A 2015 report estimates that approximately three tonnes of concrete are used each year for every person on Earth - about 22 billion tonnes. To put this in context, a recent study estimated that over the course of history we have produced 8.3 billion metric tons of concrete.
The manufacture of cement, the binding agent in concrete, consumes a lot of energy, says Fennell. Ordinary Portland cement, the most common form in concrete, is made by kilning limestone and emits about one tonne of carbon dioxide for every tonne of cement. Concrete production is responsible for about 5% of the world's human-generated CO2 emissions, according to the World Business Council for Sustainable Development.
Concrete reabsorbs some of the carbon dioxide from the atmosphere over time, Fennell notes. A 2016 study estimated that between 1930 and 2013, the equivalent of 43% of the CO2 released by lime during heating was reabsorbed by concrete products worldwide, although that percentage does not include carbon dioxide emitted by fossil fuels burned in kilns, the largest contributor to CO2 emissions during production.
Unfortunately, this absorption comes at a price, especially when cement is used in structures that have steel reinforcing bars within the concrete.
"As CO2 moves through the concrete, it changes the pH of the environment," says Fennell. Concrete loses its alkalinity and, when moisture and oxygen are present, that rebar oxidises.
As a specialist once told Langenbach: "If it's not cracked, it's not concrete".
