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Dynamic mechanical characterization of asphalt concrete mixes with modified asphalt binders
Materials Science and Engineering A 528 (2011) 6445–6454
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Dynamic mechanical characterization of asphalt concrete mixes with modified asphalt binders S. Anjan kumar ∗ , A. Veeraragavan Department of Civil Engineering, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Article history: Received 11 June 2010 Received in revised form 18 April 2011 Accepted 9 May 2011 Available online 14 May 2011 Keywords: Modified asphalt Dynamic modulus Creep Energy dissipation Rutting
a b s t r a c t The primary objective of this work is to characterize and compare the dynamic mechanical behavior of asphalt concrete mixes with styrene butadiene styrene (SBS) polymer and crumb rubber modified asphalt binders with the behavior of mixes with unmodified viscosity grade asphalt binders. Asphalt binders are characterized for their physical and rheological properties. Simple performance tests like dynamic modulus, dynamic and static creep tests are carried out at varying temperatures and time. Dynamic modulus master curves constructed using numerical optimization technique is used to explain the time and temperature dependency of modified and unmodified asphalt binder mixes. Creep parameters estimated through regression analysis explained the permanent deformation characteristics of asphalt concrete mixes. From the dynamic mechanical characterization studies, it is found that asphalt concrete mixes with SBS polymer modified asphalt binder showed significantly higher values of dynamic modulus and reduced rate of deformation at higher temperatures when compared to asphalt concrete mixes with crumb rubber and unmodified asphalt binders. From the concept of energy dissipation, it is found that SBS polymer modification substantially reduces the energy loss at higher temperatures. Multi-factorial analysis of variance carried out using generalized liner model showed that temperature, frequency and asphalt binder type significant influences the mechanical response of asphalt concrete mixes. The mechanical response of SBS polymer modified asphalt binders are significantly correlated with the rutting resistance of asphalt concrete mixes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Review of earlier work
Performance of asphalt pavement depends on the properties of materials used, design accuracy and construction quality. Material characterization is the most important task that plays a significant role in the design and performance of the pavements during the design life. Asphalt concrete mix is a composite mixture of aggregates, mineral filler and asphalt binder. Consequent to characterisation of the various ingredients of an asphalt concrete mix for its fundamental properties, the mix design is crucial as it dictates the performance of asphalt pavements during the design life. The testing of mechanical properties of mixes is not envisaged in the Superpave method of mix design. Protocols have been developed and presented in NCHRP 9-29 [26] to characterize the asphalt concrete mixes for their mechanical properties, such that resistance to rutting and fracture and the properties can be evaluated using the simple performance tester.
Asphalt concrete mixes exhibit viscoelastic solid to fluid like behavior depending upon on the time and temperature of testing. Hence the time–temperature dependent characteristics of asphalt concrete mixes are to be considered while evaluating their fundamental properties. The dynamic modulus determined under sinusoidal loading can be used to characterize the mechanical response of asphalt concrete mixes under varying time and temperature. For a viscoelastic material like asphalt concrete mixes, master curves may be developed at a reference temperature using the time–temperature superposition principles to account for variation in temperature and time of loading within the linear viscoelastic range. Stress dependent dynamic modulus predictive equation was developed using the k– non-linear elastic models [34], and it considers the effect of bulk and shear stresses on the dynamic modulus values. In 2003, Christensen et al. [10] used Hirsch model to predict the dynamic modulus of asphalt concrete mixes using binder modulus and mixture composition based on the law of mixtures. Studies [24,31,35] have shown that, dynamic moduli of asphalt concrete mixes are significantly dependent on the specimen geometry, nominal aggregate size, loading time and test temperature. It has
∗ Corresponding author. Fax: +91 44 22574250. E-mail addresses: [email protected] (S. Anjan kumar), [email protected] (A. Veeraragavan). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.05.008
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been reported that the dynamic moduli values can be increased by using stiff asphalt binders at lower binder content and lower (3%) air voids level [38]. However, variations in the measured and predicted dynamic modulus of asphalt concrete mix are found to increase with increase in temperature. Also, significant difference in dynamic modulus value measured in laboratory and field [9] have been reported. Investigations [39] also show that the effect of air voids on the dynamic modulus was not significant when compared to temperature and frequency. Modification of asphalt using high boiling point petroleum and SBS polymer increases the dynamic modulus and fatigue resistance of asphalt concrete mixes when compared to conventional asphalt concrete mixes [18]. Studies [24] show that simple performance test parameters like flow number and time are useful to differentiate asphalt concrete mixes based on the traffic level. Flow number (dynamic creep) estimation is highly dependent on the repeatability of data collected and could be misleading [20,33]. Use of failure point determined by creep stiffness could reduce the error resulting due to low data point in flow number determination [20]. The rate of deformation estimated during secondary flow using a stepwise method assuming that strain is neither held constant nor increased could also give a good estimate of flow number [33]. Rutting may be considered as the most important type of distress in asphalt pavements. Mix design and tests on asphalt concrete mixes using simple performance tests could be dutiful to build up the correlation between the mix response and rutting [26]. The mechanical behavior of asphalt concrete mixes under repeated loading can be characterized by primary, secondary and tertiary stages. Transition between these three stages can be assigned as a rut indicator [12]. It was also shown that SBS polymer modified asphalt binder with linear grafting improves resistance to rutting when compared to conventional and other modified asphalt binders [15]. The mechanical properties and temperature susceptibility of asphalt concrete mixes could be improved by using SBS polymer modified asphalt binder [2]. Continuous phase of polymer in modified asphalt was found to be reasoning behind the improved rutting and fatigue properties of modified asphalt concrete mixes [13]. The improved toughness and tenacity and also formation of network in asphalt binder depend on the type and amount of polymer used for modification which results in increased complex modulus [22,23]. It was found that rubber modification increases the high temperature properties of asphalt mixes but makes the material brittle at low temperatures and these phenomena may be due to swallowing of oil fraction and carbon black present in the rubber [1]. Polymer modification was found to be effective in increasing rutting resistance at high temperature and fatigue resistance at intermediate temperatures. This behavior was essentially due to micro-structural changes related to the development of polymer-rich phase and molecular arrangements [14,16,21,25]. The disturbed structure of polymer modified asphalt binders during shear was found to reform with time and this property imparts the ability of self-healing [7]. Essentially in polymer modification, optimization of blend composition, shear rate, temperature and time produces the best rheological properties of modified asphalt binder [8]. The above studies have shown that simple performance tests evaluate the asphalt concrete mixes for their resistance to rutting and fracture. Modification of asphalt binder enhances the resistance to permanent deformation of asphalt concrete mixes. Characterizing and understanding the mechanical response of multi-constituent material like asphalt binder is a difficult task. Modification of asphalt binder with different types of additives like polymer, crumb rubber etc., has added more complexity to this task. Limited studies were carried out on mixes with different modified asphalt binders under varying temperature and time. Also the relation between the mechanical properties of modified
Fig. 1. Aggregate gradation of bituminous concrete.
asphalt binders and asphalt concrete mixes are not completely understood. Modification is aimed at enhancing the temperature susceptibility of asphalt binders. Hence in the present study, the combined effect of time, temperature and asphalt binder type on the observed mechanical response of asphalt concrete mixes are evaluated, so as to provide the much needed information on the feasibility of modified asphalt binders for enhanced life. Investigations on the fundamental properties and mechanical characteristics will facilitate ranking of the performance of asphalt concrete mixes with different modified asphalt binders and their comparison with asphalt concrete mixes with unmodified asphalt binders. 3. Objectives The objectives of the studies are: • To compare the physical and rheological properties of modified asphalt binders with unmodified asphalt binder. • To evaluate the dynamic mechanical response of asphalt concrete mixes with modified and unmodified asphalt binders under varying temperature and time. • To investigate the creep characteristics of the asphalt concrete mixes with modified and unmodified asphalt binders under static and dynamic loading conditions. • To characterize the rutting resistance of asphalt concrete mixes with modified and unmodified asphalt binders using dry wheel tracker. 4. Experimental investigations 4.1. Materials used SBS polymer and crumb rubber modified asphalt binders and unmodified viscosity grade (VG-30) asphalt binder were used in the present investigations. The aggregate gradation for asphalt concrete mix (Fig. 1), grade-2 (bituminous concrete) recommended by Indian specification [27] was adopted. 4.2. Simple performance tests 4.2.1. Dynamic modulus (NCHRP 9-19 [26]) Tests were carried out as per AASHTO TP62 [4] using simple performance tester to characterize the dynamic modulus of
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asphalt concrete mixes. Asphalt concrete mixes with modified and unmodified asphalt binders were subjected to sinusoidal (haversine) compressive loading, varying the frequency from 25 to 0.01 Hz and temperature from 20 to 60 ◦ C. The specimens were tested under unconfined condition within the linear viscoelastic range, where the strain is between 75 and 125 micro-strains [26]. The applied stress and resulting axial strain from three on-specimen mounted displacement transducers were measured as a function of time. From the data collected, the dynamic modulus and phase angle were calculated as follows: = (E1 + iE2 )ε
Pi =
E2 =
15 (mij − moj ) j=1
= stress, E1 =
the performance of asphalt concrete mixes as per European standard 12697-22(E) [11]. This was attempted to address the benefit of modification under slow vehicular movement conditions for which the asphalt modification is aimed. Asphalt concrete mixes with modified and unmodified asphalt binders were conditioned for 6 h before the actual commencement of the test in an environmental chamber. All specimens were tested at temperature of 40 and 60 ◦ C for 10000 passes or up to rut depth of 10 mm, whichever is earlier. From the 15 local profile measurement points, the proportional rut depth was calculated using the following relationship:
(1)
where
o
εo
o
εo
(6)
cos ı, (storage modulus)
(2)
sin ı, (loss modulus)
(3)
4.3. Constrained variables
ı = phase angle. The magnitude of the complex modulus called as dynamic modulus is given by,
∗ E = (E12 + E22 )1/2
(4)
substituting (2) and (3) in (4) we get, εo
15 × h
where Pi is the measured proportional rut depth, mij is the local deformation, moj is the initial measurement at the jth location, and h is the specimen thickness.
ε = strain, and
∗ o E =
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(5)
4.2.2. Dynamic creep Tests were carried out on asphalt concrete mixes to evaluate the permanent deformation characteristics under repeated loading/dynamic creep conforming to NCHRP 9-19 [26]. The asphalt concrete mixes with modified and unmodified asphalt binders were subjected to a repeated haversine axial compressive loading. Loading time of 0.1 s with 0.9 s dwell was maintained constant and 1000 cycles were applied. The axial stress of 200 kPa and contact stress of 5 kPa was applied. Asphalt concrete mixes were tested at varying temperature from 20 to 60 ◦ C. The specimens were tested under unconfined condition. The resulting permanent axial strains were measured as a function of the number of applied loading cycles. 4.2.3. Static creep Asphalt concrete mixes with modified and unmodified asphalt binders were subjected to static axial compressive loading also known as static creep. Axial deviatoric stress of 200 kPa and contact stress of 5 kPa was applied. The asphalt concrete mixes were tested at varying temperatures from 20 to 60 ◦ C. All samples were tested under unconfined condition. The resulting permanent axial strains were measured as a function time. These loading conditions were selected so as to evaluate the relative performance of different asphalt concrete mixes without undue flowing at high temperatures due to high deviatoric stress. 4.2.4. Wheel tracker The rutting resistance of asphalt concrete mixes was evaluated using dry wheel tracker. Moving load of 700 N was applied through a rubber hosed wheel of 200 mm diameter, 50 mm width on 150 mm diameter cylindrical specimens. The speed was maintained at 53 passes per minute. The maximum load and minimum speed of equipment capability was selected to relatively evaluate
All specimens were prepared using gyratory compactor with a single aggregate gradation at a constant asphalt binder content of 5.25%. The compaction effort was maintained constant, such that the volumetric properties are comparable between asphalt concrete mixes with modified and unmodified asphalt binders. This binder content was maintained constant as the optimum percentage obtained in case of asphalt concrete mixes with unmodified asphalt binder. All specimens tested for mechanical and performance characteristics were subjected to short-term aging as per ASTM D 6925-09 [5]. 5. Results and discussions 5.1. Asphalt binder characterization Modified and unmodified asphalt binders characterized in the present study fulfilled the requirements as per Indian specifications [6,19]. Performance grading of asphalt binders were carried out as per AASHTO [3]. The temperature at which G*/sin ı ≥ 2.2 kPa is found to be higher for modified asphalt binders when compared to unmodified asphalt binder and is shown in Table 1. Significant difference in the highest critical temperature causing rutting between SBS polymer modified and crumb rubber modified asphalt binders was not observed. SBS Polymer modified asphalt binder has showed the lowest phase angle. This explains that due to elastic nature (lower phase angle), the SBS polymer modified asphalt binder results in enhanced resistance to rutting. The energy dissipated by unmodified asphalt binder is 17% higher when compared to SBS polymer modified asphalt binder. Low temperature susceptibility was reduced by 37.5% with SBS polymer modification when compared to unmodified asphalt binder. This shows that only SBS polymer modification can enhance both high temperature rutting resistance and low temperature thermal cracking resistance. 5.2. Dynamic modulus master curve The effect of time and temperature on the behavior of the viscoelastic materials can be described by fitting master curve using time–temperature superposition principle. The hypothesis behind the time–temperature principle is that the mechanical response of viscoelastic materials at high temperature and high strain rate is similar to that of low temperature and low strain rate [37]. Temperature dependency of viscoelastic materials can be accounted by the amount of shifting required at each temperature [30]. Fig. 2
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Table 1 Physical properties of modified and unmodified asphalt binders. Properties
Binder type and specification limits VG-30
Penetration at 25 ◦ C 60–70 46 Softening point (R&B) (◦ C) 80 Ductility at 27 ◦ C (cm) ◦ Elastic recovery at 15 C (%) – Viscosity, poise 5.29 ◦ Separation ( C) – After subjecting to aging in thin film oven 0.42 Loss in weight (%) 18.23 Reduction in penetration at 25 ◦ C (%) Increase in softening point (◦ C) – Elastic recovery at 25 ◦ C (%) – Performance grade after subjecting to short and long-term aging ◦ 74.5–10 Equivalent performance grade ( C)
Limits [6]
PMB
Limits [19]
CRMB
Limits [19]
60–70 45–55 75 min – 3 min (135 ◦ C) –
50–60 60 – 77 7.29 1
50–90 55 min – 70 min 2–6 (150 ◦ C) 3
30–40 56 – 68 7.87 2
<60 55 min – 50 min 2–6 (150 ◦ C) 3
1 max 48 max – –
0.19 12.72 2 60
1 max 35 max 6 max 50 min
0.35 28.57 4 48
1 max 40 max 6 max 35 min
82.2–16
81.8–10
PMB, styrene butadiene styrene polymer modified asphalt binder; CRMB, crumb rubber modified asphalt binder.
shows the dynamic modulus master curves for asphalt concrete mixes with modified and unmodified asphalt binders. As can be seen from Fig. 2, the master curves developed for asphalt concrete mixes with modified and unmodified asphalt binders overlap with each other at higher frequencies. In other words, the magnitude of shift between the asphalt concrete mixes with modified and unmodified asphalt binder at higher frequencies was not found to be significant. However, it may be observed that at lower frequencies (longer loading times) asphalt concrete mixes with SBS polymer and crumb rubber modified asphalt binders showed higher dynamic modulus values when compared to mixes with unmodified asphalt binder. This could be possibly due to the higher stiffness in the polymer and crumb rubber modified asphalt binder itself at low frequencies.
explained by the factors considered in the present study. The null hypothesis that the factors (temperature, frequency and asphalt binder type) have no significant effect on dynamic modulus is tested by calculating and comparing the F statistic against the critical F value. Experimental design formulated for the analysis of variance is as shown in Table 2. The specimens were tested at three temperatures viz., 20, 40 and 60 ◦ C, at ten frequencies for the three asphalt binder types. Statistical summary of the analysis of variance is shown in Table 3. It can be observed that at 95% level of significance, the temperature, frequency, asphalt binder types and model intercept show significant influence on the dynamic modulus values. The intercept shows that there could be unexplained variation in the measured dynamic modulus value due to the factors which were not considered in the present study.
5.2.1. Statistical inference The effect of temperature, frequency and asphalt binder type on the measured dynamic modulus values was analyzed using multi-factor ANOVA (analysis of variance). Generalized linear model was used to infer the influence of fixed factors (temperature, frequency and asphalt binder type) on the response variable (dynamic modulus). An intercept (constant term) was included in the model to account for any variation which could not be
5.2.2. Phase angle The phase angle is the shift between the applied stress and the resultant strain. It can be used to understand the viscoelastic properties of the material, and also signifies the mechanical loss. In a purely elastic response, the phase angle will be zero, whereas a purely viscous response will be indicated by a phase angle of 90◦ . Fig. 3 shows the pictorial representation of phase angle for a viscoelastic material. The effect of temperature variation on the phase lag of asphalt concrete mixes with modified and unmodified asphalt binders are shown in Figs. 4–6. It can be observed from Fig. 4 that at lower temperature of 20 ◦ C, asphalt concrete mixes with crumb rubber modified asphalt binder show a lower phase lag when compared to asphalt concrete mix with SBS polymer modified and unmodified asphalt binders. This may be due to higher storage modulus in the crumb rubber modified asphalt binders at lower temperature which exhibits elastic response. It can be observed in Fig. 5 that when the temperature is increased from 20 to 40 ◦ C, the phase lag in the asphalt concrete mix with unmodified asphalt binder increases at a higher rate when compared to asphalt concrete mixes with SBS polymer and crumb rubber modified asphalt binder mixes. However, at higher temperature of 60 ◦ C (Fig. 6), the mixes with SBS polymer modified asphalt binder exhibited lower phase lag when compared to mixes with crumb rubber and unmodified Table 2 Experimental design.
Fig. 2. Master dynamic modulus curves for asphalt concrete mixes with different binders.
Fixed factors
Levels
Temperature (◦ C) Frequency (Hz) Asphalt binder type
3 10 3
20, 40 and 60 25,20,10, 5, 2, 1, 0.5, 0.2, 0.1 and 0.01 VG-30, PMB and CRMB
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Table 3 Generalized linear model. Dependent variable: dynamic modulus Source
Sum of squares
Corrected model 8.714 × 108 8.456 × 108 Intercept 1.739 × 108 Frequency Temperature 6.832 × 108 Asphalt binder type 1.425 × 107 Error 1.031 × 108 Total 1.820 × 109 9.744 × 108 Corrected total R squared = .894 (adjusted R squared = .876)
df
Mean square
F
Sig.
13 1 9 2 2 76 90 89
6.703 × 107 8.456 × 108 1.933 × 107 3.416 × 108 7.126 × 106 1.355 × 106
49.431 623.612 14.253 251.911 5.256
0.000 0.000 0.000 0.000 0.007
Fig. 3. Phase angle under sinusoidal loading in viscoelastic material [29].
asphalt binders. This shows that modification with styrene butadiene styrene polymer imparts stiffness to asphalt binder at higher temperature. It can also be observed from Fig. 6 that variation in the phase lag in the mixes with SBS polymer modified asphalt binder was much lower than that of asphalt concrete mixes with crumb rubber and unmodified asphalt binders. This shows that temperature susceptibility of asphalt concrete mixes can be improved by SBS polymer modified asphalt binder.
5.2.3. Dissipation During the thermo-mechanical process in a viscoelastic material like asphalt concrete mixes, energy dissipation takes place. This energy transforms to heat and could be used as a measure of damping capacity of viscoelastic materials [37]. Damping energy is defined as the ratio of energy loss per cycle to maximum energy which the system can store for given amplitude. The energy loss
Fig. 4. Phase angle variation with frequency at 20 ◦ C.
Fig. 5. Phase angle variation with frequency at 40 ◦ C.
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Fig. 6. Phase angle variation with frequency at 60 ◦ C.
Fig. 7. Variation of dissipation with frequency at 20 ◦ C.
per cycle could be calculated by integrating the increment of work done over a complete cycle as follows [37]:
T
W =
0
dε dt dt
(7)
Under sinusoidal loading conditions, energy loss may be obtained using,
ε0 0 ω sin ωt cos(ωt − ı)dt
(8)
0
The maximum energy that the material can store in one cycle may be computed as follows: T/4
W=
0
dε dt dt
(9)
Similarly for an sinusoidal loading,
W=
5.3. Dynamic creep test
T
W =
polymer improves the performance of asphalt concrete mixes at higher temperatures when compared to crumb rubber and unmodified asphalt binders. The same is also true when mixes with crumb rubber modified asphalt binder were compared with unmodified asphalt binder at 60 ◦ C.
T/4
ε0 0 ω sin ωt cos ωtdt
(10)
Variation in the accumulated permanent strain in the different asphalt concrete mixes under dynamic creep with temperature can be observed in Figs. 10–12. At lower temperature (20 ◦ C), the asphalt concrete mixes with crumb rubber modified asphalt binder showed least accumulated strain (Fig. 10) when compared to mixes with SBS polymer and unmodified asphalt binders. This may be due to higher stiffness of the crumb rubber modified asphalt binder at lower temperature. However, accumulation of permanent axial strain in mixes with SBS polymer modified asphalt binder was lower than unmodified asphalt binder mixes. However, significant difference in the accumulated permanent strain values were not
0
From Eqs. (8) and (10) the damping energy may be obtained as follows: W = 2 sin ı W
(11)
where W is the energy loss per cycle, W is the maximum energy stored per cycle, is the stress, ε is the strain, and ı is the phase angle. Using the relation given in Eq. (11), damping energy at varying temperatures from 20 to 60 ◦ C for mixes with modified and unmodified asphalt binders were computed. Figs. 7–9 shows the energy dissipated in asphalt concrete mixes with different asphalt binders at different temperatures. It can be observed from Fig. 7 that at low temperature of 20 ◦ C, damping energy of crumb rubber modified asphalt concrete mix was lower than that of SBS polymer modified and unmodified asphalt binder mixes. At 40 ◦ C (Fig. 8) the asphalt concrete mixes with unmodified asphalt binder show a high dissipated energy when compared to mixes with modified asphalt binders. At high temperature of 60 ◦ C, the loss of energy in mixes with SBS polymer modified asphalt binder was significantly lower when compared to crumb rubber modified and unmodified asphalt binder mixes (Fig. 9). This shows that modification of asphalt binder with SBS
Fig. 8. Variation of dissipation with frequency at 40 ◦ C.
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Fig. 9. Variation of dissipation with frequency at 60 ◦ C.
Fig. 11. Accumulated strain at 40 ◦ C under dynamic creep for different asphalt concrete mixes.
observed between different asphalt concrete mixes. Figs. 11 and 12 shows the variation in accumulation of permanent axial strain at 40 and 60 ◦ C for different asphalt concrete mixes. It can be seen that modification of asphalt binder with SBS polymer improves the resistance of asphalt concrete mixes to dynamic creep at higher temperature (60 ◦ C) substantially when compared to mixes with crumb rubber and unmodified asphalt binders.
(Fig. 14). It can be observed from Fig. 15 that at 60 ◦ C mixes with SBS polymer modified asphalt binder exhibited higher resistance to static creep when compared to mixes with crumb rubber and unmodified asphalt binder. This shows that the temperature susceptibility of asphalt binder could be substantially improved by SBS polymer modification.
5.4. Static creep test
5.5. Creep parameters
It can be seen from Fig. 13 that the trend in accumulated strain in case of static and dynamic creep test for the modified and unmodified asphalt binder mixes are similar. As expected, the accumulated strain values were higher in case of static creep test when compared to dynamic creep test in both modified and unmodified asphalt binder mixes. It can be observed (Fig. 13) that mixes with crumb rubber modified asphalt binder showed substantially lower accumulated strain when compared to mixes with SBS polymer and unmodified asphalt binder. Similar trend was observed at 40 ◦ C
Creep parameters under dynamic and static state were estimated using the linear portion (where strain rate is constant over time) of the creep curve. Slope and intercept for both tests were calculated through regression analysis considering the linear portion of the curve. Tables 4 and 5 show the values of slope and intercept values at different temperatures for modified and unmodified asphalt binder mixes under dynamic and static creep tests respectively. These parameters indicate the permanent deformation characteristics of viscoelastic material like asphalt concrete mix [26]. Higher value of intercept essentially shows lower mod-
Fig. 10. Accumulated strain at 20 ◦ C under dynamic creep for different asphalt concrete mixes.
Fig. 12. Accumulated strain at 60 ◦ C under dynamic creep for different asphalt concrete mixes.
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Fig. 13. Accumulated strain at 20 ◦ C under static creep for different asphalt concrete mixes.
Fig. 15. Accumulated strain at 60 ◦ C under static creep for different asphalt concrete mixes.
ulus and lower resistance to creep. At a constant intercept higher slope indicate higher rate of deformation. It can be observed from Tables 4 and 5 that the slope increases with increase in temperature. At higher temperatures (60 ◦ C), asphalt concrete mixes with SBS polymer modified asphalt binder offered higher resistance to deformation and hence showed lower slope when compared to mixes with crumb rubber and unmodified asphalt binder. However at lower temperatures (20 ◦ C), asphalt concrete mixes with crumb rubber modified asphalt binder showed lower rate of deformation when compared to mixes with SBS polymer and unmodified asphalt binder mixes. This may be due to higher stiffness in the crumb rubber modified asphalt binder at lower temperature which offers higher resistance to creep [1]. 5.6. Rutting
Fig. 14. Accumulated strain at 40 ◦ C under static creep for different asphalt concrete mixes.
Figs. 16 and 17 show the rutting resistance of asphalt concrete mixes with modified and unmodified asphalt binders. At 40 ◦ C, significant difference in the rut depth was not observed in asphalt concrete mixes between modified and unmodified binders (Fig. 16). At 40 ◦ C significant difference in the modulus of asphalt
Table 4 Deformation parameters under dynamic creep test for different asphalt concrete mixes. Asphalt binder type in mix
Temperature (◦ C) 20
VG30 PMB CRMB
40
60
Slope
Intercept
Slope
Intercept
Slope
Intercept
0.33 0.23 0.21
485.79 518.04 415.48
1.51 0.86 0.89
1563.40 791.33 830.40
4.77 2.49 2.91
4909.26 1270.96 2643.69
Table 5 Deformation parameters under static creep test for different asphalt concrete mixes. Asphalt binder type in mix
Temperature (◦ C) 20
VG30 PMB CRMB
40
60
Slope
Intercept
Slope
Intercept
Slope
Intercept
1.10 0.85 0.58
2283.29 2071.24 2081.65
1.39 0.94 0.86
3745.83 3112.11 2689.23
1.48 1.02 1.05
8082.65 3684.51 5642.53
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6. Comparison of findings with previous studies
Fig. 16. Rut depth of different asphalt concrete mixes at 40 ◦ C.
concrete mixes was not found with different asphalt binders. However the modulus of asphalt concrete mixes with SBS polymer and crumb rubber modified asphalt binders were higher than mixes with unmodified asphalt binder. This could be the reason for no significant difference in rutting resistance at intermediate temperature of around 40 ◦ C. It can be observed from Fig. 17 that at high temperature of 60 ◦ C, asphalt concrete mixes with SBS polymer modified asphalt binder show higher rutting resistance when compared to mixes with crumb rubber modified and unmodified asphalt binders. The rutting resistance of crumb rubber modified asphalt concrete mixes was also found to be higher than unmodified asphalt binder mixes. From this it can be inferred that, modification of asphalt with styrene butadiene styrene polymer improves the rutting resistance of mixes at high temperatures. This high rut resistant response of asphalt concrete mixes with SBS polymer modified asphalt binder may be due to higher (G*/sin ı) of binder, lower accumulated strain as can be seen from dynamic and static creep tests.
Fig. 17. Rut depth of different asphalt concrete mixes at 60 ◦ C.
Dynamic modulus of asphalt concrete mixes with PG 64-22 (performance grade) asphalt binder at 55 ◦ C ranges from 500 to 2000 MPa for frequency variation from 0.01 to 25 Hz, respectively [39] and also reported to be 350 MPa at 0.5 Hz [24]. In the present study, the dynamic modulus values were found to range from 130 to 1300 MPa for frequency variation from 0.01 to 25 Hz and 270 MPa at 0.5 Hz in case of unmodified (VG-30) at 55 ◦ C. With different unmodified and modified asphalt binders and gradations used, the range of dynamic modulus obtained in the present investigations is found to be comparable with the earlier works. The rutting resistance of asphalt concrete mixes with polymer modified asphalt binder at 40 and 60 ◦ C showed 1.4 and 4.0 times higher rutting resistance respectively when compared to unmodified asphalt binder mixes [17,28,36]. In the present investigation, the asphalt concrete mixes with styrene butadiene styrene polymer modified asphalt binder at 40 and 60 ◦ C showed 1.1 and 4.8 times higher rutting resistance respectively when compared to unmodified asphalt binder mixes. Comparison studies [32] between unmodified asphalt binder and SBS polymer modified asphalt binder mixes at 60 ◦ C showed that mixes with SBS polymer modified asphalt binder showed 3.5 times higher rutting resistance. Experimental results are found to be comparable with the values in published literature. However differences could be because of the variation in asphalt binder type and content and also aggregate gradation with varying nominal maximum size and adopted limits. Dynamic creep tests [17] on all mixes with different aggregate gradations at 40 ◦ C demonstrated that the addition of SBS polymer modifiers enhanced the resistance to permanent deformation of asphalt concrete mixes. Similar trend was also observed in the present investigations. From creep tests it was found that addition of SBS modifiers enhanced the permanent deformation resistance by 2.1 times when compared to unmodified asphalt binder mixes. Accumulated strain for 1000 cycles was found to be 2000 microstrains whereas in the present study it was 1400 microstrains. This may be due to denser gradation of the aggregates adopted in the present study.
7. Conclusions The effect of asphalt binder modification with SBS polymer and crumb rubber on the dynamic mechanical behavior of asphalt concrete mixes has been investigated. Modification of asphalt using SBS polymer and crumb rubber reduced the high temperature susceptibility by 10 and 9.8%, respectively when compared to unmodified asphalt binder. However it is interesting to observe that only SBS polymer modification improved resistance to low temperature cracking by 1.6 times when compared to unmodified asphalt binder. Master curves developed using time–temperature superposition principle showed that, only at lower frequencies (longer loading times) modification of asphalt binder has benefit over unmodified asphalt binder. Drastic variation in the phase lag of asphalt concrete mixes with unmodified asphalt binder when compared to asphalt concrete mixes with SBS polymer and crumb rubber modified asphalt binders explained the possible reasons for the improved thermo-mechanical behavior at high temperatures through modification. Creep stiffness was found to be 3.86 times lower in case of asphalt concrete mixes with unmodified asphalt binder when compared to asphalt concrete mixes with SBS polymer modified asphalt binder. Inferences made using statistical analysis showed that energy dissipated at 60 ◦ C in case of SBS polymer modified asphalt binder was significantly lower than that of unmodified and crumb rubber modified asphalt binder.
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From the multi-factorial analysis of variance, it was found that temperature, frequency and asphalt binder type had significant influence on the measured dynamic modulus values. The benefits of asphalt binder modification reflected through mechanical response were found to correlate with high rutting resistance of asphalt concrete mixes with SBS polymer and crumb rubber modified asphalt binder at 60 ◦ C and found to be significant statistically. These results contributes to the explanations for better performance of asphalt pavements with modified asphalt binders over unmodified asphalt binder at high temperatures and longer loading times. 8. Application of the research work in practice The research work carried out here brings out advantages of asphalt binder modification in improving the high temperature properties of asphalt concrete mixes. The study demonstrates the potential application of polymer and crumb rubber modified asphalt binders in improving the performance of asphalt concrete mixes over unmodified asphalt binder mixes, and also in ranking the different asphalt binders. The analysis carried out in the present investigation helps the researchers to understand the viscoelastic behavior of asphalt binders and mixes which contributes to the desired performance. Inferences made by statistical analysis helps to appreciate the use of modified asphalt binders at high temperatures and longer loading times. Acknowledgments The authors greatly acknowledge the Ministry of Road Transport and Highways (MoRTH), Government of India and Indo-US Science and Technology Forum (IUSSTF) for the financial support provided for the researchers for the research study. References [1] A.A. Yousefi, J. Iran. Polym. 11 (2002) 303–309. [2] A. Khodaii, A. Mehrara, J. Constr. Build. Mater. 23 (2009) 2586–2592. [3] American Association of State Highway and Transportation Officials (AASHTO) M 320, Washington DC, USA, 2009.
[4] American Association of State Highway and Transportation Officials (AASHTO) TP62, Washington DC, USA, 2007. [5] American Society for Testing and Materials (ASTM): D 6925-09, West Conshohocken, PA, USA, 2009. [6] Bureau of Indian standards IS: 73, New Delhi, India, 2006. [7] C. Wekumbura, J. Stastna, L. Zanzotto, J. Mater. Civil Eng. 19 (2007) 227–232. [8] D.O. Larsen, J.L. Alessandrini, A. Bosch, M.S. Cortizo, J. Constr. Build. Mater. 23 (2009) 2769–2774. [9] D.S. Gedafa, M. Hossain, S. Romanoschi, A.J. Gisi, J. Mater. Civil Eng. (2009). [10] D.W. Christensen, T.K. Pellinen, R.F. Bonaquist, J. AAPT 72 (2003) 121–151. [11] European Standard, EN-12697-22, 2002. [12] Zhou S F., T. Scullion, L. Sun, J. Trans. Eng. 130 (2004) 486–494. [13] G.D. Airey, Int. J. Pavement Eng. 5 (2004) 137–151. [14] G.M. Morales, P. Partal, F.J. Navarro, F.M. Boza, C. Gallegos, N. Gonzalez, O. Gonzalez, M.E. Munoz, Fuel 83 (2004) 31–38. [15] H. Azari, A. Mohseni, N. Gibson, J. Transport. Res. Board. 2057 (2008) 157–167. [16] H. Fu, L. Xie, D. Dou, L. Li, M. Yu, S. Yao, J. Constr. Build. Mater. 21 (2007) 1528–1533. [17] H. Ozen, A. Aksoy, S. Tayfur, F. Celik, J. Build. Environ. 43 (2008) 1270–1277. [18] H.J. Lee, J.H. Lee, H.M. Park, J. Constr. Build. Mater. 21 (2006) 1079–1087. [19] Indian Road Congress (IRC) SP: 53, New Delhi, India, 2002. [20] J. Bausano, R.C. Williams, J. Test. Eval. 38 (2009). [21] J.F. Masson, V. Leblond, J. Margeson, J. Microsc. 221 (2005) 17–29. [22] J.N. Wang, J.D. Lin, J.M. Wang, S.H. Chen, Proceedings of the 81st Annual Meeting of the TRB, 2002. [23] J.S. Chen, M.C. Lia, M.S. Shiah, J. Mater. Civil Eng. 14 (2002) 224–229. [24] L.N. Mohammad, S. Saadeh, S. Obulareddy, S. Cooper, J. Test. Eval. 36 (2007). [25] M.A. Elseifi, G.W. Flintsch, I.L. Al-Quadi, J. Mater. Civil Eng. 15 (2003) 32–40. [26] M.W. Witczak, K. Kaloush, T.K. Pellinen, M.E. Basyouny, TRB. National Cooperative Highway Research Program (NCHRP) Report 465, Washington DC, USA, 2002. [27] Ministry of Road Transport and Highways (MoRT&H), 4th Revision, New Delhi, India, 2001. [28] O. Sirin, H.J. Kim, M. Tia, B. Choubane, J. Constr. Build. Mater. 22 (2008) 286–294. [29] R.F. Bonaquist, D.W. Christensen, W. Stump, TRB, National Cooperative Highway Research Program (NCHRP) 513, Washington DC, USA, 2003. [30] R. Bonaquist, TRB, National Cooperative Highway Research Program (NCHRP) 614, Washington DC, USA, 2008. [31] S. Bhattacharjee, R.B. Mallick, J.S. Daniel, ASCE Pavement Conf. (2008) 1–13. [32] S. Tayfur, H. Ozen, A. Aksoy, J. Constr. Build. Mater. 21 (2007) 328–337. [33] S.W. Goh, Z. You, J. Constr. Build. Mater. 23 (2009) 3398–3405. [34] T.K. Pellinen, M.W. Witczak, J. AAPT 71 (2002) 321–350. [35] V. Tandon, X. Bai, S. Nazarian, J. Mater. Civil Eng. (2006) 477–484. [36] W.G. Wong, H. Han, G. He, K.C.P. Wang, W. Lu, J. Constr. Build. Mater. 18 (2004) 399–408. [37] W.N. Findley, J.S. Lai, K. Onaran, Creep and Relaxation of Nonlinear Viscoelastic Materials, Dover Publications, New York, 1989. [38] X. Shu, B. Huang, J. Composit. B 39 (2008) 704–713. [39] Y.H. Cho, D.W. Park, S.D. Hwang, J. Constr. Build. Mater. 24 (2010) 513–519.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Dynamic mechanical characterization of asphalt concrete mixes with modified asphalt binders S. Anjan kumar ∗ , A. Veeraragavan Department of Civil Engineering, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e
i n f o
Article history: Received 11 June 2010 Received in revised form 18 April 2011 Accepted 9 May 2011 Available online 14 May 2011 Keywords: Modified asphalt Dynamic modulus Creep Energy dissipation Rutting
a b s t r a c t The primary objective of this work is to characterize and compare the dynamic mechanical behavior of asphalt concrete mixes with styrene butadiene styrene (SBS) polymer and crumb rubber modified asphalt binders with the behavior of mixes with unmodified viscosity grade asphalt binders. Asphalt binders are characterized for their physical and rheological properties. Simple performance tests like dynamic modulus, dynamic and static creep tests are carried out at varying temperatures and time. Dynamic modulus master curves constructed using numerical optimization technique is used to explain the time and temperature dependency of modified and unmodified asphalt binder mixes. Creep parameters estimated through regression analysis explained the permanent deformation characteristics of asphalt concrete mixes. From the dynamic mechanical characterization studies, it is found that asphalt concrete mixes with SBS polymer modified asphalt binder showed significantly higher values of dynamic modulus and reduced rate of deformation at higher temperatures when compared to asphalt concrete mixes with crumb rubber and unmodified asphalt binders. From the concept of energy dissipation, it is found that SBS polymer modification substantially reduces the energy loss at higher temperatures. Multi-factorial analysis of variance carried out using generalized liner model showed that temperature, frequency and asphalt binder type significant influences the mechanical response of asphalt concrete mixes. The mechanical response of SBS polymer modified asphalt binders are significantly correlated with the rutting resistance of asphalt concrete mixes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Review of earlier work
Performance of asphalt pavement depends on the properties of materials used, design accuracy and construction quality. Material characterization is the most important task that plays a significant role in the design and performance of the pavements during the design life. Asphalt concrete mix is a composite mixture of aggregates, mineral filler and asphalt binder. Consequent to characterisation of the various ingredients of an asphalt concrete mix for its fundamental properties, the mix design is crucial as it dictates the performance of asphalt pavements during the design life. The testing of mechanical properties of mixes is not envisaged in the Superpave method of mix design. Protocols have been developed and presented in NCHRP 9-29 [26] to characterize the asphalt concrete mixes for their mechanical properties, such that resistance to rutting and fracture and the properties can be evaluated using the simple performance tester.
Asphalt concrete mixes exhibit viscoelastic solid to fluid like behavior depending upon on the time and temperature of testing. Hence the time–temperature dependent characteristics of asphalt concrete mixes are to be considered while evaluating their fundamental properties. The dynamic modulus determined under sinusoidal loading can be used to characterize the mechanical response of asphalt concrete mixes under varying time and temperature. For a viscoelastic material like asphalt concrete mixes, master curves may be developed at a reference temperature using the time–temperature superposition principles to account for variation in temperature and time of loading within the linear viscoelastic range. Stress dependent dynamic modulus predictive equation was developed using the k– non-linear elastic models [34], and it considers the effect of bulk and shear stresses on the dynamic modulus values. In 2003, Christensen et al. [10] used Hirsch model to predict the dynamic modulus of asphalt concrete mixes using binder modulus and mixture composition based on the law of mixtures. Studies [24,31,35] have shown that, dynamic moduli of asphalt concrete mixes are significantly dependent on the specimen geometry, nominal aggregate size, loading time and test temperature. It has
∗ Corresponding author. Fax: +91 44 22574250. E-mail addresses: [email protected] (S. Anjan kumar), [email protected] (A. Veeraragavan). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.05.008
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been reported that the dynamic moduli values can be increased by using stiff asphalt binders at lower binder content and lower (3%) air voids level [38]. However, variations in the measured and predicted dynamic modulus of asphalt concrete mix are found to increase with increase in temperature. Also, significant difference in dynamic modulus value measured in laboratory and field [9] have been reported. Investigations [39] also show that the effect of air voids on the dynamic modulus was not significant when compared to temperature and frequency. Modification of asphalt using high boiling point petroleum and SBS polymer increases the dynamic modulus and fatigue resistance of asphalt concrete mixes when compared to conventional asphalt concrete mixes [18]. Studies [24] show that simple performance test parameters like flow number and time are useful to differentiate asphalt concrete mixes based on the traffic level. Flow number (dynamic creep) estimation is highly dependent on the repeatability of data collected and could be misleading [20,33]. Use of failure point determined by creep stiffness could reduce the error resulting due to low data point in flow number determination [20]. The rate of deformation estimated during secondary flow using a stepwise method assuming that strain is neither held constant nor increased could also give a good estimate of flow number [33]. Rutting may be considered as the most important type of distress in asphalt pavements. Mix design and tests on asphalt concrete mixes using simple performance tests could be dutiful to build up the correlation between the mix response and rutting [26]. The mechanical behavior of asphalt concrete mixes under repeated loading can be characterized by primary, secondary and tertiary stages. Transition between these three stages can be assigned as a rut indicator [12]. It was also shown that SBS polymer modified asphalt binder with linear grafting improves resistance to rutting when compared to conventional and other modified asphalt binders [15]. The mechanical properties and temperature susceptibility of asphalt concrete mixes could be improved by using SBS polymer modified asphalt binder [2]. Continuous phase of polymer in modified asphalt was found to be reasoning behind the improved rutting and fatigue properties of modified asphalt concrete mixes [13]. The improved toughness and tenacity and also formation of network in asphalt binder depend on the type and amount of polymer used for modification which results in increased complex modulus [22,23]. It was found that rubber modification increases the high temperature properties of asphalt mixes but makes the material brittle at low temperatures and these phenomena may be due to swallowing of oil fraction and carbon black present in the rubber [1]. Polymer modification was found to be effective in increasing rutting resistance at high temperature and fatigue resistance at intermediate temperatures. This behavior was essentially due to micro-structural changes related to the development of polymer-rich phase and molecular arrangements [14,16,21,25]. The disturbed structure of polymer modified asphalt binders during shear was found to reform with time and this property imparts the ability of self-healing [7]. Essentially in polymer modification, optimization of blend composition, shear rate, temperature and time produces the best rheological properties of modified asphalt binder [8]. The above studies have shown that simple performance tests evaluate the asphalt concrete mixes for their resistance to rutting and fracture. Modification of asphalt binder enhances the resistance to permanent deformation of asphalt concrete mixes. Characterizing and understanding the mechanical response of multi-constituent material like asphalt binder is a difficult task. Modification of asphalt binder with different types of additives like polymer, crumb rubber etc., has added more complexity to this task. Limited studies were carried out on mixes with different modified asphalt binders under varying temperature and time. Also the relation between the mechanical properties of modified
Fig. 1. Aggregate gradation of bituminous concrete.
asphalt binders and asphalt concrete mixes are not completely understood. Modification is aimed at enhancing the temperature susceptibility of asphalt binders. Hence in the present study, the combined effect of time, temperature and asphalt binder type on the observed mechanical response of asphalt concrete mixes are evaluated, so as to provide the much needed information on the feasibility of modified asphalt binders for enhanced life. Investigations on the fundamental properties and mechanical characteristics will facilitate ranking of the performance of asphalt concrete mixes with different modified asphalt binders and their comparison with asphalt concrete mixes with unmodified asphalt binders. 3. Objectives The objectives of the studies are: • To compare the physical and rheological properties of modified asphalt binders with unmodified asphalt binder. • To evaluate the dynamic mechanical response of asphalt concrete mixes with modified and unmodified asphalt binders under varying temperature and time. • To investigate the creep characteristics of the asphalt concrete mixes with modified and unmodified asphalt binders under static and dynamic loading conditions. • To characterize the rutting resistance of asphalt concrete mixes with modified and unmodified asphalt binders using dry wheel tracker. 4. Experimental investigations 4.1. Materials used SBS polymer and crumb rubber modified asphalt binders and unmodified viscosity grade (VG-30) asphalt binder were used in the present investigations. The aggregate gradation for asphalt concrete mix (Fig. 1), grade-2 (bituminous concrete) recommended by Indian specification [27] was adopted. 4.2. Simple performance tests 4.2.1. Dynamic modulus (NCHRP 9-19 [26]) Tests were carried out as per AASHTO TP62 [4] using simple performance tester to characterize the dynamic modulus of
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asphalt concrete mixes. Asphalt concrete mixes with modified and unmodified asphalt binders were subjected to sinusoidal (haversine) compressive loading, varying the frequency from 25 to 0.01 Hz and temperature from 20 to 60 ◦ C. The specimens were tested under unconfined condition within the linear viscoelastic range, where the strain is between 75 and 125 micro-strains [26]. The applied stress and resulting axial strain from three on-specimen mounted displacement transducers were measured as a function of time. From the data collected, the dynamic modulus and phase angle were calculated as follows: = (E1 + iE2 )ε
Pi =
E2 =
15 (mij − moj ) j=1
= stress, E1 =
the performance of asphalt concrete mixes as per European standard 12697-22(E) [11]. This was attempted to address the benefit of modification under slow vehicular movement conditions for which the asphalt modification is aimed. Asphalt concrete mixes with modified and unmodified asphalt binders were conditioned for 6 h before the actual commencement of the test in an environmental chamber. All specimens were tested at temperature of 40 and 60 ◦ C for 10000 passes or up to rut depth of 10 mm, whichever is earlier. From the 15 local profile measurement points, the proportional rut depth was calculated using the following relationship:
(1)
where
o
εo
o
εo
(6)
cos ı, (storage modulus)
(2)
sin ı, (loss modulus)
(3)
4.3. Constrained variables
ı = phase angle. The magnitude of the complex modulus called as dynamic modulus is given by,
∗ E = (E12 + E22 )1/2
(4)
substituting (2) and (3) in (4) we get, εo
15 × h
where Pi is the measured proportional rut depth, mij is the local deformation, moj is the initial measurement at the jth location, and h is the specimen thickness.
ε = strain, and
∗ o E =
6447
(5)
4.2.2. Dynamic creep Tests were carried out on asphalt concrete mixes to evaluate the permanent deformation characteristics under repeated loading/dynamic creep conforming to NCHRP 9-19 [26]. The asphalt concrete mixes with modified and unmodified asphalt binders were subjected to a repeated haversine axial compressive loading. Loading time of 0.1 s with 0.9 s dwell was maintained constant and 1000 cycles were applied. The axial stress of 200 kPa and contact stress of 5 kPa was applied. Asphalt concrete mixes were tested at varying temperature from 20 to 60 ◦ C. The specimens were tested under unconfined condition. The resulting permanent axial strains were measured as a function of the number of applied loading cycles. 4.2.3. Static creep Asphalt concrete mixes with modified and unmodified asphalt binders were subjected to static axial compressive loading also known as static creep. Axial deviatoric stress of 200 kPa and contact stress of 5 kPa was applied. The asphalt concrete mixes were tested at varying temperatures from 20 to 60 ◦ C. All samples were tested under unconfined condition. The resulting permanent axial strains were measured as a function time. These loading conditions were selected so as to evaluate the relative performance of different asphalt concrete mixes without undue flowing at high temperatures due to high deviatoric stress. 4.2.4. Wheel tracker The rutting resistance of asphalt concrete mixes was evaluated using dry wheel tracker. Moving load of 700 N was applied through a rubber hosed wheel of 200 mm diameter, 50 mm width on 150 mm diameter cylindrical specimens. The speed was maintained at 53 passes per minute. The maximum load and minimum speed of equipment capability was selected to relatively evaluate
All specimens were prepared using gyratory compactor with a single aggregate gradation at a constant asphalt binder content of 5.25%. The compaction effort was maintained constant, such that the volumetric properties are comparable between asphalt concrete mixes with modified and unmodified asphalt binders. This binder content was maintained constant as the optimum percentage obtained in case of asphalt concrete mixes with unmodified asphalt binder. All specimens tested for mechanical and performance characteristics were subjected to short-term aging as per ASTM D 6925-09 [5]. 5. Results and discussions 5.1. Asphalt binder characterization Modified and unmodified asphalt binders characterized in the present study fulfilled the requirements as per Indian specifications [6,19]. Performance grading of asphalt binders were carried out as per AASHTO [3]. The temperature at which G*/sin ı ≥ 2.2 kPa is found to be higher for modified asphalt binders when compared to unmodified asphalt binder and is shown in Table 1. Significant difference in the highest critical temperature causing rutting between SBS polymer modified and crumb rubber modified asphalt binders was not observed. SBS Polymer modified asphalt binder has showed the lowest phase angle. This explains that due to elastic nature (lower phase angle), the SBS polymer modified asphalt binder results in enhanced resistance to rutting. The energy dissipated by unmodified asphalt binder is 17% higher when compared to SBS polymer modified asphalt binder. Low temperature susceptibility was reduced by 37.5% with SBS polymer modification when compared to unmodified asphalt binder. This shows that only SBS polymer modification can enhance both high temperature rutting resistance and low temperature thermal cracking resistance. 5.2. Dynamic modulus master curve The effect of time and temperature on the behavior of the viscoelastic materials can be described by fitting master curve using time–temperature superposition principle. The hypothesis behind the time–temperature principle is that the mechanical response of viscoelastic materials at high temperature and high strain rate is similar to that of low temperature and low strain rate [37]. Temperature dependency of viscoelastic materials can be accounted by the amount of shifting required at each temperature [30]. Fig. 2
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Table 1 Physical properties of modified and unmodified asphalt binders. Properties
Binder type and specification limits VG-30
Penetration at 25 ◦ C 60–70 46 Softening point (R&B) (◦ C) 80 Ductility at 27 ◦ C (cm) ◦ Elastic recovery at 15 C (%) – Viscosity, poise 5.29 ◦ Separation ( C) – After subjecting to aging in thin film oven 0.42 Loss in weight (%) 18.23 Reduction in penetration at 25 ◦ C (%) Increase in softening point (◦ C) – Elastic recovery at 25 ◦ C (%) – Performance grade after subjecting to short and long-term aging ◦ 74.5–10 Equivalent performance grade ( C)
Limits [6]
PMB
Limits [19]
CRMB
Limits [19]
60–70 45–55 75 min – 3 min (135 ◦ C) –
50–60 60 – 77 7.29 1
50–90 55 min – 70 min 2–6 (150 ◦ C) 3
30–40 56 – 68 7.87 2
<60 55 min – 50 min 2–6 (150 ◦ C) 3
1 max 48 max – –
0.19 12.72 2 60
1 max 35 max 6 max 50 min
0.35 28.57 4 48
1 max 40 max 6 max 35 min
82.2–16
81.8–10
PMB, styrene butadiene styrene polymer modified asphalt binder; CRMB, crumb rubber modified asphalt binder.
shows the dynamic modulus master curves for asphalt concrete mixes with modified and unmodified asphalt binders. As can be seen from Fig. 2, the master curves developed for asphalt concrete mixes with modified and unmodified asphalt binders overlap with each other at higher frequencies. In other words, the magnitude of shift between the asphalt concrete mixes with modified and unmodified asphalt binder at higher frequencies was not found to be significant. However, it may be observed that at lower frequencies (longer loading times) asphalt concrete mixes with SBS polymer and crumb rubber modified asphalt binders showed higher dynamic modulus values when compared to mixes with unmodified asphalt binder. This could be possibly due to the higher stiffness in the polymer and crumb rubber modified asphalt binder itself at low frequencies.
explained by the factors considered in the present study. The null hypothesis that the factors (temperature, frequency and asphalt binder type) have no significant effect on dynamic modulus is tested by calculating and comparing the F statistic against the critical F value. Experimental design formulated for the analysis of variance is as shown in Table 2. The specimens were tested at three temperatures viz., 20, 40 and 60 ◦ C, at ten frequencies for the three asphalt binder types. Statistical summary of the analysis of variance is shown in Table 3. It can be observed that at 95% level of significance, the temperature, frequency, asphalt binder types and model intercept show significant influence on the dynamic modulus values. The intercept shows that there could be unexplained variation in the measured dynamic modulus value due to the factors which were not considered in the present study.
5.2.1. Statistical inference The effect of temperature, frequency and asphalt binder type on the measured dynamic modulus values was analyzed using multi-factor ANOVA (analysis of variance). Generalized linear model was used to infer the influence of fixed factors (temperature, frequency and asphalt binder type) on the response variable (dynamic modulus). An intercept (constant term) was included in the model to account for any variation which could not be
5.2.2. Phase angle The phase angle is the shift between the applied stress and the resultant strain. It can be used to understand the viscoelastic properties of the material, and also signifies the mechanical loss. In a purely elastic response, the phase angle will be zero, whereas a purely viscous response will be indicated by a phase angle of 90◦ . Fig. 3 shows the pictorial representation of phase angle for a viscoelastic material. The effect of temperature variation on the phase lag of asphalt concrete mixes with modified and unmodified asphalt binders are shown in Figs. 4–6. It can be observed from Fig. 4 that at lower temperature of 20 ◦ C, asphalt concrete mixes with crumb rubber modified asphalt binder show a lower phase lag when compared to asphalt concrete mix with SBS polymer modified and unmodified asphalt binders. This may be due to higher storage modulus in the crumb rubber modified asphalt binders at lower temperature which exhibits elastic response. It can be observed in Fig. 5 that when the temperature is increased from 20 to 40 ◦ C, the phase lag in the asphalt concrete mix with unmodified asphalt binder increases at a higher rate when compared to asphalt concrete mixes with SBS polymer and crumb rubber modified asphalt binder mixes. However, at higher temperature of 60 ◦ C (Fig. 6), the mixes with SBS polymer modified asphalt binder exhibited lower phase lag when compared to mixes with crumb rubber and unmodified Table 2 Experimental design.
Fig. 2. Master dynamic modulus curves for asphalt concrete mixes with different binders.
Fixed factors
Levels
Temperature (◦ C) Frequency (Hz) Asphalt binder type
3 10 3
20, 40 and 60 25,20,10, 5, 2, 1, 0.5, 0.2, 0.1 and 0.01 VG-30, PMB and CRMB
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Table 3 Generalized linear model. Dependent variable: dynamic modulus Source
Sum of squares
Corrected model 8.714 × 108 8.456 × 108 Intercept 1.739 × 108 Frequency Temperature 6.832 × 108 Asphalt binder type 1.425 × 107 Error 1.031 × 108 Total 1.820 × 109 9.744 × 108 Corrected total R squared = .894 (adjusted R squared = .876)
df
Mean square
F
Sig.
13 1 9 2 2 76 90 89
6.703 × 107 8.456 × 108 1.933 × 107 3.416 × 108 7.126 × 106 1.355 × 106
49.431 623.612 14.253 251.911 5.256
0.000 0.000 0.000 0.000 0.007
Fig. 3. Phase angle under sinusoidal loading in viscoelastic material [29].
asphalt binders. This shows that modification with styrene butadiene styrene polymer imparts stiffness to asphalt binder at higher temperature. It can also be observed from Fig. 6 that variation in the phase lag in the mixes with SBS polymer modified asphalt binder was much lower than that of asphalt concrete mixes with crumb rubber and unmodified asphalt binders. This shows that temperature susceptibility of asphalt concrete mixes can be improved by SBS polymer modified asphalt binder.
5.2.3. Dissipation During the thermo-mechanical process in a viscoelastic material like asphalt concrete mixes, energy dissipation takes place. This energy transforms to heat and could be used as a measure of damping capacity of viscoelastic materials [37]. Damping energy is defined as the ratio of energy loss per cycle to maximum energy which the system can store for given amplitude. The energy loss
Fig. 4. Phase angle variation with frequency at 20 ◦ C.
Fig. 5. Phase angle variation with frequency at 40 ◦ C.
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Fig. 6. Phase angle variation with frequency at 60 ◦ C.
Fig. 7. Variation of dissipation with frequency at 20 ◦ C.
per cycle could be calculated by integrating the increment of work done over a complete cycle as follows [37]:
T
W =
0
dε dt dt
(7)
Under sinusoidal loading conditions, energy loss may be obtained using,
ε0 0 ω sin ωt cos(ωt − ı)dt
(8)
0
The maximum energy that the material can store in one cycle may be computed as follows: T/4
W=
0
dε dt dt
(9)
Similarly for an sinusoidal loading,
W=
5.3. Dynamic creep test
T
W =
polymer improves the performance of asphalt concrete mixes at higher temperatures when compared to crumb rubber and unmodified asphalt binders. The same is also true when mixes with crumb rubber modified asphalt binder were compared with unmodified asphalt binder at 60 ◦ C.
T/4
ε0 0 ω sin ωt cos ωtdt
(10)
Variation in the accumulated permanent strain in the different asphalt concrete mixes under dynamic creep with temperature can be observed in Figs. 10–12. At lower temperature (20 ◦ C), the asphalt concrete mixes with crumb rubber modified asphalt binder showed least accumulated strain (Fig. 10) when compared to mixes with SBS polymer and unmodified asphalt binders. This may be due to higher stiffness of the crumb rubber modified asphalt binder at lower temperature. However, accumulation of permanent axial strain in mixes with SBS polymer modified asphalt binder was lower than unmodified asphalt binder mixes. However, significant difference in the accumulated permanent strain values were not
0
From Eqs. (8) and (10) the damping energy may be obtained as follows: W = 2 sin ı W
(11)
where W is the energy loss per cycle, W is the maximum energy stored per cycle, is the stress, ε is the strain, and ı is the phase angle. Using the relation given in Eq. (11), damping energy at varying temperatures from 20 to 60 ◦ C for mixes with modified and unmodified asphalt binders were computed. Figs. 7–9 shows the energy dissipated in asphalt concrete mixes with different asphalt binders at different temperatures. It can be observed from Fig. 7 that at low temperature of 20 ◦ C, damping energy of crumb rubber modified asphalt concrete mix was lower than that of SBS polymer modified and unmodified asphalt binder mixes. At 40 ◦ C (Fig. 8) the asphalt concrete mixes with unmodified asphalt binder show a high dissipated energy when compared to mixes with modified asphalt binders. At high temperature of 60 ◦ C, the loss of energy in mixes with SBS polymer modified asphalt binder was significantly lower when compared to crumb rubber modified and unmodified asphalt binder mixes (Fig. 9). This shows that modification of asphalt binder with SBS
Fig. 8. Variation of dissipation with frequency at 40 ◦ C.
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Fig. 9. Variation of dissipation with frequency at 60 ◦ C.
Fig. 11. Accumulated strain at 40 ◦ C under dynamic creep for different asphalt concrete mixes.
observed between different asphalt concrete mixes. Figs. 11 and 12 shows the variation in accumulation of permanent axial strain at 40 and 60 ◦ C for different asphalt concrete mixes. It can be seen that modification of asphalt binder with SBS polymer improves the resistance of asphalt concrete mixes to dynamic creep at higher temperature (60 ◦ C) substantially when compared to mixes with crumb rubber and unmodified asphalt binders.
(Fig. 14). It can be observed from Fig. 15 that at 60 ◦ C mixes with SBS polymer modified asphalt binder exhibited higher resistance to static creep when compared to mixes with crumb rubber and unmodified asphalt binder. This shows that the temperature susceptibility of asphalt binder could be substantially improved by SBS polymer modification.
5.4. Static creep test
5.5. Creep parameters
It can be seen from Fig. 13 that the trend in accumulated strain in case of static and dynamic creep test for the modified and unmodified asphalt binder mixes are similar. As expected, the accumulated strain values were higher in case of static creep test when compared to dynamic creep test in both modified and unmodified asphalt binder mixes. It can be observed (Fig. 13) that mixes with crumb rubber modified asphalt binder showed substantially lower accumulated strain when compared to mixes with SBS polymer and unmodified asphalt binder. Similar trend was observed at 40 ◦ C
Creep parameters under dynamic and static state were estimated using the linear portion (where strain rate is constant over time) of the creep curve. Slope and intercept for both tests were calculated through regression analysis considering the linear portion of the curve. Tables 4 and 5 show the values of slope and intercept values at different temperatures for modified and unmodified asphalt binder mixes under dynamic and static creep tests respectively. These parameters indicate the permanent deformation characteristics of viscoelastic material like asphalt concrete mix [26]. Higher value of intercept essentially shows lower mod-
Fig. 10. Accumulated strain at 20 ◦ C under dynamic creep for different asphalt concrete mixes.
Fig. 12. Accumulated strain at 60 ◦ C under dynamic creep for different asphalt concrete mixes.
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Fig. 13. Accumulated strain at 20 ◦ C under static creep for different asphalt concrete mixes.
Fig. 15. Accumulated strain at 60 ◦ C under static creep for different asphalt concrete mixes.
ulus and lower resistance to creep. At a constant intercept higher slope indicate higher rate of deformation. It can be observed from Tables 4 and 5 that the slope increases with increase in temperature. At higher temperatures (60 ◦ C), asphalt concrete mixes with SBS polymer modified asphalt binder offered higher resistance to deformation and hence showed lower slope when compared to mixes with crumb rubber and unmodified asphalt binder. However at lower temperatures (20 ◦ C), asphalt concrete mixes with crumb rubber modified asphalt binder showed lower rate of deformation when compared to mixes with SBS polymer and unmodified asphalt binder mixes. This may be due to higher stiffness in the crumb rubber modified asphalt binder at lower temperature which offers higher resistance to creep [1]. 5.6. Rutting
Fig. 14. Accumulated strain at 40 ◦ C under static creep for different asphalt concrete mixes.
Figs. 16 and 17 show the rutting resistance of asphalt concrete mixes with modified and unmodified asphalt binders. At 40 ◦ C, significant difference in the rut depth was not observed in asphalt concrete mixes between modified and unmodified binders (Fig. 16). At 40 ◦ C significant difference in the modulus of asphalt
Table 4 Deformation parameters under dynamic creep test for different asphalt concrete mixes. Asphalt binder type in mix
Temperature (◦ C) 20
VG30 PMB CRMB
40
60
Slope
Intercept
Slope
Intercept
Slope
Intercept
0.33 0.23 0.21
485.79 518.04 415.48
1.51 0.86 0.89
1563.40 791.33 830.40
4.77 2.49 2.91
4909.26 1270.96 2643.69
Table 5 Deformation parameters under static creep test for different asphalt concrete mixes. Asphalt binder type in mix
Temperature (◦ C) 20
VG30 PMB CRMB
40
60
Slope
Intercept
Slope
Intercept
Slope
Intercept
1.10 0.85 0.58
2283.29 2071.24 2081.65
1.39 0.94 0.86
3745.83 3112.11 2689.23
1.48 1.02 1.05
8082.65 3684.51 5642.53
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6. Comparison of findings with previous studies
Fig. 16. Rut depth of different asphalt concrete mixes at 40 ◦ C.
concrete mixes was not found with different asphalt binders. However the modulus of asphalt concrete mixes with SBS polymer and crumb rubber modified asphalt binders were higher than mixes with unmodified asphalt binder. This could be the reason for no significant difference in rutting resistance at intermediate temperature of around 40 ◦ C. It can be observed from Fig. 17 that at high temperature of 60 ◦ C, asphalt concrete mixes with SBS polymer modified asphalt binder show higher rutting resistance when compared to mixes with crumb rubber modified and unmodified asphalt binders. The rutting resistance of crumb rubber modified asphalt concrete mixes was also found to be higher than unmodified asphalt binder mixes. From this it can be inferred that, modification of asphalt with styrene butadiene styrene polymer improves the rutting resistance of mixes at high temperatures. This high rut resistant response of asphalt concrete mixes with SBS polymer modified asphalt binder may be due to higher (G*/sin ı) of binder, lower accumulated strain as can be seen from dynamic and static creep tests.
Fig. 17. Rut depth of different asphalt concrete mixes at 60 ◦ C.
Dynamic modulus of asphalt concrete mixes with PG 64-22 (performance grade) asphalt binder at 55 ◦ C ranges from 500 to 2000 MPa for frequency variation from 0.01 to 25 Hz, respectively [39] and also reported to be 350 MPa at 0.5 Hz [24]. In the present study, the dynamic modulus values were found to range from 130 to 1300 MPa for frequency variation from 0.01 to 25 Hz and 270 MPa at 0.5 Hz in case of unmodified (VG-30) at 55 ◦ C. With different unmodified and modified asphalt binders and gradations used, the range of dynamic modulus obtained in the present investigations is found to be comparable with the earlier works. The rutting resistance of asphalt concrete mixes with polymer modified asphalt binder at 40 and 60 ◦ C showed 1.4 and 4.0 times higher rutting resistance respectively when compared to unmodified asphalt binder mixes [17,28,36]. In the present investigation, the asphalt concrete mixes with styrene butadiene styrene polymer modified asphalt binder at 40 and 60 ◦ C showed 1.1 and 4.8 times higher rutting resistance respectively when compared to unmodified asphalt binder mixes. Comparison studies [32] between unmodified asphalt binder and SBS polymer modified asphalt binder mixes at 60 ◦ C showed that mixes with SBS polymer modified asphalt binder showed 3.5 times higher rutting resistance. Experimental results are found to be comparable with the values in published literature. However differences could be because of the variation in asphalt binder type and content and also aggregate gradation with varying nominal maximum size and adopted limits. Dynamic creep tests [17] on all mixes with different aggregate gradations at 40 ◦ C demonstrated that the addition of SBS polymer modifiers enhanced the resistance to permanent deformation of asphalt concrete mixes. Similar trend was also observed in the present investigations. From creep tests it was found that addition of SBS modifiers enhanced the permanent deformation resistance by 2.1 times when compared to unmodified asphalt binder mixes. Accumulated strain for 1000 cycles was found to be 2000 microstrains whereas in the present study it was 1400 microstrains. This may be due to denser gradation of the aggregates adopted in the present study.
7. Conclusions The effect of asphalt binder modification with SBS polymer and crumb rubber on the dynamic mechanical behavior of asphalt concrete mixes has been investigated. Modification of asphalt using SBS polymer and crumb rubber reduced the high temperature susceptibility by 10 and 9.8%, respectively when compared to unmodified asphalt binder. However it is interesting to observe that only SBS polymer modification improved resistance to low temperature cracking by 1.6 times when compared to unmodified asphalt binder. Master curves developed using time–temperature superposition principle showed that, only at lower frequencies (longer loading times) modification of asphalt binder has benefit over unmodified asphalt binder. Drastic variation in the phase lag of asphalt concrete mixes with unmodified asphalt binder when compared to asphalt concrete mixes with SBS polymer and crumb rubber modified asphalt binders explained the possible reasons for the improved thermo-mechanical behavior at high temperatures through modification. Creep stiffness was found to be 3.86 times lower in case of asphalt concrete mixes with unmodified asphalt binder when compared to asphalt concrete mixes with SBS polymer modified asphalt binder. Inferences made using statistical analysis showed that energy dissipated at 60 ◦ C in case of SBS polymer modified asphalt binder was significantly lower than that of unmodified and crumb rubber modified asphalt binder.
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From the multi-factorial analysis of variance, it was found that temperature, frequency and asphalt binder type had significant influence on the measured dynamic modulus values. The benefits of asphalt binder modification reflected through mechanical response were found to correlate with high rutting resistance of asphalt concrete mixes with SBS polymer and crumb rubber modified asphalt binder at 60 ◦ C and found to be significant statistically. These results contributes to the explanations for better performance of asphalt pavements with modified asphalt binders over unmodified asphalt binder at high temperatures and longer loading times. 8. Application of the research work in practice The research work carried out here brings out advantages of asphalt binder modification in improving the high temperature properties of asphalt concrete mixes. The study demonstrates the potential application of polymer and crumb rubber modified asphalt binders in improving the performance of asphalt concrete mixes over unmodified asphalt binder mixes, and also in ranking the different asphalt binders. The analysis carried out in the present investigation helps the researchers to understand the viscoelastic behavior of asphalt binders and mixes which contributes to the desired performance. Inferences made by statistical analysis helps to appreciate the use of modified asphalt binders at high temperatures and longer loading times. Acknowledgments The authors greatly acknowledge the Ministry of Road Transport and Highways (MoRTH), Government of India and Indo-US Science and Technology Forum (IUSSTF) for the financial support provided for the researchers for the research study. References [1] A.A. Yousefi, J. Iran. Polym. 11 (2002) 303–309. [2] A. Khodaii, A. Mehrara, J. Constr. Build. Mater. 23 (2009) 2586–2592. [3] American Association of State Highway and Transportation Officials (AASHTO) M 320, Washington DC, USA, 2009.
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