WPI

New Approach for Determining Permeability of

Hot Mix Asphalt

Worcester Polytechnic Institute (WPI)

University of Massachusetts at Dartmouth

WPI

Topics

FLaboratory permeability

FUse of a different approach

FField permeameter

FRecommendations

WPI

Why the concern?

0

10

20

30

40

50

60

70

80

90

100

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4

Sieve Size

Per

cen

t P

assi

ng

0.075 0.15 0.3 0.6 1.18 2.36 4.75 9.5 12.5 19

0

10

20

30

40

50

60

70

80

90

100

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6

Sieve Size

Per

cent

Pas

sing

0.075 0.15 0.3 0.6 1.18 2.36 4.75 9.5 12.5 19 25

More interconnectedvoids in coarse graded mixes

WPI

Objective

FDetermine a test procedure for evaluation of permeability of HMA

WPI

Use of Laboratory Permeability Test

FFalling Head Permeameter used

FSamples saturated before testing

FCoefficient of permeability is determined, assuming Darcy’s law is valid

WPI

Main Objectives of Mix Design

FWhat do we actually want to know?– Is the pavement going to let in too much

water?

FAt the construction air voids, is the pavement going to let in too much water?

FFor the gradation I have, what is the desirable construction air voids such that the pavement does not let in too much water?

WPI

Approach

FDetermine the amount of interconnected air voids present in the mix

F Interconnected air voids are actually water accessible air voids

FDetermine the amount of water accessible air voids in an accelerated fashion.

WPI

Concept of Porosity

FPorosity is defined as F the % air void in the compacted sample

that is accessible to water. FA sample is vacuum sealed inside a bag

and a density, ρ1 is calculated by using the method outlined in ASTM T130-01.

FThe same sample while under water is opened and a second density, ρ2 is determined.

WPI

Concept of Porosity

F% Porosity =

Fρ1= the vacuum sealed density of compacted sample

Fρ2= density of the vacuum sealed sample after opening under water

X100?2

?1?2%P

−=

WPI

Permeability and Porosity

FWater flows through accessible voids or pore spaces in a pavement.

FHence, the rate of flow must be related to the amount of water accessible voids, or porosity, in some way.

FTherefore, the permeability or coefficient of permeability must be a function of porosity.

WPI

y = 2E-05e0.5417x

R2 = 0.55

y = 3.66E-05e0.44x

R2 = 0.80

0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090.0100.0110.0120.0130.0140.0150.0160.0170.0180.0190.0200.0210.0220.0230.0240.0250.0260.0270.028

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Porosity, VTM, %

Co

effi

cien

t o

f P

erm

eab

ility

, cm

/s

VTM SSD Porosity Expon. (VTM SSD) Expon. (Porosity)

9.5 mm and 12.5 mm NMAS

WPI

Interpretation of data

FThe critical porosity corresponding to a critical permeability can be determined.

FAs recommended by Florida DOT researchers, considering a critical permeability of 10-3 cm/second,

- a critical porosity of 7 percent is determined.

WPI

Interpretation of Data

FTherefore, mix design samples can be checked for permeability potential

F by conducting porosity tests on samples compacted to construction voids (as determined by SSD method),

Fand a maximum allowable porosity value of 7 percent can be used.

WPI

Can we get an indication of porosity before we start?

FPorosity has been defined as total amount of water accessible voids in a mix.

FHow can one estimate the porosity from knowledge of mix gradation and air voids?

FMultiple regression analysis conducted with porosity, air voids and aggregate gradation data.

WPI

683

.897

.804

.7981.406

CountNum. Missing

|R|R Squared

Adjusted R SquaredRMS Residual

Regression SummaryPorosity vs. 2 IndependentsRow exclusion: perm #1_12.5_9.5.svd

2 526.694 263.347 133.281 <.0001

65 128.432 1.976

67 655.126

DF Sum of Squares Mean Square F-Value P-ValueRegression

Residual

Total

ANOVA TablePorosity vs. 2 IndependentsRow exclusion: perm #1_12.5_9.5.svd

3.472 1.225 3.472 2.834 .00611.260 .080 .861 15.686 <.0001

-.127 .028 -.251 -4.575 <.0001

Coefficient Std. Error Std. Coeff. t-Value P-Value

InterceptAir Voids, SSD

PP2.36

Regression CoefficientsPorosity vs. 2 IndependentsRow exclusion: perm #1_12.5_9.5.svd

-4

-3

-2

-1

0

1

2

3

4

Res

idua

l Por

osity

2 4 6 8 10 12 14 16Fitted Porosity

Residuals vs. FittedRow exclusion: perm #1_12.5_9.5.svd

Porosity = 3.47 + 12.6 * VTM – 0.127 * PP 2.36

WPI

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

0 1 2 3 4 5 6 7 8 9 10 11 12

Voids in Total Mix, %

Po

rosi

ty, %

25 PP 2.36 30 PP 2.36 35 PP 2.36 40 PP 2.36 45 PP 2.36

WPI

Critial air voids for mixes with different gradationsPercent passing the 2.36 mm sieve Critical Air Voids

25 530 635 6.540 745 7.5

WPI

Note

– For a mix with approximately 45 percent passing the 2.36 mm sieve

– A porosity of 7 percent corresponds to VTM of 7 percent

– Most of experiences from the past (in the pre-Superpave era) has been with mixes with 40-45 percent passing the 2.36 mm sieve

WPI

Note

– A density of 93 percent of TMD or VTM of 7 percent has often been recommended and used, without any significant permeability problem.

– Therefore, the selection of 7 percent porosity as a critical porosity seems to be justified.

WPI

0

1

2

3

4

5

6

7

8

9

10

11

20 25 30 35 40 45 50

Percent Passing the 2.36 mm Sieve

Po

rosi

ty, %

5 % VTM 6 % VTM 7 % VTM 8 % VTM

WPI

List of critical percent passing the 2.36 mm sieve for specific air voidsAir Voids Allowable percent

passing the 2.36 mm sieve5 > 256 > 317 > 418 > 45

WPI

Permeability or Porosity?

FWhich parameter should be used during mix design to prevent mixes from being excessively permeable –

F Is it permeability test or is it porosity test?

FMeasuring permeability is a more direct approach.

FPorosity shows a very good relationship with permeability

WPI

Permeability or Porosity?

FPorosity is a good mix design parameter candidate

FArgument becomes stronger when one considers the coefficient of variation of the permeability and the porosity tests.

FThe average CV for permeability testing is about three times the average CV of porosity.

WPI

Permeability or Porosity?

FHence, as a regular test procedure, because of better repeatability,

FPorosity seems to be more appropriate than permeability.

WPI

What about field permeability?

FCan we use it as a quality control tool?

FCan we use it for getting a true picture of flow through porous media?

FCan we use it to determine permeability in different directions?

WPI

Field Permeameter

14 cm19.0 cm

16 cm

1.5 cm

PlexiglasStandpipes(3 tiers)

19.0 cm

3.0 cm

2.0 cmFlexibleBase

Plastic Base to holdweights

Plastic base to holdstandpipes

8.9 cm hole

3.8 cm

8.9 cm

WPI

Field Testing

15 cm 15 cm

Testing for eachlocation

Core fromeachlocation

PressurizedMembraneon side ofsample

Head of water

Top layer

New Field Permeameter

New Permeameter

FSame equipment for lab and field test

FBasically falling head or constant head test

FCuts down variability of results from using different equipment in lab and field

PROTOTYPE

Mobile Cart F LaptopF On-board water

pressure tank (20 gal, 30 psi precharge)

F Core rigF Valve assemblyF Permeameter in

foreground

NEW PERMEAMETER

GeneralF Laptop PCMCIA data

acquisitionF Differential/gauge

pressure for head pressure, LVDT for volume change

FieldF 4” ID, 5” OD ring

cored 3” Deep (100mm x 63.5mm)

LabF 6” diameter by 4”

tall specimen (150mm x 100mm)

PROPOSED PROCEDURE (Field)

F Core & remove a ½” ring 3” deepF Set permeameter in grooveF Saturate, charge actuator, & apply confining

pressure equal to or higher than max head pressure to be applied

F Start data acquisitionF Open valve to apply head pressureF Stop data acquisition when actuator is full

downF Bleed pressures, remove permeameter &

patch “moat”

PROPOSED PROCEDURE (Lab)

FSame as field except without the coring and patching

FTest will need to be done over a floor or sink drain

FConfining cell for lab is interchangeable with smaller field cell via quick disconnects

4” CORE BARREL INSIDE 5” CORE BARREL

VALVE ASSEMBLY

Tank Relief

Tank Fill (standard

hose fitting)

Core barrel water

(standard hose)

High pressure regulator (saturation, actuator charge &

confining)

Actuator charge / confining selector

Precision low pressure regulator

(head pressure)

Drain valves

PERMEAMETER ASSEMBLY

F Upper box assembly has electronics, actuator, and LVDT

F Lower permeameter cell

TOP VIEW OF PERMEAMETER ASSEMBLY

WPI

Recommendations

FUse porosity as an indicator of mix permeability

FEstimate porosity and use estimated porosity to select gradation or desirable construction air voids

FConduct porosity test on mix design samples at construction air voids

WPI

Comments

Questions

Suggestions