Rebar: more than just a vehicle for daytrading Chinese grandmothers:
From The Conversation:
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.
Cheap and effective, in the short term at least.Luigi Chiesa/Wikimedia Commons, CC BY-SA
Reinforced concrete competes against more durable building
technologies, like steel frame or traditional bricks and mortar. Around
the world, it has replaced environmentally sensitive, low-carbon options
like mud brick and rammed earth – historical practices that may also be more durable.
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.
Many engineers and architects point to the natural affinities between
steel and concrete: they have similar thermal expansion
characteristics, and concrete’s alkalinity can help to inhibit rust. But
there is still a lack of knowledge about their composite qualities –
for example, in regard to sun-exposure-related changes in temperature.
The many alternative materials for concrete reinforcement – such as
stainless steel, aluminium bronze and fibre-polymer composites – are not
yet widely used. The affordability of plain steel reinforcement is
attractive to developers. But many planners and developers fail to
consider the extended costs of maintenance, repair or replacement.
There are technologies that can address the problem of steel corrosion, such as cathodic protection, in which the entire structure is connected to a rust-inhibiting electric current. There are also interesting new methods to monitor corrosion, by electrical or acoustic means....MORE
