Reinforced concrete

Reinforced concrete (RC) (also called reinforced cement concrete or RCC) is a composite material in which concrete's relatively low tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength or ductility. The reinforcement is usually, though not necessarily, steel reinforcing bars (rebar) and is usually embedded passively in the concrete before the concrete sets. Reinforcing schemes are generally designed to resist tensile stresses in particular regions of the concrete that might cause unacceptable cracking and/or structural failure. Modern reinforced concrete can contain varied reinforcing materials made of steel, polymers or alternate composite material in conjunction with rebar or not. Reinforced concrete may also be permanently stressed (concrete in compression, reinforcement in tension), so as to improve the behaviour of the final structure under working loads. In the United States, the most common methods of doing this are known as pre-tensioning and post-tensioning.

For a strong, ductile and durable construction the reinforcement needs to have the following properties at least:

  • High relative strength
  • High toleration of tensile strain
  • Good bond to the concrete, irrespective of pH, moisture, and similar factors
  • Thermal compatibility, not causing unacceptable stresses (such as expansion or contraction) in response to changing temperatures.
  • Durability in the concrete environment, irrespective of corrosion or sustained stress for example.
Talbruecke-Bruenn 2005-08-04
A heavy reinforced concrete column, seen before and after the concrete has been cast in place around its rebar cage

History

Expo58 building Philips
The novel shape of the Philips Pavilion built in Brussels for Expo '58 was achieved using reinforced concrete

Joseph Monier was a French gardener of the nineteenth century, a pioneer in the development of structural, prefabricated and reinforced concrete.[1] François Coignet was the first to use iron-reinforced concrete as a technique for constructing building structures.[2] In 1853, Coignet built the first iron reinforced concrete structure, a four-story house at 72 rue Charles Michels in the suburbs of Paris.[2] Coignet's descriptions of reinforcing concrete suggests that he did not do it for means of adding strength to the concrete but for keeping walls in monolithic construction from overturning.[3] In 1854, English builder William B. Wilkinson reinforced the concrete roof and floors in the two-storey house he was constructing. His positioning of the reinforcement demonstrated that, unlike his predecessors, he had knowledge of tensile stresses.[4][5][6]

Joseph Monier, a French gardener and known to be one of the principal inventors of reinforced concrete, was granted a patent for reinforced flowerpots by means of mixing a wire mesh to a mortar shell. In 1877, Monier was granted another patent for a more advanced technique of reinforcing concrete columns and girders with iron rods placed in a grid pattern. Though Monier undoubtedly knew reinforcing concrete would improve its inner cohesion, it is less known if he even knew how much reinforcing actually improved concrete's tensile strength.[7]

Before 1877 the use of concrete construction, though dating back to the Roman Empire, and having been reintroduced in the early 1800s, was not yet a proven scientific technology. American New Yorker Thaddeus Hyatt published a report titled An Account of Some Experiments with Portland-Cement-Concrete Combined with Iron as a Building Material, with Reference to Economy of Metal in Construction and for Security against Fire in the Making of Roofs, Floors, and Walking Surfaces where he reported his experiments on the behavior of reinforced concrete. His work played a major role in the evolution of concrete construction as a proven and studied science. Without Hyatt's work, more dangerous trial and error methods would have largely been depended on for the advancement in the technology.[3][8]

G. A. Wayss was a German civil engineer and a pioneer of the iron and steel concrete construction. In 1879, Wayss bought the German rights to Monier's patents and in 1884, he started the first commercial use for reinforced concrete in his firm Wayss & Freytag. Up until the 1890s, Wayss and his firm greatly contributed to the advancement of Monier's system of reinforcing and established it as a well-developed scientific technology.[7]

Ernest L. Ransome was an English-born engineer and early innovator of the reinforced concrete techniques in the end of the 19th century. With the knowledge of reinforced concrete developed during the previous 50 years, Ransome innovated nearly all styles and techniques of the previous known inventors of reinforced concrete. Ransome's key innovation was to twist the reinforcing steel bar improving bonding with the concrete.[9] Gaining increasing fame from his concrete constructed buildings, Ransome was able to build two of the first reinforced concrete bridges in North America.[10] One of the first concrete buildings constructed in the United States, was a private home, designed by William Ward in 1871. The home was designed to be fireproof for his wife.

One of the first skyscrapers made with reinforced concrete was the 16-story Ingalls Building in Cincinnati, constructed in 1904.[6]

The first reinforced concrete building in Southern California was the Laughlin Annex in Downtown Los Angeles, constructed in 1905.[11][12] In 1906, 16 building permits were reportedly issued for reinforced concrete buildings in the City of Los Angeles, including the Temple Auditorium and 8-story Hayward Hotel.[13][14]

On April 18, 1906 a magnitude 7.8 earthquake struck San Francisco. The strong ground shaking and subsequent fire destroyed much of the city and killed thousands. The use of reinforced concrete after the earthquake was highly promoted within the U.S. construction industry due to its non-combustibility and perceived superior seismic performance relative to masonry.

In 1906, a partial collapse of the Bixby Hotel in Long Beach killed 10 workers during construction when shoring was removed prematurely. This event spurred a scrutiny of concrete erection practices and building inspections. The structure was constructed of reinforced concrete frames with hollow clay tile ribbed flooring and hollow clay tile infill walls. This practice was strongly questioned by experts and recommendations for “pure” concrete construction using reinforced concrete for the floors and walls as well as the frames were made.[15]

The National Association of Cement Users (NACU) published in 1906 “Standard No. 1”,[16] and in 1910 the “Standard Building Regulations for the Use of Reinforced Concrete”.[17]

Use in construction

SagradaFamiliaRoof2
Rebars of Sagrada Família's roof in construction (2009)

Many different types of structures and components of structures can be built using reinforced concrete including slabs, walls, beams, columns, foundations, frames and more.

Reinforced concrete can be classified as precast or cast-in-place concrete.

Designing and implementing the most efficient floor system is key to creating optimal building structures. Small changes in the design of a floor system can have significant impact on material costs, construction schedule, ultimate strength, operating costs, occupancy levels and end use of a building.

Without reinforcement, constructing modern structures with concrete material would not be possible.

Behavior of reinforced concrete

Materials

Concrete is a mixture of coarse (stone or brick chips) and fine (generally sand or crushed stone) aggregates with a paste of binder material (usually Portland cement) and water. When cement is mixed with a small amount of water, it hydrates to form microscopic opaque crystal lattices encapsulating and locking the aggregate into a rigid structure. The aggregates used for making concrete should be free from harmful substances like organic impurities, silt, clay, lignite etc. Typical concrete mixes have high resistance to compressive stresses (about 4,000 psi (28 MPa)); however, any appreciable tension (e.g., due to bending) will break the microscopic rigid lattice, resulting in cracking and separation of the concrete. For this reason, typical non-reinforced concrete must be well supported to prevent the development of tension.

If a material with high strength in tension, such as steel, is placed in concrete, then the composite material, reinforced concrete, resists not only compression but also bending and other direct tensile actions. A composite section where the concrete resists compression and reinforcement "rebar" resists tension can be made into almost any shape and size for the construction industry.

Key characteristics

Three physical characteristics give reinforced concrete its special properties:

  1. The coefficient of thermal expansion of concrete is similar to that of steel, eliminating large internal stresses due to differences in thermal expansion or contraction.
  2. When the cement paste within the concrete hardens, this conforms to the surface details of the steel, permitting any stress to be transmitted efficiently between the different materials. Usually steel bars are roughened or corrugated to further improve the bond or cohesion between the concrete and steel.
  3. The alkaline chemical environment provided by the alkali reserve (KOH, NaOH) and the portlandite (calcium hydroxide) contained in the hardened cement paste causes a passivating film to form on the surface of the steel, making it much more resistant to corrosion than it would be in neutral or acidic conditions. When the cement paste is exposed to the air and meteoric water reacts with the atmospheric CO2, portlandite and the calcium silicate hydrate (CSH) of the hardened cement paste become progressively carbonated and the high pH gradually decreases from 13.5 – 12.5 to 8.5, the pH of water in equilibrium with calcite (calcium carbonate) and the steel is no longer passivated.

As a rule of thumb, only to give an idea on orders of magnitude, steel is protected at pH above ~11 but starts to corrode below ~10 depending on steel characteristics and local physico-chemical conditions when concrete becomes carbonated. carbonatation of concrete along with chloride ingress are amongst the chief reasons for the failure of reinforcement bars in concrete.[18]

The relative cross-sectional area of steel required for typical reinforced concrete is usually quite small and varies from 1% for most beams and slabs to 6% for some columns. Reinforcing bars are normally round in cross-section and vary in diameter. Reinforced concrete structures sometimes have provisions such as ventilated hollow cores to control their moisture & humidity.

Distribution of concrete (in spite of reinforcement) strength characteristics along the cross-section of vertical reinforced concrete elements is inhomogeneous.[19]

Mechanism of composite action of reinforcement and concrete

The reinforcement in a RC structure, such as a steel bar, has to undergo the same strain or deformation as the surrounding concrete in order to prevent discontinuity, slip or separation of the two materials under load. Maintaining composite action requires transfer of load between the concrete and steel. The direct stress is transferred from the concrete to the bar interface so as to change the tensile stress in the reinforcing bar along its length. This load transfer is achieved by means of bond (anchorage) and is idealized as a continuous stress field that develops in the vicinity of the steel-concrete interface.

Anchorage (bond) in concrete: Codes of specifications

Because the actual bond stress varies along the length of a bar anchored in a zone of tension, current international codes of specifications use the concept of development length rather than bond stress. The main requirement for safety against bond failure is to provide a sufficient extension of the length of the bar beyond the point where the steel is required to develop its yield stress and this length must be at least equal to its development length. However, if the actual available length is inadequate for full development, special anchorages must be provided, such as cogs or hooks or mechanical end plates. The same concept applies to lap splice length mentioned in the codes where splices (overlapping) provided between two adjacent bars in order to maintain the required continuity of stress in the splice zone.

Anti-corrosion measures

In wet and cold climates, reinforced concrete for roads, bridges, parking structures and other structures that may be exposed to deicing salt may benefit from use of corrosion-resistant reinforcement such as uncoated, low carbon/chromium (micro composite), epoxy-coated, hot dip galvanised or stainless steel rebar. Good design and a well-chosen concrete mix will provide additional protection for many applications. Uncoated, low carbon/chromium rebar looks similar to standard carbon steel rebar due to its lack of a coating; its highly corrosion-resistant features are inherent in the steel microstructure. It can be identified by the unique ASTM specified mill marking on its smooth, dark charcoal finish. Epoxy coated rebar can easily be identified by the light green colour of its epoxy coating. Hot dip galvanized rebar may be bright or dull grey depending on length of exposure, and stainless rebar exhibits a typical white metallic sheen that is readily distinguishable from carbon steel reinforcing bar. Reference ASTM standard specifications A1035/A1035M Standard Specification for Deformed and Plain Low-carbon, Chromium, Steel Bars for Concrete Reinforcement, A767 Standard Specification for Hot Dip Galvanised Reinforcing Bars, A775 Standard Specification for Epoxy Coated Steel Reinforcing Bars and A955 Standard Specification for Deformed and Plain Stainless Bars for Concrete Reinforcement.

Another, cheaper way of protecting rebars is coating them with zinc phosphate.[20] Zinc phosphate slowly reacts with calcium cations and the hydroxyl anions present in the cement pore water and forms a stable hydroxyapatite layer.

Penetrating sealants typically must be applied some time after curing. Sealants include paint, plastic foams, films and aluminum foil, felts or fabric mats sealed with tar, and layers of bentonite clay, sometimes used to seal roadbeds.

Corrosion inhibitors, such as calcium nitrite [Ca(NO2)2], can also be added to the water mix before pouring concrete. Generally, 1–2 wt. % of [Ca(NO2)2] with respect to cement weight is needed to prevent corrosion of the rebars. The nitrite anion is a mild oxidizer that oxidizes the soluble and mobile ferrous ions (Fe2+) present at the surface of the corroding steel and causes them to precipitate as an insoluble ferric hydroxide (Fe(OH)3). This causes the passivation of steel at the anodic oxidation sites. Nitrite is a much more active corrosion inhibitor than nitrate, which is a less powerful oxidizer of the divalent iron.

Reinforcement and terminology of beams

Rebarbeams
Two intersecting beams integral to parking garage slab that will contain both reinforcing steel and the wiring, junction boxes and other electrical components necessary to install the overhead lighting for the garage level beneath it.

A beam bends under bending moment, resulting in a small curvature. At the outer face (tensile face) of the curvature the concrete experiences tensile stress, while at the inner face (compressive face) it experiences compressive stress.

A singly reinforced beam is one in which the concrete element is only reinforced near the tensile face and the reinforcement, called tension steel, is designed to resist the tension.

A doubly reinforced beam is one in which besides the tensile reinforcement the concrete element is also reinforced near the compressive face to help the concrete resist compression. The latter reinforcement is called compression steel. When the compression zone of a concrete is inadequate to resist the compressive moment (positive moment), extra reinforcement has to be provided if the architect limits the dimensions of the section.

An under-reinforced beam is one in which the tension capacity of the tensile reinforcement is smaller than the combined compression capacity of the concrete and the compression steel (under-reinforced at tensile face). When the reinforced concrete element is subject to increasing bending moment, the tension steel yields while the concrete does not reach its ultimate failure condition. As the tension steel yields and stretches, an "under-reinforced" concrete also yields in a ductile manner, exhibiting a large deformation and warning before its ultimate failure. In this case the yield stress of the steel governs the design.

An over-reinforced beam is one in which the tension capacity of the tension steel is greater than the combined compression capacity of the concrete and the compression steel (over-reinforced at tensile face). So the "over-reinforced concrete" beam fails by crushing of the compressive-zone concrete and before the tension zone steel yields, which does not provide any warning before failure as the failure is instantaneous.

A balanced-reinforced beam is one in which both the compressive and tensile zones reach yielding at the same imposed load on the beam, and the concrete will crush and the tensile steel will yield at the same time. This design criterion is however as risky as over-reinforced concrete, because failure is sudden as the concrete crushes at the same time of the tensile steel yields, which gives a very little warning of distress in tension failure.[21]

Steel-reinforced concrete moment-carrying elements should normally be designed to be under-reinforced so that users of the structure will receive warning of impending collapse.

The characteristic strength is the strength of a material where less than 5% of the specimen shows lower strength.

The design strength or nominal strength is the strength of a material, including a material-safety factor. The value of the safety factor generally ranges from 0.75 to 0.85 in Permissible stress design.

The ultimate limit state is the theoretical failure point with a certain probability. It is stated under factored loads and factored resistances.

Reinforced concrete structures are normally designed according to rules and regulations or recommendation of a code such as ACI-318, CEB, Eurocode 2 or the like. WSD, USD or LRFD methods are used in design of RC structural members. Analysis and design of RC members can be carried out by using linear or non-linear approaches. When applying safety factors, building codes normally propose linear approaches, but for some cases non-linear approaches. To see the examples of a non-linear numerical simulation and calculation visit the references:[22][23]

Prestressed concrete

Prestressing concrete is a technique that greatly increases the load-bearing strength of concrete beams. The reinforcing steel in the bottom part of the beam, which will be subjected to tensile forces when in service, is placed in tension before the concrete is poured around it. Once the concrete has hardened, the tension on the reinforcing steel is released, placing a built-in compressive force on the concrete. When loads are applied, the reinforcing steel takes on more stress and the compressive force in the concrete is reduced, but does not become a tensile force. Since the concrete is always under compression, it is less subject to cracking and failure.

Common failure modes of steel reinforced concrete

Reinforced concrete can fail due to inadequate strength, leading to mechanical failure, or due to a reduction in its durability. Corrosion and freeze/thaw cycles may damage poorly designed or constructed reinforced concrete. When rebar corrodes, the oxidation products (rust) expand and tends to flake, cracking the concrete and unbonding the rebar from the concrete. Typical mechanisms leading to durability problems are discussed below.

Mechanical failure

Cracking of the concrete section is nearly impossible to prevent; however, the size and location of cracks can be limited and controlled by appropriate reinforcement, control joints, curing methodology and concrete mix design. Cracking can allow moisture to penetrate and corrode the reinforcement. This is a serviceability failure in limit state design. Cracking is normally the result of an inadequate quantity of rebar, or rebar spaced at too great a distance. The concrete then cracks either under excess loading, or due to internal effects such as early thermal shrinkage while it cures.

Ultimate failure leading to collapse can be caused by crushing the concrete, which occurs when compressive stresses exceed its strength, by yielding or failure of the rebar when bending or shear stresses exceed the strength of the reinforcement, or by bond failure between the concrete and the rebar.[24]

Carbonation

Concrete wall cracking as steel reinforcing corrodes and swells 9058
Concrete wall cracking as steel reinforcing corrodes and swells. Rust has a lower density than metal, so it expands as it forms, cracking the decorative cladding off the wall as well as damaging the structural concrete. The breakage of material from a surface is called spalling.
Concrete wall cracking as its steel reinforcing cracks and swells 9061v
Detailed view of spalling probably caused by a too thin layer of concrete between the steel and the surface, accompanied by corrosion from external exposure.

Carbonation, or neutralisation, is a chemical reaction between carbon dioxide in the air and calcium hydroxide and hydrated calcium silicate in the concrete.

When a concrete structure is designed, it is usual to specify the concrete cover for the rebar (the depth of the rebar within the object). The minimum concrete cover is normally regulated by design or building codes. If the reinforcement is too close to the surface, early failure due to corrosion may occur. The concrete cover depth can be measured with a cover meter. However, carbonated concrete incurs a durability problem only when there is also sufficient moisture and oxygen to cause electropotential corrosion of the reinforcing steel.

One method of testing a structure for carbonatation is to drill a fresh hole in the surface and then treat the cut surface with phenolphthalein indicator solution. This solution turns pink when in contact with alkaline concrete, making it possible to see the depth of carbonation. Using an existing hole does not suffice because the exposed surface will already be carbonated.

Chlorides

Chlorides, including sodium chloride, can promote the corrosion of embedded steel rebar if present in sufficiently high concentration. Chloride anions induce both localized corrosion (pitting corrosion) and generalized corrosion of steel reinforcements. For this reason, one should only use fresh raw water or potable water for mixing concrete, ensure that the coarse and fine aggregates do not contain chlorides, rather than admixtures which might contain chlorides.

RebarCloseup
Rebar for foundations and walls of a sewage pump station.
Paulins Kill Viaduct in Hainesburg, NJ
The Paulins Kill Viaduct, Hainesburg, New Jersey, is 115 feet (35 m) tall and 1,100 feet (335 m) long, and was heralded as the largest reinforced concrete structure in the world when it was completed in 1910 as part of the Lackawanna Cut-Off rail line project. The Lackawanna Railroad was a pioneer in the use of reinforced concrete.

It was once common for calcium chloride to be used as an admixture to promote rapid set-up of the concrete. It was also mistakenly believed that it would prevent freezing. However, this practice fell into disfavor once the deleterious effects of chlorides became known. It should be avoided whenever possible.

The use of de-icing salts on roadways, used to lower the freezing point of water, is probably one of the primary causes of premature failure of reinforced or prestressed concrete bridge decks, roadways, and parking garages. The use of epoxy-coated reinforcing bars and the application of cathodic protection has mitigated this problem to some extent. Also FRP (fiber-reinforced polymer) rebars are known to be less susceptible to chlorides. Properly designed concrete mixtures that have been allowed to cure properly are effectively impervious to the effects of de-icers.

Another important source of chloride ions is sea water. Sea water contains by weight approximately 3.5 wt.% salts. These salts include sodium chloride, magnesium sulfate, calcium sulfate, and bicarbonates. In water these salts dissociate in free ions (Na+, Mg2+, Cl, SO42−, HCO3) and migrate with the water into the capillaries of the concrete. Chloride ions, which make up about 50% of these ions, are particularly aggressive as a cause of corrosion of carbon steel reinforcement bars.

In the 1960s and 1970s it was also relatively common for magnesite, a chloride rich carbonate mineral, to be used as a floor-topping material. This was done principally as a levelling and sound attenuating layer. However it is now known that when these materials come into contact with moisture they produce a weak solution of hydrochloric acid due to the presence of chlorides in the magnesite. Over a period of time (typically decades), the solution causes corrosion of the embedded steel rebars. This was most commonly found in wet areas or areas repeatedly exposed to moisture.

Alkali silica reaction

This a reaction of amorphous silica (chalcedony, chert, siliceous limestone) sometimes present in the aggregates with the hydroxyl ions (OH) from the cement pore solution. Poorly crystallized silica (SiO2) dissolves and dissociates at high pH (12.5 - 13.5) in alkaline water. The soluble dissociated silicic acid reacts in the porewater with the calcium hydroxide (portlandite) present in the cement paste to form an expansive calcium silicate hydrate (CSH). The alkali–silica reaction (ASR) causes localised swelling responsible for tensile stress and cracking. The conditions required for alkali silica reaction are threefold: (1) aggregate containing an alkali-reactive constituent (amorphous silica), (2) sufficient availability of hydroxyl ions (OH), and (3) sufficient moisture, above 75% relative humidity (RH) within the concrete.[25][26] This phenomenon is sometimes popularly referred to as "concrete cancer". This reaction occurs independently of the presence of rebars; massive concrete structures such as dams can be affected.

Conversion of high alumina cement

Resistant to weak acids and especially sulfates, this cement cures quickly and has very high durability and strength. It was frequently used after World War II to make precast concrete objects. However, it can lose strength with heat or time (conversion), especially when not properly cured. After the collapse of three roofs made of prestressed concrete beams using high alumina cement, this cement was banned in the UK in 1976. Subsequent inquiries into the matter showed that the beams were improperly manufactured, but the ban remained.[27]

Sulphates

Sulfates (SO4) in the soil or in groundwater, in sufficient concentration, can react with the Portland cement in concrete causing the formation of expansive products, e.g., ettringite or thaumasite, which can lead to early failure of the structure. The most typical attack of this type is on concrete slabs and foundation walls at grades where the sulfate ion, via alternate wetting and drying, can increase in concentration. As the concentration increases, the attack on the Portland cement can begin. For buried structures such as pipe, this type of attack is much rarer, especially in the eastern United States. The sulfate ion concentration increases much slower in the soil mass and is especially dependent upon the initial amount of sulfates in the native soil. A chemical analysis of soil borings to check for the presence of sulfates should be undertaken during the design phase of any project involving concrete in contact with the native soil. If the concentrations are found to be aggressive, various protective coatings can be applied. Also, in the US ASTM C150 Type 5 Portland cement can be used in the mix. This type of cement is designed to be particularly resistant to a sulfate attack.

Steel plate construction

In steel plate construction, stringers join parallel steel plates. The plate assemblies are fabricated off site, and welded together on-site to form steel walls connected by stringers. The walls become the form into which concrete is poured. Steel plate construction speeds reinforced concrete construction by cutting out the time-consuming on-site manual steps of tying rebar and building forms. The method results in excellent strength because the steel is on the outside, where tensile forces are often greatest.

Fiber-reinforced concrete

Fiber reinforcement is mainly used in shotcrete, but can also be used in normal concrete. Fiber-reinforced normal concrete is mostly used for on-ground floors and pavements, but can also be considered for a wide range of construction parts (beams, pillars, foundations, etc.), either alone or with hand-tied rebars.

Concrete reinforced with fibers (which are usually steel, glass, plastic fibers) or Cellulose polymer fibre is less expensive than hand-tied rebar. The shape, dimension, and length of the fiber are important. A thin and short fiber, for example short, hair-shaped glass fiber, is only effective during the first hours after pouring the concrete (its function is to reduce cracking while the concrete is stiffening), but it will not increase the concrete tensile strength. A normal-size fiber for European shotcrete (1 mm diameter, 45 mm length—steel or plastic) will increase the concrete's tensile strength. Fiber reinforcement is most often used to supplement or partially replace primary rebar, and in some cases it can be designed to fully replace rebar.

Steel is the strongest commonly available fiber, and comes in different lengths (30 to 80 mm in Europe) and shapes (end-hooks). Steel fibers can only be used on surfaces that can tolerate or avoid corrosion and rust stains. In some cases, a steel-fiber surface is faced with other materials.

Glass fiber is inexpensive and corrosion-proof, but not as ductile as steel. Recently, spun basalt fiber, long available in Eastern Europe, has become available in the U.S. and Western Europe. Basalt fibre is stronger and less expensive than glass, but historically has not resisted the alkaline environment of Portland cement well enough to be used as direct reinforcement. New materials use plastic binders to isolate the basalt fiber from the cement.

The premium fibers are graphite-reinforced plastic fibers, which are nearly as strong as steel, lighter in weight, and corrosion-proof. Some experiments have had promising early results with carbon nanotubes, but the material is still far too expensive for any building.

Non-steel reinforcement

There is considerable overlap between the subjects of non-steel reinforcement and fiber-reinforcement of concrete. The introduction of non-steel reinforcement of concrete is relatively recent; it takes two major forms: non-metallic rebar rods, and non-steel (usually also non-metallic) fibres incorporated into the cement matrix. For example, there is increasing interest in glass fiber reinforced concrete (GFRC) and in various applications of polymer fibres incorporated into concrete. Although currently there is not much suggestion that such materials will replace metal rebar, some of them have major advantages in specific applications, and there also are new applications in which metal rebar simply is not an option. However, the design and application of non-steel reinforcing is fraught with challenges. For one thing, concrete is a highly alkaline environment, in which many materials, including most kinds of glass, have a poor service life. Also, the behaviour of such reinforcing materials differs from the behaviour of metals, for instance in terms of shear strength, creep and elasticity.[28][29]

Fibre-reinforced plastic/polymer (FRP) and glass-reinforced plastic (GRP) consist of fibres of polymer, glass, carbon, aramid or other polymers or high-strength fibres set in a resin matrix to form a rebar rod, or grid, or fibres. These rebars are installed in much the same manner as steel rebars. The cost is higher but, suitably applied, the structures have advantages, in particular a dramatic reduction in problems related to corrosion, either by intrinsic concrete alkalinity or by external corrosive fluids that might penetrate the concrete. These structures can be significantly lighter and usually have a longer service life. The cost of these materials has dropped dramatically since their widespread adoption in the aerospace industry and by the military.

In particular, FRP rods are useful for structures where the presence of steel would not be acceptable. For example, MRI machines have huge magnets, and accordingly require non-magnetic buildings. Again, toll booths that read radio tags need reinforced concrete that is transparent to radio waves. Also, where the design life of the concrete structure is more important than its initial costs, non-steel reinforcing often has its advantages where corrosion of reinforcing steel is a major cause of failure. In such situations corrosion-proof reinforcing can extend a structure's life substantially, for example in the intertidal zone. FRP rods may also be useful in situations where it is likely that the concrete structure may be compromised in future years, for example the edges of balconies when balustrades are replaced, and bathroom floors in multi-story construction where the service life of the floor structure is likely to be many times the service life of the waterproofing building membrane.

Plastic reinforcement often is stronger, or at least has a better strength to weight ratio than reinforcing steels. Also, because it resists corrosion, it does not need a protective concrete cover as thick as steel reinforcement does (typically 30 to 50 mm or more). FRP-reinforced structures therefore can be lighter and last longer. Accordingly, for some applications the whole-life cost will be price-competitive with steel-reinforced concrete.

The material properties of FRP or GRP bars differ markedly from steel, so there are differences in the design considerations. FRP or GRP bars have relatively higher tensile strength but lower stiffness, so that deflections are likely to be higher than for equivalent steel-reinforced units. Structures with internal FRP reinforcement typically have an elastic deformability comparable to the plastic deformability (ductility) of steel reinforced structures. Failure in either case is more likely to occur by compression of the concrete than by rupture of the reinforcement. Deflection is always a major design consideration for reinforced concrete. Deflection limits are set to ensure that crack widths in steel-reinforced concrete are controlled to prevent water, air or other aggressive substances reaching the steel and causing corrosion. For FRP-reinforced concrete, aesthetics and possibly water-tightness will be the limiting criteria for crack width control. FRP rods also have relatively lower compressive strengths than steel rebar, and accordingly require different design approaches for reinforced concrete columns.

One drawback to the use of FRP reinforcement is their limited fire resistance. Where fire safety is a consideration, structures employing FRP have to maintain their strength and the anchoring of the forces at temperatures to be expected in the event of fire. For purposes of fireproofing, an adequate thickness of cement concrete cover or protective cladding is necessary. The addition of 1 kg/m3 of polypropylene fibers to concrete has been shown to reduce spalling during a simulated fire.[30] (The improvement is thought to be due to the formation of pathways out of the bulk of the concrete, allowing steam pressure to dissipate.[30])

Darby, A., The Airside Road Tunnel, Heathrow Airport, England,

Another problem is the effectiveness of shear reinforcement. FRP rebar stirrups formed by bending before hardening generally perform relatively poorly in comparison to steel stirrups or to structures with straight fibres. When strained, the zone between the straight and curved regions are subject to strong bending, shear, and longitudinal stresses. Special design techniques are necessary to deal with such problems.

There is growing interest in applying external reinforcement to existing structures using advanced materials such as composite (fiberglass, basalt, carbon) rebar, which can impart exceptional strength. Worldwide, there are a number of brands of composite rebar recognized by different countries, such as Aslan, DACOT, V-rod, and ComBar. The number of projects using composite rebar increases day by day around the world, in countries ranging from USA, Russia, and South Korea to Germany.

See also

References

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  3. ^ a b Condit, Carl W. (January 1968). "The First Reinforced-Concrete Skyscraper: The Ingalls Building in Cincinnati and Its Place in Structural History". Technology and Culture. 9 (1). doi:10.2307/3102041. JSTOR 3102041.
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  9. ^ Mars, Roman. "Episode 81: Rebar and the Alvord Lake Bridge". 99% Invisible. Retrieved 6 August 2014.
  10. ^ Collins, Peter (1920–1981). Concrete: The Vision of a New Architecture. McGill-Queen's University Press. pp. 61–64. ISBN 0-7735-2564-5.
  11. ^ McGroarty, John Steven (1921). Los Angeles from the Mountains to the Sea. 2. Los Angeles, CA: American Historical Society. p. 176.
  12. ^ Annual Report of the City Auditor, City of Los Angeles, California for the Year Ending June 30. Los Angeles, CA: Los Angeles City Auditor. 1905. pp. 71–73.
  13. ^ Williams, D. (February 1907). "What Builders are Doing". Carpentry and Building: 66.
  14. ^ W.P.H. (April 19, 1906). "Reinforced Concrete Buildings at Los Angeles, Cal". Letters to the Editor. Engineering News-Record. 55: 449.
  15. ^ Austin, J. C.; Neher, O. H.; Hicks, L. A.; Whittlesey, C. F.; Leonard, J. B. (November 1906). "Partial Collapse of the Bixby Hotel at Long Beach". Architect and Engineer of California. Vol. VII no. 1. pp. 44–48.
  16. ^ Standard Specifications for Portland Cement of the American Society for Testing Materials, Standard No. 1. Philadelphia, PA: National Association of Cement Users. 1906.
  17. ^ Standard Building Regulations for the Use of Reinforced Concrete. Philadelphia, PA: National Association of Cement Users. 1910.
  18. ^ Qiu, Jianhai. "Emerging Corrosion Control Technologies for Repair and Rehabilitation of Concrete Structures". WebCorr Corrosion Consulting Services. page 1. Retrieved 23 December 2016.
  19. ^ "Concrete Inhomogeneity of Vertical Cast-In-Situ Elements In Frame-Type Buildings".
  20. ^ Simescu, Florica; Idrissi, Hassane (December 19, 2008). "Effect of zinc phosphate chemical conversion coating on corrosion behaviour of mild steel in alkaline medium: protection of rebars in reinforced concrete". Science and Technology of Advanced Materials. National Institute for Materials Science. 9 (4). Archived from the original on 2016-04-07.
  21. ^ Nilson, Darwin, Dolan. Design of Concrete Structures. the MacGraw-Hill Education, 2003. p. 80-90.
  22. ^ "Techno Press". 2 April 2015. Archived from the original on 2 April 2015.
  23. ^ Sadeghi, Kabir (15 September 2011). "Energy based structural damage index based on nonlinear numerical simulation of structures subjected to oriented lateral cyclic loading". International Journal of Civil Engineering. 9 (3): 155–164. ISSN 1735-0522. Retrieved 23 December 2016.
  24. ^ Janowski, A.; Nagrodzka-Godycka, K.; Szulwic, J.; Ziółkowski, P. (2016). "Remote sensing and photogrammetry techniques in diagnostics of concrete structures". Computers and Concrete. 18 (3): 405–420. doi:10.12989/cac.2016.18.3.405. Retrieved 2016-12-14.
  25. ^ "Concrete Cancer". h2g2. BBC. March 15, 2012 [2005]. Retrieved 2009-10-14.
  26. ^ "Special Section: South West Alkali Incident". the cement industry. British Cement Association. 4 January 2006. Archived from the original on October 29, 2006. Retrieved 2006-11-26.
  27. ^ "High Alumina Cement". Archived from the original on 2005-09-11. Retrieved 2009-10-14.
  28. ^ BS EN 1169:1999 Precast concrete products. General rules for factory production control of glass-fibre reinforced cement. British Standards Institute. 15 November 1999. ISBN 0-580-32052-9.
  29. ^ BS EN 1170-5:1998 Precast concrete products. Test method for glass-fibre reinforced cement. British Standards Institute. 15 March 1998. ISBN 0-580-29202-9.
  30. ^ a b Arthur W. Darby (2003). "Chapter 57: The Airside Road Tunnel, Heathrow Airport, England" (PDF). Proceedings of the Rapid Excavation & Tunneling Conference, New Orleans, June 2003. p. 645. Archived from the original (PDF) on 2006-05-22 – via www.tunnels.mottmac.com.

Further reading

  • Threlfall A., et al. Reynolds's Reinforced Concrete Designer's Handbook – 11th ed. ISBN 978-0-419-25830-8.
  • Newby F., Early Reinforced Concrete, Ashgate Variorum, 2001, ISBN 978-0-86078-760-0.
  • Kim, S., Surek, J and J. Baker-Jarvis. "Electromagnetic Metrology on Concrete and Corrosion." Journal of Research of the National Institute of Standards and Technology, Vol. 116, No. 3 (May–June 2011): 655-669.
  • Daniel R., Formwork UK "Concrete frame structures.".
Arch bridge

An arch bridge is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side. A viaduct (a long bridge) may be made from a series of arches, although other more economical structures are typically used today.

Carbonation

Carbonation refers to reactions of carbon dioxide to give carbonates, bicarbonates, and carbonic acid. In chemistry, the term is sometimes used in place of carboxylation, which refers to the formation of carboxylic acids.

In inorganic chemistry and geology, carbonation is common. Metal hydroxides (MOH) and metal oxides (M'O) react with CO2 to give bicarbonates and carbonates:

MOH + CO2 → M(HCO3)

M'O + CO2 → M'CO3In reinforced concrete construction, the chemical reaction between carbon dioxide in the air and calcium hydroxide and hydrated calcium silicate in the concrete is known as neutralisation.

Central Plaza (Hong Kong)

Central Plaza is a 78-storey, 374 m (1,227 ft) skyscraper completed in August 1992 at 18 Harbour Road, in Wan Chai on Hong Kong Island in Hong Kong. It is the third tallest tower in the city after 2 International Finance Centre in Central and the ICC in West Kowloon. It was the tallest building in Asia from 1992 to 1996, until the Shun Hing Square was built in Shenzhen, a neighbouring city. Central Plaza surpassed the Bank of China Tower as the tallest building in Hong Kong until the completion of 2 IFC.

Central Plaza was also the tallest reinforced concrete building in the world, until it was surpassed by CITIC Plaza, Guangzhou. The building uses a triangular floor plan. On the top of the tower is a four-bar neon clock that indicates the time by displaying different colours for 15-minute periods, blinking at the change of the quarter.

An anemometer is installed on the tip of the building's mast, at 378 metres (1,240 ft) above sea level. The mast has a height of 102 m (335 ft). It also houses the world's highest church inside a skyscraper, Sky City Church.

Charwelton BT Tower

Charwelton BT Tower is a telecommunication tower built of reinforced concrete at Charwelton near Byfield, Northamptonshire, England. It is 118 metres (387 ft) tall and one of the few British towers built of reinforced concrete. It is a landmark for miles around.

EN 10080

The EN 10080: Steel for the reinforcement of concrete is a European Standard. This standard is referenced by EN 1992.

Fiber-reinforced concrete

Fiber-reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers – each of which lend varying properties to the concrete. In addition, the character of fiber-reinforced concrete changes with varying concretes, fiber materials, geometries, distribution, orientation, and densities.

Glass fiber reinforced concrete

Glass fiber reinforced concrete or 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.

Jaroslav Josef Polívka

Jaroslav Josef Polivka (20 April 1886 – 9 February 1960), Czech structural engineer who collaborated with Frank Lloyd Wright between 1946 and 1959.

Jaroslav Josef Polivka a.k.a. J. J. Polivka Civil Engineer was born in Prague in 1886. He received his undergraduate degree in structural engineering at the College of Technology in Prague in 1909. He then studied at the Federal Polytechnic Institute in Zurich, Switzerland and at the Prague Institute of Technology, where he earned a doctoral degree in 1917. After serving in First World War, he opened his own architectural and engineering office in Prague and developed his skills in stress analysis of reinforced concrete, pre-stressed reinforced concrete and steel structures. Polivka became an expert in photo-elastic stress analysis, a technique that examines small-scale transparent models in polarized light.

In Prague Polivka worked together with avant-garde Czech architect Josef Havlíček on the Habich Building (1927–28) and Chicago Building (1927–28).

Polivka designed the structural frame of the Czech Pavilion at the Paris International Exhibition of 1937 collaborating with renown Czech architect, Jaromír Krejcar and Czech engineer René Wiesner. Two years later, he worked with Czech architect Kamil Roškot to design another Czech Pavilion at the 1939 New York World's Fair. In 1939 Polivka immigrated to the United States and took a position as research associate and lecturer at the University of California, Berkeley. In 1941, he and Victor di Suvero co-invented a structural design technique that received a patent for improvements in structures. Polivka with his son Milos translated into English Eduardo Torroja’s ‘Philosophy of Structures’ book published in 1958.

In 1946 Polivka began to work with Frank Lloyd Wright collaborating on several major projects until Wright's death in 1959. For Wright’s projects Polivka performed stress analyses and investigations of specific building materials. They worked on a total of seven projects, two of which were built: the Johnson Wax Research Tower, 1946-1951 at Racine Wisconsin and the Guggenheim Museum, 1946-1959 in New York City for which Polivka managed to design out the gallery ramp perimeter columns initially required. Their other well-known design proposal was the reinforced concrete Butterfly Bridge (proposed at a world record span of 1000-ft )at the Southern Crossing of the San Francisco Bay (1949–52).

Polivka performed the photoelasticity for the Podolsko Bridge is an arch bridge that spans the Vltava between Podolsko and Temešvár in Písek District, Czech Republic. At the time of its completion in 1943, it was the longest arch bridge in Czechoslovakia.

He died in Berkeley, California.

Pipe (fluid conveyance)

A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases (fluids), slurries, powders and masses of small solids. It can also be used for structural applications; hollow pipe is far stiffer per unit weight than solid members.

In common usage the words pipe and tube are usually interchangeable, but in industry and engineering, the terms are uniquely defined. Depending on the applicable standard to which it is manufactured, pipe is generally specified by a nominal diameter with a constant outside diameter (OD) and a schedule that defines the thickness. Tube is most often specified by the OD and wall thickness, but may be specified by any two of OD, inside diameter (ID), and wall thickness. Pipe is generally manufactured to one of several international and national industrial standards. While similar standards exist for specific industry application tubing, tube is often made to custom sizes and a broader range of diameters and tolerances. Many industrial and government standards exist for the production of pipe and tubing. The term "tube" is also commonly applied to non-cylindrical sections, i.e., square or rectangular tubing. In general, "pipe" is the more common term in most of the world, whereas "tube" is more widely used in the United States.

Both "pipe" and "tube" imply a level of rigidity and permanence, whereas a hose (or hosepipe) is usually portable and flexible. Pipe assemblies are almost always constructed with the use of fittings such as elbows, tees, and so on, while tube may be formed or bent into custom configurations. For materials that are inflexible, cannot be formed, or where construction is governed by codes or standards, tube assemblies are also constructed with the use of tube fittings.

Precast concrete

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 lifted into place ("tilt up"). In contrast, standard concrete is poured into site-specific forms and cured on site. Precast stone is distinguished from precast concrete using a fine aggregate in the mixture, so the final product approaches the appearance of naturally occurring rock or stone. More recently expanded polystyrene is being used as the cores to precast wall panels. This is lightweight and has better thermal insulation.

Precast is used within exterior and interior walls. By producing precast concrete in a controlled environment (typically referred to as a precast plant), the precast concrete is afforded the opportunity to properly cure and be closely monitored by plant employees. Using a precast concrete system offers many potential advantages over onsite casting. Precast concrete production can performed on ground level, which helps with safety throughout a project. There is greater control over material quality and workmanship in a precast plant compared to a construction site. The forms used in a precast plant can be reused hundreds to thousands of times before they have to be replaced, often making it cheaper than onsite casting when looking at the cost per unit of formwork.Many state and federal transportation projects in the United States require precast concrete suppliers to be certified by either the Architectural Precast Association (APA), National Precast Concrete Association (NPCA) or Precast Prestressed Concrete Institute (PCI).

There are many different types of precast concrete forming systems for architectural applications, differing in size, function, and cost. Precast architectural panels are also used to clad all or part of a building facades or free-standing walls used for landscaping, soundproofing, and security walls, and some can be prestressed concrete structural elements. Stormwater drainage, water and sewage pipes, and tunnels make use of precast concrete units.

The New South Wales Government Railways made extensive use of precast concrete construction for its stations and similar buildings. Between 1917 and 1932, they erected 145 such buildings. Beyond cladding panels and structural elements, entire buildings can be assembled from precast concrete. Precast assembly enables fast completion of commercial shops and offices with minimal labor. For example, the Jim Bridger Building in Williston, North Dakota, was precast in Minnesota with air, electrical, water, and fiber utilities preinstalled into the building panels. The panels were transported over 800 miles to the Bakken oilfields, and the commercial building was assembled by three workers in minimal time. The building houses over 40,000 square feet of shops and offices. Virtually the entire building was fabricated in Minnesota.

To complete the look of the four precast wall panel types — sandwich, plastered sandwich, inner layer and cladding panels — many surface finishes are available. Standard cement is white or grey, though different colors can be added with pigments or paints. The color and size of aggregate can also affect the appearance and texture of concrete surfaces. The shape and surface of the precast concrete molds have an effect on the look: The mold can be made of timber, steel, plastic, rubber or fiberglass, each material giving a unique finish.

Rebar

Rebar (short for reinforcing bar), known when massed as reinforcing steel or reinforcement steel, is a steel bar or mesh of steel wires used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and aid the concrete under tension. Concrete is strong under compression, but has weak tensile strength. Rebar significantly increases the tensile strength of the structure. Rebar's surface is often deformed to promote a better bond with the concrete.

The most common type of rebar is carbon steel, typically consisting of hot-rolled round bars with deformation patterns. Other readily available types include stainless steel, and composite bars made of glass fiber, carbon fiber, or basalt fiber. The steel reinforcing bars may also be coated in an epoxy resin designed to resist the effects of corrosion mostly in saltwater environments, but also land based constructions. Bamboo has been shown to be a viable alternative to reinforcing steel in concrete construction. These alternate types tend to be more expensive or may have lesser mechanical properties and are thus more often used in specialty construction where their physical characteristics fulfill a specific performance requirement that carbon steel does not provide. Steel and concrete have similar coefficients of thermal expansion, so a concrete structural member reinforced with steel will experience minimal stress as the temperature changes.

Reinforced concrete column

A reinforced concrete column is a structural member designed to carry compressive loads, composed of concrete with an embedded steel frame to provide reinforcement. For design purposes, the columns are separated into two categories: short columns and slender columns.

Saint-Jean-de-Montmartre

The Church of Saint-Jean-de-Montmartre is located at 19 Rue des Abbesses in the 18th arrondissement of Paris.

Situated at the foot of Montmartre, it is notable as the first example of reinforced concrete in church construction. Built from 1894 through 1904, it was designed by architect Anatole de Baudot, a student of Viollet-le-Duc and Henri Labrouste. The brick and ceramic tile-faced structure exhibits features of Art Nouveau design while exploiting the superior structural qualities of reinforced concrete with lightness and transparency. The Art Nouveau stained glass was executed by Jac Galland according to the design of Pascal Blanchard. Interior sculpture was by Pierre Roche.

The reinforced concrete structure followed a system developed by the engineer Paul Cottancin.

Construction was attended by skepticism over the properties of the new material, which violated rules laid down for unreinforced masonry construction. A lawsuit delayed construction, resulting in a demolition order that was not resolved until 1902, when construction was resumed.

There is a guided tour of the church on every fourth Sunday of the month at 4:00 PM.

Structural steel

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.

Most structural steel shapes, such as I-beams, have high second moments of area, which means they are very stiff in respect to their cross-sectional area and thus can support a high load without excessive sagging.

T-beam

A T-beam (or tee beam), used in construction, is a load-bearing structure of reinforced concrete, wood or metal, with a 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 (vertical section) of the beam below the compression flange serves to resist shear stress and to provide greater separation for the coupled forces of bending.The T-beam has a big disadvantage compared to an I-beam because it has no bottom flange with which to deal with tensile forces. One way to make a T-beam more efficient structurally is to use an inverted T-beam with a floor slab or bridge deck joining the tops of the beams. Done properly, the slab acts as the compression flange.

Tallinn TV Tower

The Tallinn TV Tower (Tallinna teletorn) is a free-standing structure with an observation deck, built to provide better telecommunication services for the 1980 Moscow Summer Olympics regatta event (see Sailing at the 1980 Summer Olympics). It is located near the suburb Pirita, six km north-east of the Tallinn city center. With its 313 m (1030.2 ft), the TV Tower is the tallest building in Tallinn. The tower was officially opened on 11 July 1980. The viewing platform at a height of 170 metres was open to the public until 26 November 2007, when it was closed for renovation. Having been repaired, the tower began receiving visitors again on 5 April 2012. The building is administered by the public company Levira (formerly Estonian Broadcasting Transmission Center Ltd) and is a member of the World Federation of Great Towers.The architects were David Baziladze and Juri Sinis, the engineers – Vladimir Obydov and Yevgeny Ignatov. The construction work was supervised by Aleksander Ehala.

The cornerstone was laid on September 30, 1975, and the building was inaugurated July 11, 1980 (although the first transmission took place in 1979). The tower body was constructed of reinforced concrete rings 50 cm thick that weigh a total of 17,000 metric tons, and the total tower weight is approximately 20,000 tons. The tower survived a fire during the construction stage.

The observation deck on the 21st floor, originally designed to have a rotating section, is located 170 m above ground, and has a diameter of 38 m. The Tower was closed to the public on November 26, 2007. Before it was closed, tickets were priced at 60 Estonian kroon and, aside from an infrequently used concrete and metallic staircase, the observation deck was accessed by two elevators. Vilnius TV Tower has a similar architectural design but features a rotating observation deck 165 m above ground.

The structure consists of a 190-metre reinforced-concrete tower and a 124-metre metal mast on top of it. Under the tower is a two-storey building with equipment rooms, entrance halls and a conference centre. The diameter of the tower at its base is 15.2 metres and the wall thickness is 50 cm. The diameter of the tower from 140 metres upwards is 8.2 metres. A total of 10,000 m3 of concrete and 1,900 tons of steel were used in the TV Tower construction.

Tallinn TV tower was reopened on April 5, 2012 with completely new interior design made by KOKO Arhitektid.

Local guide books advertise the observation deck's views of Tallinn and extending to the Gulf of Finland. The tower is described as having a 1980s Soviet feel and a restaurant is located on the observation floor. Bullet holes dating from the Soviet coup attempt of 1991 are still visible at the base of the tower.

Tolsford Hill BT Tower

Tolsford Hill BT Tower is a telecommunication tower built of reinforced concrete at Tolsford Hill on the North Downs near Folkestone, Kent. Tolsford Hill BT Tower is one of the few British towers built of reinforced concrete and is 67.36 metres ( 221 ft) high.

Wotton-under-Edge BT Tower

Wotton-under-Edge BT Tower is a 76.2 metres ( 250 ft) tall telecommunication tower built of reinforced concrete at Wotton-under-Edge in Gloucestershire, UK. Wotton-under-Edge BT Tower is one of the few British towers built of reinforced concrete.

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