Long-term Field Performance of WMA Pavements

Wednesday, October 25, 2017

1:00-2:30PM ET

TRANSPORTATION RESEARCH BOARD

The Transportation Research Board has met the standards and requirements of

the Registered Continuing Education Providers Program. Credit earned on

completion of this program will be reported to RCEP. A certificate of completion

will be issued to participants that have registered and attended the entire session.

As such, it does not include content that may be deemed or construed to be an

approval or endorsement by RCEP.

Purpose Discuss NCHRP Research Report 843. Learning Objectives

At the end of this webinar, you will be able to: • Compare field performance of WMA with control HMA pavements in

transverse cracking, longitudinal cracking, rutting, and moisture susceptibility

• Identify significant material property determinants that have both engineering and statistical impacts on the long-term field performance of HMA and WMA pavements

• Identify laboratory tests that can be integrated into mix design and evaluation to improve the long term performance of asphalt pavements

• Describe the critical roles of other facts, such as pavement structure and climate, on long term performance of asphalt pavements

NCHRP Research Report 843: Long-Term Field Performance of Warm Mix Asphalt Technologies NCHRP Project 09-49A

NCHRP is a State-Driven Program

– Suggest research of national interest

– Serve on oversight panels that guide the research.

• Administered by TRB in cooperation with the Federal Highway Administration.

• Sponsored by individual state DOTs who

Practical, ready-to-use results • Applied research aimed at

state DOT practitioners • Often become AASHTO

standards, specifications, guides, syntheses

• Can be applied in planning, design, construction, operations, maintenance, safety, environment

Today’s Speakers

• Shihui Shen, Pennsylvania State University • Shenghua Wu, University of South Alabama • Weiguang Zhang, Southeast University, China

TRB Webinar Long-Term Field Performance of Warm Mix

Asphalt Pavements NCHRP 9-49A Project

Research Team

Washington State University Pennsylvania State University

Louisiana State University, Louisiana Transportation Research Institute

October 25, 2017

Long-term Field Performance Comparison between HMA and WMA

Shihui Shen Ph.D., Associate Professor Department of Engineering

Pennsylvania State University, Altoona szs20@psu.edu

Phone: (814) 940-3326

• Introduction & Objectives

• Project Scope and Research Approach

• Results of Field Performances • Volumetrics

• Transverse Cracking

• Wheel-path Longitudinal Cracking

• Rutting & Moisture Susceptibility

2

Outline

Introduction

3

• Rapid growth in the use of WMA • Limited research on long-term performance of WMA

• How WMA compares with HMA in terms of specific performance

• What are the critical material properties that could have significant impact on the long-term performance of WMA

• Better understanding of WMA technologies for full implementation Long-term

Performance

Research Objectives

• Investigate the long-term field performance of WMA:

- Transverse cracking

- Wheel-path longitudinal cracking

- Rutting and moisture damage

• Identify the material and engineering properties of WMA pavements that are significant determinants of their long-term field performance

4

• Introduction & Objectives

• Project Scope & Research Approach

• Results

• Volumetrics

• Transverse Cracking

• Wheel-path Longitudinal Cracking

• Rutting & Moisture Susceptibility

5

Outline

Project Scope

6

• 28 pavement projects o 22 in-service projects +1 HVS = 40 HMA-WMA pairs 4-9 years pavement age

o 5 newly constructed in 2011/2012 = 8 HMA-WMA pairs

Climate Zone

In-serv. New Total

D/F 5 1 6

D/N F 3 1 4

W/F 9 1 10

W/N F 6 2 8

28

7

0

5

10

15

20

Organic Chemical Foaming

WMA Type

17

0

2

4

6

8

10

12

Climate Zone

Dry Freeze Wet Freeze Dry No-Freeze Wet No-Freeze

6 4

No.

of P

roje

cts

No.

of P

roje

cts

0

2

4

6

8

10

12

(4,5) (5,7) (7, 9)

Pavement Age

10

0

5

10

15

20

25

Pavement Type

Flexible PCC or Cement Stabilized Base

No.

of P

roje

cts

No.

of P

roje

cts

12

18 10 8

11

7

20

8

Scope: Project Distribution

8

0

5

10

15

20

< 3 million > = 3 million

Traffic (ESALs)

18

10

02468

101214

Yes No N/A

Use of Anti-stripping

12 12

4

02468

10121416

Yes No N/A

Use of RAP

10

No.

of P

roje

cts

No.

of P

roje

cts

No.

of P

roje

cts

14

4

Scope: Project Distribution

Research Approach

9

Selection of WMA Candidate Projects

Field Characterization (Field cores, plant mix)

Laboratory Characterization

(cores, recovered binders)

Distress Survey: Transverse and longitudinal cracking,

rutting

FWD test

Volumetrics: aggregate gradation, AC, Gmm, Gmb

Material properties: binder PG, MSCR, IDT, Hamburg, etc.

Material Characterization: Mixture

10

Mixture Test

IDT Dynamic Modulus/Creep

Compliance

Fatigue- IDT Fracture at

Room Temp

Thermal Cracking-IDT Fracture at Low

Temp

Rutting /Moisture Hamburg

Testing Conditions

Temp.: −4, 14, 32, 50, 68, 86ºF; Frequency: 20, 10, 5, 1, 0.1, 0.01 Hz Duration: 100 seconds

Temp.: 68ºF Loading rate: 2 in./min

Temp.: 14ºF Loading rate: 0.1 in./min

Temp.: 122ºF Wet condition

Material Properties

Dynamic modulus; Creep compliance

IDT strength; Fracture work density; Vertical failure deformation Horizontal failure strain

IDT strength; Fracture work density; Vertical failure deformation; Horizontal failure strain

Rut depth; Stripping inflection point (SIP); Cycles

References/Standards Wen et al. 2002 AASHTO T322 Wen 2012 AASHTO T324

Volumetric property check: • Gradation • AC content • Gmm and Gmb • Air voids • VMA, VFA • Film thickness

Material Characterization: Asphalt Binder

11

Binder Test PGs Rutting: MSCR

Fatigue: Monotonic at Room Temp

Thermal Cracking: Monotonic at Low

Temp

Testing Conditions

Different temp depending on the test (DSR, BBR)

Stress: 0.1, 3.2kPa Temp.: 98% Reliability from LTPP Bind

Temp.: 68ºF Shear strain rate: 0.3 s-1

Temp.: 41ºF Shear strain rate: 0.01s-1

Material Properties

PG; BBR stiffness; m-value

Jnr0.1, Jnr3.2; R0.1, R3.2

Maximum stress; Fracture energy; Failure strain

Maximum stress; Fracture energy; Failure strain

References/Standards

AASHTO MP1/T240/T313

AASHTO T350 Wen et al. 2010 Wen 2012

Research Approach

12

Lab Test Results Field Test Results

Field Characterization: Two-Rounds of Distress Survey Transverse Cracking

Reflective Surface-initiated

13

14

Field Characterization: Two-Rounds of Distress Survey Wheel-path Longitudinal Cracking

Observation: • 2 to 4 feet away from the

shoulder or centerline. • Field cores indicate top-down

fatigue cracks.

15

Field Characterization: Two-Rounds of Distress Survey Rut Depth

Other Data Collection Activities

16

• Pre-construction o mix design, existing pavement structure,

location, existing pavement distress, etc.

• Construction information o plant modification, mix/compaction temp., in-

place density, raw materials.

• Post-construction o QA/QC data, procurement of cores, FWD, etc.

• Introduction & Objectives

• Project Scope and Research Approach

• Results • Volumetrics

• Transverse Cracking • Wheel-path Longitudinal Cracking • Rutting & Moisture Susceptibility

17

Outline

In-place Volumetric Properties HMA vs. WMA

18

• Asphalt content: almost identical

• Air voids of field cores: WMA > HMA

• Introduction & Objectives

• Project Scope and Research Approach

• Results • Volumetrics

• Transverse Cracking • Wheel-path Longitudinal Cracking • Rutting & Moisture Susceptibility

19

Outline

Field Transverse Cracking HMA vs. WMA, 2014/2015 (4-9 years)

20

Field Transverse Cracking HMA vs. WMA, Y2014/2015 (4-9 years)

21

0

5

10

15

20

H > W H = W H < W

6 31

2

No. of Pairs

Effect of Aging on Transverse

Cracking

22

• Introduction & Objectives

• Project Scope and Research Approach

• Results • Volumetrics • Transverse Cracking • Wheel-path Longitudinal Cracking • Rutting & Moisture Susceptibility

23

Outline

Field Wheel-path Longitudinal Cracking Y2013/2015 (4-9 years)

24

Field Wheel-path Longitudinal Cracking HMA vs. WMA, Y2014/2015 (4-9 years)

25

0

5

10

15

20

H > W H = W H < W

1 39

4

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7 8 9 10

Whe

el-p

ath

Long

itudi

nal C

rack

Le

ngth

, ft/

200f

t

Pavement Age, years

Wheel-path Longitudinal Cracking Effect of Pavement Age

26

HMA WMA

• Introduction & Objectives

• Project Scope and Research Aproach

• Results • Volumetrics • Transverse Cracking • Wheel-path Longitudinal Cracking • Rutting & Moisture Susceptibility

27

Outline

Field Rutting Performance Y2014/2015 (4-9 years)

28

Field Rutting Performance HMA vs. WMA, Y2014/2015 (4-9 years)

29

• 1/16” , as a practical criterion to determine if any significant difference between the HMA and WMA pavements.

0

5

10

15

20

H > W H = W H < W

3 39

1

No. of Pairs

Field Rut Depth Effect of Pavement Age

30

00.05

0.10.15

0.20.25

0.30.35

0.40.45

0 1 2 3 4 5 6 7 8 9 10

Seco

nd-R

ound

Rut

Dep

th, i

n.

Pavement Age, years

TN SR 125 WA SR 12

HMA WMA

Moisture Susceptibility

31

• Moisture damage was not observed • Both WMA and HMA pavements • Consistent with NCHRP 9-49 findings

• Use SIP to evaluate field cores

Moisture Susceptibility Lab Hamburg Wheel Track Test Results

Projects Red : No anti-strip agent Black : W/ anti-strip agent Green : Info not available 32

33

Relationship between Rutting and Cracking Performance

Dry Freeze Climatic Zone

Rutting Dominates

34

Not Conclusive

Relationship between Rutting and Cracking Performance

Dry Non-Freeze Climatic Zone

35

Cracking Dominates

Relationship between Rutting and Cracking Performance

Wet Freeze Climatic Zone

36

Cracking & Rutting

Relationship between Rutting and Cracking Performance

Wet Non-freeze Climatic Zone

Significant Determinants of Long-term Field Performance

Shenghua Wu Ph.D., Assistant Professor

Dept. of Civil, Coastal, and Environmental Engineering University of South Alabama, Mobile, AL

shenghuawu@southalabama.edu Phone: (251) 460-6174

Objectives

• Identify the material and engineering properties of WMA pavements that are significant determinants of their long-term field performance

- Paired ranking method

- Statistical Partial Least Square (PLS) Method

- Binary Logistic (BL) Regression

38

Methodology: Statistics Based Methods

39

• Paired Ranking o Ranking of variables and field performance; o Comparison between variable and performance

ranking, find significant determinants.

• Partial Least Square (PLS) o Solve collinearity issue; o Works well for small sample size; o Crack propagation model, significant determinants.

• Binary Logistic (BL) o Probabilistic-based; o Analyzing data with two categorical responses; o Crack initiation model, significant determinants.

Significant Determinants for Transverse Crack: Potential Variables Considered

40

Predictor Variables Unit Range IDT strength, 14°F Mpa 2.17-5.56 Creep compliance (D1), -4°F 1/Gpa 0.04-0.16 Creep compliance (D2), 14°F 1/Gpa 0.05-0.18 Creep compliance (D3), 32°F 1/Gpa 0.07-0.71 m-value, creep compliance N/A 0.11-0.46 Fracture work density, 14°F Mpa 0.02-0.13 Air voids % 1.8-9.1 Binder low PG °C -6.5 to -28.4 Binder stiffness, BBR Pa 21.5-414.2 m-value, BBR, -6°C N/A 0.23-0.48 Asphalt content % 4.2-10.0 Effective binder content % 6.7-14.4 Percent Passing # 4 Sieve % 45-82.2 Percent Passing # 8 Sieve % 24.9-57.6 Percentage Passing #200 sieve % 2.8-11.9 NMAS mm 9.5-12.5 Pavement age year 2-7 Low temperature hour hour 0.1-1269.8 HMA thickness inch 2.0-14.3 Overlay thickness inch 1.0-6.0 AADTT N/A 10-3380

Material

Climate Structure Traffic

Significant Determinants for Transverse Cracking (Based on 2nd Round Results)

41

24 25 22 23

0

5

10

15

20

25

30

35

No.

of P

airs

Positive Negative

Mix E* (14°F)

Mix work density (14°F)

BBR m-value (10.4°F)

Mix horizontal failure strain (68°F)

Transverse Crack Propagation Model

42

Y = e 8.825−11.665X1−9.587X2+0.0033X3−0.267X4−1.047X5+0.0006X6 Y = transverse crack length, ft/200 ft; e = base of natural logarithm; X1 = fracture work density (14°F), Mpa; X2 = creep compliance (32°F), 1/GPa; X3 = average yearly low-temperature hour, hr (from AASHTOWare Pavement ME Design); X4 = percentage passing No. 200 sieve, %; X5 = overlay thickness, in.; X6 = AADTT.

(a) Predicted vs. Measured (b) Model Validation

Transverse Crack Initiation Model

43

𝑃𝑃� 𝑦𝑦𝑖𝑖 = 1 = 11+𝑒𝑒− −8.42+0.0019X7−0.284X8+1.52X9+0.867X10

where 𝑃𝑃� = probability of initiation of transverse crack, %; yi = ith pavement section; e = base of natural logarithm; X7 = average yearly low-temperature hour, hr (from AASHTOWare Pavement ME Design); X8 = percentage passing No. 200 sieve, %; X9 = IDT strength (14°F), MPa; and X10 = pavement age, year.

(a) Predicted vs. Measured (b) Model Validation

Significant Determinants for Transverse Cracking

44

1st Round Ranking Mix Fracture Work Density, 14°F, (-) Mix E*, 14°F, (+) Mix IDT Strength, 68°F, (+) Binder BBR Stiffness, 14°F, (+) Mix Vertical Failure Deformation, 68°F, (-) Mix Horizontal Failure Strain, 68°F, (-) 2nd Round Ranking Mix Fracture Work Density, 14°F, (-) Mix E*, 14°F, (+) Binder BBR m-value, 14°F, (-) Mix Vertical Failure Deformation, 68°F, (-) Statistical PLS Model, Crack Propagation Creep Compliance, 32°F, (-) Average Yearly Low-temperature Hour, (+) AADTT, (+) Overlay Thickness, in., (-) Percentage Passing No. 200 Sieve, %, (-) Mix Fracture Work Density, 14°F, (-) Statistical PLS Model, Crack Initiation Pavement Age, (+) IDT Strength, 14°F, (+) Average Yearly Low-temperature Hour, (+) Percentage Passing No. 200 Sieve, (-)

Significant Determinants for Longitudinal Cracking: Potential Variables Considered

45

Predictor Variables Unit Range IDT strength, 68°F MPa 1.2-3.9 Work density, 68°F MPa 0.05-0.21 Dynamic Modulus, 70°F, 1Hz MPa 706.3-8448.1 Mixture Fracture Energy, 68°C MPa 0.002-0.095 Air Voids % 1.8-9.1 Percent Passing # 4 Sieve % 45-82.2 Percent Passing # 8 Sieve % 24.9-57.6 Percent Passing #200 Sieve % 2.8-11.9 NMAS mm 9.5-12.5 G*sin(phase angle) kPa 666.3-5170.6 Binder Fracture Energy MPa 0.5-15.5 Effective Binder Content % 6.7-14.4 Total Pavement Thickness mm 50.8-406.4 Overlay Thickness mm 19.3-152.4 AADTT N/A 5-450000 Pavement age year 0.05-7

Material

Structure Traffic

Significant Determinants for Longitudinal Cracking (Based on 2nd Round Results)

46

15 13

17 17

0

4

8

12

16

20

24

No.

of P

airs

Positive Negative

Mix IDT strength(68°F)

Mix creep compliance(68°F)Mix horizontal failurestrain (68°F)Mix vertical failuredeformation (68°F)

Longitudinal Crack Propagation Model

47 (a) Predicted vs. Measured (b) Model Validation

Y = 𝑒𝑒(−0.73402+0.00874𝑋𝑋1+2.01589𝑋𝑋2+0.39481𝑋𝑋3+0.00149𝑋𝑋4−1.31441𝑋𝑋5)

where Y = field longitudinal crack, ft/200 ft; e = base of natural logarithm; X1 = total pavement thickness, mm; X2 = IDT strength (68oF), MPa; X3 = in-place air voids, %; X4 = AADTT; and X5 = percentage passing No. 200 sieve, %.

Longitudinal Crack Initiation Model

48 (a) Predicted vs. Measured (b) Model Validation

𝑷𝑷� 𝒚𝒚𝒊𝒊 = 𝟏𝟏 = 𝟏𝟏𝟏𝟏+𝒆𝒆−(𝟑𝟑.𝟓𝟓𝟏𝟏+𝟎𝟎.𝟎𝟎𝟎𝟎𝟑𝟑𝑿𝑿6−𝟎𝟎.𝟏𝟏𝟎𝟎𝟓𝟓𝑿𝑿7−𝟎𝟎.𝟎𝟎𝟓𝟓𝟎𝟎𝟎𝟎𝑿𝑿8−𝟐𝟐𝟐𝟐.𝟐𝟐𝑿𝑿9+𝟎𝟎.𝟐𝟐𝟏𝟏𝟓𝟓𝑿𝑿10)

where P = probability of initiation of top-down crack, %; X6 = pavement age, year; X7 = percentage passing No. 200 sieve, %; X8 = overlay thickness, mm; X9 = fracture work density (68°F), MPa; X10 = binder fracture energy (68°F), MPa; and yi = ith pavement section.

Significant Determinants for Longitudinal Cracking

49

1st Round Ranking Mix Horizontal Failure Strain, 68°F, (-) Mix Vertical Failure Deformation, 68°F, (-) Mix IDT Strength, 68°F, (+) Mix Creep Compliance, 14°F, (-) 2nd Round Ranking Mix Horizontal Failure Strain, 68°F, (-) Mix Vertical Failure Deformation, 68°F, (-) Mix IDT Strength, 68°F, (+) Mix Creep Compliance, 14°F, (-) Statistical PLS Model, Crack Propagation Total Pavement Thickness, (+) Percentage Passing No. 200 Sieve, (-) In-place Air Voids, (+) Mix IDT Strength, 68°F, (+) AADTT, (+) Statistical PLS Model, Crack Initiation Pavement Age, (+) Binder Fracture Energy, 68°F, (+) Overlay Thickness, (-) Mix Fracture Work Density, 68°F, (-) Percentage Passing No. 200 Sieve, (-)

Significant Determinants for Rut Depth: Potential Variables Considered

50

Variable Unit Range RRI NA 769.8-18718 In-place air voids % 1.8-9.1 Asphalt film thickness μm 10.5-33.3 Effective binder content % 7.5-13.8 High temperature performance grade (PG) °C 60.3-96.6 MSCR Jnr0.1 kPa-1 0.03-1.47 MSCR Jnr3.2 kPa-1 0.03-1.62 MSCR Jnrdiff % 1.12-47.6 MSCR R0.1 % 4.6-69.4 MSCR R3.2 % 2.8-68.3 Mixture dynamic modulus, 30°C, 0.1Hz MPa 186-2009 Percent Passing # 4 Sieve % 45-82.2 Percent Passing # 8 Sieve % 24.9-57.6 Percentage Passing # 200 sieve % 2.8-11.9 NMAS mm 9.5-12.5 Pavement age month 49-110 AADTT NA 5-57000 Overlay thickness mm 19-76 Total HMA thickness mm 51-363 Mixing temperature °C 107-168 Accumulated hour of air temperature>25°C hour 87-10944

Material

Climate Structure Traffic

Significant Determinants for Rut Depth (Based on 2nd Round Results)

51

24 24

34 34

23

0

5

10

15

20

25

30

35

40

No.

of P

airs

Positive Negative

Jnr0.1

Jnr3.2

Hamburg RuttingResistance Index

Binder high temp PG

Mix E* (86°F)

Rutting Resistance Index (RRI)

52

Hamburg Wheel-Tracking Test AASHTO T324 (50˚C)

RRI = N × (1 - RD) N = number of passes at completion of test RD = rut depth at completion of test, inch.

RRI 2554 5243 2976

Rut Depth Predictive Model

53

Y =− 0.489776 + 0.119943 ln X1 − 0.062497ln (X2) + 0.018386ln (X3) +0.043839ln (X4) + 0.086717ln (X5)

Where,

Y = field-measured rut depth, inch; X1 = pavement age, month; X2 = RRI; X3 = AADTT; X4 = total HMA thickness, mm; and X5 = overlay thickness, mm.

(a) Predicted vs. Measured (b) Model Validation

Significant Determinants for Rut Depth

54

2nd Round Ranking Mix Rutting Resistance Index, (-) Binder High PG, (-) Jnr0.1, Jnr3.2, (+) Mix E*, (-) Mix Creep Compliance, (+) Statistical Model Mix Rutting Resistance Index, (-) Pavement Age, (+) AADTT, (+) Total HMA Thickness, (+) Overlay HMA Thickness, (+)

Summary and Findings

Weiguang Zhang Ph.D., Associate Professor School of Transportation

Southeast University, Nanjing, China wgzhang@seu.edu.cn

Phone: (86)13912956989

• Transverse cracks were found to initiate from the top surface of the pavement, but often overlapped with transverse cracks in existing asphalt layer Transverse cracking could be a combination of thermal

and reflective cracking.

• Transverse cracking performance between HMA and WMA Comparable for the majority of HMA and WMA

pavements Mostly seen in pavements with four or more years of

age

Findings: Transverse Cracking

56

• Mixture’s fracture work density obtained from IDT testing at 14°F is recommended as a significant determinant; The Higher fracture work density, the less transverse cracking.

• Other indicators: mix E* (14°F), mix IDT strength (68°F),

mix creep compliance (32°F) and mix vertical failure deformation (68°F) values.

Findings: Transverse Cracking

57

• Cracks were found to initiate from surface of pavement May be indicative of top-down fatigue cracking.

• Performance comparison between HMA and WMA Comparable for the majority of HMA and WMA

pavements Cracks start to develop mostly at age of 4 years; more

cracking is seen with 6+ years.

Findings: Wheel-path Longitudinal Cracking

58

• The mixture’s IDT strength at 68°F is recommended as the significant determinant. The relatively lower IDT strength, the less longitudinal cracking.

• Other indicators: the mix vertical failure deformation

(68°F) and horizontal failure strain (68°F) from IDT test, and mix creep compliance (14°F) values.

Findings: Wheel-path Longitudinal Cracking

59

Findings: Rutting & Moisture Susceptibility

• Rutting performance between HMA and WMA pavements is mainly comparable.

• Field rut depth starts to build up as early as 3 years; and becomes more differentiable (more than 0.1”) with 6 or more years service.

• Based on field investigation, no moisture-related distress was found in both HMA and WMA pavements.

• Based on laboratory HWT test results, most of mixes without an anti-stripping agent exhibited SIPs The use of anti-stripping agent may be beneficial overall for both

HMA and WMA mixtures. 60

Findings: Rutting & Moisture Susceptibility

• RRI value from Hamburg wheel tracking test is recommended as significant determinant for rutting performance. Higher RRI, lower rut depth in the field.

• Other indicators: mix E* (86°F), mix creep compliance (86°F), binder PG, and percent recovery.

61

• Reduced production temperature of WMA can have an influence on field density. The in-place density of WMA is generally lower than HMA

pavements

• Distress distribution appears to be climatic related. Within dry zones (freeze or non-freeze), rutting appears to be

the major type of distress, regardless of HMA or WMA Within wet freeze zones, cracking is the dominant distress

type Within wet non-freeze zones, cracking and rutting can both

happen. U

Findings: Overall

62

Effect of moisture on cracking!!.

Acknowledgements • NCHRP 9-49A project

• Dr. Ed Harrigan and panel members

• Field work partners, Braun Intertec. Inc & Bloom Companies, LLC

• Statisticians from PSU and WSU

• FHWA mobile lab, State DOTs and local agencies

• Past NCHRP WMA project teams, universities, graduate students, ……

63

Thank You! Any questions?

64

Today’s Participants

• Ed Harrigan, Transportation Research Board, eharriga@nas.edu

• Shihui Shen, Pennsylvania State University, szs20@psu.edu

• Shengua Wu, University of South Alabama, shenghuawu@southalabama.edu

• Weiguang Zhang, Southeast University, China, wgzhang@seu.edu.cn

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