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Formworks and Accessories Requirements for Concrete Casting

Formwork and falsework shall be so constructed that they will support the loads imposed on them by the fresh concrete together with additional stresses imposed by vibrating equipment and by construction traffic, so that after the concrete has hardened the formed faces shall be in the positions shown on the drawings within the tolerances set as described in the specifications of Cast in Place Concrete.

Formwork shall be so designed that it may be easily removed from the work without damage to the faces of the concrete.  It shall also incorporate provisions for making minor adjustments in position, if required, to ensure the correct location of concrete faces.  Due allowance shall be made in the position of all formwork for movement and settlement under the weight of fresh concrete.

Where overhangs in formwork occur, means shall be provided to permit the escape of air and to ensure that the space is filled completely with fully compacted concrete.

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Formwork shall be provided for concrete surfaces at slopes of 30o to the horizontal or steeper.  Surfaces at slopes less than 20o may be formed by screeding.  Surfaces at slopes between 20o and 30o shall generally be formed unless the Contractor can demonstrate to the satisfaction of the consultant that such slopes can be screeded with the use of special screed boards to hold the concrete in place during vibration.

Horizontal or inclined formwork to the upper surface of concrete shall be adequately secured against uplift due to the pressure of fresh concrete. Formwork to voids within the body of the concrete shall also be tied down or otherwise secured against floating.

The internal and external angles on exposed faces of concrete shall be formed with fillets and chamfers of 25mm size, unless shown otherwise on the Drawings.

Formwork shall not be re-used after it has suffered damage which is sufficient to impair the finished surfaces of the concrete.

Where circumstances prevent easy access within the form for cleaning and inspection, temporary openings for this purpose shall be provided through the formwork. Shear keys shall be provided in all construction joints of the size and shape indicated on the Drawings.

Where precast concrete elements are proposed by the Contractor and agreed by the consultant, they shall comply with the requirements of precast concrete specification.  Such elements shall be set true to line and level within the tolerances prescribed for the appropriate class of finish as described in precast concrete specification and fixed so that they cannot move when concrete is placed against them.

formwork for concrete works

Formwork Supports

Ground supports shall be properly founded on footings designed to prevent settlement. Supports for formwork may be bolted to previously placed concrete provided the type of bolt used is acceptable to the consultant.  If metal ties through the concrete are used in conjunction with bolts, the metal left in shall not be closer than 75mm to the face of the concrete.

Formwork Joints

Joints in formwork for exposed faces shall, unless otherwise specified, be evenly spaced and horizontal or vertical and shall be continuous or form a regular pattern.

All joints in formwork including formwork for construction joints shall be tight against the escape of cement and fines.  Where reinforcement projects through formwork, the form shall fit closely round the bars. Refer below table for minimum periods for formwork removal:

 

Position of formwork Minimum period for temperatures over 10°C Strength to be attained
Vertical or near vertical
faces of mass concrete

Vertical or near vertical faces of reinforced walls, beams and columns

Soffits of arches, beams and slabs (props left in position)

Props to soffits of arches, beams and slabs

24 hours
48 hours
10 days
14 days
 

 

 

0.2C
0.3C
0.8C
C
C is the nominal strength for the class of concrete used.

 

Special finish using Controlled Permeability Formwork (CPF) liner

The requirement for a special finish shall be as for traditional formwork finishes except that a Controlled Permeability (CPF) liner shall cover the formwork.

Controlled Permeability Formwork (CPF) shall be used on all surfaces as detailed on the drawings.

The CPF liner shall have independent third party certification and have a current British Board of Agreement (BBA) certificate.

The CPF liner shall be tested as recommended by the BBA and certified by them to:

  1. Have a compression of less than 10 percent under a pressure of 200kPa.
  2. Have a minimum pore size of less than 0.030mm.
  3. Have a minimum water relocation capacity of 0.35l/m2.
  4. Result in bleed water from the liner which is free from cement and fine aggregate particles.

The concrete cast against the CPF liner shall have an even uniformly textured matt finish and shall be free of blowholes and other surface blemishes.  The use of the CPF liner shall improve the performance of the concrete as follows:

  • The mean SURFACE STRENGTH for the CPF cast face shall exceed that for the control face by at least 70%.
  • The mean 10 MINUTE ISAT result for the CPF cast face shall be not more than 15% of that for the control face.
  • The mean DEPTH OF CARBONATION for the CPF cast fame shall be not more than 15 percent of that for the control face.
  • The mean CONCENTRATION OF CHLORIDES at a depth of 11mm from the CPF cast face shall be not more than 15% of that for the control tape.

The above shall be established by tests on a specially cast panel using the procedures recommended by the BBA.

The CPF liner shall be used once only.  Release agents shall not be used with the liner and any residual release agent remaining on forms from previous use shall be removed.

To ensure conformity with the performance requirements the CPF liner is to be used in accordance with the manufacturers’ technical guidelines.

The CPF liner shall unless otherwise directed be left in place on the concrete after formwork removal for the curing period specified by the consultant.  It shall be kept wet and covered with plastic sheeting to promote efficient curing.

Curved surfaces

For curved surfaces where F2, F3 or F4 finishes are called for, the formwork face shall be built up of splines cut to make a tight surface which shall then be dressed to produce the required finish.

Alternatively single curvature surfaces may be faced with plastic or plywood linings attached to the backing with adhesive or with escutcheon pins driven flush.  Linings shall not bulge, wrinkle or otherwise deform when subjected to temperature and moisture changes.

Formwork Preparation

Before any reinforcement is placed into position within formwork, the formwork shall be thoroughly cleaned and then dressed with a release agent.  The agent shall be either suitable oil incorporating a wetting agent, an emulsion of water suspended in oil or low viscosity oil containing chemical agents.  The Contractor shall not use an emulsion of oil suspended in water nor any release agent which causes staining or discoloration of the concrete, air holes on the concrete surface, or retards the set of the concrete.

In order to avoid colour differences on adjacent concrete surfaces, only one type of release agent shall be used in any one section of the Works.

In cases where it is necessary to fix reinforcement before placing formwork, all surface preparation of formwork shall be carried out before it is placed in position.  The Contractor shall not allow reinforcement to be contaminated with formwork release agent.

Before placing concrete, all dirt, construction debris and other foreign matter shall be removed completely from within the placing area.

Before concrete placing commences, all wedges and other adjusting devices shall be secured against movement during concrete placing and the Contractor shall maintain a watch on the formwork during placing to ensure that no movement occurs.

Formwork Removal

Formwork shall be carefully removed without shock or disturbance to the concrete. No formwork shall be removed until the concrete has gained sufficient strength to safely withstand any stresses to which it may thereby be subjected.

The minimum periods which shall elapse between completion of placing concrete and removal of forms are given in above table and apply to ambient temperatures higher than 10oC.

Alternatively, formwork may be removed when the concrete has attained the strength set out in table above provided that the attained strength is determined by making test cubes and curing them under the same conditions as the concrete to which they refer.

Compliance with these requirements shall not relieve the Contractor of his obligation to delay removal of formwork until the removal can be completed without damage to the concrete.

If the Contractor wishes to strip formwork from the underside of arches, beams and slabs before the expiry of the period for supports set out above, the formwork shall be designed so that it can be removed without disturbing the supports.  The Contractor shall not remove supports temporarily for the purpose of stripping formwork and subsequently replace them.

As soon as the formwork has been removed, bolt holes in concrete faces, other than construction joints, which are not required for subsequent operations shall be completely filled with mortar sufficiently dry to prevent any slumping at the face.  The mortar shall be mixed in the same proportions as the fine aggregate and cement in the surrounding concrete and with the same materials and shall be finished flush with the face of the concrete.

Stripping of Formwork:

The consultant shall be notified in writing before the removal of any formwork. Concrete shall be thoroughly wetted as soon as the forms are first loosened and shall be kept wet during the removal operations and until the curing media is applied. Potable water supply with hoses having the fine fog spray attachments shall be ready at each removal location before operations are recommended.

The period of time elapsing between the placing of the concrete and the striking of the formwork shall be approved by the consultant after consideration of the loads likely to be imposed on the concrete and shall in any case be not less than the periods shown in the relevant section above. Re-shoring to beams and slabs shall be placed immediately after stripping formwork.

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