10.1 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 good practice [6-5A(28)]. The tolerances on the shapes, lines and dimensions shown in the drawing shall be within the limits given below:
| a) Deviation from specified dimensions of cross-section of columns and beams | mm |
|---|---|
| b) Deviation from dimensions of footings: | |
| 1) Dimensions in plan | mm |
| 2) Eccentricity | 0.02 times the width of the footing in the direction of deviation but not more than 50 mm |
| 3) Thickness | mm or 0.05 times the specified thickness, whichever is less |
These tolerances apply to concrete dimensions only, and not to positioning of vertical reinforcing steel or dowels.
10.2 Cleaning and Treatment of Formwork
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 without coating the reinforcement.
10.3 Stripping Time
10.3.1 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.
10.3.2 While the above criteria of strength shall be the guiding factor for removal of formwork in normal circumstances where ambient temperature does not fall below C, the following striking period may deem to satisfy the guideline given in 10.3.1:
| Type of Formwork | Minimum Period Before Striking Formwork | |
| For Concrete Made Using OPC | For Concrete Made Using Cement Other than OPC or Using Cementitious Materials Like Fly Ash and Slag | |
| (1) | (2) | (3) |
| a) Vertical formwork to columns, walls, beams | 16-24 h | 16-24 h |
| b) Soffit formwork to slabs (Props to be refixed immediately after removal of formwork) | 3 days | 7 days |
| c) Soffit formwork to beams (Props to be refixed immediately after removal of formwork) | 7 days | 10 days |
| d) Props to slabs: | ||
| 1) Spanning up to 4.5m | 7 days | 10 days |
| 2) Spanning over 4.5 m | 14 days | 14 days |
| e) Props to beams and arches: | ||
| 1) Spanning up to 6 m | 14 days | 14 days |
| 2) Spanning over 6 m | 21 days | 21 days |
NOTE — Utmost care shall be taken to provide props. The props shall be provided immediately after stripping each shuttering panel and not after stripping all the panels of the entire slab.