I am in the process of building a new shop (24' x 30') and I have a
question regarding the concrete floor. They just poured the slab
yesterday and I noticed that the guy didn't put in any expansion
joints. Is this going to cause a problem for me down the road? The
4" slab is on frost footings 36" down and he used 1/2" rebar and wire
mesh. Any advice would be appreciated.
It'll crack. If you want it to crack in predictable places, have someone
come in with a wetsaw and put the grooves in the floor. A slab that big
will definately crack.
My garage floor happens to be 24'x30' as well, and it's divided into
quarters with joints (troweled in, but that's not an option for you now).
The two I just checked have cracks in the bottom of 'em, as designed.
If you didn't specify joints, he may not come in and do it for nothing, but
it's odd that any flatwork guy wouldn't strongly suggest that you put
them in at the time of the pour.
One way or the other, if you don't get joints in there, your floor _will_
crack where you don't want it to.
Hmmmmm... my garage floor is 26x36 and it has no joints cut into it. It's
poured with mesh in it only - no rebar. It's almost 10 years old and no
cracks. Most of the two and three car garages in my neck of the woods do
not have joints cut into them, and we live in upstate NY - plenty of frost
issues here. Cracks in the floors are not a common problem.
If you read this: http://www.johnbridge.com/control_joints.htm
I think you will see that joints on reinforced concrete
aren't required for runs of less than 40 feet. An expansion
joint should be provided next to an existing structure. You
didn't provide all the necessary info to determine if YOUR
installation should have had them.
BTW, did you mean "contraction" joints, rather than
"expansion" joint, Alex? (You'll see what I mean when you
read the link I provided)
On 5 Jun 2004 06:28:02 -0700, firstname.lastname@example.org (Alex) wrote:
They normally will not but in any expansion joints ...I do not think I
have ever saw a normal garage or basement for that matter that had
I can gaureetee you a couple of things about concrete...
1. it will get hard
2 it will crack
All the joints do however is control the cracks..not stop them....
I do not think you should have a problem.... (But I am sure not an
expert on concrete slabs IN your area....
I had 3 contractors come out to bid on my garage slab. 28x36. All 3 of
them had all kinds of opinions and difference in "the way we do a slab"
but all of them said "95% of all small slabs will crack". I did not ask
what a large slab was but assume it to be in excess of 6 - 10 inches thick.
Due to water problems in the construction area it had to be a floating
slab and has enough rebar to build a bridge, as well as the mesh. It
did not crack for 3 years and after it did crack they do not pose any
problem as they have stayed "hairline", I am not getting any breakout
nor are they getting any larger after 4 more years. soil preparation
under the slab is at least as important as the slab itself.
Best I can tell is the only difference between a "controlled" crack and
an uncontrolled crack is ascetics. YMMV
The spacing of control joints is a factor of three things:
The depth of the concrete;
The tensile strength of the concrete (the reinforcing steel contributes
to the tesile strength);
The water to cement ratio of the concrete.
In the case of your slab, there will probably be cracks. I can't say
for sure, not knowing all the particulars. It might be good insurance
to have someone cut the control joints, but depending on the concrete
that was used it mey be too late to be effective. The higher the cement
content, and lower the water to cement ratio, the faster it hydrates,
and what ever shrinkage cracking happens occurs during the initial
hydration. However, the higher the cement content, and lower the water
to cement ratio, the less cracking you have.
The other option is to wait until the shrinkage cracks appear, (if they
do), and seal them with a silicone caulk.
It depends. You generally don't need expansion joints. You'll likely
get some shrinkage, but the effect depends on how much steel is in the
slab. In my years of experience with concrete, using #10 wire mesh
(6" x 6") is a waste of money. The tensile strength of each strand of
wire is not enough to help much at all. If you put 1/2" rebar on 12"
centers criss-crossing, it's unlikely it will crack significantly at
all. If there's mesh in the slab and rebar in teh frost footings only,
then you can expect cracking, but how much depends on the slump of the
material poured (how "runny" it was), and other factors. If it had
"fiber-mesh" fiberglass reinforcement in the concrete, that's stronger
than mesh any day and will reduce cracking dramatically.
If you are nervous, there's no problem cutting a slit down the center
both ways to make 4 slabs 12 x 15 feet. You don't need to cut all the
way through -- 2 inches will suffice. That will create a stress point
that will make it sufficiently probable to crack there and not elsewhere.
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