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RCC Floor

Types & Construction of RCC Slabs

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RCC Floor

  1. 1. RCC Floor Hrishi Desai 0813
  2. 2. Introduction A Reinforced Concrete Slab is the one of the most important component in a building. It is a structural element of modern buildings. Slabs are supported on Columns and Beams. RCC Slabs whose thickness ranges from 10 to 50 centimetres are most often used for the construction of floors and ceilings Thin concrete slabs are also used for exterior paving purpose. In many domestic and industrial buildings a thick concrete slab, supported on foundations or directly on the sub soil, is used to construct the ground floor of a building. In high rises buildings and skyscrapers, thinner, pre-cast concrete slabs are slung between the steel frames to form the floors and ceilings on each level. While making structural drawings of the reinforced concrete slab, the slabs are abbreviated to “r.c.slab” or simply “r.c.”. !
  3. 3. Types of Design For a suspended slab, there are a number of designs to improve the strength- to-weight ratio. In all cases the top surface remains flat, and the underside is modulated: Corrugated Slab, usually where the concrete is poured into a corrugated steel tray. This improves strength and prevents the slab bending under its own weight. The corrugations run across the short dimension, from side to side. Ribbed Slab, giving considerable extra strength on one direction. Waffle Slab, giving added strength in both directions.
  4. 4. Waffle Slab Corrugated Slab Single Ribbed Slab
  5. 5. Construction A concrete slab can be cast in two ways: It could either be prefabricated or cast in situ. Prefabricated concrete slabs are cast in a factory and then transported to the site ready to be lowered into place between steel or concrete beams. They may be pre-stressed (in the factory), post-stressed (on site), or unstressed. Care should be taken to see that the supporting structure is built to the correct dimensions to avoid trouble with the fitting of slabs over the supporting structure. In situ (locally on site) concrete slabs are built on the building site using formwork. Formwork is a box-like setup in which concrete is poured for the construction of slabs.
  6. 6. For reinforced concrete slabs, reinforcing steel bars are placed within the formwork and then the concrete is poured. Plastic tipped metal, or plastic bar chairs are used to hold the reinforcing steel bars away from the bottom and sides of the form- work, so that when the concrete sets it completely envelops the reinforcement. Formwork differs with the kind of slab. For a ground slab, the form-work may consist only of sidewalls pushed into the ground whereas for a suspended slab, the form-work is shaped like a tray, often supported by a temporary scaffold until the concrete sets.
  7. 7. Materials used for the formwork The formwork is commonly built from wooden planks and boards, plastic, or steel. On commercial building sites today, plastic and steel are more common as they save labour. On low-budget sites, for instance when laying a concrete garden path, wooden planks are very common. After the concrete has set the wood may be removed, or left there permanently. In some cases formwork is not necessary – for instance, a ground slab surrounded by brick or block foundation walls, where the walls act as the sides of the tray and hardcore acts as the base.
  8. 8. Span – Effective Depth ratios Excessive deflections of slabs will cause damage to the ceiling, floor finishes and other architectural details. To avoid this, limits are set on the span- depth ratios. These limits are exactly the same as those for beams. As a slab is usually a slender member the restriction on the span-depth ratio becomes more important and this can often control the depth of slab required in terms of the span – effective depth ratio is given by, Minimum effective depth = span/(basic ratio x modification factor) The modification factor is based on the area of tension steel in the shorter span when a slab is singly reinforced at midspan, the modification factors for the areas of tensions and compression steel are as given in the figure 2 and 4 of the code.
  9. 9. Process Shuttering The process of constructing the RCC slab commences by erecting the centring and shuttering. Wooden shuttering is usually provided for the purpose although steel shuttering is recommended for getting a good under surface of the slab. Once the shuttering has been laid in a level, it has to be cleaned properly.
  10. 10. Reinforcement - The reinforcement bars are then laid as per design in both directions and tied properly with binding wire. Mixing concrete - All raw materials, cement, sand, coarse aggregates and water should be measured as per requirement of mix design The raw materials are then mixed uniformly, while ensuring that a proper water-cement ratio is maintained which is just enough to impart sufficient workability needed to pour concrete. Ideally, the mixing should be done in a mechanical mixer. The concrete mixer should be allowed to rotate for at least 2 minutes.
  11. 11. Pouring Concrete The concrete is then laid continuously to the required thickness and compacted using a needle vibrator to ensure that the cement slurry adequately fills the gaps between aggregates. Vibrated effectively such that no air gaps are left in the concrete After the concrete has been poured, it should be finished with a steel float to ensure that the top surface is finished smooth and to make fine adjustments for uniform slab thickness.
  12. 12. Curing Curing is critical for a good quality concrete slab and is is commonly done by flooding the slab with water after dividing the slab into smaller portions with cement mortar partitions. The curing of slab should begin the next day of casting and continue for at least 14 days for adequate concrete strength. Inadequate curing will result in poor quality concrete and can undo the benefit of an appropriate mix and good quality mixing and compaction
  13. 13. De-shuttering The last, but very important step. The side shuttering of the slab can be removed after 48 hours. The bottom shuttering should not be removed before 14 days. The under surface of the concrete should be cleaned with steel brushes such that no impurities are left on the surface. Although the slab should be constructed under the supervision of a professional, the owner should oversee that all the precautions are being taken during construction
  14. 14. Advantages Easy to construct, the raw materials are easily available in almost all places. Fire resistant Provides permanent durable roofing, capable of multi-storied construction Very versatile and can be cast in any form. Being a conventional option, the skill level for casting RCC slabs is commonly available Requires very low maintenance. Limitations A typical 120 mm slab weighs 300kg/m2.  This has to be designed for a live load of at least 150kg/m2. This makes the self weight of the slab as 50% of the design load resulting in 50% structural efficiency. The concrete in the tension zone does not take any stress and thus it is expensive to provide the same in the tension zone. The embodied energy of a conventional RCC slab is high because of its two main energy intensive constituents cement and steel.
  15. 15. Hrishi Desai 0813 Thank You!