Characterisation of complex polymer modified bitumen with
rheological parameters
Laurent Porot
1, Stefan Vansteenkiste
2, Michalina Makowska
3, Xavier
Carbonneau
4, Jiqing Zhu
5, Sjaak Damen
1, Kees Plug
61 Kraton Polymers B.V a subsidiary of Kraton Corporation, Amsterdam, the
Netherlands laurent.porot@kraton.com
2 Belgium Road Research Center, Sterrebeek, Belgium 3 Aalto University, Aalto, Finland
4 Colas CST, Magny les Hameaux, France 5 VTI, Linköping, Sweden
6 Strukton Civiel West, Scharwoude, the Netherlands
Word counts: 4930
Corresponding author: Laurent Porot
Road Materials and Pavement Design Volume 22, 2021 - Issue sup1: EATA2021 – Vienna https://doi.org/10.1080/14680629.2021.1910070
Characterisation of complex polymer modified bitumen with
rheological parameters
The use of modifiers for bituminous binders has become common practice to enhance the performances of asphalt materials. As a result, the binder becomes more complex, with various phase morphologies. Conventional testing methods may not always be suitable. As example, the European standardisation committee is looking at different ways to characterise Polymer modified Bitumen. The RILEM Technical Committee 272-PIM ‘Phase and Interphase behaviour of innovative bituminous Materials’, with its Task Group TG1, is evaluating different test methods for complex binders. The aim is to evaluate how these tests could address complex bituminous binders. An inter-laboratory program was conducted with 17 laboratories. Seven binders were evaluated, two were neat bitumen and two PmB. In addition to conventional property measurements, an extensive effort was made on investigating various rheological parameters obtained from Dynamic Shear Rheometer measurements. Some initial results are presented and discussed in view of the future revision of the European product standard EN 14023 for PmB.
Introduction
Over the last two decades, more and more modifiers have been used to enhance the physical or chemical properties of bituminous binders. Such modifiers may be liquid additives or polymer-based, viscous or solid particles (PIARC, 1999). As a result, binders used for asphalt applications, are becoming more complex materials, showing potential different phase morphologies (Nahar, et al., 2014) (Soenen, et al., 2008). In this context, the Rilem TC 272 PIM ‘Phase and Interphase behaviour of innovative bituminous Materials’ and more specifically the Task Group 1, TG1, is focusing on the suitability of testing methods for complex bituminous binders. The TC started in 2016 and is expected to end in 2021. The experimental plan included seven bituminous binders, in two groups, one on polymer modification, and one on liquid additives in bitumen. A total of 17 laboratories participated. The testing program was run between April 2019 and April 2020 with various tests performed, including empirical properties and more advanced characterisation with extensive work on Dynamic Shear Rheometer, DSR.
The use of DSR has gained popularity since the 90’s to fully characterise the visco-elastic behaviour of bituminous binders (Anderson, et al., 1994). For assessing Polymer modified Bitumen, it has demonstrated many advantages in comparison to conventional testing (Lu, et al., 1995) (Youtcheff, et al., 2004). Since then, it has been widely adopted in research on asphalt binder materials (Airey, 2003), and various methods to interpret the data have been developed (Porot, 17-18 Sept 2018).
In the US, DSR measurements are now part of bituminous binder purchase specifications, known as Performance Grading system (AASHTO M320, 2015) (Planche, et al., 1996), supplementing the previous specifications based solely on viscosity (NCHRP, 2001). The high temperature domain was initially characterised with |G*|/sin, as measured on original and after short term aging; although now, in many states in the US, the Multi Stress Creep Recovery, MSCR, test (AASHTO TP70, 2013) has been implemented as to surrogate the high temperature PG criteria (D'Angelo, et al., 11-14 March 2007). The intermediate temperature is addressed with the loss modulus |G*|sin after long term aging.
In Europe, the Bitumen Test Validation European project, Bitval (BitVal, 2006), was established in a response to questions raised through the CEN committee. It addressed test methods for bituminous binders including DSR and compared with conventional basic properties. Eurobitume also initiated a comprehensive data collection on various bituminous binders, during 2006-2009. Various test methods stimulated industry discussion for CEN specification (Soerensen, et al., 2012). In view of the revision of the European standards for bituminous binders, different parameters are considered with DSR, focusing mainly on equi-modulus when |G*| equal to 5 MPa, 50 kPa or 15k Pa (Durand, et al., 2021).
In the CEDR Transnational Road Research Program (call 2013) FunDBits, Functional Durability-related Bitumen Specification was established to continue the effort of the Bitval project by reviewing new literature data (Nicholls, et al., 2016). The objective was to correlate the physical properties of bituminous binders, as measured by a wide range of tests, to the corresponding asphalt mixture characteristics. Latter, research was carried out for the whole temperature range, including binder properties at low, intermediate and high temperatures. The
binder tests included a detailed description of the possible benefits of rheological testing.
DSR was already part of previous Rilem activities. The Rilem TC 152 PBM “Performance of Bituminous Materials” and TC 182 PEB “Performance testing and evaluation of bituminous materials“, performed round robin tests on binder rheology in early 2000. As example, the TC 182 PEB and its TG1 evaluated four binders at different conditioning aging with 17 participating laboratories (Sybilski, et al., 2004). The reproducibility, was established to be in the range of 10 % for the shear modulus and 5 % for the phase angle.
Increasingly researches and studies are including DSR measurements in their experimental plan and extensive literature can be found on the benefits of using DSR to fully characterise bituminous binders. The aim of the paper is not necessary to make a literature review on DSR measurements. It is more on investigating how different parameters can address the complexity of wide range of bituminous binders. It focuses on analysing the DSR data collected for PmB in the Rilem PIM TG1 in perspectives of the different approaches and previous findings. Only oscillatory measurements are included hereby, while further testing, such as multi stress creep recovery test, will be discussed in separate publications. The raw data from each lab was collected and analysed by one lab, avoiding artefact from different computing methods. In this respect, analysis included,
The validity of the data through Black Space,
Analysis of parameters, |G*|/sin, |G*|sin along with equi-modulus parameters at different aging conditions and cross-over parameters, Comparison of fundamental and basic properties.
Experimental plan
Materials
Within the Rilem PIM TG1 seven binders were evaluated in two groups, one for polymer modification and one for liquid additives, including neat bitumen for reference comparison. The results discussed here are only related to the group 1 for PmB. Table 1 provides the list of all materials, the ones in italic being not discussed in this paper.
Table 1. List of Materials evaluated in Rilem PIM TG1
Label Binder Grade EU Grade US
Bit1 Neat bitumen 35/50 35/50 PG 70-22
PmB1 Standard PmB 25-55/70 PG 76-16
PmB2 Highly modified PmB 25-55/85* PG 82*-28 Bit2** Neat bitumen 70/100 70/100 PG 64-22
Blend1 Bit1 + bio-based additive 70/100 PG 64-28
Blend2 Bit2 + REOB 70/100 PG 64-28
Blend3 Bit3 + paraffinic oil 70/100 PG 64-28
* the measured values were higher than the maximal class from respective specification ** Bit2 was used to produce the PmB2
The choice of the PmB was made to have two significantly different binders. The PmB1 was commercial produced PmB, specifically selected for the Rilem PIM TG2 looking at asphalt mix characteristics to have a low temperature fracture for asphalt mix in the range of – 20 C. The PmB2 was a lab produced PmB with 7.5 % of high vinyl linear SBS polymer ensuring a rich polymer phase to enhance rutting and fatigue properties (Habbouche, et al., 2020). The two neat bitumen were from the same source, with Bit1 having similar consistency to the PMBs at intermediate temperature, based on penetration value at 25 C, and the Bit2 having been used to produce the PmB2.
Testing
The different binders were all artificially aged in the laboratory with short term aging, through Rolling Thin Film Oven Test, RTFOT, at 163 C according to EN 12607-1, and further with Pressure Aging Vessel, PAV, for 20 h at 100 C, according to EN 14769. The binders, at the different aging conditioning, were extensively tested by the different laboratories with conventional properties and more fundamental properties measurements (Porot, et al., 2020). Later, only the DSR results for the two PmBs and two neat bitumen are further analysed and discussed, the other results being subject of further publications. Over the 17 labs, 12 generated DSR data, but not on all binders. Table 2 shows the experimental matrix for each binder with the total number of results collected for DSR measurements. The labs, which are not numbered in the table, participated in the Rilem PIM TG1 but did not performed DSR on those binders. In total 11 labs performed measurement on original binder, four after RTFOT and six after RTFOT+PAV.
Table 2. Experimental matrix with lab participation for DSR measurement Bit1 PmB1 PmB2 Bit2 Original RTFOT PAV
Total 10 9 11 9 11 4 6 Lab1 x x x x x x x Lab2 x x x x x x Lab3 x x x x x Lab5 x x Lab8 x x x x x x Lab9 x x Lab10 x x x x x Lab11 x x x x x Lab12 x x x x x x Lab13 x x x x x Lab14 x x x x x Lab16 x x x x x x
Each laboratory performed DSR measurement using their own protocols. Most of them used two plate-plate geometries: 8 mm plate with 2 mm gap for intermediate temperatures and 25 mm plate with 1 mm gap for the high temperatures, and run in frequency sweep at different temperatures. No specific instructions were given, in order to assess the variability of the results between labs for the various binders. Measurements were made at different frequencies
with a range of 0.1 to 10 Hz and at least 1.59 Hz (10 rad/s) recorded for all labs. Temperatures were in the range of 10 to 40 C (even starting sometime at 4 C) with 8 mm plate for intermediate temperature and 30 to 82 C for the 25 mm plate, up to 100 C for some labs. Additionally, some data was available from 4 mm plate measurements at low temperature varying from -30 till 10 C.
Results
Basic properties
Between six and nine laboratories measured the penetration value at 25 C and softening point temperature for the four binders. Figure 1 displays these basic properties, with points for each laboratory values. The two PmBs had penetration value at 25 C in the same magnitude of range as Bit1, while the softening point temperature was more discriminant with the highly modified PmB having the temperature recorded, by eight labs, between 88 and 93 C. It was noted that the scatter in the results was higher with the high softening point temperature, as recorded for PmBs. For comparison, the reproducibility in EN 1427 is 5 C when measured in glycerol and 2C in water on pen grade bitumen.
Basic properties of the four original binders
DSR initial consistency analysis
The raw DSR data from each laboratory was collected with storage and loss moduli and used to determine the complex shear modulus, |G*| and phase angle, . As measurements were performed at different frequencies and temperatures, the first analysis was made using the Black Space on the original binders. It allows displaying all data without the need of shifting models as it would be needed with master curves. It helps identifying any deviation from time temperature superposition principle and any single data set out of the range from the other ones. The data are plotted with |G*| on x-axis and on y-axis, as the range of values are greater for |G*| and, especially for PmB, there is no unicity of values. Figure 2 displays the Black space for the four binders and all laboratories. The shear modulus were recorded between 103 to 109 Pa.
With Bit1, only one lab reported measurements clearly out of other data range, thus it was not included later in further analysis. Also one laboratory
recorded for the four binders, values for low temperature greater than 109 Pa, while, for other labs, it never achieved this value. For PmBs, the variability was higher for low |G*| values as compared to intermediate and high |G*| values / intermediate and low temperatures. In other words, the lower the shear modulus, the higher the scatter it was. One explanation may be related to the morphology / phase behaviour of the binders and limits of linear viscous elastic domain. The low |G*| range is, indeed, in the range where polymer modification has a greater effect and then, measurement more sensitive to the sample morphology. For all binders, the middle range of measurements for each geometry displayed less scatter than the extreme range of temperature conditions.
While the neat binders shown continuous smooth concave curves, the PmBs had clear rubber plateau starting around 60 , and being more pronounced for PmB2, highly modified bitumen, than for PmB1. Comparing the two neat binders, Bit1 & Bit2, the profiles were very similar and almost overlap, with only a contracture of the curves. Comparing PmB2 with Bit2, which was used as base bitumen, from a shear modulus above 106 Pa, the curves had similar curvature shape with only a shift up.
Similar analyses were also made on aged binders, however the number of dataset was lower than for original ones and less interesting especially considering the validity of the data set for each lab.
Black Space for the four binders, original state
The two labs having been identified out of the range of the others, from initial analysis, were not included later in the interpretations. Based on this first analysis of the raw DSR measurement data, a further interpretation was done by extracting measurements at same frequency of 1.59 Hz, equivalent to 10 rad/s, as the reference frequency most often used for computing DSR parameters. This was done for each lab using the same calculation and each parameter was computed
by interpolation between different measurement points. This avoided using shifting model that may induce some artefact in calculation, when laboratories are not using the same model or the same shifting factors. The parameters are reported in Table 3 with the formula used to determine them.
Table 3. Determination of DSR parameters at 1.59 Hz (10 rad/s)
Parameter Criteria Formula
High PG true T |G*|/sin, 1kPa on original, 2.2 kPa RTFOT
Temperature linear and |G*|/sin in log
Int PG true T |G*|*sin 5000 kPa RTFOT+PAV
Temperature linear and |G*|*sin in log
Equi |G*| T and |G*| at 5 MPa, 50 kPa, 15 kPa
Temperature and linear and |G*| in log
Cross over T, |G*| = 45 Temperature and linear and |G*| in log
DSR PG criteria
The SHRP-1 program in the US was the first step in introducing the use of DSR data in purchase specification system for bituminous binders (Anderson, et al., 1994). The initial Performance Grade specification framework used, amongst other, DSR parameters for high temperature with |G*|/sin minimum 1 kPa on original and minimum 2.2 kPa after RTFO, and for intermediate temperature with |G*|sin maximum 5000 kPa on RTFO+PAV binder. In this paper, the true PG temperatures were determined by interpolating the DSR data, as measured at 1.59 Hz, exactly equal to each criteria.
Figure 3 displays the true high PG temperatures on original and after RTFOT, the softening point temperatures are added for comparison; the errors bars are for max and min values. It is worth noting that, on original binders, there were between seven and eight data points, while after RTFOT only between 3 and 4 values. For all binders the two true high PG temperatures were in the same magnitude of range, with a slight difference for PmB2, which displayed higher values. With softening point, the neat binders shown an increase after RTFOT, while it was negligible for the PmBs. Overall the High PG temperatures can distinguish between neat bitumen and PmB as can softening point temperature.
DSR PG true high T criteria for the four binders
Figure 4 shows bars representing the true intermediate PG temperature after RTFOT+PAV, and the penetration value at 25 C on original and after RTFOT+PAV marked. The variability is consistent between the neat binders and the PmBs at approximatively +/- 2 C. Interestingly, on the penetration value at 25 C of original binders, limited difference were observed between Bit1 and the PmBs, the |G*|sin criteria can distinguish more clearly the performance. The PmB2 displayed |G*|sin value close to the one from Bit2, which was used as base bitumen in its formulation. The penetration value after RTFOT+PAV seems to follow a similar trend like the PG parameter at differentiating between PmBs as compared to original binder.
DSR PG true intermediate T criteria for the four binders
DSR equi-consistency parameters
Europe is also anticipating using DSR to characterise bituminous binders. It was initiated earlier in 2000 with BitVal project and later Eurobitume data collection. For this purpose, in this study, it was decided to investigate different parameters to characterise the rheological behaviour, with temperature and phase angle at equi- modulus. The first approach, based on the Eurobitume data collection, was considering the shear modulus equal to half of the highest value limits for each geometry, as set in EN 14770. For the 8mm plate, it was |G*|=5 MPa and for the 25 mm plate |G*|=50kPa (Durand, et al., 2021). More recently in Germany, was developed criteria at |G*|=15kPa as a surrogate to the softening point temperature on original binder (Alisov, et al., 2020). The parameters for the three criteria were determined with temperature and phase angle on all binders.
Figure 5 displays the comparison, for original binders, for high service temperature criteria with all data points of labs. The softening point temperature range is shown as reference and reported as “SoftPt T”. The plain marks are for |G*|=50 kPa and the shadow marks for |G*|=15 kPa. As seen in the analysis of the Black Space, the lower the shear modulus, the more scatter between labs. This is even more valid for the PmB1. At comparing average values between criteria 50 kPa or 15 kPa, the latter displayed higher temperature and phase angle for Bit1, Bit2 and PmB1, but higher temperature / slightly lower phase angle for PmB2. With the two neat bitumen, the phase angles were in the same magnitude of range. The PmB1 was distinguished from the Bit1 mostly by the phase angle while
temperature was similar. Comparing with softening point temperature, while the 15 kPa criteria seemed to correlate for the neat bitumen, with PmBs, both 15 kPa and 50 kPa criteria did not correlate as well as for the neat bitumen. The analyse of these criteria needs to consider both temperature and phase angle, which may be greater challenge to address the high service temperature with a single parameter, as the PG criteria is able to address.
DSR high temperature equi-modulus for the four original binders On Figure 6 the temperatures and phase angles at |G*|=5 MPa are displayed for the four binders at the three conditioned states for all labs. The plain marks are for original (7 to 8 labs), the shadow marks after RTFOT (3 to 4 labs) and the empty marks after RTFOT+PAV (5 to 6 labs). All binders were in the same range of intermediate temperature for original state, varying between 15 and 20 C. And interestingly, the phase angles were close to 60 for the neat bitumen and 45 for the PmB. The variability between labs was in a magnitude of 3 to 5 C, and the phase angle was more consistent, between 2 and 4 .
For the neat bitumen, for each state, phase angle was in the same range and they distinguished with a shift in temperature by 5 C on original to 8 C on long-term aging. With PmB, the phase angles were lower when compared to plain bitumen, meaning more elastic behaviour, and temperatures were still in an intermediate range, about 20 C for PmB1 and 16 C for PmB2. After aging, there was a shift with higher temperature and lower phase angle, meaning stiffer and more elastic behaviour.
Overall, the analyse of this criteria needs to consider both temperature and phase angle making it more difficult to address with a single parameter for intermediate service temperature, as with using the PG criteria.
DSR intermediate temperature equi-modulus for the four binders
Other parameters
Other parameters may be considered. For intermediate temperature, the cross-over parameters, when the phase angle is equal to 45, is a criteria of interest as it addresses the transition between a predominant elastic behaviour, where < 45 to a predominant viscous behaviour, where > 45 (Porot, et al., July 2016). The cross-over temperature can address the consistency, how stiff the binder is, and the cross-over modulus, the temperature susceptibility. An example of this parameter is described in British specification series 900 (although at different conditions) and is gaining more interest in the US as well (Garcia Cucalon, et al., 2019). Figure 7 displays the cross-over parameters, temperature and shear modulus, for the four binders at the different aging states, at frequency of 1.59 Hz. Overall, the temperatures were in the range of intermediate temperature, the softer the grade, the lower the temperature. With aging, this temperature increased consistently for all binders, about 5 C after RTFOT and further 10 C after PAV. Interestingly, this was something not observed with other parameters. An example for PmB, there were limited changes on softening point temperature after aging. One explanation may be the fact that at high temperature, the polymer phase behaviour is more predominant. The shear modulus, is somehow more complex to analyse, however as a trend, with aging the value was almost divided by a factor of two for each aging step.
DSR Crossover parameters for the four binders at 1.59Hz
Discussion
The use of DSR has gained in popularity to characterise bituminous binder. It brings more fundamental properties through consistency and visco-elastic components when compared to empirical properties. By performing the measurements in various conditions of temperatures and / or frequencies, it generates multiple possibilities of parameters for service temperatures.
Figure 8 is comparing the different foreseen parameters for high service temperature at different aging states, clockwise from original, after RTFOT and RTFOT+PAV. The four binders have been selected as a wide range of grades from soft to hard, from standard to high modification. The accuracy / variability
of each parameters has been analysed previously, only the average values are reported. The relevance of each parameter is more visible in the radar graph. For each parameter, the greater the difference between binders, the more discriminating in distinguishing different grades it is. Softening point temperature and PG criteria |G*|/sin are more discriminating to distinguish grades and complex bitumen when compared to modulus temperature. The equi-modulus parameters need to be analysed in combination with phase angle which is missing in the radar analysis.
DSR parameters comparison for high service temperature
Figure 9 displays in the same way the average values of the different parameters for intermediate temperature with equi-modulus at 5 MPa, cross-over and PG criteria |G*|sin. Again the greater the difference between binders, the more discriminating the parameter. The equi-modulus temperature at |G*|=5 MPa alone hardly distinguishes between the neat bitumen and complex polymer modified bitumen. Moreover, PmB2 and Bit2, which was used as base bitumen, had similar temperature; and PmB1 and Bit1 had similar temperatures. The same applied for the PG criteria, which is only valid after RTFOT+PAV. The cross-over temperature was better at distinguishing polymer modification in the three aging states.
DSR parameters comparison for intermediate service temperature Table 4 provides the different average temperatures for each criteria for the four different binders on original, after RTFOT and after RTFOT + PAV.
Table 4. Criteria temperature for all binders
Bit1 Bit2 PmB1 PmB2
Original binder Average Average Average Average
Softening point T 54 °C 46 °C 74 °C 91 °C T at |G*|/sinδ =1kPa 75 °C 66 °C 81 °C 106 °C T at |G*| 50kPa 47 °C 40 °C 48 °C 51 °C T at |G*| 15kPa 55 °C 47 °C 57 °C 65 °C T at |G*| 5MPa 21 °C 15 °C 20 °C 16 °C T at Cross over 11 °C 4 °C 17 °C 11 °C
After RTFOT Average Average Average Average
Softening point T 59 °C 52 °C 75 °C 93 °C T at |G*|/sinδ =2.2kPa 74 °C 66 °C 83 °C 101 °C T at |G*| 50kPa 51 °C 44 °C 54 °C 54 °C T at |G*| 15kPa 59 °C 51 °C 64 °C 69 °C T at |G*| 5MPa 23 °C 17 °C 24 °C 18 °C T at Cross over 15 °C 8 °C 25 °C 14 °C
After RTFOT + PAV Average Average Average Average
Softening point T 67 °C 60 °C 78 °C 102 °C T at |G*| 50kPa 59 °C 51 °C 64 °C 63 °C T at |G*| 15kPa 68 °C 59 °C 74 °C 80 °C T at |G*|sinδ =5000kPa 26 °C 19 °C 24 °C 20 °C T at |G*| 5MPa 28 °C 21 °C 29 °C 23 °C T at Cross over 23 °C 15 °C 37 °C 25 °C
Conclusion
In an attempt to understand the fundamental material behaviour and answer to the question on suitability of testing methods, the RILEM TC 272 PIM and its TG1 is looking at the phase morphology of complex bituminous binders. Amongst other testing, DSR measurements, in oscillatory mode, have been extensively investigated, especially for polymer modified bituminous binders. With data set from 12 laboratories, on four different binders, over the 17 participating laboratories on all binders, different DSR parameters were all compared together. As compared to empirical properties, DSR confirms bringing more information in a wide range of testing conditions and can better distinguish between binders.
PmBs showed more complexity in the low shear modulus range / high temperature domain, combining intrinsic stiffness value and visco-elastic balance. This enables to distinguish better between PmB and unmodified bitumen. Equi-modulus parameters can address differences, but they need both, temperature and phase angle values. The lower the equi-modulus, the higher the potential scatter. While PG criteria with a single parameter can better distinguish between binders than equi-modulus.
Also with intermediate temperature, equi-modulus parameter requires the use of two values, temperature and phase angle as compared to single PG criteria. The cross-over temperature is an interesting path to better distinguish and characterise the visco-elastic rheology of complex binders.
More data has been collected and is under analysis and interpretation, especially on the low temperature domain, comparing Bending Beam Rheometer and 4mm plate DSR, as well with multi strain creep recovery test.
Acknowledgment
This work was developed by the task group TG1 within the RILEM TC 272 PIM. The authors would like to acknowledge the active support of participating laboratories and experts having provided data and analyse especially M. Makowska from Aalto University, S. Vansteenkiste from Belgium Road Research Center, V. Mouillet from CEREMA, X. Carbonneau from Colas, P. Apostollidis from Delft Technical University, C. Raab from Empa, E. Chailleux from Université Gustave Eiffel, D. Menshing from FHWA, L. Porot from Kraton, Runhua Zhang from New Hampshire University, H. Soenen from Nynas and A. Margaritis from Antwerp University, K. Plug from Strukton, L. Tsantilis from Politecnico di Torino, J. Zhu from VTI, M. D Elwardany from Western Research Institute.
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