BRE IP3/06/3 PDF

Introduction

This information paper provides guidance on forecasting the service life of reinforced concrete structures and components at the design stage, and describes methods available to enhance service life where required.

A number of techniques can be used to give a service life forecast. For example, service life can be predicted based on models of the deterioration of concrete in different environments (Box 1). However, in order to provide a relatively simple approach, the BRE service life design system uses a version of the factorial approach used in BS ISO 15686-1.

The following equation (taken from BS ISO 15686-1) provides a summary of the factors that can affect the service life of a component:

ESLC = RSLC × A × B × C × D × E × F × G (equation 1)

RSLC (reference service life) refers to the period of time that the component or assembly can normally be expected to last under specified service conditions. For concrete, the RSLC will generally be derived from BS 8500-1 which gives options for service lives of 50 or 100 years.

ESLC (estimated service life) is the expected life of the particular component in its specific environment allowing for the factors

A to G: A = Quality of materials factor

B = Design level factor

C = Work execution level factor

D = Indoor environmental conditions factor

E = Outdoor environment factor

F = In use conditions factor

G = Maintenance conditions factor.

In using equation 1, factors A to G should be applied by the designer based on the particular circumstances of the project, previous experience and available information. The factorial system allows the designer to identify deviations from the norm or factors leading to a specific concern. It is a guide to help select appropriate approaches to meeting the required service life. The designer should use experience to inform decisions and give ‘best' rather than conservative estimates. If in doubt, expert assistance should be sought.

Generally speaking, factors should be left at 1 when the designer has no information that the design or conditions are contrary to that assumed in the codes and standards used. Table 1 provides additional guidance on the use of these factors in the context of reinforced concrete construction. An example of using the factorial approach for a hypothetical concrete element is given in Box 2.

The iterative nature of service life design means that it cannot readily be illustrated in a simple schematic diagram by which a designer can ensure that the required service life design is met. However, by using the approach outlined here, it is expected that designers will come to understand better the choices they face and will have to make fewer or no iterations.

Where the ESLC falls below that required, strategies for increasing service life can be used to assist the designer in selecting or modifying the approach used in protecting the structure. These are discussed below.