Before moving forms were introduced for concrete work, an average of nine days had to elapse from the time of pouring to the striking of forms. New methods have enabled concrete buildings to be raised much more quickly
MOVING FORMS were used to build these silos for cement at West Thurrock, Essex. This photograph shows the work at the stage when the walls had reached a height of 20 feet. From the main platforms are hung lower platforms used for finishing the previously poured section. To the left is the concrete hoist, with mixing plant at the foot.
FEW things have wrought so vast, a change in the civil engineering world as reinforced concrete. Each development brings still further change, and things once considered impossible become standard practice. One such development is the introduction of moving forms and the perfection of the method.
Many people still consider it impossible to put up buildings so rapidly that the concrete is still green when it leaves the forms, and that the building is often completed long before the concrete has attained its ultimate strength. This can, however, be done with moving forms, and it has been done in scores of instances.
Forms, formwork, and shuttering mean the same — the wooden, sometimes sheet-steel, casings erected as moulds into which the liquid concrete is poured. When the concrete has hardened, the forms are struck and the concrete structure is left. With ordinary fixed forms the time allowed between pouring the concrete and striking the forms averages nine days. With moving forms it averages nine hours. When moving forms are used the forms are not struck, but move upwards, and only nine hours elapse from the time that the concrete is poured to the time that it emerges at the bottom of the mould and is left unsupported.
It seems incredible that green concrete can thus be left unsupported while more and more concrete is being added above it, without the whole structure collapsing. The man who first attempted to do this was the late J. S. Metcalf of Canada. The first work he attempted was a silo of four small bins. That was in 1900 and the silo is still in use. His tackle was crude, but the method was sound and, with various improvements, has been universally adopted.
The basic principle of the method is that concrete is poured into a shallow mould having the same external and internal wall plan as the structure, the mould being arranged so that it can be moved slowly upwards. The upward movement is continued (generally over a period of several days) until the required height for the building has been reached.
There must be a series of fixed elements on which the forms can move. These elements are generally steel rods, of ¾ in. to 1¼ in. diameter, built into the wall as the work proceeds. The wall thus grows out of itself, as it were. Attached to each rod by a screw-jack arrangement is a heavy timber yoke weighing some 2½ cwt., and this yoke carries the working platform and the forms into which the concrete is poured. By turning the jack, the yoke, platform and forms can be moved upwards on the fixed rods at any desired speed. The yokes are spaced out round the structure so that the load on each is the same. On a large scheme there may be as many as 600 yokes and jacks, and sometimes the jacks are power-operated by a series of motors and shafts.
The main platform covers the entire area of the structure, and it is on this platform that the major part of the work is done during construction. As soon as the foundations have been completed, the formwork is built up for the whole of the walls, whatever the shape or size of the structure, and the platform is laid. When jacking begins, the forms and platform are lifted together. The steel reinforcement is stored on the platform in racks and fixed as the platform moves upward, and the concreters follow on the heels of the steel fixers. The concrete is mixed on the ground and lifted to the platform in a vertical hoist erected alongside the structure, and it is distributed and poured into the forms at the required points by chutes or hand-trucks.
The forms are generally four feet deep. To prevent the upward movement from disturbing the newly placed concrete they are tapered, that is, they are the wall thickness apart at the top and ¼ in. farther apart at the bottom. A lower platform is hung from the yokes to allow finishing operations. The concrete that is exposed as the form moves upwards is still green — that is, it has not finally set. The surface can be easily rubbed down with a wooden float to remove irregularities and board marks, and small holes can be filled. If the nature of the building calls for some further treatment of the wall surfaces, such as a cement wash, stippling or painting, this work also is done from the lower platform.
CONCRETE BUILDING erected in 1935 at Highgate, London. Moving forms of a special type were used for pouring the walls of this block of flats. The forms (at the top of the building) were independent of the walls. Thus they could be detached at each floor level to allow the floors to be cast in one piece.
Moving form construction furnishes a good example of teamwork. The layout of the yokes and of the jacking system is taken into account at an early stage in the planning of a scheme, and the design of the structure, and particularly of the reinforcement, is often varied to fit in with the moving form system. Each section of the work — namely, foundations, formwork construction, erection of mixer, hoist and other plant, steel fixing, concreting, jacking and finishing — has to be considered and the whole programme carefully planned.
The most careful supervision and coordination are required when jacking begins. After the forms and platform have been built, the first four feet of wall are concreted, and at the same time the fixed rods are put in position. An interval of several hours is allowed for the concrete to set. Then the yokes are adjusted, the platform is levelled and jacking starts. The jacking team must be well drilled and jacking must proceed strictly to plan or to a system of signals. The effect of a set of jacks at one end of the platform being raised at a greater speed than at the other would be disastrous. The number of jacks for each man depends on circumstances.
First, each jack is tightened, and after the whole of the platform has been carefully levelled and the forms plumbed, a complete round of jacking is made, each jack being given a quarter turn; or, say, a lift of ⅛ in. Another similar round generally frees the forms, and from then on the upward movement continues with as few interruptions as possible.
The speed of jacking is from seven to ten feet a day, which means that the structure is being built at this rate — that is, 50 to 70 feet in a week. There is no limit to the size of the building in plan.
When the roof level has been reached, the forms are stopped and fixed to the walls, and the main working platform is used as the shuttering for the concrete roof slab. Grooves are left in the walls where floors occur, and the floors are built in after the moving formwork has been raised clear. Any internal work, such as fixing, joinery, plumbing, or painting can also proceed inside the building beneath the working platform. This work can follow upwards in the wake of the concreting.
Moving forms were first used on simple silos having high vertical walls and no floors, windows or similar complications; but it was soon found possible to apply the method to other types of buildings, such as factories and flats. The building must be high if the method is to prove economical.
THE SILOS illustrated at the top of this page reached a height of 69½ feet five days later. With other methods generally nine days must elapse before the concrete can be left unsupported; with moving forms new concrete is poured continuously and is left unsupported after nine hours.