Thursday, August 20, 2009

Origins of Concrete


It may surprise some to learn that concrete was first invented by the ancient Romans and that it was in common use by about 50 BC.

Roman concrete, like any concrete, consisted of mortar and an aggregate. The mortar (a hydraulic cement) was a mixture of limestone and a special kind of volcanic dust called Pozzolona which contained a high element of alumina and silica. The Pozzolona created an exceptionally strong bond with the aggregate. The aggregates varied and were a mixture of rubble from the remains of demolished buildings and pieces of rock.

The discovery of concrete meant that instead of cutting and shaping large blocks of stone to enable structural stability, Romans could now simply “pour” concrete onto a formwork resulting in the hardened concrete taking the shape of the container. The “pouring” process is not as we know it today but rather a process of applying layers of the mixture to achieve the final shape.

The Pantheon

Roman concrete was remarkably strong and could span long distances when formed into arches, vaults and domes. The Pantheon is an example of Roman construction excellence where the upper dome consisted of alternate layers of light tufa (a soft volcanic stone) and pumice with a density of 1350 kg/m3 and foundations of travertine as an aggregate with a density of 2200 kg/m3. The tufa was used to face the concrete. The Pantheon's dome is the largest unreinforced dome in the world.

As Roman concrete was easy to work, cheap to make and enabled shortened construction time its invention resulted in an expansion of Roman cities and provinces. As the concrete was fireproof it was safer than wooden arches and supports.

Roman concrete could set under water which led to improved bridge and harbour design.

Roman concrete had however one defect – it was unsightly as it showed an ugly surface once the formwork was removed. The Romans however overcame this problem by covering concrete surfaces with other materials such as marble or brickwork.

Bruce Lyle
Membership Services Manager


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