Risk Based Inspections : Rate of Deterioration Score

Many defects are known to take many years to develop to the point where they require maintenance or present a risk to structural integrity or public safety. However, the structure specific characteristics, i.e. construction form, details susceptible to deterioration and the overall construction material are significant considerations in determining the rate of deterioration for observed deterioration mechanism(s). Table 1 below provides the anticipated deterioration rates for typical construction forms for bridges, culverts, gantries, retaining walls, subways and masts and their respective construction material types. Three classifications were used to distinguish between the different deterioration rates as follows:

• Slow (S) – Given mild exposure circumstances structural forms and materials with a slow rate of deterioration would require over 120 years to reach a condition score of 40 or below.

• Medium (M) – Given mild exposure circumstances structural forms and materials with a medium rate of deterioration would require between 60 – 120 years to reach condition score 40 or below.

• Fast (F) – Given mild exposure circumstances structural forms and materials with a fast rate of deterioration would require less than 60 years to reach condition score 40 or below.

Note: The list presented in th table below is not exhaustive. Where a particular combination of material and structural form is not available, a close approximate combination can be selected or engineering judgement can be used in estimating the rate of deterioration.

In addition to the structure specific characteristics the rate of deterioration is significantly influenced by the exposure severity, i.e. external influences that may cause rapid or progressive deterioration, damage or failure. Four classifications are used to distinguish between the different severities of exposure, i.e. mild, moderate, severe and very severe, and a detailed description of these is provided in Table 2.

Structures that are not currently included in Table 1 will be treated as follows:

• ‘Cat Lighting’ will be treated as ‘Mast’,

• ‘Chamber’, ‘Pipe Subway’ and ‘Tunnel’ will be treated as ‘Subway’,

• ‘River Wall’ will be treated as ‘Retaining Wall’, and

• ‘Vault’, ‘Footbridge’ will be treated as ‘Bridge’.

The Rate of Deterioration Score, RoDS, takes account of the above considerations and can be selected from Table 2 which is implicitly based on a 6 year PI interval and distinguishes between the different deterioration rates and exposure severities.

Table 1. Rate of Deterioration Depending on Structure Characteristics