BALANCED MIX DESIGN APPROACHTravis Patton, P.E.

Flexible Pavements

Initial Thoughts

Dry mixes are prevalent in our industry.

Restrictive specifications

and general notes are not

long term solutions.Lowering N-design

and raising lab molded

densities will only

help so much.

Continuing to increase binder

replacement without addressing mix

performance is not sustainable.

We need to increase our

understanding of the factors

which drive mix performance to

help us better optimize our mixes

2

We prescribe material qualities and characteristics, but don’t always get

the performance we are after

Current State of Affairs

3

“Recipe” driven volumetric designs are

based on history and what we think works

What constitutes a good cake? Is it the individual steps to make it, or is it the end product?

What defines a good cake? It Tastes Good

What defines a good mix? Lasting Performance

LET’S STOP USING THE RECIPE TO DETERMINE IF THE CAKE IS GOOD

What’s more important; cracking resistance or rutting resistance?

Can I have both?

Economics or performance?

Have we placed too many restrictions trying to prescribe what “good hot mix” should look like on paper?

Why not specify performance?

Current State of Affairs

4

Eliminate the restrictions and focus on

PERFORMANCE

What is a Balanced Mix Design?

Optimize

o Make the best or most effective use of

(a situation, opportunity, or resource)

Balance

o Being in proper arrangement or

adjustment, proportion

Balanced Mix Design

o Optimize the mix to provide needed performance and balance

between stability and durability.

5

Optimized Mix Design Approach – Framework

Material Selection

Aggregates

Binder, Modifiers & Additives

Recycled Materials

Optimize JMF

Gradation

Binder Content

Volumetrics

Mix Performance Evaluation

Stability

Durability

Adjustments

Workability

Mixing

Compaction

Segregation

6

Optimized Mix Design Approach – Framework

Mix Performance Evaluation

Stability

Durability

Indicator of cracking resistance Indicator of

rutting resistance

7

Balanced Mix Design Approach - PG 64-22

y = 2E-07x12.635

R2 = 0.9829

y = 698467x-2.4408

R2 = 0.9505

0

5000

10000

15000

20000

25000

4.0 4.5 5.0 5.5 6.0 6.5 7.0

Asphalt Content (%)

Nu

mb

er

of

Pa

ss

es

(1

2.5

mm

Ru

t D

ep

th)

0

500

1000

1500

2000

2500

3000

3500

4000

4500

OT

Nu

mb

er o

f Cy

cle

s

HWT

OT

5.3

5.4

300

5.7

5.4

8

OT Data Analysis

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Cri

tic

al F

rac

ture

En

erg

y l

b-i

n/i

n^

2

Crack Progression Rate

Averages

PFC (6)

SMA-C (16)

SMA-D (45)

SMA-F (5)

SMAR-F (9)

SP-C (36)

SP-D (6)

TOM-C (53)

TOM-F (3)

Type D (15)

9

Example

10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 0.25 0.5 0.75

Cri

tica

l F

ract

ure

En

ergy, lb

s-in

./in

.2

Crack Progression Rate

• SuperPave Type D

• PG 70-22

• Limestone Aggregate

SAC B

Stripping Inflection Point

Mix Performance Space Diagram

From: Dr. Bill Buttlar, University of Illinois

0

2

4

6

8

10

12

14

0 200 400 600 800 1000 1200

Ham

bu

rg R

ut

Dep

th (

mm

)

DC(T) Fracture Energy (J/m2)

SOFT MIX

(reflective crack

control)

SUPER MIX

(SMA, high

traffic surface

mixes)

POOR MIX

(non-surface, low

traffic or temporary

use only)

STIFF MIX

(bottom layers of

full depth

pavements)

11

Balanced Mix Design Approach – Limestone Rock Asphalt

12

0

2

4

6

8

10

12

14

16

18

20

22

24

26

30 35 40 45 50 55

Can

tab

ro L

oss

, %

Hveem StabilityType I D Fine

Specification Development

13

There is a lot of work at the national

level already in progress towards

utilizing this type of approach

Some form of a balanced mix design framework is probably in our future

Specification Goals:• Relax constraints

• Design by specifying end performance

HWT OT

Skid

IFI

MSCR

ΔTc

VMA

NCAT Track

14

NCAT Track

15

One section designed traditionally. The other section

designed using BMD.

District Implementation

16

WICHITA

FALLS PARIS

ATLANTA

TYLER

DALLAS

LUFKIN

FORT

WORTH

WACO

BRYAN

BEAUMONT

HOUSTON

YOAKUM

CORPUS

CHRISTI

PHARR

LAREDO

SAN

ANTONIO

AUSTIN

BROWNWOOD

SAN

ANGELO

EL PASO

ODESSA

LUBBOCK

AMARILLO

ABILENE

CHILDRESS

Have not use BMD

Used BMD