Journal of Highway and Transportation Research and Development Vol.1, No. 1(2006)27

Study on Permeability Coefficient of Asphalt Mixture*

JingChen((^f:)" , Fupu Li (^S#),Jinan Shen (tfc±%)

(Research Institute of Highway of the Ministry of Communications. Beijing 100088. China)

Abstract:Premature damage may arise from excessive permeability of asphalt mixture.There was no requirement for permeability during

asphalt mixture design in the early specification. The theory of permeability of mixture is discussed, and Darcy s law is found applicable to

asphalt mixture and the falling head test is suitable for dense asphalt mixture. The relation between permeability and air voids of AC and

SMA is presented in this paper through research in laboratory. A modified permeameter is introduced. Characteristics of permeability coef-

ficient of asphalt mixture are also analyzed. A standard of coefficient of permeability suggested to be adopted during asphalt mixture design

is submitted.

KGy words: asphalt mixture; permeability

0 Introduction

It' s very important to measure the permeability of mix-

tures during the asphalt mixture design.This test can avoid

producing mixture with high permeability which may cause

water damage of asphalt pavement. There is no requirement

for the permeability of asphalt mixture during the asphalt

mixture design in the old edition of Technical Specifications

for Construction of Highway Asphalt Pavements (JTJ032-

94) .Through the research on the permeability of mixtures in

the lab, the authors propose a standard of coefficient of per-

meability when designing the coarse-graded asphalt mixtures.

1 Research on the theory of permeability

1.1 Law of permeability Henry Darcy,a French waterworks engineer,established

the fundamental theory of permeability for soils ,

q = K-i-A, (1)

where: q is rate of flow, cm /s; K is coefficient of perme-

ability, cm/s; i is hydraulic gradient; A is total cross-section-

al area, cm 0

Coefficients of permeability of some materials are pre-

sented in Table 1

Darcy' s law is applicable to medium such as silt, fine

sand and medium sand through which the flow of water is

laminar. The flow of water is turbulent when the water is

transmitted through coarse sand, gravel and pebble. Darcy' s

law is invalid in a turbulent water flow condition. Tab.l Permeability coefficients of some materials

Type of material Coefficients of permeability (cm/s)

silt 500xKr6~100xl(T5

fine sand 100xl(r5~500xl0-5

medium sand 500xl0-5~200xl(T4

coarse sand 200 xlO"4- 500xl0"4

gravel 500xl0-4~100xl0-3

pebble 100xKT3~500xi0-3

In the research, permeability of asphalt mixture is stud-

ied. The maximum allowable permeabilities suggested by

NCAT are 100 x 10"5 cm/s and 150 x 10 "5 cm/s for fine-

graded mixture and coarse-graded mixture, respectively.The

value put forward by NCAT is in the range of permeability of

fine sand shown in Table 1,which means Darcy' s law is ap-

plicable to asphalt mixture where the flow of water is lami-

nar.

1.2 Approaches and calculation of permeability coeffi-

cient of asphalt mixture

Two approaches are used to test the permeability of a

material when Darcy' s law is valid, one is constant head test

the other is falling head test.The constant head test and the

falling head test are most applicable for relatively high per-

meable materials and less permeable materials, respective-

ly. Approaches used for materials are shown in Table2.

An investigation of SUPERPAVE mixture by G. W.Ma-

upin showed that permeabilities of most mixtures are be-

tween 10" cm/s and 10" cm/s, only a few are between

Manuscript received November 6,2005

* Scpported by the Research on Preventive Techniques for Avoiding Premature Failure of Expressway (No.200131822362)

E~ mail address : j. chen @ rioh. en

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28 Journal of Highway and Transportation Research and Development

10~ cm/s and 10" cm/s.Results of the above mentioned

researches and Table2 indicate that falling head test is appli-

cable for dense - graded mixtures. Equation for calculating

the coefficient of permeability when using a constant head

test is as follows:

Tab.2 Applicable approaches for permeability of

different material

Permeability Coefficient( cm/s)

1 10"' 10"2 10"3 10"4 10"5 10"6 1(T7 1Q-8 io-9

High permeable Less permeable Not permeable

Constant head test

Falling head test

A't h2 (2)

where, K is coefficient of permeability, cm/s; a is cross area

of stand pipe, cm ; L is length of specimen, cm; A is cross

area of specimen, cm ; t is time over which head is allowed

to fall,s; /ij is water head at beginning of test,cm; h2 is wa-

ter head at end of test,cm.

The method of Permeability described in Field Test

Methods of Subgrade and Pavement for Highway Engineering

(JTJ059-95) is one kind of falling head test.This method is

simple for testing in field, equation for his method is as fol-

lows:

C,„ = to — t.

x60 (3)

Tab.3 Permeabilities test

where, Cw is coefficient of permeability of pavement, rriL/s;

Vi is volume of water of first reading, mL, usually 100 mL;

V2 is volume of water of second reading, mL, usually 500

mL; tx is time at first reading, s; t2 is time at second read-

ing, s.

1.3 Selection of Permeameter

Permeameter described in Field Test Methods of Sub-

grade and Pavement for Highway Engineering ( JTJ059-95) is

improved in the study. The diameter of the bottom is enlarged

to 22 cm, the height of the permeameter becomes 51.5 cm, a

ring with the same area as the bottom is introduced to deter-

mine the seal area.

2 Research in lab

2.1 Mixtures for test

Mixtures of AC25I, AC20I, modified AC16I, AC13I,

SMA13 and SMA16 are selected for compare. Median of gra-

dations is used for designing these mixtures. Permeability

tests were performed on specimens prepared by roller com-

pactor to determine the relationship between permeabilities

and air void content of different mixtures.

2.2 Results

Permeability coefficient was calculated by equation

3. Status of permeability of all specimens was described dur-

ing the test. Results of different mixtures are shown in Table

3.

data of R.I.O.H.inlab

AC13I

Air voids( %)

^(mL'min- )

Status of permeability

10.4 8.4 7.4 6.7 6.0

629 462 262 83 0

completely penetrative completely penetrative completely penetrative completely penetrative not penetrative

SMA13

Air voids (%)

^(mL'min-1)

Status of permeability

9.6 8.1 7.0 6.5 4.55

1 702 1 458 1 292 740 52

completely penetrative completely penetrative completely penetrative completely penetrative not penetrative

Modified AC16I

Air voids (%)

Cw(mL'min )

Status of permeability

10.2 8.3 6.0 5.0 4.0

1 605 289 0 0 0

completely penetrative completely penetrative not penetrative not penetrative not penetrative

AC25I

Air voids( %)

C„,(mL-min_1)

Status of permeability

7.1 5.0 3.8 2.7 1.9

592 140 12 24 3

completely penetrative completely penetrative not penetrative not penetrative not penetrative

AC20I

Air voids( % ) 8.4 7.8 7.6 5.6 4.6 4.9 3.7 3.0

Cw(niL'imn~ ) 650 409 410 44 14 11 7 28

Status of completely completely completely not not not not not

permeability penetrative penetrative penetrative penetrative penetrative penetrative penetrative penetrative

Air voids( %) 11.6 10.5 9.3 8.2 7.2 4.6 3.5

C^CmLTnin"1) 1949 1 846 1 435 272 117 115 9

Status of completely completely completely completely completely completely not

permeability penetrative penetrative penetrative penetrative penetrative penetrative penetrat ive

SMA16

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Jing Chen ,etal: Study on Permeability Coefficient of Asphalt Mixture 29

3 Data analysis

Permeabilities of all mixtures are proportional to air

void content.Air void content should be controlled to avoid

producing excessively permeable mixtures.A regression was

developed for each mixture for the air void content vs.pe-

rmeability . Regressions are shown in Table4 where Y is per-

meability of Cw (mL/min) and x is air void content (% ) .

Tab.4 Regressions of air void content versus Cw

of different mixtures in lab

Type of mixtures Regressions R

AC13I Y = 149.88x-881.18 0.977 5

SMA13 r=2 311.31n(*)-3 424.6 0.975

Modified AC16I Y = 79.768*2 - 897.8* + 2 408.8 0.984 6

SMA16 Y = 0.070 l*4'965 0.941 6

AC20 Y = 35.97x2 - 302.21a: + 622.63 0.995 7

AC25 Y= 33.189*2 - 188.24* + 258.37 0.999 7

As shown in Table 4, the regressions produced an R

ranging from 0.941 6 to 0.999 7, which means a good

agreement between air void content and permeability. Regre-

ssions in Table4 also show there is a unique permeability -

air void content relationship for each type of mixture. When

different mixtures have the same air void content, the larger

the size of mixture is, the more the permeability is because

distribution of air voids inside mixtures are different.

Comparing permeability coefficient K calculated by

equation2 with permeability coefficient Cw calculated by

equation3, two values of all mixtures have a good agreement

in the linear regression with an R of 0.998 7. The Figure 1

shows Cw vs. K and the regression is as equation 4.

2 500

~c 2 000

0.000 0.005 0.010 0.015 0.020 0.025 0.030 K(cm-s-1)

Fig.l C„ versus K

Cw=75 4l9xK + 4.6. (4)

Conversing the maximum allowable permeability K of

NCAT to Cw by equation 4, the maximum allowable perme-

ability Cw are 80 mL/min and 118 mlVmin for fine-graded

and coarse - graded mixtures, respectively. These values can

be taken into account when establishing the criteria of per-

meability. The plot of air void content versus permeability of

Cw is shown in Figure 2.

^2000

■| 1500

p 1 000

o ? 500

SMA13 /-"SMA16

' / Aci6I

A -V-jAC %C-~- AC13I ^<^Z

0 1 23456789 Air void content(%)

10 11 12 13

Fig.2 Air void content versus C„

In the Figure 2, when AC20 and AC25' s air void cont-

ent of is more than 5% and AC16I' s is more than 8% , Cw

will increase rapidly, while permeability of Ac 131 doesn' t

change so rapidly as other mixtures. Trend lines in Figure 2

show Cw of mixture will increase rapidly when air void cont-

ent exceeds certain value and Cw of large size mixture will

change more than that of small size.

AS shown in Figure 2, mixtures except AC25 are almost

not permeable when air void content are in the design

range.For example, the air void content requirement for AC

mixtures is 3 % through 5 % , Cw of mixtures except AC25 is

less than 120 mL/min and most Cw are zero. As shown in

Table 3, for conventional dense - graded mixtures such as

AC13I,modified AC16I and AC20I,specimens of 5 cm thick

are not penetrative when air void content is less than 6%,

which means water will not be transited into the lower pave-

ment layers. When air void content is between 7% and 8% ,

all specimens are penetrative and permeability is very

large.The specimen of 5 cm thick prepared by roller com-

pactor is not applicable for AC25 which has a nominal maxi-

mum size of 26.5 cm, this may cause excessive permeability

of AC25.For SMA13 and SMA16, Cw is less than 50 mL/

min when design air void content is 4% . Cw of SMA mix-

tures will be more than 110 mL/min if air void content is

more than 6% and C„ will also increase rapidly if air void

content exceeds 8 %, which means permeability of SMA is

more susceptive to air void content than conventional dense-

graded AC mixtures and the permeability limit to SMA

should be less than AC.

4 Limit to permeability during mixture design

Two situations are taken into account when establishing

limit to Permeability. One is for conventional dense-graded

mixtures with nominal maximum size less than 19 mm and

the other is for SMA.

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30 Journal of Highway and Transportation Research and Development

4.1 Mixtures of AC

Range of air void content and permeability is shown in

Table 5 when specimens changed from penetrative to water-

proof.

Tab.5 Specimens from waterproof to penetrative

Type of mixtures Range of air void content (%) Range of C„ (mL • min ~')

AC13I 6.0-6.7 0-83

Modified AC16I 6.0-8.3 0-289

AC20I 5.6-7.6 44-410

SMA13 4.6-6.5 52-740

SMA16 3.5-4.6 9-115

As shown in Table 5, if air void content is between 3 %

and 5% or between 4% and 6%, AC 131 and modified

AC 161 are not penetrative and AC20I changes from water-

proof to penetrative. According the results of this study and

other researches, Cw of AC13I, AC16I, and AC20I will be

less than 100 mL/min and even be zero when the design air

void content is between 3 % and 5 % . Considering actual air

void content maybe more than 6% and the limit to perme-

ability is set for the first time, the limit to Cw of AC mixtures

with nominal maximum size less than 19 mm should be 120

mL/min.

4.2 Mixtures of SMA

The design air void content of SMA is 3% through 4%

and 4.5% for heavy-duty roads, so permeability of mixtures

with 4% through 4.5% air void content will be checked. As

shown in Table 5, specimen of SMA13 is not penetrative with

a Cw of 52 mL/min when the air void content is 4.6% .Air

void content of SMA 16 is between 3.5% and 4.6% when

the specimen is not penetrative and the Cw is 9 mL/min

through 115 mL. According these results, Cw of SMA will be

less than 60 mL/min when the design air void content is

4% .Considering actual air void content maybe 4.5% and

the limit to permeability is also set for the first time, the limit

to C„, of SMA should be 80 mL/min.

5 Conclusions and recommendations

The following are the conclusions and recommendations

for the study:

(1) Darcy' s law is applicable to asphalt mixture.

(2)Falling head test is more suitable for dense-graded

mixtures than constant head test.

(3)There is a good linear regression of K in equation 2

versus Cw in equation 3.

(4) Permeability is associated with the gradation and

maximum size of mixtures.

(5) Permeability will increase rapidly when air void

content is more than 7 % or 8 % . The larger the size of mix-

ture is, the more permeable the mixture will be when air void

content increases.

(6) Maximum allowable permeabilities of Cw should be

80 mL/min and 120 mL/min for SMA and AC mixtures with

a nominal maximum size less than 19 mm, respectively.

References

[l] Hong Yu-kang, Engineering Petrology and Soil Mechanics.Be-

ijing:People s Communication Publish, 1998.

[2] G W Maupin, Jr. .Investigation of test methods, pavements, and

laboratory design related to asphalt permeability. Virginia Trans-

portation Research Council, VTRC00-R24,2000.

[3] JTJ059-95, Field Test Methods of Subgrade and Pavement for

Highway Engineering.

J. Highway Transp. Res. Dev. (English Ed.) 2006.1:27-30.

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