Reinforced concrete

Concrete is a combination of small aggregate (sand), large aggregate (gravel), a binding agent or matrix, and water. Historically, lime was used as a matrix, mostly for mortars that had no large aggregate. In 1774 the British engineer John Smeaton added crushed iron-slag to the usual quicklime-sand-water mix, making the first modern concrete for the foundations of the Eddystone Lighthouse off the English coast. Fifty years later, a new matrix was discovered. Portland cement, a calcium silicate cement made with a combination of calcium, silicon, aluminum, and iron, is the basis of modern concrete. In 1824, the English stonemason Joseph Aspdin made it by burning (on his kitchen stove) finely ground limestone and clay, then grinding the combined material to a fine powder. It was named for its original use in a stucco that imitated Portland stone. However, the burnt clay yielded silicon compounds that combined with water to form a much stronger bond than lime. It was to revolutionize the architectural and engineering world.

For the next thirty years or so, plain concrete, because of its tremendous compressive strength (resistance to crushing), was used for walls. Sometimes it replaced brick as fire-resistant covering for iron-framed structures. Reinforced concrete, developed first by the French, combines concrete’s compressive strength with the tensile strength (resistance to stretching) of metal—at first, iron and later steel—reinforcing bars or wire. The first person to employ such construction was the Parisian builder François Coignet, for drainage and building works throughout the 1850s. His own all-concrete house in Paris (1862) survives.

The new material was also investigated by the Parisian market gardener Joseph Monier, who was granted a patent in 1867 for garden pots made of cement mortar reinforced with a cage of iron wire. Over the next decade, he built concrete water-storage tanks, patented several ideas for bridges, and promoted reinforced concrete for floors, arches, railroad ties, and bridges. Not being an engineer, he was not permitted to build bridges in France, so he sold his patents to German and Austrian contractors Wayss, Freitag and Schuster, who built the first reinforced concrete bridges in Europe.

Monier exhibited his plant pots at the 1867 Paris Exposition, where they caught the attention of the builder François Hennebique (1842–1921), who then looked for applications in the building industry. Beginning with reinforced concrete floor slabs in 1879, by 1892 he had developed a complete building system of columns, beams, and floors, which he applied to an apartment building in Paris. Having patented the system, he wound up his contracting firm to become a consulting engineer. The structural and esthetic implications of monolithic reinforced structures were staggering. Because concrete is in effect a liquid, the only limitations placed upon the plasticity of architectural shapes lay in the buildability of the formwork (known as shuttering) that held it in place while it set. The remaining major technological step was taken by yet another Frenchman, Marie Eugène Léon Freyssinet (1879–1962), who perfected pre-stressed concrete, allowing the construction of even greater spans.

It was left to Auguste Perret (1874–1954) to address the problem of a suitable esthetic for the new material, because (of course) there was no historical precedent. Although he never became an architect in the strictest academic sense, Perret was one of the pioneers of French modern architecture, in whose work we see the first rational expression of reinforced concrete. Perret developed Hennebique’s structural system, but while the latter had disguised his buildings’ reinforced concrete skeletons with masonry, from as early as 1903 Perret—he called himself a “builder in reinforced concrete”—began frankly expressing them as part of the architecture. That approach reached its pinnacle in his war memorial church of Notre Dame du Raincy, Paris, of 1922–1923, claimed by architectural historian Peter Collins to be the most revolutionary building of the first quarter of the twentieth century. Raincy was a model for other churches by Perret, including St. Therese, Montmagny (1925); a chapel at Arcueil (1925); and St. Joseph, Le Havre (begun in 1950). Perret’s carefully designed shuttering, producing “off-form” surfaces that needed no further finishing work, inspired the so-called Brutalist architecture movement, mostly British, of the late 1950s, as well as Japanese architecture right to the end of the twentieth century.

Beyond what may now be described as orthodox reinforced concrete construction, a number of engineers—the German Ulrich Finsterwalder, the Italian Pier Luigi Nervi, and the Spaniards Eduardo Torrojay Miret and Felix Candela—pushed the versatile material to its technological limits, developing the cantilever and the thin shells that may be regarded as the ultimate concrete form.