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Evaluation of the characteristics of Trinidad Lake Asphalt and Styrene–Butadiene–Rubber compound modified binder
Construction and Building Materials 202 (2019) 614–621
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Evaluation of the characteristics of Trinidad Lake Asphalt and Styrene–Butadiene–Rubber compound modified binder Jun Liu a,b, Kezhen Yan a,⇑, Jenny Liu b, Dong Guo a a b
College of Civil Engineering, Hunan University, Changsha 410082, China Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO 65401, United States
h i g h l i g h t s The SBR and TLA compound modified binders had better low-temperature flow properties than TLA single modified binders did. The concentration combination of 2% SBR and 20% TLA was recommended. The workability was degraded by introducing additives but all the studied binders met the Superpave specification.
a r t i c l e
i n f o
Article history: Received 17 October 2018 Received in revised form 2 January 2019 Accepted 9 January 2019
Keywords: Trinidad Lake Asphalt (TLA) Styrene–Butadiene–Rubber (SBR) Compound modified binder Rheological properties Non-load related cracking
a b s t r a c t A comprehensive experiment involving different Trinidad Lake Asphalt (TLA) and Styrene–Butadiene–R ubber (SBR) compound modified binders was conducted to investigate the rheological and aging properties of TLA and SBR compound modified binders. Four TLA (5%, 10%, 20%, and 30%) and four SBR concentrations (0%, 2%, 3%, and 4%) were selected. Dynamic shear rheometer (DSR) and bending beam rheometer (BBR) tests were performed to evaluate the rheological properties. Conventional tests (penetration, softening point, and ductility tests) were conducted on TLA and SBR compound modified binders with different aging states (i.e., original, RTFO aged, and PAV aged) to investigate the physical and durability properties. Rotational viscosity tests were performed to evaluate the workability. The results indicated that compound modified binders with TLA and SBR could improve the deformation resistance of binders by increasing the early stiffness and prolong the service life of the corresponding pavement by improving the durability, compared with base asphalt binder. The addition of TLA could degrade the lowtemperature flow properties, but the degradation effect could be offset by adding SBR. The combination of 2% SBR and 20% TLA was recommended based on the comprehensive analysis of test results. The workability was degraded by introducing additives, but all binders studied met the Superpave specification. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Modification on some neat (non-modified) asphalt cements is required in order to meet specifications, producing modified asphalt binder. The use of modified binder has increased significantly in recent years mainly due to the increased demand on hot mix asphalt (HMA) pavements, development of Superpave asphalt binder specifications, environmental and economic pressure, and public agencies’ willingness to pay higher-cost for longer pavement service life [1–4]. Trinidad Lake Asphalt (TLA) is a unique natural binder, which has been served as a kind of asphalt binder modifier to improve high-temperature performance for several decades. Similar to most ⇑ Corresponding author. E-mail address: [email protected] (K. Yan). https://doi.org/10.1016/j.conbuildmat.2019.01.053 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
of the modifiers such as polymers, TLA was found to increase the strength of the flexible pavement. The stiffness and Marshall stability of asphalt mixture could be improved by incorporating of TLA [5]. The TLA modified mixtures were usually applied in asphalt overlay project to help obtain better permanent deformation resistance [6]. TLA was blended with asphalt binder with a pen grade of 160/220 to produce the mixed asphalt binder with lower penetration value for a surface course in England [7]. TLA was also added to PG 76-28 binder along with steel slag in an overlay project in Colorado, the US for the shortage of quality aggregate [8]. The composition of TLA typically includes mineral matter and asphalt binder, and thus the mineral matter was served as fine aggregates or fillers in that Colorado project. Additionally, TLA was also incorporated into asphalt mastic which was served as waterproofing materials on steel bridge decks [9]. Liao et al. investigated the rheological properties of TLA modified binder. Their results
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indicated that the addition of TLA into asphalt binder could bring an improvement on rutting resistance, and the optimal dosage of TLA was found to be in the range of 20–33% [10]. Although lots of previous studies [5–11] indicated that the introducing of TLA into asphalt binders or mixtures could bring better permanent deformation resistance which is usually related to high-temperature performance, the results of pilot tests of this study (the ductility test and equivalent brittle point results, presented in following sections) and previous study [12] showed that the addition of TLA could degrade the low temperature cracking resistance of the asphalt binder. Therefore, it is critical to find a modifier or a technology that can incorporate with TLA to have the compound modified binder that can benefit from TLA regarding high-temperature performance without compromising thermal cracking resistance. Styrene-butadiene rubber (SBR) is a kind of widely used modifier that can improve the low-temperature ductility and elastic recovery of asphalt binder, which in turn improving the thermal cracking resistance [13–15]. According to Yildirim, the ductility of asphalt binder could be increased by adding the SBR, which making the asphalt binder more flexible at low temperatures and thus more capable of resisting thermal cracking [16]. Ruan et al. indicated that the incorporating of SBR into asphalt binder could bring an increase in complex modulus at high temperatures, and could bring a decrease in complex modulus at low temperatures [17]. In addition, SBR was also applied as modifier along with other modifiers to produce compound modified binder. Zhang and Yu investigated the properties of SBR and polyphosphoric acid (PPA) compound modified binder, and the results indicated that the addition of SBR could improve the thermal cracking resistance of the PPA modified binder [18]. Zhang et al. investigated the properties of SBR and montmorillonite (MMT) compound modified binders, and indicated that the addition of SBR and MMT into asphalt binder formed a newly network, which can improve the high-temperature properties and storage stability of the asphalt binder [19]. Zhu et al. combined SBR with Nano-materials to modify the asphalt binder, and both the thermal and the ultraviolet (UV) aging resistance of the asphalt binder were improved by adding the modifiers [20]. However, according to Salehfard et al., the low-temperature ductility of SBR modified binder would decrease substantially after short-term aging. The authors attributed the reduction to the SBR molecules, which are easy to be oxidized [21]. This drawback of SBR may be eliminated by incorporating TLA into SBR modified binder. According to Li et al., the addition of TLA can effectively improve thermal-oxidative or UV aging resistance of asphalt binder [22]. Considering the characteristics of TLA and SBR modified asphalt binders, it is promising to incorporate TLA with SBR to obtain the compound modified binder which can not only show great permanent deformation resistance benefiting from the addition of TLA but also possess remarkable low-temperature flexibility due to the incorporation of SBR. Although the characteristics of TLA and SBR modified binders were separately investigated in many previous studies, no study looked into the properties of TLA and SBR compound modified binder. Thus, a comprehensive investigation of the characteristics of TLA and SBR compound modified binder is essential to provide better understanding for promoting its engineering application. The primary objectives of this study are to: 1. Evaluate the conventional (physical), and rheological properties of compound modified binders with different dosages of TLA and SBR to see if the compound modified binders have better low-temperature properties than those of TLA single modified binders (without SBR), without degrading the permanent deformation resistance.
2. Investigate the aging properties of compound modified binders with different dosages of TLA and SBR to see if the compound modified binder would show better durability properties. 3. Determine the optimal contents of TLA and SBR for the compound modified binder based on the testing results. Conventional tests (Penetration, softening point, and ductility tests), rotational viscosity tests, dynamic shear rheometer (DSR) tests, and bending beam rheometer (BBR) tests were conducted on compound modified binders with different TLA and SBR dosages at different aging states to achieve the above objectives. 2. Materials and preparation The base asphalt used in this study was the AH 70 which was produced by SINOPEC Shanghai, China. Table 1 lists some of its’ basic properties. The SBR used in this study was SBR 1502 with 40 mesh. The basic properties of TLA used in this study are listed in Table 2. Four TLA concentrations (i.e. 5%, 10%, 20%, and 30%) and four SBR concentrations (i.e. 0%, 2%, 3%, 4%), by weight of base asphalt, were selected in this study based on previous studies [23,24]. According to the recommendations from previous studies [23,24], the compound modified binders were prepared as follows. Considering SBR is less compatible with petroleum asphalt binder than TLA does, SBR modified binders were produced firstly. Then, TLA was added to SBR modified binders to produce TLA and SBR compound modified binders. A shearing mixer was used to mix binders. The base asphalt and TLA were heated until they became fluid before mixing. SBR was added slowly into liquid base asphalt with stirring speed of 2000 rpm. After about 20 min of mixing, the stirring speed was changed to 5000 rpm, and TLA was added. Another 30 min of stirring was needed to obtain the compound modified binders. The temperature was kept at 160 °C during the whole mixing process. 3. Experimental details Penetration tests at 15 °C, 25 °C, and 30 °C, ductility tests at 5 °C, and softening point tests were conducted according to ASTM D5, D113, and D36, respectively. By linearly regressing the logarithm of Penetration (P) against temperature (T), the intercept (K) along with slope (A) were obtained to calculate the equivalent softening point (T800 , Eq.1) and equivalent brittle point (T1;2 , Eq.2). T800 and T1;2 are conventional and empirical indices to evaluate the high and low-temperature performance, respectively [24]. The asphalt binder with higher T800 value and lower T1;2 value shows better high-temperature performance and low temperature cracking resistance.
T800 ¼
lg800 K A
ð1Þ
T1;2 ¼
lg1:2 K A
ð2Þ
Table 1 Basic properties of base asphalt binder. Properties
Units
Test results
Specification limits
Penetration (at 25 °C) Softening point Ductility (at 15 °C) Rotational viscosity (at 135 °C)
0.1 mm °C cm cp
69.7 50.2 >150 540
60 to 80 47 100 3000 RV 180
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Table 2 Basic properties of TLA.
4. Results and discussions
Properties
Units
Test results
Specification limits
Penetration (at 25 °C) Softening point Mineral filler content Density Retained Penetration after TFOT
0.1 mm °C % g/cm3 %
4.0 93 36.7 1.3072 55
0–5 90 33–38 1.3–1.5 50
The rotational viscosity at 135 °C of the studied binders was measured according to ASTM D4402 using a Brookfield viscometer to evaluate the pumping ability at the HMA plant. The DSR tests were applied to investigate the rheological properties of the compound modified binders based on AASHTO T 315. The complex modulus (G*) which indicates the stiffness of tested binder at desired temperature and frequency of loading and phase angle (d) which reflects elastic and viscous components of complex modulus were obtained. The G* is as importance as the resilient modulus of the base to determine the performance of the pavement [27]. The test geometry with 25 mm diameter plate and 1 mm gap was used. The strain control model was applied (a constant strain of 12% was applied) and the applied loading frequency was 1.59 Hz (10 rad/s). Temperature sweep test (temperature range from 40 °C to 80 °C with an increment of 2 °C/min) was conducted on each studied binder. Besides, the failure temperatures which defined as the temperature at which the rutting parameter (G*/sin d) of binder equal to 1.0 kPa were determined by conducting performance grade (PG) determination tests. The BBR tests were conducted to evaluate the thermal cracking resistance of TLA and SBR compound modified binders according to AASHTO T 313. A constant load is applied to a sample, and the creep stiffness (S) and m-value at 60 s at three different temperatures (i.e., 6 °C, 12 °C, and 18 °C) were recorded. The asphalt binder with low S value and high m-value shows better low temperature cracking resistance. Superpave specified that the S should be lower than 300 MPa and m-value should be higher than 0.3 at the predicted lowest pavement temperature for the asphalt binder that could be applied to the field. Rolling thin-film oven (RTFO) tests were conducted on the studied compound modified binders following the guide of the ASTM D2872. The samples were kept at the RTFO for 85 min at 163 °C. Pressure aging vessel (PAV) tests were performed on RTFO residues to simulate the long-term aging process according to ASTM D6521. The temperature was kept at 100 °C, and the air pressure was kept at 2.1 MPa during the 20 h of testing. Retained Penetration (RP) at 25 °C, softening point increment (DS) and viscosity aging index (VAI) were measured or calculated to characterize the aging performance of studied binders. A higher value of DS or VAI reflects more significant influence of aging, while a higher value of RP means lower influence. The indices are recommended to be calculated by Eqs. (3)–(5).
Fig. 1(a) presents the test results of penetration. As shown, the penetration of modified binder regardless of TLA modified binder or TLA and SBR compound modified binder, was much lower than that of base asphalt, indicating that TLA has harden effect on asphalt binder. Besides, it can be seen that the compound modified binders with a specific SBR concentration showed a gradual decrease in penetration with the increase of TLA concentration,
(a) Penetration results
(b) Softening point results
DS ¼ Softening point of aged binder
RP ¼
Softening point of unaged binder
ð3Þ
Penetration of aged binder 100% Penetraction of unaged binder
ð4Þ
VAI ¼ lglg ga 103 lglg gb 103
ð5Þ
where ga (gb Þ stands for the viscosity of asphalt sample after (before) aging.
(c) Ductility results Fig. 1. Conventional tests results of compound modified bin.
J. Liu et al. / Construction and Building Materials 202 (2019) 614–621
which indicated that the binders with higher TLA concentration are stiffer than those with lower TLA concentration. This is consistent with the finding of Liao et al.’s [10], which pointed out that the additions of TLA would bring higher Modulus. However, unlike the significant harden effect of TLA, SBR showed the insignificant effect on penetration. At a specific concentration of TLA, the penetration of compound modified binder slightly decreased with the increase of SBR concentration. Fig. 1(b) illustrates the effects of TLA and SBR on the softening point. The softening point of compound modifier binder with a specific SBR concentration, as expected, increased with the increase of TLA concentration. The temperature difference between base asphalt binder and modified binder with 30% TLA was 44.8 °C, which was very significant. The addition of SBR slightly increased the softening point of the compound modified binder, though the compound modified binders with higher SBR concentration was softer than those with lower SBR concentration, as indicated by the penetration results. The similar conclusion that TLA can improve the hightemperature performance can be obtained from the equivalent softening point results. As shown in Fig. 2, the compound modified binder with 4% SBR and 30% TLA had the highest equivalent softening point value. It should be noted that the addition of SBR showed the insignificant effect on the equivalent softening point value of TLA modified binders except for the binders with 10% TLA. A further study on the microstructures of compound modified binder with different concentrations of SBR and TLA is recommended to explain this phenomenon. The ductility results are illustrated in Fig. 1(c). According to Kandha [26], the ductility of asphalt binder at low temperatures (e.g., at 5 °C) showed robust correlation with the lowtemperature performance of the pavement. The pavements containing asphalt with low ductility are likely to show poorer service than pavements containing asphalts of the same penetration but with high ductility. As mentioned in the introduction section, ductility tests indicated that the addition of TLA degraded the lowtemperature performance of binder. The ductility of compound modified binders with a specific SBR concentration decreased with the increase of TLA concentration, while that of compound modified with a specific TLA concentration increased with the increase of SBR concentration. This indicated that the SBR introduced better low-temperature properties to the compound modified binder. The similar conclusion can be obtained from the equivalent brittle point results, as shown in Fig. 3. The compound modified binder
Fig. 2. Equivalent softening point results.
617
Fig. 3. Equivalent brittle point results.
with 5% TLA and 4% SBR showed lowest equivalent value (16 °C), while the TLA modified binder with 30% TLA had the highest value (7 °C). Based on the equivalent brittle point and ductility test results, it appears that SBR can serve as the modifier that can neutralize or even eliminate the degradation effect of TLA on low-temperature performance. The viscosity is generally applied to evaluate the flow characteristics of the binder. A lower viscosity at high construction temperatures is better for facilitating pumping of the binder, mixing and compaction of HMA. The rotational viscosity at 135 °C of compound modified binders is illustrated in Fig. 4. It can be observed that the base asphalt binder had the lowest value while compound modified binder with 4% SBR and 30% TLA showed the highest value, which indicated that both additives affected the viscosity. The difference between the lowest and highest values was almost 2000cp, which was very significant. However, despite the significant increase, the highest viscosity value (2478cp) that from compound modified binder with 4% SBR and 30% TLA still met the requirement of maximum 3000cp according to Superpave specification. It seems that TLA had a more significant effect on viscosity than SBR does based on the illustration of Fig. 4, however, it’s noted that the TLA concentrations (i.e., 5%, 10%, 20%, and 30%, by weight of neat binder) applied in this study were much higher than SBR concentrations. The authors would attribute this to the mineral content containing in TLA. The viscosity of binder would be increased a lot by adding mineral fillers [10].
Fig. 4. Viscosity of compound modified binder.
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To further characterize the high-temperature performance of TLA and SBR compound modified binders, temperature sweep tests were conducted using DSR. Fig. 5 illustrates the complex modulus and phase angle of compound modified binders with different TLA and SBR concentrations. Complex modulus (G*) can be considered as the total resistance of the binder to deformation when repeated sheared [1]. Phase angle (d) is defined as the time lag between the applied stress and the resulting strain, and used to characterize the viscoelastic properties of the binder. When the viscoelastic material, for example, asphalt binder, is loaded, part of deformation is elastic, and part is viscous. As shown in Fig. 5, the complex modulus decreased while the phase angle increased with the increase of temperature. From Fig. 5(a), it can be seen that the G* value of TLA modified binders was higher than that of base asphalt binder at a specific test temperature. The G* increased with the increase of TLA concentration at a specific test temperature. The d decreased gradually with the increase of TLA concentration, indicating that modified binders with higher TLA concentration were more elastic than those with lower concentration at the desired temperature. A more elastic part in modulus is beneficial to deformation resistance in some circumstances. Comparing Fig. 5(a) and (b), it can be observed that the G* value (d value) of the modified binder with SBR was higher (lower) than that of the binder without SBR at the same temperature, indicating that the addition of SBR improved the deformation resistance of binder. However, no significant difference on G* and d was observed from Fig. 5(b)–(d) among compound modified binders with different SBR concentrations. Based on these results, the overall conclusion can be obtained that
the TLA has a significant effect on improving deformation resistance, especially when TLA concentration equal to or above 20%. The addition of SBR with specific concentrations can improve deformation resistance as well, but the effect was insignificant. The conclusion can be confirmed by the failure temperatures of various compound modified binders. As shown in Fig. 6, the modified binder with higher TLA concentration showed a higher fail temperature value. SBR had little effect on failure temperature. Fig. 7(a-c) present the BBR test results on compound modified binder at different temperatures. Vertical and horizontal dashdotted lines represent thresholds specified by Superpave, which are 300 MPa for stiffness and 0.3 for m-value, respectively. As the lowest pavement temperatures in most parts of south China is between 10 °C and 25 °C and the BBR tests are designed to be conducted at the temperature which is 10 °C higher than pavement temperature, the three testing temperatures (6°C, 12 °C, and 18 °C, which are corresponding to pavement temperatures of 16 °C, 22 °C, and 28 °C, respectively) were selected. Fig. 7(a) presents the BBR tests results at a test temperature of 18 °C. Lower stiffness value while higher m-value in low temperatures are more desirable for thermal cracking resistance. The thermal cracking resistance of TLA modified binder degraded with the increase of TLA concentration. The addition of SBR can significantly improve the thermal cracking resistance. Four binders met Superpave specification (both stiffness and m-value) at 18 °C. They all were compound modified binders with SBR. The data points for the binder with 2%SBR and 20%TLA and binder with 3% SBR and 20% TLA were almost overlapped, but they all met the Superpave spec-
(a) 0% SBR
(c) 3% SBR
(b) 2% SBR
(d) 4% SBR
Fig. 5. Complex modulus and phase angle of compound modified binder.
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(a) -18 °C Fig. 6. Failure temperature of compound modified binder.
ification. While the compound modified binders with 30% TLA all failed the specification at 18 °C, at any SBR concentrations. According to the DSR results present previously, the compound modified binders with 20% TLA and those with 30% TLA showed similar high-temperature performance. Therefore, the TLA concentration should be kept equal to or below 20% based on the BBR and DSR tests results. Besides, it should be noted that the compound modified binder with 4% SBR and 20%TLA was tested at 18 °C as well, and it showed similar (a little bit better) low-temperature performance as compound modified binder with 3% SBR and 20% TLA did. As mentioned previously, the compatibility between SBR and asphalt was poor [16]. The higher SBR concentration, the poorer compatibility. Therefore, the SBR concentration was recommended to be kept as 2% or 3%. The data for compound modified binders with 4% SBR were not included in the following discussion. Fig. 7(b) presents the BBR tests results at 12 °C. It can be found that all the tested binders met the Superpave specification except those with 30% TLA. The two data points which are located in left bottom indicated that these two corresponding binders were mvalue controlled: the binders were soft enough at 12 °C, but they had poor relaxation properties, which would result in thermal stress accumulation [25]. Fig. 7(c) gives the BBR tests results at 6 °C. All the compound modified binders met the Superpave specification, indicating that all the studied compound modified binder can be applied in the regions with lowest pavement temperature warm than 16 °C, as long as other conditions such as economic and mixing conditions allow. Fig. 8(a-c) illustrate the RP, DS, and VAI of compound modified binders, respectively. Only binders with 2% SBR and 3% SBR were studied, based on previous discussions. As shown in Fig. 8(a), TLA showed a significant effect on RP. For binders with RTFO aging, the RP increased with the increase of TLA, regardless of SBR concentration. This indicated that the addition of TLA could significantly prevent hardening during mixing and construction. SBR showed an insignificant effect on RP. The compound modified binders with 3% SBR showed slightly higher RP value than those with 2% SBR. The results were different from those froma previous study [21] which stated that the SBR modified binder was more prone to oxidation. For binders with RTFO and PAV aging, the compound modified binder with 3% SBR and 20% TLA showed the highest RP value, which indicated that it had the best durability property after long-term aging. The similar conclusion can be obtained from DS (Fig. 8(b)) and VAI (Fig. 8(c)) results. The DS/VAI decreased with the increase of TLA after short-term aging, for the compound modified binder with a specific SBR concentration. The binder with
(b) -12 °C
(c) -6 °C Fig. 7. Stiffness and m-value of compound modified binder at different temperatures.
higher SBR concentration showed slightly lower DS and/or VAI values than that with lower concentration. The compound modified binder with 3% SBR and 20% TLA showed lowest DS and/or VAI values. Based on the above statements, the concentrations of SBR and TLA were recommended as 3% and 20% respectively, for the best durability properties.
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(a) Retained Penetration results
SBR concentrations (0%, 2%, 3%, and 4%) were selected. Several laboratorial tests including conventional test (penetration, softening point, and ductility tests), rotational viscosity test, DSR test, and BBR test were conducted to investigate the rheological and aging properties of TLA and SBR compound modified binders. It can be found that the modified binders with TLA could improve the deformation resistance of asphalt binder by increasing the early stiffness and prolong the service life of the corresponding pavement by improving the durability properties. However, the addition of TLA could degrade the low-temperature flow properties, compared with the base asphalt. The compound modified binders with SBR had better lowtemperature flow properties than TLA single modified binders did. The addition of SBR could slightly improve the hightemperature deformation resistance and durability properties of compound modified binder. The concentration combination of 2% SBR and 20% TLA was recommended, since at this combination the compound modified binder showed not only good hightemperature performances and anti-oxidation properties but also good low-temperature flow properties, compared with the base asphalt as well as binders modified by TLA only. The compound modified binder with 2% SBR and 20% TLA met the Superpave BBR test specification at the testing temperature of 18 °C. Although the workability was degraded by introducing additives (indicated by the increased viscosity value), all the studied binders met the Superpave specification which specified that rotational viscosity at 135 °C should be less than 3000cp.
Conflict of interest None.
References
(b) Softening point increment results
(c) Viscosity aging index results Fig. 8. Aging indices results of compound modified binders.
5. Conclusions A comprehensive laboratorial investigation on TLA and SBR compound modified binders was presented. The asphalt binders were modified with different combinations of TLA and SBR concentrations. Four TLA concentrations (5%, 10%, 20%, and 30%) and four
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Evaluation of the characteristics of Trinidad Lake Asphalt and Styrene–Butadiene–Rubber compound modified binder Jun Liu a,b, Kezhen Yan a,⇑, Jenny Liu b, Dong Guo a a b
College of Civil Engineering, Hunan University, Changsha 410082, China Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO 65401, United States
h i g h l i g h t s The SBR and TLA compound modified binders had better low-temperature flow properties than TLA single modified binders did. The concentration combination of 2% SBR and 20% TLA was recommended. The workability was degraded by introducing additives but all the studied binders met the Superpave specification.
a r t i c l e
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Article history: Received 17 October 2018 Received in revised form 2 January 2019 Accepted 9 January 2019
Keywords: Trinidad Lake Asphalt (TLA) Styrene–Butadiene–Rubber (SBR) Compound modified binder Rheological properties Non-load related cracking
a b s t r a c t A comprehensive experiment involving different Trinidad Lake Asphalt (TLA) and Styrene–Butadiene–R ubber (SBR) compound modified binders was conducted to investigate the rheological and aging properties of TLA and SBR compound modified binders. Four TLA (5%, 10%, 20%, and 30%) and four SBR concentrations (0%, 2%, 3%, and 4%) were selected. Dynamic shear rheometer (DSR) and bending beam rheometer (BBR) tests were performed to evaluate the rheological properties. Conventional tests (penetration, softening point, and ductility tests) were conducted on TLA and SBR compound modified binders with different aging states (i.e., original, RTFO aged, and PAV aged) to investigate the physical and durability properties. Rotational viscosity tests were performed to evaluate the workability. The results indicated that compound modified binders with TLA and SBR could improve the deformation resistance of binders by increasing the early stiffness and prolong the service life of the corresponding pavement by improving the durability, compared with base asphalt binder. The addition of TLA could degrade the lowtemperature flow properties, but the degradation effect could be offset by adding SBR. The combination of 2% SBR and 20% TLA was recommended based on the comprehensive analysis of test results. The workability was degraded by introducing additives, but all binders studied met the Superpave specification. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Modification on some neat (non-modified) asphalt cements is required in order to meet specifications, producing modified asphalt binder. The use of modified binder has increased significantly in recent years mainly due to the increased demand on hot mix asphalt (HMA) pavements, development of Superpave asphalt binder specifications, environmental and economic pressure, and public agencies’ willingness to pay higher-cost for longer pavement service life [1–4]. Trinidad Lake Asphalt (TLA) is a unique natural binder, which has been served as a kind of asphalt binder modifier to improve high-temperature performance for several decades. Similar to most ⇑ Corresponding author. E-mail address: [email protected] (K. Yan). https://doi.org/10.1016/j.conbuildmat.2019.01.053 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
of the modifiers such as polymers, TLA was found to increase the strength of the flexible pavement. The stiffness and Marshall stability of asphalt mixture could be improved by incorporating of TLA [5]. The TLA modified mixtures were usually applied in asphalt overlay project to help obtain better permanent deformation resistance [6]. TLA was blended with asphalt binder with a pen grade of 160/220 to produce the mixed asphalt binder with lower penetration value for a surface course in England [7]. TLA was also added to PG 76-28 binder along with steel slag in an overlay project in Colorado, the US for the shortage of quality aggregate [8]. The composition of TLA typically includes mineral matter and asphalt binder, and thus the mineral matter was served as fine aggregates or fillers in that Colorado project. Additionally, TLA was also incorporated into asphalt mastic which was served as waterproofing materials on steel bridge decks [9]. Liao et al. investigated the rheological properties of TLA modified binder. Their results
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indicated that the addition of TLA into asphalt binder could bring an improvement on rutting resistance, and the optimal dosage of TLA was found to be in the range of 20–33% [10]. Although lots of previous studies [5–11] indicated that the introducing of TLA into asphalt binders or mixtures could bring better permanent deformation resistance which is usually related to high-temperature performance, the results of pilot tests of this study (the ductility test and equivalent brittle point results, presented in following sections) and previous study [12] showed that the addition of TLA could degrade the low temperature cracking resistance of the asphalt binder. Therefore, it is critical to find a modifier or a technology that can incorporate with TLA to have the compound modified binder that can benefit from TLA regarding high-temperature performance without compromising thermal cracking resistance. Styrene-butadiene rubber (SBR) is a kind of widely used modifier that can improve the low-temperature ductility and elastic recovery of asphalt binder, which in turn improving the thermal cracking resistance [13–15]. According to Yildirim, the ductility of asphalt binder could be increased by adding the SBR, which making the asphalt binder more flexible at low temperatures and thus more capable of resisting thermal cracking [16]. Ruan et al. indicated that the incorporating of SBR into asphalt binder could bring an increase in complex modulus at high temperatures, and could bring a decrease in complex modulus at low temperatures [17]. In addition, SBR was also applied as modifier along with other modifiers to produce compound modified binder. Zhang and Yu investigated the properties of SBR and polyphosphoric acid (PPA) compound modified binder, and the results indicated that the addition of SBR could improve the thermal cracking resistance of the PPA modified binder [18]. Zhang et al. investigated the properties of SBR and montmorillonite (MMT) compound modified binders, and indicated that the addition of SBR and MMT into asphalt binder formed a newly network, which can improve the high-temperature properties and storage stability of the asphalt binder [19]. Zhu et al. combined SBR with Nano-materials to modify the asphalt binder, and both the thermal and the ultraviolet (UV) aging resistance of the asphalt binder were improved by adding the modifiers [20]. However, according to Salehfard et al., the low-temperature ductility of SBR modified binder would decrease substantially after short-term aging. The authors attributed the reduction to the SBR molecules, which are easy to be oxidized [21]. This drawback of SBR may be eliminated by incorporating TLA into SBR modified binder. According to Li et al., the addition of TLA can effectively improve thermal-oxidative or UV aging resistance of asphalt binder [22]. Considering the characteristics of TLA and SBR modified asphalt binders, it is promising to incorporate TLA with SBR to obtain the compound modified binder which can not only show great permanent deformation resistance benefiting from the addition of TLA but also possess remarkable low-temperature flexibility due to the incorporation of SBR. Although the characteristics of TLA and SBR modified binders were separately investigated in many previous studies, no study looked into the properties of TLA and SBR compound modified binder. Thus, a comprehensive investigation of the characteristics of TLA and SBR compound modified binder is essential to provide better understanding for promoting its engineering application. The primary objectives of this study are to: 1. Evaluate the conventional (physical), and rheological properties of compound modified binders with different dosages of TLA and SBR to see if the compound modified binders have better low-temperature properties than those of TLA single modified binders (without SBR), without degrading the permanent deformation resistance.
2. Investigate the aging properties of compound modified binders with different dosages of TLA and SBR to see if the compound modified binder would show better durability properties. 3. Determine the optimal contents of TLA and SBR for the compound modified binder based on the testing results. Conventional tests (Penetration, softening point, and ductility tests), rotational viscosity tests, dynamic shear rheometer (DSR) tests, and bending beam rheometer (BBR) tests were conducted on compound modified binders with different TLA and SBR dosages at different aging states to achieve the above objectives. 2. Materials and preparation The base asphalt used in this study was the AH 70 which was produced by SINOPEC Shanghai, China. Table 1 lists some of its’ basic properties. The SBR used in this study was SBR 1502 with 40 mesh. The basic properties of TLA used in this study are listed in Table 2. Four TLA concentrations (i.e. 5%, 10%, 20%, and 30%) and four SBR concentrations (i.e. 0%, 2%, 3%, 4%), by weight of base asphalt, were selected in this study based on previous studies [23,24]. According to the recommendations from previous studies [23,24], the compound modified binders were prepared as follows. Considering SBR is less compatible with petroleum asphalt binder than TLA does, SBR modified binders were produced firstly. Then, TLA was added to SBR modified binders to produce TLA and SBR compound modified binders. A shearing mixer was used to mix binders. The base asphalt and TLA were heated until they became fluid before mixing. SBR was added slowly into liquid base asphalt with stirring speed of 2000 rpm. After about 20 min of mixing, the stirring speed was changed to 5000 rpm, and TLA was added. Another 30 min of stirring was needed to obtain the compound modified binders. The temperature was kept at 160 °C during the whole mixing process. 3. Experimental details Penetration tests at 15 °C, 25 °C, and 30 °C, ductility tests at 5 °C, and softening point tests were conducted according to ASTM D5, D113, and D36, respectively. By linearly regressing the logarithm of Penetration (P) against temperature (T), the intercept (K) along with slope (A) were obtained to calculate the equivalent softening point (T800 , Eq.1) and equivalent brittle point (T1;2 , Eq.2). T800 and T1;2 are conventional and empirical indices to evaluate the high and low-temperature performance, respectively [24]. The asphalt binder with higher T800 value and lower T1;2 value shows better high-temperature performance and low temperature cracking resistance.
T800 ¼
lg800 K A
ð1Þ
T1;2 ¼
lg1:2 K A
ð2Þ
Table 1 Basic properties of base asphalt binder. Properties
Units
Test results
Specification limits
Penetration (at 25 °C) Softening point Ductility (at 15 °C) Rotational viscosity (at 135 °C)
0.1 mm °C cm cp
69.7 50.2 >150 540
60 to 80 47 100 3000 RV 180
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Table 2 Basic properties of TLA.
4. Results and discussions
Properties
Units
Test results
Specification limits
Penetration (at 25 °C) Softening point Mineral filler content Density Retained Penetration after TFOT
0.1 mm °C % g/cm3 %
4.0 93 36.7 1.3072 55
0–5 90 33–38 1.3–1.5 50
The rotational viscosity at 135 °C of the studied binders was measured according to ASTM D4402 using a Brookfield viscometer to evaluate the pumping ability at the HMA plant. The DSR tests were applied to investigate the rheological properties of the compound modified binders based on AASHTO T 315. The complex modulus (G*) which indicates the stiffness of tested binder at desired temperature and frequency of loading and phase angle (d) which reflects elastic and viscous components of complex modulus were obtained. The G* is as importance as the resilient modulus of the base to determine the performance of the pavement [27]. The test geometry with 25 mm diameter plate and 1 mm gap was used. The strain control model was applied (a constant strain of 12% was applied) and the applied loading frequency was 1.59 Hz (10 rad/s). Temperature sweep test (temperature range from 40 °C to 80 °C with an increment of 2 °C/min) was conducted on each studied binder. Besides, the failure temperatures which defined as the temperature at which the rutting parameter (G*/sin d) of binder equal to 1.0 kPa were determined by conducting performance grade (PG) determination tests. The BBR tests were conducted to evaluate the thermal cracking resistance of TLA and SBR compound modified binders according to AASHTO T 313. A constant load is applied to a sample, and the creep stiffness (S) and m-value at 60 s at three different temperatures (i.e., 6 °C, 12 °C, and 18 °C) were recorded. The asphalt binder with low S value and high m-value shows better low temperature cracking resistance. Superpave specified that the S should be lower than 300 MPa and m-value should be higher than 0.3 at the predicted lowest pavement temperature for the asphalt binder that could be applied to the field. Rolling thin-film oven (RTFO) tests were conducted on the studied compound modified binders following the guide of the ASTM D2872. The samples were kept at the RTFO for 85 min at 163 °C. Pressure aging vessel (PAV) tests were performed on RTFO residues to simulate the long-term aging process according to ASTM D6521. The temperature was kept at 100 °C, and the air pressure was kept at 2.1 MPa during the 20 h of testing. Retained Penetration (RP) at 25 °C, softening point increment (DS) and viscosity aging index (VAI) were measured or calculated to characterize the aging performance of studied binders. A higher value of DS or VAI reflects more significant influence of aging, while a higher value of RP means lower influence. The indices are recommended to be calculated by Eqs. (3)–(5).
Fig. 1(a) presents the test results of penetration. As shown, the penetration of modified binder regardless of TLA modified binder or TLA and SBR compound modified binder, was much lower than that of base asphalt, indicating that TLA has harden effect on asphalt binder. Besides, it can be seen that the compound modified binders with a specific SBR concentration showed a gradual decrease in penetration with the increase of TLA concentration,
(a) Penetration results
(b) Softening point results
DS ¼ Softening point of aged binder
RP ¼
Softening point of unaged binder
ð3Þ
Penetration of aged binder 100% Penetraction of unaged binder
ð4Þ
VAI ¼ lglg ga 103 lglg gb 103
ð5Þ
where ga (gb Þ stands for the viscosity of asphalt sample after (before) aging.
(c) Ductility results Fig. 1. Conventional tests results of compound modified bin.
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which indicated that the binders with higher TLA concentration are stiffer than those with lower TLA concentration. This is consistent with the finding of Liao et al.’s [10], which pointed out that the additions of TLA would bring higher Modulus. However, unlike the significant harden effect of TLA, SBR showed the insignificant effect on penetration. At a specific concentration of TLA, the penetration of compound modified binder slightly decreased with the increase of SBR concentration. Fig. 1(b) illustrates the effects of TLA and SBR on the softening point. The softening point of compound modifier binder with a specific SBR concentration, as expected, increased with the increase of TLA concentration. The temperature difference between base asphalt binder and modified binder with 30% TLA was 44.8 °C, which was very significant. The addition of SBR slightly increased the softening point of the compound modified binder, though the compound modified binders with higher SBR concentration was softer than those with lower SBR concentration, as indicated by the penetration results. The similar conclusion that TLA can improve the hightemperature performance can be obtained from the equivalent softening point results. As shown in Fig. 2, the compound modified binder with 4% SBR and 30% TLA had the highest equivalent softening point value. It should be noted that the addition of SBR showed the insignificant effect on the equivalent softening point value of TLA modified binders except for the binders with 10% TLA. A further study on the microstructures of compound modified binder with different concentrations of SBR and TLA is recommended to explain this phenomenon. The ductility results are illustrated in Fig. 1(c). According to Kandha [26], the ductility of asphalt binder at low temperatures (e.g., at 5 °C) showed robust correlation with the lowtemperature performance of the pavement. The pavements containing asphalt with low ductility are likely to show poorer service than pavements containing asphalts of the same penetration but with high ductility. As mentioned in the introduction section, ductility tests indicated that the addition of TLA degraded the lowtemperature performance of binder. The ductility of compound modified binders with a specific SBR concentration decreased with the increase of TLA concentration, while that of compound modified with a specific TLA concentration increased with the increase of SBR concentration. This indicated that the SBR introduced better low-temperature properties to the compound modified binder. The similar conclusion can be obtained from the equivalent brittle point results, as shown in Fig. 3. The compound modified binder
Fig. 2. Equivalent softening point results.
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Fig. 3. Equivalent brittle point results.
with 5% TLA and 4% SBR showed lowest equivalent value (16 °C), while the TLA modified binder with 30% TLA had the highest value (7 °C). Based on the equivalent brittle point and ductility test results, it appears that SBR can serve as the modifier that can neutralize or even eliminate the degradation effect of TLA on low-temperature performance. The viscosity is generally applied to evaluate the flow characteristics of the binder. A lower viscosity at high construction temperatures is better for facilitating pumping of the binder, mixing and compaction of HMA. The rotational viscosity at 135 °C of compound modified binders is illustrated in Fig. 4. It can be observed that the base asphalt binder had the lowest value while compound modified binder with 4% SBR and 30% TLA showed the highest value, which indicated that both additives affected the viscosity. The difference between the lowest and highest values was almost 2000cp, which was very significant. However, despite the significant increase, the highest viscosity value (2478cp) that from compound modified binder with 4% SBR and 30% TLA still met the requirement of maximum 3000cp according to Superpave specification. It seems that TLA had a more significant effect on viscosity than SBR does based on the illustration of Fig. 4, however, it’s noted that the TLA concentrations (i.e., 5%, 10%, 20%, and 30%, by weight of neat binder) applied in this study were much higher than SBR concentrations. The authors would attribute this to the mineral content containing in TLA. The viscosity of binder would be increased a lot by adding mineral fillers [10].
Fig. 4. Viscosity of compound modified binder.
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To further characterize the high-temperature performance of TLA and SBR compound modified binders, temperature sweep tests were conducted using DSR. Fig. 5 illustrates the complex modulus and phase angle of compound modified binders with different TLA and SBR concentrations. Complex modulus (G*) can be considered as the total resistance of the binder to deformation when repeated sheared [1]. Phase angle (d) is defined as the time lag between the applied stress and the resulting strain, and used to characterize the viscoelastic properties of the binder. When the viscoelastic material, for example, asphalt binder, is loaded, part of deformation is elastic, and part is viscous. As shown in Fig. 5, the complex modulus decreased while the phase angle increased with the increase of temperature. From Fig. 5(a), it can be seen that the G* value of TLA modified binders was higher than that of base asphalt binder at a specific test temperature. The G* increased with the increase of TLA concentration at a specific test temperature. The d decreased gradually with the increase of TLA concentration, indicating that modified binders with higher TLA concentration were more elastic than those with lower concentration at the desired temperature. A more elastic part in modulus is beneficial to deformation resistance in some circumstances. Comparing Fig. 5(a) and (b), it can be observed that the G* value (d value) of the modified binder with SBR was higher (lower) than that of the binder without SBR at the same temperature, indicating that the addition of SBR improved the deformation resistance of binder. However, no significant difference on G* and d was observed from Fig. 5(b)–(d) among compound modified binders with different SBR concentrations. Based on these results, the overall conclusion can be obtained that
the TLA has a significant effect on improving deformation resistance, especially when TLA concentration equal to or above 20%. The addition of SBR with specific concentrations can improve deformation resistance as well, but the effect was insignificant. The conclusion can be confirmed by the failure temperatures of various compound modified binders. As shown in Fig. 6, the modified binder with higher TLA concentration showed a higher fail temperature value. SBR had little effect on failure temperature. Fig. 7(a-c) present the BBR test results on compound modified binder at different temperatures. Vertical and horizontal dashdotted lines represent thresholds specified by Superpave, which are 300 MPa for stiffness and 0.3 for m-value, respectively. As the lowest pavement temperatures in most parts of south China is between 10 °C and 25 °C and the BBR tests are designed to be conducted at the temperature which is 10 °C higher than pavement temperature, the three testing temperatures (6°C, 12 °C, and 18 °C, which are corresponding to pavement temperatures of 16 °C, 22 °C, and 28 °C, respectively) were selected. Fig. 7(a) presents the BBR tests results at a test temperature of 18 °C. Lower stiffness value while higher m-value in low temperatures are more desirable for thermal cracking resistance. The thermal cracking resistance of TLA modified binder degraded with the increase of TLA concentration. The addition of SBR can significantly improve the thermal cracking resistance. Four binders met Superpave specification (both stiffness and m-value) at 18 °C. They all were compound modified binders with SBR. The data points for the binder with 2%SBR and 20%TLA and binder with 3% SBR and 20% TLA were almost overlapped, but they all met the Superpave spec-
(a) 0% SBR
(c) 3% SBR
(b) 2% SBR
(d) 4% SBR
Fig. 5. Complex modulus and phase angle of compound modified binder.
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(a) -18 °C Fig. 6. Failure temperature of compound modified binder.
ification. While the compound modified binders with 30% TLA all failed the specification at 18 °C, at any SBR concentrations. According to the DSR results present previously, the compound modified binders with 20% TLA and those with 30% TLA showed similar high-temperature performance. Therefore, the TLA concentration should be kept equal to or below 20% based on the BBR and DSR tests results. Besides, it should be noted that the compound modified binder with 4% SBR and 20%TLA was tested at 18 °C as well, and it showed similar (a little bit better) low-temperature performance as compound modified binder with 3% SBR and 20% TLA did. As mentioned previously, the compatibility between SBR and asphalt was poor [16]. The higher SBR concentration, the poorer compatibility. Therefore, the SBR concentration was recommended to be kept as 2% or 3%. The data for compound modified binders with 4% SBR were not included in the following discussion. Fig. 7(b) presents the BBR tests results at 12 °C. It can be found that all the tested binders met the Superpave specification except those with 30% TLA. The two data points which are located in left bottom indicated that these two corresponding binders were mvalue controlled: the binders were soft enough at 12 °C, but they had poor relaxation properties, which would result in thermal stress accumulation [25]. Fig. 7(c) gives the BBR tests results at 6 °C. All the compound modified binders met the Superpave specification, indicating that all the studied compound modified binder can be applied in the regions with lowest pavement temperature warm than 16 °C, as long as other conditions such as economic and mixing conditions allow. Fig. 8(a-c) illustrate the RP, DS, and VAI of compound modified binders, respectively. Only binders with 2% SBR and 3% SBR were studied, based on previous discussions. As shown in Fig. 8(a), TLA showed a significant effect on RP. For binders with RTFO aging, the RP increased with the increase of TLA, regardless of SBR concentration. This indicated that the addition of TLA could significantly prevent hardening during mixing and construction. SBR showed an insignificant effect on RP. The compound modified binders with 3% SBR showed slightly higher RP value than those with 2% SBR. The results were different from those froma previous study [21] which stated that the SBR modified binder was more prone to oxidation. For binders with RTFO and PAV aging, the compound modified binder with 3% SBR and 20% TLA showed the highest RP value, which indicated that it had the best durability property after long-term aging. The similar conclusion can be obtained from DS (Fig. 8(b)) and VAI (Fig. 8(c)) results. The DS/VAI decreased with the increase of TLA after short-term aging, for the compound modified binder with a specific SBR concentration. The binder with
(b) -12 °C
(c) -6 °C Fig. 7. Stiffness and m-value of compound modified binder at different temperatures.
higher SBR concentration showed slightly lower DS and/or VAI values than that with lower concentration. The compound modified binder with 3% SBR and 20% TLA showed lowest DS and/or VAI values. Based on the above statements, the concentrations of SBR and TLA were recommended as 3% and 20% respectively, for the best durability properties.
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(a) Retained Penetration results
SBR concentrations (0%, 2%, 3%, and 4%) were selected. Several laboratorial tests including conventional test (penetration, softening point, and ductility tests), rotational viscosity test, DSR test, and BBR test were conducted to investigate the rheological and aging properties of TLA and SBR compound modified binders. It can be found that the modified binders with TLA could improve the deformation resistance of asphalt binder by increasing the early stiffness and prolong the service life of the corresponding pavement by improving the durability properties. However, the addition of TLA could degrade the low-temperature flow properties, compared with the base asphalt. The compound modified binders with SBR had better lowtemperature flow properties than TLA single modified binders did. The addition of SBR could slightly improve the hightemperature deformation resistance and durability properties of compound modified binder. The concentration combination of 2% SBR and 20% TLA was recommended, since at this combination the compound modified binder showed not only good hightemperature performances and anti-oxidation properties but also good low-temperature flow properties, compared with the base asphalt as well as binders modified by TLA only. The compound modified binder with 2% SBR and 20% TLA met the Superpave BBR test specification at the testing temperature of 18 °C. Although the workability was degraded by introducing additives (indicated by the increased viscosity value), all the studied binders met the Superpave specification which specified that rotational viscosity at 135 °C should be less than 3000cp.
Conflict of interest None.
References
(b) Softening point increment results
(c) Viscosity aging index results Fig. 8. Aging indices results of compound modified binders.
5. Conclusions A comprehensive laboratorial investigation on TLA and SBR compound modified binders was presented. The asphalt binders were modified with different combinations of TLA and SBR concentrations. Four TLA concentrations (5%, 10%, 20%, and 30%) and four
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