Deep Fill Under Garage Slab

lazerguy

Hi guys,

You've always given me good answers on my questions, now I ask your help again...

I'm building my 30ft x 30 ft garage/shop. Heaviest equipment that will go in there will be my 3/4 ton Suburban. Spot loads could be large at times, when I use an engine crane.

I am proposing to cast a 4'' slab with 7/16 re-bar every 18 in both directions.
I had first thought a 6'' slab, but a concrete guy told me that slab base matters more than slab thickness in preventing cracking.

I've excavated down to rock, which is very hard.
There are huge differences in the level of the rock bottom, requiring filling up to 4ft in some places.

Of the different fill materials available, I'm not sure which one to choose:

a) 3/4'' clean angular crushed stone ?
b) clean sand compacted?
c) natural gravel compacted?

The area where the garage is being built has very good drainage, so I don't think that drainage is an issue.
The garage will normally not be heated, and I live in Quebec Canada so typical frost depth here is 5 ft.

Which fill material would you recommend and how would you compact it, ie sheepsfoot tamper or vibrating plate?

Thanks!
Lazerguy

CJKarl

I'm no engineer, but in my opinion 3/4 stone compacted in 6-12" lifts would be my answer.

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cleveman

I think you want something as big as you can handle with some minus in it, i.e. not clean. If you can handle 3", go with it, and compact it in small layers as suggested. I wouldn't go over 6" with the layers.

The reason you want the minus is because it is not possible to compact a bunch of 1/2" ball bearings. You would know not to try to use river rock for example. Sure, 3/4" clean will compact better than 3/4" river rock because it is angular, but the smaller pieces included in the minus will really help.

If you can't handle 3", go to 2".

One final tip-don't handle the stuff any more than you must. Every time you pick up a load with a bucket and dump it and spread it, you are separating out the different sizes of the mix.

Make sure you put plastic down before you pour and have a 1'x1' perimeter with a 5/8 rod in it if you aren't using footings.

Kgmz

Use crushed with fines, otherwise know as whatever size minus.

When we pave new roads we use what is called around here to DOT specs, CSBC and CSTC.

CSBC is crushed stone base course
CSTC is crushed stone top course

CSBC is actually 1 3/4" minus
CSTC is actually 5/8" minus

The CSBC is the base of at least 10-12" deep with a 2" CSTC top course.

If you use this it will compact almost as hard as concrete. and use a vibratory plate. Also spray a little water on it as you compact to get everything to bind together when it drys.

concretemasonry

Make sure you have some moisture in the material you are compacting. The water helps lubricate the particles so the compactor better. - That is why you frequently see water wagons spraying water to make the compaction better and easier.

If you get too much water, just wait a bit so any good base material will drain the excess water.

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tkle

7/16" rebar?

Tscarborough

7/16, 9/43rds, whatever it takes.

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lazerguy

Thanks for the input guys.
I'll definitely use some kind of crushed stone, with moisture and compact it with a vibrating plate in small lifts.
Though I'm not sure I get the point of having fines in the mix. For a road/driveway surface, I do see the point. But for a slab base? Even if there are small voids in clean stone, clean stone is supposed to be virtually incompressible, so there's no way it could settle more after the slab is cast. If it can't settle, the slab will stay supported and it won't crack. Right?

concretemasonry

The stone will not compress - that is very tue.

What happens is that if you do not have the right type of fines to fill the voids, the stone can shift or roatate if there is moisture or any vibration present. this is especially true with crushed stone that is angular and can bridge temporarily very easily. The bottom line is you cannot reliably compact clean, crushed rock. It can also work into the material under it over time.

Natural clean aggregates with graded fines get their maximum density and compaction very fast and easily.

In all cases, moisture is the lubricant that makes the compaction faster and easier for all materials.

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Dick

Engineer, designer and consultant recently active domestically and internationally on construction and design in about 35 countries.

neolitic

Here, we order #53's and minus
for that kind of fill and just wet it down.
Either that or pea gravel....
it comes wet enough.

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