Ripping out concrete floor, pouring new

[Todd Brewer]

I posted a couple of threads regarding leveling an uneven floor. Most recommended tearing it out and starting over. After much thought and an attempt to grind the high spots and fill the low spots, I decided the best solution is indeed to tear out the concrete and pour new.

The shop side was done about 6 weeks ago. Very good job, nice and flat/level.

My concern is the contractor said it is better to not tie the slab (via rebar) to the foundation/stem wall. He cut off all the rebar coming out of the foundation. Also, he put expansion material around the perimeter. He also put in crack control joints about every 6 feet. The slab is about 17' x 24'.

He is coming back on Monday to do the garage side (about 22' by 24').

Do you think the slab should be tied to the foundation/stem wall? I don't think it can go anywhere, it is surrounded by the stem walls. My inclination is to tie concrete together. Climate is mild, SF bay area, so no freeze/thaw issues. Only thing that might disturb it is tree roots or earthquake. I have a lot of Italian Cypress trees near the garage but I don't see them doing any damage to sidewalks or driveways. They are very common around here.

Thoughts much appreciated!

[Jim Andrew]

Think you got a good concrete guy this time, would recommend following his recommendations.

[Richard Jones]

I would need to know more about the slab prep before I said removing rebar was smart, and probably not then. If the sub-grade isn't compacted/firm natural soil/etc., removing the steel will cause problems.

I don't have it in front of me, but I believe ACI 301 (?) speaks more to isolation joints vs. expansion at a perimeter scenario. I don't believe I've ever seen a slab like this ever get any larger than it is when it is first poured............... If you want them to move independently, the isolation joint/material is adequate. I've never met a flatwork mechanic that thought expansion joint was worth anything in a garage slab like this.

I really don't understand removing the steel, sorry..................

Confused as, well, just confused.............

[Michael N Taylor]

The slab does not need to be tied to the foundation, it is better to let each move separately. We quit using expansion joint material years ago because the concrete is never bigger than when it is poured, it shrinks as it cures. We would spray the foundation with a bond breaking material before placing the concrete or you could use polyethylene taped to the foundation and trim later. The contractor did a very good job using enough crack control joints but do not be surprised if you get some hairline cracks. In 45 years of commercial construction I know that if concrete decides to crack for what ever reason it will even if you pour it in two foot squares.

[Thomas Bank]

I've designed floating slabs, slabs tied into the foundation walls, and slabs sitting on top of a ledge in the foundation wall. Each has their place. Just randomly deciding to change an aspect of the design after the fact is rarely a good idea...

[Richard Jones]

The slab does not need to be tied to the foundation, it is better to let each move separately. I don't agree with that for every situation.

We quit using expansion joint material years ago because the concrete is never bigger than when it is poured, it shrinks as it cures. I agree with that.

We would spray the foundation with a bond breaking material before placing the concrete or you could use polyethylene taped to the foundation and trim later. I agree with that.

The contractor did a very good job using enough crack control joints but do not be surprised if you get some hairline cracks. 6' apart is plenty adequate.

In 45 years of commercial construction I know that if concrete decides to crack for what ever reason it will even if you pour it in two foot squares. Amen.

Again, floating the slab on the sub-grade is just too dependent upon other factors to make an arbitrary decision.

[Todd Brewer]

Thanks for all the opinions!

I'm still not sure about the rebar, my inclination at the moment is to tie them together.

I will insist that no expansion material be installed around the perimeter. I don't like the look, and if concrete is no bigger than when wet then it makes no sense.

Taping poly to the perimeter seems like a good idea if slab is allowed to float, but does it make sense if the slab and perimeter are tied together with rebar?

[Ole Anderson]

No sense in the poly bond breaker if you go back to tying the slab to the foundation with rebar. Of course, if they already cut the bar, you will be drilling holes in order to install the ties.

Another item to consider is slab reinforcement. Most flatwork guys will use a light duty steel mesh which really makes the slab no stronger, it just holds it together when you get temperature cracks. That is what I used when I redid my garage floor. And I didn't tie it to the foundation. But on my new exterior concrete, I had them add poly fibers to the mixer on-site, and didn't use the steel mesh. Six years later in Michigan, I don't have a crack in 2000 sf of exterior concrete. Steel mesh was 30 cents per square foot, the fiber mesh was 20 cents per square foot. I did go with 5" of concrete in my drive as I regularly run my 11 ton RV on it.

If you go with the fiber mesh, I would save it for where you want a broom finish as it is difficult to trowel to a smooth finish or where you have stamped concrete (I had all three finishes for various places).

Curing your slab is critical. Most flatwork contractors quit and leave the site as soon as they finish the concrete, and may return to sawcut the joints, either within about 24 hours while the slab is still soft, or after about a week when is it hard. They rarely tell you about curing. As soon as it sets up, but before it starts to look dry on top, cover it with poly sheeting for a week to hold the moisture in. Exterior slabs can be sprayed with a clear or white curing compound.

[Richard Jones]

No sense in the poly bond breaker if you go back to tying the slab to the foundation with rebar. Of course, if they already cut the bar, you will be drilling holes in order to install the ties.

Another item to consider is slab reinforcement. Most flatwork guys will use a light duty steel mesh which really makes the slab no stronger, it just holds it together when you get temperature cracks. That is what I used when I redid my garage floor. And I didn't tie it to the foundation. But on my new exterior concrete, I had them add poly fibers to the mixer on-site, and didn't use the steel mesh. Six years later in Michigan, I don't have a crack in 2000 sf of exterior concrete. Steel mesh was 30 cents per square foot, the fiber mesh was 20 cents per square foot. I did go with 5" of concrete in my drive as I regularly run my 11 ton RV on it.

If you go with the fiber mesh, I would save it for where you want a broom finish as it is difficult to trowel to a smooth finish or where you have stamped concrete (I had all three finishes for various places).

Curing your slab is critical. Most flatwork contractors quit and leave the site as soon as they finish the concrete, and may return to sawcut the joints, either within about 24 hours while the slab is still soft, or after about a week when is it hard. They rarely tell you about curing. As soon as it sets up, but before it starts to look dry on top, cover it with poly sheeting for a week to hold the moisture in. Exterior slabs can be sprayed with a clear or white curing compound.

All good points, not that my endorsement is needed. I personally don't like fibermesh all that much, but wire isn't any good unless you pull it up into the concrete as you pour (chairs? we don't need no stinkin' chairs!). That's what that hook is for on the back side of the concrete rake........ My mantra: pull it up, pull it up, pull it up........repeat.........

[Carl Beckett]

In 45 years of commercial construction I know that if concrete decides to crack for what ever reason it will even if you pour it in two foot squares.

I worked concrete for a few years when I was younger. The owner of the company had this exact philosophy (many years in the business) - so much so that every 3 years or so he broke up his back porch and repoured it..... (said it wasnt a good impression to have his own concrete full of cracks)

[Thomas Bank]

All good points, not that my endorsement is needed. I personally don't like fibermesh all that much, but wire isn't any good unless you pull it up into the concrete as you pour (chairs? we don't need no stinkin' chairs!). That's what that hook is for on the back side of the concrete rake........ My mantra: pull it up, pull it up, pull it up........repeat.........

I've seen enough concrete removed where the mesh was all under the concrete and just had "peaks" here and there from doing exactly that...

The wire mesh you buy in rolls is worthless. For it to do any good, you have to go to the mats and set it on chairs.

[Richard Jones]

Pretty darn hard to walk on mesh with chairs, even with the 1.7 size in 5x5 mats, heck, with 2.9 is a headache off of chairs. Not saying it isn't doable, just that's it's a pain.

I've also broken up slabs/sidewalk where the WWM was on the ground, almost reusable. You gotta pick it up...........

I will politely disagree with "The wire mesh you buy in rolls is worthless". Placed too much that worked fine if installed correctly.

[Thomas Bank]

My point about the stuff in rolls is that it is too fine a gauge. Just the weight of the concrete will push it down. The heavier gauge mats on chairs will support a person.

[Todd Brewer]

No sense in the poly bond breaker if you go back to tying the slab to the foundation with rebar. Of course, if they already cut the bar, you will be drilling holes in order to install the ties.

Work hasn't started yet, so if I tell them to not cut the rebar out of the foundation, I can have it tied to the rebar in the slab.

[Jim Andrew]

The only reason to have the slab rebared to the walls is if your base is not solid. Is the base compacted?