Lifetime & forecast: estimating how a structure will develop

Damage assessment: three criteria for every defect

During the inspection, the engineer records each damage and each defect individually. Every damage is assessed against three criteria, each on a scale from 0 to 4. A damage can therefore be insignificant for structural safety and considerable for durability.

From these individual assessments, the condition rating of the structure is calculated using a defined procedure. It ranges from 1.0 to 4.0:

Weighting: the most severe damage sets the rating

The condition rating is not an average of all defects. The most severely rated damage is decisive; the number and extent of further damage can raise the rating further. A single damage with a high structural safety rating therefore shapes the rating of an otherwise sound structure.

This has two consequences for maintenance. First, the rating shows how urgent a structure is, but not which damage lies behind it, how large it is or what its repair will cost. Second, the rating often depends on just a few locations in the structure. Anyone who wants to maintain or improve it has to keep exactly these locations in view.

This is where monitoring comes in. The measuring points sit where the governing damage sits: at the crack, at the bearing, in the area with moisture and chloride ingress. The damage that sets the rating becomes a measured quantity with a history.

Why the rating alone is not enough

The rating is a snapshot of the inspection day. A principal inspection takes place every six years, with a minor inspection three years later. In between, nobody is at the component with a measuring instrument.

Two structures with the same rating can therefore develop in entirely different ways. In one, a crack has been stable for years; in the other, it opens a little further every month. The rating does not distinguish between the two; the data series does. It is precisely this rate of change that determines urgency, and it is the basis of every forecast.

Many types of damage affect durability first, long before they reach structural safety, for example corrosion inside a prestressed concrete section. Measuring during this phase buys time: for a targeted repair while the damage is still limited, and for maintenance planning that is not caught out by the next inspection.

Looking ahead from the past and the present

A forecast connects three time horizons. The past comes from inspection reports, the history of the condition rating, as-built documents and earlier measurements. They show how the structure has behaved so far. The present comes from the current readings, together with weather data and, where available, traffic data. They show what is happening at the component today.

The trend is derived from both, adjusted for the daily and seasonal temperature cycles. When it is projected forward, the result is not a single line but a range that widens over time. If the monitoring concept defines a threshold, such as a crack width or a moisture value, the range shows the earliest point at which this threshold could be reached. That is the period in which a measure should be planned.

The forecast does not predict the next condition rating. It describes how quickly the measured quantities at the governing damage are changing, giving the inspecting engineer and the operator a sound basis for the next assessment. With every new reading, the range is checked and adjusted where necessary.