Concrete slab
Encyclopedia
Reinforced concrete
Reinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
, typically between 10 and 50 centimeters thick, are most often used to construct floors and ceilings, while thinner slabs are also used for exterior paving.
In many domestic and industrial buildings a thick concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
slab, supported on foundation
Foundation (architecture)
A foundation is the lowest and supporting layer of a structure. Foundations are generally divided into two categories: shallow foundations and deep foundations.-Shallow foundations:...
s or directly on the subsoil
Subsoil
Subsoil, or substrata, is the layer of soil under the topsoil on the surface of the ground. The subsoil may include substances such as clay and/or sand that has only been partially broken down by air, sunlight, water, wind etc., to produce true soil...
, is used to construct the ground floor of a building. In high rise buildings and skyscraper
Skyscraper
A skyscraper is a tall, continuously habitable building of many stories, often designed for office and commercial use. There is no official definition or height above which a building may be classified as a skyscraper...
s, thinner, pre-cast concrete slabs are slung between the steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
frames to form the floors and ceilings on each level.
On the technical drawings, reinforced concrete slabs are often abbreviated to "r.c.slab" or simply "r.c.".
Thermal performance
There are two main thermal considerations. The first is the question of insulating a floor slab. In older buildings, concrete slabs cast directly on the ground can drain heat from a room. In modern construction, concrete slabs are usually cast above a layer of insulationThermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
such as expanded polystyrene, and the slab may contain underfloor heating
Underfloor heating
Underfloor heating and cooling is a form of central heating and cooling which achieves indoor climate control for thermal comfort using conduction, radiation and convection...
pipes. However, there are still uses for an uninsulated slab, typically in outbuildings which are not heated or cooled to room temperature. In those cases, casting the slab directly onto a rocky substrate will maintain the slab at or near the temperature of the substrate throughout the year, and can prevent both freezing and overheating.
The second consideration is the high thermal mass
Thermal mass
Thermal mass is a concept in building design which describes how the mass of the building provides "inertia" against temperature fluctuations, sometimes known as the thermal flywheel effect...
, which applies to walls and floors, or wherever the concrete is used within the thermal envelope
Building envelope
The building envelope is the physical separator between the interior and the exterior environments of a building. Another emerging term is "Building Enclosure". It serves as the outer shell to help maintain the indoor environment and facilitate its climate control...
. It is a disadvantage where the rooms are heated intermittently and require a quick response, as the concrete takes time to warm up, causing a delay in warming the building. But it is an advantage in climates with large daily temperature swings, where the slab acts as a regulator, keeping the building cool by day and warm by night.
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, 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.
- A ribbed slab, giving considerable extra strength on one direction.
- A waffle slab, giving added strength in both directions.
Reinforcement design
- A one way slab needs moment resisting reinforcement only in its short-direction. Because, the moment along long axes is so small that it can be neglected. When the ratio of the length of long direction to short direction of a slab is greater than 2 it can be considered as a one way slab.
- A two way slab needs moment resisting reinforcement in both directions. If the ratio of the lengths of long and short side is less than one then moment in both direction should be considered in design.
Construction
A concrete slab may be prefabricatedPrefabrication
Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting complete assemblies or sub-assemblies to the construction site where the structure is to be located...
or in situ
In situ
In situ is a Latin phrase which translated literally as 'In position'. It is used in many different contexts.-Aerospace:In the aerospace industry, equipment on board aircraft must be tested in situ, or in place, to confirm everything functions properly as a system. Individually, each piece may...
. Prefabricated concrete slabs are built in a factory and 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. It is vital that the supporting structure is built to the correct dimensions, or the slabs may not fit.
In situ concrete slabs are built on the building site using formwork
Formwork
Formwork is the term given to either temporary or permanent molds into which concrete or similar materials are poured. In the context of concrete construction, the falsework supports the shuttering moulds.-Formwork and concrete form types:...
- a type of boxing into which the wet concrete is poured. If the slab is to be reinforced, the rebar
Rebar
A rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...
s are positioned within the formwork before the concrete is poured in. Plastic tipped metal, or plastic bar chairs are used to hold the rebar away from the bottom and sides of the form-work, so that when the concrete sets it completely envelops the reinforcement. For a ground slab, the form-work may consist only of sidewalls pushed into the ground. For a suspended slab, the form-work is shaped like a tray, often supported by a temporary scaffold until the concrete sets.
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.
See also
- Slab-on-grade foundations
- Hollow core slabHollow core slabA hollow core slab , also known as a voided slab or hollow core plank is a precast slab of prestressed concrete typically used in the construction of floors in multi-story apartment buildings...
(Voided slab, one-way spanning) - Voided biaxial slabVoided biaxial slabVoided biaxial slabs are reinforced concrete slabs in which voids reduce the amount of concrete.While concrete has been used for thousands of years, the use of reinforced concrete is a relatively recent invention, usually attributed to Joseph-Louis Lambot in 1848...
(Voided slab, two-way spanning) - FormworkFormworkFormwork is the term given to either temporary or permanent molds into which concrete or similar materials are poured. In the context of concrete construction, the falsework supports the shuttering moulds.-Formwork and concrete form types:...
- RebarRebarA rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...
- Concrete coverConcrete coverConcrete cover, in reinforced concrete, is the least distance between the surface of embedded reinforcement and the outer surface of the concrete...
- paving stonePaver (flooring)An interlocking concrete paver is a pre-cast piece of concrete or brick commonly used in exterior hardscaping applications. Pavers were developed in Europe and introduced into the United States in the early 1970s....
- PlattenbauPlattenbauPlattenbau is the German word for a building whose structure is constructed of large, prefabricated concrete slabs. The word is a compound of Platte and Bau...