Degradation evaluation index of asphalt pavement based on mechanical performance of asphalt mixture

Construction and Building Materials 140 (2017) 75–81

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Degradation evaluation index of asphalt pavement based on mechanical performance of asphalt mixture Ying Gao a,⇑, Dawei Geng b, Xiaoming Huang a, Guoqiang Li c a

School of Transportation, Southeast University, 2# Sipailou, Nanjing, Jiangsu 210096, China Qixia Department of Transportation, 11# Qixia Ave, Nanjing, Jiangsu 210000, China c Department of Mechanical & Industrial Engineering, Louisiana State University, Baton Rouge, LA 70803, USA b

h i g h l i g h t s
Evaluate the degradation of pavement structure with properties of asphalt mixtures.
Evaluate the combined effect of climate and traffic load on asphalt pavement.
Develop a degradation index to show the aging degree of asphalt mixtures.

a r t i c l e

i n f o

Article history: Received 19 October 2016 Received in revised form 26 January 2017 Accepted 19 February 2017 Available online 28 February 2017 Keywords: Asphalt pavement Degradation evaluation index Indirect tensile test Elastic modulus Degradation Index

a b s t r a c t Performance of asphalt pavement degrades after opening to traffic. The degradation of pavement structure is difficult to be evaluated because of the difficulties in in-situ testing and large scattering of testing results. Mechanical properties of asphalt mixtures were tested in this study to indirectly evaluate the degradation of pavement structure. Firstly, different testing methods were discussed from specimen preparation to testing results simulation of pavement structures. Indirect tensile test was finally chosen because of its easy testing process, clear stress state, stable testing results and convenience in preparation of field cored specimen. Secondly, asphalt mixtures with the same material and gradation as that of Nanjing Airport Expressway asphalt pavement were aged from 2 h to 24 h to simulate pavement at different aging period and tested. All testing results are sensitive to aging when the aging time is short and keep relatively stable when the aging time is longer. Elastic modulus of asphalt mixtures is more sensitive to aging time than that of strength and modulus at failure. Degradation Index (Id) was defined as the ratio of elastic modulus of original asphalt mixture to that of aged mixture which shows the aging degree of asphalt mixtures. Thirdly, fatigue tests were conducted on mixtures of different aging time to determine their fatigue life. Indirect tensile tests were performed on specimens with 20%–80% of fatigue life and different aging time to check the combined effect of climate and traffic loading. Id was modified based on the testing results. Ó 2017 Elsevier Ltd. All rights reserved.

1. Background Performance of asphalt pavement degrades with the effect of traffic loads and surrounding climate. Maintenance or rehabilitation is required depending on the serviceability of pavement. According to Highway Performance Assessment Standard in China, the assessment of asphalt pavement performance includes assessing the pavement surface condition, riding quality, rutting depth, skidding resistance, and pavement structure strength [1]. The pavement surface condition is estimated by calculating the ratio ⇑ Corresponding author. E-mail address: [email protected] (Y. Gao). http://dx.doi.org/10.1016/j.conbuildmat.2017.02.095 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

of pavement area with distress, which include all types of cracking, corrugation and shoving, rutting, pot holes, bleeding, raveling, and patching to area of pavement surface with and without distress. Riding quality is represented by international roughness index. Skidding resistance is calculated through side-way force coefficient. Pavement structure strength is represented by the ratio of designed deflection to in-situ measured deflection, which is also known as the bearing capability of pavement. Normally the evaluation index of pavement serviceability for maintenance focus on the characteristics related to driving safety such as the pavement surface condition, riding quality, rutting depth, and skidding resistance [2–4].These four aspects of pavement performance are very easy tobe measured with automatic test equipment. Serviceability

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evaluation index for rehabilitation is closely related to the bearing capability of pavement, which is also one of the key input parameters for pavement rehabilitation design method. Benkelman beam deflectometer, falling weight deflectometer and plate bearing tests are often used in field to evaluate the bearing capability of pavement structures [5–8]. Traffic speed deflectometers are increasingly being used to measure pavement deflections caused by a moving load at a range of driving speeds [9–11]. However the testing results of deflectometer or bearing plate represent the strength of the whole pavement structures from the subgrade, base to surface layers, but not limited to the layers of interest. Furthermore, the data of deflectometer and bearing plate are quite scattered. One of our field studies showed that the variation coefficients of deflectometer data for 5 repeated testing points at a distance within 3meters are as high as 25%, with the maximum one being 63% [12].Pavement structural strength is affected by and closely related to the mechanical properties of the pavement materials and the interfacial adhesion of layers. Interlayer damages of asphalt pavement are usually caused by moisture and play an important role in the damage of pavement material [13,14].Therefore, the bearing capability of each layer can be obtained through performing proper mechanical tests on the pavement material. The purpose of this study is to evaluate the structure strength degradation of asphalt pavement through conducting tests on asphalt pavement mixtures of different aging and fatigue period.

2. Selection of testing method The degradation of pavement structure is actually caused by the gradually accumulated damage of pavement materials under the combined action of traffic load, humidity, light and temperature in a long time period [15–19].Strength, stiffness and remaining life of asphalt mixtures decrease with time, which result in the lower bearing capability and degradation of pavement structure. Pavement fails once the damage in asphalt mixture reaches an unacceptable level. It is necessary to evaluate the strength, stiffness and residual fatigue life of existing asphalt pavement mixtures in order to evaluate the degradation of asphalt pavement. There are many testing methods used for mechanical properties evaluation of asphalt mixtures. Several rules must be obeyed to choose the proper testing method and evaluation index, which aresummarized as following: (1) The testing results should have good reproducibility and repeatability. (2) Mechanical properties obtained from the test should be the required input parameters in asphalt pavement design method. (3) The stress state of the specimen should be as simple as possible. (4) The stress and strain in the specimen should be easily measured. (5) Testing conditions such as equipment and experimental environment should not be too harsh. (6) Specimen should be easily cored from existing pavement. Micro cracks occur and develop in asphalt mixtures under repeated load which cause the degradation of material properties. Popular tests used to show these material properties in laboratory are direct or indirect tensile test, flexural beam test and compression test. Uniaxial compression test is a kind of old and matured test which shows the compressive characteristics of pavement materials. Specimens are easy to be fabricated in laboratory or be cored in field. Test is easy to perform and the equipment is available in most

laboratories. Modulus determined through static compression loading test is one of the input parameters of current Chinese asphalt pavement design method. The forthcoming revised Chinese asphalt pavement design method turns to the use dynamic modulus and phase angle, which will be determined through repeated compression loading test. Indirect tensile test is also an old and extensively used test to support the asphalt structure design and evaluate mixture properties. The indirect tensile strength is used to evaluate the fatigue stress at the bottom of continuous layers in the current Chinese asphalt design method. Most importantly, the cores obtained from existing pavement, even with thin layer, can be tested directly in the laboratory with this test method. Flexural beam test is widely used to evaluate the fatigue characteristics of asphalt mixtures. Test is easy to perform and the equipment is available in most laboratories. However, the specimens for this test are cut from large size square slabs which are quite difficult to be cored in the field. Direct tensile test shows clear stress distribution in asphalt mixture. But the test is hard to perform and the specimens for the test are as difficult to be prepared in-situ as that of flexural beam test. Some tests cannot be chosen to evaluate the properties of asphalt mixture because the size of cored specimens cannot meet the testing requirement. Specimens for uniaxial compression test should be 100 mm in diameterand150 mmin height [20]. It is better to keep the ratio of height to diameter greater than 1.5 and the diameter of specimen as large as4 times of that of the largest aggregate to avoid the size effect. Normally the thickness of each asphalt pavement layer is around 40 mm to 80 mm and the corresponding diameter of a single largest aggregate is about 16 mm to 30 mm. It is thus a challenge to core a satisfactory specimen for compression test for each asphalt layer. Indirect tensile test was finally chosen to evaluate the properties of asphalt mixture because of its easy testing process, clear stress state, stable testing results and ease in obtaining field cored specimens.

3. Specimen preparation Nanjing Airport Expressway was under maintenance during the study. Mixture used in this study was the same as that of Nanjing Airport Expressway. Specimens were cored from the Airport Expressway to validate the testing results in the laboratory. Part of cored specimens are shown in Fig. 1. The cylindrical cores are used for uniaxial compression test directly or cut along the interface of two layers and used for indirect tensile test. The slab specimens are cut into beams and used for fatigue tests. Gradation of the dense asphalt mixture with 19 mm nominal maximum aggregate size (AC20) used in the study is shown in Table 1. 70# base binder (performance grade PG64-22) was used in the mixture. Basic tested properties of the binder are given in Table 2. The asphalt mixture was designed according to the Marshall mixture design method. The optimal binder content of the mixture was 4.6%. Performance of asphalt mixture decreases with time because of the effect of weather and traffic load which cause the aging of asphalt binder, fatigue of asphalt mixture, permanent deformation on pavement and other distress. The aging and fatigue performance of asphalt mixtures were studied to simulate the structure strength degradation of in field pavement. All asphalt mixtures were put in the oven to be aged before molding a specimen. The temperature of the oven is set at 163 °C to accelerate the aging of asphalt mixtures. The aging time should be long enough to expose mixtures to different aging conditions. According to the

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Fig. 1. Cored specimens at one section.

Table 1 Gradation of asphalt mixture. Sieve size (mm)

26.5

19.0

16.0

13.2

9.5

4.75

2.36

1.18

0.6

0.3

0.15

0.075

Pass percent (%) Specification limits (%)

100 100

93.9 90–100

85.2 78–92

75.1 62–80

59.6 50–72

42.1 26–56

35 16–44

25 12–33

18 8–24

11 5–17

5.1 4–13

4 3–7

Table 2 Basic properties of the binder. Testing item

Testing results

Penetration (25 °C, 100 g, 5 S) (0.1 mm) Ductility (5 cm/min,15 °C) (cm) Softening point (B&R) (°C) Flash point(COC) (°C) Solubility (chlorylene) (%) Density at 15 °C (g/cm3) Wax content (%) Dynamic viscosity at 60 °C (Pa.s)

67 >150 49.0 322 99.94 1.036 1.5 193

research done before, stiffness of the asphalt change slowly when the aging time reaches 20 h [21]. The upper aging hour was set as 24 h in this study. Other aging hours were set as 2 h，4 h，8 h and 16 h, respectively to represent asphalt mixtures at different aging period. Fatigue tests were conducted on asphalt mixtures after aging.

4. Testing results 4.1. Effects of aging on asphalt modulus Indirect tensile tests were conducted on asphalt mixtures at different aging conditions. The testing temperature was 15 °C and the loading speed was 50 mm/min according the Chinese Standard Test Method of Bitumen and Bituminous Mixture for Highway Engineering [20]. The specimen before the test and after damage are shown in Fig. 2. The testing results aresummarized in Table 3. Table 3 shows that indirect the tensile strength, elastic modulus and modulus at failure of asphalt mixtures increase with the aging time. This result agrees with the previous studies about the performance of asphalt mixtures affected by aging time [22,23]. Asphalt mixture is composed of aggregate and binder. It is believed that the performance of the aggregates keeps unchanged with aging time, whereas the properties of the binder are sensitive to aging time. Some small molecules in the oil group evaporate at high temperature. Some molecules in the saturate and aromatic groups in the binder oxidize under high temperature aging. The content of resin and asphaltenes in the binder increases with aging time, which increases the viscosity and adhesion of binder. This leads to the improvement in load resistance of asphalt mixture, as shown in

Table 3 the increase in strength and modulus. Aging of the binder may decrease the cracking resistance of mixture. In order to evaluate the sensitivity of each testing results to aging time, a Disparity Ratio (DR) was defined as the ratio of testing results difference between aged and original mixture to testing results of the original mixture as shown in Fig. 3. Fig. 3 shows that all DR indexes increase quickly when the aging time is short (<16 h) while keep relatively stable when the aging time is long (>16 h).The elastic modulus of the asphalt mixture increases more quickly than strength and modulus at failure. Tensile strength and modulus at failure represent the mechanical properties of the asphalt mixture at the failure point which the testing results are more scatter than that of elastic modulus as shown in Table 3. While the elastic modulus represents the mechanical property of the asphalt mixture at its elastic state which is consistence with the stress condition of in field material under traffic load. The elastic modulus of mixtures is more sensitive to aging time than that of strength and modulus at failure as shown in Fig. 3.Table 3 shows that the testing results of elastic modulus have less scattering than that of strength and modulus at failure. Cored specimens from Nanjing Airport Expressway were tested with the same test method to verify the testing results of laboratory made specimen. This expressway was completed and opened to traffic in 1997. Part of the primary lane and passing lane of the expressway was rehabilitated in 2007. Pavement structure of the expressway is 4 cm SMA13 + 6 cm AC20 + 8 cm AC25 (SMA13 is stone matrix asphalt with 13 mm nominal maximum aggregate size. AC25 is dense gradation asphalt concrete with 26.5 mm nominal maximum aggregate size.). Fifteen specimens were cored at each position with different materials, lanes or construction times and tested with indirect tensile test. Testing results of the cored AC20 which have the same gradation as that fabricated in the laboratory are given in Table 4. Table 4 indicates that the modulus at failure has much larger coefficient of variation than that of strength and elastic modulus. Though the strength of the cored specimens have the lowest CV, the average value of strength at different service time periods corresponding to different aging times are too closed to be distinguished. The CVs of the elastic modulus are a little larger than that of strength but are still lower than 10%, which is quite acceptable for in-situ tests. Specimens cored from the pavement opened

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Fig. 2. Specimen during and after the indirect tensile test.

Table 3 Indirect tensile testing results. Aging time (h)

0 2 4 8 16 24 1 2 3 4 5

Strength (MPa)1

Elastic modulus (MPa)4 2

3

Modulus at failure5 (MPa)

Number of samples

Average

SD

CV(%)

Average

SD

CV(%)

Average

SD

CV(%)

1.640 1.829 2.001 2.162 2.314 2.389

0.142 0.154 0.151 0.112 0.170 0.138

8.7 8.4 7.5 5.2 7.3 5.8

588 715 876 953 1044 1075

22 37 44 47 39 56

3.7 5.2 5.0 4.9 3.7 5.2

301 325 389 460 509 517

23 31 33 41 38 35

7.6 9.5 8.5 8.9 7.5 6.8

5 5 5 5 5 5

Strength means the indirect tensile strength at the maximum load. SD means standard deviation. CV means coefficient of variation. Elastic Modulus means the tangent modulus of the mixtures at the linear range of the stress- strain curve, when the load is around 10%–40% of the maximum load. Modulus at failure means the secant modulus at the peak strength point.

to traffic in 1997 are 10 years older than specimens cored from the pavement opened to traffic in 2007. Elastic moduli of specimens in

the year of 1997 are about 20% larger than that of specimens in 2007.This result agrees with the testing tendency of specimens

DR of Elastic Modulus

DR of Tensile Strength

100.00% 80.00% 60.00% 40.00% 20.00%

100.00% 80.00% 60.00% 40.00% 20.00% 0.00%

0.00% 0

2

4

8

16

24

0

2

4

Aging time/h

100.00% 80.00% 60.00% 40.00% 20.00% 0

2

4

8

Aging time/h (c) Modulus at Failure

16

24

(b) Elastic Modulus DR of Testing Results/%

DR of Modulus at Failure

(a) Tensile Strength

0.00%

8

Aging time/h

16

90 80 70 60 50 40 30 20 10 0 0

2

4

8

16

24

Aging time/h

24

Strength

Elastic Modulus

Modulus at Failure

(d) Indirect Tensile Test Results

Fig. 3. Disparity ratio of indirect tensile testing results vs aging time.

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Y. Gao et al. / Construction and Building Materials 140 (2017) 75–81 Table 4 Testing results of cored specimens. Lane

Strength (MPa) Average

1997

Primary lane Passing lane Shoulder

2.320 2.339 2.314

2007

Primary lane Passing lane

2.289 2.402

Elastic modulus (MPa)

Modulus at failure (MPa)

CV (%)

Average

CV (%)

Average

CV (%)

3.2 2.6 2.2

1062 1005 1052

5.8 8.2 3.6

598 576 662

15.1 10.8 8.9

5 5 5

2.1 3.2

855 801

4.6 7.2

492 431

22.4 14.9

5 5

prepared in the laboratory that the elastic modulus increases with aging time. In this study, elastic modulus was chosen as a performance evaluation index of pavement material because it has low scattering and high sensitivity to aging time. TheDegradation Index (Id) was defined as the ratio of the elastic modulus of the original asphalt mixture to that of the aged mixture. Testing results of Id are shown in Fig. 4.

Id ¼

Seo Se

Number of samples

100000

Fagure Life

Open Time

10000

ð1Þ 1000

where Seo is Elastic modulus of the original asphalt mixture; Se is elastic modulus of the aged asphalt mixture. Id decreases with the increase of aging time. The decrease rate become slower as the aging time becomes longer. Id of the mixture at 16 h aging time decreases by 9% than that of 8 h, while the decrease of Id is 3% between the mixtures of 24 h and 16 h aging time. This means that aging has a limited influence on the performance of asphalt mixtures. It is believed that 20 h conditioned aging in the laboratory represents the upper aging limit in the field. ThenIdhas a lower bound which is 0.547. 4.2. Effect of aging and traffic load Performance degradation of asphalt mixtures is quite complicated under the co-work of climate and traffic load. Idshows the effect of aging. It is better if Idcould also show the effect of traffic load. For this purpose, the fatigue performance of asphalt mixtures at different aging time were studied in this paper. The stress controlled indirect tensile fatigue test was conducted on asphalt mixtures of different aging period. The load was set as half sine wave mode with the maximum stress of 0.7 MPa and frequency of 10 Hz [20].The test temperature is 15 °C. Six samples were tested for asphalt mixtures of each aging time. Average fatigue life of the asphalt mixtures at different aging time are shown in Fig. 5. Fig. 5 shows that the fatigue life of asphalt mixtures increases with aging time. The reason maybe that the modulus of the mix-

1 0.9

Id

0.8 0.7 0.6 0.5 0.4

0

2

4

6

8

10

12

14

16

18

20

Aging me /h Fig. 4. Id of asphalt mixtures at different aging time.

22

24

0

2

4

6

8

10

12

14

16

18

20

22

24

Aging me /h Fig. 5. Fatigue life of asphalt mixtures.

tures increases with aging time, which makes the mixtures become stronger and have longer life in stress controlled fatigue test. Indirect tensile tests were conducted to determine the elastic modulus of asphalt mixtures at different aging time and different period of fatigue life. First, the mixtures were aged from 2 h to 24 h to obtain mixtures of different aging conditions. Then fatigue tests were conducted and stopped at 20%–80% of the average fatigue life to simulate mixtures with different loading conditions. Indirect tensile tests were performed on specimens of different aging and loading conditions to evaluate the co-effect of climate and load. Testing results are summarized in Table 5 and Fig. 6, respectively. Table 5 and Fig. 6 shows that the elastic modulus of asphalt mixtures increases first then decreases with the increase of loading repetitions at the same aging condition. The increase in modulus at the beginning period may be caused by the further compaction of the mixture. The decrease in modulus after 20% of fatigue life shows the internal damage of the specimens caused by fatigue loading. Asphalt pavement shows the same tendency as pavement materials that prepared in the laboratory, where the performance of the pavement increases at the first one or two years after opening to traffic and decreases gradually with the serving time. Elastic modulus of the asphalt mixtures changes depending on both the load repetition and aging condition. However, its sensitivity to these two factors are quite different. Aging time shows larger effect on the elastic modulus of the asphalt mixtures. The elastic modulus increases more than 60% when the aging time increases from 2 h to 24 h, while it changes less than 15% when the loading repetitions change from 0% to 80% of the fatigue life. Normalizing the data in Table 5 based on the elastic modulus of the specimens without fatigue (zero load repetition) is given in Table 6. In Table 6, the elastic moduli of asphalt mixtures without traffic loading (0% fatigue life) were set as the baseline value at each different aging time. The elastic moduli of mixtures at different period of fatigue life were divided by the baseline value, obtaining the normalized elastic moduli. The normalized elastic moduli of the asphalt mixtures at 0 h, 4 h and 24 h aging time decrease less than 10% when their loading repetition increase from 0% to 80% of fati-

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Table 5 Elastic modulus of asphalt mixtures. Period of Fatigue life (%)

Elastic modulus with different aging time (MPa) 0h

2h

4h

8h

16 h

24 h

0 20 40 60 80

608 628 600 612 529

717 699 694 667 614

901 968 895 885 823

970 950 913 866 857

1045 1102 1022 900 923

1090 1105 1087 1010 1004

Fig. 6. Elastic modulus of asphalt mixtures at different aging and loadingperiod.

Table 6 Elastic modulus normalization. Period of fatigue life (%)

0 20 40 60 80

Aging time

Average

0h

2h

4h

8h

16 h

24 h

1.000 1.069 1.021 1.042 0.901

1.000 0.976 0.968 0.931 0.856

1.000 0.931 1.007 1.030 1.095

1.000 0.979 0.941 0.893 0.884

1.000 1.055 0.978 0.861 0.883

1.000 1.014 0.997 0.927 0.921

gue life. The biggest change of normalized elastic moduli at different period of fatigue life is 14.4%, which occurs to the asphalt mixture with 2 h aging time. It can be concluded from Table 6 that the elastic moduli of asphalt mixtures change slowly with fatigue life. This means that the degradation index, which is affected by both fatigue loading and aging, can be simplified to represent the effect of aging only. A comprehensive coefficient, a, representing the effect of fatigue loading, is thus obtained by averaging all the normalized elastic modulus data in Table 6, which gives a = 0.97. The degradation index, which is simplified by considering the effect of traffic loading by a constant a, becomes:

I0d ¼ a

Seo Se

ð2Þ

The modified I0d provides a simple way to determine the effect odf aging on the pavement degradation by performing indirect tensile test with cored samples.

1 1.004 0.985 0.947 0.923

5. Summary and conclusions Load bearing capability evaluation of asphalt pavement is quite difficult due to the difficulties in conducting the tests and the large scattering of the testing results. Mechanical properties of asphalt mixtures were characterized to indirectly evaluate the degradation of pavement structure strength. The main conclusions are as following: 1. Various widely used test methods were compared and discussed to evaluate the mechanical characteristics of asphalt mixtures. Indirect tensile test was finally chosen because of its easy testing process, clear stress state, stable testing results and ease in obtaining in-situ cored specimen. 2. Elastic modulus of asphalt mixtures determined from indirect tensile test was chosen as a performance evaluation index of pavement material because it has low scattering and high sensitivity to aging time.

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3. TheDegradation Index (Id) was defined as the ratio of the elastic modulus of the original asphalt mixture to that of the aged mixture, which shows the degree of aging in asphalt mixtures. 4. Indirect tensile tests were conducted on asphalt mixtures of different aging time and fatigue period to evaluate the combined effect of climate and traffic load. 5. It is found that the effect of fatigue loading on elastic modulus of asphalt mixtures is not as large as that of aging. Therefore, the effect of fatigue loading was represented by a constant. The degradation index was modified and simplified to only represent the effect of aging on the pavement degradation. 6. Pavement structures are quite diverse with different asphalt mixtures. Further studies need to be conducted on different type of asphalt mixtures to evaluate the degradation of pavement performance.

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