[DRIFT-O-MATIC]

I plan on using a monolithic slab for my geodesic dome home. The footers around the perimeter will be supporting a concrete block wall which will be filled with concrete and rebar. This will form the lower level on which the 2-story dome will sit. My engineer states that the block wall must be positioned in the center of the footings. Then he suggests plastering a 45 degree mortar bevel on the outside of the wall to help when installing the waterproofing. The next thing he suggested, to help with waterproofing it even more, is to embed the first course of blocks 2” deep into the wet concrete of the slab. Has anyone here ever heard of doing this latter step? Is it really feasible to do this before the concrete sets up? I figure he knows what he’s talking about, since he used to be a concrete contractor.

 

[PipeGuy]

You should take a look at using one of these waterstop products RX waterstop. I've used the RX product extensively in poured in place concrete applications and it's effective.
While embedding the block into the slab will provide a sound mechanical bond to better resist the lateral forces that try to push the wall along the surface of the slab, the technique may or may not have much effect on the joint's ability to withstand hydrostatic pressure. Using a waterstop product, particularly one with expansive properties, is a good, cost effective, means by which to provide for watertight integrity of a construction joint.

 

[Teetorbilt]

That's a new one for me. I would think that it would weaken the slab by about 30% offhand.

 

[PipeGuy]

he suggests plastering a 45 degree mortar bevel on the outside of the wall to help when installing the waterproofing.

What's the bevel supposed to do, make water run off or provide a seal? If it's a seal you're after I would expect a mortar seal to be only as good as the surfaces it's applied to are clean when the mortar is applied. I also wouldn't want a mortar used - I'd want a non-shrink grout.

 

[Glasshousebltr]

keep in mind, getting water out is as important as the seal, use drain tile.

As far as embedding your block, I would think a keyway on the footing would accomplish that.

Bob

 

[DRIFT-O-MATIC]

This is a new construction slab, so the RXwaterstop would not apply here. The bevel is just to make the waterseal (I'm assuming he means a membrane?) easier to apply. The grade is 6" below the finished floor surface of the slab, so there shouldn't be any water infiltration issues, except from sheeting off the wall.
Personally, I can't see the advantage of sinking the blocks into the concrete. I don't see how it can be done fast enough, before the concrete sets up, anyway...maybe I'm wrong.

 

[Mike Finley]

How in the hell would you embed concrete blocks 2 inches in wet concrete and have them be level all the way around?

I would think that the bevel made out of mortar would just be a mechanical attempt at helping the water off the wall and footing and not pool on the footing around the wall. I would be curious as the the effectiveness of this thinking long term though, Won't the mortar eventually separate from the blocks in places and allow water behind it and then under it?

I would think if the waterproofing process works, it works whether there is a bevel around the base or not?

 

[DRIFT-O-MATIC]

Reply from engineer

I asked the engineer what waterproofing he was talking about, and this was his reply:
"The two most common waterproofing systems I know are hot mopping (like on flat roofs) or cementatious products with epoxies in them. Home Depot carries a couple of these".

He didn't say I had to use the imbedding of the first course, only suggested it. You also need to know that I intend to dry stack all the CMU's and apply a bonding surface coat / mesh fabric to both sides, then fill with grout and rebar. So, it (imbedding) would go quicker. I just wasn't sure if it could be done before the concrete set up. Anyone know if this would work?

 

[PipeGuy]

Putting aside what the engineer says, what do you want to accomplish?

I think it would be very difficult to properly lay out and set the block before substantial set up of the concrete. If for some reason you're keen on having the block embedded in the slab to some degree, you could easily create a slightly oversized key-way in the slab at the wall (block) line when you pour the slab and then sit the block in the key way and fill any annular space with grout or mortar.

By the way, I have never seen a single foundation 'hot mopped' or had any kind of epoxy reinforced product applied. I've seen thousands of poured foundations sprayed with bituminous products, and hundreds of block walls 'parged' with mortar and sprayed. Around here anyway, drainage 'board' is also making big in-roads in the residential foundation dampproofing business.

In the end, given that "the grade is 6" below the finished floor surface of the slab", I can't imagine you'd ever have a water problem...period. Which leads me to wonder, why do you need waterproofing if the wall is above grade? What am I not understanding?

 

[DRIFT-O-MATIC]

PipeGuy, I agree with you. There should be no need to waterproof, with the grade below slab level. This is the trouble with working with a engineer in California, when my project is in Florida (he also has a license for Florida). I just could not find anyone else who was familiar with my geodesic dome home plans. I will confer with him to see if all this is really necessary. My thoughts were to just build a monolithic slab/footings, then errect my CMU walls on top of that, with the first course mortared (for leveling purposes, only). The rest of the courses would be parge coated both sides, in two lifts. Then filling all cells w/ grout, and rebar spaced as per code.

 

[PipeGuy]

My thoughts were to just build a monolithic slab/footings, then errect my CMU walls on top of that, with the first course mortared (for leveling purposes, only). The rest of the courses would be parge coated both sides, in two lifts. Then filling all cells w/ grout, and rebar spaced as per code.

Will the slab have 'L' bars along the wall line that protrude vertically up (through the slab surface) and tie to the wall rebar? If so, I really don't think there's any reason whatsoever to embed the block into the slab.

 

[DRIFT-O-MATIC]

Will the slab have 'L' bars along the wall line that protrude vertically up (through the slab surface) and tie to the wall rebar? If so, I really don't think there's any reason whatsoever to embed the block into the slab.

Yes, "L" bars will be used. I agree with you that embedding is not needed. I don't know why he even suggested it as a possibility.

Thanks, Guy

 

[DRIFT-O-MATIC]

Will the slab have 'L' bars along the wall line that protrude vertically up (through the slab surface) and tie to the wall rebar? If so, I really don't think there's any reason whatsoever to embed the block into the slab.

By the way, how are the "L" bars placed? Are they shoved into the concrete after screeding, and before floating?

 

[PipeGuy]

Ideally they're tied to a slab's reinforcement mat, with the horizontal leg of the "L" pointing towards the inside of the slab. In the abscence of a mat to tie to, I've seen them worked into the slab after bull floating and then touched up around with a hand tool. The less you have to finish around them the better.
Be careful to provide at least 3 inches of concrete around all sides of the embedded rebar.

 

[DRIFT-O-MATIC]

Ideally they're tied to a slab's reinforcement mat, with the horizontal leg of the "L" pointing towards the inside of the slab. In the abscence of a mat to tie to, I've seen them worked into the slab after bull floating and then touched up around with a hand tool. The less you have to finish around them the better.
Be careful to provide at least 3 inches of concrete around all sides of the embedded rebar.

How long is the "L" leg of the rebar? Just curious. I won't be doing the slab; but I will be doing the walls myself.

 

[PipeGuy]

I think the minimum 'lap' is typically 25 times the diameter of the bar. So a #4 'L' bar would have (2) legs at (25 x 1/2") each plus the distance along the bend. A piece of #4 bar 30" long should get you pretty close to what you'd want - plenty long if you're using #3 bar.

 

[Tom R]

Then he suggests plastering a 45 degree mortar bevel on the outside of the wall to help when installing the waterproofing. The next thing he suggested, to help with waterproofing it even more, is to embed the first course of blocks 2” deep into the wet concrete of the slab. Has anyone here ever heard of doing this latter step? Is it really feasible to do this before the concrete sets up? I figure he knows what he’s talking about, since he used to be a concrete contractor.

The beveled mortar is standard practice, - - but embedding the blocks is a cheater method I've seen only by 'rip-and-run' guys. Embedment would be more like a half inch than two inches, - - if the block were to 'set' two inches would mean your mix was already ruined.

 

[ConcreteGuy]

I think your questions have been answered, but maybe some information would help others. And BTW, being in Florida, the watertable is a legitimate concern, so don't blow off waterproofing. Try to talk with your local building inspectors about it. Anyhows...

The embedment of block is a type of key-way, as already stated. It is purely structural. If the engineer recommends it, and it is not feasible, appropriate use of re-bar doweling* and concrete keying is a good alternative. BTW, keying footing concrete has beneficial waterproofing properties, especially when combined with a "waterstop" material. Embedment of CMU has none of the above benefits, in my opinion, and is silly at best. (That's not engineering, that's "pretendgineering", lol.)

The bevel recommended where the foundation wall meets the footing is a "cant", as in roofing. The intention is to prevent a failure in waterproofing by easing the edge of a transition in substrates. This is a sound idea, but much depends upon waterproofing requirements, the product, and its application. In other words, the right product for the application, installed by skilled craftsmen, should negate the need for a bevelled transition. Lol, and despite the failure of all of the above, I've never seen a bevel specified or applied.

Most call-backs with waterproofing are because of what happens after the installation. Waterproofing is as critical as roofing. Watch the installation and then protect it.

Water never sleeps.

*ask your re-bar supplier for a "standard hook" on L bars, you have to furnish the length projecting from the footing.