Железобетонные плиты в современном строительстве.

Flat, horizontal concrete element of modern buildings

A concrete slab is a common structural element of modern buildings, consisting of a flat, horizontal surface made of cast concrete. Steel reinforced slabs, typically between 100 and 500 mm thick, are most often used to construct floors and ceilings, while thinner mud slabs may be used for exterior paving
In many domestic and industrial buildings, a thick concrete slab supported on foundations or directly on the subsoil, is used to construct the ground floor. These slabs are generally classified as ground bearing or suspended. A slab is ground bearing if it rests directly on the foundation, otherwise the slab is suspended. For multi story buildings, there are several common slab designs :
Beam and block, also referred to as rib and block, is mostly used in residential and industrial applications. This slab type is made up of pre stressed beams and hollow blocks and are temporarily propped until set, typically after 21 days. A hollow core slab which is precast and installed on site with a crane
In high rise buildings and skyscrapers, thinner, pre cast concrete slabs are slung between the steel frames to form the floors and ceilings on each level. Cast in situ slabs are used in high rise buildings and large shopping complexes as well as houses. These in situ slabs are cast on site using shutters and reinforced steel. On technical drawings, reinforced concrete slabs are often abbreviated to "r. c. c. slab" or simply "r. c.". Calculations and drawings are often done by structural engineers in CAD software.

Energy efficiency has become a primary concern for the construction of new buildings, and the prevalence of concrete slabs calls for careful consideration of its thermal properties in order to minimise wasted energy. Concrete has similar thermal properties to masonry products, in that it has a relatively high thermal mass and is a good conductor of heat. In some special cases, the thermal properties of concrete have been employed, for example as a heatsink in nuclear power plants or a thermal buffer in industrial freezers.

Thermal conductivity of a concrete slab indicates the rate of heat transfer through the solid mass by conduction, usually in regard to heat transfer to or from the ground. The coefficient of thermal conductivity, k, is proportional to density of the concrete, among other factors. However, this study concerned hollow concrete blocks and its results are unverified for concrete slabs. The actual value of k varies significantly in practice, and is usually between 0.8 and 2.0 W m−1 K−1. This is relatively high when compared to other materials, for example the conductivity of wood may be as low as 0.04 W m−1 K−1. One way of mitigating the effects of thermal conduction is to introduce insulation .

The second consideration is the high thermal mass of concrete slabs, which applies similarly to walls and floors, or wherever concrete is used within the thermal envelope. Concrete has a relatively high thermal mass, meaning that it takes a long time to respond to changes in ambient temperature. This is a disadvantage when rooms are heated intermittently and require a quick response, as it takes longer to warm the entire building, including the slab. However, the high thermal mass 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. Typically concrete slabs perform better than implied by their R value. This not only allows for better insulation but decreases the weight of slab which has a positive effect on load bearing walls and foundations.

Ground bearing slabs, also known as "on ground" or "slab on grade", are commonly used for ground floors on domestic and some commercial applications. It is an economical and quick construction method for sites that have non reactive soil and little slope. For ground bearing slabs, it is important to design the slab around the type of soil, since some soils such as clay are too dynamic to support a slab consistently across its entire area. This results in cracking and deformation, potentially leading to structural failure of any members attached to the floor, such as wall studs. In addition to filling the downhill side, this area of the slab may be supported on concrete piers which extend into the ground. In this case, the fill material is less important structurally as the dead weight of the slab is supported by the piers. However, the fill material is still necessary to support the curing concrete and its reinforcement. There are two common methods of filling controlled fill and rolled fill. Ground bearing slabs are usually supplemented with some form of reinforcement, often steel rebar. However, in some cases such as concrete roads, it is acceptable to use an unreinforced slab if it is adequately engineered .

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:
A corrugated slab is designed when the concrete is poured into a corrugated steel tray, more commonly called decking. This steel tray improves strength of the slab, and prevents the slab from bending under its own weight. The corrugations run in one direction only. A ribbed slab gives considerably more strength in one direction. This is achieved with concrete beams bearing load between piers or columns, and thinner, integral ribs in the perpendicular direction. An analogy in carpentry would be a subfloor of bearers and joists. Ribbed slabs have higher load ratings than corrugated or flat slabs, but are inferior to waffle slabs. A waffle slab gives added strength in both directions using a matrix of recessed segments beneath the slab. This is the same principle used in the ground bearing version, the waffle slab foundation. Waffle slabs are usually deeper than ribbed slabs of equivalent strength, and are heavier hence require stronger foundations. However, they provide increased mechanical strength in two dimensions, a characteristic important for vibration resistance and soil movement.

Unreinforced or "plain" slabs are becoming rare and have limited practical applications, with one exception being the mud slab They were once common in the US, but the economic value of reinforced ground bearing slabs has become more appealing for many engineers. Since unreinforced concrete is relatively very weak in tension, it is important to consider the effects of tensile stress caused by reactive soil, wind uplift, thermal expansion, and cracking. One of the most common applications for unreinforced slabs is in concrete roads.

A two way slab has moment resisting reinforcement in both directions. This may be implemented due to application requirements such as heavy loading, vibration resistance, clearance below the slab, or other factors. However, an important characteristic governing the requirement of a two way slab is the ratio of the two horizontal lengths. If where is the short dimension and is the long dimension, then moment in both directions should be considered in design. In other words, if the axial ratio is greater than two, a two way slab is required. A non reinforced slab is two way if it is supported in both horizontal axes.

A concrete slab may be prefabricated (precast), or constructed on site.

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. 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. This concept is known as concrete cover. For a ground bearing slab, the formwork may consist only of side walls pushed into the ground. For a suspended slab, the formwork 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, plastic and steel are gaining popularity as they save labour. On low budget or small scale jobs, for instance when laying a concrete garden path, wooden planks are very common. After the concrete has set the wood may be removed. Formwork can also be permanent, and remain in situ post concrete pour. For large slabs or paths that are poured in sections, this permanent formwork can then also act as isolation joints within concrete slabs to reduce the potential for cracking due to concrete expansion or movement. In some cases formwork is not necessary for instance, a ground slab surrounded by dense soil, brick or block foundation walls, where the walls act as the sides of the tray and hardcore (rubble) acts as the base.