Time to Removal of Formwork
Removal of Formwork (Stripping Time) :In normal circumstance and where various types of cement are used, forms, may generally be removed after the expiry of the following periods
For other types of cement, the stripping time recommended for ordinary Portland cement may be suitably modified. Generally, If Portland pozzolana or low heat cement or OPC with direct addition of fly ash has been used for concrete, the stripping time will be 10/7 of the period stated for OPC with 43grade cement above.
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 slabs, beam or arch as the case may be together with any live load likely to occur during curing or further construction.
For rapid hardening cement,3/7 of above periods for OPC 33 grade will be sufficient in all cases except for vertical side of slabs, beams and columns which should be retained for at least 24 hours.In case of cantilever slabs and beams,the centering shall remain till structures for counteracting or bearing down have been erected and have attained sufficient strength.
Proper precautions should be taken to allow for the decrease in the rate of hardening that occurs with all types of cement in cold weather and accordingly stripping time shall be increased. Work damaged through the premature or careless removal of forms shall be reconstructed within 24hrs.
Oiling the Surface: Shuttering gives much longer service life if the surfaces are coated with suitable mould oil which acts both as a parting agent and also gives surface protections.A typical mould oil is heavy mineral oil or purified cylinder oil containing not less than 5% pentachlorophenol conforming to IS 716 well mixed to a viscosity of 70-80 centipoises.After 3-4 uses and also in cases when shuttering has been stored for a long time, it should be recoated with mould oil before the next use.
Inspection of Form Work
The completed form work shall be inspected and approved by the Engineer-in-Charge before the reinforcement bars are placed in position.Proper form work should be adopted for concreting so as to avoid honey combing, blow holes, grout loss, stains or discoloration of concrete etc. Proper and accurate alignment and profile of finished concrete surface will be ensured by proper designing and erection of form work which will be approved by Engineer-in-Charge.
Shuttering surface before concreting should be free from any defect/ deposits and full cleaned so as to give perfectly straight smooth concrete surface. Shuttering surface should be therefore checked for any damage to its surface and excessive roughness before use.