slab/framing questions
Last Post 04 Dec 2015 04:41 PM by BadgerBoilerMN. 37 Replies.
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BadgerBoilerMNUser is Offline
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21 Nov 2015 08:58 PM
We use wire in all of our slab on grade heated slabs and heated driveways. A basement or walk-out is strictly optional, but as Jon correctly points out will control cracks and prevent most modest shear forces. Preparing the substrate is most important.
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21 Nov 2015 10:45 PM
After years of doing it, we have come to the conclusion that both mesh and glass fibres are almost useless.
If you are going to reinforce the slab, use minimum 3/8" rebar or better yet, use Helix Micro Bars.

The reinforcement is both to strength the slab and to prevent shrink cracks. Shrink cracks are the result of the concrete losing it's moisture therefore reducing the volume. As the volume reduces, the slab will crack unless it is very small, has components to prevent or spread the shrink evenly or is place with water reducer in a very low slump. While it is not impossible to place crack free concrete, it is extremely difficult so most assume some cracking will occur.

Since we have agreed many months ago that there is almost no heat gradient in an ICF wall, why would a floor slab perform differently?
Just because concrete has a small R value, "about nothing," it does not mean that the heat is "lost", it just means the upward movement is slowed down. This make recovery a wee bit slower but does not make the actual conversion of energy less effective. Unless you don't put any insulation under the slab. Adding a slab sensor will look after that recovery if programmed correctly.
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21 Nov 2015 11:57 PM
Badger -
1) if using wire mesh, do you bother lifting during the pour to center it, or is sitting on the PEX enough to give the structural support you want?
2) in terms of preparing substrate - which substrate do you prefer, and do you rent/use a compactor to 'prepare it', or what do you consider ' well prepared'
3) Im not familiar with 'heat source being controlled with "outdoor reset" rough explanation of this on a simple hot water tank hydronic system?

FBBP-
google of helix micro bars shows interesting information. My nearest supply would be across the border to Canada.... is this a product my local concrete plant would be familiar with using, that if they did not have available, I could supply and they could properly mix? Looks like the finishing has some particulars that may require a crew that has dealt with it before.

I don't mean to be hijacking the thread into other topics.... but rather than a new thread....
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22 Nov 2015 09:44 AM
I did not use wire in my own walk-out slab and would NOT let anyone pull it up if I did, since the theoretical 1.3% performance bump is not worth the risk of a saw cut in your PEX. My own sub-soil had rested for 100 years so compaction was not needed.

It doesn't pay to overthink a basement slab.

Outdoor reset i.e. weather sensitive controls will help with comfort and economy. When combined with a slab sensing thermostat, as FBBP suggests, just can't be beat.
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22 Nov 2015 10:19 AM
but does not make the actual conversion of energy less effective


Slightly more R value between your source of heat (tubing) and where it needs to get to (the room) means slightly higher water temp is needed to achieve the same output. Higher source temps may or may not result in lower conversion efficiency (definitely with a heat pump, not at all with an electric boiler or fixed temp water heater).

My understanding is that polypropylene fibers are effective in reducing plastic shrinkage during curing. But don't help with crack size control.

If you are concerned about cracks, think about where you can put control joints.
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22 Nov 2015 03:53 PM
Posted By milnerpt on 21 Nov 2015 11:57 PM
1) if using wire mesh, do you bother lifting during the pour to center it, or is sitting on the PEX enough to give the structural support you want?
2) in terms of preparing substrate - which substrate do you prefer, and do you rent/use a compactor to 'prepare it', or what do you consider ' well prepared'
3) Im not familiar with 'heat source being controlled with "outdoor reset" rough explanation of this on a simple hot water tank hydronic system?


1. From structural and crack control perspective, it should be lifted near center pour, but like Badger mentioned, the problem is keeping it there, and you really don't want to damage a tube. Even with the WWM attached, the tube does have some bouyancy. Probably best practice for the tube is to staple it down and lay the mat over top, then just lift the mat. IF you are concerned with upward heat transfer, put more insulation under the pad

2. IF it is undisturbed native soil, then it shouldn't need compacting. IF you had to dig or backfill or otherwise disturbe native soil, yes, compact it... Crushed rock with fines compacts the best, but your native soil might also compact well enough

3. Outdoor reset adjusts the source temp(boiler) based on outside temp. As the outside temp decreases the boiler temp increases. As the outside temp drops, your building looses more heat to the outside thru the envelope. By raising the source temp, you put more heat into the envelope to compensate. Once setup properly, it should keep the inside at a stable temperature as the outside temp fluctuates. Most boilers designed for heating have this feature these days. mose domestic hot water heaters do not, so you want a unit designed for heating duty(prettymuch need that for code compliance anyway) IF piggybacking the radiant system off of another boiler system using a heat exchanger, they make electronic mixing valves with an outdoor reset sensor that will vary the temperature at the mix valve...
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22 Nov 2015 06:26 PM
1) It is for a walk out. I am going to have to back fill 6" gravel in to pour the slab so it covers my footings.... which probably means I should compact it.... rent a machine? just rake it? tamp it? Obviously fines would help it settle even more, but wouldn't that affect its ability to function as a drain? (actually, more I think, maybe it would not and something with fines would be superior to washed rock?)

2) I plan on having the slab tooled (it will be 30x30) for relief control joints. Im a little torn on if I should do more for the integrity of the slab, such as mesh, rebar, or fibercrete. Maybe the more I think about it, the more I want to keep that pex away from the surface (don't lift it), and that putting mesh over it and having concrete crew walking all over it (damaging the pex) makes me nervous. Even though the fiber may not give as much structural support and crack resistance than some people feel, its 'good enough' for an enclosed basement.
-badger, you reference your own slab.... did you place anything in it for reinforcement (as you mention no use of mesh)?

3) I would be piggybacking off DHWH. ill have to talk to my provider about proper air/slab sensor... this would function the same as (or is the same as?) the 'outdoor reset'?
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22 Nov 2015 09:04 PM
Does your water situation warrant a drain inside the footing? If it does , well then you need to leave room in the stack-up for a capillary break(pea gravel) under the vapor barrier, with this leading to a drain pipe in drain rock around the inside of the footing. That drain gets piped out under the footing to a dry well or a catch basin with a pump to pump it into a drainage system.

The slab sensor is connected to your thermostat and controls the slab/room temperature by cycling the heat to the slab on and off to maintain a set temp(controls the circulating pumps and zone valves if used).

The outdoor reset system has a sensor you put outside to measure the outside air temperature. It modifies the water temp sent to the slab when the thermostat calls for heat. Since you are piggybacking a DHW system, you will need a thermal regulating valve with outdoor reset control.

What DWH are you planning on using?
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22 Nov 2015 10:15 PM
I did not use wire for my own or the dozens of basement radiant slabs I've done for others.

I use a simple air sensing thermostat for most radiant slabs. A slab sensor is rarely needed.

All condensing boilers have outdoor reset but it is rare to find any other space heating boiler with this feature and even more rare to find one as standard equipment.

We use many water heaters for space heating. Some are condensing, high efficiency and some even have outdoor reset and a heat exchanger as standard equipment e.g. HTP Versa-Hydro.

There are many ways to skin this cat.
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23 Nov 2015 10:38 PM
Water situation -
Foundation and footing have a cold joint that seeps. The concrete these are constructed with had an additive to make it waterproof (some mineral, I can look up the name later). PNW has seasonal wet/dry seasons. At the wettest time of year, the wall is dry, water seeps at the joint, dampens the footer. Some water drips off the edge, does not pool. Maybe at worst 1/2 cup of water per foot of footing at the end of a large rainstorm. Currently, it is a crawl space with lots of open air, and seems to just evaporate in the area (1200sq ft). Plan is to pour 6" gravel - 3/4 drain rock, which will come up to the level of the top of the footing. I can put a French drain in, just in case, but likely it would be overkill? The gravel area drains to a low point that goes under the footing and to daylight out the walk out part of the basement. No pump present. Plan will be for dimple sheeting running up along the footing and up the foundation, vapor barrier, 2" XPS, 4" floating concrete slab with 2" perimeter XPS.... that should make it water tight.

DWH - label states Bradford white corp - 50 gal 42000BTU/hr, model DS150S6FBN

900sq ft space will be finished with slab. heat loss calculations - I think I underestimated my 99% low, hard to calculate total R value for walls as it is still yet to be worked out.... but globally a well insulated basement below heated space in a mild climate. My calculations 8000, other 'in the know' approximate 12-14000 roughly for that space in this climate. (im thinking the bigger issue is not to undershoot and overshooting isn't as big of a problem?)

I plan for a air, and possibly slab sensor as well hooked up to thermostat, open system, potatable tubing/components, pre-fab control panel. 1 zone, 4 circuits 300', 1/2" pex. Plan is to take some pictures, draw some diagrams, create a new posting once I gather my thoughts.... then let the real criticism (and stop hijacking this thread) begin. I take it all as constructive, as it has led me to contemplate many aspects I didn't think of. Thanks for all the input.
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24 Nov 2015 01:51 PM
Posted By jonr on 21 Nov 2015 10:50 AM We also embedded #4 rebar into the stem wall when we poured and then bent them over and tied them into the rebar. This ties the slab into the foundation and locks everything together. I thought that this encourages slab cracking and the best way was to allow the slab to float/shrink freely. For example, some people put a little gravel over the footing edge so that the slab can't bond to it. I waffled on this quite a bit because of the amount of people (on the internet discussion forums)that felt it was better to float the slab, but ended up defaulting to what I know and the recommendations of tradesmen and engineers around me. I am in commercial and industrial construction and work side by side, and supply, concrete guys.....multi billion dollar companies to local flatwork guys, and I employ a structural engineer as well. Every single one of them recommended pinning the slab to the foundation. These are guys that have decades of experience each and thousands of pours under their belts...from simple slabs, to highly complex, crazy, cutting edge "formations". After discussing the pros/cons (and aggravating the hell out of them), I pinned the slab. Just for grins and giggles, and I know people appreciate seeing large engineering projects, 2 projects I'm currently involved in: https://vimeo.com/user35422462 Iowa Fertilizer, $1.8 Billion project http://www.iowafertilizer.com/ http://frib.msu.edu/content/construction-progress-photos?page=1 MSU FRIB, Atom Smasher, $730 million project. Base slab is 14' thick! Don't know how to hyperlink the links, but copy and paste will get you there..
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24 Nov 2015 03:46 PM
Every single one of them recommended pinning the slab to the foundation.


I don't see any references to this (and many to leaving it floating) - so I'm interested in the pro/con details.
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25 Nov 2015 12:24 PM
Posted By milnerpt on 23 Nov 2015 10:38 PM
Water situation -
Foundation and footing have a cold joint that seeps. The concrete these are constructed with had an additive to make it waterproof (some mineral, I can look up the name later). PNW has seasonal wet/dry seasons. At the wettest time of year, the wall is dry, water seeps at the joint, dampens the footer. Some water drips off the edge, does not pool. Maybe at worst 1/2 cup of water per foot of footing at the end of a large rainstorm. Currently, it is a crawl space with lots of open air, and seems to just evaporate in the area (1200sq ft). Plan is to pour 6" gravel - 3/4 drain rock, which will come up to the level of the top of the footing. I can put a French drain in, just in case, but likely it would be overkill? The gravel area drains to a low point that goes under the footing and to daylight out the walk out part of the basement. No pump present. Plan will be for dimple sheeting running up along the footing and up the foundation, vapor barrier, 2" XPS, 4" floating concrete slab with 2" perimeter XPS.... that should make it water tight.

DWH - label states Bradford white corp - 50 gal 42000BTU/hr, model DS150S6FBN

900sq ft space will be finished with slab. heat loss calculations - I think I underestimated my 99% low, hard to calculate total R value for walls as it is still yet to be worked out.... but globally a well insulated basement below heated space in a mild climate. My calculations 8000, other 'in the know' approximate 12-14000 roughly for that space in this climate. (im thinking the bigger issue is not to undershoot and overshooting isn't as big of a problem?)

I plan for a air, and possibly slab sensor as well hooked up to thermostat, open system, potatable tubing/components, pre-fab control panel. 1 zone, 4 circuits 300', 1/2" pex. Plan is to take some pictures, draw some diagrams, create a new posting once I gather my thoughts.... then let the real criticism (and stop hijacking this thread) begin. I take it all as constructive, as it has led me to contemplate many aspects I didn't think of. Thanks for all the input.


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30 Nov 2015 07:00 AM
Posted By jonr on 24 Nov 2015 03:46 PM
Every single one of them recommended pinning the slab to the foundation.


I don't see any references to this (and many to leaving it floating) - so I'm interested in the pro/con details.


(Paraphrasing)..Structural engineer wanted the walls tied together to counter lateral forces, hydraulic pressure, wind, and frost heave from the exterior. With no floor joist, slab is structural element, horizontally. I'm sure its more critical the higher the walls out of the ground, but engineers err on the side of caution, overkill. Field guys say that crack control is better when the concrete is one unit as opposed to separate (they all feel concrete will crack at some time). Floating slab will allow cracks to become bigger, pinned slab will limit movement. Other concerns were interior walls on a floating slab, exterior walls on foundation with nothing to hold movement together between the two if they were separate (drywall cracks, finish issues..)
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30 Nov 2015 12:47 PM
You must always accomplish engineering when required and you must always accomplish what the engineering calls for. Of course, being licensed professional engineers, what else would we recommend!

The building design has to address all of the loading that it will experience. We design/install HR heated floors in seismic zones 2b/3. We also design passive solar heated buildings with larger than code allowed roof overhangs thus requiring additional structural accommodation for the larger wind induced lateral loads. There are a multitude of ways to satisfy these engineering structural requirements without having to rigidly pin slab-on-grade or elevated ground level slabs to the footings or stem walls (e.g., wider and stronger footings). Excessive rigidity is not a good thing in seismic zones.

All of our HR slab-on-grade or HR elevated ground level slabs are floating slabs. Yes, concrete will always crack but the location of this cracking can be precisely predetermined by using concrete crack inducement strips or by cutting concrete control joints. HR heated slabs also introduce larger thermally induced cyclic loads that are not normally experienced in other buildings. When integrating HR design with slab structural design, the goal is to know precisely where the concrete cracking will occur and to limit the magnitude of this cracking with adequate reinforcement. PEX must be properly sleeved when it has to be placed where this controlled concrete cracking will occur to minimize the likelihood of having damaged PEX in the future.

This may well be a discussion about the difference between an integrated engineering design and piecemeal engineering designs... Anyhow, you will likely be just fine and good luck!
Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
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01 Dec 2015 08:38 AM
Not to get too far off topic (but I guess I am), but there is a large disconnect between engineers and what they design in the office and what actually gets built in the field (and how it performs). Up until about 6-7 years ago, engineers were designing things fairly well...meaning the intended design was what was pretty much completed in the field. Recently, however, that's not the case. The good engineers, ones who grew up in the particular industry and usually had field experience, prior to, or during their careers are gone, retired. CAD, the boon of many an engineering student, has become the bane of the field. Point and click has taken over drawings. A library of cut sections and details is the "bible" of current engineers. Subs are expected to "make it work" without much guidance from engineers (other than "intended" design). The bidding process makes it even worse...low bidder gets the job. If you can hire a fresh faced engineering student, who has never picked up a hammer, welder, rebar, etc (or had to personally correct a design mistake) to pick standard details from their library of details and copy paste into the drawings for minimum wage and let the subs work out the details, well, hey you can bid the job with less hours and costs and get the contract. You can't ask them to go through the design and make sure its actually going to work...cause they've never been in the field to see the issues or the impact other designs, trades have on their design. Its totally flipped to where issues are created by generic details and then the engineers rely on the field guys to provide or come up with a solution. Its refreshing to see and discuss details on this forum with people who seem to have the knowledge and experience to back up their design and statements (even if they expose flaws of my own)... because its certainly the exception and the not the norm in the industry today (might be a commercial/industrial construction thing and not so much residential, I'm not sure). I guess that's my rant for the day, I've used up my parenthesis allotment. I appreciate your comments and clarifications
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01 Dec 2015 11:13 AM
There is a lot of truth to what you wrote. Back in the day when the US was a manufacturing powerhouse, engineers would get hands-on experience with electrical circuits, machining, metallurgical principals, programming languages, and welding. These days MS Excel and PowerPoint seem to be the core competencies. These days experienced good engineers are often not respected or valued because they point out the failings of less competent engineers and management leadership. Superficial rhetoric has replaced substantial action in many areas. Sometimes companies, countries and people just have to lose everything before they learn what truly matters.
Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
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04 Dec 2015 04:41 PM
You had me going until you said "open" system "potable tubing" .

I outta here...
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