Study of physical and rheological properties of wax modified binders using classic and SHRP testing methods

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 20, April 2013, pp Study of physical and rheological properties of wax modified binders using classic and SHRP testing methods Shengjie Liu* School of Highway, Chang an University, Xi an , China Received 22 October 2012; accepted 30 January 2013 Considerable effect of Fischer-Tropsch (FT) paraffin on improving the original asphalt properties along with reducing the emissions and saving energy, encouraged using this additive as an effective modifier. In this study, extensive laboratory investigations have been carried out using classic and SHRP testing methods. The results of both test methods show that adding FT paraffin into original asphalt reduce penetration, ductility at 15 C, and m-value, while enhance penetration index, softening point and creep stiffness. Results indicate that there is an inflection point or inflection interval in ductility at 5 C. Rheological properties of modified specimens are investigated with DSR in temperature sweep and creep tests. Results indicate that the aforementioned parameters increase by increasing the FT paraffin content. Furthermore, at higher temperatures, increasing the FT paraffin additives content can reduce the binder viscosity and at lower temperatures less than the melting point of FT paraffin the reverse is true. Keywords: Wax modified binders, FT paraffin, Classic tests, Rheological property, Creep property, DSR In recent years, environmental awareness such as global warming and emissions has been increasing rapidly. One of the main sources of energy consumption and environmental emissions stemming from industries related to transport infrastructures, resides in the manufacturing, spreading, and conservation of asphalt mixes 1. Meanwhile, the energy consumed is a major component of pavement construction that significantly contributes to the total cost 2. In order to reduce the emissions and save energy, the asphalt industry is constantly trying to lower the mixing and compaction temperatures of the mixes by using new energy-saving technologies, while maintaining or improving the pavement performance. The warm mix asphalt (WMA) refers to technologies that allow reducing the asphalt binders mixing and compaction temperature by reducing the binders viscosity at a given temperature. Typically, the mixing temperatures of warm mix asphalt mixtures range from 100 C to 140 C (212 F-280 F ) compared to the mixing temperatures of 150 C-180 C (300 F-350 F) for hot mix asphalt 3-5. From an environmental and economic standpoint, use of WMA technologies may contribute to reduce mixing and compaction temperature, while other * lsjwork@126.com pavement properties such as crack susceptibility at low temperatures, fatigue resistance and adhesion are not dramatically affected by the use of flow improvers 5,15. Different WMA technologies were used to reduce mixing and compaction temperature 6, although these technologies are quite different, they all target the same goals, namely, lower bitumen viscosity, better mat workability, and improved workability and emissions conditions. During the several additives, FT paraffin has been used widely as an commercial wax 7,8, because of its excellent insulating characteristics. Previous research studies have taken to prove the FT paraffin have significant effects on asphalt performance Below the laying and compaction temperatures, there may be an increase in viscosity due to wax crystallization of FT paraffin, which in turn could increase the asphalt pavement resistance to plastic deformation 12. Decreasing temperature in the mixing and compaction will reduce odor emission from plants such as traditional gaseous pollutants (CO, NOx and SO 2 ), greenhouse gases (CO 2 ); and improve the working conditions at plants and paving sites 13. So the asphalt plant can be located at regions of strict air pollution regulations. The lower mixing and compaction temperature will also allow for longer haul distance and help to extend the paving season 14.

2 LIU: WAX MODIFIED BINDER 133 Numbers of wax modified binders trial projects have been implemented in many states. In spite of several researches conducted in the field of wax modified binders, potential problems and unknowns still exist. The short life of wax modified binders tends to raise logical doubts and uncertainties about their use, for it is still at a stage of standardization 16,especially the current binder specifications in China are based on penetration testing, which could not properly account for pavement performance. Thus, a thorough understanding of the properties and performance of wax modified binders is necessary in order to implement it successfully. The objective of this study was to investigate the engineering properties of wax modified binders incorporating the FT paraffin technologies. Laboratory experimental tests including classic tests and SHRP test methods were conducted to simulate the physical and rheological properties of wax modified binders. In this regard, the following objectives were supposed: (i) Effect of FT paraffin on the basic rheological characteristics of studied bitumen such as penetration, adhesion, and elastic properties, (ii) Investigating the temperature susceptibility of FT paraffin modified bitumen, Effect of FT paraffin on stiffness and cracking potential of the original bitumen using classic and Strategic highway research program (SHRP) testing methods and (iv) Comparison between the results of classic and SHRP testing methods and investigating the correlation between results. Experimental Procedure The asphalt binder selected for this investigation was a 60/80 penetration grade (PG70-22) base bitumen, which was provided by Sinopec Asphalt Co. Ltd. 60/80 bitumen has the maximum application in most regions in China. The basic properties of binders as per (JTJ ) are shown in Table 1. The additive used in this study was a product of wax-ft, which is a long chain of aliphatic hydrocarbons obtained from coal gasification using the Fischer-Tropsch (FT) process. After crystallization, it forms a lattice structure in the binder which is the basis of the structural stability of the binder containing FT paraffin. It shows high viscosity at lower temperatures, and low viscosity at higher temperatures. Table 2 shows the relevant performance indexes of this FT paraffin. For all specimens, wax modified binders in this study was prepared in the laboratory using a shear mixer, which was made by Yaxing Maching Co. Ltd., China. Firstly, the asphalt (about 400 g) was heated until became fluid in an iron container, and then the Wax-FT pellets were added into hot binders respectively when the temperature above 120 C. In order to ensure the wax-ft was adequately soluble in the asphalt, the shear speed was adjusted to 400 rpm for 30 min. And then the asphalt was immediately used for tests. Five contents (0%, 1%, 2%, 3%, 4%, 5% by weight of binder) were selected based on previous studies and field trial experiences. Then, specimens were tested by classic and SHRP testing methods. Classic tests consisted of penetration, softening point, ductility, viscosity which were carried out using related China standards. Also, dynamic shear rheometer (DSR) and bending beam rheometer test (BBR)tests were performed and obtained results were compared with classic tests results. Results and Discussion Classic tests results and analysis is a performance index that evaluates hardness and consistency of asphalt, since 1889 it has been used as a standardized phenomenological test in the highway industry and it is also the most Test items (25 C, 0.01 cm) Ductility (5 C, cm) Table 1 The properties of asphalt binder Softening point ( C) Viscosity (135 C, Pa.s) index (PI) Density (15 C, g/cm) Wax content (%) Value Test Items Congealing point ( C) (25 C, cm) Table 2 The basic properties of wax-ft (65 C, cm) Viscosity (135 C, cp) Melting temperature ( C) Density (25 C, g/cm 3 ) Value 98 <

3 134 INDIAN J. ENG. MATER. SCI., APRIL 2013 common control test for penetration grade asphalt. Figure 1 illustrates the s at three temperature (15 C, 25 C, 30 C) of wax modified binders based on results obtained from lab tests. As shown in Fig. 1, compared with the control binder (with 0% FT paraffin content), the addition of wax-ft into asphalt binder did affect the penetrations of binders. The penetrations decreased gradually by increasing wax-ft content, indicating wax-ft make the asphalt binder harder and more viscous. index (PI) index (PI) is one of the important indexes to reflect asphalt temperature sensibility, With respect to the effect of wax-ft additive,the penetration of each binder were obtained at three temperatures(15 C, 25 C, 30 C),and then penetration index (PI) is calculated as follows: PI = ( A) /(1 + 50A) where A = (log( pent ) log( pent ))/( T1 2 ) T 1 2 Here pen is the abbreviation of penetration, T 1 and T 2 are different temperatures, and T 1 > T 2. A is penetration temperature index. Figure 2 shows the statistical results of the change in the penetration index as a function of the warm asphalt additive. A general trend is found from the results that the addition of wax-ft increase the binder s penetration index, compared to the control asphalt binder. With wax-ft added into the base asphalt, this was an inflection point or inflection interval in penetration index, PI first increased with increasing wax-ft content up to 4% and then started to decrease. This phenomenon indicated that the wax-ft content is statistically insignificant on the temperature susceptibility of wax modified binders. Softening point The softening point is the temperature at which the binder begins to show fluidity. It is defined as the temperature at which a bitumen sample can no longer support the weight of a 3.5 g steel ball. The test results are shown in Fig. 3. Through the lab tests, it can be seen that the softening point is further improved by increasing the wax-ft contents. The increasing nature of the softening point indicates that wax-ft can improve the high temperature performance of asphalt, and which will also increase the high temperature stability of asphalt mixture. Furthermore, the binder and mixture have more resistance against rutting at high temperatures Ductility The results of ductility test for specimens are shown in Fig. 4. As shown in this figure, ductility Fig. 2 Relationship lines between PI and wax-ft content Fig. 1 Relationship lines between penetration and wax-ft content Fig. 3 Relationship lines between soft point and wax-ft content

4 LIU: WAX MODIFIED BINDER 135 Fig. 4 Relationship lines between ductility and wax-ft content at two temperatures (at 5 C) of the specimens showed an inflection point or inflection interval between 2% and 4%. Ductility (at 5 C) first increased with increasing wax-ft content up to 3% and then started to decrease. While ductility (at 15 C) sustainly decreased by increasing wax-ft content from 0% to 5%. It is noted that when the content of wax-ft increased from 3% to 4%, the ductility (15 C) decreased sharply from 125 cm to 79 cm. The results indicated that the addition of wax-ft would be likely to affect low temperature performance of the binder. Viscosity Viscosity at high temperature is considered to be an important property because it represents the binder s ability to be pumped through an asphalt plant, thoroughly coat aggregate in asphalt concrete mix, and be placed and compacted to form a new pavement surface. Brookfield rotational viscometer was used to test the viscosity of these virgin and wax-ft binders at seven different temperatures at a fixed frequency of 6.8 rad/s. Figure 5 illustrates the viscosity of asphalt binder decreases rapidly by increasing the temperature from 80 C to 140 C as expected. In addition, with respect to the effect of wax-ft, large variability of viscosity was observed with the change of wax-ft content at different temperatures, it can be observed that, at the temperature of 80 C, 5% wax-ft addition contributed to the highest viscosity, whereas at a temperature of 150 C, the mix with 0% wax-ft content had the highest viscosity. Further analysis was performed to investigate the significance of the effect of wax-ft content on Fig. 5 Relationship lines between viscosity and wax-ft content at different temperatures viscosity. It can be found that the temperature of 100 C is the turning point, when the temperature below it, the viscosity of asphalt increased with increase of content; whereas at the temperature above 100 C, the viscosity of asphalt decreased as the content increased. This can be explained as wax-ft is a material with different organic structures within different temperatures, and its melting point is about 100 C, when the temperature is higher than its melting point, it could dissolve into base asphalt and keep in liquid state entirely, so it could enhance the rheological properties of asphalt, however when the temperature is lower than its melting point, it will keep the crystal status and disturb molecule movement, and then the asphalt viscoelastic will strengthen. Viscosity is a generally accepted measuring factor for determine, lower viscosity indicates asphalt mixtures can be easily mixing and compacting. The viscosity of asphalt binder decreased with wax-ft content increasing at the high temperature indicating that wax-ft addition contribute to drop down the mixing and compacting temperatures of asphalt mixtures. SHRP result and analysis Rheological measurement results and analysis The DSR was suggested as a means to characterize asphalts viscoelastic properties during the Strategic Highway Research Program (SHRP), a 5-year $150 million United States research effort established and funded in The DSR is a classic rotational rheometer that applies oscillatory shear to asphalt using the parallel-plate configuration and assesses its

5 136 INDIAN J. ENG. MATER. SCI., APRIL 2013 rheological behavior through the response of the material to the imposed stresses or deformations 17. Two measured properties (complex modulus G* and phase angle δ) were transformed into two performances based properties G*sinδ and G*/sinδ to reflect the dissipated energy by the nonelastic components of the material response. The parameter G*sinδ measures the damage or the dissipated energy for a linear viscoelastic material subjected to a strain controlled load, which is an indicator of fatigue cracking for thin pavements at intermediate pavement temperatures. For a stress controlled mode of loading, the dissipated energy is G*/sinδ, which is a measure of rutting potential at high temperatures. This parameter is also used as an indicator of fatigue cracking for thick pavements at intermediate temperatures. It is imperative both parameters are chosen that best relates to the material s rheological behavior. Based on the lab tests, the results of G*/sinδ for warm asphalt binders containing different content of wax-ft are shown in Fig. 6. For different temperature conditions (at 70 C, 76 C, 82 C), the rutting factor increased by increasing wax-ft content, and the increment became more significant at a higher percentage of wax-ft, indicating content increasing contributed an promotion of rutting resistance at a high temperature. Figure 7 shows the results of the fatigue factor, G*sinδ, as a function of temperature at 25 C, 28 C and 31 C. It can be seen that G*sinδ increased by increasing wax-ft content, and the additions of 1% wax-ft showed a very rapid increase compared to the control binder, which implied that wax-ft contributed to adverse effects on fatigue resistance. As the wax-ft added into asphalt, the elastic of binder would be greater than the base asphalt, which weaken the capability of asphalt to relieve stress and reduced the ability of fatigue resistance. According to the AASHTO M 320, the G*/sinδ should be less than 1.0 kpa for the unaged asphalt, through the DSR test, it is found that the control PG70-22 binder does not meet the requirement at 76 C and 80 C, but with the addition of wax-ft is 1%, the binder can meet the criteria at 76 C, while the content is 5%, the high temperature grade will near the criteria at 80 C, showing the addition of wax-ft could improve the high temperature grade from 70 C to 76 C, even to 80 C. Results indicate that the high temperature performance and rheological behavior has increased by increasing the wax-ft content. The BBR is a three-point bending-beam experimental set-up, designed to characterize the low-temperature viscous behavior of bitumen. It can be used to evaluate how much a binder deflects or creeps under a constant load at low temperature. The creep stiffness is the ratio of a certain load applied to the variation in strain or displacement in the prepared asphalt beam and the m-value is the slope of the stress against strain curve relationship in a log scale. Lower creep stiffness and higher m-value of PAV aged binder at a low temperature usually mean a higher resistance to low temperature cracking of asphalt binders. In this study, the creep stiffness and m-value of PAV aged binders were conducted by BBR tests at T = -6 C, -12 C and -18 C and the average value was determined. Figure 8 shows the creep stiffness of WMA with different wax-ft contents at three Fig. 6 Relationship lines between Gsinδ and wax-ft content at different temperatures Fig. 7 Relationship lines between Gsinδ and wax-ft content at different temperatures

6 LIU: WAX MODIFIED BINDER 137 With the wax-ft content increased to 2%, the WMA failed the requirement at -12 C, and further, when the content was 5%, it failed the criteria at -6 C, showing that addition of wax-ft could drop the low temperature grade from -22 C to -16 C, which means the low-temperature performance of asphalt have been weakened. Fig. 8 Relationship lines between stiffness and wax-ft content at different temperatures Fig. 9 Relationship lines between m value and wax-ft content at different temperatures temperatures. Addition wax-ft into asphalt greatly increased the low temperature stiffness of wax modified binders, which will decrease the toughness of wax modified binders mixtures and increased the occurring possibility of the asphalt binder at low temperature. Similar to what was discussed above, the change trend of m-value also proved this, as shown in Fig. 9, the increasing of wax-ft content lead to decrease of m-value at three temperature. AASHTO M 320 requires the creep stiffness should be less than 300 MPa and the m-value should be greater than at the test temperature during the performance grading of the asphalt. It is noted that, for the control asphalt binder of PG70-22, it meets this requirement at -6 C and -12 C. Comparison between the results of classic and SHRP tests For the asphalt specimens, penetration steadily reduced by increasing the wax-ft content. In contrast, obtained results for G*sinδ at three temperatures increased by reduced wax-ft content. Negative correlation exists between penetration and G*sinδ. PI parameter increased by increasing the wax-ft content, whereas G*/sinδ parameter in three temperature reduced by increased wax-ft content. There was a same trend in diagrams. Therefore, PI could be used as an indicator for temperature susceptibility evaluation. The results of softening point, G*sinδ, G*/sinδ and the differences between these parameters have been discussed. The effect of wax-ft content on both parameters was to some extent similar. Therefore, in overall, it could be concluded that for studied specimens there is a positive correlation among G*sinδ, G*/sinδ and softening point. The differences among creep test at 25 C, ductility at 5 C and 15 C results have been discussed. By increasing the wax-ft content, ductility at 15 C of specimens reduced whereas stiffness increased upon increasing the wax-ft content. Ductility at 5 C first increased and then decreased by increasing the wax-ft content. Obtained results imply that there is poor relationship among creep, ductility at 5 C and 15 C. Conclusions Based on laboratory investigations and obtained results in this study, the following conclusions can be drawn: (i) The penetration, ductility at 15 C, m-value reduced gradually as the content of the wax-ft increased, while the penetration index (PI), softening point and creep stiffness results decreased. (ii) Wax-FT additives could reduce binder viscosity at high temperatures and thus allow lower mixing and laying temperatures. But when the asphalt temperatures below the wax-ft s

7 138 INDIAN J. ENG. MATER. SCI., APRIL 2013 melting point this reduced viscosity is offset again or the effect is even reversed. (iii) Both classic tests and SHRP tests arrived the same results that wax-ft can enhance the consistency of asphalt, and reduce the temperature sensitivity of asphalt through improving the high temperature performance of asphalt. However, it has negative effect on low temperature performance and fatigue resistance performance of wax-ft modified asphalt. (iv) Based on analysis between the results of classic and SHRP tests. There is a meaningful relationship between (penetration-g*sinδ), (PI-G*/sinδ), (softening point-g*sinδ), (softening point -G*/sinδ). In contrast there is no relationship between (ductility-stiffness/ m value) parameters. References 1 Das Prabir Kumar, Tasdemir Yuksel & Birgisson Björn, Constr Build Mater, 30 (2012) Hamzah Meor Othman, Jamshidi Ali & Shahadan Zulkurnai, J Cleaner Prod, 18 (2010) Kristjansdottir O, Muench S, Michael L & Burke G, J Transport Res Board, 2040 (2007) Prowell B, Warm mix asphalt, International Technology Scanning Program Summary Report, July 11(2007), ( 5 Xiao F, Amirkhanian S & Putman B, J Transport Res Board, 2180 (2010) Zaumanis M, Warm mix asphalt Investigation, Ph.D. Thesis, Riga Technical University, Kgs. Lyngby, Denmark, Biro S, Gandhi T & Amirkhaninan S J, J Constr Build Mater, 23 (2009) Mohammad L, Shadi S & Cooper S, Evaluation of Asphalt Mixture Containing Warm Mix Additive, Proc Airfield and Highway Pavements Conf, ASCE, Tasdemir Yuksel, Constr Build Mater, 23 (2009) Das Prabir Kumar, Tasdemir Yuksel & Birgisson Bjorn, Road Mater Pavement Des, 13 (2009) Edwards Y, Tasdemir Y & Isacsson U, Mater Struct, 39 (2006) Heukelom W, J Assoc Asphalt Paving Technol, Houston, 42 (1973) Li Jun, Zhang Yuxia & Zhang Yuzhen, Constr Build Mater, 22 (2008) Edwards Ylva, Tasdemir, Yuksel & Butt Ali Azhar, Mater Struct, 43 (2010) Edwards Y, Tasdemir Y & Isacsson U, Cold Regions Sci Technol, 45 (2006) Bahia H U & Anderson D A, in Physical properties of asphalt cement binder, edited by Hardin J C, (American Society for Testing and Materials), 1995, pp 1-27