Development of a Performance Grading System for Asphalt

Asian Transport Studies, Volume 2, Issue 2 (2012), 121-138 © 2012 ATS All rights reserved

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Development of a Performance Grading System for Asphalt Binders Used in Thailand Nattaporn CHAROENTHAM a, Kunnawee KANITPONG b a School of Transportation Engineering, Institute of Engineering, Suranaree University of

Technology, 111, University Ave., Suranaree, Muang, Nakorn Ratchasima, 30000, Thailand; E-mail: [email protected]

b School of Engineering and Technology, Asian Institute of Technology, P.O.Box 4, Klong Luang, Pathumthani, 12120, Thailand; E-mail: [email protected]

Abstract: Widely-used existing penetration and viscosity grading systems have been found to be insufficient in characterizing performance of asphalt binders according to many researchers. It has been discovered that asphalt binders can have different temperature susceptibility and performance characteristics even if classified in the same grade using these two grading systems. Recognizing the limitation of these systems, the Strategic Highway Research Program (SHRP) was initiated in 1987 to develop performance based tests and specifications for asphalt binders and asphalt mixture design - the so-called SUPERPAVE. SUPERPAVE tests have been introduced to measure physical properties of asphalt binders that could be directly related to field performance such as resistibility to rutting, fatigue cracking and thermal cracking. This concept has been successfully implemented in the US and European countries. In Thailand, only penetration system has apparently been used by related authorities. Nowadays, there is an initiative to improve the current grading system in Thailand by replacing it with a PG grading system. Unfortunately, complex and expensive equipment are required for the SUPERPAVE grading system, which is still unavailable in Thailand. Moreover, knowledge is likewise limited in understanding system implementation. In addition, the SUPERPAVE specification is based on pavement conditions in the USA. Therefore, this study aims to develop a performance grading system using indirectly estimated parameters related to pavement performance and conditions in Thailand. After a validation of such developed system, it was found that the proposed performance grading system can be satisfactorily used to classify the performance of binders which is also comparable with the SUPERPAVE system. It should be, however, noted that the proposed system can be used as a transition until sufficient equipment are obtained and sufficient understanding on SUPERPAVE implementation are acquired. Key Words: Performance grading system, Classification, Asphalt binder, Asphalt cement 1. INTRODUCTION In Thailand, the asphalt binder specification has been developed for a long time by the Thailand Industrial Standard Institute (TISI). Two types of grading systems, penetration and AC viscosity grading systems, are provided as found in TIS 851-2542 (Thailand Industrial Standard Institute, 1999). Typically, only a penetration grading system is specified for asphalt cement standards of the Department of Highways (DOH), the Department of Rural Roads (DOR) and the Department of Public Works and Town & Country Planning (DPT). Four penetration grades are specified as AC40-50, AC60-70, AC85-100 and AC120-250. The

CHAROENTHAM, A. and KANITPONG, K. / Asian Transport Studies, Volume 2, Issue 2 (2012), 121-138

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penetration grading specification requires the measurement and control of physical properties of asphalt binders including consistency, durability, purity and safety. However, performance-related properties such as rutting, fatigue cracking and thermal cracking are not considered in this specification. Additionally, the rheological properties of asphalt binders during the long-term aging which represent the condition of asphalt binders under further aging during a long term service period are neglected. The critical pavement condition factors which affect pavement performance such as traffic volume, traffic speed, traffic load, temperature and moisture, are not considered. For this reason, the traditional specification on asphalt binders for Thailand should be changed and direct performance-related properties of the pavement should be taken into consideration. One possible improvement is to use SUPERPAVE performance grading (PG) system, which was developed by the Strategic Highway Research Program (SHRP) in the early 1990s, to replace the existing grading system in Thailand. Nevertheless, the problem of using PG grading system in Thailand is the lack of complex and expensive equipment and knowledge limitation in understanding its development and implementation. Moreover, development of parameter criteria currently used in SUPERPAVE specification is based on existing pavement conditions in the United States. A solution to these problems is to develop an asphalt binder performance grading system that can be implemented in Thailand without using expensive equipment. Besides, the proposed parameters and criteria should be based on existing pavement performance and conditions in Thailand. Therefore, this study aims to develop a performance grading system based on indirectly estimated parameters related to pavement performance for asphalt binders used in Thailand. 2. THE CONCEPT OF PERFORMANCE GRADING SYSTEM

BASED ON INDIRECTLY ESTIMATED PARAMETERS

In order to generate specifications by considering the pavement performance, it is necessary to consider multiple performance measures (Van de Ven et al., 2004). Four key performance requirements for the asphalt binder used for the production of asphalt mixture in Thailand are identified in this study, which includes workability, permanent deformation (rutting) resistance, fatigue cracking resistance and thermal cracking resistance. Moreover, the selection of appropriate asphalt properties that merit characterization is driven by these performance requirements which are encountered during HMA production, construction and the service life of the pavement. For workability purposes, the rotational viscometer (RV) has been adopted in the SUPERPAVE specification for determining the asphalt viscosity at high temperature during the production and construction process (McGennis et al., 1994). This is to ensure that the binder is sufficiently liquefied to be pumped and mixed during the construction process. Nevertheless, such equipment is not available in many local pavement engineering divisions in Thailand. Currently, the viscosity of asphalt binders is determined by using the kinematic viscosity tested at a temperature of 135°C which is the standard test method in Thailand (Thailand Industrial Standard Institute, 1999). Normally, the rutting resistance can be increased by using stiffer asphalt binder with higher viscosity or lower penetration grade. The stiffness of asphalt binder is continuously increased during aging (Roberts et al., 1996). Therefore, the short-term aged binders that represent the


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asphalt binders in the early stages of its life immediately after placement and before long-term aging should be considered. Because the short-term aged residues of binders do not indicate the actual aging during HMA production and placement in the field, the additional controlled stiffness of original asphalt binders should then be included as a safeguard against those asphalt binders. Consequently, the stiffness of the original and the short-term aged asphalt binders at various high temperatures, depending on the climate, could be selected to control rutting. The complex shear modulus (G*) and the phase angle (δ) of binder measured by using the dynamic shear rheometer (DSR) are used to determine the rutting parameter (G*/sinδ) to control the permanent deformation or rutting in the SUPERPAVE asphalt binder specification (McGennis et al., 1994). Due to the limited availability of DSR equipment in Thailand, the stiffness of asphalt binder, Sb(t), is proposed to replace G*/sinδ in this study. The binder stiffness can be determined without direct measurements, but by using the Van der Poel nomograph (Van der Poel, 1954) as shown in Figure 1. This nomograph can determine binder stiffness modulus based on the penetration and the softening point (TR&B) at a range of temperature and loading time using the penetration index (PI). Additionally, the estimated stiffness modulus after RTFO-aged can be determined by using the developed relationship between the stiffness values before and after short-term aged by RTFO. The relationship can be used to estimate the stiffness without the modulus testing after the short-term aging with rolling thin film oven (RTFO) equipment.

Figure 1. Van der Poel’s nomograph (Van der Poel, 1954)

Generally, the asphalt binder becomes stiffer during its service life, which results in high potential of fatigue cracking susceptibility (Roberts et al., 1996). According to the SUPERPAVE specification, the fatigue parameter (G*sinδ) obtained from the DSR testing is chosen to limit fatigue cracking for long-term aged binder. The pressure aging vessel (PAV)


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has been used to simulate the long-term aged binder that occurs during 5-10 years of in-service HMA pavement (McGennis et al., 1994). For the proposed specification in this study, the estimated Sb(t) using the Van der Poel nomograph is proposed to control fatigue cracking at intermediate service temperature. In addition, the adjustment factor for the TR&B and PI can be determined to simulate the long-term aged by PAV. At very low temperatures, the binder acts as a solid material. Also, exceeding of creep stiffness can result in high potential of cracking when temperature drops rapidly (Roberts et al., 1996). The parameter of Sb and m-value obtaining by Bending Beam Rheometer (BBR) and Direct Tension Tester (DTT) testing are required to control low temperature cracking in SUPERPAVE specification (McGennis et al., 1994). In the proposed system, the estimated Sb from the nomograph and the calculated m-value are proposed as substitutions. The calculated m-value is the slope of the plot of stiffness against time in log-log scale as shown in Figure 2. Moreover, the breaking stress and/or tensile strain at break (λ) of binder are proposed to control brittleness of binders because the break normally occurs under condition of large stress and low temperature. The λ can be determined using Heukolom’s nomograph (Heukelom, 1966), which is illustrated in Figure 3. The effect of PAV aging is considered as well as the fatigue requirement with the same adjustment factors.

Figure 2. Determination of m-value

3. PROPOSED PERFORMANCE GRADING SYSTEM

3.1 Climate Data Analysis and Pavement Temperature Determination Based on the concept of performance grading system, asphalt binders should be selected on the basis of the climate in which they are intended to serve (McGennis et al., 1994). The pavement temperature is needed for the selection of binder grading system. Therefore, the pavement temperature throughout Thailand is incorporated into the proposed performance grading system. The distribution of pavement temperature, such as maximum and minimum design pavement temperature, can be determined from the relationship between pavement temperature and weather temperature. The 30 years (1997 to 2007) of weather temperature data throughout Thailand is obtained from the Thai Meteorological Department (TMD). For each year, the hottest seven-day period is determined and the average maximum air temperature for those seven consecutive days is calculated. For all recorded years, a mean and standard deviation

Log Creep Stiffness, Sb

60 Log Loading Time, s

slope = m-value


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Avg-2S.D. Avg+2S.D.

98% reliability

Avg 50% reliability Avg

are calculated. Similarly, the coldest day of each year is identified and the mean and standard deviation are computed. Besides, 98 percent reliability is considered to ensure that only a two percent chance in an average year that the actual temperature can exceed the design temperature. Figure 4 shows a distribution of high and low air temperatures for the northern region of Thailand.

Figure 3. Heukolom’s nomograph (Heukelom, 1966)

Figure 4. Distribution of high and low design air temperatures for the northern region of Thailand In order to predict the high pavement temperature, the relationship between pavement temperature and air temperature was developed for five regions in Thailand. This relationship


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was developed by measuring the air temperature and the pavement temperature at the pavement surface and at the depth of 50 mm below the surface. A linear regression analysis was conducted using the air temperature as the independent variable and the high pavement temperature as the dependent variable. It should be noted that other variables also affect the pavement temperature such as wind speed, sun radiation, moisture in the atmosphere, etc., but for the simplicity of the pavement temperature estimation, they are excluded from the model in this study. The high pavement temperature model developed for five regions in Thailand is shown in Table 1.

Table 1. Maximum pavement temperature model for five regions of Thailand

Region of Thailand TPAV@surface(Max) R2 TPAV@50mm(Max) R2

NORTH -2.131+1.073 Tair(Max) 0.944 -0.186+1.087 Tair(Max) 0.813 NORTHEAST -3.561+1.221 Tair(Max) 0.818 -1.042+1.197 Tair(Max) 0.738 CENTRAL -3.741+1.285 Tair(Max) 0.552 -8.344+1.438 Tair(Max) 0.592 EAST 0.019+1.227 Tair(Max) 0.480 -2.577+1.315 Tair(Max) 0.522 SOUTH -1.936+1.086 Tair(Max) 0.899 9.569+0.902 Tair(Max) 0.414

where TPAV@surface(MAX) = maximum pavement temperature at the pavement surface (°C) TPAV@5cm(MAX) = maximum pavement temperature at 50 mm below the surface (°C) Tair(MAX) = maximum air temperature (°C)

Table 1 indicates that the maximum pavement temperature model at the pavement surface for northern, northeastern and southern achieves a goodness-of-fit R2 of 0.944, 0.818 and 0.899, respectively. For the maximum pavement temperature model at 50 mm below the surface, R2 of northern and northeastern are 0.813 and 0.738, respectively. On the other hand, the statistic of regression analysis is insufficient for the other models. Based on the developed model, the expected maximum pavement temperatures can be calculated for each region at 50% and 98% reliability as presented in Table 2.

Table 2. Maximum pavement design temperatures for Thailand Region of Thailand %Reliability Tair(7days-Max)ºC TPAV@surface(Max)ºC TPAV@50mm(Max)ºC

NORTH 50 41 42 44 98 45 51 54 NORTHEAST 50 41 47 48 98 43 55 57 CENTRAL 50 40 48 49 98 44 57 59 EAST 50 38 47 47 98 42 56 57 SOUTH 50 38 39 44 98 40 47 53

For the minimum pavement temperature, the data is not available in Thailand, and therefore, the minimum pavement temperature at surface can be estimated by using the weather data from TMD, and the SUPERPAVE’s low temperature model at the pavement surface is shown in the following equation:

TPAV@SURFACE(MIN) = 0.286+0.692TAIR(MIN) (1)


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The minimum pavement temperature at specified depth can be calculated using the following equation:

Td(MIN) = [email protected] (MIN) - [[email protected](MIN)(d-6.4)]+0.0146(d-6.4) (2)

where Td(MIN) = minimum pavement temperature at depth d d = depth from surface, mm [email protected](MIN) = minimum pavement temperature at 6.4 mm

= 2.27+0.778TAIR(MIN) for air temperature below 0°C = 6.83+1.014TAIR(MIN) for air temperature above 0°C Table 3 shows the expected minimum pavement temperature using the SUPERPAVE models for each region at 50% and 98% reliability.

Table 3. Minimum pavement design temperatures for Thailand

Region of Thailand %Reliability Tair(Min)ºC TPAV@surface(Min)ºC TPAV@50mm(Min)ºC

NORTH 50 5 4 12 98 -1 -2 6 NORTHEAST 50 6 4 13 98 2 0 9 CENTRAL 50 10 7 17 98 4 1 11 EAST 50 12 9 19 98 8 4 15 SOUTH 50 16 11 22 98 10 5 16

Figure 5. Pavement design temperature for Thailand (98% reliability)

The performance grading system allows the users to select binder grades for climate specific to project location. Thus, performance graded number for proposed grading system is based on existing temperature condition in Thailand that covers high and low pavement temperature as shown in Figure 5. The set of binder grades can then be selected based on the temperature condition in all regions. From Figure 5, three main high temperature grades, PG58 and PG64 are proposed to cover high temperature range of Thailand. In order to consider high traffic volume and/or slow moving traffic, PG70 grade should be added. Low temperatures are

-2 to+54ºC 0 to+57ºC

+4 to+57ºC+1 to+59ºC

+5 to+53ºC


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ranged between -2ºC and +5ºC, and therefore, it is reasonable to use a system with low temperature of +8.0, +2.0, and -4.0ºC. For a full factorial system, 3 high-temperatures and a 3 low-temperatures grid results in 9 grades, which is double amount of grades used today in Thailand as shown in Table 4.

Table 4. Proposed performance grading system for Thailand Proposed Grade PG58 PG64 PG70

High Temperature Grade (HT) 58 64 70

Low Temperature Grade (LT) +8 +2 -4 +8 +2 -4 +8 +2 -4

Performance Related Property Performance Criteria

For Workability

Brookfield Viscosity Max. 0.3 Pa.s (3,000 cP) Temp=135°C

Kinematic Viscosity Max.2,700 cSt.

For Rutting Resistance

Estimated Creep Stiffness

(Un-aged) @HT, kPa S(0.0015) ≥ 27kPa 58 64 70

Estimated Creep Stiffness

(RTFO-aged) @HT, kPa S(0.0015) ≥ 60kPa 58 64 70

For Fatigue Resistance

Estimated Creep Stiffness (PAV-aged), kPa S(0.015) ≤ 60,000 kPa 33 30 27 36 33 30 39 36 33

For Thermal Cracking Resistance

Estimated Creep Stiffness (PAV-aged), kPa S(60)≤300,000 kPa 18 12 6 18 12 6 18 12 6

Estimated Creep Rate (PAV-aged) m(60)≥0.3 18 12 6 18 12 6 18 12 6

Elongation at break λ(60)≥0.02 18 12 6 18 12 6 18 12 6

3.2 Asphalt Binder used for the Proposed Grading System Development Five available sources of asphalt binder in Thailand (I, II, III, IV, and V) are selected to perform traditional tests as reference binders. The traditional tests are composed of penetration test at 25°C and softening point test (TR&B). The tests are performed at three different aging stages, including un-aged, RTFO-aged and PAV-aged. The test results are summarized in Table 5, which indicates that all reference binders can be classified as a penetration grade of 60/70 and a performance grade of 64-22. Based on the test results, the penetration values of original binder are measured between 63 and 70, unit of 0.1mm, and then reduced to 38 to 43-unit range after RTFO-aging. After the PAV aging, the penetration values are reduced to a range of 23 to 29. For softening point test, the TR&B of original binder varies between 45°C and 48°C and increased to a range of 50°C to 55°C after RTFO-aging. This yields the range of TR&B to be in between 54°C and 62°C after PAV-aging. Consequently, it can be concluded that the binders are stiffer by aging as indicated from a decreasing of penetration and an increasing of TR&B. Table 5 shows PI values at different aging stages, consisting of un-aged, RTFO-aged and PAV-aged, indicating range of values of -1.6 to -0.8, -1.5 to -0.6, and -1.3 to -0.2, respectively. 3.3 Setting of Acceptance Limit in the Proposed Grading System The acceptance limits of the proposed PG system are derived from experience and


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documented field performance. The performance-related properties must be selected and the acceptable limits for acceptance of bitumen have to be defined. The following properties are selected.

Table 5. Current asphalt binders used in Thailand

Source Pen PG

Pen@25°C (0.1mm) TR&B (°C) PI

Original RTFOT PAV Original RTFOT PAV Original RTFOT PAV

I 60/70 64-22 63 38 23 47.1 52.3 58.0 -1.4 -1.2 -1.0

II 60/70 64-22 64 39 24 46.3 52.3 57.3 -1.6 -1.2 -1.1

III 60/70 64-22 70 43 29 45.6 50.1 54.6 -1.6 -1.5 -1.3

IV 60/70 64-22 69 42 28 46.7 50.7 55.9 -1.3 -1.4 -1.1

V 60/70 64-22 68 38 24 48.5 55.0 62.0 -0.8 -0.6 -0.2

Min 63 38 23 45.6 50.1 54.6 -1.6 -1.5 -1.3

Max 70 43 29 48.5 55.0 62.0 -0.8 -0.6 -0.2

Average 67 40 26 46.8 52.1 57.6 -1.3 -1.2 -0.9

STDEV 3 2 3 1.1 1.9 2.8 0.3 0.3 0.4

85th Percentile 69 42 28 47.7 53.4 59.6 -1.1 -1.0 -0.7

3.3.1 Workability The workability of HMA mixture during mixing and compaction can be controlled by asphalt binder viscosity. Furthermore, its measurement is preferably linked to grading and facilitated checking of conformity. Therefore, the kinematic viscosity test at 135ºC, used in the existing specifications, has been proposed as an alternative. According to SUPERPAVE specification, the viscosity is set at 3Pa.s (3000 cP) using Brookfield test. The data plot of kinematic viscosity and Brookfield viscosity is presented in Figure 6. The results of both measurements correlate very well with high R2 value of 0.87. Therefore, the kinematic viscosity of 2,700 cSt which is equivalent to the Brookfield viscosity of 3Pa.s is used to determine the acceptance limit for workability in the proposed grading system.

Figure 6. Correlation between Brookfield test and kinematic viscosity test 3.3.2 Rutting Resistance As mentioned above, the Sb at typical speed in moderate climate is estimated using the PI value. The speed of 60 km/hr is selected as the typical speed based on normal traffic seen in


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Thailand. In 1971 Barksdale and coworkers had developed a chart of loading time as a function of vehicle speed and depth beneath the pavement surface. For the typical speed of 60 km/hr and a pavement surface layer thickness of 100 mm, the loading time is 0.015 seconds (Barksdale et al., 1971). Since pavement performance data is not yet available in Thailand, interviews were conducted on engineers and experts in road construction to derive acceptance limits. Based on experience and documented field performance, it should be noted that the binders of 60/70 penetration grade has worked well for moderate traffic volume roads in moderate climate zones. Based on pavement design temperature of Thailand, most of the maximum pavement design temperature is close to 58°C. A moderate climate temperature of 58°C is then proposed as a reference to determine acceptance limit of Sb. As explained earlier, the limit of Sb for rutting resistance should be considered for un-aged and RTOF-aged binders. For un-aged binders, because a decreasing of PI value can increase the temperature susceptibility of binders, a maximum PI of -0.8 (Table 5) is adopted as representative value of PI for AC60/70. Moreover, a representative TR&B of 45°C is set to cover the minimum TR&B of five reference binders. This is based on the assumption that all reference binders have performed well for rutting resistance. Using these values in solving the Van der Poel nomograph indicates that such bitumens approximately give a Sb(t=0.015 sec) value of 27 kPa. This value is proposed in the new specification table for all grades as shown in Table 4. It can be concluded that the Sb(0.015) of un-aged asphalt for all grades must be equal to or greater than 27 kPa at maximum design pavement temperature for the proposed system.

Figure 7. Effect of RTFO aging on G* at each temperature The effect of RTFO aging was considered using two parameters, G* and Sb. Based on DSR test result, as indicated in Figure 7, the G* values of RTFO-aged binder increase ranging between 1.7 to 2.9 times compared to the un-aged values. Also, the increasing of estimated Sb of RTFO-aged binders, determined by using TR&B and PI, varies from 1.5 to 2.7 as shown in Figure 8. To compromise high and low range used, the average value of 2.2 times to the un-aged is proposed to achieve a minimum stiffness of RTFO-aged binders. Therefore, the minimum Sb (t=0.015 sec) of RTFO-aged binders requires 2.2 times of 27 kPa which is approximately 60 kPa as proposed in Table 4.

1.7 ≤ G*(RTFOT) ≤ 2.9 G*(Original)


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Figure 8. Effect of RTFO aging on Sb at each temperature

3.3.3 Fatigue Cracking Resistance In order to identify stiffness limits for fatigue cracking performance criteria, the field performance data and HMA coring samples from various locations in Thailand were obtained to determine the fatigue cracking parameter. The HMA coring of 7 samples are selected based on the set of criterion which has both good fatigue performance and pavement age in range of 5 to 10 years. The performance data and general info of selected coring samples are summarized in Table 6. It indicates that sample C3 and C7 were ranked as the first and second top coring samples that showed very little damage on longitudinal cracking. Both samples therefore can be considered as samples which perform very well in terms of fatigue cracking resistance.

Table 6. Field performance data of HMA coring samples Coring

Sample HW No. Section Province

Age

(yr.)

Pavement

Thickness

(cm)

Longitudinal

Cracking

Transverse

Cracking Block cracking

Rank

%Area Severity %Area Severity %Area Severity

C1 201 204+100-203+900 Loey 5 8 15% 2 0 0 5% 1 5

C2 36 42+700-42+500 Rayong 6 5 <5% 1 <5% 1 10% 2 4

C3 21 42+700-42+500 Saraburi 7 10 0 0 0 0 5% 2 2

C4 1 485+000-485+200 Lampang 7 7 0 0 5% 1 10% 2 3

C5 1 439+000-439+200 Tak 7 17 5% 1 35% 3 25% 3 6

C6 1 313+050-313+850 KampangPetch 9 10 <10% 2 5% 3 15% 3 7

C7 101 415+500-415+300 Sokothai 10 5 5% 1 0 0 0 0 1

Asphalt binders were extracted from all samples, C1 to C7, and were tested to measure G*sinδ parameter as shown in Table 7. Based on SUPERPAVE specification, the G*sinδ parameter is chosen to limit the fatigue cracking with its maximum value of 5000 kPa. The decreasing amount of energy dissipated per load cycle by the lowering of G*sinδ value can minimize the chance of fatigue cracking occurrence. The results show that the extracted binder of sample C3 has lowest G*sinδ value at each testing temperature. It can be concluded that C3 binder performs the best in resisting fatigue cracking. The C3 binder was tested for the traditional measurements including TR&B and penetration test. The stiffness of C3 binder is estimated by using nomograph with TR&B, PI, and at the

1.5 ≤ Sb(RTFOT) ≤ 1.7 Sb(Original)


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intermediate pavement design temperature in Thailand of 21°C. The value of binder stiffness obtained from the nomograph is 64.4 MPa (or 60 MPa). Thus, the Sb of 60 MPa is proposed as maximum of Sb value of PAV-aged asphalt binder in the requirement of the proposed PG specification, as shown in Table 4.

Table 7. SUPERPAVE test results of the extracted binders Coring

Sample

Frequency Sweep, G*sinθ @T(°C) (kPa) < 5,000 kPa Rank

16 19 22 25 28 31 34 37 40

C1 15,555 12,243 9,422 6,817 4,685 3,128 1,986 1,330 794 6

C2 9,028 6,684 4,731 3,450 2,224 1,344 834 489 293 3

C3 4,151 3,043 2,053 1,354 854 537 308 181 91 1

C4 7,241 5,301 3,771 2,417 1,542 906 529 304 107 2

C5 11,089 8,330 5,964 4,021 2,632 1,595 953 550 318 4

C6 11,599 8,732 6,272 4,385 2,848 1,750 1,089 663 391 5

C7 12,846 10,529 8,457 6,554 5,050 3,712 2,619 1,823 1,281 7

3.3.4 Low Temperature Cracking Thermal cracking is not a serious concern in Thailand since the temperature is normally not very low. Therefore, the criterion in SUPERPAVE specification can be used. The Sb (t=60 sec) of PAV-aged binders must be equal to or less than 300 MPa. Also, the m-value must be equal to or greater than 0.3. The strain at break (λ) of PAV-aged binder is proposed to control binder brittleness at low temperature, using the Heukolom's nomograph (Heukelom, 1966) as displayed in Figure 3. The minimum limit of λ of 0.02 is identified based on the TR&B and PI of long term-aged binders with the loading time of 60 seconds. 4. SYSTEM VALIDATION

The proposed performance grading system developed in this study was validated to appropriately classify asphalt binders used in Thailand, and to be able to compare with the SUPERPAVE PG system. The other four binders (asphalt A, B, C, and D) are graded based on the proposed specification and compared with the SUPERPAVE specification. The traditional measurements are conducted to test the fundamental binder properties as shown in Table 8. It shows that the penetration testing results of all binders can be separated into two groups of penetration grade. Binder A and B are classified as grade 60/70, while binder C and D are classified as grade 40/50. These four binders are also graded based on the SUPERPAVE specification as presented in Table 9. It indicates that the asphalt binder A and B are in the same PG grade, PG 64-22, while binder C is graded as PG 70-16 and PG 64-16, respectively.

4.1 Parameter Validation As described above, the estimated Sb parameter by using the nomograph is proposed to substitute the G* parameter measuring from DSR test. The creep stiffness (Sb) is plotted against complex shear modulus (G*) as presented in Figure 9. It can be observed that both parameters show very high correlation for all performance aging stages which can be concluded that the Sb parameter can be used to represent G*.


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Table 8. Traditional properties of four new asphalt binders Source

Pen.@25°C (0.1mm) TR&B (°C) PI

Original RTFO PAV Original RTFO PAV Original RTFO PAV

A 67 42 26 46.8 50.1 55.7 -1.40 -1.50 -1.20

B 68 39 25 48.3 54.7 63.1 -0.90 -0.70 0.10

C 45 25 16 51.2 58.2 66.2 -1.10 -0.82 -0.20

D 44 29 20 50.2 54.2 59.2 -1.40 -1.40 -1.00

Min 44 25 16 46.8 54.6 55.7 -1.40 -1.40 -1.20

Max 68 42 26 51.2 62.0 66.2 -0.90 -0.90 0.10

Average 56 34 22 49.1 57.6 61.1 -1.20 -1.20 -0.60

STDEV 13 8 5 2.0 2.8 4.6 0.20 0.20 0.60

85th Percentile 68 41 26 50.8 59.6 64.8 -1.00 -1.00 0.00

Table 9. Classification of four new asphalt binders by using SUPERPAVE specification

Asphalt Pen

Original RTFO PAV BBR

PG T(°C)

G*

(kPa)

G*/sinθ

(kPa)

>1.0

T(°C) G*

(kPa)

G*/sinθ

(kPa)

>2.2

T(°C)G*

(MPa)

G*/sinθ

(MPa)

>5.0

T(°C)

s

(MPa)

<300

m-value

>0.3

A 67 64 1.30 1.30 64 2.29 2.29 25 4.38 4.38 -12 276 0.344 64-22

B 68 64 1.51 1.50 64 3.35 3.37 22 5.81 4.02 -12 205 0.313 64-22

C 45 70 1.13 1.13 70 3.22 3.23 28 5.73 4.10 -6 178 0.316 70-16

D 44 64 2.22 2.22 64 4.23 4.24 28 4.23 3.61 -6 171 0.378 64-16

Figure 9. Comparison between Sb and G* at different performance stages

4.1.1 Validation of Estimating Sb after RTFO Aging The study proposes to consider the effect of RTFO aging without Rolling Thin Film Oven testing by estimating the stiffness of RTFO-aged binder. The linear regression model between


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stiffness of RTFO-aged and un-aged binder has been developed based on the five reference binders as expressed in the following equation:

log (Sb(RTFO-aged)) = 0.2619+1.0482 (log Sb(un-aged)) (3)

where Sb (RTFO-aged) = stiffness of asphalt binder after RTFO aging

Sb(un-aged) = stiffness of un-aged asphalt binder was calculated using PI and TR&B at a range of temperature and loading time equal 0.015 sec

The coefficient of determination (R2) of the equation is 0.993. Consequently, the equation is verified by comparing between stiffness parameters of binders which are estimated by the equation and aged by Rolling Thin Film Oven. The comparison between estimated Sb(Estimated

RTFO-aged) and Sb(RTFO-aged) is plotted based on the testing of four binders as shown in Figure 10. It indicates that the stiffness of asphalt binder after RTFO aging can be estimated by using the developed equation (eq.3) with R2 value of 0.95.

Figure 10. Validation of estimating Sb after RTFO aging

4.1.2 Validation of Estimating Sb after PAV Aging The estimation of Sb after PAV Aging is determined in the proposed system without PAV aging of asphalt binder. The binder stiffness after PAV aging (Sb (PAV-aged)) is determined by using TR&B and PI of PAV-aged binder. In this study, it is assumed that PAV aging leads to the increase of the softening point and PI by simulated factors. The simulated factors are obtained from results of traditional test of five reference binders as presented in Table 5. The 85th percentile values of both TR&B (12°C) and PI (0.6) increasing after PAV aging are selected as simulated factors.

The simulated factors are verified by comparing between stiffness parameters of binder which are aged by pressure aging vessel and simulated PAV aging as illustrated in Figure 11. Figure 11 shows that the stiffness of asphalt binder after PAV aging can be estimated by using the simulating factors with R2 value of 0.972.


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Figure 11. Validation of estimating Sb after PAV aging

4.2 Binders Classification by Proposed Performance Grading System Four validated binders (A, B, C and D) were classified based on the proposed performance grading system as presented in Table 11. From the results, the proposed system classify binder as PG grade of 58-4, 58-10, 64-4, and 58+2 for binder A, B, C, and D, respectively.

Table 11. Classification of the validation binders by the proposed PG system

Asphalt Pen

Original Estimated RTFOT Estimated PAV Estimated PAV

GradeSb (t=0.015 sec)

≥27 kPa

Sb (t=0.015 sec)

≥60 kPa

Sb (t=0.015 sec)

≤60 kPa

Sb(t=60sec)

≤300MPa

m(60)

≥0.30

λ(60)

≥0.02

52°C 58°C 64°C 52°C 58°C 64°C 21°C 24°C 27°C 30°C -12°C -6°C 0°C -12°C -6°C -12°C -6°C

A 60/70 82 33 14 184 72 29 91.6 60.2 40.2 24.8 242 96.8 34.5 0.379 0.521 0.02 0.05 58-4

B 60/70 102 41 19 233 90 41 70.1 47.7 31.9 20.3 159 69.8 24.9 0.347 0.501 0.05 0.08 58-10

C 40/50 153 65 28 356 145 60 133 90.5 61.0 39.5 300 145 55.1 0.300 0.391 0.015 0.05 64-4

D 40/50 136 55 23 315 122 50 164 104 67.9 45.3 409 187 66.7 0.373 0.05 58+2

4.3 Comparison between Proposed Performance Grading System and SUPERPAVE

Performance Grading System In this section, the proposed PG system is verified by comparing it with the SUPERPAVE PG system. Table 12 shows the classification of four binders using both systems. The SUPERPAVE system yields classification in the same grade of PG64-22 for binders A and B, and a grade of PG70-16 and 64-16 for binders C and D, respectively. Nevertheless, the proposed system gives the proposed PG grade of 58-4, 58-10, 64-4, and 58+2 for binders A, B, C, and D, respectively. From Table12, the proposed system indicates that binder C (proposed PG64) has provided better performance than binders A, B and D (proposed PG58) at higher temperature. This is because the greater number of high temperature grade of binder, the binder can perform better in terms of rutting resistance at higher temperature. This is similar to the results obtained from the SUPERPAVE system. In addition, the proposed PG system can classify the binders even though they are in the same


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penetration grading system. It can be observed that the proposed system and the SUPERPAVE system can differentiate between binders C and D, which are classified in the same grade by traditional grading system. The results show that the high-temperature grade numbers should be different based on both grading systems. It should be noted in this study that the proposed grading system is assumed to have the original stiffness limit value equal to or greater than 27 kPa (Sb≥27kPa), which is obtained from the average Sb of current asphalt binders used in Thailand. Moreover, it is assumed that these values have a satisfactory performance. On the contrary, criterion on SUPERPAVE specifications is derived based on asphalt binder properties and pavement performance data in the US.

Table 12. Classification of the validation binders by using the SUPERPAVE system and the

proposed PG system SUPERPAVE system

Asphalt Pen

Original RTFO PAV BBR

PG T(°C)

G*

(kPa)

G*/sinθ

(kPa)

>1.0

T(°C) G*

(kPa)

G*/sinθ

(kPa)

>2.2

T(°C)G*

(MPa)

G*/sinθ

(MPa)

>5.0

T(°C)

s

(MPa)

<300

m-value

>0.3

A 60/70 64 1.30 1.30 64 2.29 2.29 25 4.38 4.38 -12 276 0.344 64-22

B 60/70 64 1.51 1.50 64 3.35 3.37 22 5.81 4.02 -12 205 0.313 64-22

C 40/50 70 1.13 1.13 70 3.22 3.23 28 5.73 4.10 -6 178 0.316 70-16

D 40/50 64 2.22 2.22 64 4.23 4.24 28 4.23 3.61 -6 171 0.378 64-16

Proposed PG system

Asphalt Pen

Original Estimated RTFOT Estimated PAV Estimated PAV

GradeT(°C)

Sb (t=0.015 sec)

≥27 kPa T(°C)

Sb (t=0.015 sec)

≥60 kPa T(°C)

Sb (t=0.015 sec)

≤60 kPa T(°C)

Sb(t=60sec)

≤300MPa

m(60)

≥0.30

λ(60)

≥0.02

A 60/70 58 33 58 72 27 40.2 -12 242 0.379 0.02 58-4

B 60/70 58 41 58 90 24 47.7 -12 159 0.347 0.05 58-10

C 40/50 64 28 64 60 30 39.5 -6 145 0.391 0.05 64-4

D 40/50 58 55 58 122 30 45.3 -6 187 0.373 0.05 58+2

The proposed PG system (Table 12) shows that binder B has provided the best performance in the resistance of fatigue cracking compared to other binders. This is because its stiffness parameter has met the limit at the lowest of intermediate pavement temperature of 24°C. Correspondingly, the SUPERPAVE system illustrates that binder B has met the limit at the lowest of intermediate pavement temperature (22°C). Furthermore, the proposed system and the SUPERPAVE system similarly present that binder A has had lower fatigue cracking resistance compared with binder B. The property of binder A has reached the limit at intermediate pavement temperature of 25°C and 27°C for the SUPERPAVE system and the proposed system, respectively. In addition, both systems have shown that both binders C and D have equally given resistibility to fatigue cracking. The property of both binders (C and D) has achieved the limit at 28°C and 30°C for the SUPERPAVE system and the proposed system, respectively. It can be concluded that the proposed system is comparable with the SUPERPAVE system to classify available binders used in Thailand, for fatigue cracking resistance purposes. Nevertheless, the intermediate temperatures of both systems are different because they are normally derived from high and low pavement temperatures. These temperatures are determined based on Thailand climate which is different from the US. This causes a difference in intermediate service temperature between the proposed system and SUPERPAVE system resulting in different grade numbers. Additionally, the result shows that low temperature grade numbers of both systems are different. This is because the grade numbers of the proposed system (e.g. -4, +2) have been


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derived based on the range of low pavement temperature in Thailand which differs from the one used in the US (e.g. -22, -16). However, both systems have depicted that the stiffness of binders A and B have passed the limit at lower pavement temperature (-12°C) than binders C and D (-6°C). Both systems can similarly infer that the low temperature cracking prevention of binders A and B is better than binders C and D. It can be concluded that the proposed system is comparable with the SUPERPAVE system in classifying available binders used in Thailand, for temperature cracking resistance purposes.

5. CONCLUSIONS AND RECOMMENDATIONS In this study, a proposed performance grading system was developed based on indirectly estimated parameters related to pavement performance for asphalt binder used in Thailand. The proposed parameters and criteria were derived based on existing pavement performance and conditions in Thailand. The main findings from the study are summarized as follows:

The Sb parameter in the proposed system can be sufficiently used instead of the G* parameter measurement from the DSR test in the SUPERPAVE system.

The simulation of aging effect in the proposed system was effectively used as a substitute for RTFO and PAV equipment in the SUPERPAVE system.

The proposed performance grading system can classify binders in Thailand having different performance characteristics within the same grade of the penetration grading system.

The proposed system is desirably comparable with the SUPERPAVE system in classifying existing binders used in Thailand.

The proposed system can be temporarily used to classify performance of asphalt binders used in Thailand while the SUPERPAVE system is still not available.

In order to additionally ensure that the proposed system can be used to classify asphalt binders in Thailand, experiments on HMA performance properties such as dynamic creep test and indirect tensile fatigue test should be conducted in the laboratory. The dynamic creep test could be conducted to measure rutting resistance of aggregate-mixed asphalt binder (HMA) property. Also, the indirect tensile fatigue test could be conducted to measure fatigue resistance of HMA. Finally, due to the limitation on the availability of modified asphalt which has only one source in Thailand, the study only provides specification for unmodified asphalt binders. Further study can be examined for both unmodified and modified asphalt binders. REFERENCES Barksdale, R.G. (1971) Compressive stress pulse times in flexible pavements for use in

dynamic testing. Highway Research Record, 345, 32-44. Heukelom, W. (1966) Observations on the rheology and fracture of bitumens and asphalt

mixes. Proceedings of the Association of Asphalt Paving Technologists, 35, 358-399. McGennis, R.B., Shuler, S., Bahia, H.U. (1994) Background of SUPERPAVE asphalt binder

test methods. FHWA, Rep. No. FHWA-SA-94-069. Washington, D.C.: Federal Highway Administration.

Robert, F.L., Kandhal, P.S., Ray Brown, E., Lee, D., Kennedy, T.W. (1996) Hot Mix Asphalt Materials, Mixture Design, and Construction. Maryland: National Asphalt Pavement


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Association Research and Education Foundation. Thailand Industrial Standard Institute (1999) Asphalt Cement for Use in Pavement

Construction. TIS 851-2542. Van der Poel, C.A. (1954) A general system describing the viscoelastic properties of bitumens

and its relation to routine test data. Journal of Applied Chemistry, 4 (5), 221-236. Van de Ven, M.F.C., Jenkins, K.J., Bahia, H.U. (2004) Concepts used for development of

bitumen specifications. Proceedings of the 8th Conference on Asphalts Pavements for Southern Africa (CAPSA’04), Sun City South Africa, September 12-16.

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Development of a Performance Grading System for Asphalt