[joelegner]

Where did you hear that reinforced concrete has a lifespan of 100 years?

As a structural engineer, I can tell you this is not correct as a general rule, but is true for structures subject to chlorides, especially marine structures like bridge piers.

One of my now-retired professors at the University of South Florida studied concrete durability for FDOT. He told me FDOT is now using a 100-year design basis for bridges. The concrete materials and additives have gotten quite good over the past 2 decades. We are learning a lot and still improving our concrete.

The key is to make a tortuous path for the chloride ions so it takes them decades to build up enough to overcome the passive film at the steel-concrete interface. The high pH of the concrete matrix causes this passive film, and it takes either acidification or chlorides to defeat it. Concrete bridge decks and roads in cold regions that use deicing salts are also damaged by chlorides.

Reinforced concrete protected from the weather in buildings would not have a 100-year lifespan forecast. If the building is properly maintained, the concrete should last much, much longer than that. I say "properly maintained", because of the Surfside Condo collapse in Florida. There a leaking plaza deck, lack of maintenance, and design flaws (columns too skinny) led to a tragic collapse of a building.

[toss1]

What baffles me (as someone working in Advanced Composites) is why not simply get rid of the steel, the steel-concrete interface, the chloride ion and corrosion problem altogether — by instead using rebar made of composites?

Rebar of fiberglass, carbon fiber, basalt, and other combinations is all redily available and has known properties.

The key issue is that steel will corrode, then expand and put the concrete in tension (which concrete sucks at reacting), causing the concrete to crack, then spall off. None of the composites do this.

Yet, despite composites being available for years, and even being cheaper than steel [0], they are being picked up at remarkably slow rates.

It seem blindingly obvious to me that everyone should have just switched some time ago. Yet, this has not happened. Why?

[0] https://ernestmaier.com/is-fiberglass-rebar-more-affordable-...

[joelegner]

Alternative reinforcement is an area of ongoing research. There are issues with cost and stiffness of the more exotic types. Research is probably slower because of the need to at all times be confident our structures are safe. We cannot freely experiment with exotic reinforcement in the built environment. It needs to be proven, first analytically then in the laboratory, then in pilot projects, and then in the legal and political forum before adoption in life safety critical applications like bridges and buildings.

[toss1]

Yes, I certainly agree that validation research needs to be done, and done absolutely solidly before wholesale switching .

Yet in this case, the existing tech is known bad (although TBF, the how-bad is well-characterized), and the new technologies are already qualified to fix this bad tech, e.g., carbon-fiber re-wrap of disintegrating steel-rebar bridge columns [0,1,2].

Certainly seems that applications like road-bed construction that require rebar, where the worst-case is a part of the road gets potholes prematurely, vs abridge or building collapsing, should already be mandated to use composite rebar. That would significantly increase the data set that can be used for real-world aging studies, with minimal risk?

[0] https://www.hj3.com/blog/dot-bridge-column-repair

[1] https://www.advancedfrpsystems.com/how-to-repair-concrete-co...

[2] https://hydratechllc.com/resources/case-study-dot-bridge-con...