Is anyone else obsessed with the concrete finish on Sejima's Yokohama apartment?

Those thin sections in Sejima's work usally rely on **Self-Consolidating Concrete** (SCC). It flows like water into tight formwork without needing mechanical vibration, which is how you avoid those honeycombing voids in such narrow profiles. If they used standard vibration on a 60mm pour, they’d risk shifting the rebar or blowing out the forms. The precision required for the formwork alone probly doubled the construction cost compared to a standard apartment block. To get that consistent pale gray, they aren't just using standard off-the-shelf Portland cement. It's often a custom blend of white cement mixed with standard gray, or they use high volumes of fly ash to keep the tone light and uniform. The lack of blotchiness comes from using non-porous formwork—likely phenolic resin-coated plywood—and applying a high-quality **penetrating sealer** immediately after curing. That stops the concrete from soaking up grime and looking dark after a few typhoons.

Living in it is a diffrent story. The insulation on those thin walls is minimal, meaning you're basially living in a thermal battery. During a humid Yokohama summer, the AC has to run constantly becasue that concrete will hold the heat long into the night. It’s a massive trade-off where the aesthetic of the "concrete shell" wins over thermal efficiency. And the lack of railings on some of those roof spaces is classic Sejima—it looks incredible, but it's a nightmare for anyone with kids or a fear of heights.

While the structural integrity is solid due to the high-strength mix and tight rebar specs, the maintenance is what keeps people up at night. With no overhangs or traditional drainage, the concrete is exposed to everything. If a hairline crack develops, there’s no wall cavity to catch the moisture; it goes right into the living space. It's lovely stuff to look at, but it requires a very specific type of tenant to live with those trade-offs.

That 60mm thickness is what gets me. I’ve heard you need a certain amount of 'cover' so the steel inside doesn't rust, but there's barely any room for that here. Are they using some kind of specialized coated rebar, or just hoping the sealer does all the heavy lifting?

Also, with those outdoor stairs being totally open to the elements, I'd be terrified of them getting mossy or slippery. Does the high-strength mix help keep the surface from getting porous enugh for stuff to grow on it, or is someone out there scrubbing it every weekend?

To answer that bit about the steel, they often use stainless steel rebar or even carbon fiber grids for these ultra-thin Japanese projects. Standard carbon steel woud blow the face off that concrete in five years with only 15-20mm of cover. Japan’s building codes are strict, but for these 'artistic' builds, they use high-density additives that make the matrix almost impermeable to salt and moisture.

As for those stairs, I'd bet they have a clear sand-textured grit mixed into the final sealer. It doesn't ruin the look but gives you just enough bite so you aren't sliding off the edge during a downpour. It’s definatly a high-maintenance beast though.