Development of Stress Sweep Rutting (SSR) test for permanent deformation characterization of asphalt mixture

Construction and Building Materials 154 (2017) 373–383

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Development of Stress Sweep Rutting (SSR) test for permanent deformation characterization of asphalt mixture Dahae Kim a,⇑, Y. Richard Kim b a b

Office of Materials and Research, South Carolina Department of Transportation, 1406 Shop Road, Columbia, SC 29201, USA Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Campus Box 7908, Raleigh, NC 27695-7908, USA

h i g h l i g h t s
Developing a Stress Sweep Rutting (SSR) test method to calibrate a permanent deformation model, shift model.
Evaluation of effect of temperature, deviatoric stress, load time, and rest period on permanent deformation.
Use of actuator displacement to collect the permanent strain data.
Comparisons of rut-depth predictions from the original TSS and the SSR methods using the LVECD program.

a r t i c l e

i n f o

Article history: Received 20 December 2016 Received in revised form 17 July 2017 Accepted 23 July 2017

Keywords: Asphalt materials Rutting Permanent deformation TSS SSR Flow number

a b s t r a c t The goal of this work is to develop a simple rutting test method, referred to as the Stress Sweep Rutting (SSR) test, which can be used to calibrate a permanent deformation model, known as the shift model, of asphalt mixtures. The effect of each shift model parameter, i.e., temperature, deviatoric stress, load time, and rest period, on permanent deformation has been explored and evaluated in an effort to minimize the test requirements of the rutting characterization. The accuracy of the SSR test method was evaluated using the Layered ViscoElastic pavement analysis for Critical Distresses (LVECD) program by predicting the permanent deformation and compared to the Triaxial Stress Sweep (TSS) method, which was originally developed to calibrate the shift model. The rut depths predicted from the SSR and TSS method using the LVECD program are in close agreement, thereby verifying the SSR test method. Published by Elsevier Ltd.

1. Introduction Permanent deformation is one of the major distresses in asphalt pavements. The permanent deformation of the asphalt surface that accumulates in the wheel-paths and is caused primarily by repeated traffic load cycles is referred to as rutting [1]. Resistance to rutting is a critical part of performance in the field, and testing for it is an important consideration. There are different tests to assess the rutting resistance of asphalt mixture. Currently the most common type of standardized laboratory test of this nature is a loaded wheel tester (LWT): numerous types of LWT equipment are available, such as the Georgia Loaded Wheel Tester, the Asphalt Pavement Analyzer (APA), the Hamburg Wheel Tracking Device, and the French Laboratoire Central des Ponts et Chaussées (LCPC) Wheel Tracker [2,3]. In an effort to identify HMA mixtures that

⇑ Corresponding author. E-mail addresses: [email protected] (D. Kim), [email protected] (Y.R. Kim). http://dx.doi.org/10.1016/j.conbuildmat.2017.07.172 0950-0618/Published by Elsevier Ltd.

may be prone to rutting, many agencies have begun using LWTs as supplements to their mixture design procedures [1]. However, these methods are simple pass-fail type test. The Superpave Shear Tester (SST) was developed as part of SHRP research to be one of the HMA mixture performance testers for Superpave mix design [4]. Although it is quite versatile and reasonably able to predict HMA pavement permanent deformation, it has not been widely adopted because of its complexity and cost [5]. Anderson et al. [6] examined the use of indirect tensile, IDT, strength, volumetric parameters and Superpave Gyratory Compaction (SGC) properties to predict rutting potential of an asphalt mixture. Cohesion was identified as an indicator of how strongly the asphalt cement binds together the aggregate particles of a given mixture. The IDT strength test is a measure of tensile strength and a good indicator of mixture cohesion. The authors found that rutting potential can be evaluated using the indirect tensile strength (IDT), compaction slope measured with the Superpave Gyratory compactor and voids in mineral aggregate (VMA). One of the laboratory test methods that assess the asphalt mixture’s rutting resistance is the flow

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number test [7]. The flow number test subjects a specimen to repeated compressive loads at a specific test temperature. The flow number is the number of cycles at which the specimen experiences tertiary flow, which is caused by shear deformation that has been identified as the major mechanism causing severe rutting in the field. However, the flow number can represent the rutting resistance of a material only in terms of ranking at a certain testing condition. The evaluation of rutting resistance at various loading and temperature conditions using the flow number test requires substantial effort that is not practical. Therefore, researchers have developed alternative testing methods to reduce the testing effort and be able to predict permanent deformation under loading and temperature conditions close to the field. One simplified alternative methodology that can be used to evaluate the rutting performance of asphalt mixtures is the Incremental Repeated Load Permanent Deformation (iRLPD) test. Azari and Mohseni [8] suggested the iRLPD test protocol that can produce minimum strain rate (MSR) mastercurves by applying three different deviatoric stresses at one effective temperature. Thus, the MSR can be calculated at any temperature and deviatoric stress from the MSR mastercurve. This test is relatively simple and requires only three specimens but can be used only to rank rutting resistance even though it includes various loading conditions. Another method is the Triaxial Stress Sweep (TSS) test [9], which is an optimized test method that is used to calibrate the shift model proposed by Choi and Kim [10] to predict rutting. The main advantage of the TSS test method is that it can account for the effects of temperature, loading time, and deviatoric stress on the permanent deformation of asphalt concrete. However, the TSS test requires four inherent tests and two replicates to eliminate variability, so the total testing time, including temperature conditioning, is two days for a single mixture, which is impractical for most users. The purpose of this study is to develop a simple test method that can predict reasonable rut depths of asphalt pavement. In order to select and verify the best test protocol, the rut depths predicted from the original TSS test and the SSR test with different rest period options were compared. All of the rutting parameters obtained from each set of tests were input into the LVECD program [11] to calculate the rut depths within all rut-susceptible layers in a pavement. The program calculates linear viscoelastic pavement responses under moving loads using fast Fourier transforms. Each individual layer rut depth value was predicted as a function of time, temperature, and traffic loading repetition. The pavement responses are then used in the shift model to predict rutting performance. Asphalt concrete rut depth predictions were carried out based on the shift model. The unbound materials for the base and the subgrade were modeled using linear elastic properties; MEPDG unbound layer model is embedded in the LVECD program to estimate the permanent deformation of the unbound materials [12]. The total rut depth is the sum of the permanent deformation values for each individual sublayer. The inputs required for the LVECD simulations are design time, structural layout, traffic, and climate; the design period for this study was assumed to be ten years. The structural layout for the simulations was a 10-cm asphalt layer on top of an aggregate base and subgrade. A single tire with standard loading of 80 kN at the center of the pavement was assumed with 700 average annual daily truck traffic (AADTT). 2. Objective The primary objectives of this study are to: 1. Develop a simple rutting test method, Stress Sweep Rutting (SSR) test, to overcome limitations of the original TSS test and to calibrate the shift model for accurate permanent deformation prediction for asphalt mixtures,

2. Predict rut depths using the proposed SSR test with LVECD program and compare the result obtained from the TSS test to verify the SSR test, and 3. Use actuator displacement for permanent strain calculation instead of LVDTs for simple SSR test. 3. Background 3.1. Shift model The shift model was developed to capture the effects of deviatoric stress, load time, and temperature on permanent strain of asphalt mixture. The shift model is composed of one permanent strain mastercurve and two shift functions, as presented in Eqs. (1) through (4). The strain mastercurve is obtained directly from a triaxial repeated load permanent deformation test with a constant deviatoric stress amplitude of 689 kPa (so called the reference test herein) and can be expressed using the incremental model form shown in Eq. (1), which can fit both the primary and secondary region behavior of asphalt concrete [9,10]. In Eq. (1), 1  b and e0 are the slope and intercept of the permanent strain versus number of loading cycles relationship in log-log space; and NI represents the number of cycles at which the secondary region begins. The shift factors are obtained by shifting the permanent strain of an individual loading block with specific load time and deviatoric stress amplitude toward the permanent strain mastercurve. Eqs. (2) and (3) present the reduced load time shift factor and vertical stress shift factor, respectively. The physical number of cycles at a given condition is converted to a reduced number of cycles using the total shift factor, which is the summation of the vertical stress shift factor and the reduced load time shift factor, as expressed in Eq. (4). The two shift factors are expressed as functions of temperature, load time, and vertical stress.

ev p ¼

e0  Nred

ðNI þ Nred Þb

;

ð1Þ

anp ¼ p1 logðnp Þ þ p2 ;

ð2Þ

arv ¼ d1 logðrv =Pa Þ þ d2 ;

ð3Þ

Nred ¼ A  N

 p1  d1 np rv : 1 Pa

ð4Þ

where

evp = viscoplastic strain (i.e., permanent strain), e0, NI, b = coefficients of the incremental model,

Nred = reduced number of cycles at reference loading conditions, N = physical number of cycles of a certain loading condition, anp = reduced load time shift factor, p1, p2 = coefficients of reduced load time shift factor, np = reduced load time, arv = vertical stress shift factor, d1, d2 = coefficients of vertical stress shift factor, rv = vertical stress, Pa = atmospheric pressure to normalize stress, and A = 10p210d2. 3.2. Triaxial Stress Sweep (TSS) test The TSS test is an optimized test method that is used to calibrate the shift model. The TSS test is composed of two types of tests to obtain the incremental model and shift model parameters, i.e., the reference test and the multiple stress sweep test. The reference test is the triaxial repeated load permanent deformation (TRLPD) test using 689 kPa deviatoric stress pulse. The shift model

D. Kim, Y.R. Kim / Construction and Building Materials 154 (2017) 373–383

requires the reference test to characterize a representative strain development curve called the permanent strain mastercurve using a single deviatoric stress, pulse time, rest period, and confining pressure. The other inherent test, the multiple stress sweep (MSS) test, is performed at different temperatures and multiple deviatoric stress levels. The protocol requires the triaxial test at three different temperatures of low, intermediate, and high (TL, TI, and TH, respectively) and three deviatoric stress levels (483, 689, and 896 kPa) in order to define the reduced load time shift factor and vertical stress shift factor, respectively. The TSS test temperatures are selected by utilizing Enhanced Integrated Climate Model (EICM) software and obtaining the permanent strain from the strain ratio model in the Mechanical-Empirical Pavement Design Guide (MEPDG) [13]. The temperature selection method is discussed later in the paper. Table 1 provides a summary of the TSS test protocol.

4. Experimental investigation 4.1. Testing plan The current TSS test protocol requires four tests (one reference test and three MSS tests) and recommends two replicates to eliminate variability, so the total testing time, including temperature conditioning, is two days for a single mixture, which is not practical. Therefore, several ways were investigated in this study to reduce these TSS test requirements. First, because the TSS test is an optimized test protocol that is developed to calibrate the shift model, effect of each parameter affecting the shift model calibration needs to be examined. The shift model contains three factors that play important roles in predicting the permanent strains of asphalt mixtures: temperature, deviatoric stress, and load time. Thus, in this study, the possibility of using fewer temperatures, lower deviatoric stress levels, and less load time was explored. Another way to simplify the TSS test protocol is to eliminate the reference test, which not only cuts the test time by about half, but reduces the number of samples needed. A reversed loading block (689–483–896 kPa) was introduced for the high test temperature (TH) in order to use the first loading block as a reference curve and therefore to eliminate the reference test. Effect of rest period was also considered to reduce the test demand because the rest period is directly related to total testing time; 10-, 5-, and 3-s rest periods were conducted for this purpose, in this study. For verification purposes, random loading history test and Layered ViscoElastic pavement analysis for Critical Distresses (LVECD) program analysis results were compared to select and verify the best SSR test. Lastly, actuator displacement was considered for collecting the permanent strain data, which simplified the instrumentation system substantially. Table 2 summarizes various test conditions to accomplish the goal of this study.

Table 1 Summary of TSS test protocol. Triaxial Stress Sweep Test Test method

Reference

MSS

Number of tests Pulse time (s) Rest period (s) Confining pressure (kPa) Deviatoric stress (kPa) Number of cycles Parameters Total samples Total testing time (hour)

1 (TH) 0.4 10 (TH) 68.95 689 600 e0, NI, and b 2 3.5

3 (TH, TI, and TL) 0.4 10 (TH) and 1.6 (TI and TL) 68.95 483, 689, and 896 200 p1, p2, d1, d2, and d3 6 4.8

375

4.2. Specimen preparation The 9.5 mm Superpave surface mix with RAP (RS9.5B) mixture was selected to run various test conditions, as shown in Table 2. The aggregate structure of the RS9.5B mixture is a coarse 9.5 mm nominal maximum aggregate size (NMAS) composed of 27% #78 coarse aggregate, 33% washed screenings, and 40% fractionated reclaimed asphalt pavement (RAP). The total asphalt content is 5.6% with PG 58–28 binder by weight of the total mixture. This loose mixture was obtained from Knightdale, North Carolina, and all of the specimens were prepared in accordance with AASHTO PP 60 [14], Fabrication of Cylindrical Performance Test Specimens Using the Superpave Gyratory Compactor. Cylindrical specimens 100 mm in diameter by 150 mm in height were cut and cored from gyratory-compacted specimens that were originally 150 mm in diameter by 178 mm in height. After obtaining specimens of the appropriate dimensions, air void measurements were taken via the CoreLok method. The air void content of a given sample fell within the target range of 6.0 ± 0.5%. In order to reduce friction between samples and loading platens, greased double latex sheets were used. The specimens were covered with latex sheets and sealed using O-rings for confined tests and the confining pressure of 69 kPa (10 psi) was applied throughout the test.

5. Effect of each test parameter used to simplify the TSS protocol 5.1. Temperature The current TSS test requires three test temperatures, as shown in Table 1; the result is utilized to define the reduced load time shift factor in Eq. (2), which has two variables. If the reduced load time shift factor could be obtained from two test temperatures instead of three, and thus the test requirements would be reduced significantly. The reduced load time shift factor is a linear logarithmic equation that can be defined with two data points from two temperatures. For this reason, the most logical temperature to eliminate would be the intermediate temperature, because the shift factors from TH and TL would cover a wide temperature range. Fig. 1(a) and (b) show the total shift factors obtained from three temperatures (TH, TI, and TL) and two temperatures (TH and TL), respectively. Fig. 1(c) shows that the reduced load time shift factors obtained from two temperatures and from three temperatures are in good agreement. A further step that could be taken to simplify the TSS test protocol would be to eliminate both the intermediate and low temperatures. However, a linear equation cannot be defined using one data point from a single temperature, so a method to define the reduced load time shift factors for one temperature would necessarily differ from the previously described method. A similar method is to apply a fixed reduced load time shift factor, obtained from a large TSS test database at North Carolina State University (NCSU), as shown in Fig. 2(a). The reduced load time shift factor line derived from two temperatures collapses exactly onto the shift factor line derived from three temperatures, while the fixed function line has a slightly gentler slope and higher y-intercept than the other two lines, as shown in Fig. 2(a). That is, the shift factor derived using two temperatures can represent the mixturespecific information, whereas the fixed reduced load time shift factor for a single temperature cannot. Fig. 2(b) and (c) show comparisons of measured strain to predicted permanent strain with three options: (1) 3 temperatures, (2) 2 temperatures, and (3) 1 temperature. The results indicate that the permanent strains predicted from two temperatures are about 5% different from those obtained from three temperatures at 54 °C; however, the permanent strain

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Table 2 Summary of testing in this study. Mix

Deviatoric Stress (kPa)

Temperature

Pulse-Rest (s)

Number of Samples

Test Time (hr)

Comments

R S 9.5B

100 (Reference), 689–483–896 (TH, TI, and TL)

Reference (54 °C) TH (54 °C) TI (40 °C) TL (20 °C) TH (54 °C) TL (20 °C) TH (54 °C) TL (20 °C) TH (54 °C) TL (20 °C) TH (54 °C) TI (40 °C)

0.4–10, 0.4–10, 0.4–1.6, 0.4–1.6 0.4–10, 0.4–1.6 0.4–5, 0.4–1.6 0.4–3, 0.4–1.6 Pulse: 0.1–1.6 Rest: 10

8

8.3

Original TSS test

4

4.1

Reversed block

4

2.5

–

4

1.8

–

4

18

Random loading

689–483–896 (TH), 483–689–896 (TL) 689–483–896 (TH), 483–689–896 (TL) 689–483–896 (TH), 483–689–896 (TL) 276–896 (Random)

2

2

(a) Total SF (100 psi) 3 Temp.

(b) Total SF (100 psi) 2 Temp. 1

0 -1 -2

483 KPa

Total SF (a total)

Total SF (a total )

1

689 KPa

-3

0 -1 -2

483 KPa 689 KPa

-3

896 KPa

896 KPa -4 1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

-4 1.0E-05

1.0E+00 1.0E+01

1.0E-04

Reduced Load Time (Sec)

(c) Reduced Load Time SF

1.0E-01

1.0E+00 1.0E+01

(d) Vertical Stress SF

0

Stress SF (aσd)

Load Time SF (aξp)

1.0E-02

2

2 1

1.0E-03

Reduced Load Time (Sec)

-1 -2 -3 -4

(1) 3 temperatures

-5 -6 1.E-06

1

0

-1

(1) 3 temperatures

(2) 2 temp. 1.E-05

1.E-04

1.E-03

1.E-02

1.E-01 1.E+00 1.E+01

Reduced Load Time (Sec)

(2) 2 temp. -2 50

70

90

110

130

150

Vertical Stress (=dev+con)

Fig. 1. Effect of number of test temperatures on (a) total shift factor (3 temperatures), (b) total shift factor (2 temperatures), (c) reduced load time shift factor, and (d) vertical stress shift factor. Note SF is shift factor; dev and con are the deviatoric and confining stresses respectively.)

predicted from one temperature is about 20% lower than the strains obtained from three temperatures. Based on this observation, two temperatures (TH and TL) are recommended to derive the reduced load time shift factor, because the use of only these two temperatures still results in relatively accurate permanent strain predictions. Proper test temperature selection is another consideration that was investigated in this. The original procedure for selecting the test temperature utilizes EICM software by combining the permanent strain values from the strain ratio model in the MEPDG [11]. First, a cumulative density graph for a specific section is produced by accumulating the permanent strain values in terms of the pavement temperature obtained from the EICM. Because the high temperature is the most important temperature for rutting performance, the 100th percentile is selected,

which is the highest pavement temperature during the analysis period. For cases where the calculated temperature is higher than 54 °C, the high temperature is adjusted to 54 °C, and the load time is increased based on the time-temperature (t-T) shift factor to compensate for the effect of change in temperature. The low temperature corresponds to the 10th percentile of the cumulative density. However, this method is somewhat complicated and requires extra EICM runs. Mohseni and Azari [15] proposed a simplified procedure to determine the effective temperature, which uses a single test temperature to simulate the rutting of asphalt mixtures in the field. This method employs the ‘degree-days’ (DD) parameter in LTPPBind that correlates well with rutting. Effective temperatures can be calculated using the DD parameter at the location the materials will be placed, as shown in Eq. (5).

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3

(a) Reduced Load Time SF

Total SF (a Total)

2 3 temperatures

1

2 temp.

0

1 temp.

-1 -2 -3 -4 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01

Reduced Load Time (Sec) 10%

10.00%

(c) @ 40°C

Measured (1) 3 temperatures (2) 2 temp. (3) 1 temp.

1%

0%

Permanent Strain

Permanent Strain

(b) Effect of Temperature @ 54°C

1

10

100

1.00%

0.10%

0.01%

1000

1

Number of Cycles

10

100

1000

Number of Cycles

Fig. 2. (a) Averaged reduced load time shift factor from the NCSU TSS database, (b) effect of number of temperatures on permanent strain prediction at 54 °C, and (c) effect of number of temperatures on permanent strain prediction at 40 °C.

T H ¼ 58 þ 7  DD  15  logðH þ 45Þ

ð5Þ

number tests generally are performed at 54 °C, 40 °C, and 20 °C. If intermediate testing is needed, the test temperature is calculated by the following Eq. (6):

where TH = test temperature, °C, DD = degree-days > 10 °C (1000) from LTPPBind v. 3.1, and H = depth of layer, mm, (0 for surface layer).

TI ¼

  TH þ TL þ3 2

ð6Þ

5.2. Deviatoric stress

Five sections were selected in the United States and Canada to compare the test temperatures obtained from the two different methods, as shown in Table 3; please note that all temperature shown in the Table 3 is for surface layer when H is zero in Eq. (5). The effective temperatures (Teff) obtained using the DD parameters are similar to those obtained from the TSS test for high temperatures (TH). Thus, the high temperature selection method can be borrowed from Moheseni’s work to simplify and facilitate the SSR test temperature selection method. In cases where the calculated TH is higher than 54 °C, the temperature should be fixed at 54 °C, because Asphalt Mixture Performance Testers (AMPTs) have limitations at extreme temperatures and the samples could be hard to work with (Choi 2013). Low test temperatures can be fixed at 20 °C because permanent strain does not develop significantly at low temperatures and flow

Deviatoric stress relates directly to tire pressure, which depends on many factors, such as tire type, tire structure, and loading conditions [16]. Assuming free-rolling conditions with an 18-kN load, the contact pressure distribution is between 414 kPa and 827 kPa [17]. Also, Choi et al. conducted a thorough literature review concluded to use 483 kPa (70 psi) based on the results [18]. In order to reduce the test requirements, the deviatoric stress shift factor was examined in this study. The vertical stress shift factor needs at least two different stress levels to be derived, as shown in Eq. (3). The total test time might be shortened by eliminating the intermediate stress level. However, the extra effort needed to run three different stress levels is fairly small due to the fact that it requires simply adding more blocks during the test procedure.

Table 3 Comparison of test temperature selection methods. Location

Teff Test

TSS Test

State

City

TH (°C)

TH (°C)

TI (°C)

TL (°C)

TH_Test (°C)

SSR Test TH (°C)

TL (°C)

NY AL AZ NC Canada

Angelica Auburn Phoenix Raleigh Manitoba

49 60 65 58 47

47 61 64 55 48

35 45 49 40 36

15 19 31 23 19

47 54 54 54 48

49 54 54 54 47

20

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Because the samples are already prepared and temperatureconditioned, the extra time required is only that needed to run the test during that loading block. Considering all these factors together, the three deviatoric stress levels selected for the TSS test protocol are reasonable. Thus, the SSR test also would use three deviatoric stress levels.

5.3. Load time The shift model uses reduced load time by applying the timetemperature superposition principle; the reduced load time is a function of physical load time and temperature. An equivalent 80-kN single-axle load with a single tire was used to determine the reasonable load time for a real pavement. The spacing between two single tires was selected to be 2.6 m. The tire contact area depends on the contact pressure, which is calculated as 0.0522 m2 for a single tire based on tire pressure of 758 kPa. The contact area is converted into an equivalent rectangular area with a length of 0.29 m and a width of 0.18 m. Another important factor that affects load time is vehicle speed, which is related directly to the duration of loading on the pavement. A lower speed will give a longer loading duration than a higher speed and, thus, will produce larger rut depths in the pavement surface. In order to suggest a reasonable pulse time for the SSR test, the same reduced load time at various temperatures was converted to different physical times by applying t-T shift factors, as shown in Table 4. In this study, the averaged mixture shift factor function was determined using the data from the NCSU database, as shown in Eq. (7).

log ðaT Þav g;mixture ¼ 0:00064T 2  0:15649T þ 5:23156 ðreference temperature ¼ 54 CÞ

ð7Þ

An example of this use of the mixture shift factor is as follows. The physical load time is 0.01 s when a vehicle with a 0.29-m contact length and speed of 30 m/s passes a specific point. If the pavement under consideration is at 65 °C, which is the maximum temperature that can be calculated in the United States and Canada from Eq. (5), a time-temperature shift is needed to correct the load time used in the SSR test, which is run at a maximum temperature of 54 °C. Using Eq. (7), 0.08 s is the physical pulse time at 54 °C that produces the same reduced pulse time as the 0.01-s pulse at 65 °C, which means that the 0.01-s pulse time at 65 °C is equivalent to the 0.08-s pulse at 54 °C. The same procedure was undertaken using 46 °C because 46 °C is more within the reliable testing range of the AMPT than 65 °C; the pulse time for this temperature was 0.4 s. Note that test devices such as the AMPT may not properly simulate the load when the loading time is less than 0.1 s [13]. Additionally, the 0.4-s pulse time can generate sufficiently large permanent deformation within a practical testing time. Therefore, the 0.4-s pulse time was employed in the TSS test protocol with the anticipation that this pulse time would be recommended for further SSR testing.

Table 4 Reduced pulse temperatures.

times

at

different

Temperature (°C)

Pulse time

65.0 54.0 46

0.01 0.08 0.4

5.4. Reference curve The reference test is one type of repeated loading permanent deformation test that applies a repeated load at a constant frequency (load time) to a test specimen for many repetitions and then measures the specimen’s permanent deformation. The shift model requires a reference test to characterize a representative strain development curve called the permanent strain mastercurve, as discussed in the background information. In order to eliminate the reference test, Kim [18] focused on the methods needed to predict a strain mastercurve from the first block of the MSS test obtained under 483 kPa, because the MSS test employs increasing stress blocks with 483, 689, and 896 kPa and only the first block has the information for both the primary and secondary regions. Experience gained through Kim’s work showed the importance of obtaining a strain mastercurve under 689 kPa [18]. Therefore, the reversed loading block protocol (689–483–896 kPa) is introduced in this study. The idea of a reversed loading block originated by simply questioning whether the stress level of the first block (483 kPa) could be switched to the second block (689 kPa) in the original TSS test protocol. This concept offers the possibility to take the strains of the first block as a reference curve instead of predicting the reference curve, if the stress level of the first block is 689 kPa. In order to verify this concept, the reversed loading block protocol was conducted for the RS9.5 mixture. Fig. 3(a) and (b) compare the TSS and SSR tests, respectively. The TSS test requires four tests (reference, TH, TI, and TL) with increasing loading blocks (483, 689, and 896 kPa), as detailed in Fig. 3(a). However, the SSR test requires only two tests (TH and TL); the high test temperature (TH) employs the reversed loading block (689, 483, and 896 kPa) and the low test temperature (TL) uses increasing deviatoric stress levels (483, 689, and 896 kPa). The reference curve for the SSR test is obtained from the first block of the high temperature result (TH), detailed in Fig. 3(b). Note that for the original reference curve the TSS test is run up to 600 cycles, as shown in Fig. 3(a), but the first block of the SSR test stops at 200 cycles. Therefore, the remaining cycles from 200 to 600 could be extrapolated from the incremental model obtained from the first block of SSR, which is shown in Fig. 3 (b) as a dashed line. The reference curves obtained from the TSS test and the SSR test with the reversed loading block are in a good agreement, as shown in Fig. 3(b). The main advantage of this approach is that it uses the first block of the SSR test as a reference curve without any correction, which reduces the testing time and required number of specimens significantly. Fig. 3(c) through (f) present details about the calibration procedure of the shift model by comparing the TSS and reversed loading block test results. The strains at the end of each block are shifted in the horizontal direction to match the equal strains on the reference curve; these shifts for each block were used to produce the total shift factors, as shown in Fig. 3(c) and (d). The total shift factors can be divided into the reduced load time shift factors shown in Fig. 3(e) and the vertical stress shift factors shown in Fig. 3(f), respectively. Overall, the reduced load time shift factors and vertical stress shift factors obtained from the two different methods are similar. The reduced load time shift factors obtained from the SSR test with the reversed loading block tend to be higher at the high reduced load time but become similar at the low reduced load time. The vertical stress shift factors can be calculated by subtracting the reduced load time shift factor from the total shift factors. The vertical stress shift factors from the SSR test are the same at high vertical stress levels and tend to be lower at low vertical stress levels. The effect of the reversed loading block on the rutting prediction is discussed later in the section, ‘Verification of Shift Model Calibrated Using SSR Test’.

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483 KPa

2.0%

689 KPa

896 KPa

689 KPa

(b) SSR

1.5%

TH

Reference

1.0%

Permanent Strain

Permanent Strain

(a) TSS

896 KPa

483 KPa

2.0%

TI

0.5%

Measured Reference

1.5%

Predicted Reference

1.0%

TH

0.5%

TL

TL 0.0%

0

200

400

600

0.0%

0

200

Number of Cycles 2

600

2

(c) TSS

(d) SSR

1

1

Total SF (a total)

Total SF (a total )

400

Number of Cycles

0 -1 -2 483 KPa -3

0

-1 483 KPa

-2

689 KPa

689 KPa

896 KPa -4 1.0E-05

1.0E-04

1.0E-03

1.0E-02

896 KPa -3 1.0E-05

1.0E-01 1.0E+00 1.0E+01

Reduced Load Time (Sec)

1.0E-04

1.0E-03

1.0E-02

1.0E-01 1.0E+00 1.0E+01

Reduced Load Time (Sec)

2

2

(e) Reduced Load Time SF

(f) Vertical Stress SF Stress SF (aσd)

Load Time SF (aξp)

1 0 -1 -2

0

-1

TSS

TSS

-3 -4 1.0E-05

1

SSR

SSR 1.0E-04

1.0E-03

1.0E-02

1.0E-01 1.0E+00 1.0E+01

Reduced Load Time (Sec)

-2 70

90

110

130

150

Vertical Stress (=dev+con)

Fig. 3. Comparison of calibration process using TSS and SSR test protocols: (a) reference curve of TSS test, (b) :reference curve of SSR test, (c) total shift factors for TSS test, (d) total shift factors for SSR test with the reversed loading block, (e) reduced load time shift factor, and (f) vertical stress shift factor (RS9.5B mixture).

5.5. Rest period The rest period is related directly to the total testing time and, thus, it also should be considered in reducing the testing demands of the original TSS test. The TSS test utilizes a 10-s rest period for the high test temperature; this long rest period makes the testing time impractical considering that a 0.9-s rest time is used for the general flow number test. Therefore, tests with reversed loading blocks with shorter rest periods (10, 5, and 3 s) were conducted for the high temperature tests in order to investigate the effect of rest period on permanent strain development. Whereas, a 1.6-s rest period was used for low test temperature for further SSR tests as that was used in the TSS test protocol. Fig. 4 shows that permanent strain values obtained from the three rest periods. The result

shows that the permanent strains are close to each other and the differences between them are minimal; which is easily smaller than the sample-to-sample variability, as shown in Fig. 4(a). Table 5 summarizes the individual shift model coefficients that obtained using the different rest periods. Fig. 3 describes the detailed calibration procedure of the shift model using the SSR test protocol. Overall, the slope (1-b) of the permanent strain mastercurve is consistent, although the slope slightly decreases as the rest period decreases. Therefore, using shorter rest period than 10-s is feasible to reduce the overall testing time. The shortest possible rest period was selected after verification as presented in the following section via a (1) random loading history comparison and (2) rut depth predictions in the structural model using the LVECD program.

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3.0%

3.0%

(b) Averaged Strains

2.0%

10s Rest-1 10s Rest-2 10s Rest-3 5s Rest-4 5s Rest-5 3s Rest-6 3s Rest-7

1.0%

0.0%

0

200

400

600

Permanent Strain

Permanent Strain

(a) SSR (Reversed Blocks)

2.0%

10s Rest

5s Rest 1.0% 3s Rest

0.0%

0

200

Number of Cycles

400

600

Number of Cycles

Fig. 4. Effect of rest period: Comparison of permanent strains with reversed loading blocks (689–483–896 kPa) for (a) individual samples and (b) averaged strains.

Table 5 Coefficients for shift model. Test Method

Deviatoric Stress (kPa)

Pulse-Rest (s)

TSS SSR

483–689–896 689–483–896 (TH) 483–689–896 (TL)

0.4–10 0.4–10 0.4–5 0.4–3

Reference Curve

Load Time SF

Vertical Stress SF

b

e0

NI

p1

p2

d1

d2

0.700 0.697 0.710 0.712

0.003 0.003 0.003 0.003

2.159 1.203 1.248 1.285

0.719 0.824 0.816 0.803

0.036 0.290 0.291 0.288

2.499 3.073 3.340 3.473

2.119 2.674 2.903 3.016

Note: SF is shift factor.

6. Verification of Shift Model Calibrated Using SSR Test 6.1. Random loading history A random loading history test was employed to verify the proposed SSR test protocol. The random loading history randomly changes the stress levels and load times, which simulates the actual field conditions of various traffic loads. A good prediction of the random loading history using the shift model, which was calibrated by the SSR test, would verify the proposed SSR test protocol and the calibration procedure for the permanent deformation model. The random load tests were performed at a high temperature (54 °C) and intermediate temperature (40 °C) with two replicates where the asphalt mixture deforms significantly. The load time distribution is from a 0.1-s to a 1.6-s and the deviatoric stress levels range from 483 kPa to 896 kPa with a 5-s rest period. The loading history was selected randomly using the ‘RAND’ function in MATLABÒ. Overall, the predicted strain values obtained using different rest periods show good correlations with the measured permanent strain values, as shown in Fig. 5. At 54 °C, the best agreement is evident between the experimental strain and predicted strain using the 5-s rest period, as shown in Fig. 5(a) and (b), respectively, and at 40 °C, the prediction with the 3-s rest period showed better results than the others, as shown in Fig. 5(c) and (d).

6.2. Rut depth predictions using LVECD program Fig. 6 shows the comparison of the predicted rut depths obtained from the original TSS test and from the SSR test with varying rest periods with LVECD program. In this figure, the legend indicates that ‘SSR’ uses two tests with reversed blocks at a high temperature, and ‘TSS’ employs four tests with increasing loading blocks. By comparing the rut depths obtained from ‘TSS 0.4-10’ and ‘SSR 0.4-10’, the effect of the reversed loading block on rut depth predictions can be investigated. The difference between

these predictions is 5.1% after a 10-year simulation, which is allowable because 10% variability is usually acceptable. Interestingly, the rut depths predicted using the different rest periods for the SSR test are almost identical to each other. Slightly lower permanent strain values were observed for all the results predicted from the SSR tests than from the original TSS test method, as shown in Fig. 6. However, the difference between the rut depth predictions are less than 5% after a 10-year simulation, which is not a significant difference considering that the SSR test requires about half the time and fewer replicates compared to the TSS test. After careful exploration of all the rest period options, a 3.6-s rest is recommended for the SSR test to make one cycle of pulse and rest to be a 4.0-s; thus, the recommendation is a 3.6-s rest period with a 0.4-s pulse.

7. Actuator displacement All of the strain measurements in the TSS and SSR test data presented here were obtained using four LVDTs equally spaced around the circumference of the specimen. However, installing these loose-core LVDTs and positioning them properly on the test specimen is time consuming, and thus, on-specimen LVDTs are not acceptable for routine testing. Therefore, one of the ideas to reduce the test requirements was to use actuator displacement to determine the permanent deformation. Use of actuator displacement, if it provides the shift model coefficients sufficiently close to those from the specimen-mounted system, offers a substantially simplified instrumentation system compared to using LVDTs [19]. In this study, the permanent strains based on actuator displacement were compared to those based on loose-core LVDTs, and each permanent strain was then used to calibrate the shift model to evaluate rut depth predictions in the structural model using the LVECD program. The actuator displacement measurements were converted to strain by dividing the measurements by the specimen length of 150 mm. The data from the LVDT-mounted specimens were converted to strain by dividing the LVDT measurements by

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10.0%

5.0%

(b) log scale (54°C)

4.0% 3.0% 2.0% Measured1 0.4-10s 0.4-3s

1.0% 0.0%

0

500

Measured2 0.4-5s

1000

Permanent Strain

Permanent Strain

(a) RS 9.5B (54°C)

1.0%

0.1%

1500

1

Number of Cycles 2.0%

1000

(d) log scale (40°C)

Permanent Strain

Permanent Strain

100

10.0%

(c) (40°C) 1.5%

1.0%

0.5%

0.0%

10

Number of Cycles

0

500

1000

1500

Number of Cycles

1.0%

0.1%

0.0%

1

10

100

1000

Number of Cycles

Fig. 5. Random loading history predictions using varying rest periods (RS9.5B).

10

Total Rut Depth (mm)

TSS 0.4-10 SSR 0.4-10

8

SSR 0.4-5 SSR 0.4-3

6 4 2 0

0

20

40

60

80

100

120

Time (Months) Fig. 6. Effect of rest period on rut depths predicted from SSR tests.

3%

Permanent Strain

X head vs. On Specimen (RS9.5B) On Specimen - TH X head - TH On Specimen - TL X head - TL

2%

1%

0% 0

100

200

300

400

500

600

Number of Cycles Fig. 7. Comparison of permanent strain values obtained from actuator displacement and loose-core LVDTs (RS9.5B).

the gauge length of 70 mm. Fig. 7 shows that the permanent strain values obtained from the actuator displacement (denoted as X head) are generally higher than those obtained using LVDTs (with a 70-mm gauge length). The difference between the permanent strain values obtained from the loose-core LVDTs and actuator displacement is caused by the compliance of the system (i.e., machine compliance). At a relatively high temperature, the modulus value of the asphalt mixture is low and the mixture is much softer than the equipment; therefore, the deformation that is due to machine compliance is negligible. However, at a low temperature, the modulus value of the asphalt sample is high and close to that of the equipment; as a result, the deformation due to machine compliance is not negligible. If the system compliance of the test machine can be characterized reliably and accurately, then the actual deformation in the sample can be computed easily by subtracting the system’s displacement in the test equipment from the total displacement experienced by the actuator. A method that employs an aluminium specimen was used to account for the effects of system compliance. The basic concept is that the actual deformation in the specimen can be computed by subtracting the displacement in the test equipment (including membrane displacement) from the total displacement experienced by the actuator. The detailed steps are as follows:
Run SSR tests using an asphalt specimen at desired temperatures (TL and TH) and calculate the strains based on actuator displacement (eactuator).
Prepare a 100-mm (4-in.) diameter by 150 mm (6-in.) tall aluminium specimen with the same end treatment (covered with greased double latex sheets) as the asphalt specimen.
Run SSR tests using the aluminium specimen at the test temperatures (TL and TH) and calculate the strains based on actuator displacement (emachine).

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Table 6 Comparison of permanent deformation test methods. Test method Strain Measure Number of tests

Reference TSS

Total samples Total test time (h) Pulse time (s) Rest period (s) Deviatoric stress (kPa) Number of cycles

SSR

TSS

Machine Displacement – 2 (TH and TL) 4 2 0.4 3.6 (TH) and 1.6 (TL) 689, 483, and 896 (TH) 483, 689, and 896 (TL) 200

LVDTs 1 (TH) 3 (TH, TI, and TL) 8 8.3 0.4 10 (TH) and 1.6 (TI and TL) 483, 689, and 896


Subtract the machine’s strain (emachine) values from the actuator’s strain (eactuator) values to calculate the strain on the specimen (eon-specimen). Note that testing with an aluminium specimen should be conducted at exact test temperatures (TL and TH for the SSR test method) and using the same end treatment. This procedure allows the displacement of the sample to be isolated from the total movement of the test equipment. Thus, the effect of machine compliance can be excluded by obtaining the real specimen displacement. This method was verified by comparing the results with the rut depth predictions obtained from the LVECD program, as given in Fig. 8. Fig. 8 shows that the rut depths predicted from the corrected actuator displacement are almost identical to those obtained from the loose-core LVDTs. It is believed that if the system compliance of the test device can be characterized reliably and accurately, the predicted rut depths of the pavement structure would be similar to those predicted using LVDTs. In general, the rut depth values predicted from the actuator displacement are slightly higher than those obtained from the loose-core LVDTs. This outcome is due to the relatively high machine compliance at low temperatures that causes high total shift factors at a low reduced frequency, which results in the high rut depth predictions in the analysis results. However, the difference between the rut depths obtained from the two types of measurements is less than 3 %, which is negligible. Furthermore, the higher side of the predictions using the machine displacement cancels out the under-predicted rut depths caused by the shorter rest period, as reported in Fig. 6. Therefore, for practical purposes, the permanent strains obtained from machine displacement can be used without any correction.

6.0

Rut Depth (cm)

Rut Depth Comparison (RS9.5B) 5.0 4.0 3.0 2.0

(1) LVDTs

(2) X head

1.0

Flow number 1 (TH) 3 0.9 0.1 0.9 483 1000

8. Comparisons of SSR, TSS, and flow number tests Table 6 presents comparisons of the different permanent deformation test protocols (SSR, TSS, and flow number). The proposed SSR test requires 4.1 h of testing time and four specimens for a single mixture, which is roughly half the TSS test requirements but more than the flow number test requirements. However, the flow number test can only rank materials whereas the SSR test protocol can be used to predict pavement rut depths. The SSR testing for a single mixture, including temperature conditioning and pressurizing, can be accomplished within a day. In short, the proposed SSR test requires shorter testing times and fewer replicates than the original TSS test, but nonetheless is able to produce relatively accurate predictions using actuator displacement for permanent deformation measurements. 9. Conclusions The TSS test is a protocol that is used to calibrate a permanent deformation model called the shift model. The TSS test is advantageous over other models because it can simulate the effects of temperature, load time, and deviatoric stress. However, the TSS test protocol requires eight specimens and two days of testing for a single mixture, which is impractical for agencies and contractors. Therefore, the SSR test proposed in this paper is designed to reduce the testing time and number of specimens used in the original TSS test. The effects of each parameter, i.e., temperature, deviatoric stress, load time, and rest period, were explored to reduce the test requirements of the TSS test and to predict relatively accurate permanent deformation. Two temperatures (TH and TL) are recommended for the SSR test by eliminating the intermediate temperature. Also, a simplified temperature selection method is specified for the SSR test. The SSR test at a high temperature uses a 0.4-s pulse time and a 3.6-s rest period with a reversed loading block (689, 483, and 896 kPa). It employs the first block as a reference curve and uses machine displacement for strain measurement to calibrate the shift model. The rut depths predicted from the SSR and TSS method using the LVECD program are in close agreement. In conclusion, the proposed SSR test requires a shorter test time with fewer replicates than the original TSS test, and uses actuator displacement instead of LVDTs, and it still produces accurate predictions of permanent deformation. Future research will primarily focus on conducting the SSR test with multiple mixtures because this study employed only one mixture to make a conclusion.

(3) Corrected X head

0.0

0

20

40

60

80

100

120

Time (Month) Fig. 8. Rut depth predictions of RS.5B mixture using LVECD program based on (1) loose-core LVDTs, (2) actuator displacement, and (3) corrected strains.

References [1] Accelerated Laboratory Rutting Tests: Evaluation of the Asphalt Pavement Analyzer, NCHRP 508, Transportation Research Record: Journal of the Transportation Research Board, Transportation Research Board of the National Academies, Washington, D.C., 2003.

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