I'm preparing to pour an 8x14 monolithic slab/footing for an addition on the house. I've had everything engineered and plans drawn. I've doweled the existing concrete according to the plan specs. It's a standard 4-inch slab, and the footings are about 24-inches below the finish floor level. I *should* have some experienced concrete finishers at the pour, but I like to know how to do the job the right way. I've read that to remove entrapped air from the concrete, you can use a vibrator on the forms or insert a vibrator into the concrete. I've also read that you can tamp the concrete with a rod. I figure that the slab is small enough that rod tamping is a viable option.

When tamping with a rod:

1. Which portions of the slab/footing should be tamped (e.g. everything, just the footing, or what)?
2. How deep should the rod penetrate into the concrete (all the way to the vapor barrier/aggregate, etc.)?
3. How far apart should each tamping site be (like six inches, a couple feet, etc?).
4. How do you know when you've tamped one spot enough?

Or is the slab large enough to justify the expense of renting a vibrator? I like doing things on the cheap (who doesn't?), but if I'm going to get honeycombed concrete because I was trying to bring in the project under budget, I'd lay out the money for the right tools (quality > $). Thanks in advance for the advice.

 

Residential concrete slabs are rarely vibrated, at least around Indianapolis. Usually the concrete is poured so wet that it doesn't really need to be further consolidated. I don't know if it's the same where you live or not. Really, the only places I would consider doing it would be near the forms, and then just to minimize honeycombing. You can accomplish the same thing by filling the forms, and then tapping all over them with a rubber mallet. The vibration of the tapping causes the voids to fill in. Commercial or industrial concrete is typically placed drier and is subject to more rigorous quality control procedures, hence the mechanical vibrating.
I'm unfamiliar with "tamping" the concrete, unless you mean using a jitterbug on the surface, or possibly "rodding" it. Rodding is similar to what you described; simply stabbing the concrete with a long, smooth steel rod. I've only seen it done when taking slump tests or making test cylinders though, and think it would be a tedious and ultimately meaningless task on a large area. If you decide to do it though, don't poke a hole in your vapor barrier.
Another option is the use of self-consolidating concrete. This is a relatively new product which negates the need for vibrating. You could call your ready mix producer and ask about it. In my opinion though, it would probably cost more than the added benefit you would receive. Good luck on your job!

Pecos

 

Yes, rodding concrete is what I meant, but it appears you got my drift. Terminology varies, I suppose. Thanks for the tip on using the mallet.

I'd assume that the concrete here (Arizona) is generally poured fairly wet, since even at 5 in the morning our summer temperatures are in the 80s.

 

Really, the wetter the concrete (added water) the weaker it is, so it's better if poured no wetter than about a 5 inch slump. I was being sort of sarcastic about how most people around here pour. I've seen a lot of contractors who think nothing of pouring an 8 inch slump, and then wonder why it cracked so badly!

There are additives called superplasticizers that allow for pouring wetter without losing strength. You may want to ask your supplier about those as well as the self-consolidating concrete.

Is the concrete slab you are pouring post-tensioned, with cables running through it? I hear they do that a lot in Texas and other sandy areas (Arizona?). If so, maybe vibrating it all would be beneficial. People do things differently in different areas, so if you've got some experienced professional help, they should be able to guide you. Good luck.

Pecos