Concrete is a compound material made from sand, gravel and cement. The cement is a mixture of various minerals which when mixed with water, hydrate and rapidly become hard binding the sand and gravel into a solid mass. The oldest known surviving concrete is to be found in the former Yugoslavia and was thought to have been laid in 5,600 BC using red lime as the cement.
The first major concrete users were the Egyptians in around 2,500 BC and the Romans from 300 BC The Romans found that by mixing a pink sand-like material which they obtained from Pozzuoli with their normal lime-based concretes they obtained a far stronger material. The pink sand turned out to be fine volcanic ash and they had inadvertently produced the first 'pozzolanic' cement. Pozzolana is any siliceous or siliceous and aluminous material which possesses little or no cementitious value in itself but will, if finely divided and mixed with water, chemically react with calcium hydroxide to form compounds with cementitious properties.
The Romans made many developments in concrete technology including the use of lightweight aggregates as in the roof of the Pantheon, and embedded reinforcement in the form of bronze bars, although the difference in thermal expansion between the two materials produced problems of spalling. It is from the Roman words 'caementum' meaning a rough stone or chipping and 'concretus' meaning grown together or compounded, that we have obtained the names for these two now common materials.
Lime and Pozzolana concretes continued to be used intermittently for nearly two millennia before the next major development occurred in 1824 when Joseph Aspdin of Leeds took out a patent for the manufacture of Portland cement, so named because of its close resemblance to Portland stone. Aspdin's cement, made from a mixture of clay and limestone, which had been crushed and fired in a kiln, was an immediate success. Although many developments have since been made, the basic ingredients and processes of manufacture are the same today.
In 1830, a publication entitled, "The Encyclopaedia of Cottage, Farm and Village Architecture" suggested that a lattice of iron rods could be embedded in concrete to form a roof. Eighteen years later, a French lawyer created a sensation by building a boat from a frame of iron rods covered by a fine concrete which he exhibited at the Paris Exhibition of 1855. Steel reinforced concrete was now born. The man normally credited with its introduction as a building material is William Wilkinson of Newcastle who applied for a patent in 1854 for "improvement in the construction of fireproof dwellings, warehouses, other buildings and parts of the same".
It is not only fire resistance that is improved by the inclusion of steel in the concrete matrix. Concrete, although excellent in compression, performs poorly when in tension or flexure. By introducing a network of connected steel bars, the strength under tension is dramatically increased allowing long, unsupported runs of concrete to be produced.
Steel and concrete complement each other in many ways. For example, they have similar coefficients of thermal expansion so preventing the problems the Romans had with bronze. Concrete also protects the steel, both physically and chemically.
Composition of Portland Cement
Portland cement is a complex mix of many compounds, some of which play a major part in the hydration or chemical characteristics of the cement. It is manufactured commercially by heating together a mixture of limestone and clay up to a temperature of 1300 to 1500°C. Although twenty to thirty percent of the mix becomes molten during the process the majority of the reactions which take place are solid-state in nature and therefore liable to be slow. Once cooled, the resulting clinker is ground to a fine powder and a small amount of gypsum (calcium sulphate dihydrate) is added to slow down the rate at which the cement hydrates to a workable level.
The work of early investigators using optical and X-ray techniques, starting in 1882 with Le Chatelier, has shown that most Portland cement clinkers contain four principal compounds. These are tricalcium silicate (3CaO.SiO2), aluminate (3CaO.Al2O3) and a ferrite phase from the (2CaO.Fe2O3 - 6CaO.2Al2O3.Fe2O3) solid solution series that at one time was considered to have the fixed composition (4CaO.Al2O3.Fe2O3). These phases were named alite, belite, celite and felite respectively by Tornebohm in 1897.
Hydration of Portland Cement
When water is mixed with Portland cement a complicated set of reactions is initiated. The main strength giving compounds are the calcium silicates which react with water to produce a calcium silicate hydrate gel (C-S-H gel) which provides the strength, and calcium hydroxide which contributes to the alkalinity of the cement. Tricalcium silicate reacts quickly to provide high, early strengths while the reaction of dicalcium silicate is far slower, continuing, in some cases, for many years. The other cement compound of particular relevance to steel reinforced concrete is tricalcium aluminate. It reacts rapidly with water to produce calcium aluminate hydrates.
The amount of tricalcium aluminate present may well be limited as in the case of sulphate resisting Portland cement, to prevent adverse reactions between the hydrate and sulphates from the environment which can result in swelling and cracking of the cement matrix.
The great advantage of tricalcium aluminate is its ability to combine with chlorides, so removing them from the liquid phase of the cement. Chloride ions, as will be seen, are one of the major causes of corrosion of embedded steel.