Rebar has been around for decades in concrete construction. It’s time for something better.
The steel reinforcement of concrete has been around for centuries and was standardized around 1950. Steel reinforcing bar—known as rebar—has a convenient property: a similar thermal expansion coefficient to the concrete surrounding it, making it a natural system to add tensile strength to concrete beams and columns. Unfortunately, steel is prone to corrosion, especially when moisture seeps inside columns and beams through cracks. That rust causes spalling and eventual failure of reinforced concrete structures. Is there a better way? Jim Anderton comments.
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Episode Transcript:
With the recent collapse of a Manhattan parking garage, plus the obvious and expensive damage to reinforced concrete infrastructure everywhere in the Western world, it amazes me that the regular and expensive failures of these structures are accepted by politicians and voters alike.
The designers and builders of the Egyptian Pyramids and the Acropolis didn’t spend the equivalent of hundreds of millions of dollars every couple of decades to keep those monuments from falling down. Yet this is the reality for reinforced concrete structures everywhere that isn’t a desert.
Why is this happening? Well, since the turn of the last century, use of steel reinforcing rods embedded in concrete has been the preferred construction technique for large structures from skyscrapers to sewer mains—for sound engineering reasons. Steel is a terrific material under tension, a quality that concrete lacks, and the thermal expansion coefficient of steel is very close to that of today’s concrete mixes, eliminating the problems of thermally induced stresses.
For the first couple of decades, it works great—until the water gets in.
And it will get in, unless the inevitable cracking in concrete columns and beams is addressed with meticulous maintenance, including durable water-resistant coatings. Water is bad, but in the Northeast, road salt is in common use, and that saline solution is the perfect electrolytic fluid to facilitate corrosion of reinforcing bar.
Unfortunately, the iron oxide produced by the corrosion of steel is greater in volume than the base metal that it corrodes, and since the bars are constrained by the concrete matrix, the pressure buildup is enormous.
This creates a phenomenon called oxide jacking. For bars near the surface, this can cause spalling, something seen commonly in aging reinforced concrete bridge structures in America. Deeper inside the column or beam, the stresses can create extensive cracking, letting even more moisture in and accelerating the process. The results are expensive to remediate and can be dangerous, as we have seen in New York City.
So, what can be done? It’s possible to use nonferrous materials for the reinforcement, but it’s expensive. Rebar can be epoxy coated or galvanized, but again this comes at a high cost. It may be possible in the future to use 3D printing with composite fillers in concrete mixes to eliminate rigid reinforcement altogether, although no one to date has found a way to do this cheaply, either.
But the problem isn’t in the engineering, I think; it’s in the politics. Current steel reinforced concrete structures in places such as the American Northeast have a couple of decades of low maintenance durability in front of them post-construction. After that, it gets expensive—but those costs are externalized to taxpayers in the next generation.
Would ratepayers tolerate construction costs that would be 50, 100 or 150 percent higher than current reinforced concrete in order to get a structure that would last maintenance-free for three or four hundred years? Politicians say no, but I’m not so sure.
Would I pay a modest toll to cross a major bridge structure that is engineered to last 500 years? I would, as a sort of gift to future generations.
And in the long run, I suspect that this would be a lot cheaper than spending billions to chip, sandblast and patch reinforced concrete structures year after year and decade after decade.