The prediction of service life of drainage facilities is difficult because of the wide range of environments encountered and the various protective measures available. Service life and durability are directly related to resistance to corrosion, abrasion, and other modes of deterioration.
Drainage facilities are usually designed for a specific service life. The design service life is sometimes defined as the expected period for which they are relatively free from maintenance. However, it can be defined to include a planned rehabilitation after a given number of years to reach the required service life as part of a value analysis approach. (See Art. 10.10.1.)
For a metal culvert, the design service life can be based on the number of years between the time it is installed and the time a perforation from either corrosion or abrasion occurs at any location in the culvert. However, this is a rather conservative approach because the consequences of small perforations are usually minimal and a single perforation can occur long before there is a general thinning of the metal. Thus, service life charts are often based on an average service life that extends life past first perforation by 25 percent or more. For a concrete culvert, the design service life is usually defined as the time between installation and when deterioration reaches the point of exposed reinforcement anywhere in the culvert.
The selection of design service life is dependent upon the use, importance, and ease of replacement of the culvert. A culvert located under a high fill or a roadway with high traffic volumes will be expensive to replace, and the replacement will disrupt traffic. Thus, such culverts are often assigned a design service life of 50 years or more. In contrast, a culvert parallel to the main road—for example, a pipe underneath an access road—will be relatively easy to replace and can be replaced with little disruption. Thus, such culverts, including those under low fill or on a minor roadway, are often assigned a shorter service life.
