Pouring a concrete pad a few bags at a time?

[MikeT]

I am contemplating building a concrete pad for an outdoor soaking tub  (like 8'x8').  The nature of the site (and my other projects on site) are such that I was mulling over the idea of pouring the pad incrementally, over multiple days.   That would allow me to hand mix without too much trouble.  So I guess my question is concerns the downsides of having a pad that essentially has multiple cold joints.  Given the nature of my project, is this a big deal?  I will be using rebar.

[MountainDon]

I poured an extension to my driveway in 4 x 6 foot sections one at a time. I used fiberboard expansion strips between each section and the welded wire mesh reinforcement in each. This has worked well and allowed me to do a section each morning before the day got hot.

I used a HF mixer and bought sand and gravel mix and bagged portland cement; much cheaper than bagged "redi-mix".

[MikeT]

Thanks for that input, Don.  I am so programmed to purchasing RediMix that I hadn't thought about mixing the ingredients.  I like this option much better!

[waggin]

I know someone who replaced their entire foundation doing 4-5' sections at a time, leaving exposed rebar on the new end to tie in the next section.  Definitely doesn't meet the continuous pour requirement, but I'm sure in the end, it was better than the early 1900's non-reinforced original.

[MikeT]

I was just thinking that for a pad for an outdoor tub, the lack of a continuous pour wouldn't be a big deal....But I will wait to see what others think..

[firefox]

I was told by my foster father years ago that you could do a layered approach as follows: Do the first layer, say an inch to an inch and a half. Rough up the surface with a piece of rebar making quqrter inch
gauges in a hash pattern no more than two inches between lines.
Moisten it before you leave and put plastic over it.
Set up your rebar or mesh the next day making sure that is raised at least a half inch over the first pour. Wet down the surface and then Sprinkle cement powder over the surface. Pour another inch to inch and a half of concrete so that you have at least a half inch over the top of the rebar. Repeat the same process but without the rebar this time covering with a minimum of inch and a half of concrete.
Finally pour a thin layer of concrete and sand mix on the concrete
after the concrete has cured for 3 or 4 days. When the last layer has had a time to cure get some raw cement and a steel trowl. Sprinkle some raw cement down and the carefully strike the concrete flat with the trowl. This brings up the water from the concrete, moistens the cement powder. This helps to provide a hardened smooth surface. Bear in mind that although this is comfortable to walk on and looks good, it may be slippery so you may want to
put some decorative hash marks or pattern down for anti slip protection.
Bruce

[diyfrank]

8x8 doesn't sound that big. How many bags are you thinking of?
I hand mix 44 #80 bags of quikcrete in a Wheelbarrow when I poured my footings. It was work but not that bad. You could do a washed aggregate and that might look ok if you did it just right.

[John_M]

What about putting in a patio of paver bricks?  You could bring in some crusher run (pretty cheap from a gravel pit) and tamp it down really well, then place your pavers down.  If you make your base thick enough (say 8-12 inches) you can drive a car on that stuff.

I'm just not a big fan of concrete pads...it's not if they'll crack, but when.  Pavers look so nice when they are installed properly and can easily be fixed if the ever get out of level, which if installed properly, is rare!

[MountainDon]

I believe most tub makers will specify that the tub be placed on a concrete slab rather than bricks. I know when my BIL put in a tub and had me build a deck around it he told me the tub supplier specified the size and thickness of the slab, other wise they would not warranty the unit.

How many gallons of water in an 8x8' tub? I have no idea, but if it's 300 lbs that would be  300 x 8.33 lbs = 2500 pounds, plus tub etc.

I've done a lot of concrete brick walks and patios and it seems that no matter how careful I was there's some spot that sinks after a year or two. I'd hate to have that happen under a tub of water. On the other hand all the 4x6 sections I've done are still solid, no cracks even though they have been driven on.    

[Redoverfarm]

John I would have to agree with Don on using the bricks.  There is more of a chance for failure given the amount of constant direct weight of the tub.  If the concrete is not appealing then there is nothing to prevent you from adding them around the exterior of the tub for that particular look.  

Most failure in concrete is poor preparation (Uneven cracks).  If you remove the top soil to solid ground, form, add reinforcement it should withstand what you are describing.  Even if the slab would crack (Even cracks) that is a sign of curing incorrectly it should not effect it's strength as a whole.  

[MikeT]

 No matter what I do, I need to do a decent site prep job, so that is a constant.  I have examined (or at least thought about this) from a number of angles, and I keep coming back to the slab.  Mtn Don's idea of creating the concrete by using the correct proportions of the ingredients as opposed to using the bags has a lot of appeal.  It really brings the cost down significantly.  Most of the pad will be invisible (covered up by the tub), so I could care less what the finish looks like (as long as it is level). 

[MountainDon]

Concrete is made from cement, sand, gravel and water. These ingredients are commonly combined in a 1:2:3 proportion to achieve maximum strength (1 part cement, 2 parts sand, 3 parts gravel). The amount of water used to mix the ingredients is by far the most important factor in determining the final strength of the concrete: Use the least amount of water that will still give you a workable mix. A little extra cement will increase strength.

A workable mix should be smooth and plastic (not wet and runny or dry and crumbly). If the mix is too wet, add small amounts of sand and gravel (in the proper proportions) until the mix is workable. If the mix is too stiff, add small amounts of water and cement until the mix is workable.

http://www.lifewater.ca/Appendix_J.htm    metric measurements but good info

[davidj]

How many gallons of water in an 8x8' tub? I have no idea, but if it's 300 lbs that would be  300 x 8.33 lbs = 2500 pounds, plus tub etc.

300 gallons is probably on the low side for a 8x8 tub - probably more like 400-500.  And the old 8x8 tub I picked up must have weighted close to 1000lb.  And don't forget 6 x 160lb of people.  So maybe 5500lb total.

To give good support to the tub, you really want it to be a solid slab.  If you do e.g. 4 x 4ftx4ft sections with some kind of expansion joint you risk it "folding" at the joints (which is typically fine on e.g a driveway but not good in this application).  So I'd guess it would be better to do a shallow pours across the whole area rather than full-depth pours over a small section, i.e. firefox's technique sounds ideal.  If the weight is carried by the outside of the tub then I think putting the rebar/mesh slightly higher would help as a perimeter load would put the top in tension and the bottom in compression (with reinforced concrete, the metal handles the tension and the concrete handles the compression).

[frazoo]

An 8x8x2' tub (not sure of your depth) would be around 128 cu/ft times 7.48 gal. per cu/ft giving you 957 gal. times 8.33 per gal yields over 7900 lbs of water.
Of course, you probably would not fill it all the way and the water's  direct weight is spread over a area of about 64 sq/ft. 

Now my head hurts, to much math, too early, no coffee yet.

frazoo

[Redoverfarm]

Just a little personal insight.  I have a 6' X 6' tub.  To properly fill it to the manufacture's recommendation it takes approximately 340 gallons to fill it.

[SouthernTier]

I know someone who replaced their entire foundation doing 4-5' sections at a time, leaving exposed rebar on the new end to tie in the next section.  Definitely doesn't meet the continuous pour requirement, but I'm sure in the end, it was better than the early 1900's non-reinforced original.

What have been people's experience with inspectors with regard to continuous pour?  There's no possible way to get a concrete truck into where I want to build my cabin, and I want a block foundation, so will need a footer.  I figure I'd mix it bag by bag using a small portable mixer.  It seems like it would be fairly continuous so long as you don't take much of a break and you always work in a continous fashion around the footer forms (i.e. work clockwise, not work an inch of elevation at a time).

[waggin]

The project I referenced was a Sergeant Schultz one:  I see notheeng; I know notheeng!