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High temperature properties of wax modified binders and asphalt mixtures
Construction and Building Materials 23 (2009) 3220–3224
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
High temperature properties of wax modified binders and asphalt mixtures Yuksel Tasdemir * Bozok University, Engineering and Architecture Faculty, Yozgat, Turkey
a r t i c l e
i n f o
Article history: Received 12 February 2009 Received in revised form 16 June 2009 Accepted 18 June 2009 Available online 15 July 2009 Keywords: FT-paraffin Polyethylene wax Dynamic mechanical analysis Bending beam rheometer French rutting tester
a b s t r a c t The influence of adding two commercial waxes (FT-paraffin and polyethylene wax) to binders of penetration grades 50/70 and 160/220 were investigated for high temperature performance of binders and asphalt concrete mixtures. Binder properties were determined by conventional test methods, dynamic mechanical analysis and bending beam rheometer testing. The high temperature properties of asphalt concrete were investigated using the French rutting tester. The addition of FT-paraffin and polyethylene wax increased the rutting resistance of mixtures for both types of binder. The FT-paraffin modified asphalt mixtures showed the best rutting resistance. Adding polyethylene wax showed the highest stiffening effect in terms of rutting factor by DMA for the binders. However, this could not be verified in asphalt mixture testing. Adding FT-paraffin improved the rutting resistance of asphalt mixtures containing low or high penetration binder, but for the mixture containing high penetration binder the specification limit was exceeded. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In many cold regions of the world, asphalt concrete pavement failures are associated with low temperature cracking and rutting. However, the mix design criteria that is related to minimized low temperature cracking is often in conflict with the criteria related to maximizing rutting resistance. The asphalt binder grade is the most important factor for low temperature performance of asphalt mixtures [1]. The softer binder has high resistance to low temperature cracking but may cause rutting at high temperatures. Asphalt binders sometimes are modified to improve low as well as high temperature performance of asphalt pavements, for instance by the use of commercial waxes. Examples of commercial waxes are Fischer Tropsch-paraffin (FT-paraffin), montan wax and polyethylene wax. Effects of commercial waxes on binder properties have been discussed for a long time [2–5]. Commercial wax mainly is used as flow improver in asphalt concrete and mastic asphalt. Wax as flow improver has a softening/viscosity decreasing effect on the binder and asphalt mixture at high temperatures (above approximately 80 °C). The main purpose of such an addition is to reduce the mixing temperature of the asphalt (by about 20–40 °C), in order to improve workability, reduce energy consumption and emissions (bitumen fume and aerosol). Below the laying and compaction temperatures, there may be an increase in viscosity due to wax crystallization, which in turn could increase the asphalt pavement resistance to plastic deforma-
* Tel.: +90 354 2421002; fax: +90 354 2421005. E-mail addresses: [email protected], [email protected] 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.06.010
tion. Advocates of this technique are of the opinion that other pavement properties such as crack susceptibility at low temperatures, fatigue resistance and adhesion are not dramatically affected by the use of flow improvers [2,3]. This paper deals with the addition of two commercial waxes to two types of binders of different penetration grade. The main purpose of the study was to investigate the high temperature performance of the asphalt mixtures and binders, but low temperature performance of binders was tested as well, using the bending beam rheometer. 2. Experimental 2.1. Binder and additives In this study, two different penetration grade binders 50/70 and 160/220 were used. They are called LP (low penetration) and HP (high penetration), respectively. The binders were modified for this study by using two types of commercial waxes, FT-paraffin (Sasobit) and polyethylene wax. FT-paraffin and polyethylene wax are denoted as wax S and wax PW, respectively. Wax S is a flow improver, which is used for asphalt concrete and mastic asphalt to reduce the mixing temperature. The polyethylene wax used in this study is normally not used as flow improver or modifier in asphalt mixtures. However, adding wax PW had shown considerable positive effect on the rheological behavior (increased stiffness by dynamic mechanical analysis (DMA)) at medium and higher temperatures in previous studies [6,7], and was therefore chosen for this particular study. Samples were prepared by adding calculated amount of additive to approximately 250 g of binder, after which the sample was heated for 30 min at 155 °C. (Prior to this sample preparation, the original binders were heated in larger buckets for about 3 h at the same temperature and were then, subdivided.) The samples were then placed in preheated blocks and homogenized by shaking for 90 s. The same treatment was applied to the base binders. Levels of 3% and 6% wax by weight of the binder were used. No compatibility problems were observed during mixing.
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Y. Tasdemir / Construction and Building Materials 23 (2009) 3220–3224 2.2. Aggregate
Table 1 Properties of aggregate.
Crushed basalt and crushed limestone were mixed and then used for the preparation of asphalt mixtures. A dense graded asphalt concrete according to Turkish road standards, with maximum aggregate size of 19 mm was prepared. The measured particle size distribution of the aggregate used is shown in Fig. 1. Aggregate properties are shown in Table 1. 2.3. Asphalt mixtures The Marshall method of mix design was used to determine the optimal binder content for the aggregate gradation used in the experiment and it was found to be 5.25% by weight. The mix design was performed only for one unmodified mixture. Compaction was done by a mechanical Marshall compactor with 75 blows per side at 135 °C for all specimens. The calculated and measured mixture properties and their comparison according to design criteria are given in Table 2. As seen from Table 2, all of the properties of mixture are within specification limits for wearing course. In order to make a meaningful comparison of the influence of wax type on permanent deformation, the same optimal binder content was chosen for all mixtures.
Properties
Basalt
Limestone
Content (%) Los Angeles abrasion (%) Sodium sulfate soundness (%) Flatness index (%)
62 11.0 2.0 24.5
38 24.0 2.0 15.6
Coarse aggregate Bulk specific gravity Apparent specific gravity Absorption (%)
2.799 2.894 1.2
2.676 2.709 0.5
Fine aggregate Bulk specific gravity Apparent specific gravity Absorption (%)
2.690 2.737 0.6
2.675 2.718 0.6
Mineral filler Apparent specific gravity
2.743
2.723
2.4. Method of analysis for binders 2.4.1. Conventional tests The standard methods used to characterize the base and wax modified binders include softening point (EN 1427), penetration (EN 1426), dynamic viscosity (EN 12596) and kinematic viscosity (EN 12595). 2.4.2. Dynamic mechanical analysis Dynamic mechanical analysis (DMA) was performed using a rheometer (RDA II, Rheometrics). A temperature sweep with 2 °C increments (from 10 to 100 °C) was applied to characterize the temperature dependence of binder parameters at the frequency of 1 rad/s and variable strains. Parallel plates with diameter 25 mm and gap of 1 mm were used. A sinusoidal strain was applied and the actual strain and torque were measured. Dynamic shear modulus (G*) and phase angle (d) were determined. 2.4.3. Bending beam rheometer test Creep tests were carried out using the bending beam rheometer (TE-BBR, Cannon Instrument Company). Test temperatures used were 15, 20 and 25 °C. A sample beam (125 mm long, 12.5 mm wide and 6.25 mm thick) was submerged into the bath at test temperature for 60 min. A constant load of 100 g was then applied to the binder beam, which was supported at both ends, and the deflection of center point was measured continuously. Creep stiffness (S) and creep rate (m) of the binders were determined at a loading time of 60 s. 2.5. Method of analysis for asphalt mixtures 2.5.1. French rutting tester Rutting was tested using the French rutting tester. This test has been used successfully in France for over 20 years to prevent rutting in asphalt concrete pavements. Researches indicate good correlation between test results of the French rutting tester and actual field performance [8,9]. The French rutting tester was developed by the LCPC (Laboratoires des Ponts et Chaussées) to simulate rutting of flexible pavements. The loading system, mounted within a temperature chamber, consists of a table carrying a steel mold with the specimen (100 mm 180 mm 500 mm) and a treadle pneumatic rubber tire of about 400 mm in diameter and an active width of about 90 mm. The standard tire pressure is 0.60 ± 0.03 MPa. The table and specimens are pressed (pneumatically)
Fig. 1. Particle size distribution of the aggregate used.
with the standard load, 5000 ± 50 N, against the tires which are moved back and forth along linear guiding rails at a maximum travel speed of 1.6 m/s. The test temperature is controlled by a thermocouple, which monitors the temperature of the specimen at a depth of about 30 mm. Two slabs can be tested simultaneously [10]. The specimens were set at room temperature for 12 h before being tested. A pre-test load of 1000 cycles was subjected to the specimens. The thickness of each slab was then calculated by averaging 15 thickness measurements taken at 15 standard positions using a gauge with an accuracy of 0.1 mm. This thickness was considered the original thickness of the slab. The slabs were then heated to the test temperature of 60 ± 2 °C for 12 h. Deterioration measures were carried out after 1000, 3000, 5000, 10,000, 30,000, 50,000 cycles. The test was stopped when the average recorded rut depth, after a series of measures, was higher than 10% and the previous results anticipated a rut depth of more than 15% at the following step. The rutting depth was reported as a percentage of the slab thickness. The percentage was calculated as the average of 15 measurements divided by the original thickness of the slab.
3. Results and discussion 3.1. Conventional binder test measurements Test results are summarized in Table 3. As shown in the table, the addition of both wax S and wax PW reduced penetration and increased softening point and dynamic viscosity. Wax S showed the largest effects. Changes in penetration and softening point were smaller for wax modified LP binders compared to wax modified HP binders, especially when wax S was used. Binder mixtures containing 6% wax S were too stiff to be tested for dynamic viscosity at 60 °C. Kinematic viscosity at 135 °C was by reduced the addition of wax S, indicating that mixing and compaction temperatures could be decreased using this type of wax in the mix. Adding wax PW, however, slightly increased kinematic viscosity and should therefore not be suitable as flow improver. Temperature susceptibility was evaluated using results obtained from the conventional binder tests used in the study. The changing of consistency of the binder with temperature is the general definition of temperature susceptibility [11]. Penetration index (PI) is frequently used as a measure of binder temperature susceptibility, but could give incorrect indications for binders containing wax. Instead, the Penetration Viscosity Number (PVN) has been suggested and used by McLeod [12]. Lower values of PI and PVN indicate higher temperature susceptibility, and asphalt mixtures containing binders with lower temperature susceptibility should be more resistant to cracking and rutting. While all wax modified binders in the study showed less temperature susceptibility compared to the base binders according to PI values, the same binders showed higher temperature susceptibility according to PVN values. This is indicated in Table 3. However, it should be noted that tests of penetration and softening
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Table 2 Summary of Marshall design results. Design parameters
Values
Board specification in Turkey Min.
Max.
Optimal binder content (%) Bulk specific gravity, Gmb Air voids, Pa (%) Voids in mineral aggregate, VMA (%) Void filled with binder, VFA (%) Stability (kg) Flow (mm)
5.25 2.473 3.66 14.60 75 1150 2.80
– – 3 – 75 900 2
– – 5 – 85 – 4
Table 3 Conventional rheological characteristics of binders. Softening point (°C)
Dynamic viscosity@60 °C (Pa s)
Kinematic viscosity@135 °C (mm2/s)
PW PW S S
60 50 40 39 29
49 56 62 74 95
218.6 480.4 817.2 1215.5 TSa
445.1 445.2 532.5 352.2 284.3
0.94 0.19 0.80 2.79 4.79
0.62 0.79 0.76 1.33 1.82
HP 3% PW 6% PW 3% S 6% S
171 121 109 95 68
40 50 52 69 92
52.7 110.3 175.3 232.6 TSa
224.9 224.9 251.3 185.5 156.3
0.86 1.21 1.45 4.58 6.65
0.50 0.91 0.84 1.45 1.99
Binder LP 3% 6% 3% 6%
a
Penetration (d mm)
PI
PVN
Too stiff to be tested.
point are empirical and, in principle, are only valid for unmodified binders. Therefore, PI and PVN may not be relevant indicators in the case of wax modified binders. 3.2. Dynamic mechanical analysis measurements DMA results showed stiffening effects for both binders at medium and high temperatures due to the addition of wax S and wax
Fig. 2. Complex modulus and phase angle as a function of temperature for binder mixtures containing 6% commercial wax.
Table 4 Effects on rutting factor by DMA. Binder
G*/sin d at 60 °C (Pa)
Upper limit temperatures at G*/sin d = 1 (°C)
LP +6% +6% HP +6% +6%
367 1620 796 77 360 152
53.9 64.0 58.7 43.2 52.0 58.7
PW S PW S
PW. Adding wax PW indicated greater effect on complex modulus and phase angle than that of wax S. As shown in Fig. 2, the phase angle decreases the most between 50 and 100 °C for binder samples containing wax PW. A corresponding plateau effect appears for the complex modulus. At temperatures in the range of 45– 85 °C, typical of high pavement service temperatures, the main distress mechanism is rutting, and therefore, dynamic modulus and phase angle are relevant. For rutting resistance, a high complex modulus value is favorable because it represents a higher total resistance to deformation. A lower phase angle is favorable as well because it reflects a more elastic (recoverable) component of the total deformation [11]. The decrement of phase angle is more pronounced for wax PW modified LP binders than that of wax PW modified HP binders. Above 60 °C, the complex modulus of wax PW modified HP binders is the same or higher compared to unmodified LP binders. In the Superpave binder specification, rutting is taken into account using a rutting factor (G*/sin d), which is solely dependent on the rheological properties of the asphalt binder. The higher the rut factor for the binder, the stiffer the asphalt concrete should be and thus more resistant to rutting [13]. G* and d are used in the specification to control rutting by limiting G*/sin d to at least 1.0 kPa for original binder. G*/sin d at 60 °C and the upper limit temperatures (at which G*/ sin d = 1) were determined, and are shown in Table 4. Wax modification increases the G*/sin d values and upper limit temperatures of the binders at the studied frequency. The highest G*/sin d values and upper limit temperatures were achieved by adding wax PW. As indicated in Table 4 and Fig. 2, wax PW modified HP bitumen may have the same rutting resistance as unmodified LP bitumen at high temperatures. 3.3. Creep test Creep tests were carried out using BBR at three different temperatures ( 25, 20 and 15 °C). The limit stiffness temperature (lower limit temperature), LST, at which S = 300 MPa were deter-
Y. Tasdemir / Construction and Building Materials 23 (2009) 3220–3224
Fig. 3. Effect of wax modification on lower limit temperatures using BBR.
mined from BBR results at the three temperatures mentioned above. As shown in Fig. 3, the limit temperatures depend on the grade of the base binder and wax modification did not show any beneficial effect. A minor negative effect was gained when adding wax PW to binder LP and HP. The addition of wax S increased the limit temperatures of both the binders studied. These temperature increments are higher for the wax modified LP binders compared to that of the wax modified HP binders. As also can be seen in Fig. 3, the low temperature performance of wax S and wax PW modified HP binders is better than that of the unmodified LP binders.
3.4. French rutting test French rutting test results for unmodified and wax modified asphalt mixtures are shown in Fig. 4. Unmodified mixtures showed the highest permanent deformation in this test. The addition of wax PW, and especially wax S, improved the rutting resistance of both mixture types. Modified LP binder mixtures showed higher rutting resistance as compared to unmodified LP and HP binder mixtures. Furthermore, the results showed that mixtures containing wax PW did not show expected improved performance, considering the positive effects indicated in binder testing (cf. Section 3.2). The reason for this unexpected performance could be that the polyethylene product used was sensitive to aging (during manufacturing of the asphalt mixtures) and therefore there could be some instability problems [14]. Adding wax S increased the rutting resistance of asphalt mixtures containing LP or HP binders, but as for the wax S modified asphalt mixture containing HP binder, it exceeded the specification limit (>10%).
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The influence of adding wax S and wax PW to binder was investigated with regard to medium temperature performance of the asphalt concrete mixture by Edwards et al. [15]. Edwards et al. performed dynamic creep testing at 40 °C to investigate asphalt concrete performance. The results gained from that study showed that the smallest strains were recorded for the asphalt mixtures containing FT-paraffin, indicating better resistance to rutting, and the largest strains for the mixture containing no additive. Adding wax PW showed considerable positive effect in binder testing by DMA, but this effect was less pronounced in dynamic creep testing on asphalt mixtures, as in this study. Hurley and Prowell [16] reported that the addition of FT-paraffin generally decreases the rutting potential of the asphalt mixes evaluated. They pointed out that the rutting potential generally increased with decreasing mixing and compaction temperatures but the mixtures containing FT-paraffin were less sensitive in this sense compared to unmodified mixtures in terms of rutting. 4. Conclusions Effects on high temperature performance of asphalt mixtures due to the addition of wax S or wax PW to low and high penetration binders were evaluated. Base on the results obtained in this study, the following conclusions were drawn. Adding wax PW indicated greater stiffening effect than adding wax S based on DMA test results, while adding wax S showed greater stiffening effect than wax PW based on conventional binder test results. There was a decrease in kinematic viscosity at 135 °C for wax S modified binders. Indicating a possible decrease in the mixing and compaction temperatures, hence reducing emissions like bituminous fumes. Above 60 °C, the complex modulus of wax PW modified HP binders is the same or higher than for the unmodified LP binder. Also, the low temperature performance of wax S and wax PW modified HP binders is better than that of the unmodified LP binder. Based on DMA and BBR results, it may be concluded that wax PW modified HP binder have higher rutting resistance at high temperatures and better low temperature performance than the unmodified LP binder. Concerning asphalt mixtures, the wax S modified mixtures showed the best rutting resistance, although the wax S modified one, containing HP binder, exceeded the specification limit (>10%). PW modification also improved binder rutting resistance
Fig. 4. Rutting test on asphalt mixtures containing 6% commercial wax.
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Y. Tasdemir / Construction and Building Materials 23 (2009) 3220–3224
when compared to base bitumens, but asphalt mixtures, containing wax PW did not show expected improved performance as indicated from binder testing by DMA. Summarizing, wax modification improved the high temperature performance of asphalt mixtures, indicating that the modification is beneficial if done properly.
Acknowledgements This study is a part of the research supported by Erciyes University Research Fund. (Project No: EUBAP-FBA-06-15). The author would like to thank Erciyes University Research Fund for giving the opportunity to perform this research. The author also expresses appreciation to Ahmet Gürkan Güngör and Fatma Orhan in General Directorate of Highways in Turkey for assisting in rutting measurements. References [1] Vinson TS, Hicks RG, Janoo VC. Low temperature cracking and rutting in asphalt concrete pavements. In: Vinson TS, editor. Roads and airfields in cold regions. New York: American Society of Civil Engineers; 1998. p. 203–48. [2] Damm KW, Abraham J, Butz T, Hildebrand G, Riebesehl G. Asphaltverflüssiger Als ‘Intelligenter Füller’ Für Den Heisseneinbau–Ein Neues Kapitel In Der Asphaltbauweise. Bitumen 2002;1:19–24 [in German]. [3] Damm KW, Abraham J, Butz T, Hildebrand G, Riebesehl G. Asphaltverflüssiger Als ‘Intelligenter Füller’ Für Den Heisseneinbau–Ein Neues Kapitel In Der Asphaltbauweise. Bitumen 2002;1:55–61 [in German].
[4] Damm KW, Abraham J, Butz T, Hildebrand G, Riebesehl G. Asphalt flow improvers as ’intelligent fillers’ for hot asphalts – a new chapter in asphalt technology. J Appl Asphalt Binder Technol 2002;2(1):36–69. [5] Butz T, Rahimian I, Hildebrand G. Modification of road bitumens with the Fischer-Tropsch Paraffin Sasobit. J Appl Asphalt Binder Technol 2001;1(2):70–86. [6] Edwards Y, Tasdemir Y, Isacsson U. Rheological effects of commercial waxes and polyphosphoric acid in bitumen 160/220-low temperature performance. Fuel 2006;85:989–97. [7] Edwards Y, Tasdemir Y, Isacsson U. Rheological effects of commercial waxes and polyphosphoric acid in bitumen 160/220-high and medium temperature performance. Construct Build Mater 2007;21(10):1899–908. [8] Aschenbrener T. Comparison of results obtained from the LCPC rutting tester with pavements of known field performance. Transportation Research Board 1994;1454:66–73. [9] Corte JF, Brosscaud Y, Simonceli JP, Caroff G. Investigation of rutting of asphalt surface layers: influence of binder and axle load configurations. Transportation Research Board 1994;1436:28–37. [10] Federal Highway Administration, Equipment French (Laboratoire Central des Ponts et Chaussées) pavement rutting tester. www.fhwa.dot.gov/pavement/ asphalt/labs/mixtures/frenchr.cfm. [11] Kim YR. Modeling of asphalt concrete. McGraw Hill: ASCE Press; 2009. p. 22–3. [12] McLeod NW. A 4-year survey of low temperature transverse pavement cracking on the three Ontorio roads. J Assoc Asphalt Paving Technol 1972;41:424–93. [13] Tarefder RA. Laboratory and model prediction of rutting in asphalt concrete. Ph.D. Thesis, University of Oklahoma, Oklahoma, USA; 2003. [14] Edwards Y, Tasdemir Y, Isacsson U. Influence of commercial waxes on bitumen ageing properties. Energy Fuels 2005;19(6):2519–25. [15] Edwards Y, Tasdemir Y, Isacsson U. Influence of commercial waxes and polyphosphoric acid on bitumen and asphalt concrete performance at low and medium temperatures. Mater Struct 2006;39(291):725–37. [16] Hurley GC, Prowell BD. Evaluation of Sasobit for use in warm mix asphalt. Auburn, USA: NCAT Report 05–06, National Center for Asphalt Technology; 2005.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
High temperature properties of wax modified binders and asphalt mixtures Yuksel Tasdemir * Bozok University, Engineering and Architecture Faculty, Yozgat, Turkey
a r t i c l e
i n f o
Article history: Received 12 February 2009 Received in revised form 16 June 2009 Accepted 18 June 2009 Available online 15 July 2009 Keywords: FT-paraffin Polyethylene wax Dynamic mechanical analysis Bending beam rheometer French rutting tester
a b s t r a c t The influence of adding two commercial waxes (FT-paraffin and polyethylene wax) to binders of penetration grades 50/70 and 160/220 were investigated for high temperature performance of binders and asphalt concrete mixtures. Binder properties were determined by conventional test methods, dynamic mechanical analysis and bending beam rheometer testing. The high temperature properties of asphalt concrete were investigated using the French rutting tester. The addition of FT-paraffin and polyethylene wax increased the rutting resistance of mixtures for both types of binder. The FT-paraffin modified asphalt mixtures showed the best rutting resistance. Adding polyethylene wax showed the highest stiffening effect in terms of rutting factor by DMA for the binders. However, this could not be verified in asphalt mixture testing. Adding FT-paraffin improved the rutting resistance of asphalt mixtures containing low or high penetration binder, but for the mixture containing high penetration binder the specification limit was exceeded. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In many cold regions of the world, asphalt concrete pavement failures are associated with low temperature cracking and rutting. However, the mix design criteria that is related to minimized low temperature cracking is often in conflict with the criteria related to maximizing rutting resistance. The asphalt binder grade is the most important factor for low temperature performance of asphalt mixtures [1]. The softer binder has high resistance to low temperature cracking but may cause rutting at high temperatures. Asphalt binders sometimes are modified to improve low as well as high temperature performance of asphalt pavements, for instance by the use of commercial waxes. Examples of commercial waxes are Fischer Tropsch-paraffin (FT-paraffin), montan wax and polyethylene wax. Effects of commercial waxes on binder properties have been discussed for a long time [2–5]. Commercial wax mainly is used as flow improver in asphalt concrete and mastic asphalt. Wax as flow improver has a softening/viscosity decreasing effect on the binder and asphalt mixture at high temperatures (above approximately 80 °C). The main purpose of such an addition is to reduce the mixing temperature of the asphalt (by about 20–40 °C), in order to improve workability, reduce energy consumption and emissions (bitumen fume and aerosol). Below the laying and compaction temperatures, there may be an increase in viscosity due to wax crystallization, which in turn could increase the asphalt pavement resistance to plastic deforma-
* Tel.: +90 354 2421002; fax: +90 354 2421005. E-mail addresses: [email protected], [email protected] 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.06.010
tion. Advocates of this technique are of the opinion that other pavement properties such as crack susceptibility at low temperatures, fatigue resistance and adhesion are not dramatically affected by the use of flow improvers [2,3]. This paper deals with the addition of two commercial waxes to two types of binders of different penetration grade. The main purpose of the study was to investigate the high temperature performance of the asphalt mixtures and binders, but low temperature performance of binders was tested as well, using the bending beam rheometer. 2. Experimental 2.1. Binder and additives In this study, two different penetration grade binders 50/70 and 160/220 were used. They are called LP (low penetration) and HP (high penetration), respectively. The binders were modified for this study by using two types of commercial waxes, FT-paraffin (Sasobit) and polyethylene wax. FT-paraffin and polyethylene wax are denoted as wax S and wax PW, respectively. Wax S is a flow improver, which is used for asphalt concrete and mastic asphalt to reduce the mixing temperature. The polyethylene wax used in this study is normally not used as flow improver or modifier in asphalt mixtures. However, adding wax PW had shown considerable positive effect on the rheological behavior (increased stiffness by dynamic mechanical analysis (DMA)) at medium and higher temperatures in previous studies [6,7], and was therefore chosen for this particular study. Samples were prepared by adding calculated amount of additive to approximately 250 g of binder, after which the sample was heated for 30 min at 155 °C. (Prior to this sample preparation, the original binders were heated in larger buckets for about 3 h at the same temperature and were then, subdivided.) The samples were then placed in preheated blocks and homogenized by shaking for 90 s. The same treatment was applied to the base binders. Levels of 3% and 6% wax by weight of the binder were used. No compatibility problems were observed during mixing.
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Y. Tasdemir / Construction and Building Materials 23 (2009) 3220–3224 2.2. Aggregate
Table 1 Properties of aggregate.
Crushed basalt and crushed limestone were mixed and then used for the preparation of asphalt mixtures. A dense graded asphalt concrete according to Turkish road standards, with maximum aggregate size of 19 mm was prepared. The measured particle size distribution of the aggregate used is shown in Fig. 1. Aggregate properties are shown in Table 1. 2.3. Asphalt mixtures The Marshall method of mix design was used to determine the optimal binder content for the aggregate gradation used in the experiment and it was found to be 5.25% by weight. The mix design was performed only for one unmodified mixture. Compaction was done by a mechanical Marshall compactor with 75 blows per side at 135 °C for all specimens. The calculated and measured mixture properties and their comparison according to design criteria are given in Table 2. As seen from Table 2, all of the properties of mixture are within specification limits for wearing course. In order to make a meaningful comparison of the influence of wax type on permanent deformation, the same optimal binder content was chosen for all mixtures.
Properties
Basalt
Limestone
Content (%) Los Angeles abrasion (%) Sodium sulfate soundness (%) Flatness index (%)
62 11.0 2.0 24.5
38 24.0 2.0 15.6
Coarse aggregate Bulk specific gravity Apparent specific gravity Absorption (%)
2.799 2.894 1.2
2.676 2.709 0.5
Fine aggregate Bulk specific gravity Apparent specific gravity Absorption (%)
2.690 2.737 0.6
2.675 2.718 0.6
Mineral filler Apparent specific gravity
2.743
2.723
2.4. Method of analysis for binders 2.4.1. Conventional tests The standard methods used to characterize the base and wax modified binders include softening point (EN 1427), penetration (EN 1426), dynamic viscosity (EN 12596) and kinematic viscosity (EN 12595). 2.4.2. Dynamic mechanical analysis Dynamic mechanical analysis (DMA) was performed using a rheometer (RDA II, Rheometrics). A temperature sweep with 2 °C increments (from 10 to 100 °C) was applied to characterize the temperature dependence of binder parameters at the frequency of 1 rad/s and variable strains. Parallel plates with diameter 25 mm and gap of 1 mm were used. A sinusoidal strain was applied and the actual strain and torque were measured. Dynamic shear modulus (G*) and phase angle (d) were determined. 2.4.3. Bending beam rheometer test Creep tests were carried out using the bending beam rheometer (TE-BBR, Cannon Instrument Company). Test temperatures used were 15, 20 and 25 °C. A sample beam (125 mm long, 12.5 mm wide and 6.25 mm thick) was submerged into the bath at test temperature for 60 min. A constant load of 100 g was then applied to the binder beam, which was supported at both ends, and the deflection of center point was measured continuously. Creep stiffness (S) and creep rate (m) of the binders were determined at a loading time of 60 s. 2.5. Method of analysis for asphalt mixtures 2.5.1. French rutting tester Rutting was tested using the French rutting tester. This test has been used successfully in France for over 20 years to prevent rutting in asphalt concrete pavements. Researches indicate good correlation between test results of the French rutting tester and actual field performance [8,9]. The French rutting tester was developed by the LCPC (Laboratoires des Ponts et Chaussées) to simulate rutting of flexible pavements. The loading system, mounted within a temperature chamber, consists of a table carrying a steel mold with the specimen (100 mm 180 mm 500 mm) and a treadle pneumatic rubber tire of about 400 mm in diameter and an active width of about 90 mm. The standard tire pressure is 0.60 ± 0.03 MPa. The table and specimens are pressed (pneumatically)
Fig. 1. Particle size distribution of the aggregate used.
with the standard load, 5000 ± 50 N, against the tires which are moved back and forth along linear guiding rails at a maximum travel speed of 1.6 m/s. The test temperature is controlled by a thermocouple, which monitors the temperature of the specimen at a depth of about 30 mm. Two slabs can be tested simultaneously [10]. The specimens were set at room temperature for 12 h before being tested. A pre-test load of 1000 cycles was subjected to the specimens. The thickness of each slab was then calculated by averaging 15 thickness measurements taken at 15 standard positions using a gauge with an accuracy of 0.1 mm. This thickness was considered the original thickness of the slab. The slabs were then heated to the test temperature of 60 ± 2 °C for 12 h. Deterioration measures were carried out after 1000, 3000, 5000, 10,000, 30,000, 50,000 cycles. The test was stopped when the average recorded rut depth, after a series of measures, was higher than 10% and the previous results anticipated a rut depth of more than 15% at the following step. The rutting depth was reported as a percentage of the slab thickness. The percentage was calculated as the average of 15 measurements divided by the original thickness of the slab.
3. Results and discussion 3.1. Conventional binder test measurements Test results are summarized in Table 3. As shown in the table, the addition of both wax S and wax PW reduced penetration and increased softening point and dynamic viscosity. Wax S showed the largest effects. Changes in penetration and softening point were smaller for wax modified LP binders compared to wax modified HP binders, especially when wax S was used. Binder mixtures containing 6% wax S were too stiff to be tested for dynamic viscosity at 60 °C. Kinematic viscosity at 135 °C was by reduced the addition of wax S, indicating that mixing and compaction temperatures could be decreased using this type of wax in the mix. Adding wax PW, however, slightly increased kinematic viscosity and should therefore not be suitable as flow improver. Temperature susceptibility was evaluated using results obtained from the conventional binder tests used in the study. The changing of consistency of the binder with temperature is the general definition of temperature susceptibility [11]. Penetration index (PI) is frequently used as a measure of binder temperature susceptibility, but could give incorrect indications for binders containing wax. Instead, the Penetration Viscosity Number (PVN) has been suggested and used by McLeod [12]. Lower values of PI and PVN indicate higher temperature susceptibility, and asphalt mixtures containing binders with lower temperature susceptibility should be more resistant to cracking and rutting. While all wax modified binders in the study showed less temperature susceptibility compared to the base binders according to PI values, the same binders showed higher temperature susceptibility according to PVN values. This is indicated in Table 3. However, it should be noted that tests of penetration and softening
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Table 2 Summary of Marshall design results. Design parameters
Values
Board specification in Turkey Min.
Max.
Optimal binder content (%) Bulk specific gravity, Gmb Air voids, Pa (%) Voids in mineral aggregate, VMA (%) Void filled with binder, VFA (%) Stability (kg) Flow (mm)
5.25 2.473 3.66 14.60 75 1150 2.80
– – 3 – 75 900 2
– – 5 – 85 – 4
Table 3 Conventional rheological characteristics of binders. Softening point (°C)
Dynamic viscosity@60 °C (Pa s)
Kinematic viscosity@135 °C (mm2/s)
PW PW S S
60 50 40 39 29
49 56 62 74 95
218.6 480.4 817.2 1215.5 TSa
445.1 445.2 532.5 352.2 284.3
0.94 0.19 0.80 2.79 4.79
0.62 0.79 0.76 1.33 1.82
HP 3% PW 6% PW 3% S 6% S
171 121 109 95 68
40 50 52 69 92
52.7 110.3 175.3 232.6 TSa
224.9 224.9 251.3 185.5 156.3
0.86 1.21 1.45 4.58 6.65
0.50 0.91 0.84 1.45 1.99
Binder LP 3% 6% 3% 6%
a
Penetration (d mm)
PI
PVN
Too stiff to be tested.
point are empirical and, in principle, are only valid for unmodified binders. Therefore, PI and PVN may not be relevant indicators in the case of wax modified binders. 3.2. Dynamic mechanical analysis measurements DMA results showed stiffening effects for both binders at medium and high temperatures due to the addition of wax S and wax
Fig. 2. Complex modulus and phase angle as a function of temperature for binder mixtures containing 6% commercial wax.
Table 4 Effects on rutting factor by DMA. Binder
G*/sin d at 60 °C (Pa)
Upper limit temperatures at G*/sin d = 1 (°C)
LP +6% +6% HP +6% +6%
367 1620 796 77 360 152
53.9 64.0 58.7 43.2 52.0 58.7
PW S PW S
PW. Adding wax PW indicated greater effect on complex modulus and phase angle than that of wax S. As shown in Fig. 2, the phase angle decreases the most between 50 and 100 °C for binder samples containing wax PW. A corresponding plateau effect appears for the complex modulus. At temperatures in the range of 45– 85 °C, typical of high pavement service temperatures, the main distress mechanism is rutting, and therefore, dynamic modulus and phase angle are relevant. For rutting resistance, a high complex modulus value is favorable because it represents a higher total resistance to deformation. A lower phase angle is favorable as well because it reflects a more elastic (recoverable) component of the total deformation [11]. The decrement of phase angle is more pronounced for wax PW modified LP binders than that of wax PW modified HP binders. Above 60 °C, the complex modulus of wax PW modified HP binders is the same or higher compared to unmodified LP binders. In the Superpave binder specification, rutting is taken into account using a rutting factor (G*/sin d), which is solely dependent on the rheological properties of the asphalt binder. The higher the rut factor for the binder, the stiffer the asphalt concrete should be and thus more resistant to rutting [13]. G* and d are used in the specification to control rutting by limiting G*/sin d to at least 1.0 kPa for original binder. G*/sin d at 60 °C and the upper limit temperatures (at which G*/ sin d = 1) were determined, and are shown in Table 4. Wax modification increases the G*/sin d values and upper limit temperatures of the binders at the studied frequency. The highest G*/sin d values and upper limit temperatures were achieved by adding wax PW. As indicated in Table 4 and Fig. 2, wax PW modified HP bitumen may have the same rutting resistance as unmodified LP bitumen at high temperatures. 3.3. Creep test Creep tests were carried out using BBR at three different temperatures ( 25, 20 and 15 °C). The limit stiffness temperature (lower limit temperature), LST, at which S = 300 MPa were deter-
Y. Tasdemir / Construction and Building Materials 23 (2009) 3220–3224
Fig. 3. Effect of wax modification on lower limit temperatures using BBR.
mined from BBR results at the three temperatures mentioned above. As shown in Fig. 3, the limit temperatures depend on the grade of the base binder and wax modification did not show any beneficial effect. A minor negative effect was gained when adding wax PW to binder LP and HP. The addition of wax S increased the limit temperatures of both the binders studied. These temperature increments are higher for the wax modified LP binders compared to that of the wax modified HP binders. As also can be seen in Fig. 3, the low temperature performance of wax S and wax PW modified HP binders is better than that of the unmodified LP binders.
3.4. French rutting test French rutting test results for unmodified and wax modified asphalt mixtures are shown in Fig. 4. Unmodified mixtures showed the highest permanent deformation in this test. The addition of wax PW, and especially wax S, improved the rutting resistance of both mixture types. Modified LP binder mixtures showed higher rutting resistance as compared to unmodified LP and HP binder mixtures. Furthermore, the results showed that mixtures containing wax PW did not show expected improved performance, considering the positive effects indicated in binder testing (cf. Section 3.2). The reason for this unexpected performance could be that the polyethylene product used was sensitive to aging (during manufacturing of the asphalt mixtures) and therefore there could be some instability problems [14]. Adding wax S increased the rutting resistance of asphalt mixtures containing LP or HP binders, but as for the wax S modified asphalt mixture containing HP binder, it exceeded the specification limit (>10%).
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The influence of adding wax S and wax PW to binder was investigated with regard to medium temperature performance of the asphalt concrete mixture by Edwards et al. [15]. Edwards et al. performed dynamic creep testing at 40 °C to investigate asphalt concrete performance. The results gained from that study showed that the smallest strains were recorded for the asphalt mixtures containing FT-paraffin, indicating better resistance to rutting, and the largest strains for the mixture containing no additive. Adding wax PW showed considerable positive effect in binder testing by DMA, but this effect was less pronounced in dynamic creep testing on asphalt mixtures, as in this study. Hurley and Prowell [16] reported that the addition of FT-paraffin generally decreases the rutting potential of the asphalt mixes evaluated. They pointed out that the rutting potential generally increased with decreasing mixing and compaction temperatures but the mixtures containing FT-paraffin were less sensitive in this sense compared to unmodified mixtures in terms of rutting. 4. Conclusions Effects on high temperature performance of asphalt mixtures due to the addition of wax S or wax PW to low and high penetration binders were evaluated. Base on the results obtained in this study, the following conclusions were drawn. Adding wax PW indicated greater stiffening effect than adding wax S based on DMA test results, while adding wax S showed greater stiffening effect than wax PW based on conventional binder test results. There was a decrease in kinematic viscosity at 135 °C for wax S modified binders. Indicating a possible decrease in the mixing and compaction temperatures, hence reducing emissions like bituminous fumes. Above 60 °C, the complex modulus of wax PW modified HP binders is the same or higher than for the unmodified LP binder. Also, the low temperature performance of wax S and wax PW modified HP binders is better than that of the unmodified LP binder. Based on DMA and BBR results, it may be concluded that wax PW modified HP binder have higher rutting resistance at high temperatures and better low temperature performance than the unmodified LP binder. Concerning asphalt mixtures, the wax S modified mixtures showed the best rutting resistance, although the wax S modified one, containing HP binder, exceeded the specification limit (>10%). PW modification also improved binder rutting resistance
Fig. 4. Rutting test on asphalt mixtures containing 6% commercial wax.
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when compared to base bitumens, but asphalt mixtures, containing wax PW did not show expected improved performance as indicated from binder testing by DMA. Summarizing, wax modification improved the high temperature performance of asphalt mixtures, indicating that the modification is beneficial if done properly.
Acknowledgements This study is a part of the research supported by Erciyes University Research Fund. (Project No: EUBAP-FBA-06-15). The author would like to thank Erciyes University Research Fund for giving the opportunity to perform this research. The author also expresses appreciation to Ahmet Gürkan Güngör and Fatma Orhan in General Directorate of Highways in Turkey for assisting in rutting measurements. References [1] Vinson TS, Hicks RG, Janoo VC. Low temperature cracking and rutting in asphalt concrete pavements. In: Vinson TS, editor. Roads and airfields in cold regions. New York: American Society of Civil Engineers; 1998. p. 203–48. [2] Damm KW, Abraham J, Butz T, Hildebrand G, Riebesehl G. Asphaltverflüssiger Als ‘Intelligenter Füller’ Für Den Heisseneinbau–Ein Neues Kapitel In Der Asphaltbauweise. Bitumen 2002;1:19–24 [in German]. [3] Damm KW, Abraham J, Butz T, Hildebrand G, Riebesehl G. Asphaltverflüssiger Als ‘Intelligenter Füller’ Für Den Heisseneinbau–Ein Neues Kapitel In Der Asphaltbauweise. Bitumen 2002;1:55–61 [in German].
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