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Introduction To Concrete

Concrete is a very strong and versatile mouldable construction material. It consists of cement, sand and aggregate (e.g., gravel or crushed rock) mixed with water. The cement and water form a paste or gel which coats the sand and aggregate. When the cement has chemically reacted with the water (hydrated), it hardens and binds the whole mix together. The initial hardening reaction usually occurs within a few hours. It takes some weeks for concrete to reach full hardness and strength. Concrete can continue to harden and gain strength over many years.

Early history
Surprisingly, concrete has a very long if somewhat episodic history. In a Neolithic settlement excavated at Yiftahel in southern Galilee in Israel, a floor of burnt lime plaster was found. It is thought to be the earliest use of concrete. The fragments of a kiln were found on the site - the lime to make the concrete may have been burnt in it. The lime had been mixed with stone and laid 30-80mm deep and given a smooth finish. Mesolithic hut floors at Lepenski Vir in Serbia (the former Yugoslavia) were also made of a lime-bound concrete. Egyptian murals from the second millennium BC depict the making of mortar and concrete. Around 500 BC, at Camiros on Rhodes, the ancient Greeks built a 600,000 litre capacity underground cistern lined with fine concrete1.

Roman Concrete

roman concreteThe above discoveries hardly point to the intensive use of concrete; otherwise, due to its durability, concrete would likely have been found at many more ancient sites. We have to turn to the Romans for the widespread use of concrete. The Romans discovered that by mixing lime and rubble with pozzolana2 sands and water, they could make a very strong building material which they called opus caementicium. It even had the added bonus of being able to set under water, so it could be used in the construction of aqueducts and harbours. Perhaps most notable of the many Roman concrete structures that are still standing today are the Coliseum and the Pantheon in Rome.

roman concreteThe Romans typically used concrete to construct walls and roofs. Forms were used with the stone and mortar being placed in alternating layers, with the mortar being pounded into each layer of stone. Finished concrete was then faced with brick or tiles. Of particular note is the dome of the Pantheon. Built in 127 AD, heavy travertine (a type of limestone) was used in the wall concrete, whilst broken pumice was used as lightweight aggregate in the 43 metre diameter dome in order to reduce the lateral thrust on the walls. Originally a temple dedicated to all the pagan Roman gods, it has since served as the Roman Catholic Church of St. Mary and the Martyrs.

From the Romans to the Victorians

After the fall of the Western Roman Empire (AD 476), construction techniques used by the Romans were generally abandoned, but not necessarily lost. Three Saxon concrete 'pan' mixers dated around 700 AD have been discovered in an excavation in Northampton. They consist of 2-3 metre diameter shallow bowls excavated in bedrock, each with a centre socket that would have held a vertical shaft. Concentric grooves in their bases are thought to have been worn by mixing paddles fixed to a horizontal beam that rotated around the centre shaft.

roman concreteThe Normans also had knowledge of concrete, and, like the Romans, used it for wall in-fill. They found that pounded tiles and bricks mixed in with lime mortar and sand produced a similar reaction to that of the Roman pozzolana (the crushed tiles and bricks providing the needed silica and alumina).
The use of hydraulic setting pozzolanic concrete in the construction of the 150 mile (240 km) long Canal du Midi has been documented. Constructed between 1667 and 1681, it links Toulouse with Sète on France's Mediterranean coast.

Development of Natural and Portland cements

At this point, to continue our historical account of concrete, we have to digress a bit and look at the development of Portland cement, the essential ingredient that binds modern concrete together.
In 1756, John Smeaton, an engineer from Leeds, was commissioned to build the third lighthouse on the Eddystone Rocks in the English Channel near Plymouth. The first lighthouse, built of timber, had burnt down, the second, also of timber, had been blown down in a gale. Smeaton chose to build his lighthouse of interlocking cut blocks of stone. He experimented with various ingredients to find a quick-setting mortar for use in the construction of the base which was washed by the sea at high tide.
His experiments led him to use a burnt lime from South Wales and a trass (volcanic tuff) from Italy. His lighthouse stood and operated for over two hundred years, from 1759 to 1876, when it was replaced by the present lighthouse. It was dismantled down to the base and re-erected on Plymouth Hoe, where it can be seen today. Smeaton outlined his researches on mortar in a book titled, A Narrative of the Eddystone Lighthouse.
In 1796, a Mr Parker of London took out a patent on a process to produce cement by heating septarian nodules found on the shore at Harwich.
The nodules were of marl, which is a mix of clay and limestone. The nodules had the right proportions of silica and alumina (from the clay) and calcium (from the limestone), that, when burnt and ground down, produced a cement which set faster and was stronger than the traditional lime mortar. He called his product Roman cement. Other producers of similar "natural" cements sprang up in the early 19th century.
In 1813, Joseph Aspdin (1788-1855), a Leeds bricklayer, bought a copy of Smeaton's book, and this likely inspired his own research into cement. In 1824, he patented his "Portland Cement". It was made by calcining limestone, mixing and slaking the burnt lime with puddled clay, then drying the mix, breaking it into lumps and burning it again (double burning), before grinding the resultant clinker down to a powder between millstones. Gypsum was added to prevent flash setting.
Supplied in barrels, his dry powder was easily mixed with water and sand to produce a strong mortar which set quickly. He developed his new cement to produce exterior renders which could be lined-out to give the appearance of Portland stone, hence the name. It was also intended for casting various architectural mouldings and features (stucco work).
Joseph's younger son William (1815-1864), fell out with his family and moved to Rotherhithe in London in 1841, where he set up a business and further developed his father's product into the Portland cement that we know. He added more limestone (the soft local chalk) to the mix and calcined it at a much higher temperature.
By the end of the 19th century, improved manufacturing techniques (e.g., horizontal rotary kilns and ball mills) had ensured sufficient consistency of product so that Portland cement overtook and superseded the production of natural cements3 .

A world transforming material

It could be justifiably argued that since the late 1800s onwards, when consistent mass produced Portland cement became readily available, that the world that we live in has been transformed by the design and construction of all sorts of concrete structures; the list is large: dams, bridges, skyscrapers, water and sewerage systems, public buildings, schools, hospitals, shopping centres, airport and rail termini, ports, factories and sporting stadia, houses, foundations, cast beams, floor slabs, walls and stair units, monuments, art and landscape projects; from the good, to the bad and the down-right ugly. Even boat hulls have been made in cast concrete.

Reinforced concrete

On its own, concrete has excellent resistance to compression (crushing), but is very poor in tension (stretching). To give it good load bearing capability when under tension, it has to be reinforced with steel bars (rebar), polymer strands or fibres. Bars and strands can be tensioned during casting of pre-cast concrete structures such as floor and bridge beams. When the concrete has set, the tension is released and the reinforcement tries to pull back to its original length, but can't, as it is now bound into the set concrete. It thus imparts a pulling force which gives the cast structure great strength.

1 See British Cement publication "Concrete Through The Ages", 1999.
2 A type of volcanic ash, named after the Pozzuoli region in Bay of Naples. It consists of meta-stable alumina-silicates that when mixed with lime and water, react with the calcium to form a type of true hydraulic setting cement.
3There has been a recent revival in interest in Natural cements as, unlike Portland cement, they have an inherent degree of flexibility and breathability. They have therefore found a niche in the restoration of 19th century stone buildings and monuments (which were originally constructed using natural mortars). It is now known that former well-intended restoration works using hard Portland cement mortar has actually resulted in damage to stonework.

Parthenon photographs © By kind permission of R McVerry