3D printing of a building is still going to require forms or something very much like them, and I see no reason to expect that it would be any quicker or cheaper than conventional methods. It might use less manpower, but you’d be better served by putting more automation into the conventional methods.
I snipped your post for brevity. Thanks for elaborating, as my post was put in general terms because I’m a lazy git. In all my years in construction, I saw surprisingly few bad concrete placements. Seems like when they do go wrong, it’s a major fuck-up, though. Speaking of shear walls, I was on a construction security detail in Cairo for the new embassy. The contractor was pumping ‘crete up to the sixth floor or so, and I noted that he was dumping the mix in from the top of the 12’ form. I stopped him and told him that he was not supposed to exceed a 3’ lift. He tells me that the chute was only two feet above the form, so it was good. :smack:
Aside: Why is that? Does the liquid concrete get too aerated if it falls too far, or does it cause problems when it impacts whatever is below it (the bottom of the form or the already-poured concrete), or what?
The short answer is that it separates and loses strength, which I talk about near the bottom of this response (which is not a short answer).
Notes about 4 foot max lift in a day, is usually about block construction. The reason why is because if you go any higher before the mortar sets – the weight of all those courses will squeeze the mud out of the lower courses. There can also be a heat consideration; here in the southwest where it gets really hot, not only can you go up only a certain number of courses, but you must also wet it down every half hour so the mortar does not dry out and shrink sticking to neither the upper nor lower block (or more likely to only one of them). In California, whatever block you lay in a day (again usually four foot max.) you must also backfill. We had a job where the contractor had not backfilled in three days and had brought in a pump and had several trucks of mud going. One guy was filling the cells, and another guy was behind him several feet with very long vibrator he was running down into each cell to get out any air bubbles and compact the fill. (after they got to the end, the guy guiding the hose would reduce the flow and fill in the last few inches and a guy with a float (or sometimes just a wooden form stake) would follow and screed off any extra mud. Anyway, either a truck, or the pump bumped a wall ahead of where they had gotten and the top twelve feet of the wall tumbled on top of the guys killing one of them. Everything that was full grouted stayed upright, even the stuff they had just backfilled. The part with hollow cells came tumbling down, and it was everything except the very first lift. Here in Arizona, the seismic code is so lax, we only need to fill the cells on four foot centers, the other cells would be filled with pour in insulation (little foam BB’s), but every four feet would be a cell with one or two #4 rebar, depending- and be full of mud. The blockies would usually just use excess mortar, then the top two cells would be fully backfilled. We didn’t even have bond beams unless you counted the top two courses. That may have all changed in the last few years however.
The 3 foot lifts are sort of an older kind of construction (more below). I have heard about mud falling too far and knocking the hell out of forms, or being a hazard for those working near the pour. But the big problem is the material separating; concrete is made of Portland cement, large aggregate (rocks), and small aggregate (sand). If you open up a chute and let it fall twelve feet (plus two more feet in the case you mention), it is possible to end up with too many rocks and not enough cement in one place—and just the opposite in another place within the form. The strength of concrete is the properly mixed combination of those elements. If they fall too far they may separate and segregation (while wrong for humans) is disastrous for concrete. I believe they discovered this over a hundred years ago pouring the base of Hoover Dam. By the time they finished that project, mankind knew a lot more about concrete than they did when it started. (The heat from the chemical reaction of the concrete firing was still present when I was younger—not sure about now, but when I was a kid it radiated heat!) They had a GIANT hopper on cables, and pretty early on they figured out they needed to drop the mud as close to where it was going as possible. I believe the hopper would often be in the mud after they opened the gate, but not before.
Anyway, the reason that you had to pour three feet at a time, was because back in the day, someone had to stand on top of the form with a long pole (or stick with a 3’ metal form stake wired to the bottom of it), and drive it into the fresh pour about a hundred times to tease the mud into every nook and cranny. No one could drive the rod deeper than about three feet, so that was the pour depth. But you could do it all day long. If you have twenty column forms, each say 3’X3’ and twelve feet tall, you would pour one yard into each column (3’X3’X3’ equaled one cubic yard). As soon as the first one was poured someone started pounding it with his rod (so to speak), the second one would have a different guy (or maybe two- depended on the workforce available), the first guy or team of guys would move to the third one poured, etcetera, etcetera. You worked it out so the guys operating the chute never had to wait for the guys doing compacting and the compactors never had to wait for the chute. After all twenty columns had three feet in them, they could put another three feet on top of that- but each three foot pour had to be compacted.
Now-a-days, they have a long metal rod that vibrates real fast. You could run it down twelve feet of wet mud easy, but it is only about six feet long. It will get the mud into every little space even though it doesn’t reach to the bottom. If you do it right, the concrete says Boise/Cascade in an oval seal after you strip the forms. The secret is to only run it enough to tease the mud into every corner of the form. If you over vibrate it, it will bust the forms apart. Every now and then you will see a column with a weird little fat spot near the bottom. They have learned to keep a few ratchet straps handy, and if the form starts to separate they shut down the compacting vibrator and put a strap around the form to be sure it doesn’t continue to expand and leak out all the mud. Unless the columns are exposed in the final design, a little goof like that is usually okay.
Round here (downtown Chicago) it seems to vary from building to building which way they decide to do it. Seems like I see more pumps these days.
Concrete can now be pumped more than 600 meters. That’s the maximum height any building in the US is allowed to be.