post and pier vs slab

[EightFingers]

I was wondering how many houses (say in HPP) have problems with cracked slabs. Less than 5%?

[DanielP]

Concrete cracks, guaranteed. Has too cause it shrinks during cure. Rule of thumb: for a 4" thick slab, cut a control joint 1" deep every 9' to keep the cracks tidy.

[Carey]

Our house slab in Keaau town was poured in '73 & withstood both the '75 & '06 earthquakes with no cracks in the house, & one hairline crack in the carport... the concrete drive (no idea when poured, but not same pour look as house) has several cracks & one slight depression area (less than a foot dia), the old front walk no cracks, the ohana slab added in '01 has a couple of hairline cracks & the lanai poured last summer has a few hairline cracks... only the drive has any deviation (noted above) due to the cracks...

Each of the slabs has a different quality look to it, the front walk & house slab LOOK high quality, with a really fine finish, the ohana looks like a pre-teen assisted the pour, the drive & lanai look like a competent, but not master, worker did the finish on the pour...

So with anything, I do believe that the QUALITY of the workmanship has more to do with a lot of this, rather than the actual material used...

ADDED: House, carport & ohana slabs are without cut joints, the drive, walk & lanai do have expansion joints...cannot say that I have EVER seen expansion joints in a house slab pour

[MarkP]

Why all this concern about cracks in concrete? Unless reinforcing steel is pre-stressed, it is a given that the concrete will crack. It has to be. To think otherwise is to defy just about all theory of mechanics as it has been practiced for the last couple of hundred years. The strain at failure of steel in tension is huge compared to the strain at failure of concrete. Put them together and start to stretch and the concrete is going to "fail" while the steel is still hardly loaded. The modulus of elasticity of the two is also vastly different. Concrete is lots stiffer. The steel has nothing to do until the concrete has cracked. You can not put in enough steel to keep the concrete from cracking unless you use enough concrete to carry the load by itself. If you do that, the steel will feel nothing and you might as well have not used it.

A lot depends on your definition of a crack and what bad thing you think a crack will cause to happen. For example there is water intrusion. A big crack will let a lot of water through but a perfect uncracked slab will also let moisture through. Concrete is essentially porous and full of little holes. Are holes worse than cracks? Mostly it depends on the relative size of each. In the case of a ferrocement water tank consider that given a certain loading and ratio of steel to concrete, there will be a certain width of "crack" around the perimeter of the tank. Let's call it 1" worth of crack. A single 1" wide crack will render the tank completely unable to hold water. However the finely spaced reinforcement of the steel mesh used results in 1,000 cracks, each of which is only .001" wide. You won't even see these cracks. Dirt, algae, and slime will block these cracks and surface tension will take care of the rest. Also consider that a golf ball would not make it through the 1" crack and at some point a water molecule won't make it through a sufficiently small crack. There are all sorts of things that go on on microscopic and molecular scales that it is very easy to overlook. Even if all cracks were prevented the tank would still weep the way an earthenware jar does due to the inherent porosity of the material.

There is absolutely nothing wrong or unusual about cracked concrete if it has been properly reinforced. Most of the cracks you are not aware of anyway. About being properly reinforced, seems I have read on this very forum or at least a similar forum that the local way of doing things is to come back after the inspector has been through but before the pour and remove a lot of the re-bar for re-use. That would cause cracking that really is a problem.

[Rob Tucker]

Mark & Daniel are up to speed on concrete. good input guys.

[kander]

Which brings up another point and that is the use of fiber mesh (plastic strings added at the batch plant)in concrete. Some contractors say that you can skip the rebar or the wire welded mesh in lieu of fiber mesh. This simply is not true. the fiber mesh does nothing more than help prevent cracking during the curing stage of concrete. Due to the fact that the concrete will not all dry at the same rate the difference between areas will cause shrinkage stress and hence the micro cracks. Fiber mesh will help alleviate that issue but does not provide concrete with the tensile strength it needs.

[DanielP]

I don't use it. 16"x16" grid of #3 rebar in slab is my choice.

[EightFingers]

So what does the cracking do to a tile floor? I would assume it would crack as well and you would have to replace some tiles (not sure how tile would "float" on a slab if at all). Carpet and wood would fare a lot better I'd guess. So, if one was to buy a slab house would one be better off looking at older houses that have already gone through the "cracking" process?

[Carey]

If you are tiling on a new slab & do not know if/how it will crack, there is the Ditra tiling underlayment that does incorporate "give" (HD has).

Older cracks should be prepped prior to tiling, based on the location, type & potential movement of the underslab....same with using tile on tilebacker...the seams gotta be prepped properly....or tile pop-off or cracks can happen (ancient systems used a very intense sub build-up to maintain those spectacular mosaic floors of yore)

[DanielP]

Ditra is an excellent product. I have used it successfully for many years. Isolation membrane