Pompeii started in San Jose

[SteveB48]

Hi All,

It's finally time to get going on my 36" Pompeii. I have the area for the slab ready for the base rock and forms. Hopefully Saturday will be pour day.

I was planning on mixing myself (I have a mixer) using 60lbs bags. 60lbs bags simply because they are easier to handle than 80lbs bags. I'm using the standard dimensions in the plans which I believe would require 37 bags.

My question is, how long would it take to mix and pour? I'm guessing between 1.5 and 2 hours. Is that reasonable? How long can it take before I have a problem with the concrete setting up before the pour is done?

thanks,
Steve

[Aquaponicsguru]

Re: Pompeii started in San Jose

If you are by yourself... about 3 hours... half that if you have a helper. If you mix the concrete to specifications, you should not have a problem with the concrete setting up. You will have enough time to screed the crete. Best to start early so you have enough time before dark to do the finish troweling after the water evaporates (when sheen is gone from the concrete). For a normal 3.5 cf mixer, you can mix three 60lb. bags at a time but make sure you push the dry cement back into the back of the mixer for the last bag before you start it again to add water.

I just finished pouring my hearth yesterday. Today I pour the insulating concrete layer.

Good Luck. (Steve-Aquaponicsguru)

[mrchipster]

Re: Pompeii started in San Jose

Another hint, pour your form full in an area and keep working away from that full area until the entire form is full.

In other words. Do not lay in successive thin layers, work full depth from one side to the other.

FYI you will want to have a helper to level it off. It is much easier to strike off the top with one person on each end of a 2x4 to get a good flat top surface.

[SteveB48]

Re: Pompeii started in San Jose

Well the slab is poured. Now I can tear it out The concrete is brittle and crumbles. I knew I was in trouble when I couldn't float it. Quikcrete calls for 4 pints of water / 60lbs bag so 1 gal for 2 bags. That just looked too dry so I gave it a bit more after mixing for awhile. I was remembering all the comments about most people adding too much water so I was being conservative. It was pretty warm and the sun was on the work site. Reading the the specs on the Quikcrete web site it says final water content between 4-7 pints / 60lbs bag. I was nowhere near 7. Maybe 4.5?

Rather than take a chance and build an oven on top of a substandard slab, I'm going to start over. I have a couple of smaller pours to make so I'll do those first. I'm also going to invest in a concrete vibrator. I've a number of pours to do so I think that's a worthwhile investment.

Thanks to Chip and Steve for their advice.

Steve

[mrchipster]

Re: Pompeii started in San Jose

Sorry to hear that it did not work out. But I am still concerned about your tear out. Concrete is very weak for at least 2 - 3 days. But if you could not float it that could be a problem.

[SteveB48]

Re: Pompeii started in San Jose

Hi Chip,

I don't have a picture that I can send you but in one corner you could scratch it with your finger and rather then just marking the surface, you could brush away cement and rock. It was acting more like it had just came out of the bag than a 1 day old pour. Saying that makes me wonder if it wasn't mixed thoroughly either. I never saw dry spots in the pour but...

Steve

[DavidApp]

Re: Pompeii started in San Jose

Hello Steve

Sorry to hear about your pour.

A thought before you do the second one. Do you have any idea how old the Quikcrete is?
I know that Portland cement has a shelf life and will not set up if it is too old.

Good luck with the next pour. Rent or borrow a mixer if you do not have one it will make it easier to get a good mix.

David

[Novaslo]

Re: Pompeii started in San Jose

Any chance you can get a U-cart? Might be worth not messing with mixing your own and just go that route. I know it is more expensive, depending on what you pour. That is what I did and also used what I think they called a 6 sack mix. Or stronger more dense concrete.

Good luck.

[SteveB48]

Re: Pompeii started in San Jose

Thanks David. We bought a whole pallet from Lowes. I don't know if that means it all came from the same batch. I have a small pour (a couple bags) I hope to do in the next couple of days and we'll see if that has the same problem. If I have no issues there, I have another one which is about 19 bags that I'll try before I get back to the oven slab. I do have a mixer. All the other concrete work in the yard was done with a pumper truck but that was more like 4 yards. I thought (still think) I can handle this.

Darin,
I thought about getting one of those concrete trailers but a friend told me that he had trouble with the rock settling out of the mix on the ride home. The equipment near me doesn't have the mixing trailers.

Steve

[DavidApp]

Re: Pompeii started in San Jose

Hello Steve

Lowes should have enough sales that age would not be the problem.

I did not measure the water that carefully. I did use a gallon bucket but often added a bit more water if the mix looked too dry. When I floated the slab I did bring some water to the surface . Maybe an indication of a wet mix. I was working in the colder months so drying out was not an issue.

Vibrating it may help.

David

[NCMan]

Re: Pompeii started in San Jose

Vibrating a thin slab like that can do more harm than good and is not needed.

[SteveB48]

Re: Pompeii started in San Jose

Vibrating a thin slab like that can do more harm than good and is not needed.

Can you elaborate on this? I had expected that around the edges that vibrating would help.

thanks
Steve

[DavidApp]

Re: Pompeii started in San Jose

Hello

I thought that vibrating helped to get the air out and consolidated the mix.

Always ready to learn something new.

David

[NCMan]

Re: Pompeii started in San Jose

Vibrating a slab around the edges can simply be done by tapping it as you go w/a hammer. Excessive vibrating causes all the ingredients to separate. There just is no need to vibrate a slab like that. Mix your concrete thoroughly, at a consistent slump (just enough water to make it workable) and place it, being sure to have your reinforcement raised up off the bottom, roughly in the middle of the slab. If it's hot out, try to keep the moisture in the slab as long as you can. I do that by covering the slab w/plastic and then cardboard. The plastic keeps the moisture in, while the cardboard keeps it in the shade for awhile. The last thing you want when pouring in hot weather is your slab drying out too fast. The longer you keep moisture in a slab, the stronger it will be. Also, spraying the slab w/water afterwards is not really recommended. It's far better to cover the slab and leave it covered for as long as you can. Excessive spraying of a slab can be detrimental in some cases and really does not help much, unless you could keep it wet continuously, which is tough to do. The best thing to do, when you are done, is to cover it the way I described, concentrating on keeping the existing moisture in the slab. Best of luck to you.

[DavidApp]

Re: Pompeii started in San Jose

Hello NCMan

Thank you.

Knew about keeping the moisture in for as long as possible not allowing it to dry too quickly but did not realize that vibrating could cause separation. Makes sense when put like that.

David