Our urban landscapes are dominated by reinforced concrete structures of all shapes and sizes – from bridges and flyovers to architecturally stunning skyscrapers and more functional buildings such as hospitals and shopping centres. We tend to take concrete for granted, without giving a thought to its history and development, to the standards that apply to concrete construction projects, or the materials used in them.
Cement, in various forms, has been used around the world for millennia. Some ancient concrete buildings are still standing, such as the Colosseum and the Pantheon in Rome, and the Roman aqueducts found across much of Mediterranean Europe. In the early 19th century came the development of Portland cement, and within a few decades, construction techniques had progressed so far that the need for concrete reinforcement was identified, with reinforcement bars becoming common by the end of that century. Reinforced concrete is now a crucial building material in almost every construction project.
This article provides an overview of reinforced concrete and its components, as well as some of its applications.
Ancient concrete buildings used forms that rely on concrete’s natural compressive strength. The arches of the Colosseum and the massive dome over the Pantheon transfer gravity loads to their foundations via columns and walls without the concrete experiencing tension or bending. Concrete structures that don’t undergo tension or bending are likely to be massive, but will perform perfectly well without reinforcement.
The reason that the arch and dome and massive walls and columns are no longer common architectural features in concrete buildings is due to the introduction of reinforcement.
A concrete beam without reinforcement would crack and fail under load; horizontal reinforcing bars take the tension that occurs in the lower portion of a bending beam, and the vertical stirrups that confine the steel help to transfer the vertical loads to the supports.
A slender concrete column without reinforcement will fail by bending or buckling, whereas the presence of a reinforcing cage within the concrete will protect against such failures.
In short, reinforcement in concrete gives it the ability to withstand bending, to span across spaces as a beam instead of an arch, and it gives a smaller volume of concrete the ability to do more – to span further and withstand greater loads.
Reinforced concrete is used in all types of construction projects, from the smallest domestic builds to massive infrastructure projects including bridges, motorways, skyscrapers, dams, runways and sea defences.
Reinforced Concrete in Houses
Looking first at small-scale house construction, reinforced concrete is commonly used in the construction of dwellings, but usually to no higher than ground level. That is, of course, unless you have grand designs for your house to star on TV and on architectural websites. The infinite possibilities that concrete provides the designer can lead to unexpected warmth, both thermal and aesthetic, amidst often brutal beauty.
Photographed by Tarry + Perry
But every house, big or small, ambitious or functional, must sit on the ground. It must have foundations, which will be deep if the ground is weak and shallow if the ground is good. In general, though, the foundations will need to be deep enough to be unaffected by seasonal variations in the characteristics of the ground. And, invariably, those foundations will require some sort of concrete, often reinforced.
A deep foundation system can use reinforced concrete piers or piles (more info on those here) – concrete poured over steel cages lowered into holes that have been drilled down to firm strata. The house is then built up off concrete beams that span from pier to pier or pile to pile.
Shallow foundation systems may consist of traditional concrete strip footings poured at the base of a trench dug around the perimeter of a building, and sometimes through the middle where internal load-bearing walls are going to be built, as well. Another type of shallow foundation is a raft slab, a thick concrete slab built on stable, level ground that can cope with the loads that will be put on it by the new house. One of the major advantages of raft foundations is the minimal excavation and below-ground work that is required, which usually results in a quicker start to above-ground work.
Steel Reinforcement in House Foundations
The very common denominator for concrete house foundations and ground slabs is that steel reinforcement will be required somewhere: at the base of a footing; steel cages in ground beams and in the piles that support them; mesh reinforcement in a slab; and bent bars to connect such elements to each other.
Large-scale reinforced structures
At the other end of the scale, large building projects require the same things, although a lot more investigation, design, excavation and, in the end, reinforced concrete, go into foundations which have to transfer infinitely more load into the ground than are exerted by any house.
And whereas for houses the use of concrete most often stops at ground level, for many large buildings, the ground is just the start.
The tallest concrete building in the UK, the Emley Moor tower in Yorkshire, is 330m (1,084ft) tall. Its foundations will have been constructed deep enough to ensure that the massive weight of the tower is transmitted into load-bearing rock strong enough to take it, and the tower and its foundation will have been designed to withstand the bending created by the forces that the storms of the past forty years have thrown at it.
The Burj Khalifa, currently the world’s tallest building, is primarily a reinforced concrete structure. Its concrete and steel foundations, which extended to 50m (164ft) depth, took 45,000m3 of concrete. The total project used 330,000m3 of concrete and 39,000 tonnes of reinforcing steel. The highest concrete walls in the building were poured with concrete that was pumped from ground level to a height of 600m, almost twice as high as the Emley Moor tower!
These concrete buildings couldn’t have reached the height they do without reinforcement.
Concrete Building Elements
Above-ground reinforced concrete elements can be divided into four basic building elements: beams and slabs, which support floor and loads and transfer them to supports; and columns and walls, the supporting elements that transfer loads vertically towards the foundations.
There is a wide variety of concrete floor types, each of which has advantages and disadvantages, and each of which is suitable for different applications.
The simplest floor type is the flat slab, a floor that is formed by pouring concrete over steel reinforcement placed on horizontal formwork. The finished product is a floor with a flat underside supported by a grid of columns. The spacing between the columns depends on many factors, including the size and strength of the columns, the weight of the concrete floor itself, and the live loads that it is designed to support. Another factor is the ability of the slab to span without sagging excessively. A thicker slab with more reinforcement may be stiffer and able to span further, meaning less columns will be required. However, that floor will be heavier (and more expensive), and the columns will need to be larger (and more expensive).
And a heavy building needs to have a foundation built to withstand its own weight, plus the loads created by the activities that will take place when it is operational. The heavier the building, the more expensive its foundations will be to construct.
The following is a summary of alternatives to the flat slab, all of which have been developed to solve various design problems.
The thinnest type of floor slab is the post-tensioned slab, which uses less steel reinforcement than a flat slab because it relies on a network of post-tensioning tendons placed within the slab to give it its strength. Because there is less steel reinforcement to be put into place, construction can be sped up. The quicker a floor can be poured, the quicker work can begin on the columns to support the next floor.
A ribbed slab – with ribs spanning between concrete band beams with a thin slab over the top of them – is generally deeper than a flat slab, but it is lighter and stiffer, and less prone to vibration. Columns can be spaced further apart in the direction of the ribs.
A waffle slab consists of interconnecting ribs topped with a thin slab. It is typically deeper than a ribbed slab and is generally supported on a square grid of columns.
A concrete–steel composite floor comprises a concrete floor poured on permanent lightweight steel formwork supported on steel beams that are connected to the concrete in such a manner that the concrete and steel works together (compositely) to support the loads. The concrete slab takes the axial compression load, leaving the downstand steel beam to take the corresponding tension, both components working together to resist the bending forces applied to the floor. Whereas building services such as air conditioning ducts and water pipes need to be suspended below the underside of concrete downstand beams and ribs, those can run parallel to the steel beams supporting a composite floor, and even pass through them in designed locations.
Prestressed hollowcore slabs are a versatile floor system – long-spanning, lightweight, quick to erect, and perfect for commercial buildings, institutional buildings, and carpark structures. They’re fabricated off-site and when lifted into place they generally have a pre-camber (slight arch) built into them that will allow the slabs to settle to horizontal under the weight of the concrete topping poured over them.
Reinforced Concrete Beams
Beams are very common elements in concrete buildings. Sometimes you can see them if the architecture of the building expresses its structure, but often they are hidden from site, concealed by ceilings or cladding.
All concrete beams are reinforced concrete elements, as concrete on its own cannot withstand bending forces. A concrete beam, whether a foundation beam poured in a trench between piles or spanning between columns on the fourth floor of a building, consists of concrete surrounding a steel reinforcement cage placed within formwork (or an excavation) that will define its shape. The beauty of concrete is that it will pour into any shape prepared for it. So, while the repeatability and functional requirements of a multi-storey building may necessitate simple horizontal rectangular beams, it is often in bridges and motorway structures and in the lobbies of landmark buildings where structures are expressed that you can see what shapes concrete can take – tapering, arching, curving, twisting. And, more often than not, those shapes are elegant representations of efficient structural design.
Reinforced Concrete Columns
You can’t build a concrete building without columns or walls. The typical office building will have several service cores containing lifts, stairways, and service risers, and a grid of columns which support the floors.
The most economical and efficient – and boring – building design of all would be one where the columns at each location on plan all lined up and the accumulating loads were transmitted vertically to the foundations directly below them. But, just to make things interesting, most building owners don’t want such a configuration.
Picture this. You are a developer who wants to build a 20-storey building that contains apartments on the top 17 storeys, a cinema complex on the second and third floors, a retail development on the ground and first floors, and a three-level car parking basement. The most appropriate floor plan and column arrangement for the residential levels would not work in the cinema complex, nor would it be acceptable in the retail development. And it would definitely not be practical in the underground carpark!
What is the solution?
Beams. Big ones.
By the time you’ve reached the basement, the beams picking up column loads and transferring them to supports in different locations will be massive. And the columns won’t be very small, either.
You can’t order a standard column from a concrete company in the same way that you can order a regular steel beam from a steel company. But concrete columns can be precast or cast on-site, they can be constructed to any size and virtually any cross-sectional shape – although round, square, or rectangular are the most popular for reasons of cost and practicality. They can be tapered or fluted, and they can be hidden or emphasised as architectural features. There is no limit to the possibilities.
As with floors, beams, and columns, there are many different types of concrete walls used in buildings.
- Blockwork is perhaps the most widely used method of constructing walls, both load-bearing and non-load-bearing, and particularly as the inner leaf of the external cavity walls of houses. Thin joint blockwork is a more contemporary alternative to regular blockwork, and it involves a specific type of concrete block, with very thin mortar jointing.
- Crosswall concrete elements are pre-formed off-site and offer a fast and cost-effective means of constructing floors and walls in buildings such as offices, hotels and student accommodation where walls are required at regular spacings. The walls are slender, load-bearing elements, joined together with hidden connectors.
- Twinwall construction involves the use of twinned precast panels separated by spacers to form walls of high-quality finish and tolerance. After the main wall reinforcement has been put in place, the prefabricated panels are dropped into position on either side of the reinforcement, temporarily secured, and then in situ concrete is poured into the cavity.
- Insulating concrete formwork (ICF) is a method of construction that uses polystyrene insulation panels as the formwork for the concrete walls. Once the vertical and horizontal wall steel reinforcement has been tied into placed, the formwork is placed and secured prior to pouring concrete into the cavity. Unlike conventional formwork, which is removed and then reused, the insulation remains in place to contribute to the wall’s permanent performance.
- Tilt-up construction, a common building method, is exactly as the name implies. A concrete wall panel is cast horizontally on-site on the ground floor slab or a separate casting area and when it has reached sufficient strength it can be craned into place (tilted up) and fixed in place to the foundation and to neighbouring wall panels. Tilt-up panels have steel reinforcement in them that is designed to give them the load-bearing, bending, and lateral strength they need to contribute to the building’s performance.
- Tunnel form construction is a system that offers a rapid construction method for buildings with a high degree of structural repetition, such as institutional buildings and apartment blocks. The method uses steel formwork designed specifically for the building and which allows slabs and walls to be cast in the same pour. The steel formwork results in a high-quality wall surface that will need minimal work to bring it to finished quality.
Unless the concrete element is an arch or a dome or a simple footing, it will need reinforcement to do its job. The most common reinforcing material is steel, because it has high tensile strength, it is ductile, it can be bent into useful shapes, and it has similar thermal expansion properties to concrete. However, there are some alternatives.
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Perhaps the most ubiquitous product on every building site, reinforcing bar (or rebar) is simply steel rods of varying diameters (6–50mm) with surfaces that are smooth or deformed with ribs, lugs, or indentations to enhance the bond between steel and concrete. Long straight lengths of large-diameter rebar are used for longitudinal reinforcement in beams and columns and smaller diameter bars are bent or curved to form stirrups (beams) and ties (columns) to provide confinement to the longitudinal bars and contribute to shear strength.
The ends of rebar can be bent into hooks and bends to anchor the ends, which is especially important for smooth bars. However, smooth bars are mostly used to confine longitudinal steel in cages, or as dowel bars that allow joints to open and close but prevent movement in other directions. The length of bars is limited by transportation constraints, but deformed bars can be lapped with other deformed bars to ensure continuity of reinforcement.
The most common method of fixing steel reinforcement is by using soft wire ties. Bars in slabs are placed on ‘chairs’ that hold them at the required level that will ensure sufficient concrete cover below and above the bars. Similarly, chairs are used to ensure that the reinforcement within a wall cavity is located so that there is sufficient thickness of concrete protecting it.
If rebar (including) mesh does not have the required minimum cover of concrete over it, corrosion problems can arise over time, particularly if the environmental conditions are harsh and unforgiving. For example, concrete structures built close to a marine environment, especially within the salt spray zone, will suffer problems when salt-laden moisture finds its way through thin concrete layer to the rebar, as corroding steel expands with an inexorable force that concrete is not able to withstand indefinitely.
For the longitudinal bars adjacent to the ‘tension face’ of a beam – the bars that will be doing the heavy lifting – it is important that they be located exactly where the designer requires them. Too close to the tension face, and the bars that give the beam its strength will potentially be compromised by corrosion in the long term. Too far from the tension face, and the strength of the beam will be less than it was designed for.
Mesh reinforcement is simply a series of rebar or wire rods laid at specific spacings, with rods welded across them at right angles to form a grid. Mesh reinforcement is selected based on slab thickness and other parameters according to code requirements. Mesh panels are placed on spacers that hold them in the correct location within the slab and adjacent panels are placed with an overlap to ensure continuity of tensile strength within the slab. ‘Flying end mesh panels’ are special panels that have the final wire left off in order to allow the necessary 350mm overlap with adjacent panels without excessive build-up of mesh and resulting problems providing the necessary concrete cover over the top of the mesh.
Where a construction project calls for a lightweight solution that can be formed into any shape or size, glass fibre reinforced concrete is a popular choice. Super-strong glass fibres are embedded in the concrete to produce a composite material with incredible strength and performance. GFRC is often used as a cladding product and is widely used in design-led construction projects and more conventional builds, too.
Whilst most reinforcement products are made from carbon steel, there are also carbon fibre composites that offer many of the same benefits as traditional steel reinforcement products, but which also have the advantage that they are much more resistant to corrosion, require less concrete and are more lightweight. This type of reinforcement is generally used for specialist applications such as sea defences, where extreme weather and corrosion resistance is needed. However, carbon fibre composites look set to increase in popularity, and researchers in Germany are currently working on ‘The Cube’ – the world’s first building where the concrete will be reinforced entirely with carbon fibre ‘yarn’ instead of traditional steel reinforcement.
In fact, the Grand Designs concrete house referred to at the start of this article was constructed using a fibre-reinforced concrete that allowed a more than 40% reduction in the steel reinforcement that would otherwise have been required.
Post-tensioning is an alternative method of reinforcing concrete, where steel cables, or tendons, are placed in a network across a slab, each tendon draped with a shallow sag between supports. After the concrete has been poured and has gained sufficient strength, the cables are tensioned and anchored securely at the edges of the slab. Post-tensioning is often used when a slab needs to be laid on ground that is likely to move, for ground slabs that require minimal cracking, for buildings requiring the thinnest of floor slabs, and is commonly used in bridge deck construction.
The most basic concrete mix comprises Portland cement, aggregate and water, poured into place and vibrated into position. However, there are probably as many different types of concrete as there are different uses for concrete. From low-tech bag mixes used for low volumes of site concrete, to a truck full of ready-mix concrete for a house floor pour, to hundreds of trucks feeding a concrete pump for an industrial warehouse floor, to a dedicated concrete plant fabricating pre-cast segments for a tunnel project – every end-use will have a concrete mix designed specifically for it. Additives can be included to entrain air bubbles, to enable longer transportation times, and to impede moisture ingress. Concrete can be coloured, its strength can be tweaked, aggregate size can be reduced to allow pumping, and its density and porosity can be adjusted. Lightweight concretes are typically used for concrete cladding and long-spanning bridges, and high-density concretes are used for radiation shielding and other heavy-duty applications. There are concrete mixes designed for contact with sewage or sea water, and for pouring underwater. And the asphaltic concrete used for roads is designed to withstand the abrasion and ravages of traffic loadings over a long period of time.
Concrete truly is ubiquitous.
This article can only scratch the surface of a topic as broad as reinforced concrete. Concrete, and the reinforcement that gives it its backbone, has been a major building material of human civilisation for millennia. We’ve aimed to provide you with a good overview of the use of reinforced concrete within all kinds of construction projects. If you’d like more information about any specific type of concrete reinforcement or accessory, or you’d like to discuss a building project in detail, please get in touch with us today – our friendly team will be happy to offer advice and guidance.