The hammer drives an arrow (wedge) into the concrete and breaks it into pieces. During the demolition of the Yurakucho Pikaderie Theater in 1958, around 50 demolition workers (known as Yamaya: a quarryman) worked day and night using this method. This method of demolition was utilized in a large-scale mobilization of labor. The beam was knocked off the stirrup by flat chisel from above, and a rod was driven between the axial main bars. The column was laid down and then the hoop was cut. Here, the reinforcing bar inside the concrete was taken out neatly.Fig. 1 shows the transition of the demolition method for reinforced concrete structures. After that shows the breaker method, the wrecking ball method and the larger breaker method. These are all types of destruction methods which proceeded but due to environmental issues and concerns, the use of wrecking ball methods have declined and larger breaker methods have gradually changed from pneumatic pressure to hydraulic pressure. When entering the 1970s, using the cutting method, disassembly by parts, use of static cracking agents, electrical indirect crushing methods and jacking methods were tested. Currently, the crushing method is considered the general-purpose method, replacing the wrecking ball methods and larger breaker methods (Fig. 1). It has been said that the speed of progress in the methods of demolition reinforced concrete structures were fairly slow, and the main reasons are speculated to be as follows:1. Demolition work on reinforced concrete structure has a short history, and development research on the demolition method started in 1970 or later.2. In the past, local residents did not often make complaints about the environmental concerns (pollutions, etc.).3. Demolition using the wrecking ball methods and breaker methods were really efficient.4. There were many issues with the ordering system of the demolition work. In other words, in order to receive orders for construction post demolition, it is often the case that the contract’s highest priority to be extremely lowly budgeted and done in a short duration. Concerns regarding pollution control was a secondary priority that came after that.5. Reinforced concrete is a strong material and is hard to demolish due to the following properties:(a) It is difficult to cut with a normal blade, such as boring (drilling a hole and enlarging an existing hole).(b) The mass and volume are very large and it is difficult to handle.(c) Would not burn and has a high melting point.(d) Even though acids and other chemicals can be invasive, it is extremely difficult to change the properties in a short time and the permeability of the liquid is small.6. The amount of demolished material (by-product) is large and there is a limitation when reusing the material.During this period, the overturn of the structure was incorporated where the floor slabs and beams were destroyed and the walls and the columns were pulled down (Fig. 2, Fig. 3). At the time, this method was named “Ootashi (=overthrow”, where demolition expertise would describe this as a “man’s flower”, meaning a man’s pride, proving their spirit and skills. On the other hand, pick hammers have begun to be used to shave/chip the floating slab foundations (slab-on-grade) and rooftop cinder concrete slab structures.After the Korean War, breakers were also used for demolition work. Initially, the breakers were loaned by the US military and weighed around 45kg. Eventually, the demolition work was finally mechanized.Prior to then, demolishing one floor took about a month, which was a slow pace. However, along with the trend of shortening the duration of construction period, demolition work gradually moved toward mechanization. Picks and breakers during that time seemed to have been used mainly Fig. 2 Schematic diagram of pulling down outer wallFig. 3 Demolition due to pulling down (around 1963)─ 36 ─3. Year 1955-1966 (Development of the Era of Hand Breaker Method)
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