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/mm² – the standard pavement mix – to 7N/mm² 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/mm² 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/m³ 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 1,400m³.