The usual materials which are employed in the preparation of form work are aluminum, steel and timber. Steel and timber are most commonly used materials.
Steel form work
Steel is used for form work when it is desired to reuse the form work in several times. The initial cost of steel form work is very high. But it proves to be economical for large works requiring many repetitions of the form work. The erection and removal of steel form work are simple and it presents a smooth surface on removal.
Steel shuttering is used for major work where everything is mechanized. The advantages of steel form work are.
When form work is required for small works requiring less repetitions, timber is preferred to steel. The timber form work is cheap in initial cost and it can be easily adopted or altered for a new use. Timber to be used as form work should be well-seasoned, free from loose knots, light in weight and easily workable with nails without splitting. Following precautions are necessary when using timber form work.
The timber is to receive wet concrete. Hence timber form work should be neither too dry nor too wet. If it is too dry, the timber will swell and get distorted when wet and honey-comb surface will appear on removal of the form work. On the other hand, if it is too wet, the timber will shrink in hot weather resulting in gaps in the form work through which concrete will flow out. Hence ridges will be formed on the concrete surface. It is found that a moisture content of about 20% is appropriate for the timber form work.
The dimensions of components of timber form work will depend upon the loads to be carried and the availability of timber sections.
Minimum nails should be used in timber form work and the nail heads should be kept projecting so as to facilitate easy removal.
The timber form work proves to be economical for buildings with minimum number of variations in the dimensions of the rooms. Thus the cutting of timber pieces is brought down to the minimum.
Plywood form work
Use of plywood as form work is becoming popular because
The plywood as form work can be re-used several times as compared to ordinary timber form work.
The plywood form work gives surfaces which are plain and smooth and hence, they may not require any further finishing treatment.
It is possible to cover up more area by using large size panel and hence, there is considerable reduction in the labour cost of fixing and dismantling of form work.
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