Types of Slabs in Concrete Construction
A reinforced concrete slab is a critical structural element in buildings that provides flat surfaces (floors and ceilings). Slabs are categorized as one-way slabs or two-way slabs based on the reinforcement given beam support, and the ratio of spans. The former is supported on two sides and has a greater than two-to-one ratio of long to short spans. The latter, on the other hand, is supported on four sides and has a lower long-to-short span ratio than two. Various circumstances and requirements need the selection of an appropriate and cost-effective concrete slab, taking into account the type of building, architectural plan, aesthetic elements, and span length. Different types of slabs considering these parameters are given below.
- Flat slab
- Conventional slab
- Hollow-core slab
- Hardy slab
- Waffle slab
- Sunken slab
- Composite slab
- Bubble deck slab
- Other types of slabs
A flat slab is made of reinforced concrete and is supported by a concrete column or caps. These types of slabs lack beams therefore it is also known as beamless slab which is supported on a column and loads are carried straight to the column. A plain ceiling is obtained in this form of construction, which has a pleasing architectural impression. Plain roofs are thought to be less fire-resistant than standard beam slab construction. Flat slabs are simple to build and require minimal formwork because their thickness is a minimum of 8 inches or 0.2m. The types of flat slabs are provided below.
- Column with no column head and slab with no drop
- Column with no column head and slab with drop
- Column with column head and slab with no drop
- Slab with drop and column with a column head
Advantages of Flat Slabs
The advantages of flat slabs are mentioned below.
- When a deep fake ceiling is not required, this slab lowers the floor-to-floor height.
- The building's height can be decreased.
- Construction takes less time.
- The shear strength of the slab is increased.
- Lowers the apparent or effective duration of the moment in the slab
Uses of Flat Slabs
The uses of flat slabs are mentioned below.
- To improve the light diffusion on the plain roof surface.
- It is a simple and cost-effective structure.
- Flat slabs can be found in parking lots, commercial buildings, resorts or places where beam projection is not desirable.
A conventional slab is the type of slab that is supported by beams and columns. As a result, the slab's thickness is small while the beam's depth is great, and the load is passed to the beam and then the column. When compared to flat slabs, this requires more formwork. There is no need for a column cap in traditional slabs. The thickness of typical slabs is 4 inches or 10 cm, but 5 to 6 inches is advised if the concrete will be subjected to large loads on a regular basis, such as motorhomes or garbage trucks. In the traditional type of slabs, reinforcement is provided by straps that are installed horizontally and vertically. The straps that are installed horizontally are known as primary reinforcement bars, while the straps that are installed vertically are known as distribution bars.
Types of Conventional Slabs
The types of conventional slabs are mentioned below.
- One-way slab: In this type of slab, to move the load in one direction, the slab is supported by a beam on two opposing sides. The longer span to shorter span ratio is greater than or equal to two. The slab can only be rotated in one direction in this case, i.e. within a path along its shorter span. Primary reinforcement resistance is supplied to uniformly distribute the load over extended intervals above temperature and shrinkage stresses. The distribution reinforcement is delivered in the long run, whereas the principle reinforcement is provided in the short term. To endure the development of pressure, the principle bars are cranked.
- Two-way slab: In this type of slab, the weight is carried in both directions and the slab is supported by beams on all sides. The longer span to shorter span ratio is less than two. In a two-way slab, because the four supporting sides are likely to bend in both directions, distribution reinforcement is included on both sides. The length and width of such slabs are both greater than 4 metres. To prevent stress generation, distribution strips are placed at both ends of two-way slabs, which are utilized in the construction of multi-level building floors.
Hollow Core Slabs
The voids or cores that run through the units give these slabs their name. Cores can be utilized as service ducts that reduce slab self-weight while optimizing structural impact. They also provide stability by minimizing the quantity of concrete used. These types of slabs are typically offered in standard widths of 1200 mm and depths of 110 mm to 400 mm, with complete length flexibility. It's a sort of precast slab that has cores running through it. The slab is strengthened by a steel strand with a diameter of 12 mm, which lasts longer. It's ideal for situations where quick construction is required. Cranes are typically used to place slab units between beams, and the gaps between units are filled with screeds. Over a 16-meter span, a hollow core slab has been found to hold 2.5 kN/m2. It's ideal for office, retail, and parking-lot buildings.
Advantages of Hollow Core Slabs
The advantages of hollow core slabs are mentioned below.
- The cost of building and the overall weight of the structure are both reduced by using these slabs.
- Due to their thickness, hollow-core slabs have excellent fire resistance and sound insulation.
- This eliminates the need to drill into slabs for electric and plumbing appliances.
- Installation of hollow core slabs is easy and less labor is required.
- The construction process is fast.
- To strengthen hollow block masonry, no additional formwork or special building equipment is necessary.
Disadvantages of Hollow Core Slabs
The disadvantages of hollow core slabs are mentioned below.
- Hollow-core ribbed slab units may damage during shipping if not handled appropriately.
- Creating a suitable relationship amongst precast members can sometimes become tough.
- Special equipment to lift and move precast units must also be arranged.
- For short spans, it is not cost-effective.
- Repairing and strengthening are difficult.
Hardy Bricks manufactures these slabs, which are widely utilized in Dubai and China. The hardy bricks are created from the hollow concrete blocks that make up the slab components. Hardy slabs use less concrete, therefore they are lighter and have a thickness of 0.27 m, which is higher than a regular slab. The installation of this type of slabs differs significantly from that of standard slabs. The measurements of this slab are 40 cm x 20 cm x 20 cm. The steps involved in the construction of a hardy slab include the installation of formwork, the positioning of hardy blocks, the insertion of reinforcement in gaps between blocks, the installation of steel mesh on the blocks, and finally the pouring of concrete. It is cost-effective for spans up to 5m in length, decreases the amount of concrete used below the neutral axis, and modest live loads are imposed. It is suitable for places where the temperatures are really high. The types of hardy slabs are mentioned below.
- One way hardy slab
- Two-way hardy slab
Advantages of Hardy Slabs
The advantages of hardy slabs are mentioned below.
- By lowering the amount of concrete under the neutral axis, the hardy slab minimizes the slab's weight.
- They're simple to put together, especially when all of the beams are hidden beams.
- With a medium live load, it's cost-effective for spans up to 5 m.
- Improves sound and heat insulation.
Disadvantages of Hardy Slabs
The disadvantages of hardy slabs are mentioned below.
- It may be damaged during transport if not handled carefully.
- It is not cost-effective for short spans.
- It's difficult to strengthen and develop them.
Uses of Hardy Slabs
The uses of hardy slabs are provided below.
- At exceptionally high temperatures, hardy slabs are utilized to keep the temperature from rising above the slab thickness.
- The heat from the walls is absorbed by special bricks containing thermocol, which is the finest insulator in sunlight.
The waffle slab, also known as a grid slab, is a reinforced concrete roof or floor containing square grids with deep sides. These slabs are utilized to create a pleasant symbolic view at hotel, mall, and restaurant entrances, as well as to install artificial lights. After removing the formwork, a hollow hole can be discovered in this type of slab. The PVC tray (pod) is first placed on the shuttering, followed by reinforcement between the pods and a steel mesh covering the pods before the concrete is stuffed. The formwork is removed when the concrete has been set, but the PVC pods are frequently left in place, leaving a hollow gap with closed outlets at one end. Waffle slabs made of concrete are commonly employed in industrial and commercial buildings, while wooden and metallic waffle slabs are used in a variety of construction projects. Grid slabs can support live loads of 4-7 KN/m2.
The types of waffle slabs are provided below. Waffle slabs are divided into the following types based on the shape of the Pods (PVC Trays).
- Triangular pod system
- Square pod system
Advantages of Waffle Slabs
The advantages of waffle slabs are given below.
- Waffle slabs are more capable of transporting bigger loads and travelling longer distances than flat slabs due to their reduced weight.
- These slabs are suitable for usage as roof and floor slabs.
- They are suitable for spans ranging from 7 to 16 metres.
- They ensure significant savings in the framework, as light frameworks are required.
Disadvantages of Waffle Slabs
The disadvantages of waffle slabs are given below.
- Waffle slabs are not commonly used in construction projects.
- The casting forms or molds required for precast models are very expensive,
- It is cost-effective when mass manufacturing of similar units is desired.
- Construction necessitates close supervision and skilled labor.
Uses of Waffle Slabs
The uses of waffle slabs are given below.
- It is commonly used in situations where large spans are required to avoid multiple columns obstructing space (such as auditoriums, and cinema halls).
- The main reason for using this technique is to achieve strong foundation characteristics such as crack and sagging resistance.
- These types of slabs can carry more weight than conventional concrete slabs.
- They can be used where the depth of beams is limited in order to achieve clear height.
Sunken slabs are the types of slabs installed beneath a restroom to conceal sewage or sewer pipes. Pipes carrying water are hidden beneath the floor in this type, and special precautions are taken to avoid leakage problems. The sewage pipe is crammed with broken coal or bricks after it is inserted into the slab. To prevent leakage or dampness, the sunken slab must be properly waterproofed and treated. It is a type of slab that is provided below the normal level of the floor at a depth of 200mm to 300mm and filled with pieces of broken brick.
It is a type of slab that is typically made of reinforced concrete cast on top of profiled steel decking. During the construction phase, the decking serves as formwork and a working area, and it also serves as external reinforcement during the slab's service life. For steel decking with a thickness of 50-60mm, the slab span can be up to 3m. Slabs with spans of 4.5m can be built if the steel decking thickness is increased to 80mm.
Bubble Deck Slabs
These types of slabs are built by first placing prefabricated plastic bubbles, then placing reinforcement between and over the plastic bubbles, and finally pouring fresh concrete. Plastic bubbles replace the ineffective concrete in the slab's centre. Bubble deck slabs reduce weight, increase strength, allow larger spans, require fewer columns, and do not require beams or ribs under the ceiling. As a result, not only does it reduce total construction costs, but it is also environmentally friendly as it reduces the amount of concrete used. A bubble deck slab is a type of slab that has voids created in it to reduce the dead weight of the building by placing balls in the slab. Bubble decks can eliminate up to 35% of structural concrete.
Other Types of Slabs
Bridges, roofs, foundations, and driveways all rely on long-lasting concrete for a solid foundation and dependable structure. Following are the various types of slabs which are most commonly used in residential buildings.
Low Roof Slabs
These types of slabs are provided above the door for storage purposes. A low roof slab is closed on all sides and open on one, and it is located beneath the actual slab and above the level of the door cob used in homes.
A projected slab or cantilever slab is one that is fixed on one side and free on the other. These types of slabs are typically built in hotels, universities, and function halls to be used as picking or dropping off zone as well as unloading and loading area. This is one of the concrete slab types.
The Waist Slab is simply a slab with an inclination angle between the two supports. These types of slabs are most commonly found on RCC stairs. The Waist Slab is supported from beam to beam, and the main stair reinforcement steel extends up to the centre of the resting beam. The thickness of the waist slab is determined by the length of the stairway. Typically, the thickness is 5′′ or 6′′.
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