Much of the post-war construction in Japan is of nearly ubiquitous concrete construction, often using prefabricated slabs. This was largely because Japan lacked much in the way of structural metals and wood for reconstruction, and because the traditional wood framed rice paper structures of the pre-war era proved to be highly prone to propagation of firestorms. Concrete is an essentially fireproof material that can be fashioned out of (mostly) local materials, preformed and prestressed in a production-line fashion, and made into essentially any shape using minimal structural steel. This use of concrete is especially apparent in Okinawa, which was devastated by Allied bombing and invasion, and in which virtually no pre-war structures remain (except in a few of the outlying islands of the Ryukyu Archipelago). Even structures such as picnic shelters/observatories and railings are made of concrete textured and painted to look like wood.
While most people think of concrete as being inert “like rock”, as an aggregate material it is really a prefractured material with a matrix to hold it together, and while it can accept considerable compressive stresses it resists tension, torsion, and combined loading very poorly. Like all composite materials, it is only as strong as the weakest material when it comes to corrosion and degradation. Concrete buildings often show evidence of degradation only a few decades after construction, and this is especially true in nations like Japan. Concrete is degraded in three basic ways; erosion due to action of wind and waves, spallation (large flakes or slabs separating off of the surface) usually due to subsurface stresses such as corrosion or freezing, and through fracture which is due to tensile and torsional stresses which are not taken up by the reinforcing structure (rebar).
Why are Japanese (and many other) concrete buildings prone to degradation while modern concrete structures in Europe and North America more resistant? There are multiple reasons:
Corrosion: Japan, as an island nation in which much of the population lives in near-marine conditions, has a corrosive environment. While the aggregate material (sand and rock) is generally highly resistant to natural chemical action, the matrix can be dissolved by acids, electrolytic action, and many oils, especially those formed by petroleum residue. Spallation or through fracture can expose the tensile fiber to the elements which can further accelerate wear by corrosive expansion, i.e. rust from the rebar expands, forcing greater internal stress in the concrete and separating the rebar from the concrete substrate.
Poor materials: Local materials, especially aggregates, were used without care for proper size, composition, and necessary hardening, making a lot of the construction of particularly soft concrete that does not resist erosion or fracture. Some of the aggregate materials used, like those containing mica, laumontite, and pyrite may actually degrade material strength and accelerate corrosion.
Poor/no sealing and hardening: Japanese post-war buildings are not generally sealed, painted, and have insufficient hardening, making them prone to both erosion and spallation. Similar effects can be seen on old military concrete structures built around the same time.
Weather: People often think of Japan as being a tropical nation; while this is true for the more southern islands and possessions, the Home Islands (Honshū, Hokkaidō, Kyūshū, and Shikoku) are hemiboreal to subtropical and experience cold and often snowy winters in the north as well as considerable rainfall and large temperature variations throughout the year. Heavy tropical storms, depositing seawater well inland, are common during the monsoon season in the more southern areas of the Home Islands and in the southern possessions.
Seismic activity: Japan is, of course, a volcanic archipelago with a high degree of geological activity. Anyone who has owned a home with a basement in a temperate zone is familiar with the damage that almost inevitably occurs due to subsidence of the ground during freeze/thaw cycles, e.g. foundation cracking. This leads back to through fracture, exposing the rebar to corrosion and the resulting effects.
Even good quality concrete structures in temperate climates require periodic maintenance (applying sealants on exposed areas, patching cracks, reinforcing areas stressed by subsidence, et cetera). Many concrete structures that are built with aesthetic requirements will apply a facade of some kind; either a surface treatment that is attached to but concealing the structure, or an actual separate non-structural facade built of wood, sheet metal, brick, or other material that can be removed and replaced. This both conceals superficial (and sometimes significant) damage to the underlying structure and may to some extent protect it from rain and climate extremes.
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