Concrete is an artificial stone made from a mixture of cement, different sizes of stones, sand and water. Various additives can be added to influence its characteristics. Because fresh concrete is fluid, it can take on any shape; additionally, it is relatively cheap. Solid concrete is durable and fireproof.

The History of Concrete

The Romans already knew a kind of concrete under the name of opus caementitium. The adhesive component was made from limestone, which was burnt at 1,000°C, and a volcanic stone (pozzolanos) found at Mount Vesuvius, the volcano near Naples. Water was added and together with stones and parts of bricks, the mixture was poured into a casing of two walls made from brick or stone, or sometimes wooden boards.

The Romans used their opus caementitium especially for water pipes, drains, domes and arches and even for dams in the sea. The most impressive example of the Roman use of concrete is the Pantheon in Rome. This temple was built in 115 to 126 AD and features a concrete dome with a diameter of 43 metres, the largest dome of its time and the largest concrete dome until the early 20th Century. It uses different aggregates at different parts of the building (the higher up, the lighter) as well as different thicknesses of the construction. Additionally, the outside of the dome has a stepped profile to distribute the weight of the dome to the walls.

At the end of the 18th Century, 'Roman Cement' was invented by the Englishman James Parker. It used burnt lime marl (a natural mixture of clay and calcium minerals) which was mixed with sand and water and got solid very fast. Today, Roman Cement is especially important for the restoration of old buildings where it is often used as plaster and prefabricated decorations on the facades of the 19th and early 20th Century. Because of its different components, normal cement that is sold today does harm to this historic cement and should not be used to repair it. That's why the European Union is supporting a research project into Roman Cement that should result in a revival of the production of this material. This will aid the proper restoration of old buildings.

In the 19th Century, the first patents for steel-reinforced concrete were made. The French nobleman Joseph Lambot tried to find a substitute for wood for the construction of boats. He came up with a boat out of cement mortar, reinforced with wire. Another Frenchman, the gardener Monier, invented flower pots made of cement and steel. The English plasterer William Wilkinson was the first to build a ceiling made from concrete and steel ropes.

Portland Cement

From about the middle of the 19th Century, a new kind of cement was used: Portland Cement. It was invented by the British Joseph Aspdin, who named it after the white-grey limestone from Portland, Dorset, UK. With this invention, it became unnecessary to search for the right natural sources of stones, as Portland Cement uses a mixture of mineral components.

To make cement, lime, lime marl, sand, clay and minerals containing iron oxide are burnt at about 1,450°C, which partly melts them but does not make them fluid. After cooling, the mixture is ground and gypsum is added, as well as other additives if needed.

Cement is a hydraulic adhesive, which means it hardens by a chemical reaction when it is mixed with water. This will happen whether the cement is in air or under water. The chemical reaction with water causes crystals to grow. Not all reactions happen at the same time: cement starts to get solid soon after mixing with water but the complete reaction takes months or even years. Most reactions are finished after 28 days and cement reaches its normal strength at this time as long as it is kept damp for at least the first seven days. Once the water is used up, the reaction stops, whether the cement is hard or not. In the process of hardening, the cement emits an amount of heat that is dependent on its mixture and on how finely ground it is.

Dry cement is porous: the small holes are either filled with water or air. Their number and size depend on the amount of water mixed with the cement and influence the compression strength of the finished cement stone. The compression strength of concrete is directly proportional to the compression strength of the cement used. In heavy and normal concrete the additives (stones) are always harder and more durable than the cement. This means that the characteristics of the concrete are solely dependent on the characteristics of the cement.

The volume of cement does not stay the same in the process of drying. Because water is consumed in the creation of the crystal network, the cement shrinks by about 25% of the total volume of the water added to the mixture. When the cement dries completely for the first time, it shrinks by another 1% but also expands and shrinks again slightly in subsequent periods of drought and dampness. While cement is setting, it produces heat and this causes it to expand. It also expands after it has set when heated by an external source.

At constant pressure, solid cement deforms over time. This process is still not completely understood but may be caused by the rearrangement of particles at the nano-scale. Cement can also be corroded by acids, strong alkali and certain salts. In the process, substances from the cement can be dissolved and deposited at the surface.

Aggregates

The bulk of concrete is made up not of cement but of various sizes of natural or artificial stones. Their characteristics influence the characteristics of the finished concrete and therefore also its use.

Normal concrete uses gravel as aggregate. Bigger stones are crushed, sorted by size and then mixed again in the right proportions. It is important that there are always different sizes of stones in the concrete so the holes between them that are filled with cement are as small as possible, because the cement is less durable than the stones. The size of the biggest stones is dependent on the dimensions of the object to be made from the concrete. The aggregates should not contain clay, organic substances, alkali, gypsum, sulphide, salt or other damaging substances.

If the concrete is supposed to be lighter, pumice stone, wood wool or chips, brick chips, polystyrene or other aggregates can be used. For heavy concrete such as is used to shield against radiation, iron ores are added to the aggregates. Different fibres - for instance glass, carbon or iron fibres - increase the durability against tension and the deformability of the concrete.

Water

Any water can be used to make concrete, as long as it does not contain substances that can do damage like many organic substances or heavy metals. The more water is used, the less resistant the concrete is. It gets more sensitive to weather, dries faster and shrinks more, possibly resulting in cracks. The amount of water needed is dependent on the aggregates and the preferred consistency of the concrete.

Additives

Various substances can be added to concrete to influence its characteristics. One of the most obvious is colour. Others delay the drying of the concrete or make it dry faster. They can also make concrete more fluid and use less water, but most additives have unwanted side effects like a higher contraction of the drying concrete or lessening of its durability. The amount of additives in the concrete must not be more than 50 grams per kilogram.

Concrete

The fluid concrete is poured into a casing which is usually made from wood and gives the concrete its shape. Then it has to be tamped to make sure that no air bubbles are left in the concrete. When the concrete is hard enough, the casing can be removed - usually after one to three days. For the first three days, the concrete has to be damp, because the cement needs water. Additionally the temperature at the building site should not be too low or too high. The preferred temperatures are between 10° and 25°C; below this the concrete hardens too slowly or not at all, while at higher temperatures it gets hard too fast. Just like cement, the solidifying of concrete is an exothermic reaction - one that produces heat.

On building sites, concrete can be used in different forms:

1. fluid concrete that is mixed on site or brought in tanks and poured into forms
2. solid concrete blocks that are used for building like bricks
3. panels for instance made of concrete and wood flakes to use under the plaster on facades
4. prefabricated building parts like whole walls, floor elements, beams or pillars

Characteristics

The compression strength of concrete is more than ten times higher than its tensile strength. Compression is taken by the aggregate grains, which are the hardest particles in normal concrete; the forces go from one grain to the next. Tension of course can't be taken by the aggregate grains because they are not directly connected; the connection is made by the cement between the grains, so the cement takes the tension - but it is not very strong. In steel-reinforced concrete, all the tensile strength is provided by steel, which should be put into the area of the concrete that is affected by tension. A concrete beam, for instance, bends under weight which causes compression on its upper side and tension on the lower side. The steel should also not be too close to the surface of the concrete so it is safe from corrosion and does not cause Concrete Cancer. The alkaline concrete protects the steel.

Steel expands when it takes tension, which leads to thin cracks in the concrete. To avoid this, the steel can be held under tension while the concrete of a beam is hardening. Once the tension is removed the steel contracts, leading to compression in the concrete. If weight is added on the beam the compression is lessened, but hardly any tension is on the concrete; this avoids cracks.

Concrete pillars that bear a flat concrete slab can develop radial cracks under too much pressure. In the end, the pillar punches through the slab. This can be avoided with various constructions.

Concrete in Architecture of the 20th and 21th Century

Apart from its importance in the construction of bridges, tunnels, foundations and many other aspects of civil engineering, concrete - and especially reinforced concrete - also was very important in the architecture of the current and last centuries. The following paragraphs illustrate the variety of concrete architecture but can certainly never give a full summary.

One of the most characteristic concrete conceptions of the modern world is Le Corbusier's Domino House of the early 20th Century. It is not a particular house but a concept for how houses in general can be built: two concrete slabs are used for floor and ceiling, concrete pillars are used to take the forces. The actual walls that shape rooms are completely unrelated to this structure. Shape and construction are no longer related to each other.

Another of Le Corbusier's important concrete designs (next to many smaller houses) is the Unité d'Habitation in Marseilles (1942-52), designed for those who lost their homes in the war. The whole apartment building stands off the ground, carried by massive reinforced concrete pillars which contain the necessary drains. The whole house resembles a set of shelves with two-storey-high apartments and corridors on only every third level. It also contains shops and a 'garden' on the roof. Although this building may seem ugly and unfriendly to today's eyes, it was designed for the people and should start a new way of living.

Frank Lloyd Wright also used the rather new material of reinforced concrete. Like Le Corbusier with his Domino concept, Wright wanted to break up the boxes that were traditional houses; he also advocated showing materials in their true form. In the 1920s, he experimented with using prefabricated concrete blocks to build houses. He used these blocks as both a decorative and a load-bearing element which was the first time this had been done.

Wright's house for Edgar J Kauffmann (1935), 'Fallingwater', for instance is a symbiosis of concrete and natural stones. All vertical elements are made of stone, while the horizontal elements are concrete. This use of materials reinforces the sculptural look of the building. A very different design built with concrete is Wright's Guggenheim Museum in New York with its huge spiral hall.

One of the most interesting architects of concrete constructions in recent times is Santiago Calatrava. His sculptural buildings and bridges are founded on mathematical calculations. This results in spectacular dynamic shapes of concrete, glass and steel that often resemble something naturally grown.

A very different architecture are Tadao Ando's simple geometric shapes made of concrete walls. He plays with light and shadow and all aspects of the location of the building. Although based on simple boxes without any ornamentation, Ando's designs create interesting spaces inside and outside of the buildings and all in the context of their surroundings.