Centering or scaffolding in house construction

Centering/Scaffolding using sarve poles and metal/wood planks. While it was a new term for me, I had a difficult time to understand why we are using such a primitive way to do the most critical part of the building!

Just to clear the air about it.

There are times when you want to pour concrete into a container such that the concrete sets in and takes the shape of the container. Once concrete sets in you could remove the container and the concrete holds its shape. You could also call this container a mould. A regular house requires such containers to normally be line based than have sphere or other complex moulds.

Such a mould has basic requirements,

1. The form or mould should be convenient to setup and remove and should be leak proof.
2. The form should withstand load and should not give way when concrete is poured into it.
3. The form should have some kind of steel or rebars inside such that it gives rigidity and strength to the structure. [structural steel or rebars or reinforcement steel rods ]

Now such a formwork is used for

1. RCC columns
2. RCC beams
3. RCC mould or roof
4. Chejja
5. Lintels
6. Stair cases
7. Some design parts of elevation if cannot be done using blocks/bricks.

Now such a formwork can be made of

1. plastic
2. metal or
3. wood planks and boards. [ most common ]

Such a formwork requires support such that it can withstand load and also provide an access to pour concrete into the formwork. Such a support is normally called as scaffolding and is accomplished using

1. Sarve Poles [ long wood based poles from trees ]
2. Metal pipes that can fit into one another to form structures [ normally not used for individual houses due to non availability with local contractors

Normally a scaffolding using metal and metal formwork gives the best results in terms of straight lines,  straight beams, straight columns. But the metal formwork is sometimes overused and has lot of bends and can result in bent beams and non-uniform beams. Such beams when not properly done result in uneven surfaces and will end up consuming more cement during plasting. So it is better to check the formwork after it is done to ensure all beams are straight to the visible eye and also use masonry tools such as plumb line, moole matta [ 90 degree measurer ]. One can also use laser levels such as bosch or other brands which can draw perfect horizontal and perpendicular lines to the ground level. This has been extensively used in my project with its own advantages and disadvantages. It is a must try for all enthusiastic people who want to try something different.

Normally good quality framework and scaffolding is not available with contractors who do centering for individual houses. Such formwork is normally available with bigger builders and apartment contractors. If you could inquire about it, it will give the best finish since most of these modules will be perfectly flat and modular. Normally centering is done using wood/metal planks and wooden poles. The individual plates are themselves  bent in some areas, and is normally nailed to one another to hold onto each other. Normally there will be some minimum leakage of concrete in such formworks. Sometimes the poles may give way but you could always check it up for proper support by shaking a few poles and testing it manually. But do not panic, Note that 90% of the houses are constructed using this primitive method owing to cost and availability. But its good to know the limitations of such a primitive system.

Normally the concrete for such formworks is filled using

1. in-site preparation by your normal laborers for small jobs like columns
2. in-site preparation by normally called as GANG[some 10+ people supplying and mixing concrete using a manual mixer] for large jobs such as roof concrete and plinth beams.
3. RCC cement which is supplied by many vendors in Bangalore at any required grade M20, M25, etc using trucks and Pumped to roof top using pumps [ available on request with RCC supplier]

All such formwork face the problem of visibility. Most often you cannot see the gaps in concrete when its covered by formwork from all sides. So use of vibrator is mandatory. Vibrator allows to compact the concrete and vibration forces concrete to move into gaps and remove air bubbles allowing a solid compaction which is must for RCC structures[ columns/beams/roof/etc ].