So I don't want my slab to crack

[ConCretin]

i keep reading threads where the poster, in mortal fear of concrete cracks takes all kinds of steps to avoid them. These steps include obsessive compaction of base material, thickening the slab to industrial levels, steel reinforcing that leaves little room for concrete, etc, etc.

The reality is that the vast majority of slab cracks are the result of shrinkage. Concrete shrinks as it sets. If shrinkage is constrained by the sub grade, penetrations, rebar or the surrounding structure, your slab will crack.

You can conceal the cracks at the bottom of control joints but the rest is a waste of time. Slabs rarely crack due to an inadequate base or any of the other factors commonly cited.

I'm not suggesting cracks aren't a problem. I'm just pointing out the reason slabs crack so we can avoid them and keep our garage slabs looking good without wasting time and effort.

 

[TommyK]

Sing it brother

ACI 302.1-04. “Even with the
best floor designs and proper construction, it is unrealistic to expect crack-free and
curl-free floors. Consequently, every owner should be advised by both the designer and
contractor that it is normal to expect some amount of cracking and curling on every
project, and that such occurrence does not necessarily reflect adversely on either the
adequacy of the floor’s design or the quality of its construction (Ytterberg1987;
Campbell et al. 1976)”.

 

[850xpeps]

i keep reading threads where the poster, in mortal fear of concrete cracks takes all kinds of steps to avoid them. These steps include obsessive compaction of base material, thickening the slab to industrial levels, steel reinforcing that leaves little room for concrete, etc, etc.



The reality is that the vast majority of slab cracks are the result of shrinkage. Concrete shrinks as it sets. If shrinkage is constrained by the sub grade, penetrations, rebar or the surrounding structure, your slab will crack.



You can conceal the cracks at the bottom of control joints but the rest is a waste of time. Slabs rarely crack due to an inadequate base or any of the other factors commonly cited.



I'm not suggesting cracks aren't a problem. I'm just pointing out the reason slabs crack so we can avoid them and keep our garage slabs looking good without wasting time and effort.

Slowing the curing process and maintaining hydration in the slab should decrease shrinkage.

Also the dryer the pour or lower slump the less shrinkage. Just my experience.

 

[Slowgsr]

Decoupling helps as well. Can shrink around the perimeter instead.

 

[brownbagg]

pour a weak 3000 with a lot of flyash, as soon as you can walk on it, cut your control joints, deep. cover with vapor barrier, hay, and keep wet for 30 days. have no square block outs, if you have to have a block out, use a five gallon bucket. all this on top of compacted base with vapor barrier, tape all the joints

 

[ConCretin]

So I'm not suggesting that a good base, thicker slabs and reinforcing aren't of value, just that they have almost no bearing on the the most common type of cracking, which is caused by shrinkage. Your base has little or no bearing, thicker slabs aren't any more resistant to cracks and reinforcing actually increases the likelihood of shrinkage cracks.

Structural cracks are very easy to avoid with a reasonably adequate base and slab thickness. In my experience, they are quite rare and generally occur in older slabs where little or no attention was paid to either.

Slowing the curing process and maintaining hydration in the slab should decrease shrinkage.

Also the dryer the pour or lower slump the less shrinkage. Just my experience.

Proper curing will definitely slow the shrinkage process and give the concrete time to gain strength and resist cracking. Most shrinkage cracks occur early even if you don't see em until later.

Minimizing water is crucial to reducing shrinkage. Water reducers are an inexpensive way to minimize water while maintaining a workable mix.

Decoupling helps as well. Can shrink around the perimeter instead.

Great point. Expansion material is often used around the perimeter of slabs placed against walls and other obstructions but it's not really for expansion at all. It's real value is 'decoupling' or isolating the slab so it can shrink without being restrained.

pour a weak 3000 with a lot of flyash, as soon as you can walk on it, cut your control joints, deep. cover with vapor barrier, hay, and keep wet for 30 days. have no square block outs, if you have to have a block out, use a five gallon bucket. all this on top of compacted base with vapor barrier, tape all the joints

The only place i'd differ brownbags is the 3000 psi mix. While it's true a higher cement mix will shrink nominally more than a mix with less cement, I think the added cement in a 4000 psi mix provides a more durable surface.

Good point about avoiding square penetrations through your slab. Re-entrant corners are a guaranteed crack waiting to happen. Vapor barriers provide a secondary function of allowing the slab to slide rather than being restrained by a rough sub grade.



It's possible to manage shrinkage and place a fairly large slab without cracks but most of us aren't willing to take the chance of having an ugly random crack show up. That's why we use fiber to slow em down, control joints to hide them and reinforcing to hold em together.

 

[Kaizen]

A fifty ton slab on a vapor barrier will not “slide”.


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[ConCretin]

A fifty ton slab on a vapor barrier will not “slide”.


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It sure will Kaizen. A slab contracts inward as it shrinks. If you have a flat sub grade covered with vapor barrier that's free of obstructions, the amount of movement can be significant. As much as an inch in a hundred feet.

This shrinkage is usually diffused/distributed by cracks at the control joints. If you do a little research, I suspect you'll find this is pretty well documented.

 

[Kaizen]

It sure will Kaizen. A slab contracts inward as it shrinks. If you have a flat sub grade covered with vapor barrier that's free of obstructions, the amount of movement can be significant. As much as an inch in a hundred feet.

This shrinkage is usually diffused/distributed by cracks at the control joints. If you do a little research, I suspect you'll find this is pretty well documented.

Yes but as long as there is not boulders it will shrink and move regardless of a vapor barrier. With that much weight and mass I doubt the plastic helps it move.


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[rsanter]

Your base has nothing to do with the short term shrinkage cracking.
You base has everything to do with long term cracking from base and sub base subsidence where you loose the structural support of the ground, from heave, sink holes, ground shift...etc

 

[johnnyradiant]

How much of the long term cracks are actually short term shrinkage cracks just eventually being telegraphed from the poor base rather than 'fresh' new cracks? Assuming the concrete itself was a sound mix and otherwise placed correctly.

The size of a slab or the area in between control cuts or joints can be sometimes pretty astounding when there is a desire to not see cracks.

Even angle iron has a little radius to it on the inside corner for strength and it is not as brittle as concrete.

 

[ConCretin]

Your base has nothing to do with the short term shrinkage cracking.
You base has everything to do with long term cracking from base and sub base subsidence where you loose the structural support of the ground, from heave, sink holes, ground shift...etc

Agreed

How much of the long term cracks are actually short term shrinkage cracks just eventually being telegraphed from the poor base rather than 'fresh' new cracks? Assuming the concrete itself was a sound mix and otherwise placed correctly.

While it's true that shrinkage cracks sometimes aren't noticeable right away, they often occur early when the concrete is drying most rapidly and the concrete has low tensile strength gain.

Cracks don't really 'telegraph' from the base. As rsanter points out, structural cracks occur when the base fails to provide uniform support for the slab.

It's possible to get temperature cracks later on as the concrete expands and contracts with heating and cooling but this is probably the least likely cause.

 

[GarageKit]

You guys mention the square that a pole barn post creates in the slab, would it be a reasonable idea to just router a curve on the bottom edges of each post, then just continue your vapor barrier up and around the post decoupling it?

 

[ConCretin]

You guys mention the square that a pole barn post creates in the slab, would it be a reasonable idea to just router a curve on the bottom edges of each post, then just continue your vapor barrier up and around the post decoupling it?

The more you can 'soften' a re-entrant corner, the less likely you are to get a crack. A round hole in a slab doesn't create the stresses that causes cracks. The closer you can get to this, the better.

 

[brownbagg]

my argument on a weak 3000 is, a straight cement mix will set up quick and always crack due to the curl. you want to slow down the hydration. you dont want the 3000 till 27 days. a 2500 mix will crack less than a 4000 and still have plenty of strength for a slab.

now personal preference I myself would pour a 4000 and saw cut the hell out of it. cracks dont bother me

 

[TheoShooter]

I was always told that you get two guarantees with concrete. It’s gonna crack and nobody’s gonna steal it.