Excerpt from The Constructor
The removal of concrete forms also called as strike-off or form stripping should be carried out only after the time when concrete has gained sufficient strength, at least twice the stress to which the concrete may be subjected to when the forms are removed. It is also necessary to ensure the stability of the remaining forms during form removal.
Concrete Form Removal Time
The rate of hardening of concrete or the concrete strength depends on temperature and affects the form removal time. For example, time required for removal of concrete in winter will be more than time required during summer.
Special attention is required for form removal of flexural members such as beams and slabs. As these members are subjected to self-load as well as live load even during construction, they may deflect if the strength gained is not sufficient to handle to loads.
To estimate the strength of concrete before form removal, the tests on concrete cubes or cylinders should be carried out. The concrete cubes or cylinders should be prepared from the same mix as that of the structural members and cured under same circumstances of temperature and moisture as that of structural member.
When it is ensured that the concrete in the structural members has gained sufficient strength to withstand the design load, only then forms should be removed. The forms should be left for longer time if possible as it helps in curing.
Removal of forms from the concrete section should not make the structural element:
- Collapse under self load or under design load
- deflect the structural member excessively in short or the long term
- physically damage the structural member when the form is removed.
The following points must be kept in mind during form removal whether the structure will be prone to:
- freeze thaw damage
- cracks formation due to thermal contraction of concrete after form striking.
If there is a significant risk of any of the above damages, it is better to delay the removal time of forms. If forms have to be removed for optimizing the concrete construction activities, then these structures must be well-insulated to prevent such damages.
Calculation of Safe Form Striking Times:
Structural members are constructed based on designed load. But before a structure is complete and subjected to all loads assumed during structural design, the structural members are subjected to its self weight and construction loads during construction process.
So, to proceed with construction activities at a quicker rate, it is essential to calculate the behavior of structure under is self load and construction load. If this can be done and structural member is found to be safe, forms can be stripped-off.
If these calculations are not possible, then following formula can be used for calculation of safe form striking times:
Characteristic strength of cube of equal of maturity to the structure required at time of form removal
This formula was given by Harrison (1995) which describes in detail the background of determination of form removal times. Another method to determine the strength of concrete structure is to conduct the non-destructive tests on structural member.
Factors Affecting Concrete Form Striking Times
The striking time of concrete forms depends on the strength of structural member. The strength development of concrete member depends on:
- Grade of concrete – higher the grade of concrete, the rate of development of strength is higher and thus concrete achieves the strength in shorter time.
- Grade of cement – Higher cement grade makes the concrete achieve higher strength in shorter time.
- Type of Cement – Type of cement affects the strength development of concrete. For example, rapid hardening cement have higher strength gain in a shorter period than the Ordinary Portland Cement. Low-heat cement takes more time to gain sufficient strength than OPC.
- Temperature – The higher temperature of concrete during placement makes it achieve higher strength in shorter times. During winter, the concrete strength gain time gets prolonged.
- A higher ambient temperature makes the concrete gain strength faster.
- Forms help insulate concrete from surroundings, so longer the forms remain with the concrete, the less loss of heat of hydration and rate of strength gain is high.
- Size of the concrete member also affects the gain of concrete strength. Larger concrete section members gain strength in shorter time than smaller sections.
- Accelerated curing is also a method to increase the strength gain rate with the application of heat.
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