When is My Concrete Floor Gonna Quit Cracking?

[dirttracker18]

The steel is to aid in tension, concrete on its own is strong in compression.

 

[bobscogin]

The difference between mesh, rebar and fiber is that mesh and rebar will help to hold concrete together after it cracks, and can help to minimize cracking and increase load bearing of concrete in tension. Concrete is only strong in compression with steel.

I'm guessing you meant to say concrete is only strong in tension or flexural loads with steel. Without steel, the tensile capacity is only about 10% of the compression capacity, so steel is required under those load conditions.

Bob

 

[buening]

buening, your post was very well written and accurate. If i were to quibble it would be your statement on a pump adding air.

Maybe it's just semantics but in my experience, a pump knocks down the air in concrete. In addition, you should never use air entrained concrete in hard troweled slabs. It causes de-lamination.

Indeed you are correct, air is actually lost in pumping. I had that backwards. A good article on air loss during pumping: http://nrmca.org/aboutconcrete/cips/21p.pdf

When air loss is an issue, I've seen contractors use conveyor trucks instead of pumping. Indeed entrained air should not be used on hard troweled slabs. Most garages in my area are floated and not hard troweled, but may vary by area and possibly slab size.

 

[buening]

Nope. They didn't cover it because when they poured, the temp was cool, then it rained and it warmed up a bit, maybe in the 70's if I were to guess..

Thats at least one of the issues then. Its my opinion that regardless of the temperature, all concrete slabs should be covered after finishing for proper curing. Its part of going the extra mile that separates a good concrete contractor and one just wanting to make a buck and get on to the next job.

 

[HotrodHR]

It is a shame these days but it is simply difficult to find people willing to perform at a quality craftmanship level these days. I was bitching to the contractor about how they installed my hardiboard and I got the song and dance that his crew was the best I was going to get in this area. He said most people don't aspire to be in construction - that most of them are either on meth or have other substance abuse problems. I don't quite believe his rationale as I believe there are good quality people in the construction industry - just maybe not around here..

Where are you located in N. Alabama?

 

[ConCretin]

The only function steel reinforcing - bar or mesh - performs in a typical 4-5" residential slab is to hold cracks together.

The concrete section isn't deep enough for the rebar to create a significant 'beam' to carry loads across. It is the sub-grade alone that supports imposed loads.

Nor does it play a significant role in crack control

 

[ConCretin]

Indeed you are correct, air is actually lost in pumping. I had that backwards. A good article on air loss during pumping: http://nrmca.org/aboutconcrete/cips/21p.pdf

When air loss is an issue, I've seen contractors use conveyor trucks instead of pumping. Indeed entrained air should not be used on hard troweled slabs. Most garages in my area are floated and not hard troweled, but may vary by area and possibly slab size.

Based on the rest of your post, I figured that it was just a slip.

Don't ask me how I know but if you 'seal' the surface of air entrained concrete before trapped air and bleed water have dissipated, they create a void just under the surface un-bonding it from the concrete below.

 

[slowthump]

I had a 30 x 40 slab poured for my garage last July. It is a floating slab with thickened edges. The underlying soil is clay mix so I had it dug out to a depth of 2' and replaced that with compacted pit run sand. Cost me an extra grand but well worth it. I have a 16 x 30 section with heated floor and that has 2" of foam under it and plastic. Contractor used 5,000 psi concrete mix with half inch rebar 2' on center. There are no control joints and not one crack has appeared yet, not even tiny hair line cracks. And that is after a Minnesota winter, albiet a warm one with lowest temp of -25.

The big reason I believe I got such good results is that I kept it wet for 7 days starting about 2 hours after final trowel. I set up lawn sprinklers at let it run day and night. The worst enemy of good concrete is it drying out during the curing process. The chemical reaction needed for concrete requires water. The water keeps the process going. At 7 days after the pour the concrete will be 90% strength if it has the necessary water and temperature. Full strength not achieved until day 28 but who wants to keep it wet for 21 days to get the final 10%.

 

[danieldd]

Where are you located in N. Alabama?

Huntsville..

 

[hockey88fan]

I have personally never seen a garage floor with control joints. I just had my 16X26 floor done last week. Undisturbed ground, 4" of stone, wired, 6" of concrete and no control joints. My fathers garage was poured in 1994 its 30X30 with no control joints and there are no cracks anywhere.

In the picture I see control joints, are you joking?

 

[Zick]

In the picture I see control joints, are you joking?

Look at the actual garage pad, those joints you are seeing are part of the driveway.

 

[hockey88fan]

Look at the actual garage pad, those joints you are seeing are part of the driveway.

Ah, never mind ;-))

 

[rburke65]

Would having your garage floor finished with a power trowel be considered a "hard troweled" floor? Thanks.

 

[Steves32]

My slab is 20x21 & has been divided into 6 sections- joints all tooled in. Slabs been down about a year.

Slab is monolithic- 6" thick w/ rebar & 8-9 inches thick under lift.

1 year later. No cracks except where they are suppose to be.

 

[ConCretin]

The chemical reaction needed for concrete requires water. The water keeps the process going. At 7 days after the pour the concrete will be 90% strength if it has the necessary water and temperature. Full strength not achieved until day 28 but who wants to keep it wet for 21 days to get the final 10%.

slowthump, you definitely built your slab the right way. Congrats on that. I would like to offer one correction though. Curing water just protects the concrete from drying - it is not used in the hydration process. The mix water is more than adequate as long as you keep it in there. Also, the rule of thumb for most concrete is 70% in seven days.

 

[KCarGuy]

I know that most pours use "Tooled" Joints...
But I went with "Cut" Joints because (A) it is less of an stopping point for my creeper or Floor Jack. (B) its cut deeper and when it does crack, its even harder to notice when it cracks deep in the Joint.