CLAUSE TWELVE
FRAMEWORK A) General
Forms shall be constructed either of approved wood or steel to conform to the shape, line and grade required. Forms shall have sufficient strength to withstand the pressure resulting from placement and vibration and the concrete, and shall be used, wherever necessary, to confine the concrete, and shape it to the required lines or to ensure against contamination of the concrete with material caving in or sloughing from adjacent excavated surfaces.
B) Design of Forms
Design of forms shall be so designed and constructed that can be properly placed and thoroughly compacted without loss of water. Formwork shall be adequately strutted, braced or tied. It shall be capable of withstanding the pressure resulting from the placing of the concrete, whatever method of compaction is employed, without undue deflection or loss of alignment. Lumber, once used in forms shall have tails withdrawn and surface in contact with concrete shall be carefully cleaned before pre use.
C) Deflection of Formwork
Location Permissible Deflection
Between adjacent framing members +1.5mm Between adjacent ties on a framing member +0.8mm Over the full depth of a vertical face +3mm Over 10 ft length horizontally +3mm
The Contractor shall make allowance in the form work for any camber specified by the Engineer. IF required the Contractor shall supply to the Engineer designs or calculation for form work he proposed to use.
d) Support
Formwork shall be so designed that the sides of the beams and slabs can be removed without disturbing the soft forms or their supports.
Props and supports shall allow accurate adjustment of the formwork to line and level and be capable of being removed in an approved sequence without injury to any portion of the structure.
Provision shall be made for removal of formwork without disturbing props required for supporting hardened concrete.
e) Clean and Oiling of Forms
At the time concrete is placed in the forms, the surface of the forms shall be free from encrustations of mortar, grout or interfere with the fulfillment of the specification requirements relative to the finish of formed surfaces before concrete is placed, the surfaces of the forms shall be oiled with a commercial form oil that will effectively prevent sticking and will not stain the concrete surface.
f) Removal of Forms
The formwork must be removed slowly, as the sudden removal of wedges is equivalent to suddenly applying load to the partly hardened concrete.
Timber, steel, cement, asphalt pots and other materials should not be stacked on any newly constructed floor without the permission of the Engineer.
Under average weather conditions (15°C) the following table will give a guide to the minimum periods after casting at which forms and supports can be removed.
Forms supporting sides of beams, walls and columns. 2 days Forms supporting secondary slabs, provided props are left in 6 days Forms supporting main slabs provided props are left in 12 days Forms supporting beams, provided props are left in 12 days Props supporting secondary slabs 24 days
Props supporting main slabs 28 days
Props supporting beams 28 days
For concrete made with rapid hardening Portland cement these times may be reduced according to the approval of the Engineer.
g) Form Ties
The material and position of any ties passing through the formwork shall be to the Engineers approval. Any holes left after removal of ties shall be filled with concrete of mortar of approved composition. Use of embedded wire ties for holding forms shall not be permitted in concrete walls to be subjected to water pressure or where the surface through which the ties extend will be permanently exposed. Wire ties may be used for holding forms for concrete walls where embankment is to be placed against both sides of the wads. Wire ties shall be cut off flush with the surface of the concrete after the forms are removed.
h) Proportions and quality of concrete
Four classes of concrete, defined below, shall be used in the various items of work:
Class A - Concrete containing not less than 400kg of Portland cement per m3 (normal mix: 1:1½:3).
Class B - Concrete containing not less than 300kg .of Portland cement per m3 (Normal mix: 1:2:4).
Class C - Concrete containing not less than 200kg .of Portland cement per m3 (Normal mix: 1:3:6).
Class D- Concrete containing not less than 150kg of Portland cement per m3 maximum size of aggregate 38mm (Normal mix: 1:4:8).
The minimum crushing strength of the various classes of concrete measured on cube sample of 15x15x15cm shall be at least as follows: Class of concrete At 7 days (kg/cm2) At 28 days (Kg /cm2)
Concrete Class A 190 280
Concrete Class B 150 210
Concrete Class C 90 140
For each grade of concrete a set of 6 cubes shall be made, three shall be tested at an age of 7 days and three at 28 days. Where sulphate resisting cement is used, the figures for strength given in the Table shall be attained by 42 days.
The Engineer may order the prompt removal of any concrete work which upon testing proves to be of inferior strength to the above specifications and the Contractor shall follow the Engineer’s instructions with regard to the above.
The Contractor will not be entitled for any payment or extension of time for any such work.
i) Placing and Compaction for Exposed Finish Concrete
Fair-face concrete shall be placed with special care and fully compacted with immersion vibrators where at all possible. The concrete shad be placed in about 15cm layers and each layer compacted by means of immersion vibrators inserted vertically at point 40cm to 50cm apart. The: rate of concrete placing shall be kept within the capacity of the vibrators rand no more than 10cm of un-vibrated concrete shall exist as a
surcharge over the vibrator at any time.
j) Hand Mixing
Where small quantities of concrete are required, the approval of the Engineer is necessary to allow mixing by hand. The proportion of cement should in this case always be increased by 1%, Hand mixing should be carried out on a clean hard surface or platform to prevent dust and mud being shoveled up with the concrete. The cement, sand and coarse aggregate should be carefully measured in their correct proportions and thoroughly mixed before water is added. Only sufficient water to make the mix workable should be used, this should be sprayed from a watering can. Mixing should continue until the color of the wet concrete is uniform.
TEST ON CONCRETE