Mech Time-Depend Mater DOI 10.1007/s11043-016-9300-5 O R I G I N A L A RT I C L E
Black curves and creep behaviour of crumb rubber modified binders containing warm mix asphalt additives Juan Gallego1 · Ana María Rodríguez-Alloza2 · Felice Giuliani3
Received: 25 February 2016 / Accepted: 3 March 2016 © Springer Science+Business Media Dordrecht 2016
Abstract Warm mix asphalt (WMA) is a new research topic in the field of road pavement materials. This technology allows lower energy consumption and greenhouse gas (GHG) emissions by reducing compaction and placement temperatures of the asphalt mixtures. However, this technology is still under study, and the influence of the WMA additives has yet to be investigated thoroughly and clearly identified, especially in the case of crumb rubber modified (CRM) binders. In order to study the effect that different types and quantities of organic waxes have on the high and intermediate temperature properties of 15 % and 20 % CRM binders, a dynamic shear rheometer (DSR) was used. Using Black diagrams, the rheological behaviour of the binders for the defined range of test temperature and frequency are summarised in a single diagram. In this way, a preliminary evaluation of the rheological behaviour in the extended domain of time and temperature can be attained as well as the effectiveness of the time– temperature superposition principle (TTSP) on the materials under study. Creep tests were also performed in order to evaluate the differences regarding mechanical response due to the addition of rubber and WMA additives, and particularly the ability to recover the strain at high temperatures. The results of this study reveal that these binders do not conform to the Time Temperature Superposition Principle (TTSP) and their rheological behaviour is strongly affected by the interaction of waxes and bituminous matrix and thus generally exhibited a higher elasticity compared to the corresponding control binder. The creep test results carried out proved the enhancement of elasticity and the resistance to permanent deformation produced by the addition of waxes. The WMA additives significantly lower the maximum deformation when compared to the control binders and slightly increased their elastic recovery.
B J. Gallego
[email protected]
1
Department of Civil Engineering: Transport, Technical University of Madrid (UPM), C/ Profesor Aranguren 3, 28040 Madrid, Spain
2
Department of Civil Engineering: Construction, Infrastructure and Transport, Technical University of Madrid (UPM), C/ Alfonso XII 3, 28014 Madrid, Spain
3
Department of Civil and Environmental Engineering and Architecture, University of Parma, Parco Area delle Scienze, 181/A, 43124 Parma, Italy
Mech Time-Depend Mater
Keywords Road materials · Pavement · Crumb rubber · Warm mix asphalt · Additive · Dynamic shear rheometer (DSR) · Phase angle · Black curves · Creep tests
1 Introduction Although hot mix asphalt (HMA) pavements containing crumb-rubber modified (CRM) binders offer an improved resistance to rutting, fatigue and thermal cracking and other benefits (Huang et al. 2002; Shen et al. 2005), these mixtures present one major drawback: the manufacturing temperature is higher compared to conventional asphalt mixtures and, therefore, greater amounts of greenhouse gas (GHG) emissions are produced. Warm mix asphalt (WMA) technology offers a solution to this drawback thanks to the use of additives which are able to guarantee lower viscosity of bitumen at mix production temperatures and, therefore, the energy consumption and the GHG emissions can be reduced (Hurley and Prowell 2005). Hence, asphalt mixtures containing CRM binders and also those with a higher content of rubber, denominated asphalt rubber (AR) mixtures with WMA additives might be an excellent, environmentally-friendly material for road construction. The influence of organic additives on CRM binders has not been identified in great detail (Rodríguez-Alloza et al. 2013, 2014) although some studies have been carried out on organic additives in bitumen focused on the crystallisation properties (Lu et al. 2005; Redelius et al. 2002), the determination of the wax content (Edward and Isacsson 2005; Lu et al. 2008), the chemical structure (Lu and Redelius 2006; Rossi et al. 2013), and their influence on bitumen and asphalt performance (Chambrion et al. 1996; Cowley and Fisher 2002; Edwards and Redelius 2003; Petersson et al. 2008; Polacco et al. 2012). The physical properties of bitumen can be measured characterising the rheological behaviour of the material. The stiffness of bitumen is time dependent and the rheology of bitumen is defined by its stress–strain–time–temperature response. Frequently, the basic rheological characteristics of bitumen in road pavement design have been characterised performing different empirical tests like the softening point which has been used to predict the high temperature performance and permanent deformation properties. The test used to predict low temperature behaviour was the Fraass breaking point and ductility test which provides information about the cohesive properties. The penetration and softening point tests are almost completely empirical and therefore do not characterise the viscoelastic behaviour of bitumen. Viscosity tests do not yet provide information on the time dependence of bitumen. These tests are therefore unable to describe the viscoelastic properties needed for a complete rheological characterisation. Modified binders cannot be accurately characterised by these conventional tests as the modification considerably alters the rheological properties. This in turn has led to rheological properties of bitumen to be determined using a dynamic shear rheometer (DSR). The data provided from this test may be presented in different forms including isothermal and isochronal plots which are rheological parameters versus loading time at specific temperatures and versus temperature at specific loading times. Master curves present the interrelationship between temperature and frequency, and the principle used to produce this curve and relate the equivalency between time and temperature is the time–temperature superposition principle (TTSP). When bitumen exhibits a simple rheological behaviour, it is said to be “thermo-rheologically simple” and this occurs with conventional unmodified bitumen. However, this thermo-rheological simplicity is not encountered in the case of modified bitumen.
Mech Time-Depend Mater
In this study, Black diagrams are presented as they do not require manipulation of the rheological data and they provide a quick and robust method. A Black diagram is a graph of the magnitude of the complex modulus (G∗ ) versus the phase angle (δ) obtained with a dynamic test. In these diagrams the frequency and the temperature are eliminated from the plot, allowing the dynamic data to be presented in a unique plot without the need to perform TTSP manipulations. A smooth curve in a Black diagram is an indicator of time–temperature equivalency. A disjointed curve indicates a breakdown of TTSP and the presence of either a high wax content bitumen, a highly asphaltene structured bitumen or a highly polymer modified bitumen (Lesueur et al. 1996; Planche et al. 2002). Creep tests were also performed to complete the rheological description of the materials. These tests can be static or repeated for a fixed number of cycles in order to evaluate the resistance to non-reversible deformation and the existence of elastic recovery properties of asphalts under traffic conditions. These tests were carried out to evaluate the differences in the mechanical response due to the addition of rubber and WMA additives, especially regarding the ability to recover the strain at high temperatures. The static methodology with reduced application time was considered to be the most adequate. Static tests involve the imposition of a step change in stress and the observation of the subsequent development in time of the strain; the stress level applied is constant and can be also increased for successive tests. The increasing shear stress enables the evaluation of the binders mechanical behaviour and their comparison; the resistance to non-reversible deformation is identified as the binders ability to contrast the propagation of the viscous flow and their behaviour can then be described by separating the delayed elastic phenomena from the effective residual deformation (Giuliani and Merusi 2010). When the material shows a completely Newtonian behaviour, there is no elastic recovery. With the addition of modifiers, the elastic properties can be improved and the delayed elastic component becomes more relevant. This depends on the capacity of the binders to store and release energy when a load is applied and then removed, which determines the consequent binders ability to recover from the strain developed during the creep phase. On the basis of this concept, the discrimination between what is actually lost and what is storage and recoverable (in a deferred time) is a consequence of the accumulation of non-reversible deformation and of the distribution of the delayed elastic and essential viscous components in binders mechanical response. Consequently, the topic of this article is the evaluation of the mechanical behaviour of CRM binders containing WMA additives. The addition of the WMA additives is indeed potentially beneficial for lowering manufacturing temperatures of CRM binders. As a result, it is necessary not only to control the rheological variations of these types of binders and avoid compromising the asphalt mixtures workability but also to identify the elastic response and resistance to permanent deformations in order to obtain the best pavement design.
2 Materials and test program The following sections describe the materials used throughout the whole investigation.
2.1 Virgin binder The virgin binder used in this study is a 50/70 penetration grade bitumen. It is widely used to produce asphalt mixtures at conventional temperatures. Table 1 summarizes the basic
Mech Time-Depend Mater Table 1 Characteristics of the 50/70 virgin binder
Table 2 Gradation of crumb rubber
Table 3 Thermogravimetric analysis of crumb rubber
Properties
Unit
Test results
Penetration (25 °C)
0.1 mm
55.4
Softening point
°C
51.1
Composition
Unit
Test results
Asphaltenes
(%)
13.8
Saturates
(%)
9.7
Naphthene-aromatic
(%)
48.5
Aromatic-polar
(%)
28.0
Sieve (mm) (UNE 933-2) Accumulated (%) 2.0
100
1.5
100
1.0
100
0.50
94.1
0.250
23.7
0.125
3.7
0.063
0.4
TGA Plasticizer + additives (%)
Rubber 4.67
Polymer (rubber) (%)
57.41
Carbon black (%)
32.22
Ash (%)
6.02
specifications of the virgin binder. The penetration grade was assessed according to UNEEN 1426 (Bitumen and bituminous binders—Determination of needle penetration) while the Softening Point was measured according to UNE-EN 1427:2007 (Bitumen and bituminous binders—Determination of the softening point—Ring and ball method). The bitumen was also subjected to a fractionation analysis as specified in the NLT 373/94 standard.
2.2 Crumb rubber modifier The crumb rubber modifier was manufactured by mechanical grinding at ambient temperature (50 % from truck tyres and 50 % from car tyres) and to ensure consistency, only one batch of crumb rubber was used in this study. The gradation of the crumb rubber is provided in Table 2 and the thermo-gravimetric analysis in Table 3, both provided by the supplier.
2.3 WMA additives The WMA technologies can be classified in three groups: organic additives, chemical additives and foaming processes. In this study, only one organic additive was chosen in order to
Mech Time-Depend Mater Table 4 Binders name and their composition
Binder name Bitumen/Rubber (%) Additive (%) Additive name B
100/0
0
–
B15
85/15
0
–
B15 + 2S
85/15
2
Sasobit
B15 + 4S
85/15
4
Sasobit
B15 + 2L
85/15
2
Licomont BS 100
B15 + 4L
85/15
4
Licomont BS 100
B20
80/20
0
–
B20 + 2S
80/20
2
Sasobit
B20 + 4S
80/20
4
Sasobit
B20 + 2L
80/20
2
Licomont BS 100
B20 + 4L
80/20
4
Licomont BS 100
undertake a thorough research on this specific technology and thus evaluate the performance of the different existing additives as well as carrying out a comparative study. The WMA additives used in this study were Sasobit® and Licomont BS 100® . Sasobit® is a Fischer–Tropsch (F-T) wax which is produced by treating hot coal with steam in the presence of a catalyst. It is a long-chain aliphatic hydrocarbon wax with the melting range between 85 °C and 115 °C, high viscosity at lower temperatures and low viscosity at higher temperatures. Licomont BS 100® is a fatty acid amide manufactured synthetically by reacting amines with fatty acids which melts between 141 °C and 146 °C (D’Angelo et al. 2008). The study of the binders included a rheological characterisation at high and intermediate temperatures. 15 % and 20 % weight of rubber was added to the 50/70 net bitumen in order to obtain the CRM binders. The dosage rates of the WMA additives referred to the bitumen weight were 2 % and 4 %. All the binders and the names referred to hereafter are listed in Table 4. The binders labelled B, B15 and B20 will be referred to as control binders as they do not contain WMA additives.
2.4 Preparation of CRM binders containing WMA additives An oil bath with a maximum temperature of 225 °C, a mixer with a maximum velocity of 15,000 rpm, fitted with a propeller agitator and a one-litre metal container for mixing was used for the preparation of the binders. The oil bath has a temperature probe which can be introduced into the mixing receptacle, allowing the temperature of the binder to be controlled with a precision of ± 1 °C. A bitumen sample of 750 g was heated at 140 °C and then placed in the oil bath. WMA additives were carefully added to the bitumen and the blends were subsequently mixed for 15 minutes at 4000 rpm, ensuring that the additive was properly incorporated into the binder. The blend was then heated to 185 °C, and the crumb rubber was added. The mixture was blended for 30 minutes at 2000 rpm then for another 30 minutes at 900 rpm at a constant temperature of 185 °C. Reheating and homogenisation were carefully carried out at a controlled temperature in order to obtain reproducible results (Anderson et al. 2000). Special attention was then paid to the thermal history and storage conditions of the test samples before testing (1 h at 25 °C ± 0.5 °C) because of their influence on rheological measurements (Soenen et al. 2006).
Mech Time-Depend Mater Table 5 Temperature shear stress couple
Test T a (°C)
Shear stress (Pa)
−10
40,000
0
20,000
10
10,000
20
5000
30
2000
40
1000
50
500
60
200
70
100
80
50
2.5 Rheological tests Frequency sweeps performed at different temperatures provide information about the trend of the complex modulus and the phase angle in function of frequency: they are dynamic tests which consist in the application of a fixed shear stress at a constant temperature while the angular frequency changes between 1 and 100 rad/s. The shear stress applied depends on the test temperature and must belong to the linear region for the analysed binder: in order to obtain significant results, shear stresses defined in the literature for similar tests were assumed and confirmed by the results of stress sweep tests. It was decided to establish the change from linear to non-linear region when the storage modulus G decreases 5 % in relation to its initial value, according to SHRP Superpave. The chosen test temperatures and shear stresses, listed in Table 5, were kept the same for all the binders. The frequency sweeps were completed from 1 to 10 Hz in the temperature range between −10 °C and 80 °C at 10 °C intervals. The plate–plate configuration, 25 mm diameter and 2 mm gap sample geometry was used. The frequency sweeps were carried out inside the linear viscoelastic region of the studied asphalts. The change of the plate’s geometry, from 8 mm diameter to 25 mm diameter, was completed between 20 °C and 30 °C, as recommended in the literature, in order to obtain results which best fit the real trend (CEN 2006) limiting the measurement error of the sensor due to the influence of the torsional stiffness of the sample at low temperatures. Regarding the creep tests, the strain curves obtained for different materials at different stress levels are not comparable in the same diagram because they depend on the shear stress applied, and the comparison is therefore only valid for the same shear stress value. The tests methodology is divided in two phases: a first phase where the shear stress is applied on the specimen for a time period of 10 seconds and a second phase where the shear stress is removed but the strain is still recorded for the next 30 seconds, in order to evaluate the elastic recovery of the binders. The tests are carried out using the DSR in the stress-controlled mode. Table 6 summarizes the test set. Tests were repeated at a temperature of 40 °C, which represent the current conditions of exposure of a road pavement. The results obtained are described in the following sections. Even though the tests were made at three different shear stresses (100, 1000 and 10,000 Pa), it was decided to plot the results at 100 Pa exclusively and in one figure to simplify and avoid repetitive results. All the rheometrical measurements were repeated three times for each sample.
Mech Time-Depend Mater Table 6 Creep test set
Phase
Duration [s]
Point of measure numbers [−]
Measurement point [s]
1 creep
10
20
0.5
2 recovery
30
60
0.5
Fig. 1 Black curves for different control binders
3 Results and discussion 3.1 Black curves All the frequency sweep tests and related Black curves showed in the paper are representative of three independent experiments that offered the same results. Figure 1 demonstrates that with higher rubber content, the Black curves lose their smooth trends. In fact, it can be observed that there are “waves” and discontinuities in the diagrams for B15 and B20. As previously explained, this is associated with the changes in the microstructures within the binders. The CRM binders show an inflection, which indicates the changes in the behaviour closely due to the presence of crumb rubber. Although the values still remain continuous with one another, based on the results shown, we should admit that the TTSP is not respected. It may also be observed that, as the rubber content increases, the phase angle becomes lower, attributing higher elasticity to the CRM binders and, at the same time, deformations become less time-dependent, highlighting a rubbery plateau. Figures 2, 3 and 4 show the Black curves for binders with 15 % rubber. Figures 5, 6 and 7 show the Black curves for 20 % CRM binders, presenting the influence of each WMA additive with increasing contents. At lower temperatures even modified binders present high stiffness, and the behaviour tends to be similar to the bituminous binder.
Mech Time-Depend Mater
Fig. 2 Black curves of B15 with Sasobit®
Fig. 3 Black curves of B15 with Licomont BS 100®
Binders with 15 % CRM and 20 % CRM, in particular, show a predominant solid-like behaviour due to the creation of an internal network between rubber particles and asphaltenes. The rubber particles are indeed relevant in number, and when they swell by absorbing the
Mech Time-Depend Mater
Fig. 4 Black curves for different 15 % CRM binders
Fig. 5 Black curves of B20 with Sasobit®
aromatic oils and the light bitumen fraction, they interact with the asphaltenes giving a rubbery behaviour to the blend. Regarding both additives, it can be observed that as the content of wax increases, the trend of Black curves does not change after the CRM modification, and they do not lose
Mech Time-Depend Mater
Fig. 6 Black curves of B20 with Licomont BS 100®
Fig. 7 Black curves for different 20 % CRM binders
their smooth trends. The addition of the WMA additives also decreases the phase angle, and so these binders exhibit a higher elasticity compared to the control binders B15 and B20. In the case of Sasobit® , the reduction of the phase angle at the high temperatures is inferior.
Mech Time-Depend Mater
Fig. 8 Creep tests for different control binders at 40 °C
This behaviour can be explained by the different microcrystalline structure of organic additives and the reduced melting range of amide groups with intervals between 120 °C and 155 °C (Licomont BS 100® ) compared to paraffinic chains that have a melting range detectable at 40 °C (Sasobit® ). The presence of the wax enhances the highly structured blend matrix because it crystallises and increases the cohesion between particles. It determines a shift from sol-type to gel-type behaviour of the bituminous binders. The explanation of this behaviour should include not only the chemistry of the wax molecules but also the shape and dimension of the crystals (Polacco et al. 2012). At high temperatures, when the bituminous matrix becomes liquid, the wax crystals and asphaltene are able to move and re-arrange, forming a better structured network. This confirms the solid-like behaviour observed in the Black diagrams, offering the marked shape of Black curves especially when 4 % of Licomont BS 100® was added. Sasobit® and Licomont BS 100® have different melting temperatures and different residual crystallinity after being mixed with the bitumen. This is directly reflected in the crystal content and therefore in the rheological effects. However, we already observed that the inapplicability of the TTSP is not necessarily due to the wax melting, but primarily related to changes in the structures built by the solid phase inside the bituminous matrix.
3.2 Creep-recovery test at 40 °C All the creep-recovery tests shown in the paper are representative of three independent experiments that gave the same results. Figure 8 shows the strain versus time diagram for the control binders when a 100 Pa shear stress is applied at 40 °C. In Table 7, the values of the final strain ratio (γr ), the maximum strain value (γmax ) and the elastic recovery ratio (γr /γmax ) are presented for the control binders. As may be observed, the addition of rubber to the base bitumen improves the elastic component behaviour of the binders. The strain is much lower for the binders containing rubber, and they show a more significant elastic recovery ratio. The difference between the elastic recovery of the B15 and B20 is insignificant at this temperature. As for the base
Mech Time-Depend Mater Table 7 Elastic recovery ratios for control binders at 40 °C
Binder
γr [%]
γmax [%]
γr /γmax [–]
B
7.4
8.1
0.91
B15
0.2
0.7
0.31
B20
0.1
0.3
0.33
Fig. 9 Creep tests for 15 % CRM binders with WMA additives at 40 °C Table 8 Elastic recovery ratios for 15 % CRM binders at 40 °C
Binder
γr [%]
γmax [%]
γr /γmax [–]
B15
0.2
0.7
0.31
B15 + 2S
0.2
0.5
0.31
B15 + 4S
0.0
0.2
0.24
B15 + 2L
0.2
0.5
0.37
B15 + 4L
0.1
0.3
0.26
bitumen, almost all the deformation energy is dissipated in internal friction or it is used to deform the liquid matrix of the bitumen and mainly produces a non-reversible flow. The diagram of strain as a function of time was also plotted for the CRM binders with the selected WMA additives in order to evaluate the changes of the resistance to non-reversible deformation due to the addition of waxes. In Fig. 9, the creep tests for 15 % CRM binders can be observed, and in Table 8, their respective elastic recovery ratios are listed. It may be noted that with the addition of the waxes, the maximum deformation becomes lower compared to the control binder B15. With the addition of 4 % of any of the waxes, the highest level of elastic recovery is achieved. Figure 10 shows the diagrams of strain as a function of time for the 20 % CRM binders with WMA additives, and Table 9 provides the summaries of the final and maximum strain value as well as the elastic recovery ratios. It can be observed that the addition of the waxes shifts the values towards zero and increases the elastic component compared to the control binder B20. The addition of 4 %
Mech Time-Depend Mater
Fig. 10 Creep tests for 20 % CRM binders at 40 °C Table 9 Elastic recovery ratios for 20 % CRM binders at 40 °C
Binder
γr [%]
γmax [%]
γr /γmax [–]
B20
0.1
0.3
0.33
B20 + 2S
0.0
0.2
0.23
B20 + 4S
0.0
0.1
0.28
B20 + 2L
0.1
0.2
0.21
B20 + 4L
0.1
0.2
0.31
Sasobit® provides a higher elastic recovery compared to the binder with 4 % Licomont BS 100® (Table 9). In terms of maximum strain ratio, the binder with 4 % Licomont BS 100® presents the lowest value.
4 Conclusions A dynamic shear rheometer (DSR) was used to investigate the effect of different types and quantities of WMA additives on the high and intermediate temperature properties of 15 % and 20 % CRM binders. A preliminary evaluation of the rheological behaviour in the extended domain of time and temperature was attained with the Black diagrams as the rheological behaviour of the binders for the defined range of test temperature and frequency were summarised in a single diagram. Creep-recovery tests were also performed in order to evaluate the differences of the mechanical response due to the addition of rubber and WMA additives, especially regarding the ability to recover the strain at high temperatures. From these test results, the following conclusions were drawn for the binders created: Black curves did not show the standard smooth trend, and the presence of rubber and wax caused the formation of inflections and disjoint features. This is in line with the fact that these binders to not conform the Time Temperature Superposition Principle TTSP. The
Mech Time-Depend Mater
inapplicability of the TTSP is not necessarily due to the wax melting, but is mainly related to changes in the structures built by the solid phase inside the bituminous matrix. The creep test results carried out to determine the recovery from a creep loading, confirmed once more the improvement of the elasticity and the resistance to permanent deformation produced by the addition of the waxes. With the increased content of rubber, the deformations are lower; this implies that the CRM binders are less deformable and have a better resistance to non-reversible deformations. The presence of the rubber particles in the binder leads to stronger elastic properties and shifted the liquid-like behaviour of the base bitumen towards a more predominant solidlike behaviour. The WMA additives significantly lowered the maximum deformation and increased the elastic recovery when compared to the control binders and will have a significant improvement regarding resistance to permanent deformations in road pavements.
References Anderson, D., Marasteanu, M., Mahoney, J., Stephens, J.: Factors affecting the variability in the SHRP binders tests. In: Proceedings of the 79th Annual Meeting of the Transportation Research Board (2000) CEN EN 14770. Bitumen and bituminous binders—determination of complex shear modulus and phase angle—dynamic shear rheometer (DSR) (2006) Chambrion, P., Bertau, R., Ehrburger, P.: Characterization of bitumen by differential scanning calorimetry. Fuel 75(2), 144–148 (1996). doi:10.1016/0016-2361(95)00178-6 Cowley, L.G., Fisher, R.G.: Use of a bitumen/wax composition. International Patent International Publication Number: WO 02/16499 A1 (2002) D’Angelo, J., et al.: Warm-mix asphalt: European practice report no FHWA-PL-08-007 (2008) Edward, Y., Isacsson, U.: Wax in bitumen. Part 1: classifications and general aspects. Road Mater, Pavement Des. 6(3), 281–309 (2005) Edwards, Y., Redelius, P.: Rheological effects of waxes in bitumen. Energy Fuels 17(3), 511–520 (2003). doi:10.1021/ef020202b Giuliani, F., Merusi, F.: Advanced rheological characterisation of non-reversible deformation and fatigue behaviour in crumb rubber modified asphalt binders. Road Mater, Pavement Des. 11, 197–224 (2010) Huang, B., Mohammad, L.N., Graves, P.S., Abadie, C.: Louisiana experience with crumb rubber-modified hot-mix asphalt pavement. Transp. Res. Rec., J. Transp. Res. Board 1789, 1–13 (2002) Hurley, G., Prowell, B.D.: Evaluation of Sasobit® for use in warm mix asphalt. NCAT report (2005) Lesueur, D., Gerard, J.F., Claudy, P., Letoffe, J.M., Planche, J.P., Martin, D.: A structure-related model to describe bitumen linear viscoelasticity. In: Proceedings of the Eurasphalt & Eurobitume Congress, Session 5, Binders-Functional Properties and Performance Testing, E&E.5.114, Strasbourg (1996) Lu, X., Redelius, P.: Compositional and structural characterisation of waxes isolated from bitumens. Energy Fuels 20(2), 653–660 (2006). doi:10.1021/ef0503414 Lu, X., Langton, M., Olofsson, P., Redelius, P.: Wax morphology in bitumen. J. Mater. Sci. 40(8), 1893–1900 (2005). doi:10.1007/s10853-005-1208-4 Lu, X., Kalman, B., Redelius, P.: A new test method for determination of wax content in crude oils, residues and bitumens. Fuel 87(8–9), 1543–1551 (2008). doi:10.1016/j.fuel.2007.08.019 Petersson, M., Gustafson, I., Stading, M.: Comparison of microstructural and physical properties of two petroleum waxes. J. Mater. Sci. 43(6), 1869–1879 (2008). doi:10.1007/s10853-007-2417-9 Planche, J.P., Lesueur, D., Hines, M.L., King, G.N.: Evaluation of elastomer modified bitumens using SHRP binder specifications. In: Proceedings of the Eurasphalt & Road Materials and Pavement Design, vol. 3 (2002) Polacco, G., Filippi, S., Paci, M., Giuliani, F., Merusi, F.: Structural and rheological characterisation of wax modified bitumens. Fuel 95, 407–416 (2012). doi:10.1016/j.fuel.2011.10.006 Redelius, P., Lu, X., Isacsson, U.: Non-classical wax in bitumen. Road Mater, Pavement Des. 3(1), 7–21 (2002). doi:10.1080/14680629.2002.9689913 Rodríguez-Alloza, A.M., Gallego, J., Pérez, I.: Study of the effect of four warm mix asphalt additives on bitumen modified with 15 % crumb rubber. Constr. Build. Mater. 43(0), 300–308 (2013). doi:10.1016/ j.conbuildmat.2013.02.025 Rodríguez-Alloza, A.M., Gallego, J., Pérez, I., Bonati, A., Giuliani, F.: High and low temperature properties of crumb rubber modified binders containing warm mix asphalt additives. Constr. Build. Mater. 53(0), 460–466 (2014). doi:10.1016/j.conbuildmat.2013.12.026
Mech Time-Depend Mater Rossi, D., Filippi, S., Merusi, F., Giuliani, F., Polacco, G.: Internal structure of bitumen/polymer/wax ternary mixtures for warm mix asphalts. J. Appl. Polym. Sci. 129(6), 3341–3354 (2013). doi:10.1002/ app.39057 Shen, J., Amirkhanian, S., Lee, S.-J.: Effects of rejuvenating agents on recycled aged rubber modified binders. Int. J. Pavement Eng. 6(4), 273–279 (2005) Soenen, H., De Visscher, J., Vanelstraete, A., Redelius, P.: Influence of thermal history on rheological properties of various bitumen. Rheol. Acta 45(5), 729–739 (2006)
