bad mix? ruined concrete? see pics

please take a look at these pic of the slab i just poured. i mixed the water in just under what the bags said was the max ammount of water and i didn't do a slump test but it seemed to hold its shape well, but also was well mixed. it was not fluid by any means, but i've read when properly mixed the concreet may seem too dry.

whats going on here? is my slab ruined? its 3 inches thick and you'll notice one of the pics shows the concrete didn't even get under a section of rebar and left it exposed. i will be putting a 1 ton brick oven on top of this slab. it has 1/2 bar in it every 9 inches.


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thanks a lot for your help.
max

 

Concrete is lazy you have to use tools to put in place, How dry was it? If you hit it with a float did the surface get smooth? It looks like it rained on it and washed all the fines away.

 

it did not rain on it.
i misted it twice and have had a tarp over it all week.


i did hit the top part with a float and had a really hard time getting it smooth so i just gave up. but you can see in the first picture how the concrete is a bit more solid. towards the bottom, i can flake off chunks of gravel with my fingers.


we did not use a jitter bug.

 

Max, you have my sympathy, I have to agree that's one sorry looking slab. Looks to me like it was too dry a mix.

This past year I've been working on a garage foundation renovation, using Sackrete and Quikrete, and I got great, solid-looking concrete. I never bother trying to measure the water, just mix it "by eye" and "by feel". When correct it has the consistency of just slightly soupy mud. You have to thoroughly scrape the sides and bottom of the wheelbarrow (or whatever) to ensure all the dry mix is completely mixed. Pros do tend to use a dryer mix I think but they have plenty of labor and power tools to deal with issues caused by the stiffer mix. You have to "work" it once it's poured to get out the voids.

You can make do with it as is but the weather is going to destroy it within a couple of years. Water will penetrate easily and the freexze/thaw cycle will just chew it up. And the exposed rebar will rust out quickly. Rebar rusting inside a slab expands and accelerates the breakdown.

Before you mix and pour any more concrete, experiment with a couple of bags using various amounts of water until you get concrete that looks right. Use the results to make garden stepping stones or something else inconsequential.

 

I've mixed a lot of bags of mix already. But I will take a ready mix truck any day. The worse mixes I've bought were box store specials. They used what looked like small river rock pebbles instead of stone and the finished look was very dark gray.

 

this slab will be totally shrouded with a brick facade and stucko, and the bottom of the slab will be protected from the whether too. if there's no whether will it retain its strenghth?

basically all i'm concerned with is if it caves in or not with a ton of brick on top of it.


m

 

What are the dimensions of the slab and what are the dimensions of the oven?

A one ton oven over a reasonable area is reaslly not a lot. Any reasonable soil can carry a 2000 pounds per square foot, so your 2000 pounds is not a strength problem.

The adequacy of the apperance is a question that only you can answer.

Your steel is only for crack control and continuity and the steel will be serviceable for many years.

The durability will depend on the conditions (temperature, moisture and freeze/thaw cucles per year). Just because it is ugly and does not look like good concrete does not mean it will fall apart in 5 or 10 years. There is a class of very ugly, porous concrete that is used intentionally and has good durability.

If you can live with the appearance, I would leave it in place and remove it down the road when it is no longer serviceable.

Dick

 

the dimensions are about 4x4.

the oven will be roughly the same dimensions and it will be permanetly fixed to this slab.

replacing the slab down the road is not an option as it would mean destroying the whole thing.

if i had mixed it with more water, would it last longer?

 

Only 4x4?

If it were mine, I'd bust it up in a heartbeat and try again. This can't require more than 4-6 sacks, depending on how big they are. Concrete takes several weeks to reach its full strength, the sooner you get started the easier it will be. I bet you can salvage the rebar. Use at least a ten pound sledge hammer. WEAR EYE PROTECTION!!

In the first picture, is that the edge of the slab? Why does it look like there's a cavity underneath it?

 

there is a cavity undernieth it because this is a table top slab, elivated 4 ft off the ground.

it is resting on three sides of block, that were also filled with this concrete mix.

 

Max, You really don't need to bust it up and start over, just pour over what you have. If you use a good 3000 lbs mix and pour it 2"deep all the way around the slab, this should do the trick. Too much water and you will have a mess, to little water and your in for real trouble. You may want to have an expert in your town look at it.

 

Max:

Will the outer edge of the brick oven rest atop the concrete block support of the concrete countertop? Or is the footprint of the oven a good bit smaller than 4x4? This makes a big difference on what type of loads the slab will support.

 

the oven has an inner shell of brick and concreet and an outer layer of a vermiculite cement mix which is obviously less strong.
this outer layer of vermiculite will be over the block legs, but the brick and cement part of the oven will only be an inch or two over the block walls.

 

That might be enough. If most of the weight of the oven is above the blocks supporting the slab, then the slab is only transferring (most of) the load in compression. Concrete has terrific compressive strength, and even poor quality concrete still has compressive strength more than adequate to support such low loadings as this, I believe.

If the weight were concentrated out in the middle of the slab, the slab would be supporting the weight in the same way that a beam does. A beam supporting a load deflects, and because of the deflection inescapably experiences tension loads on the underside of the beam. This would be a problem for a slab of
low quality concrete, as even good concrete has relatively poor performance in tension. The rebar will help, steel has great strength in tension.

Since the slab will apparently be protected from the weather, it probably will be OK to use it as is.

 

Just a novice here, but how about a post and beam under the slab, as one would support a deck?