Analysis of Compressive Characteristics of Asphalt Mixture under Freeze-Thaw Cycles in Cold Plateau Regions

JOlUnal of Highway and Transportation Research and Development Vol. 7 .No. 4(2013) 17

Analysis of Compressive Characteristics of Asphalt Mixture under

Freeze-Thaw Cycles in Cold Plateau Regions *

SI Wei (P'lw) 1 • • , MA Biao (1:b�) " WANG Hai-nian (1£#$1j'.) "

LI Ning C$'?) 1 , HU Jian (ml][l.)2

(1. Key Laboratory of Special Area Highway Engineering of Ministry of Education. Chang'an University. Xi'an Shaanxi 710064, China;

2. Second Highway Engineering of China Communications Construction Company Ltd, Xi'an Shaanxi 710065, China)

Abstract: Low average temperature, large temperature difference, and continual freeze-thaw (F-T) cycles are the typical cli

matic characteristics of cold plateau regions. F-T cycles and the asphalt-aggregate ratio of an asphalt mixture have been analyzed

using a uniaxial compressive test under F-T cycles. The results show that the compressive strength and resilient modulus de

crease with an increase in the number of F-T cycles. The mixture performance declines sharply in the initial F-T cycles and

gradually gentle after 8 -10 F-T cycles. Further, the asphalt-aggregate ratio has a significant influence on the compressive char

acteristics of the mixture under F -T cycles: a lower asphalt-aggregate ratio leads to a better compressive performance than a high

er asphalt-aggregate ratio. A higher asphalt-aggregate ratio leads to a small loss ratio of compressive performance under F-T cy

cles. The results also reveal that F-T cycles obvious impact on the compressive properties of the mixture and that the use of as

phalt concrete (AC) -13 with an optimum asphalt content (5. 5 %) or more can improve the low-temperature compressive per

formance of the mixture under F -T cycles.

Key words: road engineering; uniaxial compression test; freeze-thaw cycles; compressive performance; asphalt mixture

1 Introduction

The Qinghai-Tibet Plateau has capricious climatic

conditions. Low average temperature, large temperature

difference, and continual freeze-thaw (F-T) cycles are

the typical climatic characteristics of this cold region.

Frequent and severe F -T cycles have a significant influ

ence on the mechanical properties, durability, and per

formance of an asphalt pavement. The distresses of as

phalt concrete (AC) are related to the deterioration of

the asphalt mixture under critical local climatic and envi

ronmental conditions[l]. Asphalt pavement is exposed to

the atmosphere and thus directly suffers from environ

mental effects and vehicle loads. This in tum leads to the

formation of a comprehensive stress (including tempera

ture stress and load stress) in the asphalt pavement.

If the stress relaxation of the asphalt pavement IS

larger than the comprehensive stress growth, there is no

obvious distress in the appearance, but the interior mi-

Manuscript received March 10, 2013

cro-damage of the pavement accumulates. When the

comprehensive stress exceeds the ultimate tensile strength

of the asphalt mixture, cracks and other evident distres

ses occur under the tensile stress and the shear stress, a

long with the wheel track and spread [2J. Moreover, water

seeps into the asphalt membrane easily and decreases the

bond force between the asphalt membrane and the aggre

gate, and once water seeps into the voids, hydrodynamic

pressure and vacuum negative pressure are generated un

der the F -T cycles. With an increase in the number of F

T cycles, the internal pores and voids expand and result

in a decrease in the load capacity. Furthermore, the mi

cro damage of the mixture gradually transfonns into

cracks and other apparent distresses[3 4J.

The distresses of AC under the F -T cycles have a

significant influence on the performance and the lifecycle

of an asphalt pavement in cold regions. The compressive

property of AC suffers from the critical impact of the F-T

cycles. However, this property is one of the most impor-

� Supported by the Road and Transport R&D Project for Western Regions of China Commissioned by Ministry of Transport (NO.

2009318000027, No. 201231879210)

� � E-mail address:[email protected]

J. Highway Transp. Res. Dev. (English Ed.) 2013.7:17-22.

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18 Journal of Highway and Transportation Research and Development

tant factors considered in pavement design as it denotes

the strength and the stiffness characteristics of the mix

ture. Therefore, the compressive property as a design pa

rameter is used for calculating the deflection of AC and

the flexural tensile stress.

At present, the impact of F -T cycles has not been

considered in the asphalt pavement design in cold re

gions, which results in some differences between the de

sign and the actual pavements. Therefore, it is consider

ably difficult to accurately evaluate the compressive prop

erty of AC under the F -T cycles. Hence, in this study,

we conduct a uniaxial compressive test of an asphalt mix

ture under F -T conditions and analyze the impact of the

F -T cycles on the compressive property of the mixture.

The results provide a good reference for asphalt mixture

design, distress prevention, and some scientific guide

lines for highway construction. Furthermore, they pro

vide some suggestions for the mixture design parameters

of the existing asphalt pavement design method.

2 Materials and experimental methods

The materials used in this laboratory study were as

follows: Asphalt, aggregates, and mineral powders. The

asphalt used was SBR-modified asphalt, a typical asphalt

used in the Qinghai-Tibet Plateau, and the aggregates

and mineral powders were obtained from limestone. The

test results of asphalt are presented in table 1.

Tab. 1 Test results of asphalt parameters

Asphalt state Test item Result

15 'C 51 Penetration (100 g,O. 1 mm)

25 'C 123. 1

Softening point (R&B) ('C ) 47. 6 Original

Ductility (em) 5'C > 150 sample

Density (gi cm3) 1. 023

Flashing point ( 't: ) >260

Solubility (% ) 99. 6

Mass loss (%) 0. 2

15 'C 65. 3 After aging Vestigital penetration ratio (% )

25 'C 58. 1

Ductility (em) 5'C > 150

The asphalt concrete used in this research is an

AC -13 mixture having a nominal maximum aggregate

size of 13 mm; gradation is recommended by the specifi-

cations of Ministry of Transport of China. The optimum

asphalt content of the asphalt mixture is obtained by u

sing the Marshall test. On the basis of the Marshall test

and considering the climate and traffic conditions of cold

regions, the final optimum asphalt content of AC-13 is

determined to be 5. 5 % .

In the ACF -T cycle test, there is no standard be

cause of the differences in the climatic conditions. Con

sidering the F -T test used across the world and the actual

climate in the Qinghai-Tibet Plateau, a modified F -T test

is proposed in this paper. The meteorological data along

the Qinghai-Tibet highway reveal that the annual mean

minimum temperature is - 14. 5 CC to - 17. 4 CC; the

annual mean maximum temperature is 6. 8 CC to 8. 1 CC ;

the positive temperature regions are from May to August,

while the lowest temperature is still below 0 CC during

these periods; and the largest temperature difference be

tween day and night reaches 23 'C to 26 'C. Conse

quently, the F -T cycle test conducted in this study in

volves the following: 30 ml of water is injected in a plas

tic bag, and each specimen is sealed once it freezes. The

freezing temperature is ( - 25 ± I ) 'C and lastsl2 h.

Then, the specimens are placed into a water bath to be

thawed at (25 ± I) 'C and lasts 12 hiS 6J• The freezer

and the water bath are used for simulating the F -T cy

cles. The sample has a cylindrical shape with a diameter

and a height of 100 mm ± 2. 0 mm and is made using a

static pressure method. The laboratory test is a uniaxial

compressive test carried out using Universal Material

Tester System (UMTS) . The loads and deformations are

automatically measured using a computer. UMTS is e

quipped with an environment chamber having an accura

cy of ( ±O. I)'C. The values of (15 ±O. 5)'C and 2

mml min are adopted as the test temperatures and the

load speed, respectively, in this research.

3 Analysis of compressive strength

The asphalt mixture having the optimum asphalt

content of 5. 5 % underv.rent several F -T cycles, and the

results of its compressive strength are shown in figures 1

and 2.

Figure 1 shows that the compressive strength of the

asphalt mixture tends to decline with an increase in the

number of F -T cycles. The variation of compressive


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SI Wei, et al: Analysis of Compressive Characteristics of Asphalt Mixture under Freeze-Thaw Cycles in Cold Plateau Regions 19

Freeze-thaw cydes (cyde)

Fig. 1 Compressive strength and F -T cycles

• .t. , y=-O.�003x2+·2.2271x-l.8!j59 ..-A . R!=0.85 dC=:��/ -=-��

8 c i;x:;/� ":1// ·········· Dmapoinl,,,jj,,[,dfromtec[

, Quadratc regression line

o 4 6 8 lO 12

Freeze-thaw cyc:es (cyc:e)

Fig. 2 Loss ratio of compressive strength and F -T cycles

strength has a parabola fonn, and the parabola function

can be used for reflecting the relationship between the F

T cycles and the compressive strength. In the initial F-T

cycles, the compressive strength decreases significantly:

After 4 F -T cycles, the decline is gradual, and after 8 F

T cycles, the decline is significant. Finally, as the num

ber of F -T cycles increases further, the degradation grad

ually stabilizes.

The internal pores and air voids of mixture increase

under the repetitive F-T cycles. Water seeps into the as

phalt membrane easily and decreases the bond force be

tween the asphalt membrane and the aggregate, which

leads to mixture loosening; this can make it easy for the

mixture to fall off. When AC is placed in an environment

with moisture, especially in a saturated condition, the air

voids will be filled with water. This water turns into ice

during the freezing cycles, causing an expansion of the

ice volume. The mixture internally generates an expan

sion force, which results in the micro-damage to the mix

ture. During the thaw cycles, with the melting of the

filled ice, the bonding between the asphalt membrane

and the aggregates weakens as the water erodes. The

skeleton role of the coarse aggregates and the bonding

force of the asphalt binders have the main contribution on

the compreSSIve strength of AC. The compreSSIve

strength decreases faster in the first few F -T cycles be-

cause of the expansion of the internal pores and the air

voids[6 7J. This indicates that the F -T cycles in the early

years have had an evident influence on the compressive

strength of the asphalt mixture.

In order to estimate the impact of the F -T cycles on

the compressive strength of the mixture, the loss ratio of

the compressive strength is investigated in this paper.

The loss ratio of the compressive strength is the difference

between the unconditioned (did not undergo F -T) and

the conditioned compressive strength divided by the un

conditioned compressive strength. The variation of the

loss ratio of the compressive strength is shown in figure

2. The loss ratio of the compressive strength increases

with an increase in the number of F -T cycles and has a

parabola fonn. The variation trend of the loss ratio of the

compressive strength corresponds to the change in the

compressive strength; after 8 F -T cycles, the decline be

comes gradual.

<2 2.2 � 2 '" f 1.8 · "

� 1.0 � S l.!J 8 4.5

.. .. ;.;. ·· 0 cyc:es ----e----:-I cycles

'- -,_ 'l(-'-" .---x" , ,

/-,,;>�-__ ¥

� -- -- x ----------. 5.0 5.5 6.0 6.5

Asph�t aggregate ratio (%)

Fig. 3 Compressive strength and

asphalt-aggregate ratio

30 -..-.. .... " --x-- 2 cycles -..� -e- 11 cycLes

� ......... _____ _tt - --41

o I................... • .................... "'... " .. c ,0', ............. , 4.5 5.0 5.5 6.0 6.5

Asphllit aggregate ratio (%)

Fig. 4 Loss ratio of compressive strength and

asphalt-aggregate ratio

Figure 3 shows that for the unconditioned mixture,

the compressive strength is large when the asphalt-aggre

gate ratio is less than the optimum asphalt-aggregate ratio

(5. 5%) and the difference between them is small. With

a continual increase in the asphalt-aggregate ratio, the

compressive strength declines sharply as compared with

that in the case of the lower asphalt-aggregate ratio. After


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14 F-T cycles, the optimum asphalt-aggregate ratio (5.

5%) corresponds to the best compressive strength, and

the decline of the compressive strength in the case of the

low asphalt-aggregate ratio is larger than that in the case

of the high asphalt-aggregate ratio. For the AC, the as

phalt-aggregate ratio is 4. 5% alter 14 F-T cycles. Com

pared to that in the unconditioned mixture, in this case,

the compressive strength declines by o. 59 MFa, which is

a decrease of 26. 5% . However, for the asphalt-aggre

gate ratio of 6. 5% , the compressive strength declined by

only o. 19 MPa, which is a decrease of 11. 3% .

Figure 4 presents the variation between the loss ratio

of the compressive strength and the asphalt-aggregate ra

tio. When AC undergoes 2 F-T cycles, the decline of the

compressive strength is obvious smaller than that after 14

F -T cycles. The lowest loss ratio of the compressive

strength appears at the optimum asphalt content

(5. 5% ) . After 14 F-T cycles, the smaller asphalt-ag

gregate ratio has a larger loss ratio of the compressive

strength; with an increase in the asphalt-aggregate ratio,

the decline becomes gradual. When the asphalt-aggregate

ratio exceeds the optimum asphalt content, the loss ratio

of the compressive strength is maintained at almost the

same level. The results indicate that the loss ratio of the

compreSSIve strength decreases at a high asphalt-aggre

gate ratio.

The asphalt-aggregate ratio is closely related to the

porosity of the mixture. The larger the asphalt-aggregate

ratio is, the smaller is the mixture porosity. The asphalt

aggregate ratio also has a significant influence on the

skeleton structure of aggregates, and the asphalt binder

plays an important role in the adhesion of the asphalt

mixture.

The air voids and the internal pores expand as the

filled water freezes in the F -T cycles. The icy water in

the pores will cause a huge expansion force, which leads

to a deterioration of the mixture's internal structure.

When the asphalt-aggregate ratio is too small, it is diffi

cult to fonn a thin asphalt membrane to bind the aggre

gate particles. The bond force between the asphalt and

the aggregate is not strong. Under the considered temper

ature gradient, loading stress, and F -T cycle conditions,

the mixture structure becomes loose, which leads to a de

crease in the compressive strength [4,8 9J. With an in-

crease in the asphalt-aggregate ratio, the structure-as

phalt forms gradually. Each particle has been packed by

asphalt and generates a cohesive force between the as

phalt and the aggregate. The cohesive force increases

with an increase in the asphalt content. The flexural

characteristics reach the peak value in the optimum as

phalt content. After several F -T cycles, the compressive

strength at the optimum asphalt content is still the best

compared to that at the other asphalt-aggregate ratios.

As the asphalt-aggregate ratio continuously increa

ses, there is no more porosity to fill the asphalt. Thus,

surplus asphalt is formed. Compared with the structured

asphalt, the surplus asphalt becomes the free asphalt and

lubricant. The aggregate particles will be "pushed away"

and slide under the loading condition. This leads to a

faster decrease in the compressive strength. However,

compared with the low asphalt-aggregate ratio, at the

high asphalt-aggregate ratio, the internal pores and air

voids decrease; this reduces the influence of ero

sion[2 3J

4 Analysis of resilient modulus

The resilient modulus of AC with the optimum as

phalt-aggregate ratio (5. 5%) suffered alter the mixture

underwent the F -T cycles. The variation of the resilient

modulus and its loss ratio are shown in figures 5 and 6 ,

respectively.

:s .... .... Quad] atlc regress.on . ne ;3 '""00 L .... .... , ::; -�/!*---"'"

� BOO

K' Data l'omts Cl'-,_cctcd itl'-l11 tcst

'-::l /' -- "-.. g , �-- ::.-.,&:=.-_ .... ..... 600! �

:5 � SOD:

o 2

)=1. 147x2-25.029x-+-76:.28 R'=0.79

4 6 B :0 Free7.e-thaw cyc:es (cyc:e oJ

Fig. 5 Resilient modulus and F -T cycles

900 x o cycles ------.- 14 cydes

300 '-----�--�------� 4.) ).0 s.) 6.0 6.5 Asphalt aggregate ratio (%)

14

Fig. 6 Resilient modulus and asphalt-aggregate ratio


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51 Wei, et al: Analysis of Compressive Characteristics of Asphalt Mixture under Freeze-Thaw Cycles in Cold Plateau Regions 21

Figure 5 shows that the resilient modulus of AC

turns decreases with an increase in the number of F -T cy

cles, and the parabolic function can be used for reflec

ting this variation trend. Like the compressive strength,

the resilient modulus declines significantly in the initial

F -T cycles and then, becomes gradual as the number of

F-T cycles increases continually. After 10 F-T cycles,

the resilient modulus becomes stable. Compared with

that in the case of the unconditioned mixture, the resili

ent modulus reduces to 152. 1 MFa, which accounts for

19. 3% .

Figure 6 shows that the variation of the resilient

modulus is significant with respect to the asphalt-aggre

gate ratio, and the resilient modulus attains the maximum

value around the optimum asphalt-aggregate ratio. In the

case of the unconditioned mixture, the resilient modulus

reaches 194. 8 MFa when the asphalt-aggregate ratio is

5. 0% and 6. 0% . After 14 F-T cycles, the difference

between them is 146. 4 MFa, and the resilient modulus is

better at the asphalt-aggregate ratio of 5. 0% than at the

ratio of 5. 5% .

The resilient modulus of AC is a function of strength

and deformation. When the mixture fails the compressive

test, the mixture's total defonnation is the sum of the

compressive of air in the mixture and the defonnation of

the mixture. The air voids vary with the changes in the

asphalt-aggregate ratio. In general, larger air voids lead

to higher defonnation. With an increase in the asphalt

aggregate ratio, the surplus asphalt appears and plays the

role of a lubricant between the aggregates, which results

in an increase in the defonnation under the load [8 9J. The resilient modulus of AC is related to the defonn

ation of the asphalt and the fine aggregates, as well as

the defonnation of the coarse aggregate skeleton. AC is

one of the porous materials; when it freezes in a saturat

ed moisture environment, the internal pores expand be

cause of the expansion force. The internal pores and

structure change with an increase in the number of F -T

cycles; this has a negative impact on the aggregate inter

lock and leads to an increase in the deformation [10 11J. Consequently, the resilient modulus of AC has a de

clining trend when the mixture undergo the F -T cycles.

Furthennore, the penetration of frozen water will destroy

the structured asphalt of the aggregates, resulting in a

decline in the cohesion between the aggregates and a de

crease in the binding.

5 Conclusions

( 1) When the asphalt mixture undergoes the F -T

cycles, the compressive strength and the resilient modu

lus decrease rapidly in the initial F -T cycles and gradual

ly after 8 -10 F-T cycles. The F-T cycles have a signifi

cant impact on the compressive properties of AC.

(2) The asphalt content has an obvious influence

on the compressive strength and the resilient modulus of

the asphalt mixture. In general, the lower the asphalt

content is, the more significant is the impact of the F -T

cycles. A low asphalt content leads to better compressive

properties than a high asphalt content. The compressive

properties exhibit the peak value at the optimum asphalt

content. The degradation of compressive properties is less

when the mixture has a higher asphalt content under the

F-T cycles.

( 3) In the case of the Qinghai -Tibet Plateau, which

has rough weather conditions, the use of AC-13 with a

high asphalt content of 5. 5 % or more is expected to im

prove the performance of the asphalt mixture at a low

temperature. Therefore, an appropriately high asphalt

content can improve the compressive properties of AC.

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Documents

Analysis of Compressive Characteristics of Asphalt Mixture under Freeze-Thaw Cycles in Cold Plateau Regions