Preparing the ground to pour a slab

[3Dsigns]

Preparing to pour a slab in half of this pole barn. It has clay/sand fill dirt and has been kind of leveled out, somewhat, with a skid steer before the barn was erected. I guess that's called a "pad". It has high spots and low spots and is really not level. Anyway, I have wet it down for a couple hours and then run over it twice with this little harbor freight ********/compactor. Now, how should I go about it leveling it enough, so that I don't have 6'' of concrete on one end and 4'' on the other? What would be your process? And do I need to compact it some more? Thanks

 

[Rc_Guy]

Preparing to pour a slab in half of this pole barn. It has clay/sand fill dirt and has been kind of leveled out, somewhat, with a skid steer before the barn was erected. I guess that's called a "pad". It has high spots and low spots and is really not level. Anyway, I have wet it down for a couple hours and then run over it twice with this little harbor freight ********/compactor. Now, how should I go about it leveling it enough, so that I don't have 6'' of concrete on one end and 4'' on the other? What would be your process? And do I need to compact it some more? Thanks

Shouldn’t you have leveled it all before you compacted it?

 

[3Dsigns]

Shouldn’t you have leveled it all before you compacted it?

I don't know. Too late now.

 

[1redTA]

string and tape measure is always available

 

[Red 17]

Set your forms at the edges. Once the forms are firmly in place at the right level, use a straight edge from form to form to check the thickness. Adjust accordingly. The straight edge can be the screed used to level the concrete once it's poured. You can use string for this too, just keep it tight.

You may have to set other boards in the middle to be able to check the thickness and use the screed.

Lots of videos on line.

It's never too late until you start to pour the concrete. Extra time, effort or hiring a crew if needed will be a short term misery vs. the permanent misery of a poorly poured, uneven, cracking slab.

 

[Rusted Nut]

Thicker concrete doesn't hurt anything except your wallet, so grade or don't. Let it dry before pouring, as the clay base will shrink when it dries from the water you put on.

 

[Rc_Guy]

I don't know. Too late now.

Not to late, you level first then compact or compact then level then compact.

I would have used a transit and leveled it first.

Or buy more concrete

 

[carcruse]

Get a laser level and a rake. If it's way unlevel, get out a shovel or get another skid steer also.

 

[3Dsigns]

0k, I shot it with my Bosch rotary laser this evening. I'll pull strings on it in the morning and start pulling down the high areas with a hoe or flat point shovel

 

[carcruse]

Why use stringlines when you have a rotating laser? String lines sag. My laser came with glasses with red lenses to see the beam easier in sunlight.

 

[NUTTSGT]

string and tape measure is always available

Done that a few times and used a bubble level on the strings to double check the form boards.

0k, I shot it with my Bosch rotary laser this evening. I'll pull strings on it in the morning and start pulling down the high areas with a hoe or flat point shovel

Rotary laser, you have what you need.

Why use stringlines when you have a rotating laser? String lines sag. My laser came with glasses with red lenses to see the beam easier in sunlight.

There's that....if you have the laser, no need for strings unless you want that visual.

 

[Captain Spaulding]

What was done to the site from the beginning? If top soil and organic material wasn’t removed, it will decompose and settle resulting in cracking in the concrete. Gravel, like #53 or #73 is excellent leveling material. I filled 26” with #53 almost 30 years ago and the slab is still flat and solid.

 

[RivennHewn]

When you think you’re done compacting, do it some more.

 

[3Dsigns]

What was done to the site from the beginning? If top soil and organic material wasn’t removed, it will decompose and settle resulting in cracking in the concrete. Gravel, like #53 or #73 is excellent leveling material. I filled 26” with #53 almost 30 years ago and the slab is still flat and solid.

All that was removed and an 8000# tractor, 16000# excavator, and a 13500# forklift were driven over it several times. then about a 12000# skidsteer, and same forklift and tractor were again driven over the clay/sand pad several times..

 

[3Dsigns]

Why use stringlines when you have a rotating laser? String lines sag. My laser came with glasses with red lenses to see the beam easier in sunlight.

I bought the whole set, Bosch factory refurbished. I guess somebody forgot to put the glasses in the case.

 

[BillK]

Are you going to pour the slab yourself or have a concrete contractor do it ? If you are paying someone to do it I would suggest having them come out and look at the site and tell you what you need to do.

Just my opinion,

 

[3Dsigns]

Shot it with the rotary laser again yesterday evening and it's still dead-on. The self leveling feature is nice. I loaded up my little yanmar tractor and will take it to the site Monday
I'm going to try moving some of the dirt with a small box blade. After that I might put one of the land levelers behind it and refine it some more. Then run the compactor over it and check it again. I'm planning to build the forms myself, then my neighbor, who is a contractor, is going to pour it for me. There's no way I can pour and finish a 36x32 slab by myself, especially now that I'm nearing 65 years old.

 

[carcruse]

I bought the whole set, Bosch factory refurbished. I guess somebody forgot to put the glasses in the case.

I should have clarified that mine wasn't a Bosch.

 

[RivennHewn]

All that was removed and an 8000# tractor, 16000# excavator, and a 13500# forklift were driven over it several times. then about a 12000# skidsteer, and same forklift and tractor were again driven over the clay/sand pad several times..

You are willing to pay for all that equipment, but draw the line at a compactor? The most important step in preparing a slab?

 

[C-S-H]

There's no way I can pour and finish a 36x32 slab by myself, especially now that I'm nearing 65 years old.

You could cast the slab with self-consolidating concrete (SCC). Rent a backpack fogger for use before initial set (about 4 hrs). Cover after initial set. Start proper 7 day moist cure after final set (12 hrs), keeping the slab covered as long as practical. You will need tight and level forms.

 

[racecougar]

You are willing to pay for all that equipment, but draw the line at a compactor? The most important step in preparing a slab?

The OP shows and mentions the compactor in posts #1 and #17.

 

[RivennHewn]

The OP shows and mentions the compactor in posts #1 and #17.

Little plate compactor will only do 3-4” deep.

We only use them to tighten up a surface.

Use of a roller gives much better results.

 

[racecougar]

Of course. Hopefully the OP has compacted in appropriate lifts to match the plate compactor shown.

 

[3Dsigns]

You are willing to pay for all that equipment, but draw the line at a compactor? The most important step in preparing a slab?

Yeah I bought a brand new compactor, wasn't aware that it was not an adequate one.

 

[Scotto]

To make damn sure you're compacted, I'd rent a trench compactor/roller, like this:

It's very cheap insurance (~$250/day). The plate compactor doesn't compact more than a few inches IMO and is best for final compaction for something like pavers.

When I prepared my garage site, I had 3' stump holes to fill. I filled them in 4-6" lifts and compacted with a trench compactor each time. I also added almost a foot of fill to the whole area to level it out. It's been about 4 years and not 1 crack in the 30x40 garage.

 

[rmack898]

I drove a concrete truck in NE Florida until a year ago and I've seen a lot of good and bad concrete work.
Hire a finishing crew to do your slab, it will be money well spent. A good finishing crew make it look so easy and tempting to do it with a "Buddy that knows concrete". Do overs in concrete are extremely expensive. Hire the slab out, they will know what the site needs before placing the concrete.

 

[RivennHewn]

Yeah I bought a brand new compactor, wasn't aware that it was not an adequate one.

I’ve been doing this kinda stuff for forty-some years, and I’ve learned some expensive lessons.

Most of my jobs these days require a geotech, special inspections, and nuclear compaction testing. It all add$ up, but gives peace of mind and accountability to contractors .

Not everyone wants to spend that kind of money, which is why I preach hitting it more than you think it needs.

 

[ConCretin]

It has clay/sand fill dirt and has been kind of leveled out, somewhat, with a skid steer before the barn was erected.

Based on your post and the pics you provided, it appears like your sub base is basically native soils the have been disturbed and then roughly graded flat. It would also appears that we are talking about an inch or two of cut and fill to level things up. If this is the case, you don't need to rent any expensive equipment or purchase any exotic materials. You definitely don't need a sheep's foot roller, self consolidating concrete or a backpack fogger.

If I'm correct in my assumptions, your little compactor is just fine. Use whatever equipment you have to cut down the high spots and fill in the low spots until you are level, As long as your lifts are only a few inches deep, just run your compactor over it a few times and you're good to go. If the pad is at the elevation you need, proceed with slab prep. If it needs to come up, don't get too fussy with rough grading and import a good granular material, rinse and repeat.

It geotechnical terms the loads on the soils under a slab on grade are fairly modest but it does need to provide uniform support to the slab so be consistent with materials and methods.

Take a look at my Guide to Floor Slabs in the link below for some additional thoughts on the process. Good luck with your placement and congrats on the new shop.