# Reinforced concrete by steel, poor durability, why?

I've been reading up about reinforced concrete, as it is one if not the cheapest way how to build (with some solid foundation). I found out that it actually has quite poor durability (50-100 years or something). The reason for this, if I understand correctly, are the steel rebars, which work inside the concrete because of corrosion and deteriorate the construction by cracking the structure. If I also understand correctly, the rebar is used to make the structures more flexible and easier to build and bigger structures simply cannot be build just by using concrete (since then it would probably be easy to get longer durability as Pantheon seems to do just fine).

It might be completely dumb question but why they don't use stainless steel instead? From what I've read the cost of stainless steel is only about 4-5x more expensive. I did some very rough calculations and considering the price of land, price of work, price of concrete, price of everything what you put in the building afterwards like other engineering materials, the cost of building demolition in its life cycle and so on it seems like financially very shortsighted decision.

Or is it the case that the stainless steel would not prolong the lifetime so rapidly? If so, why?

(stainless steel should not corrode and the thermal attributes of steel and concrete seem to be almost the same)

• Do architects build for the now or for longevity? Are the cathedrals built in previous centuries likely to last longer than the sh1t designed and built now and they are already old? Dec 4, 2021 at 9:51
• Four to five times the cost can be a significant cost deterrent.
– Fred
Dec 4, 2021 at 12:33
• Only 4-5x more expensive? What in your life would you still buy if it increased in price by 500%? "it seems like financially very shortsighted decision." Only if your construction budget, maintenance budget, and demolition budget are the same budget and you are the person responsible for all three. Silly? Maybe. But there are much worse examples of that than concrete rebar. Dec 4, 2021 at 18:27
• Fred @DKNguyen I understand the point that immediately paying more is less appealing and also if you are not the one responsible for the future problem, you might not care. But it is only increase in one particular part which itself makes a fraction of overall cost so I think this could have been regulated as a rule. In the country I live these constructions will start to be decommissioned in 20-30 years but most of the people still live in them and will live in them in that time. Thus, I would say it was shortsighted to not think about it. Dec 4, 2021 at 19:06
• The steel alloy is selected to match the thermal coefficient of the cured concrete mix. Then the amount of rebar is computed. There's more steel in concrete than people realize. Sometimes 20% by weight, usually about 7%. If the steel costs 700 dollars per ton and the mix costs 70 dollars per ton, you see the problem. The common portland concrete also has a life, and it is matched to the steel and overall requirements. Getting portland to last more than a century gets expensive. Dec 4, 2021 at 21:15

1. Rebar is enclosed and surrounded by concrete, why rebar rust?
• Concrete is a man-made material, a mixture of non-cohesive ingredients bonded together by cement to form a composite, which is prone to crack because of its comparatively weak bonding strength compared to materials that are bonded by nature. Cracks supply the oxygen and moisture, that are essential for steel to rust, expand its volume, and push the crack to open up even wider until the structure has failed. So, is the reinforcing steel the sole material to be blamed for the failure, or the shortened service life?
1. Isn't the rust-resistant nature of stainless steel attractive? The answer is "Yes and No". We don't need to explain why "yes", but we shall closer examine the answer "no".
• As mentioned in the paragraph above, reinforced concrete is made by bonding the aggregates by the bonding agent - cement, which also adheres to the deformed surface of the rebar to prevent slippage to occur and make the reinforced concrete an integral element. Can stainless steel be processed to have a carbon-steel (rebar) like rough surface and deformation? I am not a metallurgist nor a steel producer, so I wouldn't make a judgment call, rather, if interested in it, you shall do more research on your own. Now, let's compare the mechanical properties to see what the answer "NO" is based on: a) Lower Yield Strength (30-58 ksi, SS vs 60-150 ksi, Rebar). b) In general, stainless steel alloys possess a wilder range of thermal expansion properties than carbon-steel (rebar), which is closer to that of concrete, thus the better compatibility between the rebar and concrete than stainless steel and concrete. c) Cost.

Over the years, many efforts and improvements have been made to reinforced concrete to concur its weakness and lengthen its service life, such as epoxy coated rebar (green rebar) and fiber/polymer reinforced concrete, which all renders the stainless steel less likely to be considered a suitable replacement for the traditional rebar.

• There are grammatical errors that confuse what you write - could you check what you are trying to say? Dec 4, 2021 at 18:28
• @SolarMike Can you point out the problems which can be many? My mean points are two folds: 1) Explain why rebar rust. 2) Why stainless steel is not likely to be considered the alternative to rebar on most occasions. Following these points, you can edit my writing to make it more readable/representable if you wish. I'll appreciate your help. English is my weakness.
– r13
Dec 4, 2021 at 18:42
• "Doesn't the rust-resistant nature of stainless steel attractive?" "Isn't the rust-resistant nature of stainless steel attractive?" or "Doesn't the rust-resistant nature of stainless steel make it attractive?" Dec 4, 2021 at 18:44
• Corrosion of reinforcing steel steel is a problem for the longevity of bridges constructed where deicing salts are used. Where I am located in Ontario Canada, The Ministry of Transportation (MTO) has issued a memo stating that Epoxy Coated Rebar is no longer to be used for bridges citing there decades of research showing that the performance between Black and Epoxy Coated is negligible and not worth the premium paid for the epoxy. In lieu of Epoxy the MTO uses either stainless steel rebar, or GFRP bars. The SS rebar has the same deformed ridges as black rebar. Dec 4, 2021 at 19:25
• @eXPRESS Exactly. Lower yield strength needs to be made up by enlarging the beam or adding more steel, which potentially causes other headaches even economy is not much of a concern.
– r13
Dec 16, 2021 at 16:34

Stainless steel rebars and galvanized rebars are indeed used in marine structures. Epoxy coated rebars are used for milder exposures. This is especially where concrete is exposed to chlorine from seawater, de-icing, or other chemicals that encourage a voltaic cell to be formed.

The cell contains four elements, cathode, anode, electrolyte, and closed circuit.

They use stainless steel only on the layers of the rebar close to the exterior surface of the concrete to keep the cost down. But care must be taken not to allow contact to steel rebars not to create a current and rusting.

Galvanized rebar has the advantage that even if some rusting occurs, it doesn't bulk up like the case of steel leading to cracks, it migrates through the bulk of concrete, not causing cracks. In the photo below white dots sprinkled down are rusted zinc.

.

• Thanks, the galvanized rebar is interesting. Do you maybe know how big cost difference is there, or why it is not used more regularly? Dec 16, 2021 at 16:19
• galvanized rebars are around 50% more expensive. it varies abit with size of the bar. Dec 17, 2021 at 6:15
• Thanks, then it is to very suprising to me why they are not used instead everywhere. According to the information I found, their structural attributes are not lessened to the normal rebars. Dec 18, 2021 at 13:57

Carbon steel rebar is fine for the great majority of concrete structures. Salt water is a problem so if you are building a sea side dock the much more expensive stainless is warranted. Epoxy and other coatings are used in less demanding situations. Generally the concrete is well encapsulated and so protected from temporary situations like deice salt used on concrete roads. Think about the next step for your idea; Nevermind the rebar , what about the exposed steel like the Golden Gate bridge ,it could have been made of stainless ( ignoring strength and welding problems) if the builders had not been shortsighted.

• I know nothing about how much you can protect the steel by protective paint (but what I took from one answer above, and you mention it also, the SS provide much less support - which I guess is also reason to not use in concrete buildings). For the bridge the cost difference would however work different as it is 100% metal and other costs also differ. Also, I guess having the steel inside the concrete poses more complicated problem. It is not that the steel that is completely useless after 50-100 years, at least as I understand it, but the damaged concrete foundation (or maybe both). Dec 16, 2021 at 16:31

### The answer is found in Engineering Economics

The time value of money tells us that money spent 50 years in the future is almost negligible today. Doubling construction costs today would be equivalent to an astronomical spend that far in the future. This is why roads, buildings, bridges etc. aren't designed to last 50/100/500 years.

This is good design practice. The parthenon is great, but a big open room without a roof isn't terribly useful, except to look at.

• Just one comment, ideally, the reinforcement concrete structures should be last as long as possible to offset the demolition and rebuilding costs. The bridges and dams are usually designed for a service life longer than 100 years. I think the concrete tall building won't be an exception.
– r13
Dec 6, 2021 at 19:10
• TigerGuy, I understand but I don't really agree. If your hypothesis would be correct (in housing construction), it would mean that now it should be cheap and effortless to build new homes, however it is not true. I don't think building constructions is the same as for example manufacturing new processors, where if the processor would last for 20 years, nobody would use it anyway. What you need is a roof over your head, this did not change for centuries. Simple shoebox for people and why pay for demolition and rebuilding of the same if it could be avoided as @r13 points out. Dec 16, 2021 at 16:16
• @express, modern homes are not built to last centuries. In Japan, homes are torn down regularly to build new ones; the government dictates the design life to be around 20 years. These are just choices. Certainly, most homes built 200 years ago are long gone. Dec 16, 2021 at 16:35
• @TigerGuy I guess Japanese with their 250% debt to GDP can afford it but states with little more sensible fiscal policy might not. :D Just kidding, I get your point, but if that is the decision then how come people are struggling to get a place to live in my country. Of course this gets off the topic completely, but if such choice is made, it must be supported by actions mitigating the fallout. That lead me to ask the question in a first place, to understand if there are engineering limitations enforcing this (and it seems there are, as user r13 points out in his answer). Dec 16, 2021 at 16:44

There are multiple avenues through which this can be approached.

One is the economics of the construction. As you've mentioned, concrete structures are far cheaper to build than steel, though the difference is highly dependent on location and context. See my answer to another question for more details on this.

Another which I also mention in that answer is that concrete requires effectively no maintenance during its service life, so long as it's done in a minimally competent manner. Steel, on the other hand, requires frequent maintenance vs. corrosion.

And while I've never seen a life-cycle cost analysis such as you've done, let's take your results at face-value, that concrete and steel end up costing roughly as much. But then, as mentioned in @tigerguy's answer, you need to remember that costs 50 years in the future must be discounted into relative irrelevance.

But still, even if you disagree with DCF, you need to question your primary assumption. Your entire point is that it's a problem that we need to demolish that structure 50 years from now. But is it a problem? Or is it an incentive for society to demolish that building and make one more adequate for its needs 50 years from now, using technology and installations more reasonable for that future? If its a bridge, it's an opportunity to build one that better matches demand at that time: if we're all in flying cars, demolish it and build a park instead. If we're still earth-bound, make it wider to accommodate ever more traffic (a bad idea!) or incorporate more modern safety measures, etc.

After all, when's the last time you've seen bridges or other infrastructure proactively updated to incorporate new safety measures or what have you? In my experience, having the bridge look like it's about to collapse is quite useful when trying to get politicians to modernize infrastructure.

• I agree with DCF and the other things but I think that there are some assumptions made which don't apply here. First there will be not much innovation to bring the cost down and this have shown already. Secondly, I don't look at it from the perspective of an individual/group and now but rather the system and long-term sustainability. Thirdly, at least now, it seems you will always need a roof over your head. And I don't really feel the new building bring any important innovations except energy efficiency. Only thing I see is increased price tag and actually less living space in them. Dec 16, 2021 at 16:39
• @eXPRESS I don't understand what you mean by "not much innovation to bring the cost down". Is that in regards to DCF? Because DCF doesn't require lower costs to make sense. It's a simple observation of the fact that I (and society as a whole) would rather spend \$1 today and \$1 fifty years from now than \$2 (or even \$1.01) today. And if you don't feel new buildings bring any important innovations over fifty years, I'd recommend moving into unrenovated buildings from the 1970s during a natural emergency or fire and/or with a physical disability. Oh, and careful not to breathe in any asbestos.
– Wasabi
Dec 16, 2021 at 20:53
• The way I understood the DCF is that, first it is observed from a personal or grouplike perspective (because you/group cares more about their profit now) and the reason of the deprecation is always because you can either get better returns elsewhere or avoid uncertainity (e.g. that you will cash out before dying). Also me traveling to Bahamas now (when young) is ofc better than traveling twice when Im old. The better return could be investment to technology or assets which yield me profit or ability to build cheaper, some benefit. However this does not really apply here. Dec 18, 2021 at 14:02
• To the other part: Im currently living in such building. Because of cheap renovation, there is no problem for you if you are disabled, no natural emergencies pose a threat (yet, cause of the concrete issue), everything is according to regulations and there is no asbestos. I simply cant see any reasonable difference and Ive been in new flats and talked to owners. Also the renovation cost will also come to nothing because of poor foundation, since no other problem is there. Otherwise, the idea about being outdated doesnt apply. (ofc people 50 years back might not know it will improve so little) Dec 18, 2021 at 14:08