Hearth concrete mixed too dry? Need to start over?

[seattlecaveman]

This weekend I was forced to mix all my concrete for my hearth by hand because Home Depot's truck was broken down and I couldn't rent the mixer. For the first time ever, I decided to follow the instructions on the back of the bags of concrete and now I think I may have mixed my concrete too dry and have ruined my hearth.

I knew it was more dry than when I use a mixer when I was shoveling it from the wheelbarrow to the hearth. My fears were confirmed when I removed the forms yesterday. I've attached photos I took of the hearth today.

I used hardiboard, so I'm fairly sure the hearth won't collapse under it own weight, but I'm concerned that it may not be able to handle the oven.

Am I going to be safe to continue with what I have or do I need to rip this out and start over?

[Mike D]

Re: Hearth concrete mixed too dry? Need to start over?

I don't think it looks that bad, some parts of my sides look like that, but its more how it hit the sides of the form. How does the top look?

-Keep it wet and covered for a week.

mike

[brickie in oz]

Re: Hearth concrete mixed too dry? Need to start over?

The holes in the edge of the concrete are the result of poor tamping or vibrating, the cement in the concrete will set just fine.

[Tscarborough]

Re: Hearth concrete mixed too dry? Need to start over?

No problems with strength, it is stronger at lower slumps. You can use sand mix to slurry the edges.

[SCChris]

Re: Hearth concrete mixed too dry? Need to start over?

Seattlecaveman, you are gold with the concrete pad, like Tscarborough states dryer is stronger in concrete, to a point anyway. Yes it would have been nice to tamp the voids out but given a week to cure, you should be good to go. You can improve the strength by covering the pad with something wet, wet burlap is often used to keep the concrete from drying to quickly, but old towels or somesuch will work just as well and since you're in Seattle premature drying might not be an issue anyway.

Chris

[seattlecaveman]

Re: Hearth concrete mixed too dry? Need to start over?

Thanks for the feedback everyone. It's raining all week here in Seattle, so I don't think I'm going to have to worry about it drying at all. I suppose that's one benefit given to me by the Great Northwest.

Once I started to screed and float the concrete, I realized how dry it was, so I made one more bag fairly wet and poured it around on the top. That's how I was at least able to get the top to flatten out. I tamped as well as I could when pouring it into the form, but I wasn't able to make it move too much.

I'm not too worried about how the sides of the pad look because I intend to cover the entire base with stone or brick once the oven is complete. I was more concerned about how weak it appeared to be since the sides felt crumbly and lightly rubbing the sides left my hands covered with sand.

Thanks for the reassurance. I hope to use this long weekend to start on the oven itself!

[Tscarborough]

Re: Hearth concrete mixed too dry? Need to start over?

Covered or not, you should fill the bugholes.

[deejayoh]

Re: Hearth concrete mixed too dry? Need to start over?

Just give it time to cure. It will be fine. I remember when I poured mine, I thought it was too wet. Turned out fine.

[Mingy]

Re: Hearth concrete mixed too dry? Need to start over?

The main floor of my house (I have a full basement) is concrete and some parts of the underneath looked worse than that. I demanded an engineer's report, and he came and inspected it and said it was ugly, but ok. He suggested the contractor trowel grout on the bad spots.

If you are not familiar with grout (not the stuff for tiles) its like a sand cement mix but *really* strong (like 8000 psi). You can mix it from milk to thick paste and it is happy. So, you could go buy a bag or two and mix it up like drywall compound and have a go at it.

By the way, you can do all kinds of things with grout. Because to can mix it real liquid, you can use it to, for example, level a row of blocks: you just shim the blocks so they are all level, build a dam out of tape or something, and pour a bit of grout in the bottom. It'll flow and set up so when you remove the tape the block will be level. They also use it to level beams: same idea, you use some cedar shims, dam the area around the beam, and pour it in. Very cool stuff.

[seattlecaveman]

Re: Hearth concrete mixed too dry? Need to start over?

I bought some quick-set cement from Home Depot that claimed it was good for concrete repair - so I figured it would be good enough for sealing the ends of my hearth. I'll see what happens tomorrow when I try to use it. (It's claims to be 9,000 psi.)

25 lb. Cement All Multi-Purpose Construction Material-02020025 at The Home Depot

[Mike D]

Re: Hearth concrete mixed too dry? Need to start over?

-I wouldn't do anything to it until you are ready to finish the outside of the stand. You can fill in the holes with whatever material you are planing on covering it with. This is not a project that you want to rush.

-Let the oven evolve. Good Luck

Mike

[deejayoh]

Re: Hearth concrete mixed too dry? Need to start over?

How is that oven going? have you put anything on the pad yet?

[seattlecaveman]

Re: Hearth concrete mixed too dry? Need to start over?

I've been really negligent and haven't posted anything... I left the pad as-is and started right away on the oven. I had hoped to be finished by now, but many long weeks at work stopped my progress through the middle of the summer. I'm currently working on course 8. (Photos are from completed course 6 [last weekend] and course 7 [today].)

I was cutting poorly-calculated angles on courses 5-7 so those courses are fairly ugly, but I redid my jig for course 8 and that one should end up better.

[deejayoh]

Re: Hearth concrete mixed too dry? Need to start over?

Thanks for the update. That looks good. It seems so impossible, but you are very close to dome closing. My experience was that the dome was less than half the work, but YMMV based on what you want to do with your enclosure.

Oh, and try my calculator spreadsheet if you want an easy way to calculate angles for your jig setup

[seattlecaveman]

Re: Hearth concrete mixed too dry? Need to start over?

Yeah, my new jig is based on the calculations from your spreadsheet. (If only you had posted that in May...) I can only get my jig up to about 30 degrees due to the restrictions of my saw, but that will be a far better result than my previous 8 degree jig.

I'm going to finish my dome, then shut down the project for the winter. Next spring I'll build up the entrance and insulate. I'll probably stucco the insulation to hold me over until I can build the final housing around it in the following year.