Concrete

[RevDisk]

Picked up an interesting book, Concrete Planet.

http://www.amazon.com/Concrete-Planet-Fascinating-Man-Made-Material/dp/1616144815

Lot of history and interesting stories on the development of concrete over the thousands of years. Roman concrete was covered extensively, as you'd imagine. A lot on the rediscovery of concrete. Then onto modern usage/architecture. Most useful and interesting part of the book was "Why we're in trouble". 

Roman concrete wasn't as strong as modern concrete, but it is more durable. Rebar is the weak point. Ditto high strength quick setting concrete. Specifically, our infrastructure is screwed because we're building structures that will maybe last 100-200 years, with significant maintenance costs. Steel rebar is awesome until it turns into rust. Then it's a liability. Quick setting concrete makes the rebar problem worse, and cracks easily. We already ditched fast setting concrete. Rebar alternates are more problematic. Rebar concrete lasts maybe 100 years. Maybe. Epoxy coated rebar is ok, doubles or more the lifespan. Fiber glass polymer is good for 200-300 but can't be bent on site. Fine for road beds, tho. Aluminum bronze alloys are best. 500 years plus, but copper is expensive and rare compared to iron.

I plan to do some concrete work in the semi near future. Basically, intend on making some of my own pavers and retaining wall "bricks". Anyone have suggestions, recommendations, insults, jokes, sources, reference material?

[Tallpine]

I plan to do some concrete work in the semi near future. Basically, intend on making some of my own pavers and retaining wall "bricks". Anyone have suggestions, recommendations, insults, jokes, sources, reference material?

Form release fluid.   

You know about those wire ties with the loops on each end, and the little twisty hooks?

[Fly320s]

I'm also interested in concrete, at least for now. Specifically, poured concrete foundations for houses and the effects of winter time pouring.

[Devonai]

I'm still holding out for nodular propagating closed cell nano-tubular carbon fiber self-terminating programmable foam lattices.

[MechAg94]

100 years is a long time.  I guess Houston isn't that old.  Other than roads/ bridges, what structures would be an issue?

[RevDisk]

100 years is a long time.  I guess Houston isn't that old.  Other than roads/ bridges, what structures would be an issue?

Buildings, tunnels, retaining walls?

[Sergeant Bob]

Buildings, tunnels, retaining walls?

Buildings indeed. Concrete is the new steel.

[geronimotwo]

I'm also interested in concrete, at least for now. Specifically, poured concrete foundations for houses and the effects of winter time pouring.

i'm not sure about concrete, but the additive they put in mortar to allow for cold weather use tends to bleed out over the next century leaving mineral like stains on the bricks.

afaik, concrete can be poured until the ground freezes solid.  otherwise, there would be the danger of building when the ground is heaved leading to warm weather settling.

[Gewehr98]

Having seen the Roman aqueducts up close and personal, they did make some good concrete!

[RevDisk]

I'm also interested in concrete, at least for now. Specifically, poured concrete foundations for houses and the effects of winter time pouring.

Colder is worse than warm. There's certain cutoffs you shouldn't approach. There are additives for cold weather pouring, but I'm not familiar with any studies on them.

[zahc]

My grandpa hand-poured his whole back patio/veranda area with vertical walls up to 5 feet tall. One single section of it crumbled and fell apart, exposing the rebar inside. He would always look at that section and remark that he wished he knew what he did differently that caused it to fail.

[280plus]

I'm not up on the details of cold pours anymore but I do know they are more expensive due to additives and use of hot water to mix rather than cold. Also, big problems pumping with trying to get it to flow through the pipe(s) without freezing and plugging. Then I know we had some issues with the surface flaking (spalling) because it didn't stay warm enough while it was curing. We did everything we could to prevent this but it still happened in a few spots. Much better to wait till it's warm.

[Hutch]

On the other hand, I was astonished to learn that freshly mixed concrete, in the truck, is exothermic, generating heat.  Too much heat is bad for the mix.  An acquaintance was a contractor for fed.gov, and rejected many, many truckloads of concrete, in Tejas, in summertime, for being over allowed temp.  He averred that the contractor would have multi-hundred point blocks of ice put thru the equivalent of a tree-shredder, the discharge going into the trucks as the mix is generated in them.  I don't know if I learned something that day, or if my acquaintance learned that I'm gullible.  I never bothered to follow up w/ research.  It's a great story, so it ought to be true.

[280plus]

I have heard of adding ice to the mix.

[MechAg94]

So the choice is to pay a whole lot more for longer life or risk problems with old structures.  Somehow, I doubt the higher cost option will win.

[Hawkmoon]

On the other hand, I was astonished to learn that freshly mixed concrete, in the truck, is exothermic, generating heat.  Too much heat is bad for the mix.  An acquaintance was a contractor for fed.gov, and rejected many, many truckloads of concrete, in Tejas, in summertime, for being over allowed temp.  He averred that the contractor would have multi-hundred point blocks of ice put thru the equivalent of a tree-shredder, the discharge going into the trucks as the mix is generated in them.  I don't know if I learned something that day, or if my acquaintance learned that I'm gullible.  I never bothered to follow up w/ research.  It's a great story, so it ought to be true.

It is true. IIRC, the max temperature on the project I've been on for the past two years was 85 degrees F. (for the concrete, irrespective of air temperature). Adding ice to the truck at the batching plant is one of the ways the suppliers use to try to keep the temperature down. Even with ice, if the batching plant is an hour from the site and the truck gets stuck in traffic -- somebody gets to make a whole bunch of mafia blocks.

The exothermic reaction isn't limited to in the truck, however. It's that exothermic reaction that cures the concrete. The heat is generated by the chemical reaction between the cement and the mix water. Temperature has to be controlled after placement, too, so the mix water won't get cooked out before the chemical reaction has been completed. This is why slabs poured in summer are typically covered with burlap and then soaked with a sprinkler.

[zahc]

There is an urban legend that the Hoover Dam is still cooling off.

I'm not sure about that but I do know that for really big structures like that, they cast in cooling pipes.

[Boomhauer]

There is an urban legend that the Hoover Dam is still cooling off.

I'm not sure about that but I do know that for really big structures like that, they cast in cooling pipes.

Urban legend. When Hoover Dam was built, it was built as a bunch of blocks poured separately. Pipes were put in each block and then chilled water from an onsite refrigeration plant was pumped through the lines. After the concrete had cooled, the pipes were then pressure grouted full and sealed off. The engineers had calculated that "normal" concrete pouring methods would not have worked, so they poured in blocks and used the water cooling trick.

It is also an urban myth that there are bodies of workers who fell in concrete and were trapped in the hoover dam. The pours in each form were not that deep.

What is true about the concrete at Hoover Dam is the concrete is getting stronger as it ages.

[Hawkmoon]

What is true about the concrete at Hoover Dam is the concrete is getting stronger as it ages.

I don't know if that's still true. The benchmark for testing concrete compressive strength is 28 days after the date of placement, and all construction specifications are based on the 28-day strength. However, it is well-known that, up to a point, the concrete will continue to harden beyond 28 days, although the rate of strength increase slows significantly. The highest gains in strength are in the first 7 days.

I'm not an expert in concrete technology, but I think there must be a point at which the hardening stops. And I'm pretty sure it was a long time ago for the Hoover Dam.

http://www.concreteconstruction.net/concrete-articles/ages-for-strength-testing.aspx

http://www.ce.memphis.edu/1101/notes/concrete/section_3_properties.html (Note the graph)

http://www.nrmca.org/aboutconcrete/cips/39p.pdf

[Boomhauer]

I'm not an expert in concrete technology, but I think there must be a point at which the hardening stops. And I'm pretty sure it was a long time ago for the Hoover Dam.

Concrete samples from the dam over the years have shown it to be getting stronger over the decades since it was built

[Firethorn]

I don't know if that's still true. The benchmark for testing concrete compressive strength is 28 days after the date of placement, and all construction specifications are based on the 28-day strength. However, it is well-known that, up to a point, the concrete will continue to harden beyond 28 days, although the rate of strength increase slows significantly. The highest gains in strength are in the first 7 days.

Not a concrete expert either, but I remember that it depends on the type of concrete.  There's been a number of threads here and elsewhere that goes into the differences between modern and ancient Roman concrete, how modern stuff is stronger but the ancient stuff more duarable, etc...

Different types of mixes mature differently, I figure the 28 day strength thing is where the general balance of time-cost from keeping the forms/temporary supports up(or otherwise not loading the structure) exceeds that of just pouring more concrete.

Thus the concrete in the hoover dam is still 'getting stronger', it's just that it's not getting signficantly stronger.  Then again, an average of 10% of the original 1-month strength a decade since construction completed would still add up to 'significant'.  In addition, they 'happened' to not use reactive aggregate, so there's no 'alkali-silica reaction', such that you don't get degregation from that, seriously reducing the amount of erosion that the dam experiences.  As a result the Hoover dam is likely to last centuries at a minimum.

[280plus]

tion',   As a result the Hoover dam is likely to last centuries at a minimum.

Oh really? Then what is that dripping noise I hear? Oh wait, that's the faucet for my tub. Never mind.