How quickly can concrete deteriorate?

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Posted on Building Restoration

In the spring of 2011, we revisited a 30-year-old North Vancouver residential high rise building and were surprised by what we found.

Only 10 months earlier, the owners had asked us to do a series of bosun’s chair drops and a detailed survey and estimate for concrete repair. Because the concrete balcony decks surfaces were not, for the most part, protected by paint or a membrane, the decks had been freely absorbing moisture. As you can see from the picture, most of the areas of repair we found were on the “slab edges” of these balcony decks, which are most exposed to moisture.

Our previous survey involved a detailed inspection of each balcony and the “sounding out” of the entire balcony surface and surrounding walls to find areas of delamination. We accessed each deck and dragged a hammer over the deck surface, allowing us to tell when an area was delaminated due to the hollow sound it gives.

What surprised us in our follow-up survey was what had taken place in a mere 10 months. The concrete condition had significantly worsened, particularly on the weather-exposed concrete slab edges on the east side of the building.

Why did the concrete significantly deteriorate in a mere 10 months when it had been faithfully weathering the elements for the past 30 years?

The answer is that concrete doesn’t deteriorate on a steady basis. In some cases, when a concrete surface gets to a certain level of deterioration, a rapid worsening can result. This is what we found here.

All concrete loses its alkalinity over time. (This process is called carbonation.) On this building, the unprotected concrete had lost enough alkalinity to leave the rebar on many of the deck edges unprotected, and the result was cracking. The cracks were further exacerbated by the “freeze / thaw” cycle. In this case, the damage caused by freezing water was more pronounced on balcony decks with “indoor / outdoor” carpet that retained water.

Once the roller coaster got over the hill, we were in for some rapid acceleration. In this case, we saw an average 16.9% increase in the areas that were delaminated over a mere 10 months. This meant a significant increase in the concrete restoration cost.

The Strata engaged us to repair the concrete, and we finished this project in the summer of 2011.