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A study of asphalt aging using Indirect Tensile Strength test
Construction and Building Materials 95 (2015) 218–223
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A study of asphalt aging using Indirect Tensile Strength test Md Rashadul Islam a,⇑, Mohammad Imran Hossain b, Rafiqul A. Tarefder a a b
Dept. of Civil Engineering, University of New Mexico, MSC01 1070, 1 University of New Mexico, Albuquerque, NM 87131, United States Civil Engineering and Construction Department, Bradley University, Jobst Hall 100, 1501 W. Bradley Avenue, Peoria, IL 61625, United States
h i g h l i g h t s One-day laboratory long-term aging is close to one-year of field aging. ITS of laboratory long-term and field aged samples increase with aging period. ITS of short-term oven aged loose sample is concave down with aging period. Brittleness increases with conditioning period under all aging modes.
a r t i c l e
i n f o
Article history: Received 24 March 2015 Received in revised form 5 July 2015 Accepted 15 July 2015
Keywords: Asphalt Concrete Aging Field conditioning Laboratory conditioning Tensile strength
a b s t r a c t While dynamic modulus, diametrical resilient modulus, and loss of ductility are the most common parameters to study aging, this study determines laboratory equivalence of field aging using an Indirect Tensile Strength (ITS) test for its simplicity and wide usages in Asphalt Concrete (AC) performance evaluation. Cylindrical samples were compacted in the laboratory, aged in the laboratory and field, and then loaded diametrically to determine ITS value and flow number. The ITS tests were conducted on six sets of compacted samples after subjecting them to 1, 5, 10, 15, 20, and 25 days of oven aging at 85 °C in the laboratory and on 11 sets of compacted samples after subjecting them to 1, 2, 3, and up to 12-months of field aging. The ITS tests were also performed on a third set of samples whose loose mixes were subjected to 8, 16, 32, 48, 72 and 100 h oven aging at 135 °C. As expected, ITS of the laboratory (both compacted and loose) and field aged samples increase and flow number decrease with the aging period. It is found in this study that one-day laboratory aging is close to approximately one-year of field aging measured in terms of ITS value. Results from loose mix aging show that the ITS value increases with the conditioning period, reaches a peak and then decreases with the conditioning period. Overall, the flow number decreases as aging intensity increases, that is, the brittleness increases with aging. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Asphalt binder is composed of organic compounds named as asphaltenes, maltenes, and resins. Due to its polar nature, asphaltenes are more likely to react with oxygen and create carbonyl and sulfoxide compounds. These compounds result in the increase of asphaltene compounds leading to the stiffening of binder, increase in stiffness and viscosity. Characterization of this aging process is a challenging topic; it is difficult to relate chemical aspects of binder aging, such as carbonyl and sulfoxide growth, to the change in physical/rheological properties of the binder and consequently to
⇑ Corresponding author. E-mail addresses: [email protected] (M.R. Islam), [email protected]. edu (M.I. Hossain), [email protected] (R.A. Tarefder). http://dx.doi.org/10.1016/j.conbuildmat.2015.07.159 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
the mixture. Therefore, the authors employed a simplistic approach which involves the relation of one of the mechanical properties (tensile strength) with aging of asphalt mixture. The relationship between the chemical aging of binder and the mixture aging is outside the scope of this study. Asphalt binder reacts with oxygen at high temperature while mixing with aggregates in the mixing plant and during the construction period. This phenomenon is known as short-term aging. After pavement construction, asphalt binder continuously reacts with oxygen which is known as long-term aging. Both the shortand long-term aging are simulated in the laboratory following the AASHTO R 30-02 [1] test protocol. Loose asphalt mixture is oven heated at 154 °C for 2 h or at 135 °C for 4 h to simulate short-term aging in the laboratory. Long-term aging is performed by subjecting the prepared Asphalt Concrete (AC) sample in an
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oven at 85 °C for 120 h (5 days). Researchers have intensively explored the effects of short- and long-term aging on the rheological properties of asphalt binder [13,19,9,27,18,15]. All of these studies focused on the rheological properties such as the shear modulus and viscosity of aged binder. Studies on aging have also been conducted using prepared AC samples. For example, Tarefder and Faisal [22] investigated the aging effect on asphalt mastics using nano-indentation test; Valtorta et al. [24] studied the long-term aging of an asphaltic plug joint of a highway bridge; Walubita [26], Arega et al. [8] investigated the fatigue behavior of AC. Change in asphalt modulus with aging are also widely explored [20,10,17]. On the other hand, not a good amount of research has been conducted to examine the aging effect on Indirect Tensile Strength (ITS). Thomas et al. [23] studied the aging effect of AC using Indirect Tensile (IDT) test device. They determined the resilient modulus changes over aging time. The ITS was not investigated in that study. However, ITS test is very cost effective in the sense of samples preparation or collection from the field, testing and analyzing the data. A study of aging using ITS test may be useful to research community. From tensile strength point of view, ITS is used to determine the tensile properties of the asphalt mixture which is correlated to the cracking properties of the pavement. A higher ITS corresponds to a stronger cracking resistance. At the same time, asphalt mixtures that are able to tolerate higher strain prior to failure are more likely to resist cracking than those unable to tolerate high strains. ITS is also an important input parameter in transverse cracking model used in the Pavement Mechanistic-Empirical (ME) Design Guide [6]. ITS is also a measure of resistance capacity to low temperature cracking of asphalt pavement. Typically, this value is determined in the laboratory using IDT samples and used as an input parameter in the design guide. However, during the service life it may be affected by several factors such as aging, moisture, freeze–thaw etc. Effect of moisture on the tensile strength of AC has been explored in the literature [11,16]; Tarefder and Ahmed, [21]; [7]. Effect of freeze–thaw on the tensile strength of AC has also been investigated [10,12]. However, very few studies examined the aging effect using ITS tests. Walubita et al. [25] studied the tensile strength using aged AC samples using axial tension test (not in diametrical mode). Lolly [14] studied the effect of short-term aging using ITS test. The loose samples were aged up to 8 h. The effect of aging for longer conditioning periods cannot be understood from that study. The above discussion concludes that the effects of short- and long-term aging on IDT are still an unsolved issue. In addition, how field aging causes a change in ITS value is an interesting research topic. The current study is conducted to understand the field, laboratory short- and long-term effects of aging on the ITS of AC using IDT test device.
2. Objectives
3. Investigation of long-term aging 3.1. Materials and sample preparation Samples of 150 mm diameter and 170 mm height were prepared using a Superpave gyratory compactor following the AASHTO T 312-07 [5] test protocol. The samples were cut into 100 mm diameter and 50 mm thick samples using a laboratory saw and coring device. The bulk densities of prepared samples and the theoretical maximum density of the loose mixture were determined following AASHTO T 166-07 [2] and AASHTO T 209-05 [3] test standards respectively. The air voids of the samples before conditioning ranged from 5.1% to 5.9% with an average value of 5.4%. A plant produced dense graded Superpave (SP) mixture, type SP-III, was used to prepare the samples. Loose mixture was collected in cooperation with the New Mexico Department of Transportation (NMDOT) during the construction period of a site. The design binder was a Performance Grade (PG) binder, PG 76-22, which was used 4.4% by the weight of the mixture. The maximum aggregate size was 19 mm. 3.2. Conditioning of samples Two types of conditioning were conducted: long-term oven aging and field aging. In the oven aging, the prepared samples were placed on a rack in an oven at 85 °C (185 °F) for 120 h (5 days) following the AASHTO R 30-02 [1] test protocol. Following that time period, the oven was turned off and the door was left open to let the oven and specimens cool down to room temperature for about 16 h. The process was repeated up to 5 times (25 days) using different batches of samples. Fig. 1 shows one batch of the unaged asphalt sample and another batch of 120 h oven aged sample. The color difference between these two batches of samples depicts the aging action of the binder. The color of binder becomes darker with aging. During the field conditioning, the AC samples were conditioned in the sun beside the Interstate 40 (I-40) instrumentation section at mile post 141, near the city of Albuquerque, New Mexico, for about a year starting from August 2013. The reason for choosing the I-40 test site was that the section measured the environmental data along with the pavement temperature. The asphalt samples were placed on a smooth flat surface as shown in Fig. 2. During the conditioning period, the samples experienced the maximum temperature of 54 °C in summer, the minimum temperature of 11 °C in winter, about 90 freeze–thaw cycles, day-night
Unaged Samples: Whitish
Aged Samples: Black
The main objective of this study is to examine the effects of aging on the ITS of AC. The flow number (vertical deformation in hundredth of an inch) is also examined to determine the ductility change with aging. Different modes of aging such as short- and long- term laboratory aging, and field aging are considered. Specific objectives are mentioned below: (a) Examine the effects of long-term laboratory oven aging on the tensile strength and flow number of AC. (b) Examine the effects of field aging on the tensile strength and flow number of AC. (c) Examine the effects of short-term laboratory aging (on loose mixture) on the tensile strength and flow number of AC.
Fig. 1. Thin cylindrical sample before and after long-term laboratory aging.
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where P, D, and t are measured peak load (lbs.), diameter (in.), and thickness (in.), respectively. The measured average diameters and thicknesses were used in the analysis. No correction was applied to record reading obtained from the test apparatus. 3.4. Results and discussion
Fig. 2. Conditioning of samples at the sun (field conditioning).
temperature differences up to 26 °C, and typical New Mexico rainfall and humidity. The damage due to traffic loading was not considered in the case of field aging, only environmental factors were considered. 3.3. Indirect tension test The ITS of the asphalt sample was determined using the IDT test device in an environmental chamber as shown in Fig. 3. AASHTO T 283-03 [4] test protocol was followed with the exception of test temperature. Uniform compressive load of 50 mm per minute was applied until failure at 20 °C. At least four replicate samples were tested for each category adopted in the study. Tensile stress develops in horizontal direction when vertical compressive force is applied. This Indirect Tensile Stress (psi) is calculated using Eq. (1).
ITS ¼
2P
ð1Þ
pDt
Fig. 3. Indirect tension test setup.
Fig. 4(a) shows the ITS of the long-term oven aged samples. It can be seen that ITS increases with the aging period. For example, ITS of the one-day aged sample (1.47 MPa) is 17.7% greater than the unaged sample (1.25 MPa). The increase in ITS for 5-day and 25-day aged samples are 28% and 31% respectively. This means that most of the aging process occurs in the first few days of conditioning. The aging effect after 5 days of aging is quite low. This sharp increase during the first few days of conditioning is probably because of the film stiffening. Further aging of the compacted mixture may not stiffen any more binder film or mastic, since some portions of the binder may not be accessible. Further investigation is still required to monitor the aging process of the inner binder. The flow umber (vertical deformation in hundredth of an inch) of the oven aged samples is presented in Fig. 4(b). It shows that the flow number decreases from 6.16 to 4.95 for the conditioning period of 25 days. This means the brittleness of the AC samples increases (ductility decreases) with aging. However, the increase in brittleness is very sharp during the first five days of conditioning. As discussed earlier, this sudden drop of flow number during the first few days of conditioning is due to the film stiffening; the inner portion of the binder may not be affected even after long periods of conditioning. The aging process of inner binder may be pursued in future studies. ITS of the field aged samples is presented in Fig. 5(a). It shows that ITS increases with conditioning in the field. The increase of the Tensile Strength is very clear although two adjacent values may look close. This is because of the aging period between two adjacent values does not differ so much (30 days). In only 30 days of field aging, dramatic change in Tensile Strength is not expected. The ITS value of the one-year aged sample is 1.49 MPa, which is 19.2% greater than that of the unaged sample. Similar to the oven aged samples, the increase in ITS flattens after certain conditioning periods. The logic may be the same as discussed earlier. The surface binder ages up very quickly and a longer period of aging may not stiffen the in-depth binder. The flow umber of the field aged samples is presented in Fig. 5(b). It shows that the flow number decreases from 6.16 to 5.31 for the conditioning period of 358 days. However, the increase in brittleness is very slow during the first 240 days (up to March 2014) of conditioning and then (from April 2014) decreases very fast, which is not expected as found in the oven aged samples. No discrete reason was found for this anomaly behavior; however, the authors suspect that during the thawing period, ductility loss is more severe. Data from Figs. 4(a) and 5(a) is compared to correlate long-term aging in the laboratory and field. It can be seen from Fig. 4(a) that the ITS of the one-day oven aged sample is 17.7% greater than the unaged sample. The ITS value of the one-year field aged sample is 19.2% greater than that of the unaged sample as shown in Fig. 5(a). Therefore, it can be said that the ITS of the one-day oven aged sample in the laboratory and the one-year field aged sample in the field are almost equal (117.7% versus 119.2% of the unaged samples). One Way Analysis of Variance (ANOVA) test was conducted to evaluate the ITS of the one-day oven aged samples and the one-year field aged samples. The null hypothesis was that the average values were equal and the alternative hypothesis was that the average values were not equal. The p-value (probability of null hypothesis to be true) of the test is found to be 0.64. As the p-value is greater than 0.05 (5%), the null hypothesis is true. Therefore, ITSs
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7 6
1.5 Flow Number
Tensile Strength (MPa)
2
1
0.5
5 4 3 2 1 0
0 0
5 10 15 20 25 Conditioning Period (Days)
0
30
5 10 15 20 25 Conditioning Period (Days)
30
(b) Flow Number
(a) Tensile Strength
Fig. 4. Indirect Tensile Strength test results of laboratory aged samples.
8 7
1.6 Flow Number
Tensile Strength (MPa)
2
1.2 0.8 0.4
6 5 4 3 2 1 0
0 0
60 120 180 240 300 Conditioning Period (Days)
360
0
60 120 180 240 300 Conditioning Period (Days)
360
(b) Flow Number
(a) Tensile Strength
Fig. 5. Indirect Tensile Strength test results of field aged samples.
of one-year field aged samples and one-day oven aged samples are statistically equal at 95% Confidence Interval (CI). If one day oven aging is considered, the ITS is 1.47 MPa; if 365 days of field aging is considered, the ITS is 1.46 MPa. The difference between these two predictions is less than 1%, which is negligible. It can be said that the ITS of one day laboratory oven aged (long-term) AC sample is close to that of approximately one year of field aged sample based on the mixture and conditions used in the study using the tensile strength as the indicator. However, aging process of binder may be dependent on binder type, climate, binder type, mix design, air voids etc., which were not studied in the current study. Regarding the failure mode of the sample, it was observed that the unaged sample did not separate into two pieces upon failure as shown in Fig. 6(a). However, the aged samples, especially after longer period of conditioning, separated into two pieces upon failure, shown in Fig. 6(b). It was found that the longer the
conditioning period the more the tendency to separate into pieces. This is because of the brittleness of the aged samples. 4. Investigation of short-term aging 4.1. Sample preparation and ITS testing Prior to the sample preparation, different batches of loose mixtures were short-term oven aged up to 100 h at 135 °C using AASHTO R 30-02 [1] test protocol. The aging period, 100 h, may not be suitable to be termed as short-term; however, it is done so considering the sample conditioning protocol in the AASHTO R 30-02 [1] test protocol. The mixture was stirred thoroughly about every two hours using a spatula as recommended in the standard. Using the oven aged loose mixture, samples were prepared using a Superpave gyratory compactor following the AASHTO T 312-07 [5]
(a) Unaged Samples
(b) Aged Samples Fig. 6. Few failed samples.
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8 7
1.5 Flow Number
Tensile Strength (MPa)
2.0
1.0 0.5
6 5 4 3 2 1
0.0 0
20 40 60 80 100 Condtioning Period (hours)
120
0 0
20 40 60 80 100 Conditioning Period (hours)
(a) Tensile Strength
120
(b) Flow Number
Fig. 7. Indirect Tensile Strength test results of short-term aged samples.
test protocol. The samples were shaped into 100 mm diameter and 50 mm height samples using a laboratory saw and coring device. The air voids of the sample ranged between 5.2% and 5.9% with an average value of 5.5%. The ITS of the asphalt samples was determined similar way of other samples discussed earlier. 4.2. Results and discussion The ITS of short-term aged sample are presented in Fig. 7(a). It shows that the ITS value increases with an increase in the conditioning period up to about 40 h. After that ITS decreases with an increase in the conditioning period. More specifically, the ITS value increases from 1.25 MPa to 1.66 MPa (133%) for the conditioning of loose mixture by 40 h. After 100 h of conditioning, the ITS value is 1.14 MPa, which is 9% less than that of unaged samples. Probably in loose mixtures, asphalt binder film or mastic becomes too oxidized during the 40 h of conditioning. After this point, binder film or mastic loses its adhesiveness and viscosity, which results in reduced tensile strength in compacted mixture. Fig. 7(b) shows the flow number of the short-term aged samples. It shows that the flow number decreases continuously with an increase in the conditioning period. This means the ductility of short-term aged samples decreases continuously with conditioning period. Unlike the compacted mixture, while heating up the loose mixture, the whole binder ages due to striation at every 2 h. This is why, the brittleness continues to increase with the conditioning period. Based on the results of short-term aged samples, it can be said that asphalt mixture should be heated as less as possible while mixing in the plant, however, no more than 40 h. After that asphalt binder film or mastic becomes too oxidized. Asphalt undergoes long-term aging after construction, which causes an increase in ITS value as shown in Fig. 5(a). This is why, asphalt roadway should be opened for traffic soon after the construction. 5. Conclusions This study investigates the effects of laboratory (long-term and short-term) and field conditioning using ITS tests on AC samples using a single SP mixture. Based on the findings of the study, the transverse cracking prediction subroutine used in the Pavement ME Design software may be calibrated/revised to account for the aging effect in asphalt pavement. Accounting for aging on the stiffness only does not calibrate the transverse crack prediction model in the Pavement ME Design software. A wide range of mixtures and a lower test temperature are recommended for future studies. Moreover, the following conclusions can be made from the current study:
(a) The ITS of one day laboratory oven aged (long-term) AC sample is close to that of approximately one year of field aged sample for New Mexico’s climate for the single mixture tested in this study. However, factors such as binder type, climate, binder type, mix design, air voids etc., may affect the aging process which are not studied here. Future research is recommended to see what happens in longer period (2, 5, 10 years and so on). (b) The ITS values of the laboratory long-term and field aged samples increase with the aging period. (c) The ITS of the short-term oven aged loose samples increases with the conditioning period; it reaches a peak and then decreases with the conditioning period. (d) The flow number decreases (brittleness increases) with the conditioning period under all kinds of aging modes.
Acknowledgements This study was funded by the New Mexico Department of Transportation (NMDOT). The authors would like to express their sincere gratitude and appreciation to Mr. Jeff Mann, Pavement Design and Management Bureau Chief at NMDOT, for being the advocate of this project and his regular support, sponsorship, and suggestions. The authors appreciate the valuable service and time of Mr. Virgil Valdez, who is the Project Manager. Special thanks go to several Project Panel members namely, James Gallegos, Materials Bureau Chief, and Parveez Anwar, State Asphalt Engineer at NMODT.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A study of asphalt aging using Indirect Tensile Strength test Md Rashadul Islam a,⇑, Mohammad Imran Hossain b, Rafiqul A. Tarefder a a b
Dept. of Civil Engineering, University of New Mexico, MSC01 1070, 1 University of New Mexico, Albuquerque, NM 87131, United States Civil Engineering and Construction Department, Bradley University, Jobst Hall 100, 1501 W. Bradley Avenue, Peoria, IL 61625, United States
h i g h l i g h t s One-day laboratory long-term aging is close to one-year of field aging. ITS of laboratory long-term and field aged samples increase with aging period. ITS of short-term oven aged loose sample is concave down with aging period. Brittleness increases with conditioning period under all aging modes.
a r t i c l e
i n f o
Article history: Received 24 March 2015 Received in revised form 5 July 2015 Accepted 15 July 2015
Keywords: Asphalt Concrete Aging Field conditioning Laboratory conditioning Tensile strength
a b s t r a c t While dynamic modulus, diametrical resilient modulus, and loss of ductility are the most common parameters to study aging, this study determines laboratory equivalence of field aging using an Indirect Tensile Strength (ITS) test for its simplicity and wide usages in Asphalt Concrete (AC) performance evaluation. Cylindrical samples were compacted in the laboratory, aged in the laboratory and field, and then loaded diametrically to determine ITS value and flow number. The ITS tests were conducted on six sets of compacted samples after subjecting them to 1, 5, 10, 15, 20, and 25 days of oven aging at 85 °C in the laboratory and on 11 sets of compacted samples after subjecting them to 1, 2, 3, and up to 12-months of field aging. The ITS tests were also performed on a third set of samples whose loose mixes were subjected to 8, 16, 32, 48, 72 and 100 h oven aging at 135 °C. As expected, ITS of the laboratory (both compacted and loose) and field aged samples increase and flow number decrease with the aging period. It is found in this study that one-day laboratory aging is close to approximately one-year of field aging measured in terms of ITS value. Results from loose mix aging show that the ITS value increases with the conditioning period, reaches a peak and then decreases with the conditioning period. Overall, the flow number decreases as aging intensity increases, that is, the brittleness increases with aging. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Asphalt binder is composed of organic compounds named as asphaltenes, maltenes, and resins. Due to its polar nature, asphaltenes are more likely to react with oxygen and create carbonyl and sulfoxide compounds. These compounds result in the increase of asphaltene compounds leading to the stiffening of binder, increase in stiffness and viscosity. Characterization of this aging process is a challenging topic; it is difficult to relate chemical aspects of binder aging, such as carbonyl and sulfoxide growth, to the change in physical/rheological properties of the binder and consequently to
⇑ Corresponding author. E-mail addresses: [email protected] (M.R. Islam), [email protected]. edu (M.I. Hossain), [email protected] (R.A. Tarefder). http://dx.doi.org/10.1016/j.conbuildmat.2015.07.159 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
the mixture. Therefore, the authors employed a simplistic approach which involves the relation of one of the mechanical properties (tensile strength) with aging of asphalt mixture. The relationship between the chemical aging of binder and the mixture aging is outside the scope of this study. Asphalt binder reacts with oxygen at high temperature while mixing with aggregates in the mixing plant and during the construction period. This phenomenon is known as short-term aging. After pavement construction, asphalt binder continuously reacts with oxygen which is known as long-term aging. Both the shortand long-term aging are simulated in the laboratory following the AASHTO R 30-02 [1] test protocol. Loose asphalt mixture is oven heated at 154 °C for 2 h or at 135 °C for 4 h to simulate short-term aging in the laboratory. Long-term aging is performed by subjecting the prepared Asphalt Concrete (AC) sample in an
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oven at 85 °C for 120 h (5 days). Researchers have intensively explored the effects of short- and long-term aging on the rheological properties of asphalt binder [13,19,9,27,18,15]. All of these studies focused on the rheological properties such as the shear modulus and viscosity of aged binder. Studies on aging have also been conducted using prepared AC samples. For example, Tarefder and Faisal [22] investigated the aging effect on asphalt mastics using nano-indentation test; Valtorta et al. [24] studied the long-term aging of an asphaltic plug joint of a highway bridge; Walubita [26], Arega et al. [8] investigated the fatigue behavior of AC. Change in asphalt modulus with aging are also widely explored [20,10,17]. On the other hand, not a good amount of research has been conducted to examine the aging effect on Indirect Tensile Strength (ITS). Thomas et al. [23] studied the aging effect of AC using Indirect Tensile (IDT) test device. They determined the resilient modulus changes over aging time. The ITS was not investigated in that study. However, ITS test is very cost effective in the sense of samples preparation or collection from the field, testing and analyzing the data. A study of aging using ITS test may be useful to research community. From tensile strength point of view, ITS is used to determine the tensile properties of the asphalt mixture which is correlated to the cracking properties of the pavement. A higher ITS corresponds to a stronger cracking resistance. At the same time, asphalt mixtures that are able to tolerate higher strain prior to failure are more likely to resist cracking than those unable to tolerate high strains. ITS is also an important input parameter in transverse cracking model used in the Pavement Mechanistic-Empirical (ME) Design Guide [6]. ITS is also a measure of resistance capacity to low temperature cracking of asphalt pavement. Typically, this value is determined in the laboratory using IDT samples and used as an input parameter in the design guide. However, during the service life it may be affected by several factors such as aging, moisture, freeze–thaw etc. Effect of moisture on the tensile strength of AC has been explored in the literature [11,16]; Tarefder and Ahmed, [21]; [7]. Effect of freeze–thaw on the tensile strength of AC has also been investigated [10,12]. However, very few studies examined the aging effect using ITS tests. Walubita et al. [25] studied the tensile strength using aged AC samples using axial tension test (not in diametrical mode). Lolly [14] studied the effect of short-term aging using ITS test. The loose samples were aged up to 8 h. The effect of aging for longer conditioning periods cannot be understood from that study. The above discussion concludes that the effects of short- and long-term aging on IDT are still an unsolved issue. In addition, how field aging causes a change in ITS value is an interesting research topic. The current study is conducted to understand the field, laboratory short- and long-term effects of aging on the ITS of AC using IDT test device.
2. Objectives
3. Investigation of long-term aging 3.1. Materials and sample preparation Samples of 150 mm diameter and 170 mm height were prepared using a Superpave gyratory compactor following the AASHTO T 312-07 [5] test protocol. The samples were cut into 100 mm diameter and 50 mm thick samples using a laboratory saw and coring device. The bulk densities of prepared samples and the theoretical maximum density of the loose mixture were determined following AASHTO T 166-07 [2] and AASHTO T 209-05 [3] test standards respectively. The air voids of the samples before conditioning ranged from 5.1% to 5.9% with an average value of 5.4%. A plant produced dense graded Superpave (SP) mixture, type SP-III, was used to prepare the samples. Loose mixture was collected in cooperation with the New Mexico Department of Transportation (NMDOT) during the construction period of a site. The design binder was a Performance Grade (PG) binder, PG 76-22, which was used 4.4% by the weight of the mixture. The maximum aggregate size was 19 mm. 3.2. Conditioning of samples Two types of conditioning were conducted: long-term oven aging and field aging. In the oven aging, the prepared samples were placed on a rack in an oven at 85 °C (185 °F) for 120 h (5 days) following the AASHTO R 30-02 [1] test protocol. Following that time period, the oven was turned off and the door was left open to let the oven and specimens cool down to room temperature for about 16 h. The process was repeated up to 5 times (25 days) using different batches of samples. Fig. 1 shows one batch of the unaged asphalt sample and another batch of 120 h oven aged sample. The color difference between these two batches of samples depicts the aging action of the binder. The color of binder becomes darker with aging. During the field conditioning, the AC samples were conditioned in the sun beside the Interstate 40 (I-40) instrumentation section at mile post 141, near the city of Albuquerque, New Mexico, for about a year starting from August 2013. The reason for choosing the I-40 test site was that the section measured the environmental data along with the pavement temperature. The asphalt samples were placed on a smooth flat surface as shown in Fig. 2. During the conditioning period, the samples experienced the maximum temperature of 54 °C in summer, the minimum temperature of 11 °C in winter, about 90 freeze–thaw cycles, day-night
Unaged Samples: Whitish
Aged Samples: Black
The main objective of this study is to examine the effects of aging on the ITS of AC. The flow number (vertical deformation in hundredth of an inch) is also examined to determine the ductility change with aging. Different modes of aging such as short- and long- term laboratory aging, and field aging are considered. Specific objectives are mentioned below: (a) Examine the effects of long-term laboratory oven aging on the tensile strength and flow number of AC. (b) Examine the effects of field aging on the tensile strength and flow number of AC. (c) Examine the effects of short-term laboratory aging (on loose mixture) on the tensile strength and flow number of AC.
Fig. 1. Thin cylindrical sample before and after long-term laboratory aging.
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where P, D, and t are measured peak load (lbs.), diameter (in.), and thickness (in.), respectively. The measured average diameters and thicknesses were used in the analysis. No correction was applied to record reading obtained from the test apparatus. 3.4. Results and discussion
Fig. 2. Conditioning of samples at the sun (field conditioning).
temperature differences up to 26 °C, and typical New Mexico rainfall and humidity. The damage due to traffic loading was not considered in the case of field aging, only environmental factors were considered. 3.3. Indirect tension test The ITS of the asphalt sample was determined using the IDT test device in an environmental chamber as shown in Fig. 3. AASHTO T 283-03 [4] test protocol was followed with the exception of test temperature. Uniform compressive load of 50 mm per minute was applied until failure at 20 °C. At least four replicate samples were tested for each category adopted in the study. Tensile stress develops in horizontal direction when vertical compressive force is applied. This Indirect Tensile Stress (psi) is calculated using Eq. (1).
ITS ¼
2P
ð1Þ
pDt
Fig. 3. Indirect tension test setup.
Fig. 4(a) shows the ITS of the long-term oven aged samples. It can be seen that ITS increases with the aging period. For example, ITS of the one-day aged sample (1.47 MPa) is 17.7% greater than the unaged sample (1.25 MPa). The increase in ITS for 5-day and 25-day aged samples are 28% and 31% respectively. This means that most of the aging process occurs in the first few days of conditioning. The aging effect after 5 days of aging is quite low. This sharp increase during the first few days of conditioning is probably because of the film stiffening. Further aging of the compacted mixture may not stiffen any more binder film or mastic, since some portions of the binder may not be accessible. Further investigation is still required to monitor the aging process of the inner binder. The flow umber (vertical deformation in hundredth of an inch) of the oven aged samples is presented in Fig. 4(b). It shows that the flow number decreases from 6.16 to 4.95 for the conditioning period of 25 days. This means the brittleness of the AC samples increases (ductility decreases) with aging. However, the increase in brittleness is very sharp during the first five days of conditioning. As discussed earlier, this sudden drop of flow number during the first few days of conditioning is due to the film stiffening; the inner portion of the binder may not be affected even after long periods of conditioning. The aging process of inner binder may be pursued in future studies. ITS of the field aged samples is presented in Fig. 5(a). It shows that ITS increases with conditioning in the field. The increase of the Tensile Strength is very clear although two adjacent values may look close. This is because of the aging period between two adjacent values does not differ so much (30 days). In only 30 days of field aging, dramatic change in Tensile Strength is not expected. The ITS value of the one-year aged sample is 1.49 MPa, which is 19.2% greater than that of the unaged sample. Similar to the oven aged samples, the increase in ITS flattens after certain conditioning periods. The logic may be the same as discussed earlier. The surface binder ages up very quickly and a longer period of aging may not stiffen the in-depth binder. The flow umber of the field aged samples is presented in Fig. 5(b). It shows that the flow number decreases from 6.16 to 5.31 for the conditioning period of 358 days. However, the increase in brittleness is very slow during the first 240 days (up to March 2014) of conditioning and then (from April 2014) decreases very fast, which is not expected as found in the oven aged samples. No discrete reason was found for this anomaly behavior; however, the authors suspect that during the thawing period, ductility loss is more severe. Data from Figs. 4(a) and 5(a) is compared to correlate long-term aging in the laboratory and field. It can be seen from Fig. 4(a) that the ITS of the one-day oven aged sample is 17.7% greater than the unaged sample. The ITS value of the one-year field aged sample is 19.2% greater than that of the unaged sample as shown in Fig. 5(a). Therefore, it can be said that the ITS of the one-day oven aged sample in the laboratory and the one-year field aged sample in the field are almost equal (117.7% versus 119.2% of the unaged samples). One Way Analysis of Variance (ANOVA) test was conducted to evaluate the ITS of the one-day oven aged samples and the one-year field aged samples. The null hypothesis was that the average values were equal and the alternative hypothesis was that the average values were not equal. The p-value (probability of null hypothesis to be true) of the test is found to be 0.64. As the p-value is greater than 0.05 (5%), the null hypothesis is true. Therefore, ITSs
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7 6
1.5 Flow Number
Tensile Strength (MPa)
2
1
0.5
5 4 3 2 1 0
0 0
5 10 15 20 25 Conditioning Period (Days)
0
30
5 10 15 20 25 Conditioning Period (Days)
30
(b) Flow Number
(a) Tensile Strength
Fig. 4. Indirect Tensile Strength test results of laboratory aged samples.
8 7
1.6 Flow Number
Tensile Strength (MPa)
2
1.2 0.8 0.4
6 5 4 3 2 1 0
0 0
60 120 180 240 300 Conditioning Period (Days)
360
0
60 120 180 240 300 Conditioning Period (Days)
360
(b) Flow Number
(a) Tensile Strength
Fig. 5. Indirect Tensile Strength test results of field aged samples.
of one-year field aged samples and one-day oven aged samples are statistically equal at 95% Confidence Interval (CI). If one day oven aging is considered, the ITS is 1.47 MPa; if 365 days of field aging is considered, the ITS is 1.46 MPa. The difference between these two predictions is less than 1%, which is negligible. It can be said that the ITS of one day laboratory oven aged (long-term) AC sample is close to that of approximately one year of field aged sample based on the mixture and conditions used in the study using the tensile strength as the indicator. However, aging process of binder may be dependent on binder type, climate, binder type, mix design, air voids etc., which were not studied in the current study. Regarding the failure mode of the sample, it was observed that the unaged sample did not separate into two pieces upon failure as shown in Fig. 6(a). However, the aged samples, especially after longer period of conditioning, separated into two pieces upon failure, shown in Fig. 6(b). It was found that the longer the
conditioning period the more the tendency to separate into pieces. This is because of the brittleness of the aged samples. 4. Investigation of short-term aging 4.1. Sample preparation and ITS testing Prior to the sample preparation, different batches of loose mixtures were short-term oven aged up to 100 h at 135 °C using AASHTO R 30-02 [1] test protocol. The aging period, 100 h, may not be suitable to be termed as short-term; however, it is done so considering the sample conditioning protocol in the AASHTO R 30-02 [1] test protocol. The mixture was stirred thoroughly about every two hours using a spatula as recommended in the standard. Using the oven aged loose mixture, samples were prepared using a Superpave gyratory compactor following the AASHTO T 312-07 [5]
(a) Unaged Samples
(b) Aged Samples Fig. 6. Few failed samples.
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8 7
1.5 Flow Number
Tensile Strength (MPa)
2.0
1.0 0.5
6 5 4 3 2 1
0.0 0
20 40 60 80 100 Condtioning Period (hours)
120
0 0
20 40 60 80 100 Conditioning Period (hours)
(a) Tensile Strength
120
(b) Flow Number
Fig. 7. Indirect Tensile Strength test results of short-term aged samples.
test protocol. The samples were shaped into 100 mm diameter and 50 mm height samples using a laboratory saw and coring device. The air voids of the sample ranged between 5.2% and 5.9% with an average value of 5.5%. The ITS of the asphalt samples was determined similar way of other samples discussed earlier. 4.2. Results and discussion The ITS of short-term aged sample are presented in Fig. 7(a). It shows that the ITS value increases with an increase in the conditioning period up to about 40 h. After that ITS decreases with an increase in the conditioning period. More specifically, the ITS value increases from 1.25 MPa to 1.66 MPa (133%) for the conditioning of loose mixture by 40 h. After 100 h of conditioning, the ITS value is 1.14 MPa, which is 9% less than that of unaged samples. Probably in loose mixtures, asphalt binder film or mastic becomes too oxidized during the 40 h of conditioning. After this point, binder film or mastic loses its adhesiveness and viscosity, which results in reduced tensile strength in compacted mixture. Fig. 7(b) shows the flow number of the short-term aged samples. It shows that the flow number decreases continuously with an increase in the conditioning period. This means the ductility of short-term aged samples decreases continuously with conditioning period. Unlike the compacted mixture, while heating up the loose mixture, the whole binder ages due to striation at every 2 h. This is why, the brittleness continues to increase with the conditioning period. Based on the results of short-term aged samples, it can be said that asphalt mixture should be heated as less as possible while mixing in the plant, however, no more than 40 h. After that asphalt binder film or mastic becomes too oxidized. Asphalt undergoes long-term aging after construction, which causes an increase in ITS value as shown in Fig. 5(a). This is why, asphalt roadway should be opened for traffic soon after the construction. 5. Conclusions This study investigates the effects of laboratory (long-term and short-term) and field conditioning using ITS tests on AC samples using a single SP mixture. Based on the findings of the study, the transverse cracking prediction subroutine used in the Pavement ME Design software may be calibrated/revised to account for the aging effect in asphalt pavement. Accounting for aging on the stiffness only does not calibrate the transverse crack prediction model in the Pavement ME Design software. A wide range of mixtures and a lower test temperature are recommended for future studies. Moreover, the following conclusions can be made from the current study:
(a) The ITS of one day laboratory oven aged (long-term) AC sample is close to that of approximately one year of field aged sample for New Mexico’s climate for the single mixture tested in this study. However, factors such as binder type, climate, binder type, mix design, air voids etc., may affect the aging process which are not studied here. Future research is recommended to see what happens in longer period (2, 5, 10 years and so on). (b) The ITS values of the laboratory long-term and field aged samples increase with the aging period. (c) The ITS of the short-term oven aged loose samples increases with the conditioning period; it reaches a peak and then decreases with the conditioning period. (d) The flow number decreases (brittleness increases) with the conditioning period under all kinds of aging modes.
Acknowledgements This study was funded by the New Mexico Department of Transportation (NMDOT). The authors would like to express their sincere gratitude and appreciation to Mr. Jeff Mann, Pavement Design and Management Bureau Chief at NMDOT, for being the advocate of this project and his regular support, sponsorship, and suggestions. The authors appreciate the valuable service and time of Mr. Virgil Valdez, who is the Project Manager. Special thanks go to several Project Panel members namely, James Gallegos, Materials Bureau Chief, and Parveez Anwar, State Asphalt Engineer at NMODT.
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