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Concrete isn’t my thing; I have a little experience with it, not much. I have to make a mount for an African sculpture, and I think it would be best to cast it in coloured concrete. The sculpture is in stone; picture a rough cylinder about 10″ in diameter and about 54″ high. The mount would have to be about 22″ tall to put the eyes on the sculpture at a a certain level, a basic rectangular shape with a footprint say 12″ by 12″. I suggested a pyramid shape to broaden the footprint, but no, the clients want a simple shape.
my questions are : first of all, the math. By my calculation, based on a five bag mix, 12 x 12 x 24 works out to about 250 lbs, unless I’m missing something?. The sculpture weighs about 300 to 325 lbs, so you have up to 575 lbs concentrated on a 12″ square area. The house has oak hardwood floors. Maybe it should have a wider pad to spread the load at least over two joists? Any thoughts on ‘tippability’?
Finally, what would be recommended for interior reinforcement? A mesh armature, or rebar? The sculpture and the base will be connected with a steel rod.
Any ideas gratefully received.
Replies
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Yo,
I'm no concrete expert either. (I didn't think it would weigh that much, but I see it's not a cylinder but a rectangle) I do think this could be kind of tippy, and if it fell on a kid or a pet, lights out.
It wouldn't hurt to use some rebar, but I doubt if it would be necessary.
Weird project, eh?
MD
*Hi Adrian, I'll put a couple of cents in the pot.Add another 50 pounds to your weight calculations. Rule of thumb, fairly accurate, concrete weighs in at approximately 150 pounds per cubic foot. If the sculpture is physically balanced and securely attached to the center of the base I don't think it will tip anywhere without some determined influence. I'd ask again if you could make it a little wider, just for safety.As for the point load, subfloor, finished floor and joists act together to spread the load out. Picture three very large football players standing in one spot, talking - no problem. The concern is that the load will be continuous and over time, a long time, would cause some permanent deflection beyond the norm. Maybe they could locate that thing directly over a joist which is directly supported from below by a wall or column or maybe put it in a corner or next to a wall on the end of a joist supported by its bearing point.Since it will be a static display and only under extremely light compression (for concrete) you probably don't need any interior reinforcement. I hear some builders don't even use steel in their foundations. I assume you'll epoxy the joining rod or use hydrolic cement for a firm connection. Personally, I would put either a couple of short pieces of rebar across the form or some welded wire just in case. You can always parge over cracks but you have to start over again if the base should crack down the middle. If that sculpture is not balanced then there could be a slight stress on the joining rod and if they are moving it around the room to find the decorator's preferred point for viewing there could be more (just a devil's advocate thought).Hope that's helpful.
*Adrian, A six foot high stone/concrete mass sitting on a 12" square base and weighing almost 600# would scare the hell out of me as far as a tipping threat!The base would not add any stability to the sculpture since they are basically the same density and size. I agree that you need a wider and probably heavier base and may need to support the floor from below if it gets too heavy.I would also use a richer mix of concrete. It would color and finish better. You could easily incorporate light re-rod from the points of your pyramid base rising into the center.
*Two thoughts-Use Ferro Concrete, or some such light weight stuff over a hollow mesh form, greatly reducing weight-make it outa wood and Faux finish it to look like concrete.-pm
*Maybe fasten a pc of painted steel or stainless plate to the bottom of the base to help stabilize it. Something like a 30x30". If you faux it, see if you can lag the base to the floor. That sculpture weighs a %$@#load. Best of luck.
*I'd put something under it besides a couple of joists. the concentrated weight of the thing bothers me. Unless the floor joists were beefed up a lot, I'd rather see a post or 2 under the thing.
*Sounds like a job for some sort of home made EIFS stuff.If solid, a 6' high, 12" diameter sculpture and base weighing 600# could be pushed over with 50# of "push" - not much, I'd anchor it all to the floor somehow.
*Adrian,Well, Patrick beat me to it, but I'll post anyway.I have mounted many of my own sculptures, and helped in the mounting of many more of the sculptures of others. It may seem that the sheer weight of a 12x12 concrete column plus stone cylinder of that size would be hard to tip, think again. There is not only the column, but the sculpture above it. I have seen sculptures go to splitsville because someone was standing it front of it, talking, and absent-mindedly barely leaned back against it. Or they reached to adjust their waistline, or to touch someone else on the arm, and brushed against it, etc.This one being in a private home, it can get the needed momentum from a minor earthquake, from a cat being chased, from a loose basketball, a thrown pillow, almost anything.My suggestion is that you tell them that if this sculpture means anything at all to them, they should let you modify the plan. Go with a modified pyramid. Not really a pyramid, just flare the bottom out some. The bottom should be a total of about twice the top. Before you complain about the size, understand this, that only gives you a total of about a 1/4 the total base size working against any tipping forces.If the sculpture is to sit anywhere where it will be approachable from all sides, the base should be flared on all sides. If it is going to be next to a wall, the back can be flared less, and you can remove the flare at the front. Straight flare, not curved would work best with a cylindrical sculpture. It will not compete with the sculpture for attention.Make a hollow base of hardwood plywood. If they are set on concrete, cover the base with Gypcrete. (I believe that's the name of it here. A drywall compound substitute that dries faster than compound, and is like concrete when set. Waterproof, etc.) To be honest, it sounds like a wooden base would compete less with the sculpture and would be complementary. Would you do us a favor and post a picture of the sculpture ?Finaly , and don't leave this out, put some weight in the bottom of the base. Those old window weights that someone was talking about a while back would work great for this. Failing that, put a concrete block or two in there. The combination of having a flared base, and having the weight in the bottom of the base should make the thing about as indestructable as you're going to get without a major installation process. If the sculpture is going to sit flat on the base, it would also be a good idea to put some weight inside the bottom of the cylinder to lower it's tipover point as well. Matter of fact the tipover point of the cylinder itself is almost more of a problem here. I would suggest you extend the base up into the cylinder and attach it as firmly as is safe, Maybe just a friction fit. Add weight to the part of the base which extends into the cylinder as well.I hope this helps.
*Thanks for the good advice. As far as being a cylinder goes, I used that as an example to give an idea of the volume; it isn't a cylinder, but a stylised face and torso, with a lot of facets. Hence the thinking that the base should be a very simple form so it doesn't take anything away from the sculpture. The sculpture is already bored for a steel rod. The original idea, BTW , was to use a tree butt, but I'm not comfortable trying to control the drying of a big stump, varnish it, etc, with all the further drying it would do in the house.I really like the idea of a steel plate attached to the bottom to widen the footprint. So now I'm thinking about pouring the concrete around an interior box,or maybe a cube of styrofoam or something, so there is meat at the top for the steel rod, and at the bottom for some added weight there, but hollow in the centre to reduce the weight, and add a 30 x 30 plate at the bottom. You are all confirming some of the squirmy feelings I had. Thanks. And still open to suggestions.
*Hi Adrian,Be careful of the balance of your project. If the top half weighs in at 300 lbs and the bottom have weighs in at 100, you had better have quiet a large base in floor area to ensure that sucker doesn't fall and kill someone.Gabe
*Adrian,If you are building this base to the customer's specs, BE SURE TO GET A RELEASE OF LIABILITY!! They want it their way, you build it their way and they get injured and the law suit comes your way.Pete
*Hey Adrian, Sculpture already has a rebar/pipe hole through it. Why not drill a hole in the floor; carry the pipe through to a sturdy mount on the floor joists. Might solve the tipping problem...Just a thought.-Claire(PS and then when they don't like the sculpture anymore, you've got yourself a flooring job!)
*Adrian,I think the concentration of the weight of the sculpture could eventually cause a skim coat on a skim-coated wooden base to crack.I'd opt for essentially a solid base. The footprint should be increased for stability, either through flaring the base (conical, trapezoidal pyramid, etc) or by the attached plate method. Also, a flared base keeps toes (and thus the owner of the toes) a extra few inches away from the sculpture.To lighten a concrete base, you can use perlite (same stuff as found bagged in garden centers) as the aggregate instead of stone. It'll knock about 30%-35% off the weight of the base without markedly affecting the strength...especially in this case where it's all in compression. I'd also add fibermesh and a cylinder of hardware cloth for reinforcement. If you end up with a "hairy" base from the fibers, burn off the offending hairs with a propane torch.To color the base, you can add a coloring pigment to the mix for an integral color. Realize the gray portland cement will muddle the color of the pigment. Should the owners opt for a bright, pure, or clean color, use white portland cement in your mix instead of the standard gray. The white portland is a couple dollars a bag more expensive.
*Thanks Mongo. I didn't know that about perlite, and the tip about the cement colour makes all kinds of sense. How would you use the fibermesh and the hardware cloth in that application? Just have it in the pour somewhere?
*If you want a smooth-sided base, here's what I'd do. Same technique as casting a cctop.Build a form out of 3/4" ply so you can cast the base inverted. Assume a finished base 12" square by 24" high. Build the form, screw the edges well so the cream of the concrete doesn't leak out when you cast the base. I'd then line the form with gloss formica. A 12" by 12" piece for the bottom of the form (top of the base) and four 12" by 24" pieces for the sides of the form (sides of base). I adhere w contact cement. The open "top" of the form will be the eventual bottom of the base.Do you want a fancy edge on the top corners of the base? If so, add molding around the perimeter of the inside bottom of the form. Think casting, so you'll want molding that's the negative of your desired edge. Want a roundover on the finished edge? Use a piece of cove molding. Add in a 45 degree cutoff to get a 45 degree chamfered edge, etc.How is the sculpture to be attached to the base? Are you embedding a rod into the base? Or are you going to embed a length of half-inch conduit through the height of the base and run a rod through later? Regardless, drill a hole, centered, in the bottom of the form, that is either the exact outside diameter of the rod or conduit that you will embed. You may want to do that before you assemble the form so you can drill from the "inside out".Make either a hardware cloth cylinder, about 20" high by 10" in diam, or a rectangle, 20" high by 8" square. I use half-inch hardware cloth. Tie the edges together securely.Mix your concrete, adding the fiber while you're mixing. Keep it on the dry side. That is, if you're familiar with concrete, which you may be, mix it properly. Most DIYers mix it too wet, which weakens the final product. Put the rod/conduit in the hole in the base of the form. If running the rod through, obviously you'll have to raise the form off the floor so the rod can penetrate the bottom of the form (stick out the top of the base).Pour in about 2" of concrete, place the hardware cloth, centered. Pour the remainder of your concrete taking care not to displace the rod/conduit or the hardware cloth.Vibrate or hammer the sides of the form thoroughly to eliminate air blubbles. When done, if using a threaded rod, press a disc of wood (say 2" diam by 1.5" deep or so. If possible, taper the edges of the disc for easy removal) into the eventual base of the form, centered around the conduit. Once the base has cured you can pop this disc out, it'll allow you to install a recessed washer and nut to capture the end of the eventual threaded rod. Or, use whatever detail will be required so you can attach that steel plate to the bottom of the base for stability.Careful detailing the open top of the pour (bottom of the base) so that it's flat..you don't want the base to wobble like a weeble.When the base has cured, unscrew the form, peel off the sides, and you should have a nice looking base with glass-smooth sides and top. You'll probably be able to see your reflection in it. Let it dry a bit longer. Any pinholes from inadequate vibrating can be left alone, or filled either with a portland cement slurry, or filled with epoxy. You can use a matching or contrasting color. If the surface is too shiny, sand it down a bit with an orbital sander to take the shine off.Certainly let the base adequately cure a few weeks before torquing down on it or you could crack it.You know, Adrian, I just realized I went way overboard on this...I apologize for the excess. I get many follow up email Q's whenever I post regarding casting concrete and am used to rifling off somewhat stock answers, detailed, because I don't know who the reader is. You know more about building than I'll ever know, I didn't mean to offend you by going overboard.I've done similar projects for small garden sculpture...the process works pretty well. I wish you success.
*Overboard? Not by a long shot...I appreciate the good advice. As for "knowing more about building than you'll ever know"...you must have me confused with the other Adrian Wilson from Cape Breton that posts here sometimes...he can get pretty opinionated. I've never seen a post from you yet that didn't make good sense and ring true. BTW, what you described is very similar to the current plan; threaded rod through a sleeve, with welded nuts bottom and maybe top to tie the plate to the concrete, with some gasket material between the two.Thanks, and to everyone else that responded.
*how do you do a good laminate job inside this form? Laminate the ply and then build the form?
*I would build the form. (With screws, as suggested) Run a pencil along all the inside corners. Dissasemble. Laminate using pieces that are beveled to fit snug to each other in the finished assembly. And re-assemble.
*Ryan,For a project like this, I'd probably laminate first. Laminate a half-sheet of ply (or whatever size required for all the form pieces) and then cut the pieces out of the large sheet. With some forms it's easier to laminate first, others after. Depends on shapes/angles/material on hand. As long as the corners are tight...
*melimine works good for smooth forms, or MDO just seal the edges with thompsons before use.david