Why reinforce concrete in the first place? Concrete is an adjective which, before becoming a noun, was already synonymous with solid. How much more solidity do you need? If all we wanted to do was compress the material, we might have been able to leave it there. Yet life is rarely that simple.
Reinforcement bar (or simply, rebar) is usually carbon steel, hot rolled and extruded with deformations to increase its surface area, allowing it to bond better. Adding it to the concrete gel allows the combined material to become something that is neither just rocky nor elastic, but both; reinforced concrete combines compressive strength (ability to be crushed) concrete with the traction (ability to stretch) and shear strength (ability to be twisted or pulled in two directions at once on different parts of the structure) steel. It can be crushed, stretched and twisted (within reason) for the lowest cost per cubic yard of any building material.
Rebar – A History
The ancient Romans added a few ingredients to the sand (in particular a pre-cooked natural volcanic powder similar to the cement of Pozzuoli outside Naples) and concreted, creating structures of great beauty that have lasted two thousand years. the Pantheon in Rome or the Alcantara Bridge in Spain serve as great examples; the Pantheon’s record as the largest unreinforced concrete structure has never been broken. Nevertheless, they encountered many of the same problems: the concrete compresses and crushes well, it can even develop cracks without falling apart, but it doesn’t stretch well at all.
Once the empire collapsed, concrete went out of fashion again until the 18th century. french gardener, Joseph Monnier, combined it with steel wire to make pots for keeping large tropical plants in greenhouses; pots that didn’t need to be fired like clay, and that didn’t break under the strain as the plants grew. By some extraordinary stroke of luck, steel and concrete chemically bond very well and expand and contract at almost exactly the same rate under temperature changes. If they didn’t, the rebar would pull away from the concrete, like soda from a can in the freezer.
At the Paris Exhibition of 1867, Francois Hennebique saw these plant pots in action and quickly realized their potential, starting his own business the same year. In 1892, he held the patent for the use of reinforced concrete in construction. Since then, the trend to build ever higher has continued and the modern world has evolved at the same rate as the buildings that house it. The great buildings of antiquity took decades, with several generations of the same family working their entire lives to complete certain cathedrals. Cologne Cathedral in Germany, for example, spent 662 years under construction. By comparison, the central core of The Shard in London, one of the tallest buildings in Europe, was completed in less than six months.
Rebar Trials and Tribulations
Reinforced concrete must ultimately tolerate the same forces that destroy mountains: weathering and erosion. As concrete deteriorates and develops cracks, water can penetrate and corrode rebar, expand and crack outer layers of concrete as rust expands the metal. This in turn exposes more rebar to the elements, which speeds up the corrosion process. Buildings must be continually assessed to ensure that they do not endanger their occupants.
Location also plays a factor. One of the biggest hazards to standard carbon steel rebar or “black” rebar is salt. Rebar naturally forms a protective outer layer under ideal circumstances, but this can be easily eroded by Cl- or chloride ions. In solution, salt or NaCl naturally forms chlorides, which easily reacts with the exterior of the rebar later and greatly accelerates the deterioration process. Long story short, salt is bad news for rebar. Anything by the sea or in the sea, or where road salting is common, needs something more.
Two solutions revolve around coating rebar with other chemicals. Epoxy coated rebar was a common practice for years, but is being phased out and banned in many areas, such as Quebec; a single nick or cut in the epoxy and the coating, rather than protecting the steel underneath, becomes a moisture trap and actually accelerates its degradation. Enamel coatings are believed to react chemically to chlorides that attack steel, suggesting that chipping would be less of a problem. However, site manager Dafydd Downing says the benefits aren’t outweighed by the price, and in the majority of cases the standard black bar remains supreme.
Only in the most extreme environments is rebar shunned for its more expensive and tougher cousin: Glass fiber reinforced plastic (GFRP) rebar. This product is touted by its producers to last a hundred years, an apparent lifespan four times that of standard rebar. This is only required in places like heavily salted rural Canadian bridges and nuclear reactors, where builders hope they won’t have to set foot again in their lifetime.
When a building reaches the end of its life and the demolition experts are sent in, what happens to this reinforcement radical? It might comfort you to know that rebar recycling is in fact in everyone’s interest.
steel fabrication is an energy-intensive process, especially when making new products from iron ore. But recycling rebar into new products cuts energy consumption by half to two-thirds. And machines that extract bones buildings are no laughing matter either.
Excavators capable of wielding a thousand tons of raw power are capable of effectively “eat” reinforced beams and pillars, then “chew” the accumulated mass until it separates into aggregate and steel. If it’s not in your budget or you’re working on a smaller scale, you can use a masonry bit on a power drill to break into a piece of concrete and switch to a rebar bit to cut pieces. individual reinforcement.
So there you have it, the simplest of introductions to the largely invisible reinforcement that sustains the modern world. The next time you pass a construction site and see the bare steel bars before they take their place in the heart of the reinforced concrete, you might want to give this feat a quick nod. engineering.