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Compacting base for slab

Quijote

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Feb 27, 2013
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179
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Greater Boston
Just a story, no questions.

My garage is getting ready for its slab. Naturally, I told my contractor to do whatever it takes to make sure the slab will never crack (in the sinking/failure sense, not the stuff you can avoid with saw cuts).

To my surprise, he said, the stuff that works best is to just soak it down.

A few weeks ago when the foundation was finished, he brought in (good) fill and brought it up to abut the proposed level. After a few rains, that fill sank a surprising amount and remained that way.

On Monday he brought in 3 dump trucks worth of base to level it up to the desired mark (about 6 to 12" worth depending on the spot and prior settling). When I got home everything was level, felt very nicely packed and he had used a small compactor to make it nice level.

Curious, since it looked so nice, last night I asked him if I should still soak it down with a hose. He said yes. Well, the amount of settling/sinking was mind blowing. I kept a garden hose at full blast for about 20 minutes on an areas about 50 square feet each and I have spots (worst in the corners and along the sides) where the base has sunk 6". Entire sections of 50+ square feet have sunk 2-3".

Unreal. Very glad we did that. The area in the middle though, including where the 2-post lift will go, was very solid and did not sink at all. You could tell quickly because along the edges the water would disappear immediately and shortly thereafter the ground would begin to sink, whereas in the middle it would pool right away.

Anyhow, it was fascinating to see.
 
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Spudland_Dave

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Maine
NOTHING Compacts like Mother Nature as you discovered...only thing better is to let it go thru a freeze/thaw cycle.
We are big believers in it...I started all the site work for my garage 3 years ago...(Plan was we would build the following year, obviously that didn't happen) Started in July of 2010 to be exact, hauled in 108 yards of -3" spread it out...let it settle & compact via rain, come September I hauled in another 54 Yards to get it leveled out with a finished grade. By the spring of 2011 it had settled a bit more, 18 yards made it perfect....when 2012 rolled around and construction became a reality, my concrete guy was impressed with my patience & planning. I'm not worried a hair about my slab.
 
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Quijote

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Feb 27, 2013
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179
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Greater Boston
Unfortunately I can't be THAT patient. But for what it's worth it will be 4000psi concrete, so I think I'll be fine.
 

jameswood

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Feb 8, 2011
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77
Location
Nebraska
It is getting too common to see construction where the site was dug out today and poured tomorrow. Good compaction cost additional time and money so like every other aspect of our society it's on the chopping block.

Takes an informed buyer like yourself to get it done right these days.
 
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Quijote

Well-known member
Joined
Feb 27, 2013
Messages
179
Location
Greater Boston
It is getting too common to see construction where the site was dug out today and poured tomorrow. Good compaction cost additional time and money so like every other aspect of our society it's on the chopping block.

Takes an informed buyer like yourself to get it done right these days.

Absolutely. I have easy access to a hose, but on new construction you'd have to truck in water and pour it. No way someone will pay for that unless specifically requested.

Anyhow, he topped it off again today with more base, and it was soaked again. It hardly moved at all this time and the water pooled for hours. I think I am good to go. He will do final leveling tomorrow, and hopefully I'll have a slab sometime this week.
 
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Thumper68

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May 16, 2013
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5,134
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Duluth MN
Mother nature Is awesome. My 32x40 only has a few surface cracks, from drying to fast. The fill sat for about 15 months before we started forming and digging the footers, once I had all the pipes, conduit and extras under the slab we used water and a plate compactor to firm things up before installing the insulation, vapor barrier and pex.

Because I do keep it heated there are no saw cuts.
 

bullnerd

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Sep 17, 2012
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Jersey
My niebor did the same, but with sand. I've known him for 20 yrs, not a single crack or saw cut in a very large shop.
 
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