The mechanism, briefly
Road salt carried in on vehicles dissolves and moves through the concrete cover to the reinforcing steel. Once the chloride concentration at the bar passes threshold, the steel corrodes, expands, and pushes the concrete apart. You see it as a hollow-sounding delaminated patch long before it becomes a spall.
The waterproofing traffic topping is the only thing slowing that clock. Membranes have a service life — typically 10 to 15 years in the GTA depending on traffic and drainage — and once they are past it, the deck below is absorbing chlorides at full rate.
How the number moves
A localized program while the membrane is still intact is surface work: crack routing, partial-depth patching, joint sealant, topping recoat. Once the deck is soaked and reinforcing has section loss, the same area becomes full-depth removal with shoring, bar replacement, formwork and a full re-waterproof — and the level below has to close while you do it.
Roughly, and with the caveat that every structure is its own case: a recoat plus targeted patching is a fraction of a full-depth reconstruction of the same bay. What changes the budget is not the unit rate; it is the quantity migrating from partial-depth to full-depth, and the shoring, access and closure costs that come with it.
- Year 1-3 deferral: more patching quantity, same repair category
- Year 4-6 deferral: partial-depth converts to full-depth, shoring enters the scope
- Year 7+: structural review, possible slab replacement, extended parking loss
The costs that never make the reserve study
Lost parking revenue and resident disruption are rarely in the engineer's number. A phased program in a residential building means valet arrangements, off-site parking leases, elevator and stair traffic changes and a substantially longer overall duration because you can only close a portion of the deck at a time.
Emergency work costs more again. Once a piece of concrete falls, you are in shoring, netting and a compressed procurement window with no leverage on price.
What we recommend to boards and asset managers
Get a condition survey with chain-drag delamination mapping and chloride sampling — not a visual walkthrough. Tie the repair year to the membrane's remaining life rather than to the reserve fund's convenience. Then tender the work with a clear unit-price schedule for hidden quantities, because on a garage there are always hidden quantities.
The best outcome is boring: a planned program executed in the year the membrane hits end of life, priced with unit rates, phased around the residents, and closed out with a warranty and a fresh survey to reset the clock.