Asphalt Review - Volume 29 Number 2 (June / July 2010)

BITUMEN
STABILISATION IN
SOUTH AFRICA
To understand bitumen stabilisation technology (materials
treated with bitumen emulsion or foamed bitumen), it is
important to know the history of this technology.
BSMs have been used in South Africa for more than 30 years.
The concept of emulsion treatment came to the fore in the
1970s when a newly constructed freeway suffered a catastrophic
premature failure. The failed cemented base material was retreated
with bitumen emulsion and most of those sections are
still in a satisfactory condition today.
The following paper, prepared by DC Collings, Loudon
International, South Africa (davecol@iafrica.com), was also
featured at AAPA’s 13th International Flexible Pavements
Conference.
Condensed version - full version available from Conference
papers 2009 - aapa@asn,com.au
The advent of foamed bitumen in South Africa
During 1993, in pursuit of an alternative to bitumen emulsion,
a Joint Venture (JV) between a large construction company,
a small design firm and a foreign manufacturing company
imported a specialised 400 t/hr foamed bitumen mixing plant
from the foreign company. The mixing plant was duly established
on site in April 1994. The anticipated construction period was
four weeks.
From the outset, everything that could have gone wrong did
go wrong. On the odd occasion when foam was produced, the
resulting bitumen stabilised material allowed the 150mm thick
base layer to be successfully paved. A rudimentary method
for operating the plant was eventually developed, allowing a
satisfactory product to be mixed at an exceedingly slow rate.
Despite this, the road is still in good condition 15 years later,
in spite of the abusive loading it has received from timber
haulage.
Two major benefits did accrue from these expensive lessons:
• foamed bitumen was shown to be a viable and highly
attractive substitute for bitumen emulsion as a stabilising
agent (the product was good, the equipment was flawed);
and
• an in-depth understanding of the problems encountered
when using an expansion chamber to produce foamed
bitumen.
The new “properly engineered” system
Following this project and a short but intensive research
and development effort by the manufacturer’s engineering
design team, the prototype spraybar for producing foamed
bitumen was subjected to a series of trials in Germany in
November 1995. The system was then mounted on a newgeneration
recycler and shipped out to South Africa for
field trials. Six weeks of continuous work followed before
the Germans declared that the system was satisfactory. The
system currently manufactured by this company, some 13
years later, is exactly the same.
Bitumen stabilisation in South Africa
Within a year of completing these trials, four South African
contractors had purchased large recycling machines equipped to
apply foamed bitumen. These contractors quickly identified the
economic advantages to be gained by substituting foamed bitumen
for bitumen emulsion in tenders calling for the construction of
an emulsion treated base (ETB). As a result, several contracts
were awarded in favour of foamed bitumen, in spite of the Road
Authority’s uncertainty as to whether or not foamed bitumen was
indeed as effective as bitumen emulsion.
Subsequent to developing the full-scale system for mounting on
large recyclers, a small laboratory unit was developed to support
the design aspect of this new technology.
TG2 Second Edition and technology development
Following the successful use of foamed bitumen the emulsion
manufacturers supported the development of two documents
through SABITA. However, these imposed a number of
limitations. Subsequently a technical guidelines document
TG2 was developed and released in 2002.
Subsequently experience in South Africa and overseas
highlighted deficiencies in TG2 and work was done to review
those guidelines.
This review took five years to complete with funding
provided by SABITA and Gautrans and the new edition of
TG2 was published in May 2009. It was titled TG 2 Second
Edition, Bitumen Stabilised Materials, A Guideline for the
Design and Construction of Bitumen Emulsion and Foamed
Bitumen Stabilised Materials.
TG2 Second Edition therefore replaces Sabita’s Manuals 14
and 21 as well as the original TG2. Regardless of whether
bitumen emulsion or foamed bitumen is applied as the
stabilising agent, the same mix design and pavement design
procedures are applicable to both and the new method for
classifying BSMs takes no account of which stabilising agent
was used in the mix.
BSM CHARACTERISTICS AND
PERFORMANCE
BSMs are pavement materials that are treated with either
bitumen emulsion or foamed bitumen. The materials treated
are either those recovered from an existing pavement or fresh
materials. Granular materials, previously cement treated
materials or reclaimed asphalt (RA) layers are included.
Where an existing pavement is recycled, old seals or asphalt
surfacing is usually mixed with the underlying layer and
treated to form a new base or sub-base layer.
ROADS JUNE 2010/JULY 2010 29