Cold mix design based on current understanding of the breaking of bitumen emulsions

A RATIONAL APPROACH BASED ON THE CURRENT UNDERSTANDING OF

THE BREAKING OF BITUMEN EMULSIONS

Didier Lesueur & Juan José Potti

COLD MIX DESIGN:

EATA 2004 – Nottingham – 6th July 2004

Introduction

› Cold mixes are Environmentally Friendly Construction Technologies

reduce energy spending (no aggregate drying) lower particle and fume emissions

› But still used in a narrow application range mostly on secondary roads

France ~ 200 kt slow setting emulsions for ~ 2 500 kt of bitumen for hot mixes

› Why is that so? In part because they are thought to be highly technical and

therefore risky

Hot and cold mixes

› Hot Mix aggregates bitumen air voids

› Cold Mix aggregates bitumen air voids water

COLD MIX The emulsion is not the final binder Water at a time-dependent amount Proportion not sufficient: Is the bitumen in the form of droplets?


Final density difficult to measure and control


Final density difficult to measure and control

Depends on the breaking process

COLD MIX The emulsion is not the final binder

HOT MIX

QUALITY CONTROL

Quality of constituents

Proportion of constituents

Final density will ensure good performance

WORKABILITY Depends on temperature

Hot and cold mixes (2)

Hot and Cold Mixes (3)

› 1st Consequence: Cold mixes need specific testing methods to measure what is usually not taken into account for hot mixes

› 2nd Consequence: Workability is controlled by the breaking of the emulsion

How do we do that?

The breaking of bitumen emulsions

› The stability of emulsions (from Schubert, AIChE 2000)

-500

-250

0

250

500

0 5 10 15 20interparticle distance (nm)

inte

ract

ion

ener

gy (k

T)

ATTRACTION

-500

-250

0

250

500


inte

ract

ion

ener

gy (k

T)

REPULSION

ATTRACTION

The breaking of bitumen emulsions (2)

-500

-250

0

250

500


inte

ract

ion

ener

gy (k

T)

REPULSION

ATTRACTION


› The Van der Waals attraction is inevitable and essentially independant of bitumen origin

› The electrostatic repulsion can be tuned by: The choice of emulsifier The ionic environment

-100

-50

0

50

100

0 2 4 6 8 10 12 14

pH

zeta

pot

entia

l (m

V)


STABLE

UNSTABLE: BREAKING


› The breaking scenario of an unstable bitumen emulsion (after Bonakdar et al., Coll. Surf. A176,2001)

t=0 t=1heure t=2 heures 10 m10 m10 m


› The breaking can be induced by the aggregates

Ca2+Mg2+

K+Na+

Cl-SO4-

0

500

1000

1500

2000

2500

conc

entra

tion

(mg/

l)

local water - pH = 7,7GE - pH = 7,1


› Consequences: Two possible scenarios when an emulsion is mixed with aggregates

› 1: The ionic environment is not modified and the emulsion remains essentially stable

› 2: The ionic environment is highly modified and the emulsion becomes unstable


No emulsion destabilisation (non-reactive aggregates)

Emulsion destabilisation (reactive aggregates): gelling

Evaporation kinetics

(days)

A. FILM FORMING

Contraction kinetics (min)

B. GEL CONTRACTION

Designing cold mixes

› Consequences: The mix design needs to take into account the breaking kinetics

› Also, emulsifier choice will guarantee good coating

› In all cases, the mix design needs to give a reasonable estimate of the long term properties

› Water content is not indicative of the breaking of the emulsion (except when almost all is gone)

› Inside the mix, breaking may not be homogeneous

Designing cold mixes (2)

› The mix design shall confirm that the emulsion breaking will permit laying down and compaction

› It is not necessary to use complicated laboratory tests on the emulsion as long as the corresponding property can be directly measured on the mix

How can we measure it on the mix?



› Adapting the emulsion reactivity through workability (after Eckmann et al., RGRA 802, 2002)

NOT WORKABLE

curing time (hours)


0

10

20

30

40

50

60

70

1 10 100 1000

number of rotations

tota

l vol

ume

(hei

ght i

n m

m)

watertotal porosity


› Need to take into account the presence of water

onset of water drainage

~ 10 %

~ 2 %


› There exists appropriate tools to estimate workability (laying down)

› Not clear yet how compaction can be predicted at the laboratory scale

› Long term mechanical properties can be measured with current means (curing procedure 14 days, 35ºC, 20 % RH representative of 1-3 years in the field - see Serfass et al., RILEM 2003)

› What about short term? Fatigue?


Conclusion: Key Messages

› Cold mixes are not that technical: We (almost) know what´s going on

› Still, they sometimes need specific testing methods (compaction, curing)

› Time to standardize the practices standardize the accepted practices do the necessary pre-normative research develop a reliable design procedure allow the road network managers to evaluate the product on a

sound basis

Engineering

Cold mix design based on current understanding of the breaking of bitumen emulsions