, which compares test results of binders at different aging conditions at the same loading level (0.25% of strain) and testing temperature (25 °C). It was observed from the comparison that the rate of stiffness reduction and rate of increase in phase angle increased with more aging. Aging clearly contributed to the brittleness of the binder and faster damage accumulation due to microcracking and macrocracking; this sensitive mechanical behavior was well captured and represented by the SAM time sweep fatigue testing. This use of sand particles, which act as an efficient load-carrying media between thin binder films,
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(a) C vs. N and C*N vs. N
(b) δ vs. N and C*N vs. N Fig. 11. SAM time sweep test results at 25 °C.
is clearly an advantage in mimicking the actual condition of binder in AC mixtures. Fig. 12 shows typical results from the amplitude sweep of SAM specimens. The testing was conducted for SAM specimens of all three levels of binder aging at a temperature of 25 °C. In all cases, it was observed that the |G*| modulus is fairly constant, showing a linear response during the initial state, accompanied by a constant phase angle. As the loading cycles increase, the SAM specimen is subjected to strains from linear to nonlinear, and then to damage. This can be observed in the steep drop in modulus and C N. Fatigue failure was determined by the number of loading cycles corresponding to the peak of the C N curve.
(a) C vs. N and C*N vs. N
Fig. 13 shows comparisons of fatigue lives between binders (unaged, short-term aged, and long-term aged) from two different testing configurations (parallel plate of binder film and cylindrical SAM). In comparison to the binder time sweep tests which rank the PAV-aged binder to be more resistant to fatigue cracking, the SAM time sweep tests ranked the samples in a more realistic way. It showed that aging will have a detrimental effect in terms of fatigue resistance. A similar ranking was observed when tested in the amplitude sweep protocol, indicating that SAM testing was independent of the loading mode. In Fig. 13(c), it can also be observed that, from the binder LAS testing, fatigue lives reduced at lower testing temperatures (which is expected); however, the binder LAS testing ranked the PAV-aged binder more resistant to fatigue cracking than unaged and short-term aged binders. This trend is contradictory to the typical belief-observation that the pavement is more susceptible to fatigue cracking as the binder becomes aged. The same trend was also reported in other studies [31,45]. On the other hand, the amplitude sweep testing of SAM specimens presented expected results. PAV-aged samples failed earlier than the unaged and short-term aged samples, and this fatigue ranking order agrees well with the results from the time sweep testing. The SAM testing induced damage in asphalt binder that was sensitive to the level of aging, which was not clearly captured by the binder DSR testing with the 2-mm parallel plate geometry. As demonstrated and discussed in previous sections, the laboratory procedure and testing of SAM specimens for binder fatigue characterization looks promising as an alternative. Due to the use of standard Ottawa sands as a load carrier between micrometerthin binder films, SAM testing could successfully evaluate viscoelastic properties and crack-induced fatigue damage of binders in a more realistic manner than conventional mm-scale parallel plate binder tests. In an attempt to further evaluate the validity of the SAM fabrication-testing as a potential specification-type method, repeatability of the SAM fatigue testing in both time sweep mode and amplitude sweep mode was examined. An unaged binder was used to fabricate 5–6 replicates of SAM specimens for each case (i.e., one amplitude sweep test and three time sweep tests at different strain levels) at an intermediate temperature (25 °C). The unaged SAM specimens were chosen for this effort since it is the most compliant compared to the RTFOT and PAV aged cases. Fig. 14 shows representative data of each fatigue testing and a summary of repeatability from all cases in a form of bar chart where mean values, standard deviations (error bars), and coefficients of variation (COV) are plotted together. As presented, amplitude sweep testing of SAM was quite repeatable with a very low value of COV (i.e., 3.5%). Although time sweep testing (fatigue testing) was less repeatable than the amplitude sweep testing, which
(b) δ vs. N, and C*N vs. N
Fig. 12. SAM sample amplitude sweep test results at 25 °C.
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(a) time sweep tests of binder at 25ºC
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(b) time sweep tests of SAM at 25 ºC
(d) LAS* tests of SAM at 25 ºC
(c) LAS tests of binders
Fig. 13. Fatigue lives from each testing with different binders.
was expected and well-known, its repeatability level seems still quite satisfactory (i.e., less than 30% overall), considering the typical COV levels of fatigue testing of materials [46]. 5. Summary and conclusions In this study, multiple fatigue-related tests (i.e., time sweep and amplitude sweep) were conducted on both 2-mm thick parallel plate binder specimens and cylindrical SAM specimens by varying the level of binder aging. Test results between the two testing configurations were compared to investigate the effects of binder length scale and aging on fatigue cracking behavior. In addition, the original SAM fabrication-testing method was revisited and revised to examine its testing efficiency and repeatability. Based on the test-analysis results, the following conclusions can be drawn: The laboratory procedure to fabricate SAM specimens was revised from its initial state of development. Testing repeatability supports the validity of the method. Because of the use of standard Ottawa sands as a load carrier between micrometerthin binder films, SAM testing is considered a useful method to evaluate fatigue-related damage characteristics of binders in a more realistic manner than conventional mm-scale parallel plate binder DSR tests at room temperatures. In the binder fatigue tests, especially in the strain-controlled mode time sweep tests for unaged and RTFO-aged cases, there
was no clear distinction for evaluating fatigue failure point. However, in the case of SAM testing, a clear indication of fatigue failure was seen irrespective of the loading mode. The binder time sweep tests erroneously ranked the PAV-aged binder to be more resistant to fatigue cracking, while the SAM time sweep tests ranked the samples in a more realistic way by showing that aging will have a detrimental effect in terms of fatigue resistance. It was observed from the fatigue testing results that SAM testing could capture the microcracking and macrocracking phases more distinctively than 2-mm thick parallel plate binder fatigue testing. This indicated that the SAM is more sensitive in capturing fatigue cracking, particularly with the influence of aging, than binder DSR testing. Test results and findings imply that the SAM can be considered a proper solid phase to evaluate the properties and damage characteristics of binders and mastics under various conditions. A follow-up study to investigate other factors, such as the influence of SAM specimen geometry and binder content for better representation of binder characteristics in addition to fatigue damage criterion such as the energy-based failure criterion is necessary to further improve the SAM testing and relevant analysis. In addition, more binders with different additives (e.g., polymer modifiers, softening agents, fillers, fibers, etc.) need to be included in the testing program to draw a more definite conclusions. The authors are currently working on it, and new findings will be presented elsewhere.
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(b) TS repeatability (0.6% strain amplitude)
(a) LAS* repeatability
(c) Fig. 14. LAS* and times sweep repeatability for unaged SAM samples.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors thank Ms Mahdieh Khedmati for helping the laser scanning microscopy testing to image film thicknesses of SAM specimens. The third author is grateful for the support received from the Fulbright Visiting Scholar Program to participate in this research. References [1] F. Moreno, M. Rubio, Effect of aggregate nature on the fatigue-cracking behavior of asphalt mixes, Mater. Des. 47 (2013) 61–67. [2] J. Sousa, J. Pais, M. Prates, R. Barros, P. Langlois, A.-M. Leclerc, Effect of aggregate gradation on fatigue life of asphalt concrete mixes, Transp. Res. Rec.: J. Transp. Res. Board 1630 (1998) 62–68. [3] F.M. Nejad, E. Aflaki, M. Mohammadi, Fatigue behavior of SMA and HMA mixtures, Constr. Build. Mater. 24 (7) (2010) 1158–1165. [4] J. Harvey, B.-W. Tsai, Effects of asphalt content and air void content on mix fatigue and stiffness, Transp. Res. Rec.: J. Transp. Res. Board 1543 (1996) 38– 45. [5] J.M. Matthews, C.L. Monismith, J. Craus, Investigation of laboratory fatigue testing procedures for asphalt aggregate mixtures, J. Transp. Eng. 119 (4) (1993) 634–654. [6] H.-J. Lee, Y.R. Kim, Viscoelastic continuum damage model of asphalt concrete with healing, J. Eng. Mech. 124 (11) (1998) 1224–1232.
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S.R. Kommidi et al. / Construction and Building Materials 237 (2020) 117588 [20] C. Hintz, R. Velasquez, C. Johnson, H. Bahia, Modification and validation of linear amplitude sweep test for binder fatigue specification, Transp. Res. Rec.: J. Transp. Res. Board 2207 (2011) 99–106. [21] C. Wang, C. Castorena, J. Zhang, Y. Richard Kim, Unified failure criterion for asphalt binder under cyclic fatigue loading, Road Mater. Pavement Des. 16 (suppl. 2) (2015) 125–148. [22] C. Wang, H. Zhang, C. Castorena, J. Zhang, Y.R. Kim, Identifying fatigue failure in asphalt binder time sweep tests, Constr. Build. Mater. 121 (2016) 535–546. [23] F. Safaei, C. Castorena, Y.R. Kim, Linking asphalt binder fatigue to asphalt mixture fatigue performance using viscoelastic continuum damage modeling, Mech. Time Depend. Mater. 20 (3) (2016) 299–323. [24] F. Safaei, C. Castorena, Improved interpretation of asphalt binder parallel plate dynamic shear rheometer fatigue tests, Int. J. Pavement Eng. 1–14 (2018). [25] W. Cao, C. Wang, A new comprehensive analysis framework for fatigue characterization of asphalt binder using the Linear Amplitude Sweep test, Constr. Build. Mater. 171 (2018) 1–12. [26] C. Wang, W. Xie, Y. Chen, A. Diab, Z. You, Refining the Calculation Method for Fatigue Failure Criterion of Asphalt Binder from Linear Amplitude Sweep Test, J. Mater. Civ. Eng. 30 (2) (2017) 04017286. [27] M. Sabouri, Y.R. Kim, Development of a failure criterion for asphalt mixtures under different modes of fatigue loading, Transp. Res. Rec. 2447 (1) (2014) 117–125. [28] B.S. Underwood, Y.R. Kim, M.N. Guddati, Improved calculation method of damage parameter in viscoelastic continuum damage model, Int. J. Pavement Eng. 11 (6) (2010) 459–476. [29] C. Wang, C. Castrorena, J. Zhang, Y.R. Kim, Application of time-temperature superposition principle on fatigue failure analysis of asphalt binder, J. Mater. Civ. Eng. 29 (1) (2016) 04016194. [30] M.A. Elseifi, I.L. Al-Qadi, S.-H. Yang, S.H. Carpenter, Validity of asphalt binder film thickness concept in hot-mix asphalt, Transp. Res. Rec. 2057 (1) (2008) 37–45. [31] C. Hintz, R. Velasquez, Z. Li, H. Bahia, Effect of oxidative aging on binder fatigue performance, J. Assoc. Asph. Paving Technol. 80 (2011). [32] A. Abbas, B.C. Choi, E. Masad, T. Papagiannakis, The influence of laboratory aging method on the rheological properties of asphalt binders, J. Test. Eval. 30 (2) (2002) 171–176.
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[33] Y. Kim, D.N. Little, R.L. Lytton, Use of dynamic mechanical analysis (DMA) to evaluate the fatigue and healing potential of asphalt binders in sand asphalt mixtures, J. Assoc. Asphalt Paving Technol. 71 (2002) 176–206. [34] Y. Kim, D. Little, R.L. Lytton, Effect of moisture damage on material properties and fatigue resistance of asphalt mixtures, Transp. Res. Rec.: J. Transp. Res. Board 2004 (1891) 48–54. [35] Y. Kim, D.N. Little, I. Song, Mechanistic evaluation of mineral fillers on fatigue resistance and fundamental material characteristics, Transp. Res. Board (2003). [36] Y. Kim, D.N. Little, R.L. Lytton, Fatigue and healing characterization of asphalt mixtures, J. Mater. Civ. Eng. 15 (1) (2003) 75–83. [37] Y. Kim, H. Lee, D.N. Little, Y.R. Kim, A simple testing method to evaluate fatigue fracture and damage performance of asphalt mixtures, J. Assoc. Asphalt Paving Technol. 75 (2006) 755–787. [38] W. Martono, H.U. Bahia, J. Dangelo, Effect of testing geometry on measuring fatigue of asphalt binders and mastics, J. Mater. Civ. Eng. 19 (9) (2007) 746– 752. [39] B. Radovskiy, Analytical formulas for film thickness in compacted asphalt mixture, Transp. Res. Rec. 1829 (1) (2003) 26–32. [40] M. Khedmati, Y. Kim, J.A. Turner, H. Alanazi, C. Nguyen, An integrated microstructural-nanomechanical-chemical approach to examine materialspecific characteristics of cementitious interphase regions, Mater. Charact. 138 (2018) 154–164. [41] AASHTO TP101, Standard method of test for estimating fatigue resistance of asphalt binders using the linear amplitude sweep, Washington, 2012. [42] K.A. Ghuzlan, S.H. Carpenter, Energy-derived, damage-based failure criterion for fatigue testing, Transp. Res. Rec. 1723 (1) (2000) 141–149. [43] S. Shen, S.H. Carpenter, Application of the dissipated energy concept in fatigue endurance limit testing, Transp. Res. Rec. 1929 (1) (2005) 165–173. [44] S.H. Carpenter, S. Shen, Dissipated energy approach to study hot-mix asphalt healing in fatigue, Transp. Res. Rec. 1970 (1) (2006) 178–185. [45] F. Zhou, P. Karki, S. Im, Development of a Simple Fatigue Cracking Test for Asphalt Binders, Transp. Res. Rec.: J. Transp. Res. Board 2632 (2017) 79–87. [46] E. Masad, V. Castelo Branco, D.N. Little, R. Lytton, A unified method for the analysis of controlled-strain and controlled-stress fatigue testing, Int. J. Pavement Eng. 9 (4) (2008) 233–246.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Fatigue characterization of binder with aging in two length scales: sand asphalt mortar and parallel plate binder film Santosh Reddy Kommidi a, Yong-Rak Kim a,⇑, Lilian Ribeiro de Rezende b a b
Department of Civil Engineering, University of Nebraska-Lincoln, Lincoln, NE, USA School of Civil and Environmental Engineering, Federal University of Goias, Goiania, Goias, Brazil
h i g h l i g h t s Fatigue characterization of binder with aging was compared at two length scales. A realistic binder film thickness was attempted using Sand Asphalt Mortar (SAM). SAM fatigue tests can sensitively capture the influence of aging on binder fatigue. SAM fatigue tests can distintively capture progressive microcracking and macrocracking phases.
a r t i c l e
i n f o
Article history: Received 21 March 2019 Received in revised form 27 October 2019 Accepted 11 November 2019
Keywords: Sand asphalt mortar Binder Fatigue Aging Linear amplitude sweep Time sweep
a b s t r a c t Fatigue-cracking resistance of asphaltic materials and flexible pavements is strongly related to the properties of the binder under aging and its interaction with aggregate particles, which are small-scale physical-mechanical characteristics of components. Currently, there are binder test methods including the AASHTO TP 101 that are widely used to evaluate fatigue characteristics of asphalt binder. Despite the scientific leap for predicting and characterizing binder fatigue behavior in pavements, there are opportunities to better mimic the true state of the binder subjected to fatigue loading in pavements. This study attempted fatigue damage characterization using sand asphalt mortar (SAM) specimens, which are anticipated to better represent the realistic geometry (such as micrometer thick film) of binder in mixtures than typical binder fatigue tests that use 2-mm thick parallel plate specimens, while testing repeatability-efficiency can still be met due to the use of standard Ottawa sands as a load carrier between binder films. Multiple fatigue-related tests (i.e., time sweep and amplitude sweep) were conducted on both 2-mm thick binder specimens and SAM specimens by varying the level of aging to investigate the effects of binder length scale and aging on fatigue cracking behavior. The laboratory procedure to fabricate SAM specimens was presented with the testing repeatability, which supports the validity of the method. It was observed from the fatigue testing results that SAM testing could capture the microcracking and macrocracking phases more distinctively and sensitively than conventional binder fatigue testing, particularly with the influence of aging. These test results and findings imply that the SAM can be considered a proper solid phase to evaluate the properties and damage characteristics of binders and mastics under various conditions. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Fatigue cracking is one of the most commonly observed distresses in asphaltic pavements. This form of cracking usually appears along the wheel path on the pavement surface, and it is mostly due to the repeated passing of heavy axle loads. The sever-
⇑ Corresponding author. E-mail addresses: [email protected] (S.R. Kommidi), yong-rak. [email protected] (Y.-R. Kim), [email protected] (L.R. de Rezende). https://doi.org/10.1016/j.conbuildmat.2019.117588 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
ity of the fatigue cracking in pavements increases as the binder ages. Several factors, such as aggregate gradation, air voids percentage and distribution, binder-aggregate interactions, variation of binder film thickness across components in asphalt concrete (AC) mixture, binder aging, etc., affect the overall AC fatigue behavior [1–4]. However, taking all these factors into consideration to understand the fatigue resistance of AC mixtures and AC pavements can be quite complex. Alternatively, researchers have conducted cyclic fatigue tests on cylindrical or beam AC samples [5–8]. More commonly, federal and state agencies usually rely on
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the results of binder tests to select appropriate fatigue damage resistant material. This is based on the widely-accepted assumption that the binder phase is critically responsible for the integrity of mixtures in pavements; it is also perhaps the weakest component that is prone to fatigue crack initiation. Traditionally, the binder characteristic that is most frequently used to evaluate fatigue was the |G*|sin(d) of the Superpave performance grade (PG) system which provides a value limit of 5,000 kPa after the laboratory long-term aging. However, several researchers have shown that the |G*|sin(d) parameter is not well correlated with field fatigue evaluation [9–14]. Bahia et al. [15] suggested that the strain level used for the PG system was relatively low when compared to the actual strain level experienced by the binder within AC mixtures. They suggested the evaluation of higher nonlinear strain levels to better assess fatigue phenomena in AC mixtures. Later, Masad and Somadevan [16] investigated the influence of localized strain distribution on the mechanical response of AC and concluded that the strain levels in the mastic and/or binder phase were quite high and most likely to be in the nonlinear region. To include the effects of high strain levels in the binder analysis, Bahia et al. [15] suggested the use of time sweep tests. The time sweep testing can simulate a more realistic fatigue process. Anderson et al. [17] conducted the time sweep tests and claimed that the soft behavior (|G*| values less than 15 MPa) of binder results in edge effects and instable flow when testing with parallel plate geometry at intermediate temperatures. They suggested that the fatigue characterization obtained by the binder time sweep tests at intermediate temperatures (e.g., 20 °C to 25 °C) would not represent true fatigue behavior. They concluded that fatigue characterization should be performed at relatively low temperatures where the binder stiffness would be large enough to reduce edge effects and to observe fatigue-associated cracking (such as micro-cracking) phenomena. Johnson et al. [18,19] proposed a much faster fatigue cracking evaluation method based on a monotonic loading procedure using the parallel plate geometry. They proposed the use of a viscoelastic continuum damage (VECD) approach to evaluate the fatigue resistance of the binders at different loadings. Hintz et al. [20] further developed the methodology to a simple strain sweep test along with a frequency sweep test, concluding that the two tests would be enough to characterize and analyze the fatigue behavior of binders using VECD theory. Later, several modifications were made to the approach by incorporating the energy-based failure criterion. Wang et al. [21,22] used maximum stored pseudo strain energy (PSE) for defining failure in linear amplitude sweep (LAS) and time sweep tests using a simplified-VECD theory for analyzing the results. A unified fatigue failure criterion was proposed by Wang et al. [21] and Safaei et al. [23] using PSE-based failure criterion to incorporate the effects of temperature, loading mode, and history. More recently, several other studies have improved understanding through experimental characterization of fatigue damage in asphaltic media [24–29]. As aforementioned, several different approaches including the AASHTO TP 101 Standard Method of Test for Estimating Fatigue Resistance of Asphalt Binders Using the Linear Amplitude Sweep have brought various advancements from the Superpave |G*|sin(d) criterion for better predicting and characterizing binder fatigue behavior in pavements. However, most of them are based on testing of binder film of usually around 2 mm, which is clearly very different from actual geometry (considered to be around 2–100 mm) [30] of binder or more specifically mastic film in AC mixtures. Furthermore, the influence of aging on binder fatigue behavior was not well addressed as a predictor of longer fatigue lives for long-term aged binders at low strain levels [31,32]. Considering the highly nonlinear and age-sensitive viscoelastic (rate-, time-, and temperature-dependent) behavior and
cracking of binders, binder fatigue testing should better mimic the true state of the binder placed in AC mixtures. Kim et al. [33–36] attempted such testing of micrometer thick film in binder samples by incorporating fatigue tests on sand asphalt mortar (SAM). They proposed a new sample fabrication methodology, where uniformly graded Ottawa sand particles were mixed with a properly estimated amount of binder to simulate arbitrary (but considered reasonable) binder film thickness. Based on their proposed test geometry (50 mm tall and 12.5 mm in diameter cylindrical rod sample) for fatigue testing using the dynamic mechanical analyzer, the fabricated SAM specimen can properly represent a micrometer scale binder film (based on the dosage of binder) with sand acting as a load bearing skeletal system. Many researchers have tried to include the effect of particles by analyzing the intermediate scale using fine aggregate matrix (FAM). However, the main advantage of the proposed SAM approach is the possibility of replicating this test in any laboratory and maintaining the same particle distribution, which in the case of FAM, it is not feasible, since each research study considers its own particles and mixing recipe. A series of previous studies showed that the SAM can successfully and sensitively evaluate material-specific fatigue damage characteristics of various binders, as demonstrated in Fig. 1(a). Also, as illustrated in Fig. 1(b), the time sweep fatigue testing of SAM samples can provide a reasonable fatigue failure point, or ‘‘transition point,” that is well matched to the peak phase angle in a strain-controlled mode [37]. Similar tests in a stresscontrolled mode were performed by Martono et al. [38]. Their analysis was based on dissipated energy ratio (DER), which indicated that the SAM samples provided a good repeatability and were free of edge effects. 2. Research objective and scope The objective of this study is to assess the fatigue characterization of asphalt binders in two different configurations—2-mm parallel plate binder film and SAM—at different levels of aging conditions. The two different configurations consequentially represent the two different length scales of the binder (i.e., 2-mm for the parallel plate binder film and 2–100 mm for the SAM). The micrometer length scale of binder in SAM would depend on the dosage of binder selected in fabricating the SAM specimens, details of which are described in Sections 3.2 and 3.3. In addition, the SAM testing method was intended to be improved from its initial development [33–37] so that testing repeatability and efficiency can be upgraded. The two different binder length scales considered in this study are expected to provide the core aspects of binder geometry on fatigue damage and binder cracking performance, particularly the influence of aging. To that end, multiple fatigue tests, including time sweep and strain sweep tests, were performed on both configurations. Mechanical responses with damage and subsequent fatigue failure were observed, and test-analysis results between the two length scales were compared. 3. Materials and sample preparation 3.1. Selection of materials A polymer modified binder (PG 64-28) was selected for this study. The binder was aged in the laboratory according to ASTM D2872 and ASTM D6521 for simulating short-term and long-term aging, respectively. The short-term aging of binder was simulated using the Rolling Thin Film Oven Test (RTFOT) where a thin film of binder is heated in the oven for 85 min at 163 °C under a continuous air flow of 4000 ml/min. The long-term aging of binder was simulated using the Pressurized Aging Vessel (PAV) experiment, in which 50 g of RTFOT-aged binder is poured into a pan and conditioned in the PAV vessel for 20 h at 2.1 MPa pressure. For preparing
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Fig. 1. SAM test results [37]: (a) strain-fatigue life of various binders; (b) a typical time sweep test result that presents progressive fatigue damage accumulation and failure.
SAM specimens, an Ottawa sand was selected. The material characteristics met the specifications of ASTM C778. The use of a standard sand (Ottawa sand in this study) for SAM fabrication is very advantageous, since it can be easily replicated in other laboratories internationally. Regarding the aging, it should be noted that only the binder was aged, and the aged binder was used to prepare SAM specimens by mixing the aged binder with the ASTM standard sand. 3.2. SAM specimen fabrication In the SAM sample preparation, a binder content of 4% by weight of total SAM mixture. It was estimated that, for a binder dosage of 4% by weight, the film thickness obtained is approximately around 5 mm using the analytical method given by Radovskiy [39]. A detailed description on determination and estimation of film thickness is described in Section 3.3. The film thickness was estimated by assuming that all the sand particles were spherically shaped and had a constant density of 2.65 g/cm3. Based on the sieve size distribution (ASTM C778) the number of particles retained in each sieve was obtained using the diameter of the particles in each respective sieve as the average of the current and the previous sieve size. As shown in Figs. 2 and 3, the SAM specimens were prepared in two stages. In the first stage (called the ‘‘mixing stage,” shown in Fig. 2) the sand particles and binder were heated separately at 135 °C for 30 min, and then thoroughly mixed at the same temperature until all sand particles were coated well with binder. The loose SAM mixture was poured into small containers, stored at 5 °C, and capped to protect it from any unnecessary aging until it was reheated to compact individual SAM testing specimens.
In a later stage (called the ‘‘compaction stage”), the loose SAM mixture was compacted to cylindrical specimens in a metallic mold. As illustrated in Fig. 3, the mold has two halves that are assembled together. Eleven grams of loose SAM and the mold were placed in the oven at 135 °C for 20 min. Based on several trial and error processes and observations by Kim et al. [33–37], the usage of 11 g of SAM mixture for compaction was chosen to make a compacted cylinder (50 mm height and 12.5 mm diameter) in a unit weight of 2.155 g/cm3, which appeared appropriate to be tested in a solid bar state and subjected to cracking due to cyclic loading. The authors are currently investigating in detail into the correct geometry size and dosage that would appropriate fatigue testing of solid bar samples. Resulting outcomes will be presented as an extension of the current work. As detailed in Fig. 3, after heating the loose SAM mixture and the mold, the SAM mixture was immediately poured into the mold and compacted using the compaction device. The compaction device allows one to apply a constant load (up to 200 lbs. force) using a side lever on the top face of the cylinder specimen. After several trial and error processes, the compaction of SAM specimens was achieved through a series of three loading rounds. In the first round, a load of 60 lbs. of force was applied for 15 s; the mold was then flipped over, and the same load was applied for another 15 s. The same procedure was followed for the second and third rounds, but the load was increased to 80 lbs. and 100 lbs. of force, respectively. This would enable the SAM specimen to be compacted evenly from both ends, which is targeted to increase testing repeatability. After the compaction process was completed, another 15 min were allowed for cooling down the mix and the mold. Then, the two mold halves were separated to take the compacted SAM specimen from the mold. The cylindrical SAM geometry can make data analysis easy by preventing complex
Fig. 2. SAM mixing process.
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Fig. 3. SAM fabrication (in particular compaction) process.
stress distributions under a torsional testing mode [33–37]. For torsional testing in the dynamic shear rheometer (DSR), two fixtures were secured to both ends of each SAM specimen using epoxy glue. The gluing was done carefully, so as not to cause any undesirable stress concentrations at either end. Moreover, during the gluing process, it was determined that the fixtures needed to be inline. To this end, a small tool was designed and fabricated to make sure the two fixtures lined up. Fig. 3 illustrates the SAM specimens with the fixtures glued, as well as the tools used to compact the specimens. The entire fabrication process (mixing, compaction, gluing, and installation) of SAM specimens was improved from its initial development [33–37].
3.3. Binder film thickness estimation in SAM Since SAM samples are used to indirectly represent a more realistic binder film thickness for analyzing the binder fatigue properties it is important to estimate the binder thickness that is being generated for a given dosage of binder when preparing the SAM samples. Two methods were incorporated for estimating the SAM binder film thickness, one using analytical formulas presented by Radovskiy [39] and the second method using image analysis of the cross-section of SAM specimens obtained using a laser scanning microscopy (LSM). LSM is a type of microscope that scans the surface of the specimen using a focused laser beam and produces considerably higher resolution images compared to conventional optical microscopes [40]. To conduct the LSM investigation, two cylindrical SAM specimens of 50 mm with different binder dosages (i.e., 6, 8, and 10%) were prepared, and smaller cylindrical pieces of 15 mm thick were carefully cut using a low speed digital diamond saw. The SAM surface was then lightly polished avoiding heat and material loss by using diamond lapping films. Then, three images were captured from each specimen. In each image, the distance between two particles filled with binder was measured at ten different locations. The film thickness was considered as a half of the distance between the two particles. As the compaction energy was fixed for all SAM specimens, the impact on film thickness variation is related only with the asphalt binder content used in SAM preparation. Table 1 presents the film thickness calculated using the analytical formulas, where the density of sand and binder was 2.650 g/cm3 and 1.000 g/cm3, respectively. Fig. 4 shows examples of images obtained from the LMS at each dosage. Table 1 also presents the range of ten measurements in each image. Both methods show that with the increase of binder content, the film thickness also
Table 1 SAM Film Thickness Estimated. Asphalt binder content (%)
Film thickness (lm) estimated Calculated using analytical formulas
Range observed by LMS images
4 6 8 10
5 9.0 12.0 15.0
N/A 10.5–28.0 22.5–70.5 43.5–97.5
increases; however, the values estimated between the two methods presented difference and generally lower than the mastic film thickness (100 lm) surrounding large aggregates reported by Elseifi et al. [30].
4. Laboratory tests and results 4.1. Frequency sweep test To characterize linear viscoelastic properties of binders in each testing scheme (i.e., 2-mm parallel plate of binder film and cylindrical SAM specimens), strain sweep tests were first performed prior to the frequency sweep test to obtain the linear limits of the samples. For binders, the strain sweep test was performed at 0 °C with a frequency of 10 Hz and at 45 °C with 0.1 Hz loading frequency. The two temperature-frequency combinations (i.e., 0 °C 10 Hz and 45 °C 0.1 Hz) represent either stiffest or most compliant testing conditions experienced by the following frequency sweep testing where loading frequencies vary from 0.1 Hz to 10 Hz at different testing temperatures. Based on the strain sweep test results, a strain amplitude of 0.01% was finally selected for binders as a level of strain that is sufficiently small to perform the frequency sweep tests for identifying the linear viscoelastic properties. Similarly, for SAM specimens, the strain sweep (LAS*) test was performed at two temperature-frequency combinations: 10 °C-10 Hz and 45 °C-0.1 Hz, since the following frequency sweep testing was conducted by varying frequencies from 0.1 Hz to 10 Hz at a range of testing temperatures from 10 °C to 45 °C. Fig. 5(a) shows the strain applied to the SAM samples during the LAS* experiment, and Fig. 5(b) shows the |G*| changes due to strain sweeping at different testing temperatures. As observed in Fig. 5 (b), the response of the SAM changes from linear to nonlinear and then to damage due to cracking as strains increase. When the SAM response is within the linear viscoelastic regime, the modulus |G*| will remain constant to follow the homogeneity concept of linearity. Based on the results a strain amplitude of 0.0001% was selected for performing the frequency sweep testing within the linear regime. After completing the frequency sweep testing, dynamic shear modulus |G*| master curves were then formed using the frequency-temperature superposition principle. Fig. 6 shows the master curves for all the samples tested (i.e., three 2-mm thick binders and three SAMs at different aging conditions: unaged, RTFOT short-term aged, and PAV long-term aged).
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Fig. 4. LSM images of SAM prepared with (a) 6%; (b) 8%; (c) 10% binder content.
Fig. 5. SAM test result: (a) strain sweep LAS*; (b) |G*| response to the LAS* at different temperature.
Obviously, aging increased the stiffness in both cases, but the behavior between the two looks somewhat different. SAM showed an asymptotic converging of stiffness at high and low loading frequencies, while parallel plate binder testing did not show such behavior at low frequencies. In addition, the slopes of stiffness over loading frequencies between the two groups are different. The slope of SAM was lower than that of the binder samples, which must be related to the contribution of sand particles in the sample. Sand particles enable the binder film to be very thin and they
induce friction and contact within the mixture, which is a typical phenomenon observed in AC mixtures. 4.2. Fatigue test (time sweep and amplitude sweep) To characterize the fatigue damage of binders in different testing schemes, two types of cyclic fatigue tests—the amplitude sweep test and the time sweep test—were conducted in this study. The test matrix is given in Table 2. It was intended to incorporate
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Fig. 6. Frequency sweep test results.
Table 2 Testing Matrix for Fatigue Characterization of Binder and SAM. Binder (2-mm Parallel Plate) Aging
Time Sweep (Controlled Strain) at 25 °C
Time Sweep (Controlled Stress) at 25 °C
Strain Sweep at three temperatures
Unaged RTFOT-aged PAV-aged
3%, 4%, 5% 4%, 5%, 6% 5%, 6%, 7%
50 kPa 75 kPa N/A
LAS (25, 20, 15 °C) LAS (25, 20, 15 °C) LAS (25, 20, 15 °C)
study. It has been applied to both the LAS test [25] and binder time sweep test [22] and was found to be a reasonable indicator of fatigue failure of binders based on the observation that C N reached its peak value at the same time when the material yielded during the LAS test. Typical results from the strain-controlled binder time sweep test (parallel plate geometry) are shown in Fig. 7. The results are from binders with different levels of aging tested at the same loading amplitude (5% strain) and testing temperature (25 °C). It can be noted that C N did not clearly show a peak value, but instead presented a transition in the slope of the C N curve when the binder was not aged or was short-term aged. Although further investigation and testing would be required to reach more definite conclusions, it is anticipated that the gradual increase in C N observed from the unaged and short-term aged binders might be due to the testing temperature of 25 °C). At this temperature, flow behavior is significant and more dominant than cracking, and the presence of polymer in the binder might resist the faster rate of damage accumulation. When the binder time sweep was conducted under the stresscontrolled mode, a different trend in |G*| and phase angle over the increased loading cycles was observed compared to the strain-controlled binder time sweep testing. In the case of the strain-controlled mode, the stress within the sample reduced as damage accumulated, implying that the sample takes less effort to deform to a particular state once the sample has experienced
SAM (50-mm tall and 12.5-mm diameter cylinder) Aging
Time Sweep (Controlled Strain) at 25 °C
Strain Sweep at 25 °C
Unaged RTFOT-aged PAV-aged
0.20%, 0.25%, 0.35% 0.25%, 0.35%, 0.45% 0.20%, 0.25%, 0.30%
LAS* (25 °C) LAS* (25 °C) LAS* (25 °C)
the effects of temperature, loading rate (i.e., frequency), loading amplitude (either stress or strain), and the influence of aging on the fatigue resistance of binders. The linear amplitude sweep (LAS) is the standardized amplitude sweep procedure (AASHTO TP 101 [41]), which is currently the most commonly used for characterizing binder fatigue. The LAS* is another form of the strain sweep test where the only difference compared to LAS is that amplitude sweep process is conducted in the logarithmic mode as shown in Fig. 5(a). The LAS* was employed for SAM specimens because the rate of increase in strains in the typical LAS protocol was not quite feasible to apply to the SAM due to the very small strains applied to the SAM. The time sweep tests were conducted with varying loading amplitudes in either strain-controlled or stress-controlled mode. The time sweep test is a conventional fatigue test performed by applying cyclic loading at constant amplitude in the DSR. It is considered somewhat impractical because it requires much time to finish and yields a relatively high variability in testing results; however, it can mimic actual fatigue loading and damage induced. To compare fatigue damage characteristics and fatigue failure (observed from different testing schemes and binders with different levels of aging), a simple indicator to represent the fatigue failure was sought. As reported in many studies [22,33,37,42–44], several fatigue failure criterion have been proposed to define a robust failure definition in binder fatigue tests. From among those, a peak in C N values, where C is the normalized dynamic modulus and N is the number of loading cycles, was selected in this
(a) C vs. N and C*N vs. N
(b) δ vs. N, and C*N vs. N Fig. 7. Strain-controlled binder time sweep test results.
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significant damage. However, in the stress-controlled mode, the sample was subjected to a constant level of stress, meaning that to maintain the same level of stress the sample must undergo larger deformation once there is significant damage. This is clearly observed in Fig. 8. There was a phase of significant decrease in |G*| values and an increase in phase angle. Once the transition point (or the peak in C N curve) occurred, there was a faster rate of drop in the |G*| value and a corresponding increase in phase angle; this indicates increased loss in structural integrity and significant deformation within the binder sample. This trend was observed in all levels of aging (i.e., unaged, RTFO, and PAV) under the stress-controlled time sweep tests. Fig. 9 shows binder LAS test results at 25 °C. In contrast to the binder time sweep test in the strain-controlled mode, the peak of C N appeared clearly. Based on the observed peaks in C N curves, fatigue lives of binders resulting from the LAS tests at three different testing temperatures were determined. An example set of data from the time sweep test conducted on a SAM specimen is shown in Fig. 10. The data is from an unaged SAM specimen subjected to a strain amplitude of 0.25% at 25 °C. During the initial stage of the test there was a rapid decrease in |G*| and an increase in phase angle. After the initial stage, one can notice in the second stage that a linear increase in phase angle and a decrease in |G*| exist. Kim et al. [37] reported that, from the observation with the X-ray computed tomography imaging, the second stage under fatigue is related to microcracking in the sample. This phase was succeeded by a transition point, beyond which there was an increased rate of damage accumulation, which is associated with macrocracking due to fatigue [37]. The macrocracking phase induced the faster rate of decrease in |G*| and a simultaneously increased rate of phase angle. At the end of the fatigue test the
(a) C vs. N and C*N vs. N
(b) δ vs. N, and C*N vs. N Fig. 8. Stress-controlled binder time sweep test results.
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(a) C vs. N and C*N vs. N
(b) δ vs. N, and C*N vs. N Fig. 9. Binder LAS test results at 25 °C.
Fig. 10. An example result of SAM time sweep testing.
sample broke apart completely, which can be witnessed by the sudden drop in the phase angle. Unlike the parallel plate binder time sweep testing under the strain-controlled mode, there was no ambiguity in defining fatigue failure from the strain-controlled SAM time sweep testing. A similar trend was also observed in RTFO- and PAV-aged SAM specimens, as shown in Fig. 11, which compares test results of binders at different aging conditions at the same loading level (0.25% of strain) and testing temperature (25 °C). It was observed from the comparison that the rate of stiffness reduction and rate of increase in phase angle increased with more aging. Aging clearly contributed to the brittleness of the binder and faster damage accumulation due to microcracking and macrocracking; this sensitive mechanical behavior was well captured and represented by the SAM time sweep fatigue testing. This use of sand particles, which act as an efficient load-carrying media between thin binder films,
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(a) C vs. N and C*N vs. N
(b) δ vs. N and C*N vs. N Fig. 11. SAM time sweep test results at 25 °C.
is clearly an advantage in mimicking the actual condition of binder in AC mixtures. Fig. 12 shows typical results from the amplitude sweep of SAM specimens. The testing was conducted for SAM specimens of all three levels of binder aging at a temperature of 25 °C. In all cases, it was observed that the |G*| modulus is fairly constant, showing a linear response during the initial state, accompanied by a constant phase angle. As the loading cycles increase, the SAM specimen is subjected to strains from linear to nonlinear, and then to damage. This can be observed in the steep drop in modulus and C N. Fatigue failure was determined by the number of loading cycles corresponding to the peak of the C N curve.
(a) C vs. N and C*N vs. N
Fig. 13 shows comparisons of fatigue lives between binders (unaged, short-term aged, and long-term aged) from two different testing configurations (parallel plate of binder film and cylindrical SAM). In comparison to the binder time sweep tests which rank the PAV-aged binder to be more resistant to fatigue cracking, the SAM time sweep tests ranked the samples in a more realistic way. It showed that aging will have a detrimental effect in terms of fatigue resistance. A similar ranking was observed when tested in the amplitude sweep protocol, indicating that SAM testing was independent of the loading mode. In Fig. 13(c), it can also be observed that, from the binder LAS testing, fatigue lives reduced at lower testing temperatures (which is expected); however, the binder LAS testing ranked the PAV-aged binder more resistant to fatigue cracking than unaged and short-term aged binders. This trend is contradictory to the typical belief-observation that the pavement is more susceptible to fatigue cracking as the binder becomes aged. The same trend was also reported in other studies [31,45]. On the other hand, the amplitude sweep testing of SAM specimens presented expected results. PAV-aged samples failed earlier than the unaged and short-term aged samples, and this fatigue ranking order agrees well with the results from the time sweep testing. The SAM testing induced damage in asphalt binder that was sensitive to the level of aging, which was not clearly captured by the binder DSR testing with the 2-mm parallel plate geometry. As demonstrated and discussed in previous sections, the laboratory procedure and testing of SAM specimens for binder fatigue characterization looks promising as an alternative. Due to the use of standard Ottawa sands as a load carrier between micrometerthin binder films, SAM testing could successfully evaluate viscoelastic properties and crack-induced fatigue damage of binders in a more realistic manner than conventional mm-scale parallel plate binder tests. In an attempt to further evaluate the validity of the SAM fabrication-testing as a potential specification-type method, repeatability of the SAM fatigue testing in both time sweep mode and amplitude sweep mode was examined. An unaged binder was used to fabricate 5–6 replicates of SAM specimens for each case (i.e., one amplitude sweep test and three time sweep tests at different strain levels) at an intermediate temperature (25 °C). The unaged SAM specimens were chosen for this effort since it is the most compliant compared to the RTFOT and PAV aged cases. Fig. 14 shows representative data of each fatigue testing and a summary of repeatability from all cases in a form of bar chart where mean values, standard deviations (error bars), and coefficients of variation (COV) are plotted together. As presented, amplitude sweep testing of SAM was quite repeatable with a very low value of COV (i.e., 3.5%). Although time sweep testing (fatigue testing) was less repeatable than the amplitude sweep testing, which
(b) δ vs. N, and C*N vs. N
Fig. 12. SAM sample amplitude sweep test results at 25 °C.
S.R. Kommidi et al. / Construction and Building Materials 237 (2020) 117588
(a) time sweep tests of binder at 25ºC
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(b) time sweep tests of SAM at 25 ºC
(d) LAS* tests of SAM at 25 ºC
(c) LAS tests of binders
Fig. 13. Fatigue lives from each testing with different binders.
was expected and well-known, its repeatability level seems still quite satisfactory (i.e., less than 30% overall), considering the typical COV levels of fatigue testing of materials [46]. 5. Summary and conclusions In this study, multiple fatigue-related tests (i.e., time sweep and amplitude sweep) were conducted on both 2-mm thick parallel plate binder specimens and cylindrical SAM specimens by varying the level of binder aging. Test results between the two testing configurations were compared to investigate the effects of binder length scale and aging on fatigue cracking behavior. In addition, the original SAM fabrication-testing method was revisited and revised to examine its testing efficiency and repeatability. Based on the test-analysis results, the following conclusions can be drawn: The laboratory procedure to fabricate SAM specimens was revised from its initial state of development. Testing repeatability supports the validity of the method. Because of the use of standard Ottawa sands as a load carrier between micrometerthin binder films, SAM testing is considered a useful method to evaluate fatigue-related damage characteristics of binders in a more realistic manner than conventional mm-scale parallel plate binder DSR tests at room temperatures. In the binder fatigue tests, especially in the strain-controlled mode time sweep tests for unaged and RTFO-aged cases, there
was no clear distinction for evaluating fatigue failure point. However, in the case of SAM testing, a clear indication of fatigue failure was seen irrespective of the loading mode. The binder time sweep tests erroneously ranked the PAV-aged binder to be more resistant to fatigue cracking, while the SAM time sweep tests ranked the samples in a more realistic way by showing that aging will have a detrimental effect in terms of fatigue resistance. It was observed from the fatigue testing results that SAM testing could capture the microcracking and macrocracking phases more distinctively than 2-mm thick parallel plate binder fatigue testing. This indicated that the SAM is more sensitive in capturing fatigue cracking, particularly with the influence of aging, than binder DSR testing. Test results and findings imply that the SAM can be considered a proper solid phase to evaluate the properties and damage characteristics of binders and mastics under various conditions. A follow-up study to investigate other factors, such as the influence of SAM specimen geometry and binder content for better representation of binder characteristics in addition to fatigue damage criterion such as the energy-based failure criterion is necessary to further improve the SAM testing and relevant analysis. In addition, more binders with different additives (e.g., polymer modifiers, softening agents, fillers, fibers, etc.) need to be included in the testing program to draw a more definite conclusions. The authors are currently working on it, and new findings will be presented elsewhere.
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(b) TS repeatability (0.6% strain amplitude)
(a) LAS* repeatability
(c) Fig. 14. LAS* and times sweep repeatability for unaged SAM samples.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors thank Ms Mahdieh Khedmati for helping the laser scanning microscopy testing to image film thicknesses of SAM specimens. The third author is grateful for the support received from the Fulbright Visiting Scholar Program to participate in this research. References [1] F. Moreno, M. Rubio, Effect of aggregate nature on the fatigue-cracking behavior of asphalt mixes, Mater. Des. 47 (2013) 61–67. [2] J. Sousa, J. Pais, M. Prates, R. Barros, P. Langlois, A.-M. Leclerc, Effect of aggregate gradation on fatigue life of asphalt concrete mixes, Transp. Res. Rec.: J. Transp. Res. Board 1630 (1998) 62–68. [3] F.M. Nejad, E. Aflaki, M. Mohammadi, Fatigue behavior of SMA and HMA mixtures, Constr. Build. Mater. 24 (7) (2010) 1158–1165. [4] J. Harvey, B.-W. Tsai, Effects of asphalt content and air void content on mix fatigue and stiffness, Transp. Res. Rec.: J. Transp. Res. Board 1543 (1996) 38– 45. [5] J.M. Matthews, C.L. Monismith, J. Craus, Investigation of laboratory fatigue testing procedures for asphalt aggregate mixtures, J. Transp. Eng. 119 (4) (1993) 634–654. [6] H.-J. Lee, Y.R. Kim, Viscoelastic continuum damage model of asphalt concrete with healing, J. Eng. Mech. 124 (11) (1998) 1224–1232.
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