Porch slab construction


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Old 06-14-13, 11:30 AM
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Porch slab construction

I'm getting a covered porch added to my house and am considering contracting the concrete slab separately from the porch cover. The cover will be supported by 4 large posts. So, I was wondering about the proper design for the slab. Most of the slab will be 4", but it will need to be thicker for at least the posts. Apparently some of the contractors around here do 12dx12wx24h piers at the post locations, but some of them just so a full thickened perimeter of 15x15 or so. So which is better? Either one would be rebar reinforced of course.

How about the main part of the slab? Should that have 6x6 mesh, or rebar, or fiber?

3000PSI?

What about air entrainment?
 
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Old 06-14-13, 01:57 PM
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Go with door #1. You want good footings for the posts and they can't be part of the slab. Fence is fine for the slab, and 3k concrete is good, too. In 4" you won't have any air entrapment to speak of.
 
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Old 06-14-13, 04:35 PM
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Around here I think they pour the pier footings as part of the slab (monolithic). Doesn't seem like a good idea to me, and would be likely to result in cracking where the 24" deep pier footing meets the 4" part of the slab, even if it's tapered.

Seems like a thickened edge monolithic slab would be better even though it may take more concrete. Maybe 12x12 or 16x16 on the edge, with rebar.
 
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Old 06-14-13, 05:52 PM
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What they did on my shop was build rebar cages to put down in the support holes. It sort of tied the slab to the footing and made it one piece. The thickened slab is fine if you don't put vertical weight on it like a roof post. It will surely crack unless you do like you say and make it a monster.
 
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Old 06-14-13, 08:06 PM
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Spend a few extra bucks per yard, and go with entrained air in your mix. About 5% to 7% will work just fine. Doing so will minimize any likelihood of surface scaling and/or spalling, with the many freeze/thaw cycles your climate is known for.

Monolithic slab design/construction will work for your situation only if the bottom of the thickened portions fall below the frost line. Otherwise, you can expect your slab and support columns to heave if a hard freeze hits following a rainy Summer and/or Fall. Which also means the porch roof junction with the house will be a moving joint, prone to leakage--not good.

Install control joints (grooved, at least 1" deep) in the slab to make any cracks that show up look like they were intentional. It would be money well-spent to go with a 6" slab if using monolithic design. Far less likely to "misbehave." I've seen far too many fiber-mesh slabs experience all kinds of cracking, so my vote would be to use heavy wire fabric for reinforcement. Or rebar, if you choose to go with a 6" slab.
 
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Old 06-14-13, 08:27 PM
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I think our frost line is 16", by the book. I think they go 24" deep on the "pier footings". Maybe that's why they don't do the thickened edge. They would have to make it too big and the cost would go way up, when they can do the pier footings and tie them in with rebar.

I guess I'll have to see how the pier footing is tied into the 4" part of the slab with rebar. Got any pics or drawings of that?

My 5-year-old son wanted the coffee guy and the running dog here do this to shut him up.
 
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Old 06-14-13, 08:38 PM
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I think I should ask for a 6" to 8" thickened edge even with the 24" deep "pier footings" on the corners where the posts will be. Or should I just get a 6" slab and not worry about the edges?

For what it's worth, I found our city building code (bolding added by me):

14.7.3.12 CHAPTER 4 FOUNDATIONS:

A. Section R401. See this section of the IRC except as provided below: Section R401.4 Soil tests. Delete the text of this section and replace with the following. Where quantifiable data created by accepted soil science methodologies indicate expansive, compressible, shifting, or other questionable soil characteristics are likely to be present at a particular location, a soil test to determine the soil’s characteristics at a particular location shall be performed.

B. Section R403 Footings. See this section of the IRC except as provided below.

(1) R403.1.3.2 See this section of the IECC and add the following sentence to the end of the section. Where the slabs-on-ground are cast monolithically and rigid insulation is used as a forming material, a minimum of 1½ inch rigid insulation shall be used. Where sandy, silty sand or sandy gravel soils are present, rigid insulation shall not be used as a forming material. Forms must be constructed to prevent the possibility of failure or collapse. Forms shall be constructed and maintained so that the finished concrete complies with section R401.2.

(2) R403.1.4 Delete the text of this section and replace with the following language. All exterior footings shall be placed at least 12 inches (305 mm) below the grade. Where applicable, the depth of footings shall also conform to sections R403.1.4.1 and R403.1.4.2.

[14.7.3.12 NMAC - Rp, 14.7.3.12 NMAC, 1-28-11]
 
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Old 06-28-13, 10:49 PM
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Update

I have been talking to a porch builder who also does the slab. The problem is that his concrete guy is unlicensed so works under the license of the porch builder. So, when I ask the porch builder questions about the slab construction, he just says he trusts the concrete guy. He can't give me any specifics about the slab except that "he does good work" and it will meet local building code. I would really like to make sure the slab is well built, probably a little better than what they typically do around here, but I can't get him to give me any specifics. When I asked about air entrainment, his answer indicated that he didn't know what it was. I'm thinking of dumping this guy and looking for another builder. What do you think?
 
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Old 06-29-13, 07:03 PM
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A porch guy who never heard of air entrainment, and who uses an unlicensed concrete guy. You are more of a gambling man than most of us are, putting your trust in (and spending your $$$ with) people with such dubious qualifications.

If I were you, I'd either do the entire job myself, or shop around for someone having a few more credentials and experience. And maybe mention to Mr. Porch Builder that a quick phone call to his concrete supplier will fill in all of the mysteries of air entrainment, including why it's needed and what it costs. I lived and worked in the Land of Enchantment for 25 years, and don't remember any geographic areas that didn't experience freezing temperatures in the winter time, under which conditions air entrainment is an absolute necessity. If your concrete supplier is in the Rio Grande Valley, you might want to check with them for the specifics on how they prevent ASR problems (alkali-silica reactivity) in their mixes--it can destroy perfectly good concrete in just a year or two. And while you're at it, ask for an explanation from the concrete guy on exactly why he isn't licensed to do work by/in the State of NM. If he's not forthcoming, do a search under his name with the licensing agency--if he was licensed at one time, and it was revoked, there are usually some very compelling reasons for that to take place.
 
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Old 06-29-13, 09:38 PM
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BridgeMan45, thanks for the help. I'm in Albuquerque and we get plenty of freezing temperatures here.

I'm starting to think I have learned enough about this project that I could design the porch cover myself, and with a little more knowledge I could call out the specs for the slab as well. This means I could contract the work for the porch cover and the slab separately. I spoke with the permit people, and they will allow that, but they want to see the drawings for the whole project together, so if this is true (I have my doubts) I'd have to have signed contracts with both of them before I could get the building permit.
 
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Old 06-29-13, 10:06 PM
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Quite possibly, things have changed with the Albuquerque Building Department's policies since I dealt with them (almost 20 years ago). But at that time, a permit was granted once the plans were submitted and approved. No signed contracts were required, as I did all of my own work as a homeowner, for a 500+ S.F. addition in Four Hills (a rather complex job, tying the new roofline into an existing one having multiple angles, and removing an exterior load-bearing wall). It doesn't make any sense for that to be the policy now, as I suspect a lot of work in the City is still done by homeowners, with no contractors involved.

Should you decide to draw up some plans, and either act as a general contractor or simply do all of the work yourself, I can provide some assistance in terms of what "what will fly" with the Building Department. Lord knows I've dealt with many of them over the last 40+ years, in multiple states.
 
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Old 06-29-13, 11:13 PM
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Thanks again Bridgeman45. Now that I think back to my conversation with them, they did say that a homeowner can submit the plans and get the permit, and the info I got from their website says a permit can be issued to a homeowner or licensed general contractor. The drawing examples they show don't look that difficult to do either. Just a Site Plan, Roof plan, and Wall Section, all drawn to scale.
 
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Old 07-01-13, 10:28 AM
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So, now I'm working on the design specs for the slab. This will be a 4" monolithic slab. I can either go with a "turned down edge" full perimeter footing, or spot footings where the patio cover posts will be. The spot footings would likely be 12"x12" and 24" deep. Our frost line is 16". We don't have much of a problem with frost heaving, but I'd like to overbuild it to make sure. Since this will be butting against the existing 4" porch slab which is part of the house foundation, I want to minimize the potential for vertical movement of the new slab. I have ruled out dowels into the existing slab because it's only 4". I believe that to pass inspection the full perimeter footing would need to be deeper than 16". I know the full perimeter footing would cost more due to more concrete and rebar, and labor. The slab is 9'-6" by 15'-6". Pros and cons?
 
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Old 07-01-13, 12:40 PM
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If I were the one doing it, I'd go with a full perimeter footing, at about 18" deep (or whatever the permit people require, if deeper). Several reasons for doing it this way are that 1), less likelihood of cracking in the slab/footing junction (assuming it's properly done, of course) and 2), you now have a built-in, somewhat impervious "dam" of continuous concrete surrounding your 4" slab. This minimizes the chance for water to migrate under the main portion, should you have a wet Fall and then a hard freeze.

You can further lessen the chance of slab cracking by reinforcing the footing to be a 2-way beam, putting two No. 4 rebars in the top and two in the bottom, continuous all the way around. Along with a heavy mesh in the slab and tapered transition between the slab and footing. As I mentioned in an earlier post, I'd spend a few extra bucks and go with a 6" slab (would require just an additional extra yard of concrete for the dimensions you listed). I'd also then use all rebar and skip the mesh, using dobies to hold the rebar mat (No. 4s at 18", both ways) near the center of the 6" slab. Doing so will make concrete placement and consolidation a much easier and more effective operation. Don't forget to remove any organic soil under the slab area, placed there by a previous owner or builder--best subsurface drainage will be provided by the decomposed granite that naturally exists everywhere between the Sandias and the Rio Grande.
 
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Old 07-01-13, 12:51 PM
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BridgeMan45, I sure appreciate your help. I like the idea of the full perimeter footing. I think this minimizes the chance of heaving or shifting, and, it allows me to put the posts for the porch cover anywhere around the perimeter. If I go this route, do you think I can get a permit for the slab independent of the porch cover, and then get a permit for the cover later, assuming that they have the drawings for the slab on file?
 
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Old 07-01-13, 01:04 PM
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Don't assume anything when it comes to government building departments. Just furnish them an extra set of slab detail plans when you apply for this permit, and request they stamp them approved as a duplicate set. Keep them in a safe place until such time as you're ready to apply for the porch permit, and submit them with that package.
 
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Old 07-01-13, 01:59 PM
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So, I'll take your answer to mean that you think I could get a permit for the slab first, independently from the porch cover? I'll call them to ask of course, but in your experience you think it's possible?
 
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Old 07-02-13, 07:27 AM
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Yes, I think that would be a logical approach to the overall project. But what I think is irrelevant; it's what the City Building Department will require that counts. If you apply for the permit with everything at one time, in one package, you will be under the gun to get all of the work finished within the permit's time-frame, putting more pressure on you. Or requiring that you apply for a permit extension, for an additional fee, of course.
 
 

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