The costs of various forms of two-stage construction are compared theoretically with those of single-stage designs of the same overall life. The assessment of the lives of flexible and concrete single-stage designs is based on evidence from the Alconbury Hill experiment. The data on the structural performance of strengthened pavements are inadequate and therefore four structural models are studied to cover the range of possible behaviour. In general a simple costing system is used, but a more detailed one is applied to some of the flexible pavements. The sensitivity of the results to traffic growth rate and financial discount rate is investigated. For a concrete pavement a long initial life seems economic, and two-stage construction is unlikely to be worthwhile for overall lives less than about 80 years. This is partly due to the inherent structural characteristics of the material and partly to the difficulties of adding a thin overlay to a concrete pavement. For flexible pavements, particularly those not having cement-bound bases, twostage construction offers greater advantages, partly due to their adaptability to changing conditions. It is doubtful whether it is worth designing for an initial life of more than about 20 years, even if much longer overall lives are intended. There are apparent advantages with overall design lives as low as 20 years, but it is uncertain whether such advantages would be realised in practice. More data are needed on the behaviour of strengthened pavements and on the cost of the strengthening operation, including the cost of delays to traffic. The numerical results should therefore be regarded as giving general indications rather than precise forecasts. (Author/publisher)
Abstract