In this blog post, I’ll walk you through how to remove, locate, and dry out cavity water. Maybe you’re a homeowner who’s noticed a dark stain on the ceiling, or a contractor responsible for repairs who needs to fix the problem. I hope this post brings you peace of mind regarding a problem that’s easy to solve.
A common problem during the warranty period for new construction
A very common type of water damage during the warranty period of a new construction project is water seeping into cavities, which appears as dark stains on the ceiling of an apartment or other space, causing the paint and plaster on the interior ceiling to become damp. This can easily occur even a year after the project is completed.
Hollow-core slabs in construction
A hollow-core slab is a reinforced concrete element containing voids that make the structure lighter while maintaining its strength and load-bearing capacity. Hollow-core slabs are used in ground floors, upper floors, and intermediate floors. The floors and ceilings of apartments in concrete apartment buildings are therefore often made of hollow-core slabs. The concrete in the slab is very dense, so it does not easily allow water to pass through, meaning water can remain in the cavity unnoticed for a long time. Drainage holes are pre-drilled into hollow core slabs at the factory, and their accessibility should be ensured on-site. However, during various stages of construction, the holes can become clogged, and water or concrete slurry can also enter the cavity, which then dries poorly inside the cavity. Another common problem is the failure of the drain plugs at the ends of the slabs.
Drilling to remove water
However, this is not usually a major problem; often, water has simply accumulated in a single cavity. Even though there may be dozens of liters of water in the cavity, it can often be as part of a moisture survey it drains out in a controlled manner. The new concrete is also highly alkaline and the cavity is nutrient-free, so mold does not thrive in the cavities, even if water has been standing there for a long time. Of course, water doesn’t always come out of the cavity when drilling, but as a general rule, you should always be prepared for a large amount of water coming out of the cavity. Protective covers, buckets, and water vacuums should therefore be ready, even if the water trail looks small. Water can also come from an adjacent cavity, even if there are no visible signs of it. The cavity may also contain fill material or pockets, creating various pockets within it, so drilling must be done at multiple points in the cavity.
Determining the cause of the leak
We always try to determine the cause of the leak during the inspection. As noted earlier, the issue is often water left in a cavity during construction, but the cause may lie elsewhere. It is important to determine the cause of the leak to prevent it from recurring. The water may also originate from, for example, the roof, an upstairs apartment, or the plumbing. It is not always possible to identify the leak, as the water flow may have already stopped by the time we arrive on site, which makes it difficult to determine the cause. In any case, high-quality documentation of the leak’s location and possible causes is essential, so a report is always provided following the inspection.
Drying
Once the water has been removed from the cavity, the drying process can begin. Typically, drying is performed using an adsorption dehumidifier, which supplies dry air (approximately 20% RH) into the cavity. Any remaining moisture in the cavity is absorbed by the dry air and exits through a second opening. The moist air separated by the machine is usually vented out through a ventilation window via an exhaust hose. In some cases, primarily in fully cast cavities, plate dryers based on heating the concrete can also be used. However, this is less common. The drying time is typically 1–2 weeks, but it depends entirely on the specific case.
Finally, the dryness of the cavity is verified by measuring the relative humidity of the cavity space, provided that the drying process was completed at least 24 hours earlier and the cavity has had time to stabilize. The humidity should be no more than 90 RH-%, but preferably below 80 RH-%. In older buildings, even lower. The readings may sound high, but when dealing with relatively new concrete, they are perfectly normal and safe results. Over time, the moisture level will stabilize toward indoor air values, i.e., 30–60 RH-%, but with new concrete and a cavity, this can take several years, even if it hasn’t gotten wet at any point since pouring.
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