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thomcat482 (Mechanical) (OP)
17 Sep 07 13:38
i am building a house in waynesvile, nc (3700 elevation). Would there be an advantage to pouring the footers and basement floor with 5000 psi instead of 3000 psi concrete. the local standard is 3000 psi at 6" for the floor, however i have see a lot of cracks in basement floors in the area.
JAE (Structural)
17 Sep 07 13:47
Higher strength concrete means you have more Portland cement in it which means you will have MORE shrinkage, not less, and thus more potential cracking.

JKW05 (Structural)
17 Sep 07 15:40
ACI 318-4.2.2 has some requirements for higher strength concrete for special exposures; e.g. low permeability or exposure to freezing, deicing agents, seawater, etc.  Since this condition doesn't qualify under these exposures, unless you need the higher compressive strength for some other structural reason (not likely for a single family residence), it doesn't appear that there is a benefit to the higher strength.

The cracking you may have seen elsewhere is probably not due to the concrete strength, but more likely due to poor subgrade compaction; lack of control joints in slabs; absence of shrinkage reinforcement; or poor curing techniques.
JKW05 (Structural)
17 Sep 07 15:48
Forgot to include this in my previous post:

Is that 6" slump?  If so, that seems a bit high to me.  ACI recommends 3"-4" for slabs and footings.  The higher water content may be a contributing to the cracking as well....

JKW
JAE (Structural)
17 Sep 07 16:06
JKW05 hits on a good point - keep your water/cement ratio low - with low resulting slumps.  Too much water in mixes is really the prime culprit in shrinkage cracks.

Rjeffery (Civil/Environmental)
17 Sep 07 16:49
Cracking in the slab may also be due to the welded wire fabric (WWF) not being in its proper position in the slab.  It is usually placed on the vapor barrior and never lifted into the required 1/3 to 1/2 the hight of the slab thickness.  The WWF will NOT prevent all cracks but will hold them tight(er} than without the WWF.  The only real advantage to placing a higher strength concrete is that it is less permiable and may set faster allowing the formwork to be removed early.
concretemasonry (Structural)
17 Sep 07 16:57
I agree with JKW05 -

Higher strength concrete will not help and could shrink more.

The old concept of more strength means better durability, but that does not always hold and you do not have the exposure. If you do have concrete exposed (especially in a basement slab) to freezing and thawing conditions, air entrained concrete could be a good addition.

If you are worried about shrinkage, do not add excess water and keep the slump to 3" maximum even though contractors do not like to work with it. Cure prperly and slowly.

All concrete will crack and what you may have seem was really micro-cracking of the surface due to finishing and/or curing.
msucog (Civil/Environmental)
17 Sep 07 19:16
never forget about what is holding up the concrete...
BigH (Geotechnical)
17 Sep 07 20:53
Make sure you remove the laitance from the slab surface as part of the finishing process.
DRC1 (Civil/Environmental)
17 Sep 07 23:10
The odtimers would design to the minimum required strength to give durability. w/c ics imprtant, but you dont want it too low. 4" to 5" is good. Concrete will crack if it is not properly cured, or the reinforcing is to low as Rjeffery noted. Well compacted subgrade is also key, as well as proper underslab drainage. Large slabs will crack no matter what if they are not jointed. All concrete will crack to some extent. Good quality workmanship is your best bet. Spend the time and money to find a quality mason.
shin25 (Structural)
18 Sep 07 14:49
Select minimum possible amount of cementitous meterials for the mix, select the construction/contraction joints carefully, provide minimum T&S reinforcing and above all provide adequate curing- you will be good to go.
concretemasonry (Structural)
18 Sep 07 17:17
Personally, I would look for a good concrete contractor that hires finishers to place and finish concerete.

A masonry contractor hires masons. Masons use mortar to lay masonry units.
DRC1 (Civil/Environmental)
18 Sep 07 21:20
Mason contractor may be a regional thing. Around here anyone who works with stone, brick, masonary units or concrete flatwork is a mason. However, I do agree that one should hire a quality flatwork finisher.
concretemasonry (Structural)
18 Sep 07 23:00
I just went to the Mirriam Webster Dictionary to get the definition of a mason and it specifically referred to the use on mortar with no mention of concrete.

Around here, we have many contractors that do both and they advertise or hold themselves out to be "concrete and masonry constractors". They even hold separate industry company memberships for both trades. These contractors also have separate agreements with both unions.

We also have contractors that do only one of the trades and have no union agreements with the other unions.
Helpful Member!  Speedycrete (Civil/Environmental)
18 Sep 07 23:05
Don't skimp on the reinforcement if you want to minimize shrinkage cracks.  Consider going from 2' pattern to 1' (or even 18").  (not a cheap option either)  Plan the control joints to be where you WANT the cracks to be.  (under walls)  Consider fiber reinforcing if you will have a floor covering.  If you need the workability, plastizer is a great help, although not a cheap addition, but better than the cost of additional powder, which doesn't help the shrinkage problem anyways, as pointed out.  Proper curing is essential.

Concrete is not cheap, and it's more expensive to do the second time.  The foundation is the start of the build process and it's intended to outlast the rest of the building, spend your money wisely there, save money on the shingles, they are much easier to do-over in 10 years.

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