Deterioration of conventionally reinforced balconies is a major concern to residential buildings due to the high cost and disruptive nature of the repairs. Repairs often require the slab edge to be chipped off, which, as well as creating pervasive noise, usually requires balconies to be closed off for several months. The technical issues surrounding the deterioration are often poorly understood, so Condominium Corporations sometimes spend hundreds of thousands of dollars on repairs which do not adequately address the root cause. This can increase costs overall and in the worst case, cause repairs to fail prematurely. This article explores the causes of the underlying deterioration and makes some recommendations for prevention and successful repair.
Steel embedded in concrete is immune to corrosion, except under two key circumstances: exposure to chlorides and carbonation.
Chlorides (salts) usually get into concrete from application of de-icing chemicals, but also salt-water spray from oceans, and in older buildings salt that was added to the concrete (or put on the formwork) during winter construction. Salt causes embedded reinforcing steel to lose its immunity to corrosion.
Carbonation is a process where carbon dioxide in the air reacts with the moisture in the concrete, forming calcium carbonate in a process that reduces the pH of the concrete. It progresses in from the outside surface, penetrating deeper into the concrete over time for many years, until it halts it progress due to the self-limiting nature of the reaction. With a reduced pH caused by this carbonation, embedded reinforcing steel loses its immunity to corrosion.
The primary cause of highly disruptive slab-edge concrete repairs is often related to the simple drip slot at the edge of the slab. Drip slots are the small grooves at the outside edge of the balcony slabs. Their purpose is to intercept water running off the edge of the balcony so that it does not dribble back onto the balcony soffit, thereby preventing damage to soffit finishes and keeping water from dripping onto people on the balcony. An unintended consequence of the drip slot is that it reduces the thickness of the concrete covering the embedded reinforcing steel. This makes the reinforcing steel directly above the drip slot more prone to corrosion than any other steel in the balcony slab (because the rest of the steel has a thicker layer of concrete over it).
The primary failure mechanism for balcony slabs is carbonation. The layer of carbonated concrete typically only reaches 10 to 25mm deep, largely on the underside of the slab (not the topside, because the wetting related to rain reduces the depth of carbonation penetration). In most areas of the slab, 10 to 25mm of carbonation is not an issue, because the steel is typically protected by 25 to 40mm of concrete. However, right at the drip slot, this cover may be reduced to 10 to 15mm, allowing carbonation to advance to the depth of the embedded steel, making it free to corrode. The reinforcing steel right above the drip slot corrodes; the corrosion product is bigger than the steel; the expansion causes the concrete to crack (typically along the drip slot); air and water get in the crack, increasing the rate of corrosion. Eventually the outer edge of the slab works loose and needs to be repaired.
This failure mechanism is characteristically different from a chloride-induced failure. If there are chlorides in the concrete, or applied to the topside of the concrete, there will be slab edge concrete deterioration (similar to carbonation-induced damage), but there will also be topside damage and/or soffit damage away from the drip slot.
To properly repair a balcony slab it is important to understand the underlying failure mechanism. Concrete repairs must be designed to avoid recurring damage by selecting an appropriate extent of removal, appropriate materials, and applying features which protect the adjacent concrete (which remains in place beside the patched areas) from suffering the same fate. Many buildings are waterproofing their balconies during repair as a preventative measure.
Waterproofing a balcony that is failing due to carbonation may be a relative waste of money as it does not address the underlying failure mechanism. It may slow down the deterioration once it starts, but is unlikely to prevent it, as the moisture from humidity in the air is sufficient to allow the embedded reinforcing steel to corrode once the concrete is carbonated. Waterproofing a balcony that is failing due to chlorides can sometimes be an good investment if the extent of remaining chloride contaminated concrete is known and managed through proper repair techniques.
For new buildings, looking to prevent concrete deterioration from happening in later years, it may be prudent to check the cover over the embedded reinforcing steel at the drip slot to be able to evaluate the risk of future deterioration. If cover is insufficient, it may be worthwhile to fill the drip slot and install a downward facing drip deflector such as a metal drip edge or an inverted bead of caulking. While these are not as attractive as a conventional drip slot, done properly they can be neat and tidy, while reducing the risk that slab-edge repairs will be needed in the future. Unfortunately, there is still little information available from product manufacturers to tell us which, if any, caulking installed in the drip slot will provide the carbonation protection that is desired.
We typically recommend a balcony condition survey, sampling at least 15% of balconies, when condos are about 15 years old, and about every 10 years after that. The evaluation budget should allow for chloride testing, carbonation testing, cover-meter testing, hammer-tap sounding and visual review. This can usually be done from within the suites, so outside stage access is not typically required. Although this evaluation will not prevent deterioration, it allows the corporation to gain a good understanding of the current situation and likely future deterioration, so they can budget appropriately in their Reserve Fund planning.
Note that some balconies are reinforced by joist chord extensions, rather than conventional reinforcing steel. Their failure mechanism is different from that described above and a different repair approach is necessary.