1.Building No.30 (completed in 1916, 7 stories with partial semi-basement).

This is the oldest reinforced concrete building on Gunkanjima and the first reinforced concrete building in Japan. It was built over 90 years ago. According to the record, it was repaired twice. First in 1928, reinforcing bars were inserted by paring the surface of the beams and the exterior walls, worn by salt wind, were repaired with waterproof mortar. Next, in 1954, the old reinforcing bars of the columns and beams were replaced with round steel bars and there sectional areas were enlarged. But our research revealed that these repairs were carried out only in the basement and on the first to fourth stories, that the fifth to seventh stories remained as before apart from surface finish. Therefore the upper stories are in far worse structural condition. The deformed reinforcing bars and the steel wire rope of the early days had been used as described above and they are so badly rusted that it is impossible to distinguish their original section. In particular, the covering concrete on the underside of the beams has spalled off, and their main reinforcing bars are revealed or can easily be exposed with a light blow because of large cracks, and the structure is now essentially functioning like a conventional masonry structure. We found steel I sections and 12-centimeter square timber props inserted in large numbers on the fifth to seventh stories in later years to prevent collapse. A neutralization test using phenolphthalein showed that neutralization has penetrated deeper than the reinforced bars and that the reinforced concrete of this building has already reaching the end of its usual life. By contrast, the main reinforcing bars below the fifth story are less rusted, and still structurally functional. The cause for such remarkable weathering on the upper floors is not only the age of the structure but also the unsatisfactory and immature technique of waterproofing (both in respect to materials and fixing). In addition, the use of sea sand as aggregate and of sea water as admixture also contributed to the accelerated deterioration of this building, which under normal conditions would have a usual life of 60 years. The following immature and ineffective architectural details were noted: no asphalt waterproofing membrane on the original building (existing asphalt waterproofing membrane proved to have been added later); inadequate rainwater drains and gutters; no penthouse over the internal stairwell (the first penthouse with stairwell was made in 1940 in building No. 66); immature detail of flashing at the opening of the reinforced concrete shutter box for rain doors and in other places. These imperfections resulted in accelerated deterioration due to the weathering in the living rooms, as well as the main structure. This was quite seperate from weathering which took place after the mine was closed and had been one of the principle technical defects of this building, causing great discomfort and inconvenience to the residents. Because leaking water ran along floor slabs into the downstairs living rooms, the residents were driven to the expedient of hanging a sloping corrugated acrylite sheet enclosed by a box gutter to catch the water and drain it to the outside using a rubber hose.

2.Nikkyu company flats -buildings No. 16, 17, 18, 19, 20- (first phase completed in 1918, nine stories).

Buildings No. 16 and 17 were completed in 1918, and the rest were enlarged several times, so the age of the original phase is sixty eight years. Repairs which involved changing the reinforcing bars and re-covering some of the columns were carried out in 1937 and almost all the columns and beams were further repaired in 1951. It is believed that only the slab and wall of principal corridor on the seaward side are original, but as the structure is hidden under a mortar rendering, it is impossible to investigate the weathering of the original parts. Rusting and subsequent expansion of the reinforcing bars damaged parts of the work carried out in 1951 -now thirty five years old- and the lower faces of the beams have lost their concrete covering, revealing reinforcement in a number of places. These buildings were originally directly exposed to the sea winds and therefore seem to have been badly damaged by weathering before No. 51 was erected between them and the sea. In the courtyard and on the landward side, it was only the columns that had their reinforcing bars replaced and their faces recovered in 1951. The concrete slabs and beams are original and especially in beams where the Kahn Bar was used, the surface concrete has blown and rusty reinforcing bars are revealed in many places. The artificial ground on the side of the base rock below the shrine and the reinforced concrete bridge were reinforced before the mine was closed with timber props, iron pipes and steels sections, in much the same way as building No. 30, whilst in other cases, failure to appreciate the need for an expansion joint at the junction of the bedrock and the building seems to have been the reason. Anyway, the blown surfaces and rusty reinforcing bars reveal that this is a building which has reached the end of its useful life as a reinforced concrete structure. Judging from the results of the neutralization test, the Nikkyu company flats, unlike building No. 30, contained a wider scattering of neutralized parts, so it would be possible to extend its useful life by repeated repair involving changing the reinforcing bars and re-covering.

3.Building No. 65 (Built in 1945 and enlarged in 1947, 1948 and 1958, nine stories with partial basement and tenth story).

In 1945 (Showa 20), just before the end of the Second World War, this building was built originally as a six story structure. As waterproofing techniques had advanced greatly by this time, the effects of weathering are not obvious, but the part built in 1945 is in far worse condition than the rest. The reason is not only that this part was exposed to severe sea winds but also that the quality of workmanship and materials at the end of the Second World War was very low. Here and there, the concrete surfaces have blown and the reinforcing bars of the beams have been revealed. The neutralization test for undamaged parts of the concrete disclosed that neutralization has not damaged areas close to the reinforcing bars except where the covering concrete is thin because of poor workmanship, and the building is still in good enough condition to be usable for a number of years, if the damaged areas were repaired. Indeed, on the landward side and in the courtyard, its frame has scarcely weathered and the waterproofing of the terrace and the roof is secure.

4.Buildings No. 56 and No. 57 (Built in 1939, reinforced concrete construction, 3 or 4 stories).

These two apartment blocks were executed at the same time and have been standing since 1939. A new structural feature, the horizontally continuous veranda supported on giant cantilevers at the end of which parapets were fitted, was adopted for the first time on Gunkanjima. It functioned as a corridor and terrace in front of the rooms and its use demonstrates that the detail required to make such a structure feasible -piped drainage through the slab- was understood by the designer, though somewhat imperfectly, since his use of it was inept. Even the parts not directly exposed to the salt wind were damaged severely, but the fact that this drainage is limited to the cantilevers and not seen i the interior and the walls indicates that the weathering arose from the immaturity of the structure and the waterproofing. The slabs have neither beam nor haunch and are cracked at each corner, so they had to be reinforced with cast iron supporting pipes. The reasons for the failure of the cantilevers are bad arrangement of the reinforcing bars and insufficient thickness of concrete. The supporting pipes began to rust after the closure of the mine, collapsed after eight years and almost disintegrated after twelve years. The time will soon come when these slabs will fall down. These two buildings were connected by two story reinforced concrete bridges, and the expansion joint where they meet was omitted. As a result one of the bridges had already collapsed and been replaced with a wooden one before the closure of the mine. However, within eight years this timber bridge was rotten and unsafe to use. Almost every reinforced concrete parapet around the edges of the roof of these buildings broke off after 10 years because the reinforcing bars at the bottom rusted. This was caused by insufficient rise of asphalt waterproofing layer and was clearly a technical mistake.

5.Other reinforced concrete buildings.

There are about 40 reinforced concrete blocks on Gunkanjima, and the condition of each building depends upon the quality of its detailing and its site. In this report we limit detailed explanations to the main buildings already mentioned, provide a brief sketch of other structures in the next section and then discuss the causes of the weathering.