Replies

1. 
   Mike_Shultz | Jun 18, 2001 09:50pm | #1

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   I am assuming 4' round and 28" deep rather than 4' long and 28" wide. Please correct me if I am wrong. In any case if this tub is free-standing, rather than buried in the ground, the walls will be subject to tension rather than compression. Concrete only has one-tenth the strength in tension as in compression. Therefore tension reinforcing will be required.

   This tub is similar to the concrete canoe that I built and raced. My canoe had 1/2" thick walls and was built over a male mold. The practical limit that most people can build to is 1" thick and even this would render it structurally on the edge without carefull detailing as to reinforcement.

   If this is cirular as I am thinking then you could build a male inverted form. Circular side made from bending plywood over wood forms. The bottom could be formed from a circle of plywood that is raised 4" above the wall framing and 8" smaller in diameter. This will leave a 4" by 4" area to chamfer with plaster to create your radiused inside edge. Cover with paste wax as a form release. (The tub should be formed with a "drag" to allow the form to be pulled away from the concrete casting. But in your case it would be easier to destroy the form after the concrete has cured rather than form compound curves in plywood for reuse.)

   After spending 100 hours to this point you would cover the entire form with two layers of steel chicken wire (1/2" max openings). The chicken wire should be spaced mid-way in the thickness of the concrete. Use wooden dowels between the form and the wire to space the wire and remove when placing concrete.

   Polyheed is a water reducing admixture that can be added to the mix to reduce the amount of water used and to reduce cracking. Use a high strength (high sack conrete) mix and hand apply to the sides working your way from the bottom of the form to the top (the bottom of the tub). Base of tub should be at least 2" thick. Vibrate the form by running a palm sander on the inside of the forms. Concrete can the be finished on the outside using a "pool" trowel.

   Tightly cover the finished tub with poly for 28 days to allow the concrete to cure. Invert the tub and formwork. Caution this will probably weigh 1200 lbs so have a safe means of handling. Remove the interior formwork. (Hopefully the circular form was constructed to be disassembled from the interior.)

   The reason that I mentioned using a male form was that I have seen people try to use female forms and they can't make the reinforcing stay in place while they are placing the concrete. Using a male form allows the inside of the tub to be formed rather than having to trowel a smooth interior surface. So -- the better the formwork the better the finished tub.

   You now have an 800 lb tub. Good Luck.

1. 
   Don_Papenburg | Jun 19, 2001 01:38pm | #2

   *Mike , What is a pool trowel?

1. 
   Mike_Shultz | Jun 19, 2001 09:13pm | #3

   *A pool trowel is a 12" long by 3" wide steel trowel that is rounded at each end. It is used for trowelling radiused concrete surfaces. Have fun and lots of luck with your project.

1. 
   D_Lambertson | Jun 20, 2001 03:33am | #4

   *Mike, with many thanks, great response. The tub will be rectangular in shape with a rounded bottom to make it easier to keep clean. The tub is in a section where there will also be a concrete shower and concrete floor with wood mat over to catch drips from tub or shower. The guy who is casting the tub wants to extend the floor for the rest, be the shower and the mat area under the tub-something about dispersing the weight more evenly. I'm less than enthusiastic since this also means more overall weight. He's experienced in doing concrete work, but has never done a tub before. Since I assume the details of this wouldn't be of general interest, I would like to mailyou outside this forum if you are willing. What difference will be make if the shape isn't round? (I didn't realize bbowthat I had posted this message in this part of the forum and have it cross posted in the technique area now). The tub will be against an 8" thick stuccoed wall and I would like to incorporate that wall into the tub. Otherwise, it's freestanding (and resting over number of steel joists over a basement). Again, many thanks and I have more questions.

2. 
   D_Lambertson | Jun 20, 2001 03:33am | #5

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   We want to make a concrete soaking tub; outside measurements 4'x28". Can anyone point me to some literature about how to do it? Or discuss it here? We're thinking about a single casting and then moving it in place. What is the thinnest wall we can practically make? Easiest way to slope the sides? A good additive to lessen the amount of water necessary. How thick should the base/bottom be? Will appreciate suggestions. Thanks