Service lives of Highway Elements

Service lives of Highway Elements:

            To develop information on service lives or life expectancies for all kinds of highways and their individual elements in various environments would be difficult. There are many variables such as soil, climate, topography, and traffic volume that will affect differently the life of essentially the same type of highway in different places. In flat country the alignment may remain unchanged for many years. On the other hand, a road in rolling or mountains areas originally built on cheap crooked alignment often becomes obsolete because of restricted speeds and is relocated long before the life of the pavement is reached. Also, the art of highway building changes so that the date of construction will influence the probable life of a new highway of given type. Finally, it is common to incorporate portions of an existing facility into any reconstruction, as when pavements are resurfaced or lanes, new roadways, or grade separations are added to increase capacity. In these situations, the practice of classifying resurfacing or reconstruction as a form of retirement might be questioned. In sum, forecasting service lives and time to retirement from historical data is difficult and uncertain.

            Studies of the service lives and life expectancies of pavements were begun about 1935 and have been updated form time to time. Some of the findings on past service lives and reasons for retirement of several pavements types are listed in table 4-1.

Table 4-1 Average Service livers and retirement causes for highway surfaces.

	Method of retirement (%of mileage)
Surface	Service life (yr)	Resurfaced	Reconstructed	Abandoned	Retired
Low type Soil Surface* Gravel or stone*	4.0 7.5	37.5 58.0	58.1 30.4	1.2 2.5	3.2 9.1
Intermediate type Bituminous surface-treated** Mixed Bituminous+**	14.0 12.0	58.5 59.8	32.6 30.3	2.5 2.0	6.4 7.9
High type Bituminous concrete++** Portland cement concrete**	17.0 25.0	57.4 66.0	27.7 22.8	2.2 1.8	12.7 9.4

* Source: Public Roads, June 1956, p20.

** Source: HRB Record 252. Pp. 1-23.

+ Thickness of surface and base less than 7 in.

+- Mixed bituminous or bituminous penetration with of surface and base 7 in or more.

Several procedures for forecasting future retirements from such historical data have been developed. One of these, the annual rate method, employs data on retirements in a single year or band of years to develop “survivor” curves. In turn, “type survivor curves” selected from a family of such curves may be fitted to the plot. This approach is illustrated in fig. 4-1. Another procedure is called the “turnover method.” It compares the accumulated units in service with the accumulated retirements.

            Methods for forecasting service lives and life expectancies till reconstruction or resurfacing, based on AASHO pavement design procedures, have also been developed. Also, remaining service lives are being predicted by measuring the strength of existing pavements.