We had a foundation poured that turned out very bad cosmetically and had cold joints due to the heat and delays in delivery. Had it evaluated by a structrual engineer and the solution was pour a 4" reinforced slab on top. Now the cracks are numerous and I wonder if the vibration from thunder is causing the slab beneath to beat the upper foundation to rubble. Any thoughts on this ? Thanks
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Concrete over concrete
- Thread starter Barndo
- Start date
kd3pc
Well-known member
you might go back to that engineer and see what he suggests you use to increase the chances of a successful pour.
I know there are good products that will chemically bond and mechanically bond these things, but sounds as if you need something that will flex more. I am not sure I buy the thunder thing, but you want to make sure.
I know there are good products that will chemically bond and mechanically bond these things, but sounds as if you need something that will flex more. I am not sure I buy the thunder thing, but you want to make sure.
dirtydrew420
Well-known member
I had the same problem in my garage, every Mason I had look at the job said concrete over concrete NEVER works. I ended up doing a demo and re pour. Very pleased with the results. Do it once do it right!
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Unless is is specifically designed and with proper separation (or bonding depending on the purpose) you will see almost all the cracks from the lower slab form on the upper slab. Concrete is great in compression, but poor in tension. Joints in the original concrete cut at the proper time would allow the concrete to release the tension from curing (concrete is never bigger than the day it is installed). Once those cracks/joints exist, the concrete has mechanical action about those lines that continue to work with loads and temperature swings. It is likely that the cured lower slab is bonded to the upper slab and not allowing it to properly release tension. This is forcing the upper slab to crack at a higher interval since it is working against the lower slab.
Reinforcing placed at 1/2 depth really only holds the panels together to ensure that there is proper aggregate interlock if it cracks. Reinforcing placed low in the slab relieves tension from loading from above and upper reinforcing relieves tension from irregular support from below (all relative to where the loading is occurring).
When we design new concrete the thickness vs. maximum panel dimension is based on section design and factors how much support is being provided by the underlying material. Concrete loves uniform support from below that can be normalized across each panel.
Reinforcing placed at 1/2 depth really only holds the panels together to ensure that there is proper aggregate interlock if it cracks. Reinforcing placed low in the slab relieves tension from loading from above and upper reinforcing relieves tension from irregular support from below (all relative to where the loading is occurring).
When we design new concrete the thickness vs. maximum panel dimension is based on section design and factors how much support is being provided by the underlying material. Concrete loves uniform support from below that can be normalized across each panel.
ConCretin
Well-known member
There are a lot of details missing but the one thing I'm sure of is that thunder has nothing to do with it. If the OP wanted to provide a few more details and pics, we might be able to offer an informed opinion.
SGKent
Banned
Expansive soil can turn a slab to pebbles. Are you sure the soil under the first pour was properly assessed and prepared? Also you say slab in one sentence and foundation right after that on top of that. It doesn't really make a lot of sense - but I do know that if you rely on anyone that they hired to assess the situation, their loyalty is to the guy who did the work and not you. I'd be getting advise from the best minds on concrete in your area that you can hire so they are loyal to you. I would also advise them going in that it could get messy so make sure they want the job. Last I'd find out if they are best buds with the guy who did the work - just to make sure I wasn't hiring their best friend. The city building department might want to take a look at this to be sure they don't want it all torn out.