ibsmurf said:
While we're talking mud, I have a question for y'all. Wifey want me to put in a slab outside the back door (walk out basement) now that the deck has been built and we know what area we have to play with. I'm thinking of going the permanent form route (each section about 4.5' x 6') and making the slab about 3 1/2" thick. Do I need wire mesh in each section, or can I get away without it? Any helpful tips you mind sharing with a poor DIY'er?
Use rebar. Concrete should never be poured anywhere without reinforcing. You can also use a fiber mesh, which is actually fiberglass that is mixed into the concrete mix by your premix guy. This sucks for finishing slabs, though, makes them hard to finish and look nice. If you are just going to put a floor or tile over it, though, fiber works good. You can make it look nice, but it is harder. If you don't do it right, your slab will be "hairy". Make sure you get about 4" of compacted 5/8 minus down on top of a compacted subgrade, then pour your mud. 3.5" is a little thin, i would go 4" minimum for your concrete thickness.
Also, a really good way to reduce cracks, this goes to '95stroker especially, since he is doing it again, is a water-reducing admixture. What happens is what is called "shrinkage cracking" that occurs when the concrete loses much of it's mass as the water cures out of it, and the concrete "shrinks" to make up for the lost space. This causes the slab to crack. WRA reduces the amount of water you have to put into the mix to keep it workable. Without the WRA, you would have big clumps of non-trowelable crap, and workability would suck. With WRA, it flows smoothly and nicely, but without having to add water to get it there. WRA's are also called "superplasticizers".
ALSO, ANOTHER good way to reduce freeze/thaw cracks is called AIR ENTRAINMENT. WHat it does is put "carbonation" into the concrete mix (for lack of a better way of putting it), which creates air cavities inside of your slab (microscopic, you'll never tell the difference in the actual concrete product). What this does is give the ice a place to go to expand and contract inside the slab, instead of breaking the slab up when it expands. It works pretty good. Up here in Commercial construction, you rarely see exterior concrete being poured without air entrainment. Air also makes the mud really nice and workable.
When you get your work done, talk to your guy and your local premix contractor to see what they would do, and ask about these two things. They will help to reduce or eliminate cracking problems.
That said, as they say, there are only a couple constants in life, things like concrete is grey, and it cracks, and skylights leak, and other jewels of wisdom and such.
Oh, gosh, I forgot the most important part! CURING. I'll say it again. CURING. Most people get a good trowel finish on their slab, and then walk away, figuring it will do it's thing and be fine. This causes uneven curing, which causes CRACKS. (IE, the top of the slab cures before the bottom, curing results in volume change, which results in stresses if the whole slab doesn't do it at once, and therefore cracks.) What you are shooting for is 75 degrees farenheit and 100% humidity on your slab, starting the day after the pour, and continuing for 3 to 7 days afterwards. That means that if it is a lot hotter than 75 degrees out, you'd better get some shade on your slab. If it is not 100% humid out, you'd better get your soaker hose out. Usually, running a soaker hose on the slab with Burlap over the top holds the cool and the moisture in just fine. You can also tarp it to hold moisture from the cure, but I really like to water cure my slabs, so I use soaker hoses and sprinklers as much as possible. Tarps hold in teh heat of hydration (chemical reaction of concrete setting) and can over-temp your mud really fast, creating cracks.
That also means that if it is a lot colder than 75 degrees out, you need an insulated tarp and blanket, along with hotter mud (more cement), hot water, or even a cold-setting admixture (non-chlorine if you have steel in your slab or it will rust it out). We have blankets that have a vapor barrier in them that keeps the moisture on the slab, and the cold out. The hydration process in concrete creates a lot of heat, so you don't have to add heat if you have good blankets, which you can rent from your local rental place. I had a slab the other day showing 68 degrees under the blanket with no additional heating other than the heat-of-hydration, and it was 22 degrees outside.
Anyway, there is more to it than just laying the mud and finishing it, that is what I am trying to say.
