08:49AM | 07/16/04
Member Since: 07/15/04
14 lifetime posts
Obviously basement slabs need good drainage underneath so a layer of crushed stone or gravel makes sense. In a normal, above-grade garage slab does a layer of sand prove adequate enough? Also, should wire mesh always be used in slabs whether they be basement or garage? Finally, should vapor barrier be used under garage slabs, for a bond break or any other purpose?

Glenn Good

04:43PM | 07/19/04
Member Since: 09/10/03
320 lifetime posts
The stone base under a slab provides 2 things:

1) Drainage

2) Solid and stable base for the slab to rest on

Sand is a problem for a base for several reasons:

1) Next to impossible to maintain the flat grade when walking and working on top of the sand as the concrete is being poured

2) Sand needs to be wet and compacted before you pour - clean washed stone does not

3) Sand will wick moisture up into it where clean stone will not

Reinforcing of some type should be used in all interior concrete slabs and most exterior slabs to prevent major cracking. It does not have to be wire. Fiber reinforced concrete works well if you do not wish to use wire.

The biggest problems with most concrete today is in the amount of water used in the mix (or added during the pour) and not "moisture curing" it. The less water the stronger the concrete and less resulting shrinkage. Shrinkage causes cracks. Concrete should be between a 3"- 4" slump for most slabs. The larger the number the more water is in the concrete. The slump is the amount the wet concrete settles (or slumps) using a cone test. Specify the slump you want when you order the concrete and remember it will change if water is added after the truck leaves the plant. Keep an eye on the finishers. Many finishers have a bad habit of adding water to make their job easier, but this practice weakens the concrete and results in more cracking.

Concrete should be poured within 1 hour from the time it leaves the plant in most cases. Longer than that, the strength will begin to weaken (especially during hot weather). If concrete begins to get hard in the truck before you finish the pour adding water will weaken it.

Concrete should be moisture cured by covering it with 6-mil polyethylene immediately after it is finished to maintain the moisture. The moisture retaining cover should remain in place for at least 7-14 days (30 days is recommended). Using curing compound will work instead of poly and it should be applied at the same time (as soon as the concrete is finished).

The polyethylene vapor barrier under the slab also serves to maintain the moisture content in the fresh concrete during the curing period resulting in stronger concrete. Do not eliminate it.


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