Vertical Rebar Layout in Block Wall Footing

99altrade

Laying out vertical rebar during concrete pour:

Several photos of formwork on this page:
Cement Block Wall Footing

Question:

First time doing this (ready-mix coming in 4 hours!). I want to get this perfect so that when I layout the block on footer I don't learn that I've screwed up and the rebars hit some blocks, and not all block cavities.

I will be using 8/8/16 block. Main wall 20' long and 2 courses high, then 4" solid block cap block, then bluestone tread cap. If my starting block is, say 4" from beginning of footer. Footing is about a foot deep:

1) First rebar should be placed where from that start line (which is where the first block will end) - some particular inches from that point to set it inside first cavity? How many inches? I have not dealt with block before.

2) I will go with the 24" on center rule, so from point suggested in answer to 1), every 24" precisely from that point, and I'll be safely in a cavity?

3) What length rebar should be used. I.e. - how far does rebar need to be set into footer and I'm assuming it should go as high as block cavities til cap allows, in this case 16" over footer?

4) If I'm unsure of where to put rebar as it regards corner turns, and other non standard features, is it alright to hammer-drill in after pour while setting block and set rebar in that way (I'll epoxy them in even if not necessary - I have extra).

Thanks!

99altrade

Also, do these verticals need to be tied to horizontal base rebar or just pour concrete and then push them in while still wet?

Thanks.

p.s. I know, I'm nuts it's just 2.5 courses high?

AGWhitehouse

1) First rebar should be placed where from that start line (which is where the first block will end) - some particular inches from that point to set it inside first cavity? How many inches? I have not dealt with block before. If you've got a block give it a measure from face of edge to center of cavity. Here is a generic block dimension for reference: http://www.masonryinstitute.com/nwmg...rod_b_pg4a.gif

2) I will go with the 24" on center rule, so from point suggested in answer to 1), every 24" precisely from that point, and I'll be safely in a cavity? Ever 16" on center as that is the block spacing. 8/8/16 blocks are actually 15-5/8" long accounting for a 3/8" grout joint.

3) What length rebar should be used. I.e. - how far does rebar need to be set into footer and I'm assuming it should go as high as block cavities til cap allows, in this case 16" over footer? Embed at least 8" into concrete and carry to full heigh of wall.

4) If I'm unsure of where to put rebar as it regards corner turns, and other non standard features, is it alright to hammer-drill in after pour while setting block and set rebar in that way (I'll epoxy them in even if not necessary - I have extra) you could do it this way, though more labor and material intensive.

__________________

AGWhitehouse

Pushing them in should suffice for your needs.

__________________

concretemasonry

You are also using more rebar than necessary. That wall needs no rebar for structural purposes. There might be a old, provincial prescriptive requirement or a very poor inspector (if inspected).

Here, a 12 or 13 course basement (8' of backfill) has rebar at 4' on center or 2' on center if the soil is bad.

Technically, you should just fill the cores with rebar and you should use grout (8"-11" slump) instead of concrete (3"-4" slump) to fill the cores completely.

It is always best to find out what shape blocks you will be laying and locate your rebars/dowels appropriately to match the block you will be laying. There is no such thing as a standard concrete masonry unit because the specification allow a wide range in the number of cores (1,2 or 3), the spacing of the webs and whether the ends of the block are smooth or open with a partial core. Also, the block are not always symmetrical. The thicknesses of the face shells and cross webs can also vary.

When dealing with concrete you must have a plan a day or so ahead to avoid the last minute compromises.

Dick

99altrade

You are so right, I'll give you that! Plan, plan, plan. Today was the day it had to get done, and that's what I was behind on. I have to run at the moment, but I just poured the footing, put in a few rebar (before I saw your message), any others I need to do (if at all) I will drill in while constructing block wall.

I don't think I'll be able to post pics today, but will post pics of the completed footer on Sunday. I hope you guys don't ream me too bad for the results!

Thanks again to you, and the other guys for all the advice, input, and education!

Master of Cold

Its probably too late for this, but...
The corners of the block wall will be an empty cavity.
Going 24" on center is right in the middle of your second block.
After an earlier discussion yesterday, about my home I did a little research today..so its still fresh. The main reason for rebar and cement in the block is to increase the tensile strength of the face of the wall against horizontal forces. For example, to keep a 2x4 or other projectile from going through the wall during a high wind event. If a large ovject like a car hits the rebared concrete block wall it is more likely to comprimise the entire wall. Instead of the vehicle taking out the blocks that were hot, and thier adjacents, the load is transferred through the entire wall, causing the block to fracture along horizontial joints. As far as vertical strength, the reinforcement does not significantly add to the load capacity of the wall.
So what the contractors do around here (from what I was told) is drill through the slab (3/4 rebar gets 1-1/4 hole) and rebar is placed in the hole and filled. Then the blocks are fed down the rebar and grouted in. The rebar is cut-off a few inches above the top block and a cap is poured. This adds about 25k to the price of a 3 bedroom home, but its an option and not code.