Pouring concrete under existing slab.

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Old 10-21-13, 03:20 PM
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Pouring concrete under existing slab.

I am trying to fill a void at the top of the pictured staircase. It was filled with dirt, but mice like to dig in and nest in there. So, I dug out a bunch of the dirt and plan to fill in with concrete. My questions...

- Which type of concrete is best for this?

- Also, how best to fill in the back portion under the slab which will be difficult to reach? Perhaps it would be easier to just pack fill dirt in the back half and then pour concrete in the front half?

This will be my first time working with concrete, so not sure what to expect. It doesn't need to look pretty... Just want to keep the mice out.
 
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Old 10-21-13, 03:26 PM
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Welcome to the forums.

I'd use rocks instead, easier to do and I've been pretty successful with that keeping critters out when dirt appealed to them. Crusher run, 3/4 -, whatever the local yard has cheap.
 
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Old 10-21-13, 03:36 PM
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Thanks for the suggestion Mitch. This is right next to the house, so I'm concerned that using rocks might create a water issue. Also, I don't own a truck and want to do this on the cheap (don't want to pay for delivery), so prefer something I can buy in bags from the local big box store.
 
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Old 10-21-13, 03:46 PM
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How far back does the void go? I'd mix a fairly dry mix of concrete and try to pack in. It's better for the existing slab to be supported than for it to span a hole.
 
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Old 10-21-13, 04:15 PM
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It goes back about 3 feet. Does it matter if I use the "quick set" concrete or just regular concrete?
 
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Old 10-21-13, 05:14 PM
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You would be amazed how many bags that would take. I'd suggest as much as possible to fill it with 8x16x4 solid cap blocks and fill any voids with mortar mix. Maybe first pouring a 2"-3" thick concrete pad to set them on. Figure your distance from pad top to underside of existing concrete in multiples of four and a quarter inches so you can stack in the cap blocks with only a little space left. The cap blocks can be scored with a carbide masonry blade in a circular saw then snapped at the score if you need to resize them.
 
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Old 10-21-13, 07:44 PM
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Thanks Ray. I was shocked when I actually calculated the number of bags of concrete I would need. I like your idea and am going to give it a shot. Seems much easier than dealing with all that concrete!
 
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Old 10-22-13, 01:49 AM
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I'd go with solid concrete myself. Poured plenty loose, and vibrated to get it all the way down to the farthest cavities. If you don't own or have access to a vibrator, just ramming the mud with a 2 x 4 after each dumping from the wheel barrow you batch it in should be adequate. Batch 2 sacks at once, to save time. Net cost and time to batch and place 10 sacks will be considerably less than partially filling with block, especially if you first do a small footing and then have to cut and trim the blocks to get them to fit. Plus the time required for mortaring in the resulting voids, and hoping for a tight enough fit at the top and ends to keep the little critters out.

No need for quick-setting concrete, either. In fact, the slower-the-better, to give you plenty of time to avoid cold joints between batches. You could even make it pretty by putting in a front and end forms, such that it looks like an extension of the walkway.
 
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