Concrete is strong when pushed and weak when pulled; steel is the opposite. Reinforced concrete, which combines the two, spread through Japanese cities after the Great Kanto Earthquake. We read the logic of the “cover” that keeps steel from rusting, the skin of exposed concrete marked by formwork and tie holes, and Tadao Ando’s museum half sunk into a hill.
What each material does well
Concrete is strong against crushing (compression) but weak against pulling (tension). Steel bars resist being pulled apart. Reinforced concrete (RC) is a material in which each covers for the other’s weakness.
When a load bears on a beam, the beam bends slightly. Its upper side is squeezed and its lower side is stretched. The concrete takes the push at the top, and steel bars placed near the bottom take the pull. That is why beams carry most of their reinforcement low down.
There is another reason the pairing works. Within everyday temperatures, steel and concrete are said to expand and contract at almost the same rate, so heat and cold do not easily pull them apart.
The unseen thickness that keeps steel from rusting
Steel inside concrete does not rust because concrete is strongly alkaline, at around pH 12 to 13, which forms a thin protective film on the bars. But as carbon dioxide from the air seeps in, the concrete gradually “neutralises” from the surface. Once the pH falls to around 9 or 10, the film breaks down and the steel begins to rust. Rusting steel swells and cracks the concrete around it.
So a certain thickness of concrete is needed around every bar. This thickness is called the “cover”. Article 79 of the Building Standards Act’s enforcement order sets a minimum cover for each part of a building.
An earthquake that spread reinforced concrete
In Japan, reinforced concrete began to be used in parts of buildings around 1900, at naval facilities in Sasebo and elsewhere, though which came first is unclear. The Mitsui & Co. Yokohama branch, completed in 1911 to a design by Otoh Endo, is said to have been built entirely in reinforced concrete and still stands as what is described as Japan’s first RC office building. It came through the Great Kanto Earthquake without injuring anyone.
That earthquake settled the matter. In 1924 the Urban Building Act’s rules were revised to set Japan’s first seismic standards. Government offices, stations, schools, hospitals, banks and department stores were built in reinforced concrete one after another, and the Dojunkai supplied some 12,000 homes.
- c. 1900First partial uses, at naval facilities in Sasebo and elsewhereMeiji 30s; the first example is unclear
- 1911Mitsui & Co. Yokohama branch, said to be Japan’s first RC office building
- 1923Great Kanto Earthquake
- 1924Japan’s first seismic standards
- 1976Row House in Sumiyoshi (Tadao Ando), an exposed-concrete house
- 1992Benesse House Museum (Tadao Ando)
Exposed concrete: a skin that remembers its formwork
Concrete is poured into forms of wood or metal and left to harden. Exposed concrete shows the surface just as it comes out of the forms, without tiles or paint. It keeps an exact record of how the formwork was assembled.
The plywood often used for formwork measures about 900 × 1,800 mm (3 × 6 shaku). The neat grid of lines on an exposed-concrete wall is the trace of the joints between these panels. The round hollows dotted across the wall are tie holes: the marks of the cones that held the ends of the steel ties keeping facing forms the right distance apart.
Look at an exposed-concrete wall, then, and you can tell the size of the formwork panels and where the ties were placed. The joints and tie holes are also a pattern laid out by the designer. In Tadao Ando’s buildings, their arrangement sets the rhythm of an entire wall.
Ando’s Row House in Sumiyoshi, Osaka (1976), is a small exposed-concrete house with a building area of 33.7 ㎡. Osaka Prefecture’s registration document notes that formwork methods were improved and became established in the 1960s, and that exposed finishes demanded great precision. Without accurately built forms and densely compacted concrete, a beautiful surface cannot be had.
Water repellents applied to exposed concrete limit the ingress of water and salts, but are said to do little to slow neutralisation, and their effect fades over the years.
A museum half sunk into a hill
Benesse House Museum on Naoshima opened in 1992. It was designed by Tadao Ando and built by Kajima Corporation, with a floor area of 3,643.4 ㎡. Half of the reinforced concrete building is buried in the ground to protect the island’s landscape.
A schematic based on the official Japanese description, “burying half of the building in the ground”. The actual shape and proportions differ.
Burying a building is a choice reinforced concrete makes possible: it withstands the pressure and damp of the earth and can enclose large spaces underground. The Chichu Art Museum on the same island, opened in 2004, can be read as taking this idea further.
An exposed-concrete wall is also a screen for catching light.
Look at the walls when you stay
When you stay in an exposed-concrete building, look at the same wall in the morning and in the evening. Low-angled light brings out the grain of the formwork, the joints and the tie holes. A wall that looked almost flat at midday recovers the memory of its forms.
Where to see this technique
- Benesse House (Naoshima, 1992, Tadao Ando): a reinforced concrete museum half buried in the ground
Sources
- General Building Research Corporation of Japan, “Neutralisation of concrete and corrosion of reinforcement”
- MLIT materials, Building Standards Act Enforcement Order, Article 79 (cover)
- Tokyo University of Science, Kagaku Forum (history of RC in Japan); Japan Federation of Construction Contractors, ACe, January 2017
- Cabinet Office disaster information, “The Great Kanto Earthquake and seismic standards”; Obayashi Corporation history
- Japan Federation of Construction Contractors, formwork materials; Osaka Prefecture, Row House in Sumiyoshi registration document
- Japan Concrete Institute paper (effect of water repellents)
- Benesse Art Site Naoshima official magazine, January 2024
