DIGEST 2006 - Sabita

Revised TRH3 imminent:
Developments in seal design
Gerrie van Zyl
Director
PD Naidoo & Associates (Pty) Ltd
The basic philosophy for
the design of stone seals
in South Africa was
developed by F.M. Hansen in
New Zealand during the mid
1930s. The principle applied
was to partially fill the
available voids around
aggregate particles to retain
the stone on the road under
traffic and expected climatic
conditions.
Road authorities in South Africa
applied and adjusted this
philosophy to suite their particular
conditions to great success, the
majority specifying the application
rate for a given situation, based
mainly on traffic and aggregate
size.
Initial efforts by the former
Committee of State Road
Authorities (CSRA) and the CSIR
to develop a single design
procedure for South African
conditions were not successful,
resulting in guideline documents
incorporating a compendium of
design methods.
During the early 1980s polymer
modified binders were introduced
to South Africa, adding to the
complexities of design.
Although not without difficulties,
the Committee of Local Transport
Officials (COLTO) succeeded, to a
large extent, in unifying the
various design approaches and
also in incorporating design
guidelines for modified binders in
the Technical Recommendations
for Highways (TRH3, Draft 1998).
By accepting that each road
authority’s design provides an
acceptable seal, the “Rational
Design” approach, as developed
by the CSIR and the Department
of Transport, was adjusted to
obtain a range of appropriate
binder application rates.
The adjusted “Rational Design”
determines the minimum volume
of binder to retain aggregate and
the maximum volume to provide
sufficient macro texture for skid
resistance, taking into account the
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