It begins!

[dbhansen]

This doesn't advance the conversation, but I'm very curious what the purpose of the gravel is in a situation like this. Not drainage, since water won't be entering the area below your slab anyway (like it would under a floating slab). Does it provide cushion against frost heave? If that's the case, then it seems like it should be UNcompacted. Compacting it in frost-free areas makes sense, to prevent settling, but there is no chance of your slab settling after it dries. It almost seems like a better idea to just dig out 6" of soil and use plywood (and maybe some steel supports?) to form the bottom of the concrete, leaving 6" of air under the plywood. Does the center of the slab need support from below?

This seems like a unique type of construction that we're doing -- a relatively small slab resting on a block wall, with lots of weight around the perimeter but little in the middle. Maybe the "normal rules" don't apply.

Sorry to hijack your thread, but it got me to thinkin'....

[mfiore]

Originally Posted by dbhansen

Sorry to hijack your thread, but it got me to thinkin'....

I'm happy for the discussion! Always better to discuss these issues before I get to that point in the build than after. Seems like a solid base below the concrete would prevent it from settling/cracking (like our sidewalks)?

[mfiore]

I think a concrete slab will work best for me. Even though I have solid block footings, the block stand will be slightly smaller (4 inches in each dimension). This will give me a little support outside the stand for future finish material (whatever that will be) in the future.

I also will explore alternative shapes for the stand (possibly an 'H' shape, see other threads). The concrete will be needed in the middle to support this.

Questions: Since I have a 48 inch block foundation, do I need 6 inches of concrete (per plans) or will 4 inches suffice?

Is there a significant difference in the support provided by 1/2" re-bar vs 3/8? When I had the block delivered, my great home improvement store sent the wrong size, and I didn't realize it until using half of it.

[mfiore]

Further questions. The guys at the local concrete yard recommended compacted sand as a base below the concrete, rather than gravel. Any thoughts or suggestions?

[dmun]

I'm suspicious of sand under footings. Sand can shift and wash out, where crushed rock stays put, and has bigger voids and less likelihood of getting silted up and not draining. But we're talking about the inside of your footings, right? It doesn't matter what you put there. Nothing is going to shift or wash out inside the block enclosure.

[brokencookie]

I have seen a lot of concrete laid over crushed rock followed by a layer of sand. The sand is generally used because it is easier to screed before pouring concrete. If you want a nice even surface so you are sure you have a 4 in ( or 5 in ect) slab, sand is cheap and easily leveled.


Bruce

[PeterW]

Sand is used as a 'blinding' layer - it fills most of the gaps in the top surface of the gravel, and stops any sharp stones puncturing a damp membrane layer which usually goes between the sub base and the concrete.

It also stops the fines in the concrete leaching downwards into the gravel, and as you say, makes it easier to get a consistent level of concrete.

Cheers

Peter

[mfiore]

I found a product called "Paver-Base". It's a fine gravel mixture, comes in 50 lb bags, and is designed to go underneath brick pavers. It compacted very well. With just a hand tamper, it was solid. You really could drive on it without leaving an impression.

I put in 4-5 inches of compacted Paver Base, covered with plastic, put up my forms, and put down the re-bar.

I now have a foundation that consists of 4 courses of concrete blocks, mortared together, resting on 12 inches of a concrete footer, the bottom of which is 48 inches below grade (6 inches below my frost line). The footing is tied to the foundation with 1/2" re-bar, with every core filled with concrete. The foundation has re-bar that will tie into the 6 inch concrete slab I've framed. I think I'm ready to pour concrete. Is there anything else I need to do before I pour the slab?

Also, there are a few cores of the foundation with re-bar sticking out that I did not fill with concrete yet. Do I need to fill them well, and let them cure prior to my concrete slab on top, or can I fill them at the same time as the concrete slab (make sense?)

Thanks again

[thebadger]

Mfiore,

Congras on the progess...

Seems like it would tie in better, but given the weight, I don't think it would matter either way but would defere to the one of the experts...

Now's the time to add a time capsule if you want. I left one core empty, put my capsule in and then add a piece of durarock over the opening to keep concrete from running in. I then market the spot on the outside so I can create a little plaque (cheesy I know but how would someone know it's there in 500 years )

Keep posting pics.

Dick

[mfiore]

Did you put the time capsule in the foundation or the stand?