Mathematical optimisation models, supported by suitable data, can assist decision making about allocating funds between alternative maintenance tasks and about the size of the maintenance budget. The maintenance optimisation problem is, in essence, to find the optimum balance between the costs and benefits of maintenance, while taking into account various constraints (Dekker 1996). For a given road segment, choices have to be made between alternative treatment types and the times to implement those treatments. Where maintenance funds are limited, there is an additional problem of balancing the competing needs of the different segments. Maintenance tends to be underfunded relative to investment because the smaller, less obvious nature of maintenance works relative to new infrastructure (Semmens 2006, Zeitlow 2006). But deferring maintenance in the short term can be expensive in the long term, a point that can be brought to the attention of decision makers by quantifying the costs of underfunding maintenance. Maintenance can be defined as ‘all the technical and associated administrative functions intended to retain an item or system in, or restore it to, a state in which it can perform its required function’ (Dekker 1996). It does not upgrade the asset. In practice, it is common to carry out small upgrades of roads such as widening or shoulder sealing together with rehabilitations. Without maintenance, roads can quickly fall into disrepair leading to increased costs for road users in vehicle operation, time, reliability and safety. If deterioration goes too far, users will be reluctant to use the road with attendant losses of the economic and social benefits the road confers. The maintenance requirements of gravel, sealed and concrete roads and of bridges differ, however, the same general economic principles apply to all. Gravel roads need to be regraded at intervals of around six months or a year to reduce roughness and resheeted at intervals of some 8 to 10 years. Concrete roads require roughening for safety reasons as usage reduces skid resistance, maintenance and repairs to joints between slabs, crack sealing, and slab replacement. Sealed roads with flexible pavements consist of layers of crushed rock with either a chip seal (a thin layer of bitumen and crushed rock) which keeps out water, or an asphaltic concrete seal (aggregate mixed with bitumen binder), which both keeps out water and adds structural strength. The term ‘flexible pavement’ refers to the fact that the pavements can deform when loads are applied and then return to their original shape. By contrast, concrete pavements are rigid. The focus of most literature on optimisation and of this paper is on sealed roads with flexible pavements. They carry most vehicle-kilometres of traffic and command the greater part of maintenance expenditure. Concrete pavements are relatively rare and relatively new, while gravel pavements are only economically warranted for lowtrafficked roads. The paper also does not address maintenance of bridges, tunnels, geotechnical structures, and roadside equipment. However, there are similarities between maintenance principles for different types of road infrastructure. For example, Morcous and Lounis (2005) apply the same maintenance optimisation techniques to bridges and Grivas et al. (1993) to concrete pavements as other authors apply to flexible pavements. Road maintenance can be categorised as: • Routine: small tasks undertaken frequently – vegetation control, repairing or replacing signs and other roadside furniture, clearing drains and culverts, repainting line markings, patching, crack sealing and pothole repair; • Periodic: larger tasks undertaken at intervals of several years or more – resealing, resurfacing, overlay, reconstruction; and • Urgent: unforeseen repairs requiring immediate attention – collapsed culverts, washaways, landslides that block roads (Burningham and Stankevich, 2005). Optimisation models for sealed roads deal with periodic maintenance and components of routine maintenance that affect roughness or the rate of pavement deterioration, in particular patching, crack sealing and pothole repair. Road providers have considerable scope to vary the types and timing of periodic maintenance interventions. Routine maintenance, on the other hand, comprises tasks that need to be carried out if a road is to remain open to traffic and generally do not vary with traffic volume and composition. For costing purposes, routine maintenance activities not being optimised are usually assumed to be a constant amount per kilometre of road or per square metre of pavement. The next section of the paper describes the components of the optimisation problem followed by discussion of the problem itself using a simplified numerical example. A large body of published literature exists on techniques for optimising pavement maintenance, mostly from the civil engineering discipline. We provide an overview to indicate the range of techniques applied. The principles for optimising the trade-off between maintenance and construction are then discussed briefly. Ways of defining and measuring the cost of maintenance deferral and the ‘maintenance deficit’ are proposed. A section is included drawing on the earlier discussion of optimisation principles showing how maintenance contracts can be specified to give contractors the incentive to provide optimal maintenance. The conclusion advances some policy implications from the paper. (Author/publisher)
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