Thus, when a concrete slab is subjected to fluctuating temperatures, it will respond more slowly to these changes and in many cases increase the efficiency of a building. {\displaystyle l_{y}} Unit of thermal conductivity is joules per second per square meter. [9] However, there are still uses for a slab that is not insulated, for example in outbuildings which are not heated or cooled to room temperature (see § Mud slabs). 1 The heat transfer follows a direct relationship with thermal conductivity and varies accordingly. Since these slabs are inevitably poured on-site (rather than precast as some suspended slabs are), it can be difficult to control conditions to optimize the curing process. New York: Van Nostrand Reinhold, 1992. The coefficient of thermal conductivity, k, is proportional to density of the concrete, among other factors. 299. [19] One of the most common applications for unreinforced slabs is in concrete roads. [11], 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. Calculations and drawings are often done by structural engineers in CAD software. It also expresses the anisotropy of the medium. However, an important characteristic governing the requirement of a two-way slab is the ratio of the two horizontal lengths. carpet).[4]. These various factors complicate the theoretical evaluation of a k-value, since each component has a different conductivity when isolated, and the position and proportion of each components affects the overall conductivity. This can also be seen in the published values for a,. A sensitivity analysis is performed to identify the governing parameters of the temperature calculation and a nonlinear optimization method is used to enhance the formulation of the thermal properties. slab" or simply "r.c.". Thermal conductivity - Designing Buildings Wiki - Share your construction industry knowledge. If Non-reinforced slabs may also be considered one-way if they are supported on only two opposite sides (i.e. Concrete has similar thermal properties to masonry products, in that it has a relatively high thermal mass and is a good conductor of heat. The thermal analysis of concrete members according to EN 1992-1-2 shows errors. A two-way slab has moment resisting reinforcement in both directions. This concept is known as concrete cover. 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 (see below).[1][2]. x In these cases, casting the slab directly onto a substrate of aggregate will maintain the slab near the temperature of the substrate throughout the year, and can prevent both freezing and overheating. On-site concrete slabs are built on the building site using formwork - a type of boxing into which the wet concrete is poured. 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. 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. Garber, G. Design and Construction of Concrete Floors. l In modern construction, concrete slabs are usually cast above a layer of insulation such as expanded polystyrene, and the slab may contain underfloor heating pipes. It is an economical and quick construction method for sites that have non-reactive soil and little slope. These slabs are generally classified as ground-bearing or suspended. [24] In other words, if the axial ratio is greater than two, a two-way slab is required. [4] Concrete has similar thermal properties to masonry products, in that it has a relatively high thermal mass and is a good conductor of heat. The formwork is commonly built from wooden planks and boards, plastic, or steel. [4] In reality, there are many factors which contribute to the effect of thermal mass, including the depth and composition of the slab, as well as other properties of the building such as orientation and windows. © 2016 The Authors. For multi-storey buildings, there are several common slab designs (see § Design for more types): On technical drawings, reinforced concrete slabs are often abbreviated to "r.c.c. This includes use as a base or "sub-slab" for a larger structural slab. A non-reinforced slab is two-way if it is supported in both horizontal axes. In addition to filling the downhill side, this area of the slab may be supported on concrete piers which extend into the ground. Typically concrete slabs perform better than implied by their R-value. One way of mitigating the effects of thermal conduction is to introduce insulation (see § Insulation). [7] 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. It can also be defined as the ratio between the heat flux and the temperature gradient. Ground-bearing slabs, also known as "on-ground" or "slab-on-grade", are commonly used for ground floors on domestic and some commercial applications. [9] In this application, a mud slab also prevents the plastic bar chairs from sinking into soft topsoil which can cause spalling due to incomplete coverage of the steel. [10] This not only allows for better insulation but decreases the weight of slab which has a positive effect on load bearing walls and foundations. Since, thermal insulation fluctuates contrariwise with density, lower density concrete offers superior insulation as compared to greater density concrete. Conductivity of concrete depends on its composition. There are two common methods of filling - controlled fill and rolled fill. Thermal conductivity is a material property that describes ability to conduct heat. [4] The primary influences on conductivity are moisture content, type of aggregate, type of cement, constituent proportions, and temperature. Thermal conductivity is the tensor property of a medium which defines its heat conductive efficiency. Unit of thermal conductivity is joules per second per square meter. A concrete slab is a common structural element of modern buildings, consisting of a flat, horizontal surface made of cast concrete. They were once common in the USA, but the economic value of reinforced ground-bearing slabs has become more appealing for many engineers. In all cases the top surface remains flat, and the underside is modulated: Unreinforced or "plain"[17] slabs are becoming rare and have limited practical applications, with one exception being the mud slab (see below). {\displaystyle l_{x}:l_{y}<2} For a suspended slab, there are a number of designs to improve the strength-to-weight ratio. c p), which means that all variables are put together and the scatter of the different physical properties can be “equalized”. y [20] This makes them economical and easy to install for temporary or low-usage purposes such as subfloors, crawlspaces, pathways, paving, and levelling surfaces.
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