Ahh... but what if you purposely built/added strategically placed threaded rods and such. I'm sure they would have to be somehow enclosed in a sleeve, a bit protected from the cement mixture. After the cement has setup/cured, slap on plates/washers over them and then add tension to provide for cyclic operations.
My thinking was more towards building a light weight inner skeleton and let the assembler build the simple outside forms from whatever they like. You would simply provide suggested measurements for them to use for the forms.
With fuel costs/shipping most likely to start marching upwards again, seems like you could save a lot in the weight area with some careful thought/engineering.
This is just off the cuff thoughts... needs a lot of revision :)
That's the "screw-thread" approach to post-tensioning. It has been used for small projects. My first experiment with PT, over 30 years ago, was done exactly that way.
Post-tensioning is usually done with plain steel rods encased in plastic tubes, and a hydraulic jack, with a strain gage to determine when the elastic limit of the steel has been reached. At that point a clamp is squeezed onto the rod, and it applies the full elastic potential of the steel to the concrete.
It's tougher with threaded rod because the rod has to be aligned and kept very straight, or the threads catch and drag the plastic tube. It works OK for short spans. Even if you thread the ends of smooth rod, it's more difficult to determine the elastic limit. But it can be done. They use torque sensors on the torque wrenches used in building car engines, for just this purpose.
All of this has to be done after the concrete has cured, of course, and there is some relaxation of the tension as the concrete shrinks. All of this is part of the engineering calculations. In something as small as a machine tool it's fairly trivial. The best bet is to wait a month or so before tensioning.
OK, here's a simplified description of what you're dealing with, using that approach. First, the steel has to be quite close to the surface of the concrete to do any good in terms of *ultimate failure*. However, with no pre-tension, the concrete will be vulnerable to surface cracking from cyclic loads, or just from tensile loads.
The usual solution to this is to avoid putting the major loads on the concrete. Those French and Italian machines I mentioned use the concrete mostly to stabilize the steel elements, preventing them from buckling by applying mostly compressive loads to the concrete, from the sides of the steel elements.
Another way to deal with it is to use fine mesh just under the concrete's surface. You get something like a ferrocement skin that way. It doesn't completely prevent cracks, but they're very fine and very shallow, and have no significant effect on the strength or stiffness of the structure. The wire mess acts as crack-stoppers, so the most you get is a little crazing of the concrete surface.
Either way, unless you use multiple layers of mesh, the strength of the structure is really just the strength of the steel that's in it. That's not necessarily bad, but it takes a lot more steel in the combination to do the job that way, because you aren't taking advantage of the concrete's compressive strength, except the light loads that are employed to keep the steel elements from buckling. And the steel will not stay attached well to the concrete if the loads are high. The bond will be subject to high sheer loads.
I'm guessing that most people here know how prestressed concrete works; post-tensioned works the same way, basically. Ferrocement is more like fiberglass in polyester or epoxy resin. The extremely short spans of unsupported concrete -- fractions of an inch -- do not get sufficiently loaded in tension for them to fail. The steel mesh comes into play as soon as tension is applied, because of the short spans. Unlike prestressed or post-tensioned structures, however, there is no pre-stressing on the mesh. So ferrocement has more compressive strength than tensile strength. It's strong enough, however, that thin sections of it actually can be bent and they spring back.
Well, you're on the right track. Lower shipping costs is what motivated the French and Italian machining centers. Also, I think that Hardinge made some machines this way a while back. It's not a bad idea but it results in a heavy machine, once the concrete is poured, that still has a lot of steel in it.
What I'm talking about is somewhat different. Stressed-steel and ferrocement produce a concrete structure, not just one that's stabilized with concrete.
This book is the best on the subject. Take a look at the chapter descriptions, and you'll see what it's all about:
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There are comparable books on prestressed- and post-tensioned structure. This one, by the same U of M prof as the one above, covers the field:
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There is a ton of free information on the web about both, but you have to watch out for the info on ferrocement. It's become the darling of the greenie-save-the-Third-World types, who want to make it out of spit and bamboo shoots.
Yup - that's post-tensioning, an extremely effective (and adjustable) method for reinforcing concrete by placing it in compression. Works great right up until some idiot decides to cut a hole in the floor and chops though a tensioning cable (Third hand at best, but I gather it happens occasionally)
In mulling this a bit more, one thought for dealing with differential expansion on long embedded metal parts (ie, ways that you are "beefing up" with concrete) is to arrange so that the part is only locked in at one point, and is physically captured by shape (a dovetail, say) but free to slide (even hit with some mold release wax before casting) elsewhere.
One thing mentioned rather early in the referenced thread was effective damping by casting epoxy granite (I think) inside innertubes or rubber bags inside hollow section steel. While it would not add much to strength, the simplicity of fabricating a frame from large square section steel and then damping it by stuffing the box sections with material might be a worthy low-end route. IIRC the rubber-encased was better than direct cast, probably due both to damping from the rubber, and not having the voids that will develop due to differential expansion of direct-cast material in a steel tube - the rubber would allow for slippage with expansion/contraction.
Then there are the granite tools which slide granite over granite on air bearings...but those seem rather far out for DIY. What I read (I don't claim to have read every post) of the threads at CNCzone seemed to have a lot of churn and not much experimenting, at least not in a functional way. The guy who's still fussing over epoxy formulations 2 years on and has evidently never even made a test sample with aggregate is over-fussy or something like that. The one guy participating in the thread (Walter?) who seemed to be willing to toss some stuff in a form and see what happened evidently gave up and moved on. AFAICT the guys in Germany that were doing this and inspired the 2-year, 3000+ post thread have been happily making actual stuff with the process while CNCzone has pondered its perfection at length...but I could have missed something.
Sorry, I didn't notice that those URLs were the same. Just click on the book titles and it will take you to listings of the chapter headings. Then click on a chapter and you'll see the subjects covered. It's a five-minute course in the engineering of both materials.
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