I live in an area where basic building materials are available but expensive - there are not many places to rent tools and things are done slowly by hand. It is also an earthquake zone.

I am thinking of building a basement/foundation out of hollow blocks, filled with cement and using rebar inside. Is it a bad idea to allow the builders to fill the cores of the hollow blocks with cement one layer at a time? I've seen back home that they wait till the entire wall is built then pour down each cavity and fill, but it's not possible to get a cement truck at the location, and I have just 2 guys to do the work.

Any advice appreciated.

  • They make special cinder blocks with notches for the horizontal re-rod. Remember to give yourself exposed re-rod at a couple of points for an Ufer ground for your electrical. – Harper - Reinstate Monica Dec 6 '19 at 15:22
  • 1
    Certainly a single pour is preferred to get best bonding through all of the concrete. However, if you are using plenty of rebar (and not running into local Building Code violations), the rebar will provide the stability from layer to layer. – Carl Witthoft Dec 6 '19 at 15:48
  • 3
    OTOH, if you are able to mix enough concrete to fill an entire layer, that should be enough to fill a couple complete vertical hollows. Consider building up the whole thing and just filling a few vertical hollows each day. – Carl Witthoft Dec 6 '19 at 15:49
  • 2
    If you pour one layer at a time stop filling 3-4” below the top of the block this will provide more strength and less chance for leaks as the separate layers won’t be at the mortar joint, I would want to fill each hole 1 at a time after the blocks are in this will be stronger and won’t be much more work than a layer at a time but have a huge increase in strength. – Ed Beal Dec 6 '19 at 18:10
  • Great tips, thanks guys – siquijor Dec 11 '19 at 11:25

What's the phrase? "The perfect is the enemy of the good."

Or something like that?

Is it better to pour them in one go? Sure. Is it enough better to make it worth a huge effort? Perhaps not, particularly if that's the way it's usually done by your laborers. The less time between pours, the better (i.e. one per day is far better than getting to it a month later.)

I would strongly encourage having "bond beams" (horizontal reinforcement) as well as vertical reinforcement. Depending on just how expensive concrete is in your area, it might be worth doing the reinforcement as a "post and beam" rather than filling every core hole. That does require blocking off the bottom of each bond beam where it runs over empty cores below. It gets you a strong framework with less material.

  • They could potentially do post and beam, but the examples I have seen are awful - the rebar not connected properly, and they pour the posts about a meter of cement at a time (not stirred, or agitated to get air out either).... it's really a mess. But could do a ring beam at the top of the blocks? – siquijor Dec 11 '19 at 11:26

Placing the rebar and grouting the cells is not random. There are several issues to consider for a structurally sound installation: 1) size, spacing and grade of rebar, 2) type of grout, 3) lap of rebar splices.

1) I too live in a seismically active area and for an 8” wide wall about 8’-10’ high we use #5 rebar at 24” on center each way. Place one bar vertically in each corner plus one on each side of each door and window opening plus one horizontally in the very bottom and very top cell. I’d use two horizontal bars minimum over 4’ openings and smaller. (The amount of rebar depends on height, load, etc. You’ll want to contact an architect or structural engineer to calculate loads.)

Rebar is meant to be on the tension side of the wall with adequate space (about 1”) around it for grout to flow and bond to it.

2) What you put in the hollow cells is called “grout”, not cement or mortar. Grout comes in “fine” and “course”. You’ll probably want “fine”, because it will flow better around rebar, etc. Grout is very liquidity and flows like tomato soup. Make sure the slump is between 8” and 10” so it will flow everywhere and bond to the rebar. (If it’s too stiff or if you use cement or mortar it will not bond properly and the rebar will be useless.

3) Rebar is in tension, so you’ll need to lap splices a minimum of 30 bar diameters. So, for a 5/8” rebar, you’ll need to lap them about 19”-20” minimum. Also, make sure the rebar coming out of the foundation is on the same spacing as your vertical bars so they can be lapped. (Make sure the bars out of the foundation is where you want the bars in the hollow blocks.) Also, make sure the rebars extend 30 bar diameter around all corners.

Someone mentioned holding each horizontal course of grout down a few inches to help with strength and prevent water leaks. Don’t do that. If water leaks in it will settle in the cavity and create mold. Keep each course of grout level so if there is a leak it can dry out through the sand crack.

Someone mentioned installing the grout in the horizontal cells first. This can’t happen because grout is very liquidity and will flow from cell to cell seeking its own level.

Laying concrete block is tricky and takes some skill. I’d get someone with experience to help/guide.

  • Thank you for that...it's incredibly hard to find anyone with experience of doing things "properly" here... it's in Asia, fairly remote. But if I can give directions they will do a decent job. – siquijor Dec 11 '19 at 11:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.