The
Jiangyin Bridge was recently surfaced by Anderson Asphalt
Ltd. This Suspension Bridge
is the longest in Mainland China and the fourth longest
in the world. It has a span of 1,385m with 14m wide, dual
3-lane carriageways, a 1.5m central reservation and two
2.2m wide maintenance lane/footways hung either side off
the main cross section girder. The area of the main carriageway
was approximately 41,000 sq. m, with a mastic asphalt surface
of approximately 48mm thickness. Additionally the maintenance
lane/footway of 6,000 sq. m was surfaced with mastic
asphalt of 28mm thickness.
The demanding specification for the main carriageway required
a surfacing design giving 15 years durability and accommodating
a temperature range of +70°C to -15°C. Additionally,
the composite material was to show no sign of softening at 70°C.
A three stage technical design appraisal was devised: first,
a series of laboratory tests was carried out on materials to
assess their suitability and compliance with the specification
and performance criteria. The second stage comprised testing
the completed surfacing system. This was to ascertain the compatibility
of all components, their ability to perform over a temperature
range of +70°C to -15°C, and the adhesion and compatibility
with the underlying paint system on the steel deck.
The final stage entailed assessing the optimum
design that would best meet all the stated environmental
and traffic conditions over a 15 year period by finite
element (FE) analysis of the surfaced steel deck. Based
on this programme, the surfacing mix proposal was for mastic
asphalt with an Asphaltic Cement comprised of 70% Trinidad
Lake Asphalt and 30% 60/70 pen bitumen.
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The
material was mixed in a custom designed production unit
of 20 tonne
capacity which accurately combines and mixes the bitumen,
the Trinidad Lake Asphalt (TLA) and fine limestone. The
mixer was mounted on calibrated load cells with thermostatic
temperature controls which maintain the mixture at between
170°C and 210°C. After mixing, the mastic was discharged
into 1 tonne capacity steel moulds. Once cooled and tested,
the moulds were struck and blocks of mastic transferred
to Mainland China.
On site, the mastic blocks were broken up, fed into mobile transporters and melted.
Hot dried stone was added, the temperature brought up to 200°C and the mix
transported to the bridge deck for paving. The mastic asphalt was poured in front
of a purpose made laying machine, including a spreader and automatic leveller,
which ran along the top of formers. No vibration was used. Bitumen coated stone
chippings, which enhance skid resistance, were spread immediately behind the
newly laid mastic by means of a mechanical chip spreader.
Mike Sida of Anderson Asphalt Ltd concluded, “While mastic asphalt is considered
an 'old' product, it competes with the current-state-of-the 'black-art' bituminous
surfacing products by providing a durable, hard wearing, cost-effective surface.
Once the appropriate design characteristics have been established, the material
works well. It has been found to be most effective when the asphaltic cement
content of the mix is modified by adding TLA. It has been shown that it continues
to work, virtually without attention, for more than 20 years. For the ultra-sceptical,
which unfortunately is the category most of us are compelled to fall into nowadays,
seeing that it works is believing”. |
