Presidential Address 2003: Why concrete cracks — geological factors in concrete failure

The Romans gave us the term ‘concrete’ — from concretus , meaning 'grown together'. Their concrete structures were of considerable complexity and included the 43 m diameter dome of the Pantheon temple (second century AD). The Romans found that lime blended with powdered volcanic ash was superior to lime alone for structures. They exported concrete widely - making it with a special volcanic ash from Pozzuoli. This concrete was made with pumice as aggregate to make it light in weight. However, the earliest concrete can be traced back some 7000 years to the Etruscans, Greeks and Egyptians. In the eighteenth century it was found that lime made with limestone containing clay would set strongly and durably under water. Therefore, calcined septarian nodules also made a useful cement and then Aspdin, working in Wakefield, happened on the limestone and clay recipe for portland cement. Today some 12 000 000 tons of this cement is used annually in Britain. This, combined with 60 000 000 tons of aggregate and 6 000 000 tons of water, completes the concrete. Good quality rock is needed as the aggregate, but rocks often contain minor components that introduce considerable difficulties. These substances include various sulphides, sulphates, strained or finely divided silicates and some dolomitic rocks. Opal, chert and various other forms of silica have been found to be particularly troublesome. They react with alkalis in the cement to produce a swelling gel that causes cracking in the structures. This process can take up to 80 years to develop. The concrete can also be damaged by the atmosphere, hydrosphere and ground conditions. Atmospheric CO 2 and CO 2 disssolved in water react to form carbonates, sometimes deleteriously, sometimes beneficially. Sulphates also provide a hazard. In particular large amounts of the mineral thaumasite can be developed with almost total loss in strength of the concrete.