The problem of reinforced concrete

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In itself, concrete is a very durable building material. The magnificent Pantheon in Rome, the largest unreinforced concrete dome in the world, is in excellent condition after nearly 1,900 years. And yet, many concrete structures of the last century – bridges, highways and buildings – are collapsing. Many concrete structures built during this century will be obsolete before its end.

Considering the survival of ancient structures, this may seem curious. The critical difference is the modern use of steel reinforcement, known as rebar, hidden in the concrete. Steel is made mostly of iron, and one of the unalterable properties of iron is that it rusts. This ruins the durability of concrete structures in ways that are hard to detect and expensive to repair.

Although repair may be warranted to preserve the architectural heritage of iconic 20th century buildings, such as those designed by reinforced concrete users like Frank Lloyd Wright, it is debatable whether this will be affordable or desirable for the vast majority of structures. The writer Robert Courland, in his book concrete planet, estimates that the cost of repairing and rebuilding concrete infrastructure in the United States alone will be in the trillions of dollars – to be paid by future generations.

Old bridges need new money to be replaced.
Phil’s 1stPix/Flickr.com, CC BY-NC

The steel frame was a spectacular innovation of the 19th century. Steel bars add strength, allowing the creation of long cantilevered structures and thinner, less supported slabs. This speeds up construction times, as less concrete is needed to pour such slabs.

These qualities, driven by assertive and sometimes misleading promotion by the concrete industry in the early 20th century, led to its massive popularity.

Reinforced concrete competes with more sustainable construction technologies, such as steel framing or traditional bricks and mortar. Around the world, it has replaced eco-friendly, low-carbon options like mud brick and rammed earth – historic practices that may also be more sustainable.

Early 20th century engineers believed that reinforced concrete structures would last a very long time – perhaps 1,000 years. In reality, their lifespan is more like 50 to 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.

Many engineers and architects point to the natural affinities between steel and concrete: they have similar thermal expansion characteristics, and the alkalinity of concrete can help inhibit rust. But there is still a lack of knowledge about their composite qualities – for example, with regard to temperature changes related to sun exposure.

The many alternative materials for concrete reinforcement – such as stainless steel, aluminum bronze and fibre-polymer composites – are not yet widely used. The affordability of plain steel rebar is attractive to developers. But many planners and developers do not take into account the prolonged maintenance, repair or replacement costs.

Cheap and effective, at least in the short term.
Luigi Chiesa/Wikimedia Commons, CC BY-SA

There are technologies that can solve the problem of steel corrosion, such as cathodic protection, in which the entire structure is connected to a rustproof electric current. There are also exciting new methods for monitoring corrosion, electric Where acoustic means.

Another option is to treat the concrete with a rust inhibiting compound, although these can be toxic and unsuitable for buildings. There are several new non-toxic inhibitors, including compounds extracted from bamboo and “biomolecules” derived from bacteria.

Fundamentally, however, none of these developments can solve the inherent problem that putting steel inside concrete ruins its potentially high durability.

The environmental costs of reconstruction

This has serious repercussions on the planet. Concrete is the third largest contributor to carbon dioxide emissions, after automobiles and coal-fired power plants. The manufacture of cement is solely responsible for around 5% of global CO₂ emissions. Concrete also constitutes the largest proportion of construction and demolition waste and accounts for approximately one third of all landfill waste.

Concrete recycling is difficult and expensive, reduces his strength and can catalyze chemical reactions that accelerate decomposition. The world needs to reduce its concrete production, but that won’t be possible without building more sustainable structures.

Recovery of rebars: expensive work.
Anna Frodesiak/Wikimedia Commons

In a recent article, I suggest that the widespread acceptance of reinforced concrete may be an expression of a traditional, dominant, and ultimately destructive view of matter as inert. But reinforced concrete is not really inert.

Concrete is commonly perceived as a stone-like, monolithic and homogeneous material. In fact, it is a complex mixture of baked limestone, clay materials and a wide variety of rocky or sandy aggregates. Limestone itself is a sedimentary rock composed of shells and corals, the formation of which is influenced by many biological, geological and climatological factors.

This means that the concrete structures, for all their superficial stone qualities, are actually made up of the skeletons of sea creatures ground down with rock. It takes millions and millions of years for these sea creatures to live, die and turn into limestone. This timescale contrasts sharply with the lifespan of contemporary buildings.

Steel is often perceived as being equally inert and resistant. Terms such as “Iron Age” suggest ancient durability, although Iron Age artifacts are relatively rare precisely because they rust. If the structural steel is visible, it can be serviced – for example, when the Sydney Harbor Bridge is painted and repainted several times.

However, when encased in concrete, the steel is hidden but secretly active. Moisture penetrating 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 rebar up to four times its size, enlarging cracks and forcing concrete to fracture in a process called chipping, better known as “concrete cancer”.

Concrete cancer: not pretty.
Sarang/Wikimedia Commons

I suggest we need to change our way of thinking, recognize concrete and steel as vibrant and active materials. It is not about changing facts, but rather reorienting how we understand and act on those facts. Avoiding waste, pollution and unnecessary reconstruction will require think well beyond disciplinary conceptions of time, and this is especially true for the building and construction industries.

The collapsed civilizations of the past show us the consequences of short-term thinking. We should focus on building structures that stand the test of time – lest we end up with bulky, abandoned artifacts that are no more fit for their original purpose than the statues of Easter island.

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