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  • Concrete reinforced with steel is the foundation of our modern society.

  • Reinforcement within concrete creates a composite material, with the concrete providing strength

  • against compressive stress while the reinforcement provides strength against tensile stress.

  • But, while steel reinforcement solves one of concrete's greatest limitations, it creates

  • an entirely new problem: Corrosion of embedded steel rebar is the most common form of concrete

  • deterioration.

  • So what are we doing about it?

  • Hey I'm Grady, and this is Practical Engineering.

  • On today's episode, we're testing out some innovations in concrete reinforcement.

  • Although unprotected steel is naturally prone to corrosion, or rusting, when it gets embedded

  • into concrete, certain factors usually work to protect it.

  • First is the obvious protection of simply being shielded from the outside environment

  • by a relatively impermeable and durable material.

  • Water and contaminants usually can't make their way through the concrete to the steel.

  • The second form of protection is the alkaline environment.

  • The high pH of normal concrete creates a thin oxide layer on the steel that provides protection

  • from corrosion.

  • But, in some cases, this protection isn't enough.

  • One of the main sources of corrosion to rebar is salt.

  • Whether through exposure to saltwater near a marine environment or application of deicing

  • salts to make roadways safer during the winter, these chloride ions can make their way through

  • the concrete, corroding the steel reinforcement.

  • And when steel corrodes, it creates iron oxide that expands inside the concrete.

  • This expansion generates stress, sometimes called oxide jacking, and is the one of the

  • primary causes of concrete deterioration.

  • So, how do we prevent these chloride ions and other contaminants from reaching the steel

  • and causing corrosion?

  • The first line of defense is cover.

  • Cover is the minimum distance between the outside surface of the concrete and the reinforcing

  • steel.

  • And, depending on exposure and application, certain codes specify different amounts of

  • concrete cover, generally between 25 and 75 millimeters or 1 to 3 inches.

  • Cover is one of the reasons good concrete work takes so much effort before the concrete

  • ever shows up on the job site.

  • Installing strong formwork and lots and lots of wire tying all the reinforcement together

  • help to make absolutely sure that, through all the jostling and walking over and general

  • chaos that comes when it's time to actually place concrete, the rebar stays where it was

  • designed to be embedded within the final product.

  • Neglecting these steps can cause rebar to sink to the bottom of a slab or come too close

  • to an outside surface before the concrete cures, eventually leading to premature corrosion

  • of the reinforcement due to lack of cover.

  • But, even with adequate cover, a crack in the concrete can allow contaminants and water

  • into direct contact with the reinforcement.

  • And it won't surprise you to learn that cracks in concrete aren't all that rare.

  • Most concrete shrinks as it cures which can lead to cracks.

  • Changes in temperature also cause expansion and contraction which can lead to cracking.

  • Concrete can also crack under normal, expected loading conditions due to the way the steel

  • takes up stresses within the material.

  • One way to solve this issue is by prestressing the rebar, a topic I discussed briefly in

  • a previous video and something I'd like to dive deeper into in the future.

  • But today I want to show another option for reducing these cracks.

  • Fiber reinforced concrete is pretty much exactly what you'd expect it be.

  • It's not a new idea by any means, but our understanding and use of different kinds of

  • fibers within a concrete mix continues to grow.

  • Adding glass, steel, or synthetic fibers to concrete can provide a lot of benefits, but

  • one of the most important is crack control.

  • I constructed three nearly identical reinforced concrete beams to show how this works, and

  • I let them cure for about a week.

  • The first one only has steel rebar as reinforcement.

  • I'm using my hydraulic press to test out the strength of each beam and see how it performs

  • prior to failure.

  • And I'm using tons as a measurement of force on these beams, just because that's what

  • the gauge says, but the units are completely arbitrary to the demo.

  • If you prefer SI, just pretend these are metric tonnes.

  • As I increase the load on the beam, you see cracks starting at only around 3 tons.

  • These cracks form because steel stretches a little bit as it takes up the tensile stress

  • in the concrete.

  • The beam is holding the load just fine and isn't even close to failure, but concrete

  • can't stretch along with the steel so it has to crack.

  • You can imagine how these cracks could let water and air into contact with the reinforcement

  • and eventually deteriorate the concrete.

  • Those cracks are the important part of this demo, but I went ahead and increased the load

  • until the beam failed because, hey, that's what hydraulic presses are good for right?

  • For these next two beams, I included fibers in the concrete mix: one beam has steel fibers

  • and the other has glass fibers.

  • The steel rebar and fibers team up to resist tensile stresses in the beams.

  • The rebar provides large scale reinforcement to resist tension across the entire structural

  • member, and the fibers provide small scale reinforcement to resist localize tension that

  • causes cracking.

  • When I load these beams to 3 tons, you can't see a single crack.

  • In fact, for both of these beams, I didn't see any cracks form until almost double that.

  • and even then the cracks were much smaller.

  • Both beams failed at about the same load as first, one, which I expected.

  • Like I said, the fibers don't really add much overall strength to the beam, but you

  • can easily see they could go a long way in preventing corrosion of steel rebar.

  • You may be wondering why are we even using steel for reinforcement at all?

  • Steel is relatively inexpensive, well-tested, and strong, but there are lots of other materials

  • that with excellent mechanical properties that don't face this issue of corrosion.

  • For very corrosive environments, we sometimes use epoxy-coated rebar or even stainless steel,

  • but there are some emerging alternatives like Fiber Reinforced Polymers or FRP bars.

  • This is reinforcement made of basalt, remelted volcanic rock forced through tiny nozzles

  • to create fibers that are extremely strong.

  • Options like this often cost cost more than steel rebar, in some cases a lot more.

  • But, the major impediment to the use of these newer, more innovative types of reinforcement

  • isn't just the cost.

  • It's easy to see that those additional costs may be offset by the increased lifespan of

  • the concrete.

  • Another inhibition comes simply from the lack of widespread use.

  • Innovation happens slowly in civil engineering because the consequences of failure are so

  • high.

  • Gaining confidence in a design has as much to do with engineering theory as it does to

  • simply seeing how well similar designs have performed in the past.

  • But many engineering disasters have come not at the expense of bad design, but actually

  • bad maintenance, so long-term durability can be just as important to public safety as other

  • design criteria.

  • We'll certainly be seeing more innovative ways to reinforce concrete in the future,

  • including the options I mentioned in this video.

  • Thank you for watching, and let me know what you think!

Concrete reinforced with steel is the foundation of our modern society.

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