Formwork Of Concrete
Formwork Of Concrete
1). General :-
The formwork shall be designed and constructed so as to remain sufficiently rigid during placing and compaction of concrete, and shall be such as to prevent loss of slurry from the concrete. For further details regarding design, detailing, etc, reference may be made to IS 14687.The tolerances on the shapes, lines and dimensions shown in the drawing shall be within the limits given below:
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FORMWORK PERMISSIBLE TOLERANCES
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S.No.
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Particulars
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Tolerances
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I.
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Deviation from specified
dimensions of cross- section of columns and beam
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+12 mm
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-6 mm
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II.
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Deviation from dimensions of
footings
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1).Dimensions in plan
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+50 mm
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-12 mm
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2).Eccentricity
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0.02 times the width of the
footing in the direction of deviation but not more than 50 mm
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3).Thickness
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+/- 0.05 times the specified
thickness
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These tolerances apply to concrete dimensions only, and not to positioning of vertical reinforcing steel or dowels.
2). Cleaning and Treatment of Formworks
All rubbish, particularly, chippings, shavings and sawdust shall be removed from the interior of the forms before the concrete is placed. The face of formwork in contact with the concrete shall be cleaned and treated with form release agent. Release agents should be applied so as to provide a thin uniform coating to the forms with out coating the reinforcement.
3). Stripping time
Forms shall not be released until the concrete has achieved a strength of at least twice the stress to which the concrete may be subjected at the time of removal of formwork. The strength referred to shall be that of concrete using the same cement and aggregates and admixture, if any, with the same proportions and cured under conditions of temperature and moisture similar to those existing on the work.
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FORMWORKS
AND REMOVAL PERIODS AS IS 456:2000
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S.No.
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Type of Formworks
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Minimum Period Before Striking Formwork
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1
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Vertical formwork to columns, walls, beams
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16-24 hours
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2
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Soffit formwork to slabs (Props to be
refixed immediately after removal of formwork)
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3 Days
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3
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Soffit formwork to beams (Props to be
refixed immediately after removal of formwork)
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7 Days
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4
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Props to slabs
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a). Spanning up to 4.5 m
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7 Days
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b). Spanning over 4.5 m
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14 Days
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5
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Props to beams and arches
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a). Spanning up to 6 m
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14 Days
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b). Spanning over 6 m
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21 Days
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A). While the above criteria of strength shall be the guiding factor for removal of form work, in normal circumstances where ambient temperature does not fall below 15 deg. centigrade and where ordinary Portland cement is used and adequate curing is done,following stripping period may deem to satisfy the guideline given in 11.3:
For other cements and lower temperature, the stripping time recommended above may be suitably modified.
B). The number of props left under, their sizes and disposition shall be such as to be able to safely carry the full dead load of the slab, beam or arch as the case may be together with any live load likely to occur during curing or further construction.
C). Where the shape of the element is such that the form work has re-entrant angles, the form work shall be removed as soon as possible after the concrete has set, to avoid shrinkage cracking occurring due to the restraint imposed.
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