FORMWORK

I. DEFINITION

• A concrete form is a temporary boarding, sheeting or pans used to produce the desired shape and size of concrete.

• Forms should be watertight, rigid, and strong enough to sustain the weight of concrete. It should be simple and economically designed to be removed easily and reassembled without damage to them or to the concrete.

II. FACTORS CONSIDERED IN THE SELECTION OF FORMS

1. COST OF MATERIALS.2. THE CONSTRUCTION AND ASSEMBLING COST.3. THE NUMBER OF TIMES IT COULD BE USED.4. STRENGTH AND RESISTANCE TO PRESSURE AND TO TEAR AND WEAR.

III. CLASSIFICATION OF FORMS

• BY MATERIAL1. WOOD2. METAL3. PLASTIC4. COMPOSITE

• BY SHAPE

1. STRAIGHT2. CIRCULAR, ETC.

• BY USE1. FOUNDATION2. WALL3. STEPS

4. BEAMS & GIRDERS5. SLAB6. SIDEWALKS, ETC.

IV. PLYWOOD AS A CONSTRUCTION FORM• Plywood is a versatile construction material.• The introduction and satisfactory result brought about by plywood forms almost absolutely resulted in the limited use of tongue and groove (T&G) wood board.

• ADVANTAGES OFFERED BY PLYWOOD:

1. Economical both in materials and labor.

2. Has a plain, even surface with uniform thickness.

3. It offers fitted joints, eliminates dressing, planing of the surface which is normal to wooden board forms.

4. The laminated cross-grained of plywood has made the board stronger and free from warping.

5. Lightweight, handy and fast to work on.

6. It produces smooth finish of concrete that sometimes need little or no plastering at all.

METAL FORMS• are seldom used in building construction.• extensively used in road construction.• adopted on precast and prestressing plants as mould for those

flat and wider members.• generally made out of G.I. sheet, or black iron sheet, supported

by flat and angle bars designed to be assembled and locked by means of clamp, bolts and nuts, etc.

V. COSTCost is the primary consideration in selecting the kind of

materials to be used as a form. Cost is a broad term in construction, which under this particular item refers to:

•Initial investment•Assembling cost•The number of times it could be used.•Durability of the materials to resist pressure and tear and wear.

VI. CONSTRUCTION, ERECTION AND SECURING OF FORMS

Forms shall be guarded against bulging and sagging failure that occurs during the process of pouring. Forms shall be substantially strong to resist the weight and horizontal pressure of fresh concrete.

• Small structures consisting of small footings, columns and beams for one or two story building wherein (3/16”) 6 mm thick plywood is satisfactorily used supported by 2 x 2 wood frame and ribs.

• Medium size constructions are those having concrete column, beams, and concrete floor slab generally of 2 to 3 stories high. Forms are made out of (1/4” or 1/2”) 6 or 12 mm thick plywood is employed as form supported by 2 x 2 and 2 x 3 wood frame and ribs.

• Constructions having massive structures uses forms of various thickness that range from 6 mm to 19 mm thick plywood (1/4” to 3/4”) supported by lumber of sizes from 2 x 2 to 2 x 4.

2 Types of Wood Framing in Plywood Forms:

• Longitudinal ribs• Perpendicular ribs

COLUMN FORMS• CIRCULAR• SQUARE OR RECTANGULAR

BEAM & GIRDER FORMS• composed of only three parts. One bottom form and two side forms ordinarily made of plywood board supported by 2” x 2” and 2” x 3” wood frame.

WALL FORMS

• Wall forms above the ground or floor level is usually in pair strong enough to resist the lateral pressure of concrete. Wall forms should be guarded against bulging, which is the usual failure. The most effective way of securing wall form is the use of bolts and knots.

SLAB FORMS

• TRADITIONAL SLAB FORMWORK• TIMBER BEAM SLAB FORMWORK• METAL BEAM SLAB FORMWORK• MODULAR SLAB FORMWORK

Forms are properly secured in position by means of cleats, braces, twisted tie wire, bolts, clamps or nails.

• For small structures, forms are erected and secured by means of common wire nails not totally driven down leaving a protruding head for pulling off by the aid of hammer or wrecking bar.

• Sometimes this method is not sufficient when the structure is massive that the employment of those above mentioned accessories are necessary to prevent bulging and sagging of the forms.

When tie wire is used, they are twisted to tighten the forms and the projecting end are cut when forms are taken down leaving the other portion of the wire embedded inside the concrete. If bolts are used, they may be greased before the concreting so that they could be driven out of the concrete easily when the forms are removed.

VII. GREASING OF FORMS

• The purpose of greasing the forms is to make the wood waterproof.• It also prevents adherence of concrete into the pores of the wood.• Crude oil is the most economical and satisfactory material for this purpose. The crude oil is mixed with no. 40 motor oil to a proportion of 1:3 mixture or with varying viscosity according to the temperature.• Metal forms also need oil application to prevent rust formation.