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
314 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.


Moderator: Construction Systems, Foundations, and Masonry & Stone

For more information about me and/or my qualifications please visit my website at:


Post a reply as Anonymous

Photo must be in JPG, GIF or PNG format and less than 5MB.


type the code from the image


Post_new_button or Login_button

All bookworms need a good bookmark that inspires them to keep reading. To make this colorful bookmark, cut a rectangular p... It turns out that many bath and kitchen cleansers contain chemicals that are dangerous to the skin and eyes, and often pro... So often we paint tiny nooks white to make them appear larger, but opting for a dark, dramatic wall color like this one—Be... Chocolate-colored walls and large window frames allow the exposed wood beams to take center stage in this small screened p... If you're not crazy about the idea of commingling plants and pool, this modern variation may be more to your liking. The s... Yes, a freestanding garage can become its own tiny house. Artist Michelle de la Vega has all the comforts of a modern resi... There’s nothing like a new set of cabinet hardware to refresh a room. The possibilities are endless: Go modern, rustic, or... Pursue what's known as the stack effect. To achieve it, open the windows on both the upper and lower floors, and as warm a... Like no other floor type, a checkerboard design works wonders to underscore the retro kitchen theme. Vinyl flooring, ceram... Twine lanterns add pops of crafty—but sophisticated—flair to any outdoor setting. Wrap glue-soaked twine around a balloon ... When securely fastened to a tree or the ceiling of a porch, a pallet and some cushioning make the ideal place to lounge. V... Incorporate nature into your lighting scheme by securing a dead tree in a concrete mold and draping your pendant lamp from... For the cost of a can of exterior paint , you can totally transform your porch. Paint the floor a hue that complements yo... In this urban apartment, a standard-issue patio became a serene and green perch by replacing the typical concrete with gro... If you put the washing machine in the mudroom, you can stop the kids from walking through the house in dirty, grass-staine...
Newsletter_icon Google_plus Facebook Twitter Pinterest Youtube Rss_icon