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85-704: The Des Plaines River Valley Bridge is a post-tensioned concrete girder toll bridge in the northeastern portion of the U.S. state of Illinois . It carries Interstate 355 (I-355) over the Des Plaines River , the Chicago Sanitary and Ship Canal , the Illinois and Michigan Canal , several railroad lines, Bluff Road, New Avenue and a forest preserve. It is officially named the Veterans Memorial Bridge . There are title plaques on

170-467: A corrosion -inhibiting grease , usually lithium based. Anchorages at each end of the tendon transfer the tensioning force to the concrete, and are required to reliably perform this role for the life of the structure. Unbonded post-tensioning can take the form of: For individual strand tendons, no additional tendon ducting is used and no post-stressing grouting operation is required, unlike for bonded post-tensioning. Permanent corrosion protection of

255-414: A nanometer . This means in general not only that surfaces with the potential for chemical bonding need to be brought very close together, but also that these bonds are fairly brittle, since the surfaces then need to be kept close together. In dispersive adhesion, also known as physisorption , two materials are held together by van der Waals forces : the attraction between two molecules, each of which has

340-508: A broad range of structural, aesthetic and economic requirements. Significant among these include: a minimum number of (intrusive) supporting walls or columns; low structural thickness (depth), allowing space for services, or for additional floors in high-rise construction; fast construction cycles, especially for multi-storey buildings; and a low cost-per-unit-area, to maximise the building owner's return on investment. The prestressing of concrete allows "load-balancing" forces to be introduced into

425-408: A bulk of such particles. When considering identical particles, this is called cohesive force. When discussing adhesion, this theory needs to be converted into terms relating to surfaces. If there is a net attractive energy of cohesion in a bulk of similar molecules, then cleaving this bulk to produce two surfaces will yield surfaces with a dispersive surface energy, since the form of the energy remain

510-404: A chance of overwhelming the total adhesive force between the surfaces. If failure does occur at an interface containing a viscoelastic adhesive agent, and a crack does propagate, it happens by a gradual process called “fingering”, rather than a rapid, brittle fracture. Stringing can apply to both the diffusive bonding regime and the chemical bonding regime. The strings of molecules bridging across

595-406: A factor of 13. Lateral adhesion is the adhesion associated with sliding one object on a substrate such as sliding a drop on a surface. When the two objects are solids, either with or without a liquid between them, the lateral adhesion is described as friction . However, the behavior of lateral adhesion between a drop and a surface is tribologically very different from friction between solids, and

680-504: A foreign species, rather than dissociate and mix with the other. If this is true, then it follows that when the interfacial tension is high, the force of adhesion is weak, since each species does not find it favorable to bond to the other. The interfacial tension of a liquid and a solid is directly related to the liquid's wettability (relative to the solid), and thus one can extrapolate that cohesion increases in non-wetting liquids and decreases in wetting liquids. One example that verifies this

765-427: A heightened concentration of polymer tails extending across the interface. Scission is easily achieved by ultraviolet irradiation in the presence of oxygen gas, which suggests that adhesive devices employing diffusive bonding actually benefit from prolonged exposure to heat/light and air. The longer such a device is exposed to these conditions, the more tails are scissed and branch out across the interface. Once across

850-471: A large scale mechanical bond, velcro forms one on a medium scale, and some textile adhesives (glue) form one at a small scale. Two materials may form a compound at the joint. The strongest joints are where atoms of the two materials share or swap electrons (known respectively as covalent bonding or ionic bonding ). A weaker bond is formed if a hydrogen atom in one molecule is attracted to an atom of nitrogen , oxygen , or fluorine in another molecule,

935-476: A phenomenon called hydrogen bonding . Chemical adhesion occurs when the surface atoms of two separate surfaces form ionic, covalent, or hydrogen bonds. The engineering principle behind chemical adhesion in this sense is fairly straightforward: if surface molecules can bond, then the surfaces will be bonded together by a network of these bonds. It bears mentioning that these attractive ionic and covalent forces are effective over only very small distances – less than

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1020-609: A proof-loaded, redundant and monitorable pressure-containment system. Nuclear reactor and containment vessels will commonly employ separate sets of post-tensioned tendons curved horizontally or vertically to completely envelop the reactor core. Blast containment walls, such as for liquid natural gas (LNG) tanks, will normally utilize layers of horizontally-curved hoop tendons for containment in combination with vertically looped tendons for axial wall pre-stressing. Heavily loaded concrete ground-slabs and pavements can be sensitive to cracking and subsequent traffic-driven deterioration. As

1105-419: A protective sleeve or duct which is either cast into the concrete structure or placed adjacent to it. At each end of a tendon is an anchorage assembly firmly fixed to the surrounding concrete. Once the concrete has been cast and set, the tendons are tensioned ("stressed") by pulling the tendon ends through the anchorages while pressing against the concrete. The large forces required to tension the tendons result in

1190-626: A railway bridge constructed 1946 in the UK . By the 1960s, prestressed concrete largely superseded reinforced concrete bridges in the UK, with box girders being the dominant form. In short-span bridges of around 10 to 40 metres (30 to 130 ft), prestressing is commonly employed in the form of precast pre-tensioned girders or planks. Medium-length structures of around 40 to 200 metres (150 to 650 ft), typically use precast-segmental, in-situ balanced-cantilever and incrementally-launched designs . For

1275-426: A region of slight positive and negative charge. In the simple case, such molecules are therefore polar with respect to average charge density , although in larger or more complex molecules, there may be multiple "poles" or regions of greater positive or negative charge. These positive and negative poles may be a permanent property of a molecule ( Keesom forces ) or a transient effect which can occur in any molecule, as

1360-508: A result, prestressed concrete is regularly used in such structures as its pre-compression provides the concrete with the ability to resist the crack-inducing tensile stresses generated by in-service loading. This crack-resistance also allows individual slab sections to be constructed in larger pours than for conventionally reinforced concrete, resulting in wider joint spacings, reduced jointing costs and less long-term joint maintenance issues. Initial works have also been successfully conducted on

1445-532: A significant permanent compression being applied to the concrete once the tendon is "locked-off" at the anchorage. The method of locking the tendon-ends to the anchorage is dependent upon the tendon composition, with the most common systems being "button-head" anchoring (for wire tendons), split-wedge anchoring (for strand tendons), and threaded anchoring (for bar tendons). Tendon encapsulation systems are constructed from plastic or galvanised steel materials, and are classified into two main types: those where

1530-503: A single tendon duct, with the exception of bars which are mostly used unbundled. This bundling makes for more efficient tendon installation and grouting processes, since each complete tendon requires only one set of end-anchorages and one grouting operation. Ducting is fabricated from a durable and corrosion-resistant material such as plastic (e.g., polyethylene ) or galvanised steel, and can be either round or rectangular/oval in cross-section. The tendon sizes used are highly dependent upon

1615-574: Is polydimethyl siloxane rubber, which has a work of self-adhesion of 43.6 mJ/m in air, 74 mJ/m in water (a nonwetting liquid) and 6 mJ/m in methanol (a wetting liquid). This argument can be extended to the idea that when a surface is in a medium with which binding is favorable, it will be less likely to adhere to another surface, since the medium is taking up the potential sites on the surface that would otherwise be available to adhere to another surface. Naturally this applies very strongly to wetting liquids, but also to gas molecules that could adsorb onto

1700-592: Is a highly versatile construction material as a result of it being an almost ideal combination of its two main constituents: high-strength steel, pre-stretched to allow its full strength to be easily realised; and modern concrete, pre-compressed to minimise cracking under tensile forces. Its wide range of application is reflected in its incorporation into the major design codes covering most areas of structural and civil engineering, including buildings, bridges, dams, foundations, pavements, piles, stadiums, silos, and tanks. Building structures are typically required to satisfy

1785-455: Is also the mechanism involved in sintering . When metal or ceramic powders are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one. Diffusive forces are somewhat like mechanical tethering at the molecular level. Diffusive bonding occurs when species from one surface penetrate into an adjacent surface while still being bound to the phase of their surface of origin. One instructive example

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1870-437: Is an initial increase of resistance to motion, indicating that the stopping time was sufficient for the surfaces to restructure. Some atmospheric effects on the functionality of adhesive devices can be characterized by following the theory of surface energy and interfacial tension . It is known that γ 12 = (1/2)W 121 = (1/2)W 212 . If γ 12 is high, then each species finds it favorable to cohere while in contact with

1955-418: Is fed into a greasing chamber and then passed to an extrusion unit where molten plastic forms a continuous outer coating. Finished strands can be cut-to-length and fitted with "dead-end" anchor assemblies as required for the project. Both bonded and unbonded post-tensioning technologies are widely used around the world, and the choice of system is often dictated by regional preferences, contractor experience, or

2040-430: Is given for a pair of surfaces exhibiting diffusive bonding to restructure, the more diffusion will occur, the stronger the adhesion will become. The aforementioned reaction of certain polymer-on-polymer surfaces to ultraviolet radiation and oxygen gas is an instance of hysteresis, but it will also happen over time without those factors. In addition to being able to observe hysteresis by determining if W > γ 1 + γ 2

2125-423: Is most commonly used for the fabrication of structural beams , floor slabs , hollow-core slabs, balconies , lintels , driven piles , water tanks and concrete pipes . Post-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned after the surrounding concrete structure has been cast. The tendons are not placed in direct contact with the concrete, but are encapsulated within

2210-432: Is no single theory covering adhesion, and particular mechanisms are specific to particular material scenarios. Five mechanisms of adhesion have been proposed to explain why one material sticks to another: Adhesive materials fill the voids or pores of the surfaces and hold surfaces together by interlocking . Other interlocking phenomena are observed on different length scales. Sewing is an example of two materials forming

2295-506: Is that of polymer-on-polymer surfaces. Diffusive bonding in polymer-on-polymer surfaces is the result of sections of polymer chains from one surface interdigitating with those of an adjacent surface. The freedom of movement of the polymers has a strong effect on their ability to interdigitate, and hence, on diffusive bonding. For example, cross-linked polymers are less capable of diffusion and interdigitation because they are bonded together at many points of contact, and are not free to twist into

2380-428: Is that once the initial compression has been applied, the resulting material has the characteristics of high-strength concrete when subject to any subsequent compression forces and of ductile high-strength steel when subject to tension forces . This can result in improved structural capacity and/or serviceability compared with conventionally reinforced concrete in many situations. In a prestressed concrete member,

2465-468: Is the energy of cleaving species 1 from species 2 in a medium of species 3. A basic understanding of the terminology of cleavage energy , surface energy, and surface tension is very helpful for understanding the physical state and the events that happen at a given surface, but as discussed below, the theory of these variables also yields some interesting effects that concern the practicality of adhesive surfaces in relation to their surroundings. There

2550-445: Is the interfacial energy. This methodology can also be used to discuss cleavage that happens in another medium: γ 12 = (1/2)W 121 = (1/2)W 212 . These two energy quantities refer to the energy that is needed to cleave one species into two pieces while it is contained in a medium of the other species. Likewise for a three species system: γ 13 + γ 23 – γ 12 = W 12 + W 33 – W 13 – W 23 = W 132 , where W 132

2635-465: Is the most popular structural material for bridges, and prestressed concrete is frequently adopted. When investigated in the 1940s for use on heavy-duty bridges, the advantages of this type of bridge over more traditional designs was that it is quicker to install, more economical and longer-lasting with the bridge being less lively. One of the first bridges built in this way is the Adam Viaduct ,

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2720-486: Is the protection afforded to the end-anchorage assemblies of unbonded tendons or cable-stay systems, as the anchorages of both of these are required to retain the prestressing forces. Failure of any of these components can result in the release of prestressing forces, or the physical rupture of stressing tendons. Modern prestressing systems deliver long-term durability by addressing the following areas: Several durability-related events are listed below: Prestressed concrete

2805-419: Is the restraint of the crack. By providing the otherwise brittle interfacial bonds with some flexibility, the molecules that are stringing across the gap can stop the crack from propagating. Another way to understand this phenomenon is by comparing it to the stress concentration at the point of failure mentioned earlier. Since the stress is now spread out over some area, the stress at any given point has less of

2890-414: Is true, one can also find evidence of it by performing “stop-start” measurements. In these experiments, two surfaces slide against one another continuously and occasionally stopped for some measured amount of time. Results from experiments on polymer-on-polymer surfaces show that if the stopping time is short enough, resumption of smooth sliding is easy. If, however, the stopping time exceeds some limit, there

2975-417: Is undertaken for three main purposes: to protect the tendons against corrosion ; to permanently "lock-in" the tendon pre-tension, thereby removing the long-term reliance upon the end-anchorage systems; and to improve certain structural behaviors of the final concrete structure. Bonded post-tensioning characteristically uses tendons each comprising bundles of elements (e.g., strands or wires) placed inside

3060-403: The adhesive abilities of the feet of various arthropods and vertebrates (most notably, geckos ). By intermixing periodic breaks into smooth, adhesive surfaces, the interface acquires valuable crack-arresting properties. Because crack initiation requires much greater stress than does crack propagation, surfaces like these are much harder to separate, as a new crack has to be restarted every time

3145-428: The adjacent surface. Un crosslinked polymers ( thermoplastics ), on the other hand are freer to wander into the adjacent phase by extending tails and loops across the interface. Another circumstance under which diffusive bonding occurs is “scission”. Chain scission is the cutting up of polymer chains, resulting in a higher concentration of distal tails. The heightened concentration of these chain ends gives rise to

3230-405: The application, ranging from building works typically using between 2 and 6 strands per tendon, to specialized dam works using up to 91 strands per tendon. Fabrication of bonded tendons is generally undertaken on-site, commencing with the fitting of end-anchorages to formwork , placing the tendon ducting to the required curvature profiles, and reeving (or threading) the strands or wires through

3315-511: The authorities of building codes or standards, but rather exist to promote the understanding and development of prestressed concrete design, codes and best practices. Rules and requirements for the detailing of reinforcement and prestressing tendons are specified by individual national codes and standards such as: Adhesion Process of attachment of a substance to the surface of another substance. Note 1 : Adhesion requires energy that can come from chemical and/or physical linkages,

3400-468: The availability of alternative systems. Either one is capable of delivering code-compliant, durable structures meeting the structural strength and serviceability requirements of the designer. The benefits that bonded post-tensioning can offer over unbonded systems are: The benefits that unbonded post-tensioning can offer over bonded systems are: Long-term durability is an essential requirement for prestressed concrete given its widespread use. Research on

3485-552: The bridge allows the endangered Hine's Emerald Dragonfly to fly safely beneath the bridge, away from the flow of traffic. Post-tensioned concrete This compression is produced by the tensioning of high-strength "tendons" located within or adjacent to the concrete and is done to improve the performance of the concrete in service. Tendons may consist of single wires , multi-wire strands or threaded bars that are most commonly made from high-tensile steels , carbon fiber or aramid fiber . The essence of prestressed concrete

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3570-441: The compounds observed to experience van der Waals forces had no multipoles at all. London suggested that momentary dipoles are induced purely by virtue of molecules being in proximity to one another. By solving the quantum mechanical system of two electrons as harmonic oscillators at some finite distance from one another, being displaced about their respective rest positions and interacting with each other's fields, London showed that

3655-433: The concrete volume (internal prestressing) or wholly outside of it (external prestressing). While pre-tensioned concrete uses tendons directly bonded to the concrete, post-tensioned concrete can use either bonded or unbonded tendons. Pre-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned prior to the concrete being cast. The concrete bonds to the tendons as it cures , following which

3740-404: The concrete wall to form a series of hoops, spaced vertically up the structure. When tensioned, these tendons exert both axial (compressive) and radial (inward) forces onto the structure, which can directly oppose the subsequent storage loadings. If the magnitude of the prestress is designed to always exceed the tensile stresses produced by the loadings, a permanent residual compression will exist in

3825-478: The contact angle is low are considered of higher adhesion per unit area. This approach assumes that the lower contact angle corresponds to a higher surface energy. Theoretically, the more exact relation between contact angle and work of adhesion is more involved and is given by the Young-Dupre equation . The contact angle of the three-phase system is a function not only of dispersive adhesion (interaction between

3910-1042: The design and construction of prestressed concrete structures. In the United States, such organizations include the Post-Tensioning Institute (PTI) and the Precast/Prestressed Concrete Institute (PCI). Similar bodies include the Canadian Precast/Prestressed Concrete Institute (CPCI), the UK's Post-Tensioning Association, the Post Tensioning Institute of Australia and the South African Post Tensioning Association. Europe has similar country-based associations and institutions. These organizations are not

3995-415: The dispersion effect has another useful consequence. Consider a single such dispersive dipole , referred to as the origin dipole. Since any origin dipole is inherently oriented so as to be attracted to the adjacent dipoles it induces, while the other, more distant dipoles are not correlated with the original dipole by any phase relation (thus on average contributing nothing), there is a net attractive force in

4080-413: The ducting. Following concreting and tensioning, the ducts are pressure-grouted and the tendon stressing-ends sealed against corrosion . Unbonded post-tensioning differs from bonded post-tensioning by allowing the tendons permanent freedom of longitudinal movement relative to the concrete. This is most commonly achieved by encasing each individual tendon element within a plastic sheathing filled with

4165-426: The durability performance of in-service prestressed structures has been undertaken since the 1960s, and anti-corrosion technologies for tendon protection have been continually improved since the earliest systems were developed. The durability of prestressed concrete is principally determined by the level of corrosion protection provided to any high-strength steel elements within the prestressing tendons. Also critical

4250-399: The end-anchoring of the tendons is released, and the tendon tension forces are transferred to the concrete as compression by static friction . Pre-tensioning is a common prefabrication technique, where the resulting concrete element is manufactured off-site from the final structure location and transported to site once cured. It requires strong, stable end-anchorage points between which

4335-431: The energy of this system is given by: While the first term is simply the zero-point energy , the negative second term describes an attractive force between neighboring oscillators. The same argument can also be extended to a large number of coupled oscillators, and thus skirts issues that would negate the large scale attractive effects of permanent dipoles cancelling through symmetry, in particular. The additive nature of

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4420-451: The form of post-tensioned anchors drilled into the dam's concrete structure and/or the underlying rock strata. Such anchors typically comprise tendons of high-tensile bundled steel strands or individual threaded bars. Tendons are grouted to the concrete or rock at their far (internal) end, and have a significant "de-bonded" free-length at their external end which allows the tendon to stretch during tensioning. Tendons may be full-length bonded to

4505-638: The function of adhesive devices , because they do not require either surface to have any permanent polarity. They were described in the 1930s by Fritz London , and have been observed by many researchers. Dispersive forces are a consequence of statistical quantum mechanics . London theorized that attractive forces between molecules that cannot be explained by ionic or covalent interaction can be caused by polar moments within molecules. Multipoles could account for attraction between molecules having permanent multipole moments that participate in electrostatic interaction . However, experimental data showed that many of

4590-422: The function of various kinds of stickers and sticky tape fall into the categories of chemical adhesion , dispersive adhesion , and diffusive adhesion . In addition to the cumulative magnitudes of these intermolecular forces, there are also certain emergent mechanical effects . Surface energy is conventionally defined as the work that is required to build an area of a particular surface . Another way to view

4675-538: The gap would either be the molecules that had earlier diffused across the interface or the viscoelastic adhesive, provided that there was a significant volume of it at the interface. The interplay of molecular scale mechanisms and hierarchical surface structures is known to result in high levels of static friction and bonding between pairs of surfaces. Technologically advanced adhesive devices sometimes make use of microstructures on surfaces, such as tightly packed periodic posts. These are biomimetic technologies inspired by

4760-464: The interface, the tails and loops form whatever bonds are favorable. In the case of polymer-on-polymer surfaces, this means more van der Waals forces. While these may be brittle, they are quite strong when a large network of these bonds is formed. The outermost layer of each surface plays a crucial role in the adhesive properties of such interfaces, as even a tiny amount of interdigitation – as little as one or two tails of 1.25 angstrom length – can increase

4845-410: The interfacial bonds. As an additional consequence, increasing surface area often does little to enhance the strength of the adhesion in this situation. This follows from the aforementioned crack failure – the stress at the interface is not uniformly distributed, but rather concentrated at the area of failure. Some conducting materials may pass electrons to form a difference in electrical charge at

4930-622: The internal stresses are introduced in a planned manner so that the stresses resulting from the imposed loads are counteracted to the desired degree. Prestressed concrete is used in a wide range of building and civil structures where its improved performance can allow for longer spans , reduced structural thicknesses, and material savings compared with simple reinforced concrete . Typical applications include high-rise buildings , residential concrete slabs , foundation systems , bridge and dam structures, silos and tanks , industrial pavements and nuclear containment structures . First used in

5015-406: The joint. This results in a structure similar to a capacitor and creates an attractive electrostatic force between the materials. Some materials may merge at the joint by diffusion . This may occur when the molecules of both materials are mobile and soluble in each other. This would be particularly effective with polymer chains where one end of the molecule diffuses into the other material. It

5100-404: The late nineteenth century, prestressed concrete has developed beyond pre-tensioning to include post-tensioning , which occurs after the concrete is cast. Tensioning systems may be classed as either monostrand , where each tendon's strand or wire is stressed individually, or multi-strand , where all strands or wires in a tendon are stressed simultaneously. Tendons may be located either within

5185-569: The latter being reversible when enough energy is applied. Note 2 : In biology, adhesion reflects the behavior of cells shortly after contact to the surface. Note 3 : In surgery, adhesion is used when two tissues fuse unexpectedly. Adhesion is the tendency of dissimilar particles or surfaces to cling to one another. ( Cohesion refers to the tendency of similar or identical particles and surfaces to cling to one another.) The forces that cause adhesion and cohesion can be divided into several types. The intermolecular forces responsible for

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5270-432: The longest bridges, prestressed concrete deck structures often form an integral part of cable-stayed designs . Concrete dams have used prestressing to counter uplift and increase their overall stability since the mid-1930s. Prestressing is also frequently retro-fitted as part of dam remediation works, such as for structural strengthening, or when raising crest or spillway heights. Most commonly, dam prestressing takes

5355-452: The molecules in the liquid and the molecules in the solid) but also cohesion (interaction between the liquid molecules themselves). Strong adhesion and weak cohesion results in a high degree of wetting , a lyophilic condition with low measured contact angles. Conversely, weak adhesion and strong cohesion results in lyophobic conditions with high measured contact angles and poor wetting. London dispersion forces are particularly useful for

5440-407: The next individual microstructure is reached. Hysteresis , in this case, refers to the restructuring of the adhesive interface over some period of time, with the result being that the work needed to separate two surfaces is greater than the work that was gained by bringing them together (W > γ 1 + γ 2 ). For the most part, this is a phenomenon associated with diffusive bonding. The more time

5525-507: The random movement of electrons within the molecules may result in a temporary concentration of electrons in one region ( London forces ). In surface science , the term adhesion almost always refers to dispersive adhesion. In a typical solid-liquid-gas system (such as a drop of liquid on a solid surrounded by air) the contact angle is used to evaluate adhesiveness indirectly, while a Centrifugal Adhesion Balance allows for direct quantitative adhesion measurements. Generally, cases where

5610-463: The same. This theory provides a basis for the existence of van der Waals forces at the surface, which exist between any molecules having electrons . These forces are easily observed through the spontaneous jumping of smooth surfaces into contact . Smooth surfaces of mica , gold, various polymers and solid gelatin solutions do not stay apart when their separating becomes small enough – on the order of 1–10 nm. The equation describing these attractions

5695-432: The square pillars at the north and south entrances to the bridge. The bridge is 1.3 miles (2.1 km) long. The bridge consists of 34 piers from 10 to 75 feet (3 to 23 m) in height. A lower level bridge was also built for maintenance purposes, and to carry a bicycle trail that will connect other bicycle trails in the area. The total height of the bridge ranges from 80 to 100 feet (24 to 30 m). The height of

5780-553: The strands is provided by the combined layers of grease, plastic sheathing, and surrounding concrete. Where strands are bundled to form a single unbonded tendon, an enveloping duct of plastic or galvanised steel is used and its interior free-spaces grouted after stressing. In this way, additional corrosion protection is provided via the grease, plastic sheathing, grout, external sheathing, and surrounding concrete layers. Individually greased-and-sheathed tendons are usually fabricated off-site by an extrusion process. The bare steel strand

5865-408: The strength of an adhesive contact is its shape. Adhesive contacts of complex shape begin to detach at the "edges" of the contact area. The process of destruction of adhesive contacts can be seen in the film. In concert with the primary surface forces described above, there are several circumstantial effects in play. While the forces themselves each contribute to the magnitude of the adhesion between

5950-579: The structure to counter in-service loadings. This provides many benefits to building structures: Some notable building structures constructed from prestressed concrete include: Sydney Opera House and World Tower , Sydney; St George Wharf Tower , London; CN Tower , Toronto; Kai Tak Cruise Terminal and International Commerce Centre , Hong Kong; Ocean Heights 2 , Dubai; Eureka Tower , Melbourne; Torre Espacio , Madrid; Guoco Tower (Tanjong Pagar Centre), Singapore; Zagreb International Airport , Croatia; and Capital Gate , Abu Dhabi UAE. Concrete

6035-845: The surface area in between each post is elevated above the smooth surface, like a roof supported by columns. Because of these attractive dispersive forces between the PDMS and the smooth substrate, the elevated surface – or “roof” – collapses down onto the substrate without any external force aside from the van der Waals attraction. Simple smooth polymer surfaces – without any microstructures – are commonly used for these dispersive adhesive properties. Decals and stickers that adhere to glass without using any chemical adhesives are fairly common as toys and decorations and useful as removable labels because they do not rapidly lose their adhesive properties, as do sticky tapes that use adhesive chemical compounds. These forces also act over very small distances – 99% of

6120-482: The surface energy is to relate it to the work required to cleave a bulk sample, creating two surfaces. If the new surfaces are identical, the surface energy γ of each surface is equal to half the work of cleavage, W: γ = (1/2)W 11 . If the surfaces are unequal, the Young-Dupré equation applies: W 12 = γ 1 + γ 2 – γ 12 , where γ 1 and γ 2 are the surface energies of the two new surfaces, and γ 12

6205-404: The surface in question, thereby occupying potential adhesion sites. This last point is actually fairly intuitive: Leaving an adhesive exposed to air too long gets it dirty, and its adhesive strength will decrease. This is observed in the experiment: when mica is cleaved in air, its cleavage energy, W 121 or W mica/air/mica , is smaller than the cleavage energy in vacuum, W mica/vac/mica , by

6290-475: The surface of the tendons is critical to the pre-tensioning process, as it determines when the tendon anchorages can be safely released. Higher bond strength in early-age concrete will speed production and allow more economical fabrication. To promote this, pre-tensioned tendons are usually composed of isolated single wires or strands, which provides a greater surface area for bonding than bundled-strand tendons. Unlike those of post-tensioned concrete (see below),

6375-425: The surfaces, the following play a crucial role in the overall strength and reliability of an adhesive device. Stringing is perhaps the most crucial of these effects, and is often seen on adhesive tapes. Stringing occurs when a separation of two surfaces is beginning and molecules at the interface bridge out across the gap, rather than cracking like the interface itself. The most significant consequence of this effect

6460-432: The surrounding concrete or rock once tensioned, or (more commonly) have strands permanently encapsulated in corrosion-inhibiting grease over the free-length to permit long-term load monitoring and re-stressability. Circular storage structures such as silos and tanks can use prestressing forces to directly resist the outward pressures generated by stored liquids or bulk-solids. Horizontally curved tendons are installed within

6545-546: The tendon element is subsequently bonded to the surrounding concrete by internal grouting of the duct after stressing ( bonded post-tensioning); and those where the tendon element is permanently de bonded from the surrounding concrete, usually by means of a greased sheath over the tendon strands ( unbonded post-tensioning). Casting the tendon ducts/sleeves into the concrete before any tensioning occurs allows them to be readily "profiled" to any desired shape including incorporating vertical and/or horizontal curvature . When

6630-418: The tendons are stretched. These anchorages form the ends of a "casting bed" which may be many times the length of the concrete element being fabricated. This allows multiple elements to be constructed end-to-end in the one pre-tensioning operation, allowing significant productivity benefits and economies of scale to be realized. The amount of bond (or adhesion ) achievable between the freshly set concrete and

6715-406: The tendons are tensioned, this profiling results in reaction forces being imparted onto the hardened concrete, and these can be beneficially used to counter any loadings subsequently applied to the structure. In bonded post-tensioning, tendons are permanently bonded to the surrounding concrete by the in situ grouting of their encapsulating ducting (after tendon tensioning). This grouting

6800-662: The tendons of pre-tensioned concrete elements generally form straight lines between end-anchorages. Where "profiled" or "harped" tendons are required, one or more intermediate deviators are located between the ends of the tendon to hold the tendon to the desired non-linear alignment during tensioning. Such deviators usually act against substantial forces, and hence require a robust casting-bed foundation system. Straight tendons are typically used in "linear" precast concrete elements, such as shallow beams, hollow-core slabs ; whereas profiled tendons are more commonly found in deeper precast bridge beams and girders. Pre-tensioned concrete

6885-677: The use of precast prestressed concrete for road pavements, where the speed and quality of the construction has been noted as being beneficial for this technique. Some notable civil structures constructed using prestressed concrete include: Gateway Bridge , Brisbane Australia; Incheon Bridge , South Korea; Roseires Dam , Sudan; Wanapum Dam , Washington, US; LNG tanks , South Hook, Wales; Cement silos , Brevik Norway; Autobahn A73 bridge , Itz Valley, Germany; Ostankino Tower , Moscow, Russia; CN Tower , Toronto, Canada; and Ringhals nuclear reactor , Videbergshamn Sweden. Worldwide, many professional organizations exist to promote best practices in

6970-598: The van der Waals bonds by an order of magnitude. The strength of the adhesion between two materials depends on which of the above mechanisms occur between the two materials, and the surface area over which the two materials contact. Materials that wet against each other tend to have a larger contact area than those that do not. Wetting depends on the surface energy of the materials. Low surface energy materials such as polyethylene , polypropylene , polytetrafluoroethylene and polyoxymethylene are difficult to bond without special surface preparation. Another factor determining

7055-491: The wall concrete, assisting in maintaining a watertight crack-free structure. Prestressed concrete has been established as a reliable construction material for high-pressure containment structures such as nuclear reactor vessels and containment buildings, and petrochemical tank blast-containment walls. Using pre-stressing to place such structures into an initial state of bi-axial or tri-axial compression increases their resistance to concrete cracking and leakage, while providing

7140-420: The work necessary to break van der Waals bonds is done once surfaces are pulled more than a nanometer apart. As a result of this limited motion in both the van der Waals and ionic/covalent bonding situations, practical effectiveness of adhesion due to either or both of these interactions leaves much to be desired. Once a crack is initiated, it propagates easily along the interface because of the brittle nature of

7225-466: Was predicted in the 1930s by De Boer and Hamaker: where P is the force (negative for attraction), z is the separation distance, and A is a material-specific constant called the Hamaker constant . The effect is also apparent in experiments where a polydimethylsiloxane (PDMS) stamp is made with small periodic post structures. The surface with the posts is placed face down on a smooth surface, such that

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