casted-concrete post for garden entry
We are planning to have a fence constructed around our garden this summer, and my wife would like to include mosaic covered posts at the entry way. The other posts will be wood.
I’m quite comfortable with the mosaic work and the rest of the fence project, but I need some direction regarding the concrete posts.
The posts will stand just over 4′ tall, they will be obilisque in shape (a la the Washinton monument), tapering from about 10″ at the base to 6″ at the top. My thought is that I will cast them in place, standing up, beginning with an 8″ round auger hole 4′ deep (below frost line).
For those not keeping up with the math, that’s about 3.2 cu. ft. of concrete per post, including the below-grade pier (about 480 lbs.)
My question is: Do I need a proper “footing” at the bottom of the cylindrical pier, or can I just
1: set my rebar column in the hole. 2) fill the 8″ hole with concrete and let it start to set up. 3) set my form over the rest of the rebar column, and continue to fill with concrete.
I’m also wondering about the rebar column. I have no idea how much rebar to use or exactly how to construct it.
Any input would be greated appreciated. Thanks!
Replies
Others can advise on the concrete, but have you considered making the posts out of treated exterior plywood, over a standard wood or steel post? This would make it far easier to control the shape of the thing. The plywood could be covered with a stucco-like finish prior to the mosaics.
Edited 5/12/2004 2:53 pm ET by Dan
Dans idea sounds much better, especially when DW declares "that just doesn't look right there" (been there done that!)
Won't work, the plywood will move seasonally and the grout will crack. You could use durock, though.
Unless you have some unusually weak soil, like a peat bog, you don't need a footing to support only the column's own weight. Think of the footing for a house. It supports the concrete, and the entire weight of the house and the snow on the roof and it only covers a surface area twice as large as the wall footprint itself. So a post by itself needs no extra width.
You don't say what your climate is, but the post must go below the frost line. If you want to be certain it won't move you should go 1½ times the frost line deep. The top 2/3 should be smooth, like formed in a Sonotube. The bottom can be rough and should be slightly wider. The idea is that as the ground freezes it can clamp onto the post. So by having it smooth it clamps less forcefully, and by having the bottom wider it sort of holds itself down. I would want to make the part in the ground at least equal to 10" square. You don't want the ground to raise up under a lip as it freezes and jack the post up.
If you don't have frost, you'll need at least 2 or 3 feet to keep tight enough hold on the base to keep it from tipping.
If you are not attaching a gate, then three or four pieces of 3/8 or 1/2" rebar would be pleanty. Space them about 2 to 3 inches (no less than 2") from the outer face. If the column were to be load bearing you'd need rings of rebar around the bundle to resist the bulging stress, but for a small free standing column you don't need them. I'd brace up the forms all ahead of time and make one pour. Letting the bottom stiffen a bit is ok, but if it starts to get hard you'll have a "cold joint" and you won't get a bond. For maximum strength and minimum shrinkage cracking go on the stiffer side rather than the wetter side. You could also add a little extra portland to the mix to make it stronger. 6 bags per cubic yard rather than the standard 5
I don't know what to recommend as a release agent that would not also interfere with the tile adhering later.
Keep the forms on at least 3 days and keep the outsides damp, sprinkling them with water every day if needed. A few days longer would be better. You want a good hard surface that won't powder away after you've applied the tile.
MR. Andy,
I'll second everything Wayne said and just add. . .
Use a wax based release agent on your form and you can clean it with acetone. But wait 30 days before cleaning.
Pam, the antistick food spray, will work and clean off with laquer thinner.
The insides of your forms shoud be smooth and slick like formica.
Because this is a long skinny form it will be hard to consolidate the pour with too stiff a mix, even with a vibrator. I would use a very stiff mix made with water and add a slump reducing admix to it to make it just a little soupy. Kinda like beans and rice in thick gravy.
I don't know how small a unit of slump reducer you can buy, but a gallon is enough for several YARDS of concrete.
Be sure and hold enough stiff mix back so you can hand mold the pyramid cap.
When using the concrete vibrator, (reccommended to prevent bubbles from marring the surface,) Always run it down to the bottom as fast as you can.
If the mud is like warm gravy and beans(|:>) pull the vibrator out with a hand over had method, with each hand grabbing the hose a foot below the other, at a one pull per second beat. In other words, pull it out a foot per second.
If the mud doesn't flow somewhat flat under it's own weight, use a foot per 2 seconds beat.
If it's very stiff use a foot per 3 second beat.
Ideally, the vibrator wand should reach the top of the mud at the same time as the pea to marble sized air bubbles do.
Since this is a small pour of an object that has a high suface to volume ratio, the easiest way to insure a proper cure is to wrap the whole thing air-tightly in clingwrap. This way you won't have to worry about missing a misting moment. Misting a missing moment. Minting a missing minuet.
SamT