Testing and Characterization of Compacted Asphalt Pavement Materials

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1 Testing and Characterization of Compacted Asphalt Pavement Materials Manfred N. Partl 1, Hervé di Benedetto 2, Francesco Canestrari 3, Hussain U. Bahia 4 1 Empa, Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland 2 ENTPE, Ecole Nationale des Travaux Publics de l Etat, Lyon, France 3 Universita Politecnica delle Marche, Ancona, Italy 4 University of Wisconsin-Madison, Wisconsin, USA ABSTRACT: This paper summarizes some key achievements and findings by the RILEM TC 206 ATB advanced testing and characterization of bituminous materials, focusing on interlaboratory tests on compacted asphalt pavement materials. Results from wheel tracking tests on materials from real motorways well as interlayer shear bond testing on materials taken from specially constructed test sections are presented. New developments in asphalt mixture testing, such as the application of image analysis techniques, are also taken into consideration, in particular with respect to compaction effects. Results show that performance related testing of bituminous mixtures must be considered also with respect to the whole pavement system and depends on variation in compaction. It was also found that test methods developed for one type of bituminous materials do not a priory apply to another type of bituminous materials and may even lead to completely misleading conclusions as compared to real life pavement performance. 1 INTRODUCTION Increasing traffic loads and decreasing public tolerance regarding interruptions of traffic flow from road construction and maintenance as well as growing environmental requirements and global awareness of the necessity for sustainable use of material and energy resources encouraged in a progressive way new initiatives for developing improved road materials and innovative structural pavements in the last decades. During this process it turned out that traditional test methods are no longer a priori applicable and that understanding and modeling of materials has to follow new performance based directions. In particular, this means that testing and characterization of asphalt materials must focus more intensively on bridging the gap between small scale lab condition and real scale field condition also providing better information for structural considerations. Hence, key questions in this regard are: How to consider the material behavior with respect to its function in the pavement structure? How to make sure that the materials tested in the lab really correspond to the material in the field, for instance in terms of compaction and anisotropy effects? How to take into account differences between different test and laboratories? In the following, some activities in that direction are summarized which have been conducted by RILEM technical committee TC 206 ATB advanced testing and evaluation and characterization of bituminous materials during the last few years. Selected inter-laboratory

2 results from wheel tracking tests, interlayer bond tests and image analysis studies for comparing lab compaction are presented. All studies include material from real motorways or field test sections trying to make the link between laboratory and field conditions. 2 WHEEL TRACKING TESTS 2.1 Background and methodology Rutting is one of the main failure modes in pavement structures subjected to mechanical loading and ultimately reduces riding comfort and safety level of the pavement, e.g. by increasing the risk of aquaplaning. Rutting in asphalt pavements is caused by sufficiently high amount of accumulated repeated short-term loadings by passing vehicles, particularly during summerlike warm weather periods, thus creating permanent deformation in the highly time and temperature dependent material of the different pavement layers and, consequently, eventually producing rutting on the pavement surface. Reliable testing, understanding and assessing permanent deformation behavior of bituminous pavement materials are therefore crucial for pavement construction. Different test methods have been developed and used in different countries to evaluate permanent deformation of bituminous mixtures. Some methods focus directly on the bitumen behavior in order to correlate its creep behavior to the permanent deformation of the asphalt material. Others are based on mechanical property measurement of the bituminous mixtures and on empirical simulation of the effect of a wheel rolling over the surface of a bituminous layer. Among these devices, most commonly used is the Wheel Tracking Tester (WTT) generally consisting of a pneumatic or solid rubber wheel that conducts backwards and forward motions mostly on laboratory compacted specimens. Specimen sizes, testing conditions (load, frequency, temperature, etc.) and type of wheel may vary considerably depending on the WTT device types. In the frame of this RILEM study, two experimental campaigns were organized to compare different wheel tracking tests. The first campaign A was limited to bituminous mixtures made with pure bitumen. The mixtures were tested with different types of WTT (Figure 1), the French pneumatic wheel tracking tester FWTT, which is a large size device according to EN , and two different wheels tracking tester WTT-I and WTT-II with solid rubber wheels which were both small size devices according to EN For the second campaign B only the French Wheel tracking tester (FWTT) was used to characterize a mixture with polymer modified bitumen. These campaigns are introduced in the papers by Perraton et al. (2011) and Gabet et al. (2011).. a) b) c) Figure 1. WTT devices used in the inter-laboratory tests.a) FWTT (large size device: EN ), b) WTT-I (Small size device: EN ) and c) WTT-II (Small size device: close to the EN )

3 2.2 Comparison of Different Wheel Tracking Tests: Campaign A Seven laboratories from seven different countries were involved in this testing program. Three bituminous layer systems A, B and C were selected for testing (Table 1). They were composed of two or three different bituminous mixtures compacted to different thicknesses. Bituminous systems A with stone mastic asphalt SMA and B with mastic asphalt MA were standard motorway pavements from Swiss highway A1 and A3 (location: Birrfeld in Canton Aargau) whereas system C with French standard half-granular asphalt concrete BBSG was extracted from the circular test track at LCPC (IFSTTAR) in Nantes (France). From the results in Figure 2, it can be seen that the three rutting devices produce much more rutting for system B than for the other two systems. In fact, the typical maximal rut depth requirement of 10% relative to the specimen thickness after cycles for FWTT is already achieved by system B after 300 cycles. In addition, the rutting curves obtained from the three types of device, are different. The two small wheel size devices WTT-I and -II produce a rutting rate that increases at the end of the tests even for systems B and C. Hence, loading conditions of the small size devices seem to be more severe than with FWTT, because of the higher stress level and the more concentrated wheel action, which may create a punching effect at the load location, thus increasing the amount of permanent deformation. Nevertheless, ranking of the three investigated systems by the different WTT was quite the same. Best performance was obtained with system C, worst with system B. Systems A and C perform equivalently and can be considered good in terms of the requirements of the European standard whereas system B indicates poor performance that is not complying with the standard specifications. WTT results for B are in contradiction with field experience, since both systems A and B performed well after 10years in service and did not show any apparent differences. This clearly demonstrates that a test method, which may be meaningful for certain asphalt concrete types of material, may not be conclusive a priori for other types of materials, such as asphalt mastic, and even lead to misleading conclusions compared to real life pavement performance. Table 1. Layer characteristics for tested systems (BC: Bitumen content, T RB : Ring & Ball Temperature) System Layer Thickn. (mm) Type Bitumen BC (% -Weight of Aggregate) Penetration (0.1mm) T RB ( C) A Surface 42 SMA 11S: Stone Mastic Base 80 AC 22: Asphalt Concrete B Surface 45 MA 11: Mastic Asphalt Bond 36 MA Base 73 AC C Top BBSG 0/14 Béton bitum / semi grenu : Base 80 GB 0/14: Grave Bitume /50 ---

4 Figure 2. Rutting results for Systems A, B, C, determined with different WTT. 2.3 Comparison of Different Wheel Tracking Tests: Campaign B This inter-laboratory test involved seven laboratories from five different countries. Reproducibility of the French Wheel Tracking Test (FWTT) for an asphalt concrete made by one laboratory with 5.55 %-weight of plastomeric EVA (Ethyl-Vinyl-Acetate) polymer modified bitumen was investigated. The asphalt concrete AC14 can be classified as a BBSG (Béton Bitumineux Semi-Grenu), according to the French Standard (see Table 1). This mixture is known for its potential to excessive heating of the sample due to friction during WTT and sticking of the binder to the wheel. Hence, reproducibility of the wheel tracking tests for a given anti-overheating and anti-sticking procedure was investigated. The anti-overheating procedure requires two temperature sensors (T ext and T int ) for each specimen, aiming at keeping the temperatures measured inside the samples between 58 C (resp. 48 C) and 62 C (resp. 52 C) throughout the test. In order to avoid overheating, FWTT heating is stopped and temperature chambers are opened manually in case of overheating risk. This is the reason for the oscillating T ext, T int curves of the example shown in Figure 3a). The anti-sticking procedure consist of lubricating the tire with a highly greasy soap, a mixture of Glycerized Sodium Oleate GSO, and a sulfurized cooking paper laid on the specimen and also pasted with GSO. Details of both procedures are described in detail by Gabet et al. (2011). Figure 3 a) clearly shows that it was quite difficult to apply the anti-overheating procedure and that sticking may have considerable influence on the rutting behavior. In this case anti-sticking procedure reduced rut depth after cycles by about 30%. On the other hand, when comparing the results of all the labs according to Figure 3 b), analysis has shown that such antisticking procedure had statistically no influence on the results of the performed campaign. This led to the conclusion that anti-sticking procedure may only be necessary in cases of stronger sticking effects. As for the anti-overheating procedure, it was found that strictly respecting this procedure is quite difficult, as it requires constant actions in the laboratory and may increase test duration by more than two weeks. Hence, improvement of the apparatus by automatic thermal regulation with an additional cooling system appears necessary.

5 Figure 3. a) Rut depths and temperatures measured inside the samples with and without anti-sticking procedure as a function of the number of cycles as determined by lab 7; b) Mean values and standard deviations of the results from the different labs. 3 INTERLAYER BOND PROPERTIES 3.1 Background and Methodology Interlayer bond between pavement layers is a key factor in achieving the intended functionality and long term performance of pavements. Sufficient interlayer bond is necessary to meet performance challenges and increasing requirements for bearing capacity, durability and environmental compatibility. Hence, it is not surprising that interlayer shear behavior has recently attracted intensified research interest worldwide (Stöckert (2001), Canestrari et al. (2005), Kruntcheva et al (2006), Raab et al. (2009), Mohammad et al (2009), Collop et al (2009)) and has also become one focus of the RILEM asphalt testing evaluation activities. In order to compare the different test procedures for assessing the interlayer shear bonding properties of asphalt pavements, an inter-laboratory test was organized (Piber et al 2009) with material form a specially constructed trial section near Ancona (Italy). The aim of this interlaboratory test was twofold: Determine the repeatability and reproducibility of interlayer shear tests proposed by the majority of the different participants. Determine correlations between different test procedures and evaluate the influence of different test conditions. The pavement of the newly constructed trial section was composed of two asphalt concrete layers according to EN with 70/100 bitumen, i.e. a lower layer of 70mm thick AC 16 covered by a 30mm thick surface layer of AC11. Three different interface conditions were chosen. The first pavement was laid without interface treatment and the others with a precoating of a SBS polymer modified and a conventional cationic emulsion respectively. Fourteen laboratories from eleven countries participated in this study and carried out shear tests on 150mm diameter specimens at different temperatures and loading speeds with a pure shear device according to Leutner (1979). Direct shear tests with normal load and 100mm diameter specimens were also conducted (Canestrari et al. (2005)). The maximum shear load and the corresponding displacement were measured and the shear bond strength was calculated as a function of the following parameters: diameter, test temperature, test speed, stress applied normal to the interface and age of the specimen.

6 3.2 Results and discussion The evaluation of the precision showed that the quality of the data within each laboratory (repeatability standard deviation s r ) and among different laboratories (reproducibility standard deviation s R ) were closely related to the mean values of the shear bond strength τ regardless of specimen diameter, test speed and temperature. It was found that the precision of the shear test in case of an absolutely homogeneous area of the trial section was: Repeatability standard deviation: s r = 0.05 τ (MPa) Reproducibility standard deviation: s R = 0.12 τ (MPa) Figure 4a) presents the mean shear strength from 616 cores of all three pavements tested at a speed of 50 mm/min and temperatures of 10 C, 20 C and 30 C. It follows, that the shear strength of 100 mm cores is higher than the shear strength of 150 mm cores, here about 14%. The results show that the presence of emulsion reduces the scatter of shear strength at the interface, since R 2 was lowest in case of no emulsion (R 2 =0.86) whereas for modified and cationic emulsion R 2 was 0.99 and 0.94 respectively. In addition, according to Figure 4.b), for uniform interface roughness regardless of the chosen interface treatments and, the interlayer shear strength appears to follow the same temperature dependency with only little difference between the chosen emulsions but significant improvement by a factor of 2.5 when using emulsions. b) Figure 4. a) Influence of the diameter on shear strength for 50mm/min at 10 C, 20 C and 30 C; b) Influence of temperature on shear strength for 50mm/min and D=150mm 4 COMPACTION 4.1 General background and methodology Compaction methods influence the mechanical properties of asphalt mixes by creating variation in the internal material structure and therefore in their mechanical response. Characterizing structural variation is key to both mix design and understanding differences in material behavior in the field and in the lab. Hence, inter-laboratory studies regarding the effect of compaction methods on the structure and air void distribution in asphalt were conducted with thirteen laboratories from eight countries using image processing and analysis software that was newly developed for this purpose and allowed analyzing pictures taken by ordinary flatbed scanner or digital cameras. Comparison of steel sector compactor (TU Braunschweig), Marshall hammer, Hveem kneading compactor and Superpave gyratory compactor was performed on a mixture produced by LCPC in France (now IFSTTAR) for construction of a field testing section.

7 Imaging analysis software was developed as collaboration between the University of Wisconsin at Madison and Michigan State University. It analyzes two dimensional images determining frequency of angular occurrence of aggregates and fitting also a harmonic function to aggregate orientation following a procedure proposed by Tashman et. al. (2001). Figure 5a) shows a standard histogram produced by dividing individual aggregate orientation into 10 degree intervals. It also shows the same frequency of occurrence per angle represented as line graph rather than a bar chart with the addition of a harmonic fit. This fit clearly identifies the predominant aggregate orientation angle δ within the image, defined by horizontal location of the maximum of the sinusoidal wave. It also characterizes the intensity of anisotropy, defined by the amplitude A. Both characteristic values may be used for comparing the different compaction methods as shown in Figure 5 b) 4.2 Results and discussion According to the fits of harmonic function in Figure 5 b), distinct differences in aggregate orientation from various compaction methods can be observed. The Gyratory compactor showed the highest tendency to anisotropy, followed by the steel sector compactor, while the Marshall hammer showed the smallest A value and was closest to isotropy of the four methods. Field compaction was assessed at two locations in the vertical plain of the pavement cross-section. From the curves F10 and F20, it can be seen that the predominant aggregate orientation δ was quite similar in both cases and closest to the Gyratory and Hveem compactor. However, the comparison between the different compaction methods was not conclusive in terms of the amplitude A, since both field locations showed quite different intensities of anisotropy. This result demonstrates difficulties in producing homogeneous compaction in the field. Figure 5. a) Conversion from histogram representation to harmonic fit; b) Aggregate orientation histograms as line graph for different lab compactors in comparison to two field samples of the same pavement 5 CONCLUSIONS Results with wheel tracking tests and interlayer shear bond demonstrate that bituminous mixtures must be assessed and evaluated not only with respect to the materials but also with respect to their structural function and spatial characteristics within the pavement system. In order to establish conclusive links between laboratory investigations and field performance it is necessary to consider both mechanical and structural properties. Inter-laboratory testing, evaluation and knowledge exchange are indispensible for reducing the risk of errors and

8 misinterpretation. This is particularly true for new and innovative methods with little experience. Some of the main conclusions of the joint RILEM TC 206 ATB effort read as follows Inter-laboratory rutting tests with different wheel tracking test devices (WTT) clearly demonstrated that a test method that may be meaningful for asphalt concrete types of material may not be conclusive a priori for other types of materials, such as asphalt mastic, and must be carefully validated before extrapolated use in order to avoid misleading conclusions regarding real life pavement performance. The loading conditions of the small wheel size rutting test devices seem to be more severe and produce faster rutting than FWTT. Hence, rutting curves from small and large WTT devices are noticeably different. Precision of the interlayer shear test according to Leutner appears closely related to the mean values of the shear bond strength τ ; for a homogeneous interface area standard deviation repeatability was s r = 0.05 τ and reproducibility standard deviation was s R = 0.12 τ Shear strength of 100 mm diameter cores may be 14% higher than of 150 mm cores Interlayer treatment with emulsions may increase the interlayer shear bond strength by a factor of 2.5 regardless of the temperature 2-D imaging techniques can successfully and reliably be used to characterize aggregate structure of asphalt concrete mixtures and for characterizing the effect of compaction methods on the structure of the materials. However, due difficulties in producing homogeneous compaction in the field, additional studies will be required to determine which of these laboratory compaction methods most accurately reproduces the internal structure parameters seen in field compacted specimens. 6 REFERENCES Canestrari, F., Ferrotti, G., Partl, M.N. Santagata, E. (2005) Advanced Testing and Characterization of Interlayer Shear Resistance, Transportation Research Record No. 1929: Collop AC, Sutanto MH, Airey GD, Elliot RC (2009) Shear Bond Strength between Asphalt Layers for Laboratory Prepared Samples and Field Cores, Construction & Building Materials 23(6): Gabet T., Di Benedetto H., Perraton D., De Visscher J., Gallet T., Bankovski W., Olard F., Grenfell J., Bodin D., Sauzéat C., (2011) French wheel tracking round robin test on a polymer modified bitumen mixture, RILEM Materials and Structures, 2011, DOI /s x, p15 Kruntcheva, M. R; Collop A. C.; Thom, N. H. (2006) Properties of asphalt concrete layer interfaces, J. of Materials in Civil Engineering 18 (3): p Leutner, R. (1979). Untersuchungen des Schichtenverbunds beim bituminösen Oberbau, Bitumen 3: 84 91(in German) Mohammad, L. N., Bae A., Elseifi, M. (2009) Effect of Tack coat materials and application rate on the interface shear strength, Proc. of Int. Conf. Mairepav6, Vol II, Torino, Italy: Perraton D., Di Benedetto H., Sauzéat C., De La Roche C., Bankowski W., Partl M. & Grenfell J., (2011) Rutting of Bituminous Mixtures: Wheel Tracking Tests Campaign Analysis, RILEM Materials and Structures, [DOI /s y], Accepted 2010 Piber, H., Canestrari, F., Ferrotti, G., Lu, X., Millien, A., Partl, M.N., Petit, C., Phelipot-Mardelle, A., Raab, C. (2009) RILEM Inter-laboratory Test on Interlayer Bonding of Asphalt Pavements, Proc. of 7th Int. RILEM Symp. ATCBM09, Vol 2, Rhodes, Greece: Raab, C., Partl, M.N. Abd El Halim A. O. (2009): Evaluation of Interlayer Shear Bond Devices for Asphalt Pavements, Baltic J. of Road and Bridge Engineering. Vol 4 No 4: Stöckert, U. (2001). Schichtenverbund Prüfung und Bewertungshintergrund, Straße+Autobahn 11: (in German) Tashman, L., Masad, E., Peterson, B., and Saleh, H. (2001). "Internal Structure Analysis of Asphalt Mixes to Improve the Simulation of Superpave Gyratory Compaction to Field Conditions," Journal of the Association of Asphalt Paving Technologists, Vol. 70: