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DanG[_2_]

On 4/26/2012 10:55 PM, wrote:
I'm wanting to put a 20 foot concrete pad in front of my garage. Right
now, the garage floor is concrete, but the driveway in front is just
dirt. However, under that dirt, there is a bed of 2" to 3" rock, which
I put there about 6 years ago. What I want to do is just make forms and
have a just have a few yards of concrete delivered, to trowel myself.
The base dirt is well packed and solid from years of driving on it.

To add gravel, I will have to disturb this well packed dirt with the
rock underneath. (or the drive will be higher than the garage floor).
Not only will this disturb the well packed base, but will double the
cost of this job, because of the cost of the gravel, and the cost to
rent some sort of machinery to dig up this dirt and rock base, since I
already tried a shovel and with that rock base, the shovel wont even go
in the ground unless it's after a rain when it's muddy, and even then
the rock base could not be dug up with just a shovel.

My plan was to simply make forms, remove the small wooden ramp I made at
the front edge of the concrete garage floor (I left the dirt about 4
inches lower than the garage floor intending to pour concrete, but made
that ramp out of stacked 2x8's to drive in and out of the garage). Add
a few bricks with some rebar on top, And pour/finish the concrete.
A fairly simple job. However, someone told me I must put dravel under
it first. Is this gravel really needed? If it is, why? After all, I
already have the rock base, which is very solid, and rock is like
gravel. It's just that dirt has filled in around the rock so it just
looks like dirt now.

One other thing. For a 20 foot long by 12 foot wide strip of concrete,
how many joints should I cut in the concrete? I'm thinking just one, so
each section is 10ft. I plan to place one of those black fiber strips
where it meets the garage slab. I'll probably just trowel the joint in
the middle, to avoid having to rent a concrete saw.

Finally, did I figure this correctly?

20 X 12 = 240 sq ft. At 4 inches think, I estimated that I need
roughly 3 cubic yards of concrete. (Actually 2.96 cu yd). Is this
right? (I'm not the best at math).

Thanks

Your math is correct as far as it goes. It is VERY difficult to
maintain a uniform thickness. The main way to accomplish is to
carefully and accurately grade a granular subgrade material, here it
would be "fill sand". Unless you have this type of tolerance, be very
prepared for additional yardage. There is nothing worse than running
out and waiting for delivery of a 1/2 yard to finish. Much cheaper to
send back a 1/2. A truly uniform 4" slab is much stronger than one that
varies from 4 to 6 (think about tire ruts from the concrete truck if you
are backing into your forms). How are planning to pour?

Gravel capillary break is a best practice method under a floor slab, not
under exterior pavement. It does no harm, but will add to the expense
that is not necessary. What is important is proper compaction of the
subgrade. If is gets goey when it is wet, this sounds like clay - wet
clay expands. A typical commercial spec would require removal and
replacement with a select material (red select here) or lime
stabilization of the clay that is compacted to 90-95 Proctor density.
I am fairly sure you are fine to go ahead with what you have.

Wire mesh is NOT reinforcement. It keeps cracked concrete from getting
bigger cracks. In practice, it is very difficult to keep at the proper
location. Artificial or steel fiber is considered an equivalent. Cost
depends on dosage per cubic yard.

4" concrete does not gain appreciably by using #4 bar. If the bar is
run both directions there is not sufficient depth of concrete for
minimum concrete cover. Reinforcement is usually placed in the tension
dimension of the concrete which is at the bottom 1/3 point with 2" of
concrete cover from earth forming (bottom).

Your tar joint at the existing is good practice. It is an expansion
joint. If the concrete is not trapped, there is no need for more
expansion. Tooled or sawn joints are contraction joints. These help
the concrete break in a straight line when the shrinkage forces form.
Make sure that contraction joints are T/4 - at least 1/4 of the slab
thickness. Concrete should always have contraction joints at least
every 12' both directions.

YOu need to cure the concrete at least the first 3 days. You need to
keep vehicles off the concrete at least 7 days. Make sure you use air
entrainment for any exterior concrete. I would order 3500# with air.
Let the bleed water come out before any trowel work of any type. If you
don't know concrete, see about getting an informed buddy or assistant.

--

___________________________________

Keep the whole world singing . . .
Dan G
remove the seven