Rebar dowels from foundation walls to concrete porch slab?
lgr123
| Posted in General Discussion on
What are the pros and cons of using rebar dowels between foundation walls and a concrete porch slab? (yellow arrows in diagram)
The foundation walls extend below frost depth and the backfill under the porch will be clean rock, so frost heave is unlikely. Are there any other concerns?
I’m asking because the rebar dowels where the porch slab meets the house are making waterproof details difficult. I’m wondering if we even need rebar dowels at all (our builder does not normally use them).
The walls have already been poured with dowels, but we could cut them off.
Replies
Rebar dowels provide additional structural stability by connecting the porch slab to the foundation walls. This helps distribute loads evenly and prevents differential settling, which could otherwise lead to cracks or damage in the slab.
Are there any other concerns?
Yes - restrained shrinkage cracking. This concrete slab will shrink. Tying it to the foundation along both edges will manifest shrinkage cracking near the middle of the slab.
I would either:
Omit the dowels along the house foundation wall, or;
Cut/install a control joint down the middle of the slab.
Hmm... good point.
A control joint at 6' parallel to the house would impede water drainage away from the house, though, I think?
Omitting the dowels at the house is possible. But I would feel better omitting the dowels at the outer stem walls. Would that have the same effect?
Are there any other options?
In case it matters, this is a structural slab, 12' wide and 6" thick. The rebar schedule isn't finalized yet, but it will have at least #4 on 12" centers, maybe more in some areas.
Water intrusion at control joints can be managed by a properly configured joint with sealant AND adequate slope away from the house for water drainage.
The shrinkage cracking is GOING to occur regardless of the reinforcing that is used.
The more rigid slab-to-wall connection will be at the outside wall as the slab will over-pour the wall. With or without the dowels there will be restraint against shrinkage at that wall. The turn-down at the house wall offers a bit more flexibility or relief from the shrinkage although the soil against the turn-down (or haunch) will resist that shrinkage.
Whether you employ a control joint or not, the cracking will be inevitable and you'll be fighting water intrusion anyway. At least a properly configured control joint will provide a barrier against the water, assuming that it's properly maintained.
I would define adequate slope (in concrete slabs) to be no less than 1/4" per foot, or a 3"+ drop in 12' from the house wall to exterior slab edge. While it may seem like a lot of drop, it'll be hardly noticeable except at each end.
I'm with @ct yankee on all points.
Why the 6" thick porch slab? Are you parking heavy equipment on it? For normal residential use, a 5" thick slab is overkill. The reinforcement is for management of shrinkage cracking. And a couple of layers of #4's doesn't make it a structural slab, especially if the mentioned reinforcing is centered in the slab. That kind of reinforcing is a wasted expense. Properly supported and located welded wire fabric or an appropriate amount of fiberglass reinforcing added to the concrete mix will do the job better & cheaper.
You are compacting the crushed stone fill in not more than 6" lifts to at least 95% maximum Proctor, right?
@thedoktor51:
I agree with "Properly supported and located welded wire fabric."
The challenge is when the placing crew steps all over the wire and isn't inclined to return to its proper position.
This is why I prefer bars over mesh. Of course, one can use a few bars on good sand chairs and then tie the mesh to it. More than just a couple places!
Fibers add toughness more than flexural strength. I'd add some but not a heavy dose.
Key things:
- If the extra 1-2" of concrete is really insignificant compare to the total cost, just do it. Going from 4" to 6" on a 12' x 40' porch adds about 3 yards of concrete.
- Put 4x4 mesh on 1/2" (#4) bars tied to 3" high sand chairs. Puts the reinforcing closer to mid-depth (assuming 6" slab) and the mesh closer to the surface (but still with good cover) - helps with controlling/distributing shrinkage cracks, as does fiber.
- Don't overdue the mix - 3,000 psi mix is fine. Higher strength = more cement = greater propensity for shrinkage (also CO2 demand).
- Slope the slab away from the house - 1/4" per foot min.
- Saw-cut 6' o/c both ways. Or 6' x 8'. Close to square is better.
- Finish and CURE correctly.
- After 28+ days lightly powerwash, dry, and apply a good sealer.
- After 6 months clean all control joints, apply a quality joint sealant over foam caulking rope (ethafoam rod).
More good stuff from @ct yankee- all excellent points. I'd add a couple of things to his key points:
1. Don't add too much water to that 3000# mix! It only takes a gallon or two of excess water to significantly reduce the strength and REALLY make concrete shrinkage a major problem. You should target a 4" to not more than a 6" MAXIMUM slump of your freshly-mixed concrete at the chute. If your concrete sub doesn't want to work with a mixture that stiff, have a water-reducing admixture added at the batch plant. Then you can safely use concrete with a soupy 8" slump or so and not kill the concrete.
2. Given your deep frost walls on the exterior side of your drawing, that slab is going to be permanently exposed to freeze-thaw conditions that, over time, will destroy the concrete. Make sure your concrete has an air-entraining admixture added to it at the batch plant. The plant will know the proper amount to add to get your mix in the 3%-6% range.
I am more concerned with the installation of the stone backfill than I am the previous reinforcing installation & control joint discussion. If that stone isn't properly compacted (in shallow, loose lifts with powered compactors, NOT a Bobcat or similiar), it will, over time, consolidate itself in a completely unregulated manned, leaving you a varying-depth void under your unreinforced (or in this case, improperly reinforced, concrete slab. As soon as that edge-supported AND ground-supported slab looses it's support somewhere, it becomes a structural slab that requires appropriate reinforcing steel in the proper locations to keep it from falling apart. This reinforcing needs to be designed by a structural engineer and will need to be installed by a competent rodman crew. A residential concrete crew will probably be unable or unwilling to do this work. I'm guessing that a one-way concrete slab design will be the most cost-effective solution. Both one-way and two-way slab designs will have (4) separate layers of reinforcing steel located in very specific locations to address both positive and negative moments in the slab.
They aren't common in your market, but placing waxed cardboard void forms with a coverboard under your pending structural slab would be well-considered if that's the direction you choose. A 4" void form will be more than adequate. A planned and regulated depth void under your slab is a bunch easier to deal with than a mother-nature-provided irregular one. And with a true structural slab, you won't have to deal with those pesky control joints cut in the slab!
what waterproofing are you using that can't handle a penetration? for peel and stick sheet-applied WP there are standard penetration details for every manufacturer. [mostly involving extending WP 3-4 inches out onto the rebar [or whatever other penetration.]
Thanks for the comments, lots of good ideas and many things to consider.
The slabs will be at least 6" thick because they are structural, spanning wall to wall, above rooms beneath and also above backfill. The slab is structural above the backfill because the backfill will settle. We aren't compacting the backfill to any significant degree due to concerns about overstressing the basement wall. It's not a textbook perfect approach, but I haven't heard any engineering concerns about it.
Yes, the slabs are being engineered. The engineer seems to think that the right amount of #4 will make them structural, given the short spans and light loading. It will be more than #4 on 12" oc, not sure how much more yet.
The waterproofing membrane is H&B Textroflash. It can handle penetrations with the compatible sealant, but I'd rather avoid penetrations if they're aren't necessary.
Good to hear about the backfill & reinforcing issues. Sounds like you have a design professional on your team who knows their stuff. The one concern I have with the reinforcing is finding a proficient ironworker/rodman team that knows how to properly install the reinforcing. I strongly recommend your structural engineer inspect the reinforcing install before the concrete is installed. If you get a price from potential installers based on X$/SF, I'd probably run away from them as fast as possible. BTDT with structural concrete in residential work. Residential concrete crews aren't used to installing that kind of reinforcing steel and probably will walk off the job when they see what their install is- you probably ought to be talking with commercial contractors for this work.
And don't forget about the 1/4" per foot slope to drain and to use entrained air in the concrete for your porch slab. You don't need either for interior stuff- save some $$$.
I am curious why you are using Textroflash for waterproofing. It's a flashing product, not a waterproofing product. You are doing a blindside waterproofing installation that won't be accessible after construction is complete- you have just one chance to get it done right. Use proper products installed properly!
Best of luck!
In addition to a concrete sub who knows reinforcing, the concrete placing crew must ALSO understand reinforcing so it doesn't get displaced as they walk on and around the bars.
“[Deleted]”
Absolutely! It's especially critical in a structural slab, where the bottom-most and topmost layers of reinforcing are supposed to be close to the respective surfaces of the slab. My recollection is 1" clear, top & bottom.
Any tips on a better waterproofing approach? Textroflash is what the builder always uses. It has a low perm rating and is tough, so it seems like it checks all the boxes. If there is a better product or approach I would consider it. Cost isn't a factor, since this is a critical detail to get right.
I've attached a drawing of the current plan for waterproofing. Textroflash is the pink line. The blue line is brick flashing for the brick veneer, which will be the main line of defense against water getting behind the concrete slab.
Now that I see a more complete picture of what all is going on, the use of Textroflash (or PolyGuard) makes sense. Make sure you use their pre-formed inside and outside corner pieces where the membrane turns horizontal at corners. I would recommend a second layer of the flashing that is installed over the first layer that's protecting your floor framing. It needs to lap the top edge of the first layer of flashing material by a couple of inches and be secured to the wall framing with a stiff termination bar & approved sealant along the top edge of the flashing/term bar. You'll need a row of blocking behind the term bar. the horizontal edge below the brick should ideally extend slightly beyond the face of the brick, then trimmed flush.
I'm not a big fan of Mortar Net. It just moves the problematic mortar droppings & buildup higher in the wall. I prefer the CavClear product in the ENTIRE wall cavity that's 1/4" thinner than the thickness of the cavity for the full height of the wall. They also make the head joint fiber mesh weeps like H&B carries. The weep products are both excellent. Make sure you have matching weeps at the top of the brick wall! No thru-wall flashing needed at the top of the wall. The CavClear product is 16" wide so it fits between your rows of wall ties and can be installed just ahead of the brick.
Is the second layer of flashing a protection course for the first layer?
The CavClear looks great, I hadn't heard of it before. Weeps top and bottom make sense for air flow. Should the termination bar be above the concrete slab, behind the brick?
After you mentioned waterproofing vs. flashing, I did some research and found that waterproofing membranes include a test for hydrostatic pressure, whereas Textroflash does not. I also noticed that waterproofing membranes absorb less water in the ASTM D570 test (0.1% vs. 0.77%). The waterproofing membranes also appear to be thicker than the flashing (60 mils vs. 40 mils)
Henry Blueskin WP200, Polyguard 650, and WR Meadows Mel-Rol are all waterproofing membranes, I think. Carlisle CCW-705 is sort of a cross between flashing and waterproofing (it includes a hydrostatic test, but only for a 22" column of water instead the "full" ASTM D5385 test at 100 psi/231 feet of water).
The main drawback of the waterproofing membranes compared to Textroflash appears to be lower puncture resistance and tensile strength, which might matter during rebar installation and the pour. Maybe a protection course of drain mat could help.
Maybe one of these waterproofing membranes would be better than Textroflash?
I made a comparison table (attached).
Edit: WR Meadows Air Shield is another option. It seems to be more of a flashing product than a waterproofing product (more like CCW 705) but it says it can be used for protecting against water infiltration at deck-to-wall intersections and foundation sill plates.
To answer your questions, I offer the following:
1. re: "second layer of flashing...": Yes plus some other necessary stuff. The first layer is mostly for moisture protection of the vulnerable wood framing. Note that it would be a good idea to turn down the bottom edge onto the face of the foundation wall at least 1/2". I'm curious why you aren't waterproofing your foundation walls in locations that have habitable space behind them. All the waterproofing membranes you mentioned are good. I've only worked with the PolyGuard 650. It requires a primer be installed first. It's damn tough stuff. I wouldn't worry too much about accidental penetrations. The Air Shield is most likely another good option- I haven't used it.
2. Going back to the second layer of flashing, I prefer to separate the moisture management of the various systems. The first layer of flashing should terminate at least 8" above the top of the slab. The second layer forms the NECESSARY bond breaker function between the concrete slab and the brick. The termination bar for the second layer just needs to be a couple of inches higher to get a good lap.The joint where the brick meets the concrete is the most vulnerable location in your wall for wind-blown moisture penetration. Consider forming a small concrete step here for the brick to sit on and to tweak your brick coursing. You want full-course brick at your window & door heads. If you don't, it will look like crap and cost you extra for all the custom cutting of brick and wasted material. This location is where it all starts.
3. One more important thing: in a previous reply I mentioned using preformed corner pieces from the flashing manufacturer. These are actually critical items and save you on labor costs when you use the alternatives instead. If you don't do anything here other than attempt to fold the flashing around the multiple intersecting surfaces, you WILL end up with a hole in your flashing here. Holes in flashing tend to leak.
Adding a small step in the concrete to prevent wind blown water into the bottom layer of brick is a great idea. I'll ask the mason how high it needs to be to make the brick line up at doors and windows.
The basement wall is waterproofed, I just didn't show it in the drawing. There is a fluid-applied (hot spray) polymer-modified asphalt membrane covered by Delta MS dimple board. The photo shows what it looks like in the porch areas.
Are there any preformed flashing corner boots that work with 3-5/8" brick (modular brick)? The Mortarnet end dams will probably work, since they're 4"x4"x3" or 4"x6"x3" and can be flipped around with the 3" on the bottom if necessary, but the corner boots are 6" wide at the bottom. I guess they could be cut to fit?
As far as the pre-formed corner boots/end dams, I'd suggest going with the CompleteFlash End Dams- as long as they're chemically-compatible with whatever self-adhesive flashing membrane you end up with. I prefer to stay with products from the same manufacturer- the compatibility issue usually goes away. What you are trying to seal is the point hole location where the multiple vertical surfaces meet at the horizontal surface, such as at inside or outside corners. The regular thruwall flashing membrane needs to be trimmed to have a complete membrane out to the finished face of brick. The adhesive in the TextroFlash is pretty aggressive. Snap a line for the brick face location and install it just to, or no more than 1/4" back from the line. Trimming that stuff after installation is a PITA in the best case!
Using modular brick raises some issues in both the vertical and horizontal directions. In the vertical dimension, what is the lowest level of the planned brick elevation vs. the elevation at the porch, assuming you are going to install the brick at the top of your foundation wall, not the porch elevation? If starting at the top of foundation, you will have a step in the brick where the porch starts & stops. You need to have the thru-wall flashing continuous at this kind of transition. Consider having a brick expansion joint at this location- it's a stress riser as the brick goes thru it's normal dimensional changes. Modular brick courses are increments of 2 5/8", 5 1/4" and 8". Small differences, up to 1/2" (short) or so, can be made up by the brick mason with adjustments to the bed course of mortar.
The trickiest issue will be in the horizontal plane. Depending on your local Building Code, (or good practice if you don't have one), there may be requirements for Continuous Insulation on the exterior of your building with a minimum R-Value, and a minimum depth of air cavity behind the brick. Keep in mind the difference in depth between typical residential brick vs. that of modular brick (2 5/8"). They aren't the same. Those dimensional differences make a difference in complying with CI & air gap requirements.
A couple of other detail items I just caught- the XPS insulation on your rim joist & the EPDM gasket under the sill plate. Cut the EPS board 1/2" short of the actual framing members in both directions. Stick it in place, as generally centered as you can, with an insulation-compatible adhesive. Liquid Nails is a no-go. Canned spray form like Great Stuff works great. Seal around the edges of the board with spray foam (like Great Stuff). If insulating the interior walls of the foundation, cover your sill plate & top of wall with the XPS. Don't insulate the interior of the wall with fiberglass batts unless you like mold problems. XPS is flammable & emits toxic gasses when burning. You need to protect it with not less than a 15 minute fire barrier..
On your sill plate gasket, run (3) continuous beads of acoustical caulk both below it and below your sill plate.
I'd like to use modular brick if possible. The mason prefers using it, and I prefer how it looks. But it is 3-5/8" deep and the brick ledge is only 4" deep. So we'll have to overhang the brick by 5/8" to get a 1" air gap. There shouldn't end up being a visual overhang though, because we plan to add a 1/2" to 3/4" thick polymer-modified vertical concrete overlay on the foundation walls to clean them up. Strength-wise, a small brick overhang should be fine, because the brick will never go higher than 12 feet anywhere on the house. Using Queen brick wouldn't require the overhang, but I don't like the look of Queen. See attached diagrams for the two sizes I'm talking about.
The brick coursing will be challenging where the porches meet the house (see attached screenshot of the 3D model of the house -- the brown walls are supposed to be brick veneer, but the model doesn't show it). There will be brick veneer on top of the porches, which will be about 15" above the brick veneer on the rest of the house.
This transition of brick from standard wall to porch will also make flashing & waterproofing difficult. I was thinking about using a 14" tall inside corner boot at the bottom of this transition, and another outside corner boot at the top, overlapping them in a shingle fashion.
Yes, the XPS will be air-sealed to the rim board with Loctite PL300 foamboard adhesive and/or Great Stuff foam. Maybe the Window & Door Great Stuff so it can expand and contract.
For fire protection of the XPS in the rim board, there will be a fire-rated drop ceiling in the basement (something like the Armstrong Fire Guard system) so the rim board area should be mostly protected. We also plan to cover the XPS in the rim board with 5.5" of Rockwool, which gives some additional protection (but probably wouldn't be enough without a fire-rated ceiling). We're also installing a residential fire sprinkler system.
My bad on the depth of modular brick- it's 3 5/8" as you stated & the drawing indicated. Your challenge is going to be making that horizontal joint between the bottom edge of the brick and the cement parging be both neat in appearance & maintainable. I'm not too concerned with the 5/8" overhang required to get the required 1" air space. Just use 3/4" CavClear in the air space and you'll be good to go. The mason will be able to get his fingers into the air gap space to place the brick and his trowel in to trim the extruded mortar. You do need to have the thruwall flashing extend out to the outside face of the brick, and with the overhang, there is nothing to support the last part of the membrane. And the sticky side of the membrane is going to want to droop and adhere to the concrete wall face. That won't play well with your waterproofing materials or the concrete parging that follows later. Consider terminating the cement parging with a stucco casing bead 3/8" or 1/2" below the bottom of the brick and installing a stainless steel drip with a 45 degree (down) hemmed bend in it fastened to the top of the concrete wall. Tapcons every couple of feet will easily do the fastening. The flashing membrane laps over the stainless drip. This gives you the needed bond-break between the brick and the substrate and a neat, clean recess for sealant that won't obstruct the weeps in the brick. And it's maintainable.
I had a thought on a possible way to address the coursing issue between the top of your foundation wall and the top of your porch slab. As you may be aware by now, your current 12” dimension doesn’t play nice with modular brick running bond coursing. I suggest starting your brick on the foundation wall only with a rowlock course of modular brick with an over-thickness bed joint. The overthickness bed joint will take care of small irregularities in the top of foundation wall and accommodate a small ledge cast into the top of your porch slab as previously suggested. The rowlock course plus’s (3) regular running courses should course out with regular running bond coursing on the porch.
The next challenge you will have is coursing out with your residential door openings. Play around with another rowlock course that looks pleasing &try to avoid cutting running bond brick at openings. It looks really ugly IMHO, wastes a lot of brick & is labor-intensive. Think wasted $$$$ with bad appearance.
Looks like you need a structural Eng to look at this and give you some plans/details instead of this fragmented spaghetti party of comments.
I agree completely- the sooner, the better too.
With that said, the OP has a bunch of other issues that are WAY outside the area of normal expertise and professional service of a structural engineer. BTDT. He could use some professional guidance in those areas as well.
No problem on the depth of the modular brick—it's 3 5/8" as you mentioned and the drawing indicates. Your challenge will be ensuring that the horizontal joint between the bottom edge of the brick and the cement parging looks neat and remains maintainable. The 5/8" overhang required for the 1" air space shouldn't be an issue. Use 3/4" CavClear in the air space, allowing the mason to place the brick and trim the mortar with ease. Make sure the through-wall flashing extends to the outside face of the brick. With the overhang, support for the membrane's last part is lacking, causing the sticky side to droop and stick to the concrete wall, which could interfere with waterproofing or later concrete parging. Consider ending the cement parging with a stucco casing bead 3/8" or 1/2" below the brick and installing a stainless steel drip with a 45-degree downward hemmed bend, fastened to the top of the concrete wall with Tapcons every few feet. The flashing membrane should lap over the stainless drip, providing a bond-break between the brick and substrate, a clean recess for sealant, and unobstructed weeps in the brick, making it maintainable.
I agree 100% with every point. Well said! The trick will be finding tradesmen willing & at a fair price, to do, what is supposed to be, industry standard work as well-described here.
Rebar dowels connect foundation walls to concrete porch slabs, making the structure strong and stable. They reinforce connections, preventing movement over time and supporting the reliability and durability of the whole structure.