RCC Column Construction Process – 4 Steps

The term RCC stands for reinforced cement concrete. A column manufactured using RCC is referred to as a RCC column. It  is a composite material made of cement, sand, aggregate and steel bars. 

Basically a RCC column is a compression member which takes on its longitudinal axis. The bending moment on the column is due to the wind forces and the sudden massive loads like earthquakes etc.

The size, shape, quantity of the materials used like cement, sand ,aggregate, reinforcement details etc  are mentioned in the detailed drawing by the structural engineer as per the load acting on the column and also following the code provisions.

It is the backbone of the entire construction therefore it is necessary to understand the methods and techniques in order to construct it with utmost care.

However codes are available to assist in each and every step of its construction .

RCC column construction

Construction of the RCC column in the field is done in four stages. We will discuss each stage in detail .

The four main steps involved in RCC column construction that ensure a strong and durable structure for any building project  are

1. Column Layout Work.
2. Column Reinforcement Work.
3. Column Formwork 
4. Pouring Concrete,

Before that design of rcc column must be taken by a good structural engineer. Code standards have to be followed to keep with the practices of building code. According to theory, concrete and reinforcement have some limitations

Column layout work

One of the most important aspects of any building project is the column layout. The column layout determines how the load of the structure is distributed and supported, as well as how the space is divided and utilized. 

The first step is to prepare the column layout drawing , using it the location of columns are marked practically in the field. In the drawing, column locations are shown associated with grid-line with dimensions. Marking of the columns on the ground is done by laying the rope as per the grid lines shown in drawing.

Usually in any building construction the columns are placed at regular intervals and symmetric locations to simplify design and construction. 

Best Practices for Column Layout

• Aligning columns with walls and partitions to reduce interference and increase flexibility
• Minimizing the number and size of columns to reduce cost and increase usable space.
•  Maximizing the span and spacing of columns to reduce bending moments and deflections
• Considering the effects of wind, earthquake, fire, and other loads on the column layout
• Coordinating with other disciplines such as mechanical, electrical, plumbing, etc. to avoid clashes and conflicts.

Column Reinforcement Work

After the column layout works, dig the column pits according to the dimensions and the depth as specified in the drawing.

But be careful, when doing the formwork, analysis and arrangement of bars. The column failure can occur for variety of reasons. It can be a buckling or shear failure. The common reasons are explained.

Now the second important step is to place the column reinforcement bars in the pits and tie them with binding wire.  It also involves adding additional steel bars, also known as rebar, to a column in order to increase its load-bearing capacity and resist forces such as compression, tension, and bending.

The reinforcement bars should be properly spaced and aligned according to the design specifications.

Important Points

• Check the numbers and diameter of vertical bars
• The spacing between vertical bars to be maintained properly
• Development length which depends on the diameter of the bar to be checked 
• Check for the lapping in alternate bars to come at the same height and not coming inside the beam or slab.
• Lapping at l/3 or 2l/3 of column
• The spacing between stirrups 
• The hook bends properly at right angles
• Check the stirrups corner

Reinforcement Work Steps

•   Designing the reinforcement which will depend on the specific requirements of the project and the loads that the column will be subjected to. This may involve calculations to determine the number, size, and placement of the steel bars.
• Preparing the column surface to be clean without any loose debris or rust .
• The steel bars called rebars are placed in a specific pattern around the column, with spacers used to ensure that they are properly positioned. The bars are then secured together using wire or other connectors.

Column lapping

The need for column lapping is to provide continuity to transfer loads from one bar to another. When providing lapping in the column all the rebars must be lapped in the zone B, as shown in figure.

The highest and bottom portion of the column, i.e. zone-A ( L/4 length ) must to be avoided as there’ll be the most moment within this zone because  because lateral forces acting on the column will create the maximum moment in this area.

Staggered bars

All the lapped bars must staggered in zone B as shown in figure. In no case fifty percent of the rebars are staggered at the same level.  If all the bars are staggered at the same level then the column may be subjected to a failure.

Stirrups spacing

The spacing of the stirrups within the lapping zone should to be minimal compared to the regular column c/c stirrup spacing. The designed standard spacing for the  stirrup is 175 c/c. 

Placing rebars

When tying  the rebars within the lapping zone, the joggled part of the bar must  be placed over the inner surface of the lowest bar (refer to the drawing above ) to obtain a consistent clear cover from the concrete surface

If we tie the joggled portion of the rebar over the outer surface of the lowest bar, then the lapping zone would invade the concrete cover space provided for the bars.

Reinforcement spacing

When the column height increases, the reinforcement is reduced because the load placed on the column is reduced. As a result, compared to upper floors, the ground floor column reinforcement is primarily higher. Therefore, using the  reinforcement detailing will change from the bottom to the top of the building .

Column framework

Columns are crucial  in the construction as they transfer the loads from top to bottom. So it is necessary to align them vertically so that they transfer load properly and remain in position throughout concreting.

Guidelines while Shuttering

• Holding the concrete shuttering firmly in place and for proper alignment, a concrete pad is cast before fixing the shuttering. The starter cures for a day or two so that it is hard enough to fix the shuttering around it.
• Column box or shuttering for columns is made of plywood sheets or steel sheets fabricators with adequate stiffeners.
• Thin films of oil/grease  are applied to the inner surface of shuttering to easily remove from a column. After the concrete hardens.
• Shuttering is properly aligned with its verticality and diagonals are checked to ensure accuracy in dimensions.
• Formwork is thoroughly supported  before pouring the concrete. So that it does not move horizontally or vertically during concreting.
• Gaps in shuttering joints to seal with plaster or a piece of wood are provided to prevent leakage of a slurry.
• Appropriate space to be provided in the inner face of the shutter and reinforcement by fixing cover blocks.
• It is preferable to remove shutters after 24 hrs of casting. If needed to be removed earlier, it should not be removed within sixteen hours.
• The removal of shutters is handled gently without jerks so that edges of columns will be safe without damages.
• Support is given to column shuttering to prevent it from moving during concreting.
• Diagonals of the shuttering to be  checked to ensure dimensional accuracy.

Pouring Concrete

Concreting of the rcc column is done either manually or with the help of the machine or pump.

Stepwise procedure of concreting

• With the help of concrete mixers the material is made ready for concreting, however this is helpful for  less quantity  and for larger quantities, ready-mix concrete (RMC) is available.
• placing concrete either by the pump or manually depends on the type of the project..
• Concrete must be poured up to the slab bottom; the remaining column gets concrete during pouring of slab & beam.
• Mechanical vibrators are to be compulsorily , care to be taken to avoid excess vibration which results in segregation. Each layer is to compacted thoroughly.
• Target slump to be 160 mm.
• The height of pouring concrete should not exceed 1.5 m.
• Avoid construction joints in the column.
• check proper cover.
• The temperature should be below 30 degrees while pouring concrete.
• After the pouring and vibrating, the horizontality and verticality of the column is checked.

curing/wet covering

It is the old practice of covering the column with the impervious bags like jute bags and frequently spraying them with water to keep the concrete damp. This process is helpful to reduce the heat of hydration of the concrete and increase its strength.