Garage slab


R

ralph_ricketts

I want to build a garage. My site is about four feet lower than the
driveway. I will build stem walls on footings to start that will reach
the elevation of the existing drive and will totally enclose the new
slab. I read on this site that pouring a slab on top of 5/8 crushed
rock with no additional compaction of the crushed rock is an
acceptable method of slab construction. I would like to spend the
extra money on the crushed rock and then not have to worry about
meeting any other compaction requirements if possible. I would like
verification that this is an acceptable method of slab construction.
Thank you for any help or other ideas or suggestions about my
project.
Regards, Ralph
 
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R

ralph_ricketts

It all depends upon your soil. Sand, clay, etc. If you have a good
clay base you've got half the battle won. 3 sack or 5 sack is a
consideration as well. Hopefully the more astute on this subject will
chime in with more help than I can offer. My expertise comes from
having a daddy-in-law that is a contractor and tuning him out most of
the time. <G>

---

"Don't kid yourself, George W. Bush is still the President.
If you don't believe that, just ask Barak Obama".

An independent old cuss
Thanks for the reply. The area is older fill and native soil. So how
does the clay soil help?
 
T

Tom Cular

Perhaps this will help a bit.
Suggested Concrete Mix Ratios
The strength of concrete varies with the amount of cement and water in each
yard. Trailered Ready-mix concrete is not rated by pounds per square inch
(PSI) strength, but by the amount of cement sacks in each yard. The
following information represents a rule of thumb for trailered ready-mix
strengths and suggested uses.

Fence Posts
4-sack mix (four 94# bags of cement per yard): This mixture has a PSI of
between 200011 and 250011 and is recommended for things like setting fence
posts.

Foot Traffic
5-sack mix (five 9411 bags): PSI of between 2500# and 3000# and is
recommended for things like walking surfaces such as sidewalks.

Auto Traffic
6-sack mix (six 94# bags): PSI of between 30011 and 35011 and is recommended
for things like driving surfaces such as driveways.

Cold Climate Tip
In areas of multiple freeze/thaw cycles, there should be 1 sack of cement
added to each of these recommendations and add an air entraining agent.



And keep the water content as low as possible, 3-4 " slump MAX before any
additives, add a plasticizer (Super P) if necessary for workability. More
water means more shrinkage and more CRACKS! Super P will allow you to pour
(Pea Soup) without harming the final product and it's effects dissapear in
about 30 min. depending on the brand.
 
J

jloomis

Good list of sack mix......
john

"Tom Cular" wrote in message


Perhaps this will help a bit.
Suggested Concrete Mix Ratios
The strength of concrete varies with the amount of cement and water in each
yard. Trailered Ready-mix concrete is not rated by pounds per square inch
(PSI) strength, but by the amount of cement sacks in each yard. The
following information represents a rule of thumb for trailered ready-mix
strengths and suggested uses.

Fence Posts
4-sack mix (four 94# bags of cement per yard): This mixture has a PSI of
between 200011 and 250011 and is recommended for things like setting fence
posts.

Foot Traffic
5-sack mix (five 9411 bags): PSI of between 2500# and 3000# and is
recommended for things like walking surfaces such as sidewalks.

Auto Traffic
6-sack mix (six 94# bags): PSI of between 30011 and 35011 and is recommended
for things like driving surfaces such as driveways.

Cold Climate Tip
In areas of multiple freeze/thaw cycles, there should be 1 sack of cement
added to each of these recommendations and add an air entraining agent.



And keep the water content as low as possible, 3-4 " slump MAX before any
additives, add a plasticizer (Super P) if necessary for workability. More
water means more shrinkage and more CRACKS! Super P will allow you to pour
(Pea Soup) without harming the final product and it's effects dissapear in
about 30 min. depending on the brand.
 
H

HerHusband

Hi Ralph,
I want to build a garage. My site is about four feet lower than the
driveway. I will build stem walls on footings to start that will reach
the elevation of the existing drive and will totally enclose the new
slab. I read on this site that pouring a slab on top of 5/8 crushed
rock with no additional compaction of the crushed rock is an
acceptable method of slab construction. I would like to spend the
extra money on the crushed rock and then not have to worry about
meeting any other compaction requirements if possible. I would like
verification that this is an acceptable method of slab construction.
I did something similar when I built our garage, though I only had a foot
to build up, not four. We formed and poured the footings and stem walls
ourselves, then hired out for the actual pour.

It is my understanding that fill of any kind should be compacted in layers.
I believe it's 6 inches max for each layer. Filling the space in one lump
is likely to settle over time. I rented a plate compactor to compact my
gravel base, quick and easy.

I don't know what size your garage is going to be, but my 24'x28' garage
worked out to roughly a single dump truck load for a 6" layer (12.5 yards).
If you're going four feet deep, that's about eight dump truck loads. Dump,
spread, compact. Dump, spread, compact. Repeat... :) If you want to save
some labor, have the gravel delivered by conveyor trucks that can shoot the
gravel wherever you need it. Or rent a bobcat and have fun driving it
around! :)

Be sure to lay down a layer of 6 mil plastic sheeting before the concrete
pour too to keep moisture from coming up through the floor.

I also recommend adding fibermesh to the concrete for the floor slab. It
really helps minimize cracking. My slab is about 12 years old now and I
only have a single small hairline crack in one of the doorways. I wouldn't
even know it was there other than it's discolored from water running down
in it.

Good luck!

Anthony
 
J

jloomis

I do not see a problem using crushed rock.
I also would be careful as to "when" the rock is put in the concrete
interior wall.
The concrete should be allowed to set a bit before putting full load on the
wall is my concern.
Also if the rock has fines in it, that will need to settle also.
If the rock is clean it may not need that.
If it has fines I would use water to settle the fines......in layers.....
Rake out the rock as it is dumped in, and then wet it.....
I know you may not want to do this since I am not sure where the water will
go.
I usually have very porous soils here, and water is gone.
Now about a vapor barrier.....
Maybe one is not needed?
If a vapor barrier is used, the rock can or will perforate it too?
Always my concern.
I also usually put sand on my vapor barrier to allow the water in the
concrete to migrate throughout the pour.
In this case, the sand may leave in the perforations and make voids....
I have used fill sand with a 99% compaction for backfilling stem walls also.
In this case the vapor barrier does not get perforated, and then the sand I
put on top of the barrier
does not migrate either.
We then flood the sand with water to settle it also.
Sometimes we have brought trucks in with a sand mix only to flow the sand
out into the stem wall area.
It just makes putting sand in much easier.
The water does leave, and I do compact the sand with a vibratory plate
sometimes.
So many details.....
John

wrote in message

I want to build a garage. My site is about four feet lower than the
driveway. I will build stem walls on footings to start that will reach
the elevation of the existing drive and will totally enclose the new
slab. I read on this site that pouring a slab on top of 5/8 crushed
rock with no additional compaction of the crushed rock is an
acceptable method of slab construction. I would like to spend the
extra money on the crushed rock and then not have to worry about
meeting any other compaction requirements if possible. I would like
verification that this is an acceptable method of slab construction.
Thank you for any help or other ideas or suggestions about my
project.
Regards, Ralph
 
T

Tom Cular

HerHusband said:
Hi Ralph, Snip.

I did something similar when I built our garage, though I only had a foot
to build up, not four. We formed and poured the footings and stem walls
ourselves, then hired out for the actual pour.

It is my understanding that fill of any kind should be compacted in
layers.
I believe it's 6 inches max for each layer. Filling the space in one lump
is likely to settle over time. I rented a plate compactor to compact my
gravel base, quick and easy.

I don't know what size your garage is going to be, but my 24'x28' garage
worked out to roughly a single dump truck load for a 6" layer (12.5
yards).
If you're going four feet deep, that's about eight dump truck loads. Dump,
spread, compact. Dump, spread, compact. Repeat... :) If you want to save
some labor, have the gravel delivered by conveyor trucks that can shoot
the
gravel wherever you need it. Or rent a bobcat and have fun driving it
around! :)

Be sure to lay down a layer of 6 mil plastic sheeting before the concrete
pour too to keep moisture from coming up through the floor.

I also recommend adding fibermesh to the concrete for the floor slab. It
really helps minimize cracking. My slab is about 12 years old now and I
only have a single small hairline crack in one of the doorways. I wouldn't
even know it was there other than it's discolored from water running down
in it.

Good luck!

Anthony
Anthony is correct in stating that fill should be placed in 6-8" lifts and
compacted, the type of compaction equip. depends on the soil type. If it's
sandy or granular, a vibratory plate tamper will do fine, if it's a clayey,
fill an impact tamper (jumping jack) will do better. The poly moisture
barrier not only retards moisture from rising to the surface, but also holds
the moisture in the concrete through the curing process and will help the
concrete CURE not DRY.
 
T

Tom Cular

I agree with John on the sandy layer between the crushed stone and the
moisture barrier, if the stone punctures the barrier it's worthless. We also
use the plastic barrier here to limit radon infiltration. Some areas of the
country , including some areas in this state don't need that.
 
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P

phildcrow

I want to build a garage.
Thank you for any help or other ideas or suggestions about my
project.
Regards, Ralph
Ralph,
What area of the country are you in? This will have a large effect on the foundation required for your garage. In southern Louisiana, we have to be very careful about the settling of foundations (mostly because the soil is very much akin to the gumbo we love to eat). It may be worth spending a few bucks (about $2K around here) on an engineered design, particularly giventhat you want to fill 4 feet.

As far as the fill and slab, I agree with other posters here. For most areas of the country, the most readily available and cost effective structuralfill is some sort of sand/gravel mixture. Recycled concrete is another popular option if you're a greenie (not a pejorative term, BTW). Deposit in layers and compact, put down a vapor barrier and pour your slab. Layer thickness and type of material can be specified by your engineer if you chooseto go that route.

Good luck and let us know what you decide.
Phil
 

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