Roman technology: aqueducts, roads and concrete

On top of the Pont du Gard the water channel drops just 2.5 cm in 456 m, a slope of about 1 in 18,000.

Water that runs downhill for 50 km

A Roman aqueduct is mostly not the tall arches people picture. It is a covered stone channel that carries water from a spring to a city using nothing but gravity, so every metre of it has to slope gently downhill. Rome’s first, the Aqua Appia of 312 BCE, ran about 16.6 km, and all but about 90 metres of it was underground. Arches were used only where the line had to cross a valley. The Pont du Gard in southern France carries the aqueduct of Nîmes, more than 50 km long and built in the 1st century CE, across the river Gardon. It stands 48.77 metres above the river. Its engineers varied the gradient along the route; a single even slope would have made the bridge impossibly tall, and by changing it they lowered the bridge by about 6 metres.

Surveyors set out these slopes with simple instruments. Vitruvius describes the chorobates, a long wooden table with water channels and plumb lines for finding level, and the groma, a cross on a pole for laying out straight lines and right angles. Inside the city the water flowed into distribution tanks and then through lead pipes; the Latin for lead, plumbum, gives us the word plumber. Sextus Julius Frontinus, appointed water commissioner in 97 CE, wrote a book on Rome’s water supply. In it he describes the nine aqueducts of his day, complains about illegal taps that stole water, and compares the aqueducts favourably with the "idle pyramids" of Egypt.

Why arches stand

Stone is very strong when squeezed and weak when stretched. A flat stone beam over a gap bends, and its underside is pulled apart, so beams can only span short distances. An arch turns the load into compression. Each wedge-shaped stone, or voussoir, presses on its neighbours, and the weight travels round the curve and down into the piers. While an arch is being built it cannot stand on its own, so builders rest the stones on a curved timber frame called centering. Once the keystone at the top is in place, the frame can be taken away and the arch carries itself. The Romans did not invent the arch, but they used it on a huge scale, stacked in tiers as at the Pont du Gard.

Roads

The Via Appia, the "queen of roads", was begun in 312 BCE by the censor Appius Claudius Caecus, the same man behind the first aqueduct. It ran 132 Roman miles, about 195 km, from Rome to Capua, and was later extended to Brundisium on the heel of Italy. Roman roads were built for armies first: as straight as the land allowed, with a cambered surface so rain ran off into ditches at the sides. Textbooks often show a neat four-layer cross-section, but that model comes from Vitruvius’ description of floors, and excavated roads vary a great deal with local stone and soil. Stone milestones along the way gave the distance from the city.

Concrete

Roman concrete, opus caementicium, was in wide use from about 150 BCE. It mixed lime with volcanic ash, especially from around Pozzuoli near Naples, and lumps of stone or broken brick. It was laid in layers rather than poured, and it could even set under water, which let builders make harbours in the open sea. Its greatest monument is the dome of the Pantheon in Rome, finished under Hadrian around 125 CE. The dome spans about 43 metres and is still the largest unreinforced concrete dome in the world. Builders used lighter stone in the mix higher up so the dome weighed less where it was thinnest.