Centering and Shuttering

CENTERING AND SHUTTERING

Centering is used to support horizontal members. Hence, formwork for floor beams and slabs is known as centering. Shuttering is used to support vertical members. Hence, formwork for columns, footings, or retaining walls is known as shuttering. In general these are temporary formworks in which the concrete is placed and allowed to hardened

REQUIREMENTS OF CENTERING AND SHUTTERING

The general requirements of shuttering and centering are as follows:

❖ The surface of the form should be smooth

❖ It should be able to withstand all the loads coming on it

❖ It should be able to retain its shape (horizontal and vertical bracing is done for this)

❖ It should have minimum deflection under load

❖ It should be re-usable and should not be costly

❖ It should be water-proof so that it does not absorb water from the fresh concrete

❖ It should facilitate easy stripping during the time of removal

MATERIALS USED FOR CENTERING AND SHUTTERING

Steel and timber are commonly used as shuttering and centering material, former for its efficiency and later for its cost-effectiveness.

Steel: Panels are fabricated from thin steel plates and small steel angles are used along the edges to stiffen these plates. Clamps, nuts, or bolts are used to hold the steel panels together. Steel is considered the best material for centering and shuttering. The reasons are explained below:

❖ Steel provides a watertight formwork so that strength of the cement pnis concrete is preserved

❖ It can bear the load coming on it easily

❖ It can be used for vertical, horizontal, or any type of formwork

❖ It provides a levelled surface of concrete. Desired appearance based on architectural recommendations is easy to achieve with the help of steel formwork

❖ As the bottom surface of the structural member is levelled, less thickness of plaster is required

Timber: Timber is commonly employed by contractors, as it is cheap and easily available. Timber should be seasoned while using for shuttering. It should be light- weighted and should not have loose knots. Also timber should have smooth and even surface, especially when used for exposed concrete surfaces. Some of the points to be considered while using timber planks for centering or shuttering are listed below:

❖ The size and thickness of planks may not be uniform. Because of this non- uniformity, timber planks do not have watertight joints. Water and cement slurry can flow from the joints which reduces the strength of the concrete and as a result structural member.

❖ The surface of the RCC member can be uneven. Thus, more thickness of plaster is required.

❖ If timber planks cannot bear the load of concrete coming on it, then deflection may occur in the planks. In some cases, it may even lead to breakage.

Temporary Brick Masonry: It is sometimes used in mud for the vertical supports of the beam sides. Temporary brick masonry should be avoided as far as possible due to the following reasons:

❖ The brick masonry soaks the cement slurry from the member. Thus, the strength of the member is greatly affected.

❖ Moreover, the brick masonry formwork cannot bear the weight of vibrator, so proper compaction of the structural member is not possible. This again reduces the strength. here

❖ An uneven surface of RCC member is obtained requiring more thickness of plaster.