Concrete questions ...

[bob_mp]

Greetings,
I am building a 22x44 garage in the Bay Area, CA. I am finalizing (upgrading) the plans for the concrete work, which hopefully starts in 2 weeks.

The garage is designed with a perimeter foundation with a floating slab which has a reinforced edge. I am planning on installing a 2 post lift and getting an epoxy floor coating. Attached is a drawing showing the foundation and the slab.

I'd appreciate some feedback on the plan and have some basic questions.

Here's what I am planning:

1. The slab thickness will be 6" with #4 rebar on 12" centers.
2. The rebar will be supported by chairs.
3. 6 sack concrete (3500-4000 PSI).
4. Max slump for the slab is 3", for the foundation, 4".
5. Stegowrap 15 mil vapor barrier underneath the slab (no penetrations)
6. Control joints cut to a depth of 1.5"
7. Curing the concrete for 7 days with a sprinkler (or few)

Some questions:
1. Should I add fiber? I understand it helps control cracks early on, but is probably of limited utility afterwards, given the rebar. It's not very expensive to add. Does it create any issues for an epoxy floor, given that I will be grinding it down anyway?

2. What is the role of mid-range water reducers? Is that something that my engineer should specify to use or not use? Do they provide better workability without reducing strength?

3. What, if anything, should be done between the lower edge of the slab and the foundation? Is it just the vapor barrier? I read somewhere that it's a good idea to have a 1/4" of sand at the top of the foundation and below the vapor barrier to add as a bearing surface to avoid putting the slab in tension. Is this a good idea?

4. I am planning on using a reputable concrete company. That said, should I ask them to perform a slump test?

5. Should I avoid having a control joint between the posts of the lift?

Anything else I should be doing?

Much obliged for the help.

Bob

 

[brownbagg]

they no way you can place a slab at three inch slump, either go with an high range super p or increase the cement to a true 4000 and place at about 5 1/2 no more than 5 1/2. that would give you a good finish and still be in the 4000 range

 

[Leevon]

I'll do my best here. I'm not a concrete contractor day-by-day but a construction management professional that deals with concrete on a regular basis.

1) No need for the fiber. Yes, it will cause problems with your surface coatings, especially if you grind it.

2) Mid-range reducers are just one type of admixture, maybe the most common but they can help with workability and consistency by reducing the water-cement ratio or basically adding slump without adding water. I would defer to your contractor, they will have a preferred mix design for this type of work, provided it meets the criteria above let them make the call. The worst thing you could do is force it on a contractor who is unfamiliar.

3) Your detail looks good all around, but particularly in relation to the slab and footing interface. I've never seen a sand break, that doesn't mean they don't have a use but I've seen millions of square feet of slabs and the slab always touches the footer, although many times it extends to the outside edge and across the entire footer unless a knee wall is needed, as in your case. I would add that typically there would be expansion joint material at the slab edge in this condition as well.

4) If the contractor is reputable, then leave the slump test up to them. They should (and will) reject a bad batch but if you request a test, it may start to mess with the finisher's confidence and possibly override decades of experience. A pissed off finisher is always worse than a high slump, please trust me on this one. Especially if you haver mid-range admixtures, that affect the slump greatly. I'd give them the spec, deal with the expectation and outcome in your contracts and agreements, whatever your method.

5) I'd stick with the control joints, usually a 10'x10' pattern unless you have housekeeping issues at the bays, then just go around them but it will crack where it wants. Control joints are amazingly effective at reducing unwanted cracks. I'd rather have a joint with quality self-leveling caulk then a crack any day. Expansion joints are recommended by ACI at 50' intervals. I'd be tempted to run a full depth expansion joint at mid bay on the long dimension, with a caulk joint but that may be overkill.

6) Vapor barrier "Stegowrap". This product itself is widely used, and no different than most of the others on the market. It's pretty simple, but you must make sure and lap the joints (I think that Stego requires 12") and use joint tape (their brand if you're concerned about warranty). NO PENETRATIONS without taping and make sure the edges are secure as well. One hole the size of your pinky can allow enough vapor transmission to wreak havoc on special flooring types.

A residential concrete guy could probably chime in here and shoot a couple of holes in my feedback for you but I think you're on the right track. Stick to the regular program, don't over-engineer and you'll be good!

 

[bob_mp]

Thanks for the feedback. A couple of followup questions...

"I've never seen a sand break, that doesn't mean they don't have a use but I've seen millions of square feet of slabs and the slab always touches the footer, although many times it extends to the outside edge and across the entire footer unless a knee wall is needed, as in your case. I would add that typically there would be expansion joint material at the slab edge in this condition as well."

Do I understand correctly that I should place the vapor barrier on the top of the footer and extend the vapor barrier up between the side of the slab and the expansion joint material? Perhaps trim it after curing the concrete?

"no way you can place a slab at three inch slump, either go with an high range super p or increase the cement to a true 4000 and place at about 5 1/2 no more than 5 1/2. that would give you a good finish and still be in the 4000 range"

Is the 5 1/2 the slump in inches or the number of sacks of concrete? In the case of high range super p, does one specify the water to concrete ratio (or strength), the use of the the admixture, and increase the amount of acceptable slump? Is there any downside to using the high range super p, aside from perhaps cost?

Thanks again,

Bob

 

[brownbagg]

5 1/2 inch slump is a nice workable mix while still maintaining a good psi in the 4000 range using a six sack mix. super p does not effect the water cement ratio, but by adding a super p to a three you can get a 6-7 slump and 45 minute later be back at a 3 with no effect to the strength.

I could tell you how super p works but then i have to kill you. if you search super p on you tube, it has a nice video on it.