SubscribeAirfield pavement (runway, taxiways and aprons) designs have changed little in the last 20 to 30 years, since the publication of FAA design Guide and A Guide to Airfield Pavement Design and Evaluation (1st Edition, PSA, 1989), even though aircraft traffic at most airports has risen significantly, and the latest generation of Boeing and Airbus aircraft are much more damaging than their predecessors.
In the UK, BAA published its Pavement Design Guide for Heavy Aircraft Loading in 1993, intended to provide increased design reliability for very heavily loaded airfield pavements.
The second edition of A Guide to Airfield Pavement Design and Evaluation was published by Defence Estates in 2006 and incorporates the results of research and development work undertaken by a team from TRL and WSP.
It is possible to take advantage of the changes in the document, and to a lesser extent of the BAA design guide, to produce more economic pavement designs with a lower environmental impact.
The second edition of A Guide to Airfield Pavement Design and Evaluation includes:
- Higher strength concretes in rigid pavements.
- Higher strength hydraulically bound materials in flexible pavements.
- Crack and seat for rehabilitation of existing concrete pavements.
- Modern undercarriage configurations.
Taking advantage of these changes can result in:
- More cost effective constructions.
- Pavements better suited to modern aircraft mixes.
- Reduced environmental impact - through reductions in natural aggregate use, reduced cement and bit men use, and resulting reductions CO2 emissions, and lower traffic movements for supply of materials.
- Reductions in earthworks costs due to thinner pavements.
However, taking advantage of these changes is not necessarily straightforward. For instance, high strength concrete is relatively easy to produce by removing water and using a plasticiser or super-plasticiser. Delivering a consistent material to site, so that it can be laid to the required quality, is much more difficult.
The entire supply chain needs to be carefully considered, from initial delivery of the constituents, through mixing, and on to site transportation and the actual laying process.
Designers and contractors need to fully aware of the implications of changing from traditional concrete mixes. Both the 2nd Edition of A Guide to Airfield Pavement Design and Evaluation and the BAA design guide recognise potential weaknesses at longitudinal joints in concrete, and recommend thickening slabs or dowelling in certain situations.
An alternative to the use of steel dowels or a thickened slab is a keyed longitudinally joint, which can give significant cost savings and reductions in environmental impact by eliminating the use of steel.
Traditionally keyed joints have had a poor record, probably due to a combination of shape and underdesign.
If advantage is taken of increased pavement thickness for heavy loads, or high concrete strength, and high performance profiled joint can be designed that will replace dowels or thickening of the pavement.
In recent years a new failure mechanism has appeared in heavily loaded airfield pavements, involving spalling of longitudinal joins due to rotation of the joint caused by wheels trafficking parallel to the joint. This failure mechanism is not covered by any existing design guide, but requires consideration from first principles if it is to be avoided.
Another way of reducing cost and environmental impact is the use of local materials for bases and sub-bases in both rigid (concrete) and flexible pavements.
By using locally won materials, including materials already in situ, a stabilised solution can be designed and specified to meet the requirements of the selected design method.
Working as a sub-consultant for the pavement structural design on Heathrow Airport Terminal 5, WSP were able to utilise principles on which the BAA design guide is based together with a high performance keyed joint, consideration of joint rotation and in situ stabilised materials to produce significant reductions in pavement thickness and cost.
Much work to airfield pavements in the UK is strengthening of existing concrete pavements by overlaying with asphalt.
Traditionally this produces a maintenance problem through reflection cracking, and although various treatments have been developed they are expensive and not necessarily effective.
Initial cracking of concrete slabs, in a technique known as crack and seat, has proved effective overlaying roads.
Building on work by TRL for Defence Estates a WSP team incorporating TRL and staff from Illinois University has undertaken research for BAA demonstrating that the crack and seat method gives a structurally sound solution for very heavily loaded airfield pavements.
Figure 1: High Performance Keyed
Working for Costain WSP has used crack and seat techniques together with a Stone Mastic Asphalt surface course, to design an extremely rapid and cost-effective rehabilitation method for the concrete runway ends at Kent (Manston) International Airport.
There have been failures of new pavement constructions and associated design methods in the last 20 years that demonstrate. An example is concrete block paving - where following early claims of enormous contributions to the pavement strength it is now generally accepted that blocks add very little to airfield pavements, and have significant construction and operational problems.
In hindsight, it can be seen that there was a failure to understand the behaviour of the construction, to appreciate its effect on the research results, and to understand the implications in the context of the overall pavement system.
The key to successful implementation of advanced pavement designs is connected thinking between design, specification and construction methodology. Without a detailed understanding of the overall system, from the background to the design methods and their limitations, through the reasons for material specifications and on to the challenges of the construction process, it is easy to design solutions that cannot be constructed without parallel advances in the construction process.