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Hey John – Ray and I are about to replace our driveway. – 5 inches of concrete reinforced with rebar. Ha ha!
But, you probably don’t care if it doesn’t last 200 years! Skyscrapers, not so much.
I saw a documentary about the Lake Pontchartrain Causeway that included discussion of reinforcing concrete. For the pillars in that structure they kept the steel under tension, essentially stretching it, while the concrete cured. When they released the tension afterwards they got the effect of the steel, in trying to un-stretch, creating permanent compression along the vertical line of the pillar. They do the same thing to allow concrete to be used as a span in the bridge sections.
Wonder if Concretevol will show up on this one.
Nothing is harder on concrete than water in a crack turning to ice.
This is what is called “prestressed concrete“. Because concrete is strong only in compression, by prestressing the reinforcing bars, it is kept in compression even when it is not loaded. This can help in earthquakes when vertical acceleration can unload the structure.
I both welcome and fear that.
So while I’m reading murder mysteries, I’m glad you’re keeping us up on the technical literature, John! Thanks!
Yes, reinforcing steel (“rebar”) is prone to rust, and this happens especially near saltwater or in places with severe freeze-thaw cycles. This is particularly an issue for bridges up north, because the bridge decks freeze quick (air below the bridge deck gets colder than the ground below the roadway) and because they use salt to clear snow and ice from the bridges.
There are a lot of treatments for the steel to inhibit rust. You can get epoxy-coated rebar, which is a very popular option right now. You can get stainless rebar, which is really expensive. This has led to experiments with in-between products, such as stainless-coated rebar, and galvanized rebar.
Then there are a host of non-steel options that are also in play, so far only for specialty applications, but some are showing a lot of promise. There is fiberglass-reinforced concrete, in which lots of individual fibers are dropped into the concrete mix. There is carbon-fiber reinforcement.
Other approaches include chemical coatings or chemicals to mix in the concrete to reduce cracking or slow the ability of water to penetrate in the concrete to the depth where the rebar is (usually 3 inches or so).
So there is a lot of current excitement in the concrete industry.
John, thanks for this review. I have two new tabs open right now and am reading about fascinating historical projects. Cool.
Hey I think that was a microaggression….. :)
Hi Judge! Another method used to achieve this type of compression is “post tensioning”. There are variations to this as well but the basic principle is to have multiple steel tendons inside of plastic sleeves that travel from one side of a slab or beam to another. After the concrete is poured and reaches the desired minimum strength the tendons are then put under tension which compresses the slab. Post tensioning is the more common method for multistory concrete structures. Here are a few pictures of tendons during installation in a beam and the ends after the slab was cast.
The part I find extra cool is the way that tensioning done along one dimension will provide increased strength in the others. For example the bridge span is tensioned end to end, and that makes it better able to support the force of the weight rolling across which is applied at 90 degrees to the tensioning.
I’ve estimated jobs that have what is essentially fiberglass rebar around MRI installations.
Do you have any feelings about this error, John? ;-)
Do you think that much of our downtowns will be here in a couple centuries? I’d have thought that we’d want a lot more rebuilding after the Roaring Twenties take hold; construction costs will be lower, and the benefits greater. It seems likely that this will hold in a century’s time, too.
I am a frequenter of Ohio Stadium, which was completed in 1929. Although its original outer surfaces have now been covered due to an expansion project about 10 years ago, building professionals marveled at the lack of spalling (surface deterioration) on the 80-year exposed concrete surfaces. It was explained to me that the curing techniques used in 1929 were superior than those today, likely due to the expense of taking the time to are the concrete properly. This a more recent example of building techniques with concrete lost over time.
Yes that is cool. Notice also the profile of the tendons in that beam picture. They are engineered to provide lifting force when tensioned mid-span (between columns) of the beam. This is done on slabs as well.
Someone should write a concrete post or 2 to discuss this further! (shameless I know)
Having been completed in 1929, it would have been built before the introduction of “high strength” concrete cement, which came onto the market in the early 1930s according to the book. The new cement had greater compressive strength, cured quicker, and was easier to pour. It was not until 1987 that a study revealed that this cement cracked at a faster rate than the older formulation, allowing the rebar to rust. In the late 1980s and 1990s improved formulations were introduced to reduce these problems, but we were left with a legacy of 50 years of less durable concrete in place.
The Ohio Stadium was lucky to have been built before the new concrete appeared.
This is discussed in chapter 10 of the book. Glass Fibre Reinforced Polymer (GFRP) rebar is stronger than steel and weighs one quarter as much. Besides MRI or other applications where electromagnetic considerations are important, it has been used in roadbeds and bridge decks. It costs more, but increases life and reduces maintenance cost. One problem is that unlike steel rebar, it cannot be bent to shape at the job site, but must be ordered pre-bent. This is a problem in projects which do not have precise blueprints.
Carbon fibre rebar is also being tested. It seems to have many of the same properties as GFRP. There is also a non-ferrous rebar made of a substance called aluminum bronze, which can be bent as needed and is 35% less expensive than stainless steel.
It isn’t so much the skyscrapers (although I suspect the owners will not be happy if the Petronas Towers or Burj Khalifa have to be taken down and rebuilt after 120 years) as all of the mundane highway bridges which may not make it to 100 years, especially in climates with freeze/thaw cycles and where deicing salt is used. In addition to the cost of eventual replacement, there’s an ongoing cost for inspection and repairs, which is something politicians are tempted to neglect in favour of shiny new projects. When people talk about “crumbling infrastructure”, literally crumbling reinforced concrete is part of the problem.
Among other things, I love building with my hands (currently working on an outbuilding). And I spend time futzing with concrete – often using rebar or scrap in postholes and foundations.
Yesterday I poured concrete over some rolled-up scrap chainlink fence to provide a minor support role. The result was tough, indeed.
I am afraid that I build much like I cook, which is to say, I never like repeating a recipe. On the other hand, I keep learning cool things.
I learned just today how modern engineering is making things much cheaper and easier.
Here is an example: roofs.
They used to have these whopping big rafters – 2x12s or 2x10s. Expensive, hard to handle, and not that strong.
Now thanks to a better understanding of how best to use wood and steel, factories pump out “custom” trusses that are made of mere 2x4s shaped into triangles. Huge improvement in every respect.
I see there is an update just last month to the notion that the Egyptians invented concrete:
http://www.geopolymer.org/archaeology/pyramids/are-pyramids-made-out-of-concrete-1/
I have no informed opinion about this subject. I just find it intriguing and wouldn’t be surprised if it were true.
The Pyramids have been proven to be not built of concrete? Aw, darn. That was a favorite theory of mine.
A 2008 study examined stone from the Giza pyramids and found that it contained numerous fossils of marine creatures in the sedimentary rock. The calcination and fine grinding processes required to make cement destroy all fossils, so fossils are never found in concrete (apart from in large pieces of aggregate, which do not occur in the pyramid stone). Further, the species and distribution of the fossils in the pyramid stone match that of the limestone quarries where the pyramid building stone is believed to have been cut. There are also visible tool marks on the pyramid blocks which match those found in the quarry.
As to the samples tested in 2006 and found to resemble concrete, Zahi Hawass, Egypt’s Minister of Antiquities at the time, observed that the samples were taken illicitly and exported illegally from Egypt and hence were not taken under the supervision of an egyptologist knowledgeable about the Giza site. He further notes that the lower levels of the pyramids have been repaired from Roman times to the present day, often with concrete. It is thus entirely possible the sample tested was from one of these repairs.
To me, the most persuasive argument against the concrete pyramid theory is one not mentioned in the book. The Giza pyramids were built early in Egypt’s history, with many more monuments built subsequently. None of those monuments show any evidence of the use of concrete. Over Egyptian history there is a continuity in building technology with no evidence of “lost technologies”. So why, if the Egyptians had a mature concrete technology which worked on the scale of the pyramids and so simplified their construction, did they never use it again in the millennia which followed?
Most highway bridges in America are required to have an inspection every two years. This requirement was broadened in 1991, so that is why there were so many surprises during the 1990s regarding bridge conditions and the looming costs of infrastructure rehabilitations.
The worst of those bridges got addressed. There are still many awaiting major rehab projects. The fact that transportation funding has been in decline throughout the new century does not help the situation.
Sounds interesting, I got a copy of the book.
Any comments on the problems noticed on the Oakland Bay Bridge?
You! You and your . . . facts.
Could be that the Ancient Egypt equivalent to CIA deemed the possession of this technology as such a “game changer” that they classified the technology as Top Secret. Bureaucracy then took care of the rest.
These new innovations are incredibly exciting. Wonder if graphene will ever be tested as a wonder additive.
John, I want to thank you for this post from nearly two years back — I bought my dad the Concrete Planet book and he loved it. He’s a hard guy to buy books for, since he does not like fiction (!!!) but he found this book deeply fascinating and just finished it. (It took him more than a year to start it, but once he did, he really enjoyed it.)