Concrete

• 1. Introduction

  ›

  Concrete is the second-most-used substance on Earth, behind only water. Every year, more than 10 billion tonnes of it pass through human hands. As of 2006, roughly 7.5 billion cubic meters were made annually, more than a full cubic meter for every person alive. It is the most widely used building material and the most-manufactured material in the world, yet most people walk across it without a second thought. The word comes from the Latin concretus, meaning compact or condensed, the past participle of concrescere, built from con-, together, and crescere, to grow. So how does a fluid slurry of stone, sand, and powder grow into something that holds up a high-rise? Why did the Romans pour domes that still stand two thousand years later? And why does the powder at its heart account for about 8% of the world's yearly carbon emissions? The answers begin with a chemical reaction that releases its own heat.

• 2. The Chemistry Of Setting

  ›

  Hydration is the reaction that turns wet sludge into stone-like material. Water meets dry Portland cement, and over several hours the mixture hardens into a solid matrix that binds everything together. The process is exothermic, meaning it gives off heat, so ambient temperature decides how quickly concrete sets. This window of working time is what lets concrete be cast in forms and also tooled. Abrams' law governs the result. A lower water-to-cement ratio yields stronger, more durable concrete, while more water gives a freer-flowing mix with a higher slump. The reaction itself is many concurrent reactions at once. Silicates and aluminates interlink through polymerization and bond to the sand and gravel, producing calcium-silicate hydrate, written C-S-H in cement chemist notation. Over 90% of a mix's final strength is typically reached within four weeks. The remaining 10% can take years or even decades. Once hydration has occurred, it is irreversible. Over decades, calcium hydroxide in the concrete absorbs carbon dioxide and turns into calcium carbonate, strengthening the material further. That same carbonation lowers the pH of the cement pore solution, which can corrode embedded reinforcement bars. The first three days of hardening are critical, and fast drying from wind can crack the surface before it has gained strength.

• 3. Rome's Volcanic Secret

  ›

  Opus caementicium was the name the Romans gave their concrete, made from quicklime, pozzolana, and an aggregate of pumice. They used it extensively from 300 BC to AD 476. Laid as arches, vaults, and domes, it hardened into a rigid mass free from the internal thrusts that troubled builders working in stone or brick. This freedom is why the period is called the Roman architectural revolution. The Colosseum was built largely of concrete, and the Pantheon carries the world's largest unreinforced concrete dome. Modern tests show opus caementicium reached a compressive strength of about 200 kilograms per square centimeter, similar to modern Portland-cement concrete. Its tensile strength, though, was far lower, because it had no reinforcing steel. The long-term durability traces to pyroclastic volcanic rock and ash in the mix. Crystals of strätlingite, a calcium aluminosilicate hydrate, grew during curing and gave the material greater fracture resistance than modern concrete. Against seawater the advantage is sharper still. The pyroclastic materials react with seawater over time to form Al-tobermorite crystals. Researchers have proposed that hot mixing, which leaves lime clasts in the final product, gave Roman concrete a self-healing ability. The Baths of Caracalla in Rome still stand as one example, and the Pont du Gard in southern France wears masonry cladding over a concrete core.

• 4. The Long Revival

  ›

  In Britannia after the Roman Empire, the quality of concrete fell. An 8th century mortar mill survives in Northamptonshire, but low kiln temperatures, a lack of pozzolana, poor mixing, and an Anglo-Saxon habit of timber construction all dragged the craft down. It was never fully lost. From the 11th century in England, stone church and castle construction raised demand for mortar, and grinding and sieving improved through the 12th century. Bartholomaeus Anglicus described mortar-making in his De proprietatibus rerum of 1240. An English translation from 1397 reads that lime is a stone burnt, and by mixing it with sand and water, cement is made. Across the Atlantic, John L. Stephens recorded Mayan concrete at the ruins of Uxmal, dated AD 850 to 925, in his Incidents of Travel in the Yucatán. He wrote that the roof was flat and had been covered with cement, and that the floors were cement, in some places hard, in others crumbling underfoot. Vitruvius preserved the old knowledge in his treatise De architectura. A copy at Saint Gall Abbey was rediscovered by Poggio Bracciolini in 1414, and the first printed edition appeared in 1486. The modern breakthrough came at Smeaton's Tower, the third Eddystone Lighthouse, built by John Smeaton in Devon between 1756 and 1759. He pioneered hydraulic lime in concrete, using pebbles and powdered brick as aggregate.

• 5. Inventing The Modern Mix

  ›

  Joseph Aspdin patented a method for producing Portland cement in England in 1824. He chose the name for its likeness to Portland stone, quarried on the Isle of Portland in Dorset. His son William carried the developments into the 1840s and earned recognition for what became modern Portland cement. Joseph Monier invented reinforced concrete in 1849, and in 1853 François Coignet built the first reinforced concrete house. Monier went on to design and build the first reinforced concrete bridge in 1875. The next leap belonged to Eugène Freyssinet, a French structural and civil engineer who pioneered prestressed and post-tensioned concrete. His idea was to compress concrete components with tendon cables, either during or after fabrication, so they could resist the tensile forces that develop in service. Freyssinet patented the technique on the 2nd of October 1928. The reinforcing story has two halves. The use of iron reinforcement was introduced in the 1850s by French industrialist François Coignet, but it was not until the 1880s that the German civil engineer G. A. Wayss used steel. Steel and concrete form a strong bond and complement each other, the concrete strong in compression and the steel strong in tension. Together they act as a single structural element across slabs, walls, beams, columns, and foundations.

• 6. What Goes In The Mix

  ›

  Aggregate makes up the bulk of any concrete mixture, from coarse gravel and crushed limestone or granite down to fine sand. The aggregate is nearly always stronger than the binder, so varying its sizes fills the gaps, reduces the amount of costly binder needed, and lowers the price without weakening the result. Recycled aggregates from construction, demolition, and excavation waste increasingly stand in for natural stone. Admixtures are powders or fluids added during mixing, usually at less than 5% by mass of cement, to give concrete traits plain mixes cannot reach. Accelerators like calcium chloride speed hardening, though chlorides can corrode steel and are banned in some countries. Retarders such as sugar, sodium gluconate, citric acid, and tartaric acid slow setting for large pours. Air-entraining agents trap tiny bubbles that resist freeze-thaw damage, though each 1% of air may cut compressive strength by 5%. Superplasticizers increase workability and lower water content by 15 to 30%. Mineral admixtures recycle industrial waste straight into the mix. Fly ash from coal plants can replace Portland cement by up to 60% by mass. Ground granulated blast furnace slag from steelmaking can replace it by up to 80%. Silica fume, a by-product of silicon and ferrosilicon alloys, has particles 100 times smaller than fly ash. Stranger experiments exist too. Researchers at Japan's University of Kitakyushu found that washed and dried recycled diapers can replace some sand, and a model home was built in Indonesia to test the diaper-cement composite.

• 7. Many Concretes

  ›

  Asphalt concrete swaps the cement binder for bitumen and surfaces most roads, parking lots, and airports. In North America it is called blacktop or pavement, and in the United Kingdom and Ireland tarmac or rolled asphalt. The Belgian inventor and U.S. immigrant Edward De Smedt refined the process, and asphalt mixtures have been laid since the start of the twentieth century. Beyond bitumen, the variety is wide. Geopolymer concrete uses aluminosilicates and avoids lime entirely, sidestepping a major source of carbon dioxide. Graphene enhanced concrete adds a small dose, typically under 0.5% by weight, of chemically engineered graphene. Pervious concrete leaves out most fine aggregate, holding 15 to 25 percent interconnected air voids so rainwater drains through pavement into the soil and recharges aquifers. Sulfur concrete needs neither cement nor water, using sulfur as its binder. Microbial concrete enlists bacteria. Bacillus pasteurii and B. sphaericus can induce calcium carbonate to precipitate in cracks and add compressive strength, and one strain, Bacillus sp. CT-5, can cut reinforcement corrosion by up to four times. Nanoconcrete packs Portland cement particles no larger than 100 micrometers with silica particles no larger than 500 micrometers, filling voids to raise strength for foot and highway bridges. There is even a tube forest design, mimicking the elliptical hollow osteons of mammalian cortical bone, that can be 5.6 times more resistant to cracking than standard concrete by trapping cracks step by step.

• 8. The Carbon Reckoning

  ›

  Every tonne of cement produced releases, on average, one tonne of carbon dioxide into the atmosphere. The cement made for concrete accounts for about 8% of worldwide emissions each year, compared with global aviation at 1.9%. Two steps in the kiln do most of the damage. First comes the decarbonation of limestone at around 950 degrees Celsius, then the combustion of fossil fuel to reach the sintering temperature of clinker near 1450 degrees Celsius. Calcination accounts for roughly 60% of the greenhouse gases and combustion the other 40%. Because cement is only a fraction of a finished mix, a tonne of concrete is estimated to emit about 100 to 200 kilograms of carbon dioxide. Cement kilns are extremely large, complex, and dusty installations. Even efficient ones need 3.3 to 3.6 gigajoules of energy to make and grind a ton of clinker, and many burn difficult wastes such as used tires as fuel. Pioneer manufacturers have claimed lower carbon intensities, reaching 590 kilograms of carbon dioxide equivalent per tonne of cement. Researchers chase the same goal through the mix. Raising mineral admixture replacement by 10% lowered the global warming potential of slag by 1.1 kilograms of carbon dioxide equivalent per cubic meter, and fly ash by 17.3 kilograms. Grinding concrete carries its own hazard. Cement dust can cause silicosis, kidney disease, and skin irritation, and a U.S. silica rule took effect on the 23rd of September 2017 for construction, limiting breathable crystalline silica to 50 micrograms per cubic meter over an 8-hour workday.