Expanses of new taxiway and apron are required for the planes
serving Terminal 5. Aircraft loadings are high – over 50% of planes
will be long haul aircraft, among which will be the new generation
of super jumbos, such as the Airbus A380.
During construction, naturally occurring terrace gravels were
removed over large areas of the site and incorporated into the
works elsewhere. London clay is a far weaker sub-base material than
the naturally occurring gravel subgrades on the existing Heathrow
airfield. Design and construction of the Terminal 5 aircraft
pavements on clay accordingly presented a number of
challenges.
Low subgrade strength coupled with the extremely high aircraft
traffic loads resulted in a substantial pavement depth being
required. Mott MacDonald developed an integrated design and cost
model to evaluate a variety of pavement options and obtain the best
value solution. Options included rigid pavements, flexible
pavements and ground improvement.
We found the best value solution to be a rigid pavement consisting
of pavement quality concrete (PQC) on a cement bound base. Initial
design calculations indicated that the PQC depth would be
approximately 800mm.
However, this depth was well beyond the capabilities of slip-form
paving machines at that time. Slip-forming involves extruding
concrete into a desired shape. Airports use slip-forming machines
to swiftly and precisely lay large concrete paving slabs. Our
challenge was to reduce the PQC depth to enable the slab to be
safely slip-formed in a single layer.
High strength concrete
Our design and cost model showed that by increasing the strength of
the PQC we could significantly reduce the pavement depth. An
increase in the flexural strength from 6N/mm
2 – the
standard pavement mix – to 7N/mm
2 would result in an
average reduction of 80mm. Applying this reduction to the 1 million
square metres of Terminal 5 aircraft pavements would result in
substantial cost, programme and environmental benefits. Reducing
the pavement depth also resulted in less clay having to be
transported from the main site.
To achieve a 7N/mm
2 strength concrete the design and
construction teams commenced mix development in 1999. Increased
strength was obtained by reducing the water/cement ratio, resulting
in a very dry concrete mix. Tight control of raw materials –
aggregate moisture content and grading, and cement and PFA chemical
properties – was critical to achieving consistent strength. The
Terminal 5 concrete mix had a total cementitious content of
380kg/m
3 with 30% pulverised fly ash, and contained
limestone aggregate.
All members of the supply chain were involved from an early stage
to ensure that a workable high strength mix could be achieved. A
large number of laboratory and full scale production trials were
carried out and the concrete was tested on a number of other BAA
airfield projects at Heathrow, Gatwick and Stansted prior to use at
Terminal 5.
Trials showed that to achieve success the concrete batching and
paving processes had to be integrated into a single system. A
dedicated PQC batching plant was established on site.
Thrifty design
Another innovation was the use of recycled concrete in the
pavement’s cement bound base layer. Combined with reducing slab
thickness through the use of high strength concrete, it is reckoned
that 27,000 truck movements were eliminated.
To further minimise the volume of PQC required, the pavement was
designed for the specific traffic forecast in each area. For
example, areas adjacent to the terminal buildings were designed for
aircraft tugs only and detailed forecasts were used to design each
taxiway and groups of aircraft stands. Central sections of the
stands, subject to higher loading, were of deeper construction than
the edges. This innovation reduced the PQC volume by
1,400m
3.
