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DaveKrueger

04:08PM | 10/06/03
Member Since: 10/05/03
3 lifetime posts
Bvbasement
Hello,

I am adding a 24x28 ft 2-story addition to my house. The footings have rebar and are about 20"-24" thick, but the contractor did not use any wire mesh reinforcement in the 4"-thick slab.

Now, three months later as they are getting ready to put in the carpet, I noticed there is a 1/16" wide crack that spans almost the entire length of the slab. The contractor says it's a shrinkage crack and not to worry about it, but I'm concerned anyway.

I live in North Alabama and the soil is largely clay. Is it normal for a slab to crack that quickly and how likely am I to have problems since no wire mesh was used?

Thanks for any help!

Glenn Good

11:09AM | 10/08/03
Member Since: 09/10/03
314 lifetime posts
Generally fiber reinforced concrete is comparable to non-fiber concrete with wire mesh. The shrinkage cracks are fairly common and may not be a problem if they are not very wide. You can use a self leveling polyurethane caulk to fill the crack. If you experience a great deal of cracking, the concrete was not done properly or the sub grade was not prepared properly.

One problem that occurs all too often is that a concrete finishing crew will add water to the concrete after it has been delivered to the job. This in itself is not the problem however. The problem is that they have a tendency to add too much water. While wetter concrete is easier for the finishers to place, it will shrink more as it dries causing shrinkage cracks. The stiffer the concrete the less shrinkage you will experience and the stronger the concrete will be.

There is a test used for concrete as it is being poured called a “slump test”. It is used to tell how wet the concrete is as it is being poured. A metal cone is inverted and filled with the mixed concrete before it is placed. Then the cone is carefully removed and the distance the concrete slumps is then measured. The further it slumps the wetter the mix. Concrete for a building slab should not be more than a 4” slump. A slump between 3-4 inches is preferred. Most residential contractors do not have this test performed on concrete they use even though it is a simple and inexpensive test that would save many homeowners a great deal of aggravation. The reason many do not use this test is the finishers complain the concrete is too stiff to place easily making their job more difficult. The more difficult their job the more money they will ask for doing it.

Glenn www.consultationdirect.com

DaveKrueger

02:52PM | 10/08/03
Member Since: 10/05/03
3 lifetime posts
Glenn,

Thanks for the detailed reply. This contractor has been excellent and was quite clear that he insists on using a mix with less water for added strength. I've read about fiber reinforced concrete and some folks hold the opinion that the fiber reduces the cracking but doesn't do anything once the crack has actually happened. I just wanted to hear a second opinion on my specific case.

-Dave

Glenn Good

02:55AM | 10/09/03
Member Since: 09/10/03
314 lifetime posts
Dave,

It is true the fibers will not help after a crack has formed. If the sub grade is solid and has been pepped properly, narrow cracks should not be a problem. The rough and irregular shape between the two sides of the crack will help keep them locked together and prevent vertical shifting provided you do not have a large number of cracks.

Wire is of more benefit if the slab is not contained to prevent lateral movement (for example a slab poured on top of a grade with no turned down footing or walls around its perimeter. If the crack was wide (greater than 1/8”) and continuous the wire could also be of more help if the sub grade is subject to movement. However if the concrete was poured correctly and the sub grade is in good shape you should not experience this.

Glenn www.consultationdirect.com

DaveKrueger

02:59PM | 10/09/03
Member Since: 10/05/03
3 lifetime posts
Thanks, Glenn.

I see what you mean, now. The reinforced footings are deep (including a re-inforced beam down the center), the soil was very tight, and there was a good layer of gravel under the slab. There is only the one crack so far.

Thanks for the elaboration.

-Dave

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