5.2 Specification Types
It is possible to specify requirements in three distinctly different ways, by defining either:
- processes and products to be used, e.g. roller passes to compact base (method based specification)
- the performance/functional characteristics of a pavement as a whole without reference to the properties of the constituent part (performance specification)
- the properties of each and every constituent part ‘used’ and the required end-product state of each part (end-product, attribute specification).
Each method has the following advantages and disadvantages.
- The method based specification method allows better control (lower risk) on the final outcome, but may prevent innovation and not encourage commitment to the best outcome.
- The performance specification method has much in its favour because it encourages innovation and commitment, but there are currently considerable risks and difficulties in the definition of short-term and long-term performance characteristics of the pavement as a whole.
- The end-product specification method is the most common approach as it places no restriction on the processes which can be adopted by the road contractor or product supplier and hence provides more opportunities for innovation. However, it does place restriction on end-product requirements of the constituent parts, which can be in terms of either:
- composition (e.g. particle shape and texture, particle size distribution, Atterberg limits, etc.)
- behavioural or performance-based characteristics (e.g. skid resistance, strength, stiffness, permeability, etc.).
Many current specifications are hybrids of all three specification types, but the future is seen to be in the increased specification of performance characteristics. The following sections describe specifications for typical granular products currently used in road construction in Australia.