DIY concrete slab in sections?

[casperz]

I'm researching doing my own concrete work for a 40x70 slab to save some
labor costs.

I've gotten bids in the range of $4.00 to $4.50 sq/ft for a 4" slab, 8" x 24" perimeter footer. The quotes include #3 and #4 rebar 18" O.C. and 3500 psi concrete. The non-labor costs including concrete, rebar and equipment rental charges seem to be about 1.90 sq/ft. It looks like I can save about 2.10 to 2.60 a sq/ft doing it myself.

I have poured small slabs before but nothing this large. My concern is the final finish. I can hire enough finishers that could handle the entire pour at once but I quickly get to the $4.00 sq/ft mark.

I'd like to pour the footer and then come back and pour the slab in sections that one or two people could easily handle like 16'x16' sections or smaller. Of course each section would have rebar tying it into the previous sections.

From a structural standpoint, this technique would seem to have built in control joints.

Has anyone ever done this?

 

[Mandres]

how could you possibly keep it level?

 

[Vicious_Cycle]

Sounds simple enough, but you will pay extra for trucking if you do it that way.

You might just need to bite the bullet and spend the money in this case.

 

[casperz]

how could you possibly keep it level?

Just like any other large pour. You would screed from the existing slab to
the forms on the new side. I could shoot the elevation of the forms for each new section with my transit.

I know large slabs are done this way such as airport runways, just
wanted to see if anyone had tacked a smaller job this way.

Trucking costs are a good point but still would not add $2 sq ft.

 

[1320stang]

My father did the 60x120 shop he used to have this way. We didn't have a problem with the water table and it was on a flat area that had been well compacted. I don't think you can get as smooth of a surface this way, but his shop was an uninsulated barn for storing equipment in, more like a farm machine shop type deal, not something you'd keep a Ferrari in. He built another shop, this one was insulated and it was professionally poured, very nice, more like my polished garage floor.

 

[boiler7904]

A 40 x 70 slab split into 16 x 16 sections would mean something like 12 individual pours. Way unproductive and wasteful few reasons:

1. You'll get hit with a charge for having less than a normal minimum load which is usually in the neighborhood of 5 yards - you'll be at just over 3 per section.

2. 12 individual pours = 12 days of work per man. Even if you split it into 6 sections and had 2 guys, you'd have 12 man days invested.

3. You have to set up and clean up at the end of each pour = lots of repitition that you'll have to pay the finishers for.

4. You'll have a lot of extra steel (and labor) to tie the sections together.

5. You delay moving on to the next step with each additional day of pouring the slab.

Personally, I'd bit the bullet and hire a full crew of finishers to knock it out in one day - two at the most. It will be more cost effective and it will keep the project moving forward.

 

[1320stang]

Well, he's talking about doing it himself. Either he has really, really good friends, or he'll not have them long. My concrete company has a 2 yard minimum.

 

[wilbilt]

I can see the benefits to this approach from a cashflow standpoint. In my case, the cost of the slab is the biggest obstacle to having a usable garage/shop. Every other component of the project can be purchased one piece at a time if need be, but that giant, gleaming hunk of concrete cannot.

The minimum order here for a truck is 3 yards. Towable 1-yard toters are available, but cleaning and returning the trailer cuts into finish time if you are working alone. I very nearly had a yard of sidewalks get away from me on a warm day due to cleaning the mixer and the trip to return it (30 miles there and back).

I am leaning heavily toward a pole building, for a few reasons.

The holes can be dug one at a time, or a few at a time, and the poles set when convenient.

No need to pour a footing all at once. Trenches "grow" and tend to consume more concrete than estimated. This is a problem if there is not any cash for another truck on pour day.

Once the building is enclosed, the floor can be poured in sections, based upon the help and cash available.

With the pour going on inside a partially-enclosed structure, finish time availability is increased on warm days due to less exposure to direct sunlight.

In the long run, time and costs are increased, but lump sum outlays are broken into manageable chunks. If I could fork out $10-$20K for labor and concrete, I would do it in a heartbeat. Since I cannot, I have to look at the alternatives.

 

[casperz]

All great input, thanks guys!

I do plan to do this pour with two other guys. None of us have much
concrete experience but I have done smaller slabs.

To answer some of your downsides I could change the size to 40 x 80 that
would be 8 20x20 pours. Add another day for the footings. Each pour would
take just under 5 cu/yds. The redi-mix minimum is 4 cu/yds so I'm ok there.

One section a day. Not a big deal as far as extra days. Sure I'd love to have
a crew do it and if I won the lottery I would. I could also downsize my
plans and have a smaller shop but I think it's a fair trade 9 days to save
about $6000. A delay of even 2 weeks is not a big deal for me the end
result is I have a good sized shop.

Extra steel to tie things together is about $800 tops.

The other bonus is I can spread the pours out if need be to fit our
schedule and finances.

I plan to use VCT tile or Racedeck so the finish is not too important as long as I'm level and flat.
I see a lot of upsides to this for my situation but I'm still looking for the downsides and willing to listen.

More questions:

I plan to finish the gap between old and new sections like a control joint,
anyone have any suggestions for anything different?

Is 20x20 too far apart for control joints on a 4" slab with 12" O.C. #3 rebar?

Anyone have any experience with power trowels?

Could three guys do a 20x40 slab in a day using one of these? That would cut it to 4 pours.

 

[Charles (in GA)]

Put money in the bank till you have enough and then hire the crew. In reality, you are not getting ahead, it will take you as long, since you will be doing it piecemeal, as if you saved up for it, and the entire job will be better in the end. Do you plan on doing a metal building or a stick built structure. In any case you need to calculate the standard dimensions so the stud spacing comes out correctly with no odd ones at the ends or corners. You will have to figure out where to put anchor bolts if a steel building or if you are using anchor bolts on a stick structure. You have a fair amount of advance planning to do here, and all the more important since you are not pouring all at once.

You could use concrete keys, those metal strips that have the offset in them, keys the slabs together so they don't shift. Will give you an edge like a form board, but then again the form boards are just as good a solution.

Charles

 

[ownsaglock]

Just a tip- if you decide to do it in sections(I wouldnt) dig under the previous poured slab about 6"x6" the full length and make sure you get concrete from the new slab pour in this area. It forms a sort of keyway that keeps the slabs from heaving uneven at the joints.

 

[bobbyd]

20x20 is much too far apart for control and contraction joints. The old standard use to be 225 sq ft or 15x15. The industry is quickly headed toward a recommended 8x8 ft grid to minimize cracking.

Like you said, keeping it flat when pouring isn't an issue. But concrete can undergo a phenomenom called curling that tends to be worse at corners and edges and is caused by differential drying and shrinkage between the top and bottom of the slab. The more edges you have, the greater the chance to have a problem there.

Your 40x70 slab represents about 2800 sf. Good finishers can handle up to about 2000sf each. Considering the edges that have to be ran, three good finishers, yourself and one friend ought to be able to place this slab in one shot if the weather is favorable.

Using your $2.00/sf labor price your paying $5600 in labor. Let's assume the pour will take 10 hours so you'd be paying 50 manhours for the crew example above. That's about $112 per hour you'd be paying for finishers. I'd be talking to a different concrete contractor. Slabs of this type should go for no more than $0.65 per sq ft for placing and finishing. That would bring your labor total down to about $1800, a much more attractive prospect.

 

[V-10 Killer]

20x20 is much too far apart for control and contraction joints. The old standard use to be 225 sq ft or 15x15. The industry is quickly headed toward a recommended 8x8 ft grid to minimize cracking.

My brother finishes concrete and he told me the same thing. When we do my garage this summer, we will do no more than 120 sq ft sections.

 

[casperz]

All good information guys. I'll get some bids on placing/finishing only. If I could
get it for .65 sq/ft I'd go for that.

If I use a 8x8 grid what is the best product to use to fill the seams?

20x20 is much too far apart for control and contraction joints. The old standard use to be 225 sq ft or 15x15. The industry is quickly headed toward a recommended 8x8 ft grid to minimize cracking.

Like you said, keeping it flat when pouring isn't an issue. But concrete can undergo a phenomenom called curling that tends to be worse at corners and edges and is caused by differential drying and shrinkage between the top and bottom of the slab. The more edges you have, the greater the chance to have a problem there.

Your 40x70 slab represents about 2800 sf. Good finishers can handle up to about 2000sf each. Considering the edges that have to be ran, three good finishers, yourself and one friend ought to be able to place this slab in one shot if the weather is favorable.

Using your $2.00/sf labor price your paying $5600 in labor. Let's assume the pour will take 10 hours so you'd be paying 50 manhours for the crew example above. That's about $112 per hour you'd be paying for finishers. I'd be talking to a different concrete contractor. Slabs of this type should go for no more than $0.65 per sq ft for placing and finishing. That would bring your labor total down to about $1800, a much more attractive prospect.

 

[bobbyd]

For your garage project I would place contaction joints at 10' on center each using an early entry sof-cut saw (10' so it comes out even on your 40x70). This should be done the same day as the pour. Wait long enough that the saw creates a clean line but not so long that there's a chance it will start cracking. You want just a little roughness on the edge of cut. Don't let them talk you into sawing the next day, it's too late. Pay attention to the guy running the saw, you'll want some one that will pay attention and make sure the lines are straight. Request that they use black or grey chalk in their chalk line when they snap the saw cut lines. If they don't have black or grey, insist on blue. Red will stain fresh concrete and be there a long time.

Once it's sawed and has cured for about a week, fill the joints with a self leveling 1-part polyurethane sealant grey in color. Overfill the joint just slightly. Once the sealant has cured (about a week), take a razor scraper and trim the joint flush with the surface. I've done this in other garages and the joints just disappear.

Note that if you plan on putting a sealer on the floor, make sure you do the sealant first. If you seal the floor and then try and caulk the joints, it won't stick. Most water based curing compounds won't affect the sealant so make sure it gets applied as soon as the finishing is done on pour day. Clean the joint thoroughly with a wire brush prior to appling the sealant.

Here is a link for an applicable sealant:

http://www.chemrex.com/documents/sl1_tdg.PDF