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The Problem With Reinforced Concrete
By Guy Keulemans
Published on The Conversation, June 17, 2016
By itself, concrete is a very durable construction material. The magnificent Pantheon in Rome, the world’s largest unreinforced concrete dome, is in excellent condition after nearly 1,900 years. And yet many concrete structures from last century – bridges, highways and buildings – are crumbling. Many concrete structures built this century will be obsolete before its end.
Given the survival of ancient structures, this may seem curious. The critical difference is the modern use of steel reinforcement, known as rebar, concealed within the concrete. Steel is made mainly of iron, and one of iron’s unalterable properties is that it rusts. This ruins the durability of concrete structures in ways that are difficult to detect and costly to repair.
While repair may be justified to preserve the architectural legacy of iconic 20th-century buildings, such as those designed by reinforced concrete users like Frank Lloyd Wright, it is questionable whether this will be affordable or desirable for the vast majority of structures. The writer Robert Courland, in his book Concrete Planet, estimates that repair and rebuilding costs of concrete infrastructure, just in the United States, will be in the trillions of dollars – to be paid by future generations.
Steel reinforcement was a dramatic innovation of the 19th century. The steel bars add strength, allowing the creation of long, cantilevered structures and thinner, less-supported slabs. It speeds up construction times, because less concrete is required to pour such slabs.
Early 20th-century engineers thought reinforced concrete structures would last a very long time – perhaps 1,000 years. In reality, their life span is more like 50-100 years, and sometimes less. Building codes and policies generally require buildings to survive for several decades, but deterioration can begin in as little as 10 years.
Concrete is commonly perceived as a stone-like, monolithic and homogeneous material. In fact, it is a complex mix of cooked limestone, clay-like materials and a wide variety of rock or sandy aggregates. Limestone itself is a sedimentary rock composed of shells and coral, whose formation is influenced by many biological, geological and climatological factors.
However, when embedded in concrete, steel is hidden but secretly active. Moisture entering through thousands of tiny cracks creates an electrochemical reaction. One end of the rebar becomes an anode and the other a cathode, forming a “battery” that powers the transformation of iron into rust. Rust can expand the rebar up to four times its size, enlarging cracks and forcing the concrete to fracture apart in a process called spalling, more widely known as “concrete cancer”.