Voided biaxial slabs, sometimes called biaxial slabs or voided slabs, are a type of reinforced concrete slab which incorporates air-filled voids to reduce the volume of concrete required. These voids enable cheaper construction and less environmental impact.[ citation needed ] Another major benefit of the system is its reduction in slab weight compared with regular solid decks. Up to 50% of the slab volume may be removed in voids, resulting in less load on structural members. [1] This also allows increased weight and/or span, since the self-weight of the slab contributes less to the overall load.
Concrete has numerous applications in building construction, but its use for horizontal slabs is limited by its relatively high density which reduces the maximum span. [2] The usual method of rectifying this disadvantage is to incorporate some kind of reinforcement, which enables concrete slabs to be used for a broad range of spans and loading conditions. [3] Traditional approaches to structural reinforcement involve embedding another material inside the concrete, however, biaxial slabs provide an alternative solution in the form of a two-way slab which incorporates orthogonal concrete "beams" within the slab. [4] This allows greater support in both horizontal directions in order to transfer weight to a vertical member. [5]
The general concept of voided biaxial slabs relies on voids created within the concrete at the time of casting. This creates an internal array of hollow boxes in the slab, which acts as grid of horizontal supports for the flat surface on top. Another advantage is the reduction in weight, achieved by removing mass which does not directly transfer weight to a vertical member. Typical solid slabs have a loading capacity of around one-third of their own weight, which can create problems for long spans and high loadings. [2] By reducing the weight of the slab without compromising its structural strength, it is possible to create a thicker slab to support more weight over a longer span.
Hollow-core slabs, also known as voided slabs, initially appeared as one-way elements in Europe during the 1950s, and are still commonly manufactured in precast form for applications where fast construction and low self-weight are required. [2] [6] Waffle slabs are a common type of hollow-core slab which use the same principle as voided biaxial slabs. However, their voids are placed on the underside of the slab rather than embedded within the slab, leading to lower shear strength and fire resistance. [7] There has been a range of proprietary implementations of voided biaxial slabs, including the use of polystyrene blocks as a filler material in the voids. [6] However, many implementations have suffered from flexural cracking and lack of shear resistance. [2] [6]
All voided biaxial slabs incorporate an array of rigid void formers which contain air within the voids. These void formers are most commonly made of plastic such as high-density polyethylene, and may use recycled materials. [7] The void formers are produced in a variety of shapes depending on the design of the slab. Common designs include spheres, boxes, ellipsoids and toroids. [6]
The voids are usually placed in a grid-like arrangement, temporarily supported by a framework which is eventually enveloped in concrete. [7] This framework has been implemented in various ways, but the most efficient method uses a steel mesh in order to reduce material use and create an optimal geometric proportion between concrete, reinforcement, and voids. [2]
The voids are positioned in the middle of the cross section, where concrete is least beneficial to the structure. The integrity of the solid layers is maintained, as the top and bottom of the slab can experience particularly high stresses. This enables the slab to effectively resist both positive and negative bending moments. [2]
Since the underside of the slab is flat it may be finished to create an interior ceiling, in contrast to the contoured underside of waffle slabs.
Some vendors of voided biaxial slabs supply prefabricated components which are quicker to install onsite. Prefabricated slabs also have the advantage of a smooth underside suitable for use as a ceiling without further finishing. Varying degrees of prefabrication are available, including entire slabs. [1] Prefabricated modules commonly consist of a fully cast piece of slab, including all components encased in concrete. This technique consists of a "bubble-reinforced sandwich" of reinforcing mesh and voids cast in concrete. A contiguous layer of smooth finish concrete is then poured onsite, along with the addition of structural anchoring to fix the modules together. [8]
Voided biaxial slabs cast onsite take longer to construct than prefabricated slabs, but are sometimes cheaper. In a typical casting procedure, a decking of formwork is constructed out of metal or wood. This provides temporary support for the voids and the curing concrete. After the decking is constructed, reinforcing mesh is installed to support the voids. Alternatively, the voids and mesh may be supplied as a prefabricated module. Since the air in the voids is of lower density than the surrounding concrete, it tends to float to the surface of the concrete. To ameliorate this, the slab may be cast in multiple layers so that the mesh is initially anchored and is then able to restrain the voids from floating upwards in later pours. [9]
In 2017 the BubbleDeck system caused controversy due to the collapse of a parking garage at Eindhoven airport in the Netherlands. [10] This was due to insufficient shear strength at the interface between the precast concrete slabs, potentially caused by high temperatures during construction. [11] After the incident an investigation was started among buildings using the same flooring system, leading to the closure of several buildings in the Netherlands, including one at the University of Rotterdam and a school building under construction in Hoeven. [12]
Investigations according to Eurocodes have concluded that voided biaxial slabs may be modelled like solid slabs. To which degree depends on the shape of the voids. [13] This is considered an advantage over one-way ribbed slabs, which must be calculated as an array of beams.
Compared to traditional solid slabs, the reduced self-weight of biaxial slabs allows for longer spans and/or reduced deck thickness. The overall mass of concrete can be reduced by 35–50% depending on the design, [1] as a consequence of reduced slab mass, as well as lower requirements for vertical structure and foundations. Biaxial slabs commonly span up to 20 metres at a thickness of around 500 mm.[ citation needed ] The added strength also reduces the acoustic transmittance of the slab for low frequencies.
The reduced mass of biaxial slabs also results in a more environmentally friendly product which produces less CO2 emissions both in its construction and indirectly through the reduction of surrounding structural support. Total carbon emissions may be reduced by up to 41%. [1] Slabs are one of the greatest consumers of concrete in many buildings, [14] so reducing the slab mass can make a relatively large difference to the environmental impact of a building's construction.
Biaxial slabs may be marginally cheaper than solid slabs, partly due to the lower mass. If using prefabricated versions, labor can also be significantly reduced, resulting in faster and cheaper construction. This can yield time savings of up to 40% compared with traditional solid slabs. [1] However, this is heavily dependent on the particular system, and systems relying on onsite placement of void formers requires much more labor than solid slabs. [2]
Compared to one-way hollow-core slabs, biaxial slabs are more resistant to seismic disturbance. One-way decks are supported by a combination of walls and beams, leading to a relatively rigid structure which increases the risk of progressive collapse. [15]
One of the most significant differences between solid slabs and voided biaxial slabs is their resistance to shear force. Due to a lower volume of concrete, the shear resistance is also reduced. [2] For slabs using spherical voids, the shear resistance is approximately proportional to the volume of concrete, as the geometry of the voids causes efficient transfer of force to load-bearing parts, enabling all the concrete to be effective. Other shapes of voids, with flat or flattened surfaces, will result in more concrete and/or less strength. This relates especially to shear capacity, where the capacity of a slab with boxes can be 40% lower than for a slab of identical height using spherical voids. For punching shear, the capacity of a slab with spherical voids can be 600% higher than for a box slab. In some cases where greater shear resistance is required in a localised area (such as junctions with piers or walls), the voids may be omitted, leading to a partially solid slab. [13]
Reinforced concrete, also called ferroconcrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having higher tensile strength or ductility. The reinforcement is usually, though not necessarily, steel reinforcing bars and is usually embedded passively in the concrete before the concrete sets. However, post-tensioning is also employed as a technique to reinforce the concrete. In terms of volume used annually, it is one of the most common engineering materials. In corrosion engineering terms, when designed correctly, the alkalinity of the concrete protects the steel rebar from corrosion.
A floor is the bottom surface of a room or vehicle. Floors vary from simple dirt in a cave to many layered surfaces made with modern technology. Floors may be stone, wood, bamboo, metal or any other material that can support the expected load.
Prestressed concrete is a form of concrete used in construction. It is substantially "prestressed" (compressed) during production, in a manner that strengthens it against tensile forces which will exist when in service. It was patented by Eugène Freyssinet in 1928.
This page is a list of construction topics.
A shear wall is an element of a structurally engineered system that is designed to resist in-plane lateral forces, typically wind and seismic loads.
A plate girder bridge is a bridge supported by two or more plate girders.
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.
Formwork is molds into which concrete or similar materials are either precast or cast-in-place. In the context of concrete construction, the falsework supports the shuttering molds. In specialty applications formwork may be permanently incorporated into the final structure, adding insulation or helping reinforce the finished structure.
Fireproofing is rendering something resistant to fire, or incombustible; or material for use in making anything fire-proof. It is a passive fire protection measure. "Fireproof" or "fireproofing" can be used as a noun, verb or adjective; it may be hyphenated ("fire-proof").
An orthotropic bridge or orthotropic deck is typically one whose fabricated deck consists of a structural steel deck plate stiffened either longitudinally with ribs or transversely, or in both directions. This allows the fabricated deck both to directly bear vehicular loads and to contribute to the bridge structure's overall load-bearing behaviour. The orthotropic deck may be integral with or supported on a grid of deck framing members, such as transverse floor beams and longitudinal girders. All these various choices for the stiffening elements, e.g., ribs, floor beams and main girders, can be interchanged, resulting in a great variety of orthotropic panels.
Structural steel is a category of steel used for making construction materials in a variety of shapes. Many structural steel shapes take the form of an elongated beam having a profile of a specific cross section. Structural steel shapes, sizes, chemical composition, mechanical properties such as strengths, storage practices, etc., are regulated by standards in most industrialized countries.
Precast concrete is a construction product produced by casting concrete in a reusable mold or "form" which is then cured in a controlled environment, transported to the construction site and maneuvered into place; examples include precast beams, and wall panels, floors, roofs, and piles. In contrast, cast-in-place concrete is poured into site-specific forms and cured on site.
A hollow core slab, also known as a voided slab, hollow core plank or simply a concrete plank is a precast slab of prestressed concrete typically used in the construction of floors in multi-story apartment buildings. The slab has been especially popular in countries where the emphasis of home construction has been on precast concrete, including Northern Europe and former socialist countries of Eastern Europe. Precast concrete popularity is linked with low-seismic zones and more economical constructions because of fast building assembly, lower self weight, etc.
Glass fiber reinforced concrete (GFRC) is a type of fiber-reinforced concrete. The product is also known as glassfibre reinforced concrete or GRC in British English. Glass fiber concretes are mainly used in exterior building façade panels and as architectural precast concrete. Somewhat similar materials are fiber cement siding and cement boards.
Composite construction is a generic term to describe any building construction involving multiple dissimilar materials. Composite construction is often used in building aircraft, watercraft, and building construction. There are several reasons to use composite materials including increased strength, aesthetics, and environmental sustainability.
In structural engineering, the open web steel joist (OWSJ) is a lightweight steel truss consisting, in the standard form, of parallel chords and a triangulated web system, proportioned to span between bearing points.
A T-beam, used in construction, is a load-bearing structure of reinforced concrete, wood or metal, with a capital 'T'-shaped cross section. The top of the T-shaped cross section serves as a flange or compression member in resisting compressive stresses. The web of the beam below the compression flange serves to resist shear stress. When used for highway bridges the beam incorporates reinforcing bars in the bottom of the beam to resist the tensile stresses which occur during bending.
The Filigree Wideslab method is a process for construction of concrete floor decks from two interconnected concrete placements, one precast in a factory, and the other done in the field. The method was developed during the late 1960s by Harry H. Wise as a more efficient and economic construction process than conventional cast-in-place technologies.
Structural clay tile describes a category of burned-clay building materials used to construct roofing, walls, and flooring for structural and non-structural purposes, especially in fireproofing applications. Also called building tile, structural terra cotta, hollow tile, saltillo tile, and clay block, the material is an extruded clay shape with substantial depth that allows it to be laid in the same manner as other clay or concrete masonry. In North America it was chiefly used during the late 19th and early 20th centuries, reaching peak popularity at the turn of the century and declining around the 1950s. Structural clay tile grew in popularity in the end of the nineteenth-century because it could be constructed faster, was lighter, and required simpler flat falsework than earlier brick vaulting construction.
Arching or compressive membrane action (CMA) in reinforced concrete slabs occurs as a result of the great difference between the tensile and compressive strength of concrete. Cracking of the concrete causes a migration of the neutral axis which is accompanied by in-plane expansion of the slab at its boundaries. If this natural tendency to expand is restrained, the development of arching action enhances the strength of the slab. The term arching action is normally used to describe the arching phenomenon in one-way spanning slabs and compressive membrane action is normally used to describe the arching phenomenon in two-way spanning slabs.
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