Form Work

   INTRODUCTION

   Form-work is a temporary structure. It is a mould or die used to support and shape the concrete            until it attains sufficient to carry its own weight. The form-work holds the concrete until it hardens      to required shape and size. Once the concrete develops adequate strength, the forms are removed.

• Form-work constitutes 30% of the cost and 60% of the time in concrete construction
• Quality of concrete finish and soundness of concrete depends very much on the form work system.
• Desired shape of concrete is not possible if form-work not done properly.
• Form-work should be properly designed, fabricated and erected to receive concrete.
• Accidents happen because of the faulty form-work and scaffolding or staging.

2       REQUIREMENTS OF FORM-WORK

        There are essential requirements for form-work. Any good form-work should satisfy all of the                following   requirements.

         Containment

         Form-work must be capable of shaping and supporting the fresh concrete until it becomes hard.

         Strength   

         Form-work must be strong enough to withstand the dead weight of fresh concrete placed on it. Also    form-work should be strong enough to bear the imposed loads after concrete placing.

         Resistance to Leakage

         All joint in the form face should be either close fittings or covered with some sheet material to            make these grout tight. Grout leakage can lead two weak concrete. It can cause honeycombing of        the surface & may produce projecting fins (snobs), which later have to be removed and touched up.

   

          Accuracy

          Form-work must be accurately set out so that the resulting concreting elements is in the right place     and is of the correct shape and dimension. The degree of accuracy concerned will be consistent           with the item being cast.

          Easy of Handling

          Form panels and units should be made so that they can be easily handled by hand or mechanical         means. In addition to all form work must also be designed and constructed to include facilities for       adjustment, leveling, erecting and striking without damage to the formwork or concrete.

          Form-work must be capable of gradual and easily remove.

          MATERIAL USED FOR FORM WORK

          Plywood

          This is by far the most common material used for the facing panel. It is easily cut to shape on site,       and if handled and stored carefully. It is commonly used 8’×4’ sheet size. Thickness is 15mm. it is     used for slab form-work, column boxes, staircase form-work and beams etc. This 15 mm thick             board can be used for 6 times. Applying mold oil, can increase usable of plywood board as well as     the quality of structure.

          Advantages of Using Plywood Sheets.

•             Good surface finish can be obtained.
•             Plywood form-work is economical because this can be used repeatedly although the cost of plywood is higher than class II timber planks.
•             Since the plywood sheet covers large area, the time taken to construct the entire shuttering is less. 
•       It can be easily sawn to the required shape.

          Disadvantage of Using Plywood Sheets

•       Even after removing the shuttering, adhered plywood particles can be seen on the surface when normal plywood used.
•       Peeling of layers with the time.
•       Due to the slenderness of the sheet large numbers of strengthening cleats are required, otherwise form-work gets deflected

         Timber

         This is the most common material used for bracing members to the form face. Usually use 2×2, 3×5     timbers for form-work.

         GI pipes

         It was used to tighten the form work. This is a 50mm diameter bar.

         Ties

         Form-work to vertical members such as walls normally requires ties to resist the pressure of the          concrete. These ties connect the outer framing member of one panel with those of the opposite side    of the walls so.

         U Jack 

         This used to support the columns and beams.

         Power saw

          This used to cut the plywood as well as timbers.

         Plumb-bob

          This used to check the verticality of columns.

        Other Material used for form-work

•      Concrete nails & wire nails.
•      Thread bars
•      Clamp (Fix, Free)
•      Adjustable steel props
•      Chain
•      Mould oil
• 
  

    CLEANING AND TREATING OF FORM WORK

    All rubbish, particularly chippings, shavings and saw dust, was removed from the interior of the         forms before the concrete is placed. The form work in contact with the concrete was cleaned and         thoroughly wetted or treated with an approved composition (mold oil such as grease mixed with         diesel) to prevent adhesion between form work and concrete.

      Before concrete poured, the form-work is carefully examined to ensure the followings.

• Removal of all dirt, shavings, sawdust and other refuse by brushing and washing.
• The correct location of tie bars bracing and spacers, and especially connections of bracing.
• All panels are well sheeted and all supporters are well tightened.
• Whether cover blocks are properly placed.
• Whether column is checked for verticality.

FORM WORK FOR COLUMNS

Column box is done by using plywood boards. To avoid plywood joints, it is normal practice to cut the sheets length wise in widths to match the column faces. The corners of the form are usually formed to have a 45 degree.

In our site most of them were rectangular shape. In the case of the rectangular column, the form-work consists of four shutters or panels made out of plywood board nailed securely to timber planks

Material used to column form work,

• 8’×4’ 15 mm thick Plywood board
• 2×2 timber
• 50 mm diameter GI pipes
  
• Corn tie
• U jack
• Plumb-bob
• Speed level to check off-set

First, the surface was roughened for the purpose of well bonding with later concrete. Then the four shutters were fixed together and GI pipes were hung with the nails fixed to the plywood sheet using binding wires in each side vertically. Then the horizontal pipes also fixed to the board by using the Form ties. The adjustable jacks were fixed between the horizontal GI pipes and to a fixed position of the floor. The cover blocks were also tightened to the reinforcement at the top.

Placing the column and check verticality

To place the form-work in right place, first should have to fix the kickers to the top of slab surface considering the offset line which drawn by quantity surveyor. 

After constructing the form-work for columns it is necessary to check for correct verticality   before concreting.

For small scale form-works a simple center plumb can be used to check the verticality.

Two center plumbs and a steel tape is required for this process. Center plumbs were hanged at two positions in the same plane with a known distance (i.e. 200 mm) measured from the inner surface of the plywood sheet. The distance from the plywood sheet to the thread of the center plumb is measured using the steel tape at both top and the bottom of the form work.

Calculations for a plywood form-work is as follows.

Offset distance from the column edge                        =          200 mm

Plywood thickness                                                      =          15 mm

Distance between the plywood and the plumb (X)    =          200-15

                                                            X                     =          185 mm

If the verticality of the column is accurate the distance between the plywood and the center plumb thread should be 185 mm everywhere. Jack supports or locking systems of the form-work can be adjusted to obtain the correct verticality in case of a mismatch.

FORM WORK IN BEAMS AND SLAB

Ina typical reinforced concrete building frame, the slab are the primary load carrying element. They are transfer their load to secondary element such as walls or beams.

Beams can be grouped in two categories. When located in the interior of the structure are usually T beams and when on the perimeter are L beams.

DESIGN AND CONSTRUCTION OF BEAMS

• Beam form-work consist of open through section and because it is not closed at the top. It needed more support to maintain the soffit and also provide lateral support to the sides.
• Beam soffit must be thickened timber (supported by 2×2 timber)
• Beam side 15 mm plywood with supported by braces.
• Beep beams (over 600 mm) should have wales and ties.
• Adjustable jacks must be placed under the beam bottom. (Distance between each supports approximately 1’6” or 2’)

SLAB

Form-work for slabs consist of sheathing made of plywood, which rest on joists a joists are supported by stringers and stringers are supported by on shores (U jacks and scaffold) which carry the weight of entire system. Figure x shows a typical slab form with its components.

Steps which carried out for constructing slab form-work

• Place the beam bottoms and elevate to offset line using adjustable jacks which
  
  supported by beam bottom.
• Fix the beam sideboards.
• Place 2 set of scaffolds (for one height slab) to support the soffit.
• Place U jacks in four side of each scaffolds.
• Adjust that U jacks up to stringer bottom level.                                                                                             

Calculate U jack level

Soffit height                                        = 2388 mm

Stringer height                                    = 27 mm

Height of joist (GI pipe)                     = 50 mm

Thickness of plywood                         = 15 mm

Reduce height to string bottom level  = 2296 mm 

(Note: Soffit height measured by 1000 mm level from F.F.L)

• After adjust 2 U jacks of sides that near to beams, then using thread other all U jacks can be adjustable
• Then place the 3×5 timber (stringer) on top of the U jack.
• After that place GI pipes in 1’ intervals.

Before laying the plywood, should have to check followings:

• Stringers are positioned and set in line and adjusted to the correct level
• All stringers are plumb.
• Stringers joints are supported.
• Stringers are seated centrally on U head.
• Scaffold bracing are properly fixed.
• All supports are horizontally and diagonally braced.                   

• Then lay the 15 mm thick plywood on top of the GI pipe and place the side boards in beams.

When sheeting,

• Edges are properly sealed (It will prevent swelling).
• Stick surface tape to joints if necessary.
• Check that joints are sealed, level and tight.
• After laying all the plywood boards, mold oil should be applied.
• Now is ready to do the reinforcement.

STOP BOARD CONSTRUCTION FOR SLABS AND BEAMS

FORM-WORK IN STAIRCASE

Stair form-work must support the weight of concrete. The weight of the throat of the stair and the steps will have to be supported. Because of the slope of the stair, some of the force is transmitted sideways. All form-work must be well tied together to prevent sideways movement. 

WALL FORM-WORK

Wall form-work is a simpler than for other concrete units as the actual forces against it are less, most of the load being carried vertically downwards. The panels at both sides are held in position by ties. Ties are also used as spacer, arranging wall thickness. Wall support systems are usually sloping props at satisfactory intervals.

REMOVAL OF FORM-WORK

Time of form-work removal depends on the following factors.

• Type of cement
• Whether it is Ordinary Portland Cement (OPC)
• Ration of concrete mix
• Rich ration concrete gain strength earlier as compared to weak ratio of concrete.
• Weather condition
• Hydration process acceleration is hot weather condition as compared to cold and humid weather conditions

Form-work removal period

Column formwork                  - 24hrs

Soffit formwork of slab          - 4 days

Soffit formwork of beams      - 10 days

Props of slab                           - 10 days

Props to beam                         - 14 days