What is Striking Formwork?
The process of removal of formwork or shuttering in the process of casting concrete is known as striking. Once the concrete has achieved the initial recommended strength, to support the self-weight and any imposed loads, the shuttering is removed for further curing. Soffit forms are usually left in place for longer periods compared to side forms and removed last in case of slabs and beams.
Factors Affecting Period of Striking of Formwork
Period elapsed before the removal of formwork depends on various factors and will be different job to job. Some of the important controlling factors that determine the time required before removal of shuttering are as follows:
Type of Concrete Used
Concrete containing blast furnace slag (GGBS) or pulverized-fuel ash (PFA) require longer striking period compared to normal Portland cement concrete. These concretes will take longer time to gain strength compared to OPC. In case of rapid hardening cements, this is reduced significantly as the concrete attains in strength much faster pace.
Weather of Casting Concrete
During cold weathers, the rate of setting and attaining stiffness is retarded thus increasing the striking time. For example, Ground Slabs take more time compared to usual weather concreting, delaying trowelling operations. Often higher strength concrete is used to reduce the period of striking in these conditions.
Exposure conditions of the Site
As the mean temperature increases, the time required for removal of form reduces as the concrete attains strength at an increased rate, provided cracking is controlled.
Method of Curing of Concrete
Efficiency of curing can reduce the striking time. Curing of concrete should start as soon as the formwork has been removed. The concrete should be secured with insulations as a protection against temperature falls.
Type and Dimensions of Structure
Structural members like beams require more time before striking due to relatively bigger cross sections and being more critical element. Side forms are removed much sooner for curing, as in walls and column sides.
Admixtures or Treatment to Concrete
The Striking time can be brought down to the required value by adding admixtures to the concrete. Common example for these admixtures are accelerators which reduces the setting time of concrete.
General Considerations While Removing Formwork
- In case of slabs and beams, soffit formwork may be removed when the in-situ strength of concrete is 10 N/mm2 or two times the stress to which it will be subjected, whichever is the greater;
- As a best practice shuttering must not be detached until concrete is strong enough, to be self-supporting & capable to carry imposed loads;
- In order to access the in-situ strength of concrete, pull-out tests is performed or cubes are tested, which are cured under the same conditions as the in-situ concrete, achieved by temperature-matched curing by which cube samples are immersed in water whose temperature is matched to the structure;
- Proprietary quick-strip systems can be used to permit the removal of soffit shuttering without disturbing the propping;
- Especially in cold weather concreting the formwork to beam sides, walls and columns if struck at early ages the concrete will still be ‘green concrete’ and can be easily damaged.
- Striking must be carried out with care, to avoid damage to arise and projections, and it may be necessary to protect some of the work from damage immediately after removing the forms.
- The formwork must be removed slowly as the sudden removal of supports is equivalent to a shock load on the partly hardened concrete. Careful removal is also less likely to damage the formwork itself;
- Timber formwork is a good insulator in its own right, so in winter it is particularly important to avoid thermal shock to the warm concrete when timber or insulated steel forms are removed and the concrete is exposed to the cold air;
- If the formwork is not required elsewhere, it may be convenient to leave it in place until the concrete has cooled from its high early temperature.
Minimum time before Striking Formwork
Depending on Strength development of Concrete a general guide for the removal of formwork is given below for a Mean Air Temperature of 10 deg. C. Respective codes should be referred for recommended values.
|Structural Component||Time in day(s)|
|Sides to Beams & Walls||1.5|
|Soffit forms of Slabs||5|
|Soffit forms of Beams||8|
|Props to Slabs||8|
|Props to Beams||12|