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Clutchcargo 07-24-2008 11:45 AM

Filling concrete cinder blocks
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I'm going to be adding a level to my sunroom to make a walk in closet for the master bedroom. The existing foundation is fieldstone below grade and cinder block above (4 or 5 courses).
I talked to a structural engineer and he said I should fill the open cell cinder blocks with concrete. So the existing sunroom will get demoed to get access to the top of the blocks. The demo not that big of a deal because we were going to restructure for different window placement anyway and it's only 7' x 11'.

Now the questions because I've never done anything with concrete.
What mix do I use or does it come premixed in the bag (add water and voila, you’ve got concrete)?
Is there any benefit to dropping in vertical pieces of rebar?
Any idea on how much concrete I'll need?
Anything that I’m missing?

47_47 07-24-2008 12:26 PM

You can get ready measured concrete mix and add water and mix. You can mix it in a wheelbarrow or mortar tray with a hoe that has holes in it. For grouting the walls, you want the concrete a little on the loose side to help it flow into the cores. Pour the concrete in lifts and use a stick to remove the air pockets.

Concrete is strong in compression, the rebar is to take care of the tension . The advantage of the rebar is if you can't finish the pour all at once, adding the rebar will give you a continuous connection. I doubt you'll need it in 4-5 courses, for strength, ask the engineer.

Measure the wall and calculate the volume lx wx h for grouting block I divide the width by 2. Convert to yards³. There are approx. 54) 60# bags of redimix per yard.

Layout and install the j bolts for your sill plate before the concrete sets.

Mix the concrete close to the walls, so you don't have to move it too far and get a few friends to help.

Clutchcargo 07-24-2008 02:39 PM

63 Attachment(s)
Thanks 47,
What's the spacing on the J-Bolts?
Is there a trick to pouring the concrete into the wall or is it a slow scoop and fill process?

47_47 07-24-2008 03:28 PM

I do not know the required spacing, but on your 11' wall I'd use 4 and on the 7' use 3. If you can, for these short walls, lay out your framing and install the bolts where they'll wind up near the middle of a bay and not under a stud. Use 12" j bolts and leave enough exposed from the concrete for the pt sill, flat and lock washer and nut. For a 2x sill I'll leave the bolts about 2¾" high. The sill seal is usually only 1/4" high, but will compress.

I've always used a spade and shoveled it into the block.

buletbob 07-24-2008 05:44 PM

Here on the island Being in the wind zone the Foundation bolts are 5/8" dia. spaced 12" from each end and in from each corner then 4' oc there after.

Ron6519 07-24-2008 06:16 PM


Originally Posted by Clutchcargo (Post 142665)
Thanks 47,
What's the spacing on the J-Bolts?
Is there a trick to pouring the concrete into the wall or is it a slow scoop and fill process?

Spacing of the bolts is related to your local code. As for the concrete filler in the blocks, I would fill every two courses so you know it's full. With 5+ courses, you will have voids if you wait until you're at the top.
The rebar should be secured into the footing to be useful.
There should also be wire mesh in between the courses, again, by code.

concretemasonry 07-24-2008 10:29 PM

What you are really doing is grouting a wall. Grout is a concrete mix with no aggregate over 1/4".

Grout must be very wet so it can be poured. To give you an idea, concrete mix is 3-4" slump (from a 12" cone height) and grout must be 8" to 11" slump.

Pour the grout half way, rod it and then pour the next half a hour or so later and rod it. Wait an while for it to settle, top it off after it is settled.

The extra water makes sure the wall is filled completely. the excess water is absorbed by the block, causing the early consolidation or settlement.

Clutchcargo 07-25-2008 06:54 AM

63 Attachment(s)
I don't know if drilling into the field stone will be possible to secure the rebar. That would require a 36-40" drill bit. Does that exist? I imagine getting access to the bottom portion of the cinder blocks will be difficult as the placement of the blocks are staggered.
Somebody told me to use a concrete vibrator to make sure that all voids are removed. Do I need this or will working it with a stick do the same thing? Is it available for rent?

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