Under no circumstance the forms are removed until the concrete has hardened sufficiently. Under normal weather conditions, the concrete is left in place as long as possible up to a maximum of 28 days.
Figure 22.2 Formwork for square column
Figure 22.3 Formwork for beam and slab
However, the period to which the concrete should be left in place depends on the temperature of air, the shape and position of structural member, the load condition and the type of cement used. The curing period is reduced in case of use of rapid-hardening cement, low temperature, low water-cement ratio and high loads. In such cases early removal of forms may be permitted.
In order to roughly assess the hardness of concrete, the concrete has to be struck and if a metallic sound is heard, it can be presumed that the concrete has hardened. However, for other reasons such as re-use of forms, early use of the structure and need to cool the concrete in massive concrete the forms may be removed subjected to the condition that the concrete is adequately hardened.
In normal conditions with the atmospheric temperature is above 20°C, normal cement is used and the type of structure, the forms may be removed after a curing period as recommended in Table 22.1.
Table 22.1 Type of structure and curing period
After adequate curing, forms should be stripped or removed with great care so as not to damage the concrete wedges, vertical supports etc. In order to prevent sudden application of load on the structure, the forms should be slackened gradually.
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