Partial-depth vs. full-depth concrete repair: how deep the work has to go

Depth is a diagnosis, not a preference

People assume the depth of a repair is about how big the damaged area is. It is not. A small spall can sit over concrete that is chloride-contaminated all the way through, and a large-looking area of drummy concrete can be nothing more than the cover letting go while everything below it is sound. What decides the depth is where the damage actually is in the thickness of the section: how deep the chloride reaches, whether the near mat of steel or both mats are corroding, and how much cross-section the bar has lost. You cannot read any of that off the surface.

This is why two contractors can look at the same balcony and quote completely different work. One sets the depth by the size of the spall and quotes a shallow patch. The other waits for the chloride profile and the sounding survey and quotes what the section actually needs. The second quote is often higher, and it is often right. Depth is a measurement, and treating it as a preference is how repairs come back.

How the corrosion drives the depth question

The reason depth matters at all is the way steel corrodes in concrete. Sound concrete is highly alkaline, and that alkalinity keeps a thin passive film on the reinforcing steel that stops it rusting. Two things break that film. Carbonation, where carbon dioxide from the air slowly neutralizes the alkalinity from the surface inward, and chloride, where salt migrates through the concrete and reaches the bar. On the Metro Vancouver coast, chloride is the dominant driver: road salt tracked onto parkades, and sea-salt aerosol on waterfront balconies.

Corrosion of ordinary reinforcing steel tends to begin once the chloride at the bar reaches roughly 0.2 percent by mass of the cement, with published values ranging from about 0.17 to 0.26 percent depending on the concrete mix. Once it starts, the rust that forms takes up several times the volume of the steel it came from, and that expansion cracks the cover off from the inside. The spall you see is the end of a process that started at the bar. The depth question is really asking how far into the section that process has gone: is it only at the near mat, or has the chloride reached the far mat too? The rebar corrosion and spalling guide covers the mechanism in detail.

What partial-depth repair reaches

Partial-depth is the everyday repair on a Metro Vancouver balcony edge or parkade soffit. The crew chips out the delaminated cover down to and slightly behind the first mat of reinforcing steel, cleans the bar to bright metal, treats or splices it, and rebuilds the section with a polymer-modified repair mortar matched to the substrate. The bottom of the slab and the concrete on the far side of the bar are left in place, so the element keeps carrying its load through the sound material that remains. That is why partial-depth work usually needs no shoring, cures relatively quickly, and can be phased into small barricaded zones on an occupied building.

It only works when the damage really is confined to that top layer. If the chloride profile is high near the surface and drops off before the far steel, and the bar still has enough sound cross-section, partial-depth removes the contaminated concrete and leaves clean material protecting the rest of the section. Push it past those limits and you have a shallow repair sitting over live corrosion.

The quality of a partial-depth repair lives in the preparation, not the mortar. The bar has to be exposed fully around its circumference, which usually means chipping a little behind it so the mortar keys around the steel instead of just butting the front face. The steel has to be cleaned to bright metal, and the surrounding concrete has to be prepared to a defined roughness, an ICRI concrete surface profile, so the mortar bonds. A repair that debonds and sounds hollow within a season is almost always a preparation failure, not a depth failure, and it gets misdiagnosed as one.

When the work has to go full-depth

Full-depth repair removes the concrete through the entire thickness of the element, past both mats of steel, and recasts it from one face to the other. It is the answer when the chloride has driven through the full section, when the bottom mat of reinforcing steel is corroding as well as the top, when the sounding survey shows delamination on the soffit as well as the top surface, or when the bar has lost so much cross-section that it needs full replacement to the engineer\'s detail.

The important consequence on a suspended slab is structural. Removing concrete through the full thickness interrupts the load path, so a structural engineer specifies temporary shoring to hold the load while the section is open, and the review of how much can be opened at once comes before any concrete is removed. This is more cost, more phasing, and tighter oversight than partial-depth work, and it is not a decision the crew makes on site.

The risk on full-depth work is different in kind from partial-depth. A partial-depth repair fails through a wrong depth call or bad bonding, which is a durability problem that shows up over months. A full-depth repair, done wrong, fails through under-shoring or a bad removal sequence, which is a structural problem that can show up immediately. That is why the shoring design and the sequence of removal are the heart of a full-depth scope, and why an engineered removal plan, not just an engineered repair, is essential.

The two repair depths compared

The survey that sets the depth

None of this is a guess on a specific building. Before anyone quotes a repair, the engineer sounds the concrete on both faces with a hammer or chain drag to map where the cover has delaminated, runs half-cell potential readings to ASTM C876 to find where the steel is actively corroding, and drills for chloride samples at several depths so the lab can build a chloride-versus-depth profile.

The half-cell readings are worth understanding, because they show corrosion the eye cannot. A reference electrode is moved across the surface and reads the electrical potential of the steel below. As a broad guide, readings more negative than about -350 mV against a copper/copper-sulphate electrode indicate a greater than 90 percent probability of active corrosion, and readings more positive than about -200 mV indicate less than a 10 percent probability. Combined with the chloride profile, this tells the engineer not just whether the steel is corroding but how deep and how widely. On a suspended slab, that picture plus the rebar condition is exactly what decides whether the repair stops at the near mat of steel or goes through the section. The repair vs. replace guide picks up where full-depth repair stops making sense and replacement takes over.

Why Metro Vancouver conditions push repairs deeper

The coastal climate works against shallow repairs in a specific way. The region rarely gets a hard freeze-thaw cycle inland, but it is wet for months at a time, and concrete that stays damp does not dry out and stop the chloride from migrating. Salt keeps moving deeper into a wet slab instead of staying near the surface. On a parkade, the winter road-salt load adds fresh chloride every year. On a balcony near the water, sea-salt aerosol never lets up. The result is that Metro Vancouver slabs tend to develop deeper chloride profiles than a drier climate would produce over the same years, which is one reason the region sees more through-section contamination and more full-depth repair than owners expect.

The leaky-condo legacy compounds it. A generation of buildings went up with water-management details that did not hold, and the water that got into those assemblies carried chloride to the steel. Failed balcony membranes and dead parkade traffic membranes are still the most common reason water reaches the bar. That is why fixing the water source is part of every honest repair scope, whatever the depth: rebuild the section and re-waterproof, or the corrosion simply starts again.

Realistic building scenarios

The depth call is clearer in real situations than in the abstract. Here are four that map onto common Metro Vancouver conditions. In more than one, the honest answer is the simpler partial-depth repair, and in one the honest answer is to wait for data before committing to anything.

Decision framework: eight questions that set the depth

Getting the depth right the first time

The most expensive mistake on a Metro Vancouver strata slab is choosing partial-depth to save money when the damage needed full-depth. A shallow repair over chloride that runs through the section looks fine on handover and starts spalling from the underside within a year or two, and now the strata is paying for a second, deeper repair on top of the first, plus the shoring and engineering the full-depth work needed anyway. The cheaper method is only cheaper when it matches the depth of the actual damage.

This is where strata funding realities collide with building science. In BC, corporations with five or more lots must obtain a depreciation report under the Strata Property Act, and major repairs are normally funded by a special levy that needs a 3/4 vote at a general meeting. A council under pressure to keep the levy small has an incentive to approve the shallow repair, and a contractor who wants the job has an incentive to quote it. That is exactly the situation where a repair gets set by budget instead of by data. Confirm the funding rules and vote thresholds with your licensed strata manager or a strata lawyer, and treat any quote that sets the depth without a chloride profile as a financial risk, not a bargain. That is the whole reason the depth call belongs to the engineer who ran the profile, and why our building restoration crews work to the engineer\'s repair depth and shoring design rather than deciding it on the slab.

Questions to ask before you approve the scope

You do not need to read a chloride report to pressure-test a repair proposal. You need to make the contractor show the data behind the depth and account for the safety of the removal. These are the questions that separate a diagnosed repair from a guessed one, and the answers that should stop you approving it.

How the depth fits the rest of the repair decisions

The depth question does not stand alone. Once the engineer has set the depth, the next decisions follow from it. How the concrete gets put back, sprayed, formed, or hand-applied, depends partly on the depth and volume of the repair, which our shotcrete vs. form-and-pour guide works through. Whether to add corrosion protection to the steel you are leaving in place, sacrificial anodes at patch edges or a wider system, depends on how contaminated the surrounding concrete is, which the cathodic protection vs. patch repair guide and the corrosion inhibitor vs. galvanic anode guide cover.

Read as a set, these guides describe one continuous engineering conversation: whether the element can be saved, how deep the repair has to go, how to stop the corrosion coming back, and how to rebuild the section. The depth call sits near the front of that sequence, and getting it right on real data is what keeps the rest of the project from becoming a repeat.