A study to compare virgin and target asphalt binder obtained from various RAP blending charts

Construction and Building Materials 224 (2019) 109–123

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A study to compare virgin and target asphalt binder obtained from various RAP blending charts Dharamveer Singh, Burhan Showkat ⇑, Dheeraj Sawant Civil Engineering Department, Indian Institute of Technology Bombay, Mumbai 400076 India

h i g h l i g h t s
This work compared the rheological and chemical properties of virgin AC30 binder and equivalent grade AC10 binder obtained by blending RAP binder.
The proportion of RAP binder to be blended with AC10 was obtained using viscosity and PG blending charts.
RAP proportion was also determined based on performance tests such as LAS and MSCR.
RAP proportion obtained by various contrasting methods was compared.

a r t i c l e

i n f o

Article history: Received 21 September 2018 Received in revised form 19 April 2019 Accepted 6 July 2019

Keywords: Reclaimed asphalt pavement Blending chart LAS MSCR FTIR

a b s t r a c t Use of reclaimed asphalt pavement (RAP) material in mixes is on the rise. Currently, RAP proportion to reach the target binder grade is determined by absolute viscosity and performance grade (PG) methods. However, it is necessary to compare the RAP proportion recommended by the two methods. Further, it is essential to ascertain whether the RAP blended target binder is equivalent to virgin target binder based on various rheological and chemical properties. In this study different percentages (i.e. 0%, 15%, 25%, 40% and 50% by weight of binder) of RAP binder was blended with base binder (AC10). RAP proportion to achieve target grade (AC30) was determined based on absolute viscosity, PG, LAS and MSCR tests. Absolute viscosity and PG methods recommended the RAP proportion of 48% and 32% respectively. Thereafter, average of the two RAP proportions was blended with AC10 binder to obtain target grade of AC30 (AC30_RAP). AC30_RAP was observed to have a higher fatigue life than AC30 binder at lower strains and the trend reversed at higher strain. Higher recovery (R) and lower non-recoverable creep compliance value (Jnr) was exhibited by AC30. FTIR spectroscopy indicated that AC30_RAP had higher sulphoxide and carbonyl content. LAS and MSCR tests recommended the RAP proportion of 35% and 48% based on fatigue and rutting performance respectively. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Utilization of recycled asphalt pavement (RAP) material is advantageous from technical, economic and environmental perspective [1–4]. Since RAP is obtained from the pavement that has experienced diverse weather conditions, extreme solar heat, varied temperature, traffic loading and unloading, the binder extracted from RAP is aged and stiff which necessitates performing grade adjustment during mix design of asphalt mixes [5,6]. One of the most common methods of grade adjustment is selecting an appropriate softer grade of base binder so as to ensure that RAP blending does not hamper overall performance of asphalt mixes [5,7]. Mac⇑ Corresponding author. E-mail addresses: [email protected] (D. Singh), burhanshowkat524@gmail. com (B. Showkat). https://doi.org/10.1016/j.conbuildmat.2019.07.038 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

Daniel and Anderson [1] have recommended a three-tier system to choose the base binder for different proportions of RAP. They suggest that the grade of base binder should remain unaltered if <15% RAP binder is utilised. Further, if the percentage of RAP binder is 15% to 25%, the base binder has to be reduced by one performance grade. Likewise, if percentage of RAP binder is >25%, preparation of blending chart is recommended. However, some of the studies have indicated that PG grades recommended by the three-tier system may be contradictory due to variation in RAP source, which necessitates the construction of blending charts, even at the first and the second tiers so that the agencies can have more confidence on selection of base binder [8,9]. Two different approaches are recommended for the selection of proportion of RAP binder: (i) Absolute viscosity method and (ii) PG method [7,10]. Both of these methods adopt graphical approach, and assume full blending of RAP binder and base binder.

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1.1. Absolute viscosity method This method consists of determining the absolute viscosity of RAP binder and base binder in accordance with ASTM D2171 [11]. Fig. 1 illustrates the typical plot. The absolute viscosity at 60 °C of base binder is plotted as point A and the absolute viscosity at 60 °C of RAP binder is plotted as point C. Points A and C are then connected by a straight line. A horizontal line is drawn through the target (blend) viscosity intersecting the component viscosity line AC at point B. The vertical projection of point B yields the estimate of RAP proportion to meet the target blend viscosity. 1.2. PG method This method consists of determining the high, intermediate and low temperature PG grades of RAP binder and base binder in accor-

dance with ASTM D6373 [12]. Fig. 2 illustrates the typical plot. High temperature PG of RAP binder is plotted as point A and high temperature PG of base binder is plotted as point C. Points A and C are then connected by a straight line. A horizontal line is drawn through the target (blend) viscosity intersecting the component viscosity line AC at point B. The vertical projection of point B yields the estimate of RAP proportion to meet the target blend high PG. Intermediate temperature PG of RAP binder is plotted as point F and intermediate temperature base binder is plotted as point D. Points D and F are then connected by a straight line. A horizontal line is drawn through the target (blend) viscosity intersecting the component viscosity line DF at point E. The vertical projection of point E yields the estimate of RAP proportion to meet the target blend intermediate PG. Similarly, low temperature PG of RAP binder is plotted as point I and low temperature base binder is plotted as point G. Points G and I are then connected by a straight line. A

Fig. 1. Typical blending chart based on absolute viscosity method.

Fig. 2. Typical blending chart based on PG method.

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horizontal line is drawn through the target (blend) viscosity intersecting the component viscosity line GI at point H. The vertical projection of point H yields the estimate of RAP proportion to meet the target blend low PG. Currently, absolute viscosity and PG methods are used for estimating the RAP proportion. Since in India viscosity grading is followed instead of PG, it is essential to compare the RAP proportion obtained by the two methods. However, since viscosity measurement is a discrete point measurement at a single temperature, it does not account for the variable temperature existent in the field. Hence this method may fall short of safeguarding against fatigue and low temperature [13]. Moreover, it is not deemed a fitting parameter for modified binders and excludes the long-term aging of binders in the field [14]. Similarly, superpave rutting parameter, G*/Sind, which evaluates the high temperature PG of a binder and thereby aids in the construction of high temperature PG blending chart, is considered inadequate to correctly evaluate the performance of modified binders [15–17]. Non-recoverable creep compliance, Jnr, is a strong competitor to G*/Sind in that this parameter is blind to modification which makes it more discriminating and a better evaluator of rutting resistance [18]. Hence, once the required RAP proportion is determined by absolute viscosity and PG methods, further study is needed to evaluate whether the binder blended with the obtained RAP proportion has a similar or equivalent performance as that of neat target binder. For instance, if AC30 is the target binder and AC10 binder when blended with x% of RAP binder (AC10 + x%RAP) is equivalent to neat AC30 binder based on absolute viscosity and PG temperatures, it is significant to evaluate whether neat AC30 and AC10 + x%RAP will have similar rheological and chemical performance. Further, due to inherent limitations of absolute viscosity and high temperature PG methods and the accompanying development of advanced rheological tests, an attempt is required to determine the RAP proportion based on performance parameters such as rutting and fatigue with the aid of advanced rheological tests and compare them with the currently adopted absolute viscosity and PG methods. In lieu with that, the current study compares RAP proportion obtained by absolute viscosity and PG methods. AC10 binder was blended with 0%, 15%, 25%, 40% and 50% of RAP binder and RAP proportion to obtain AC30 binder was determined by absolute viscosity and PG methods. Since viscosity grading, instead of PG is followed in India, the PG method has been restricted to high temperature PG only. A detailed comparative study between the neat AC30 and equivalent RAP blended AC10 binder based on various rheological and chemical properties was also performed. Lastly, RAP proportion based on advanced rheological tests such as LAS and MSCR was determined and compared with the RAP proportion determined by absolute viscosity and high temperature PG methods.

2. Objectives

3. Materials and experimental design This study used three binder types: AC10, AC30 and RAP binder (Table 1). AC10 and AC30 are commonly used binders in India. RAP material belonged to the surface layer of Mumbai-Nashik Highway and was six years old. The asphalt binder existent in RAP material was unmodified penetration grade 50/60. Two stage process was adopted for the extraction of binder from RAP material. The first stage involved the separation of RAP aggregate from binder using centrifuge extractor and TCE as solvent [19]. The second stage involved the recovery of binder from the TCE solvent using rotary evaporator [20]. To ensure that no residual TCE remains in the extracted binder, FTIR spectroscopy was conducted and no traces of TCE were found. The extracted RAP binder was characterised by conducting preliminary tests and Table 1 illustrates the obtained results. The penetration and softening point values of RAP binder were obtained as 24 pen (2.4 mm) and 68 °C respectively. Moreover, the ductility and elastic recovery values were 21.2 cm and 15% respectively. A slight elastic recovery of RAP binder indicates the formation of asphaltene fraction which acts as bodying agent [13]. Fig. 3 illustrates the experimental plan adopted for this study. The target binder grade was AC30. It was planned to use different proportions of RAP binder with softer grade AC10 binder to achieve AC30 binder. The four blends were: AC10 + 15%RAP, AC10 + 25% RAP, AC10 + 40%RAP and AC10 + 50%RAP prepared using a mechanical mixer. The blending temperature was maintained at 160 ± 3 °C and a rotational speed of 500 rpm was adopted for a span of 30 min. Homogeneity of the blends was ensured by motioning the blade along the depth of the blending container. In addition, virgin AC10 and AC30 binders were also tested. The array of tests can be referred to in the experimental flowchart (Fig. 3). Absolute viscosity and high temperature PG were determined for AC30 and AC10 binder blended with 0%, 15%, 25%, 40% and 50% of RAP binder. The pumpability of the binders during the production was elucidated based on the Brookfield viscosity measured at elevated temperature. Further, viscosity was used in evaluating the temperature susceptibility of the binders. The fatigue cracking behaviour of asphalt binders was evaluated using viscoelastic continuum damage mechanics (VECDM) based linear amplitude sweep test (LAS) test [25,26]. In order to compare the behaviour of the binders over a range of temperature and frequencies, temperature and frequency sweep tests were performed on the binders and master curves were constructed by time–temperature superposition principle (TTSP). The rutting resistance of the binders was determined using the multiple stress creep and recovery test (MSCR) [27,28]. Percent recovery (R) and non-recoverable creep compliance (Jnr) were evaluated at the stress levels of 0.1 kPa and 3.2 kPa to account for both the linear and non-linear behaviour of binders. The chemical analysis was conducted based on Fourier Transform Infrared Spectroscopy (FTIR) methodology [29–34].

The specific objectives of the present study were: 1. Comparison of absolute viscosity and PG method to determine the proportion of RAP to achieve a target binder grade. 2. Comparison of rutting, viscoelastic, fatigue and chemical properties of neat target binder and target RAP blended binder (RAP proportion determined by absolute viscosity and PG methods). 3. Comparison of RAP proportion obtained based on fatigue and rutting performance with that obtained using conventional approach of absolute viscosity and PG methods.

Table 1 Characteristics of binders used in the study. Physical Properties

AC10

AC30

RAP

Standard code

Penetration, @ 25 °C, 100 g, 5 s Softening point (R&B), °C Ductility, @ 27 °C, cm Elastic recovery, @ 27 °C, %

80 40 75 0

45 47 40 5

24 68 21.2 15

ASTM ASTM ASTM ASTM

D5 [21] D36 [22] D113 [23] D6084 [24]

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Fig. 3. Experimental flow chart.

4. Laboratory testing 4.1. Absolute viscosity Absolute viscosity was determined for AC30 and AC10 binder blended with 0%, 15%, 25%, 40% and 50% of RAP binder using Canon-Manning vacuum viscometer in accordance with ASTM D2171 [35]. The results of this test were utilised to generate the blending charts and consequently to determine the percentage of RAP binder to meet the target viscosity in accordance with ASTM D4887 [7]. This test is also used to grade binders based on viscosity classification. 4.2. Superpave high temperature PG grade Rutting parameter, G*/Sind, provides the estimation of high temperature PG grade of a binder. In this study, G*/Sind value was measured at 58 °C, 64 °C, 70 °C and 76 °C for AC30 and AC10 binder blended with 0%, 15%, 25%, 40% and 50% RAP binder and eventually the critical temperature and corresponding PG grade was determined, in accordance with ASTM D6373 [12]. DSR was employed for the PG grade determination by performing the test in compliance with ASTM D7175 [36]. The results of this test were utilised to generate the blending charts and consequently, determine the percentage of RAP binder to meet the target PG temperature, in accordance with ASTM D4887 [7]. 4.3. Brookfield viscosity

ity of AC30 and AC10 binder blended with varying percentages of RAP binder (i.e. 0%, 15%, 25%, 40% and 50%) was measured using Brookfield viscometer at 120 °C, 135 °C, 150 °C, 165 °C and 180 °C in accordance with ASTM D4402 [37]. Equiviscous method was used to find the mixing and compaction temperature. The thermoplastic behaviour of binder was captured by measuring the temperature susceptibility in accordance with ASTM D2493 [38]. 4.4. Temperature and frequency sweep test Temperature and frequency sweep test was conducted to construct the master curve. Master curve is a vital technique to study the interrelationship between the binder stiffness and reduced frequency over a range of frequencies and temperatures. Timetemperature superposition principle (TTSP) aids in the construction of master curve and works on the principle of timetemperature equivalence. Frequency sweep test was conducted on AC30 and AC10 binder blended with 0%, 15%, 25%, 40% and 50% of RAP binder. The test was conducted at temperatures of 50 °C, 60 °C, 70 °C, 80 °C and 90 °C over the frequency range of 1 rad/s to 100 rad/sec. 25 mm diameter plate with 1 mm gap constituted the testing geometry and 4% strain was applied during the test. Master curve for complex modulus and phase angle was plotted at a reference temperature of 70 °C. Master curves were constructed using Christensen-Anderson Mathematical (CAM) model. Eq. (2). expresses the model

"

Brookfield viscosity test was conducted to determine the apparent viscosity of the binder at elevated temperature. Viscos-




xc G ðxÞ ¼ Gg 1 þ xr 

ðlog2=R #R=log2 ð1Þ

D. Singh et al. / Construction and Building Materials 224 (2019) 109–123

where G ðxÞ = Predicted complex shear modulus at a frequency of x; Gg = Glassy modulus (assumed to be equal to 1 GPa); xc = Crossover frequency at reference temperature; xr = Reduced frequency at reference temperature; and R = Rheological index (shape factor) given by the ratio of glassy modulus (Gg ) and complex shear modulus (G Þ at crossover frequency (xc Þ. The amount of shifting required at each test temperature to shift to the reference temperature was evaluated using the William-Landel-Ferry (WLF) equation expressed in Eq. (3)

logaðT Þ ¼

C 1 ðT  T r Þ C2 þ T  T r

the ageing that occurs due to ketones (C = O) at 1700 cm1 peak. ISO describes the ageing due to sulfoxide (S = O) at 1030 cm1 peak. The denominator in each of the Eqs. (13) and (14) describes the aliphatic structure.

ICO ¼

Area around 1700 cm1 Area around 1460 cm1 and Area around 1345 cm1

ð4Þ

ISO ¼

Area around 1030 cm1 Area around 1460 cm1 and Area around 1345 cm1

ð5Þ

ð2Þ

where logaðT Þ = Shift factor; C 1 and C 2 = Constants; T = Testing temperature; and T r = Reference temperature. Reduced frequency ðxr Þ is a function of shift factor and is evaluated as per Eq. (4) logaðTÞ

xr ¼ x  10

ð3Þ

where x = Testing frequency and other terms as previously described. 4.5. Linear amplitude sweep test (LAS) This test was conducted to determine the resistance of the binder to fatigue damage. AC30 and AC10 binder blended with 0%, 15%, 25%, 40% and 50% of RAP binder were long term aged and LAS test was conducted using DSR, in accordance with AASHTO TP101 [39]. This test aided in evaluating the fatigue life of the tested binders. 4.6. Multiple stress creep and recovery test (MSCR) This test was conducted on short term aged AC30 binder and AC10 binder blended with varying percentages of RAP binder (i.e. 0%, 15%, 25%, 40% and 50%) and eventually the elastic response and stress dependence of binders was determined. This test was conducted in accordance with ASTM D7405 [40]. In order to ascertain the stress sensitivity of AC30 and AC10RAP blended binders, the percent difference in non-recoverable creep compliance, Jnr_diff, was evaluated. For a better rutting performance, this value should be <75% in accordance with AASHTO MP19 [41]. 4.7. Fourier Transform Infrared Spectroscopy (FTIR) FTIR test was conducted on the AC30 binder and AC10 binder blended with 0%, 15%, 25%, 40% and 50% of RAP binder in solution form, with the concentration of solution being 75 g/l in accordance with the procedure drafted in Belgian Road Research Centre report ME 83/13 [34,42]. The instrument used for FTIR spectroscopy was Bruker 3000 Hyperion Microscope with Vertex 80 FTIR system. This test aids in the identification of functional groups with the aid of FTIR indices as depicted in Table 2. Two indices were utilised for studying the behaviour of the binders. They are: ISO and ICO. The calculation of the indices is in accordance with the Eq. (14) and (15) respectively. ICO describes

5. Results and analysis 5.1. Absolute viscosity measurement Fig. 4 illustrates the results of the absolute viscosity test for AC30 and AC10 blended with RAP binder at varying percentages of 0%, 15%, 25%, 40% and 50%, respectively. It can be observed that the addition of RAP binder to neat AC10 binder leads to the stiffening of the binder as is indicated by increase in the viscosity. For example, the viscosity increases from 1038 Poise for neat AC10 binder to 3050 Poise for 50% RAP modified AC10 binder with gradual increments at the intermediate percentages (1448 Poise at 15%, 1714 Poise at 25% and 2575 Poise at 40%). This can be attributed to the higher stiffness of RAP binder blending with a softer AC10 binder leading to an overall increase in stiffness and consequently the viscosity. 5.2. High temperature PG grade measurement Fig. 5 shows the plot for high temperature grading for AC30 binder and AC10 blended with different percentages of RAP binder. It can be seen that the addition of RAP binder resulted in a higher performance grade for the binder. For example, the continuous grade increased from 66 °C for virgin AC10 binder to 68 °C, 71 °C, 73 °C and 75 °C for 15%, 25%, 40% and 50% RAP content respectively. The high temperature PG grade for AC30 was obtained as 70 °C. It essentially means that the addition of RAP binder makes the binder resistant to rutting at high temperatures. Table 3 depicts the values of high temperature grade for AC10 blended at various percentages and AC30 binder. The continuous grade represents the temperature at which the rutting parameter (G*/sind) is exactly equal to 1.0 kPa. Below this temperature, the binder is considered to have failed in rutting in accordance with ASTM D6373 [12]. The PG grade is the binder grade assigned to AC10 blend and neat AC30 and is based on the average 7-day maximum pavement temperature at 6 °C intervals. No change in high temperature PG grade was observed up to the 15% of RAP modification. Beyond 15% RAP, one grade bump was observed which remains sustained till 50% RAP modification. 6. Blending analysis to determine RAP proportion The proportion of RAP binder to be added to a softer binder grade of AC10 to obtain a stiffer binder grade of AC30 was determined using the absolute viscosity and high PG methods. 6.1. Blending analysis based on absolute viscosity and high temperature PG

Table 2 FTIR indices for different chemical compositions. Peak (cm1)

Lower limit (cm1)

Upper limit (cm1)

1700 1030 1460 1375

1660 994 1400 1350

1753 1047 1525 1390

(Carbonyl) (Sulfoxide) (Aliphatic structures) (Aliphatic structures)

113

Due to instrumentation limitation, absolute viscosity test could not be conducted for RAP binder for complete compliance with ASTM D4887 [7]. However, an approximate methodology was followed to determine the proportion of RAP content that will provide the target blended binder grade of AC30. This method is based on

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Fig. 4. Absolute viscosity of AC30 and AC10 at varying RAP percentages.

Fig. 5. Rutting parameter for AC30 and AC10 binder at varying RAP percentages.

Table 3 High temperature grades for AC30 and AC10 binder at varying RAP percentages. Grade

Continuous Grade PG Grade

AC10 with RAP (%)

AC30

0

15

25

40

50

65.5 64

68.2 64

70.5 70

73.3 70

75.7 70

74 70

the determination of absolute viscosity of neat and blended binder and plotting them on the blending chart with abscissa as RAP binder percentage and ordinate as absolute viscosity of binder (Fig. 6) The optimum RAP binder content, whose addition to AC10 will result in AC30 binder based on absolute viscosity, was determined by comparing the results of neat AC30 binder and AC10 binder blended with varying percentages of RAP binder. Absolute viscosity of AC30 was obtained as 2949 Poise. From Fig. 6 it can be observed that the absolute viscosity of AC10 binder containing approximately 48% of RAP binder is equal to that of neat AC30 binder based on viscosity equivalence. ASTM D4887 [7] provision was followed for plotting the blending charts. This method is based on high temperature grade of the virgin and RAP binder. For determining RAP content at which AC10 blended binder will be equal to AC30 based on PG grade equiva-

lence, high temperature grade for AC10 blends was compared with that of the virgin AC30 binder to get an approximate value. Fig. 7 shows the blending chart as per ASTM D4887 [7] based on the high temperature grade of the binder. The high temperature grade of neat AC10 binder (64 °C) was plotted on 0% RAP content. The high temperature grade of RAP binder (82 °C) was plotted on 100% RAP content. The two points were connected by a straight line. The target high temperature grade was set at the high temperature PG grade of neat AC30 binder (70 °C). From the results it can be seen that AC10 blended with RAP binder content of approximately 32% gives the high temperature grade similar to AC30 binder (PG grade equivalence). 6.2. RAP binder proportion based on absolute viscosity and PG method From blending analysis based on viscosity and PG grading, it was concluded that in order to attain the target binder grade of neat AC30 binder, 48% of RAP binder needs to be blended with neat AC10 binder. Similarly, a minimum of 32% of RAP binder needs to be blended with neat AC10 binder so that the high temperature PG grade equivalence is maintained with AC30 binder. Hence a difference is observed in the proposed RAP proportions by the two methods. In order to attain a consensus between the viscosity and high temperature PG grade and for further comparative study,

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115

Fig. 6. Blending chart based on absolute viscosity method.

Fig. 7. Blending chart based on high temperature PG grade method.

a RAP binder percentage of 40%, as a mean of the two obtained RAP proportions, was considered in this paper. 7. Performance comparison of control AC30 and AC30 binder obtained after blending RAP

mixing and compaction temperatures for the two compared binder are in close vicinity. This further indicates that RAP proportion of 40%, determined from absolute viscosity and PG blending charts, is acceptable for attaining the target AC30 binder grade. 7.2. Temperature susceptibility

Binder grade of AC30 was targeted by blending AC10 binder with RAP proportion determined from absolute viscosity and high temperature PG blending charts. RAP proportion was obtained as 40%. In order to evaluate whether AC10 binder blended with 40% RAP (hereafter referred as AC30_RAP) has similar rheological and chemical properties as that of neat AC30 binder, a number of rheological and chemical tests were conducted. 7.1. Brookfield viscosity Fig. 8 illustrates the Brookfield viscosity of AC30 and AC30_RAP at varying temperatures of 120 °C, 135 °C, 150 °C, 165 °C and 180 °C. A gradual decrease in the viscosity is observed with the increase in temperature for both AC30 and AC30_RAP. On comparing, it is observed that the values of viscosity obtained for the two binders are very close thereby indicating that AC30_RAP is equivalent to AC30 binder based on Brookfield viscosity. Table 4 represents the mixing and compaction temperatures obtained for AC30 and AC30_RAP. Equiviscous method was retorted to for calculating the values [38]. It is observant that both

The change in consistency of AC30_RAP with temperature was captured using the A-VTS relationship (Eq. (2)). Brookfield viscosity aided in the determination of A-VTS relationship. VTS represents the slope and is the measure of temperature susceptibility. It was observed that AC30_RAP had a marginally higher VTS value than AC30 binder thereby indicating that its degree of consistency is more prone to temperature changes than AC30 binder. Fig. 9 shows the plot of log-log viscosity vs. log temperature, the slope of which is the VTS. It is observed that the plots for AC30 and AC30_RAP diverge at higher values of log temperature thereby indicating the existing difference in temperature susceptibilities albeit marginal. 7.3. Frequency and temperature sweep test Fig. 10 indicates the master curve for AC30 and AC30_RAP, obtained at a reference temperature of 70 °C. As is evident from the curve, the increase in complex modulus with reduced frequency is observed for both the binders, thereby indicating the

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Fig. 8. Brookfield viscosity of AC30 and AC10 blended binder at varying temperatures.

Table 4 Mixing and compaction temperatures from equiviscous method. Binder

AC30 AC10 + 40%RAP

Mixing Temp. (°C)

Compaction Temp. (°C)

Lower limit

Upper Limit

Lower limit

Upper Limit

153 152

158 157

142 142

147 146

The temperature dependency of viscoelastic behaviour of the AC30 and AC30_RAP was evaluated using the plot of log shift factor (determined for plotting the master curve) and testing temperature as is depicted by Fig. 11. It is observed that a general consensus exists between the shift factor values of the two binders indicating identical processes of relaxation for both the binders. 7.4. Fatigue performance through linear amplitude sweep test

increase in stiffness throughout the reduced frequency range. On comparing the modulus change of AC30 and AC30_RAP, it was observed that the values are near at lower frequencies of loading and as the frequency of loading increases, the two curves diverge from each other. This further indicates that at low frequency (which is equivalent to high temperature) the behaviour of AC30 and AC30_RAP are identical and as the frequency increases (or temperature decreases), the difference in the behaviour develops with AC30 having a higher complex modulus, and hence the stiffness. The identical behaviour at high temperature (low reduced frequency) may be due to the better mobilisation of stiffer RAP binder in AC10 at high temperatures, thereby pushing the behaviour close to that of AC30 in terms of stiffness.

Fig. 12 indicates the effective shear stress versus the effective shear strain curve for the AC30 and AC30_RAP. Strain dependency and damage to binder during shear loading can be studied using this plot [43]. It was observed that the stress response increases with the increase in the applied strain up to a certain point, beyond which the stress response shows a decrease. Decrease in the stress response is indicative of damage initiation in LAS test. Both AC30 and AC30_RAP have a sharp peak thereby indicating a significant dependency on applied shear strain. The failure strain for AC30 and AC30_RAP was 7% and 8.01% respectively. Hence, the damage initiation is early in AC30 as compared to AC30_RAP. Fig. 13 illustrates the plot of integrity parameter (C) versus the damage intensity (D) for AC30 binder and AC30_RAP with 40% of RAP binder. This curve represents the transition from no damage

Fig. 9. Temperature susceptibility plots of AC30 and AC30_RAP.

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117

Fig. 10. Master curve for different binder types.

Fig. 11. Temperature dependency plot for different binder types.

Fig. 12. Effective shear stress versus shear strain plot for different binder types.

(where C = 1) to complete damage (where C = 0). The rate of decrease of integrity parameter, C, with the increase in damage intensity, D, further depends on the magnitudes of C1 and C2 in accordance with Eq. (6). C1 value was obtained as 0.027 and 0.033 for AC30 and AC30_RAP respectively. C2 was obtained as

0.644 and 0.594 for AC30 and AC30_RAP respectively. It can be observed that C1 value for AC30 was marginally lower than the corresponding value for AC30_RAP. However, C2 value for AC30 was marginally higher than the corresponding value for AC30_RAP. As is evident from Fig. 13, the integrity loss rate is higher for AC30

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Fig. 13. Variation of integrity parameter with damage intensity for different binder types.

as compared to AC30_RAP binder indicating that AC30 is more prone to fatigue failure. This observation is in agreement with the strain response curve (Fig. 12) where the failure strain of AC30 was lower than that for AC30_RAP. Fig. 14 shows the fatigue life of AC30 and AC30_RAP at varying strain levels and at a frequency of 10 Hz. In order to ascertain the strain level at which the fatigue life of the tested binders became equal, a plot of fatigue life ratio was constructed against the varying strain levels. Fatigue life ratio was defined as the ratio of fatigue life of AC30_RAP to fatigue life of AC30 binder at each strain level. Fig. 15 illustrates the resulting plot. It was observed that at 7.23% strain, the fatigue life of both AC30 and AC30_RAP become equivalent. This further assisted in reporting the observation that the fatigue life of AC30_RAP was higher than that of AC30 at strain levels of up to 7.23%. Above 7.23% (particularly at strain levels of 10% and 20%) the fatigue life of AC30 was marginally higher. Eventually when the strain is increased to a higher magnitude, the reversal of fatigue life trend is observed as is evident from higher value of fatigue life of AC30 at 10% and 20% strain (Fig. 15). This indicates that AC30 can be adopted for the roads that are expected to carry higher magnitude of traffic (which generates greater degree of strain) and AC30_RAP can be adopted for low volume roads.

7.5. Rutting performance through multiple stress creep and recovery test (MSCR) MSC R test was performed to compare the elastic response of AC30 and AC30_RAP binder at stress levels of 0.1 and 3.2 kPa. ASTM D7405 [40] specifies that the stress level of 0.1 and 3.2 kPa conforms to linear viscoelastic region and non-linear viscoelastic region respectively. Elastic response is indicative of the rutting performance of the binder. Percent recovery (R), at stress level of 0.1 kPa, was observed to be 2.83% and 1.43% for AC30 and AC30_RAP respectively. A lesser percent recovery of AC30_RAP, when compared to AC30 binder, indicates that the binder has a lesser elastic recovery potential and hence is more prone to rutting failure. The percent recovery at a stress level of 3.2 kPa was observed to be 0.2% and 0.7% for AC30 and AC30_RAP respectively. The negative values can be attributed to the factors such as inertia and data sampling and is particularly pronounced when the recovery is small. Overall, a decrease in percent recovery was observed for both the binders with the increase in stress level from 0.1 to 3.2 kPa. Fig. 16 illustrates the variation of Jnr for the two binder types considered for the study. Jnr was measured at 64 °C and at two creep loadings of 0.1 and 3.2 kPa. Creep load of 0.1 kPa was applied

Fig. 14. Variation of fatigue life with strain.

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Fig. 15. Plot of fatigue life ratio versus strain level for different binder types.

Fig. 16. Jnr for different binder types.

to capture the linear behaviour of the binders and 3.2 kPa was applied to capture the non-linear behaviour of the binders. The Jnr value, at creep loading of 0.1 kPa, was found to be 3.7 kPa1 and 2.9 kPa1 for AC30_RAP and AC30 respectively. Likewise, Jnr value, at creep loading of 3.2 kPa, was found to 4.0 kPa1 and 3.2 kPa1 for AC30_RAP and AC30 respectively. The obtained values indicate that AC30 binder has lower Jnr than AC30_RAP when measured at the end of creep and recovery cycle, in both linear and non-linear region. Lower is the Jnr value, more is the tendency of the asphalt binders to return to their original shape and hence, better is the rutting resistance. Hence, it is concluded that AC30 binder has a high rut resistance as compared to AC30_RAP, even though they are equivalent based on absolute viscosity and high temperature PG grade. Percent difference in non-recoverable creep compliance (Jnr_diff) was evaluated to check the adherence to AASHTO MP19 [41] which specifies that Jnr_diff for a binder should not exceed 75%. Jnr_diff for AC30 and AC30_RAP was obtained as 8.53% and 7.14% respectively. The obtained values were well within the specified limit of 75%. Based on this criterion it is concluded that both AC30 and AC30_RAP are not rutting susceptible. AASHTO MP19 [41] aided in the classification of AC30 and AC30_RAP in compliance with Table 5. The suitability of the binder is classified for different traffic loading conditions based on the Jnr

Table 5 Grading of binder for Different Traffic-Loading Conditions (AASHTO MP19) [41] Traffic level (ESAL) and load rate

Designation

Meaning

Jnr value at 3.2 kPa1

>30 million and <20 km/h >30 million or <20 km/h 10–30 million or 20– 70 km/h <10 million and >70 km/h

E

Extremely high traffic loading Very high traffic loading High traffic loading

0.0–0.5

Standard traffic loading

2.0–4.0

V H S

0.5–1.0 1.0–2.0

value at 3.2 kPa1. Both AC30 and AC30_RAP was designated as ‘‘S”. This in turn means that both these binders can be used at the locations where standard traffic loading is expected. 7.6. Chemical characterization using Fourier Transform Infrared Spectroscopy (FTIR) Fig. 17a illustrates the FTIR spectrum of AC30 and AC30_RAP. The comparison between the two binders was evaluated based on the sulfoxide content (around 1030/cm) and carbonyl content (around 1700/cm) and aid in understanding the difference

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Fig. 17. FTIR results a) FTIR plot for different binder types b) Comparison of FTIR indices.

between the two binders based on their chemical composition. It was observed that a significant difference exists in the chemistries of AC30 and AC30_RAP. Fig. 17b illustrates the ISO and ICO values for AC30 and AC30_RAP. It can be seen that ISO and ICO values for AC30_RAP are 0.212 and 0.082 respectively, which are higher than the corresponding values of ISO and ICO for AC30 binder (0.036 and 0.008 respectively). This is indicative of the oxidative nature of RAP which manifests as the dominant component in the AC30_RAP thereby causing the values of ISO and ICO to increase. 8. RAP proportion based on fatigue and rutting performance of binder An attempt was made to calculate the percent RAP binder content to be added to AC10 binder, at each strain level, so that the resulting blend will have the same fatigue life as that of AC30 binder. Fig. 18 graphically illustrates the fatigue life of AC10 binder blended with varying percentages of RAP proportion of 0%, 15%, 25%, 40% and 50% respectively. Since the fatigue life of the binder is the function of strain level, decrease in the fatigue life was observed with the increase in percent strain. Exponential equation was fitted to the obtained data at each strain level. The fitted equations were then utilised to evaluate the RAP proportion at each strain level which when blended with AC10 binder will exhibit the fatigue life equivalent with AC30 binder. Table 6 depicts the obtained RAP proportion at each strain level. It was observed that the RAP proportion, required for the fatigue life equivalence with AC30 binder, increased with the increase in percent strain. RAP proportion of 32% was required at low strain level

of 0.1% whereas the RAP proportion increased to 40% for strain value of 20%. This may be due to the fact that at higher strain level, more resistance has to be mobilised by the binder to arrest the crack growth and tolerate the damage. More RAP proportion was observed to enhance the fatigue life of binder. This result was consistent with the findings of Mannan et al. (2015) [44]. As a consensus between the varying percentages of RAP binder at different strains, the arithmetic average value was considered. The value was obtained as 35%. Hence, for fatigue life equivalence with AC30 binder, RAP binder percentage of 35% needs to be added to AC10 binder. This value defines the lower limit of the RAP binder to be added to AC10 in order to satisfy the fatigue life. Moreover, 40% RAP binder added to AC10 binder, based on absolute viscosity and high temperature PG equivalence, can be deemed safe in fatigue. Fig. 19 illustrates the non-recoverable creep compliance value (Jnr) obtained at varying percentages of RAP binder in AC10 under the creep stress of 0.1 and 3.2 kPa. The plot was utilised to ascertain the percentage of RAP binder required to be blended with AC10 so that it has the same Jnr value as that of AC30 binder. It was observed that the RAP binder percentage of approximately 48% when blended with AC10 binder will exhibit the same Jnr as that of AC30 in the linear region of the binder. However, in non-linear region, RAP binder percentage of approximately 47% when blended with AC10 binder will exhibit the same Jnr as that of AC30 binder. The decrease in the RAP binder percentage for Jnr equivalence indicates that there is a decrease in Jnr gap between AC10 blended binder and AC30 binder during transition from linear to non-linear region, although an increase is observed when a single binder is considered. The non-linear region represents the

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Fig. 18. Fatigue life of AC10 binder at varying RAP proportions and varying strains.

Table 6 RAP proportion based on LAS test.

*

Strain (%)

Equation of fitted exponential trend line

Coefficient of correlation, R2

Targeted fatigue life of AC30_RAP binder, Nf (y)

Computed RAP proportion, % (x)

0.10% 1.00% 2.50% 5.00% 10.00% 20.00%

y = 106e0.0828x y = 5732.7e0.0644x y = 661.08e0.0571x y = 129.02e0.0516x y = 25.184e0.046x y = 4.9164e0.0405x

0.93 0.89 0.85 0.82 0.77 0.71

13749187.80 44009.78 4475.45 794.13 140.92 25.01

32* 32* 33* 35* 37* 40*

Values rounded off to nearest whole number.

Fig. 19. Plot of Jnr with varying RAP proportion.

high degree of strain which simulates the high axle load on the pavement. The non-linearity generally becomes negligible at small strains where the behaviour can be approximated as linear. However, to be on a safer side, the optimum RAP binder percentage for Jnr equivalence between AC30 and AC10 blended binder can be chosen as 48% so as to ensure better rut resistance in both the linear and non-linear region

9. Comparison of RAP proportion determined based on fatigue, rutting, absolute viscosity and PG methods In the current study, RAP proportion was determined by absolute viscosity and high temperature PG methods. The values were obtained as 32% and 48% respectively. Additionally, in this study, the percentage of RAP binder required for fatigue life equivalence,

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Table 7 Summary of RAP proportions based on various methods. Test method

Performance parameter

RAP proportion

Absolute viscosity High temperature PG LAS MSCR

– Rutting Fatigue life Rutting

48% 32% 35% 48%

based on LAS test, was evaluated which was variable at each strain level and for simplification, the arithmetic mean was considered. The value was found to be equal to 35%. It suggested that any RAP binder modification of above 35% in AC10 binder will generate a better fatigue life than virgin AC30 binder. Percentage of RAP binder required for Jnr equivalence was evaluated by conducting the MSCR test. Two values of RAP binder modification percentages, quantitatively 48% and 47%, were obtained, the former conforming to linear and the latter to the non-linear region, respectively. To be on the conservative side, a value of 48% was chosen. It suggested that any RAP binder modification of above 48% in AC10 binder will generate a better rutting resistance than virgin AC30 binder. Hence, it is concluded that the RAP proportion obtained by absolute viscosity method (48%) is higher than the RAP proportion suggested by LAS test (35%) and exactly matches with the RAP proportion suggested by Jnr using MSCR test (48%). Moreover, RAP proportion obtained by high temperature PG method (32%) is lower than the RAP proportion suggested by Jnr using MSCR test (48%). PG method could not be evaluated based on fatigue life since fatigue is a concern at intermediate temperature and intermediate PG grading was not conducted in this study. Table 7 summarises the obtained results. 10. Conclusions In this study, RAP proportion to achieve the target grade of binder (AC30) was determined by absolute viscosity and PG methods, and compared with RAP proportion obtained from LAS and MSCR tests. Further, a comparative study of various rheological and chemical properties between virgin AC30 and AC10 binder blended with obtained RAP proportion was conducted. This study has following conclusions to offer: 1. RAP proportion to achieve target AC30 binder was obtained as 32% based on absolute viscosity method. Based on high PG temperature method, the RAP proportion was obtained as 48%. In order to satisfy both absolute viscosity and high temperature PG, average of the two RAP proportions (40%) was blended with AC10 binder (AC30_RAP) and considered for further study. 2. Brookfield viscosity values for AC30 and AC30_RAP over a range of tested temperatures were close. Moreover, viscosity at the test temperature of 135 °C, which serves as the indicator of pumpability of binder, was marginally lower for AC30_RAP as compared to AC30 binder (Difference of 7 °C). 3. Master curves for AC30 and AC30_RAP showed a divergence at high reduced frequency range (equivalent to low temperature) with AC30 displaying a stiffer behaviour as compared to AC30_RAP. However, at lower values of reduced frequency (equivalent to high temperature) the master curves fused together in unison indicating identical complex modulus values for the two binders. 4. Higher fatigue life (Nf) of AC30_RAP was observed at lower strain level and reversal of fatigue life occurred at higher strain level with AC30 leading. 5. Recovery potential of AC30 binder was found to be higher than AC30_RAP in both the linear and non-linear region thereby illustrating the dominance of stiffer RAP binder. Moreover,

6. 7.

8.

9.

lower Jnr values were exhibited by AC30 binder. This indicated higher rut resistance of AC30 when compared to AC30_RAP although the two binders were equivalent based on absolute viscosity and high temperature PG. Both AC30 and AC30_RAP was found suitable for standard loading conditions (‘‘S”). FTIR revealed significant differences in the chemical characteristics of AC30 and AC30_RAP. This may be due to the dominant nature of the oxidised RAP in AC30_RAP binder. The percentage of RAP binder to be added to AC10 binder for being equivalent to AC30, based on fatigue life, was found to increase with the increase in percent strain during LAS test. RAP proportion of 35% was observed to be the minimum for equivalence with AC30 binder, based on fatigue life. The percentage of RAP binder to be added to AC10 binder for being equivalent to AC30, based on Jnr, was obtained as 48% in linear region. This was an exact match with RAP proportion determined from absolute viscosity method (48%) but was largely different than the RAP proportion determined by high temperature PG method (32%). This indicates that the determination of RAP proportion by high temperature PG method may not be sufficient from the point of view of rutting performance and the construction of blending chart based on Jnr has to be sought for.

The present study was undertaken to compare the RAP proportion obtained by constructing the blending charts based on absolute viscosity and high temperature PG grade method. This was followed by comparison of rheological and chemical properties of virgin target binder (AC30) and blended target binder (AC30_RAP). Lastly, RAP binder proportion to achieve target binder grade (AC30) was determined based on LAS and MSCR tests. It is recommended that further studies be carried out for construction of blending charts based on intermediate and low temperature PG and rheological parameters such as fatigue life (Nf) and non-recoverable creep compliance (Jnr). Moreover, the RAP source used for this study is only 6 years old. The study can be carried out with relatively older RAP sources (10–15 years). Declaration of Competing Interest The authors declares no potential conflict of interest for the submitted manuscript. References [1] R. McDaniel, R.M. Anderson, Recommended use of reclaimed asphalt pavement in the superpave mix design method: Technician’s manual, NCHRP Rep. No. 452, Project D9-12 FY’97, Transportation Research Board, Washington, DC, 2001. [2] I.L. Al-Qadi, M. Elseifi, S.H. Carpenter, Reclaimed Asphalt Pavement—A Literature Review. FHWA-ICT-07-001, Illinois Center for Transportation, Chicago, 2007. [3] A. Copeland, Reclaimed asphalt pavement in asphalt mixtures: State of the practice (No. FHWA-HRT-11-021), 2011. [4] G. Page, in: Florida’s Experience in Hot Mix Asphalt Recycling, Hot Mix Asphalt Technology, (Spring), 1988, pp. 10–16. [5] D. Singh, D. Sawant, Understanding effects of RAP on rheological performance and chemical composition of SBS modified binder using series of laboratory tests, Int. J. Pavement Res. Technol. 9 (3) (2016) 178–189. [6] P. Kandhal, K. Foo, Design in recycled hot mix asphalt mixture using superpave technology, in: R.N. Jester (Ed.), Progress of superpave: evaluation and implementation, ASTM STP 1322, American Society for Testing and Material, 1997. [7] ASTM D4887/D4887M-11: Standard Practice for Preparation of Viscosity Blends for Hot Recycled Asphalt Materials, West Conshohocken, PA, 2016. [8] E.Y. Hajj, P.E. Sebaaly, R. Shrestha, Laboratory evaluation of mixes containing recycled asphalt pavement (RAP), Road Mater. Pavement Des. 10 (3) (2009) 495–517. [9] D. Singh, D. Sawant, F. Xiao, High and intermediate temperature performance evaluation of crumb rubber modified binders with RAP, Transp. Geotech. 10 (2017) 13–21.

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