Re: concrete patio
I'll toss another opinion in for a 4" slab, for a patio, that's plenty. I'd also like to add that I've poured a lot of patio and sidewalk slabs at 4" and have never used reinforcing wire or rebar, it just isn't necessary for a non stressed piece of concrete. If this were a driveway, then the story would be completely different (minimum of 6" thick with wire mesh ). Expansion joints are important, however you don't have to use any type of separation material to do them, a simple seaming tool will do the trick. If you don't like that look, then pour the patio in smaller segments, allowing each segment to cure before pouring the next. Expansion joints give the slab a place to crack so that it doesn't crack throughout the field.
The calculation is width times length times thickness to determine the number of cubic yards of concrete you'll need. To make this very simple, 4" is 1/3 or .34 of a foot, so it would be 15x20x.34 = 102 cubic feet. There are 27 cubic feet in a yard of concrete so you divide 102 by 27 and you come up with 3.78 yards. If you're doing this yourself, I agree with the others about getting it truck delivered and rounding the quantity up to take care of any thickness variations in the pour. Personally, I like to have a redi-mix truck come that mixes the concrete on site rather than a batch truck that brings a single load to you. The reason for this is that a redi-mix truck will only charge you for what you use and there will be no leftovers, whereas the batch truck will charge you for any overage and you'll run short if you miscalculate. This could also be done with a U-cart hopper IF you've got help, IF the supplier isn't too far away, and IF you've built your forms properly.
I also agree that a 4 yard pour is a pretty daunting task that is not suited to an inexperienced DIY'r without the proper tools. That is not to say that a DIY'r can't pour concrete, just that you'll want to learn how to do it on a much smaller pour, say 1/4 to 1/2 yard to get the feel for working the concrete with the proper tools. Your four yard patio will require at least two people, preferably four or more with at least one experienced person on site to orchestrate and oversee the work. That experienced person can also check that the forms are properly set and the area is ready for concrete before the truck arrives. If you have much distance to go, then hiring a pump may be in order, however you can move a lot of concrete with a couple contractor sized wheelbarrows in short order.
And to end this long message, be sure that you've set your forms so that the patio slab is at least 1" below any doorway and has an absolute minimum of 1/8" per foot of fall away from the house, a 1/4" would be much better.
Last edited by A. Spruce; 06-20-2009 at 10:53 AM.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!