Deterioration Modelling should consider renewal date as part of the equation instead of just install date

In deterioration modelling, the key question is whether the model is predicting deterioration of the original asset or deterioration of the asset in its current post-renewal state.

Install date only

Using install date means asset age is simply:

Age=Model Date−Install Date\text{Age} = \text{Model Date} - \text{Install Date}

This works reasonably well where assets have not undergone interventions that materially reset their condition.

For example, a sewer installed in 1970 and assessed in 2026 has an age of 56 years. A deterioration model based on install date treats it as a 56-year-old asset regardless of whether it was rehabilitated in 2015.

The main risk is that the model can systematically overstate deterioration and failure probability for renewed assets.

Install date + renewal date

Where renewal materially restores asset condition or service life, the deterioration clock can instead be based on the most recent relevant intervention:

Effective Age=Model Date−max⁡(Install Date,Renewal Date)\text{Effective Age} = \text{Model Date} - \max(\text{Install Date},\text{Renewal Date})

So if that same 1970 pipe was structurally relined in 2015:

2026−2015=11 years2026 - 2015 = 11 \text{ years}

The deterioration model then treats it as approximately an 11-year-old renewed asset, rather than a 56-year-old original pipe.

However, that simple reset is only defensible if the renewal effectively creates a new deterioration lifecycle.

The important distinction

Not all "renewals" are equivalent.

So we don't want to use a generic field called Renewal Date as an unconditional age reset. We need to also consider the renewal type.

A better deterioration framework is:

Condition(t)=f(Install Date,Intervention History,Intervention Type,Material,Environment,Observed Condition)\text{Condition}(t) = f( \text{Install Date}, \text{Intervention History}, \text{Intervention Type}, \text{Material}, \text{Environment}, \text{Observed Condition} )

Three modelling approaches

Option 1 — Install date only

Simple and transparent. Where Info360 Asset is right now.

Useful where renewal history is incomplete or unreliable, but renewed assets will appear artificially old.

Option 2 — Effective install date

Define:

Effective Install Date={Renewal Date,if qualifying renewal existsInstall Date,otherwise\text{Effective Install Date} = \begin{cases} \text{Renewal Date}, & \text{if qualifying renewal exists}\\ \text{Install Date}, & \text{otherwise} \end{cases}

Then model deterioration from effective age.

This is often a very practical asset-management approach.

Option 3 — Intervention-aware deterioration model

Retain the original install date and explicitly model interventions.

For example:

Ct+=Ct−−ΔCrenewalC_{t^+} = C_{t^-} - \Delta C_{\text{renewal}}

followed by a potentially different post-renewal deterioration rate:

dCdtpost≠dCdtoriginal\frac{dC}{dt}_{post} \neq \frac{dC}{dt}_{original}

This lets you represent something such as a 50-year-old host pipe with a 10-year-old liner rather than pretending the entire asset is simply 10 years old.

For water and wastewater infrastructure, this is generally the more defensible conceptual model when the intervention history is good enough.

Why this matters for risk and rehabilitation modelling

Suppose two identical pipes were installed in 1970:

An install-date-only model may give both approximately the same deterioration probability.

But operationally, their likelihood of structural failure may be very different.

At the same time, assigning Pipe B an age of 11 can also be too optimistic if the 2015 intervention only addressed one failure mode. A liner might significantly improve structural performance while leaving other risks—connections, external loading, hydraulic capacity, corrosion mechanisms, host-pipe interaction, etc.—partly dependent on the original asset.

Recommended approach for an asset-management deterioration model

I propose maintaining three separate concepts rather than overwriting install date:

Original Install Date\boxed{\text{Original Install Date}} Latest Qualifying Renewal Date\boxed{\text{Latest Qualifying Renewal Date}} Effective Deterioration Age\boxed{\text{Effective Deterioration Age}}

Then derive effective age according to intervention type.

That preserves asset history while giving the deterioration model the appropriate starting point.