The Manta floor is a so-called ribbed cassette floor. Each element consists of two concrete beams (ribs) with a thin floor slab between them on top. The ribs are on the underside. The element width is 1200 mm. The floor slab is 55 mm thick and tensioned between the ribs.
Much less so than, for example, a screed floor, these floors also sometimes use calcium chloride as a (drying) accelerator added during the production of the elements. The calcium chloride causes the reinforcement to rust away, meaning it can no longer adequately absorb the tensile forces. The corrosion process causes the reinforcement in the Manta floor to expand and crush the surrounding concrete. The load-bearing capacity of the ribs decreases, and the Manta floor can fail. This problem is also known as "concrete rot."
The reinforcement in the Manta floor is primarily intended to absorb the tensile forces in the concrete. These tensile forces often occur at the bottom of the floor system, which is why most of the reinforcement is located there. Clearly, this concrete damage negatively impacts the safety and functioning of the ground floor. Therefore, the floor no longer meets its expected and designed lifespan of fifty years.
Rust spots are visible as brown patches on the concrete and dented concrete. Rusting causes the reinforcement to expand, cracking the concrete cover and reducing the diameter of the reinforcement. This exposes the reinforcing bars, accelerating the rusting process, partly due to the oxygen and moisture content in the crawl space. A dry and well-ventilated crawl space is important!
Concrete rot isn't caused solely by the addition of calcium chloride! In some cases, the reinforcement is too close to the surface, or the concrete isn't sufficiently compacted, allowing a humid environment to damage the reinforcement. The damaged reinforcement corrodes, expands, and causes concrete damage to the ribs of the floor elements.
