Proper preparation for basement floor slab

[arbitrader]

I have contracted to have a new home built and am getting stonewalled by a very large nationial homebuilder.

They have poured footings and when asked if they intend to compact the 4 inch stone base prior to pouring the 4 inch thick basement slab they indicated that they don't do this because they are building on undisturbed soil.

I told the project manager that I am in agreement with his statement if the soil was indeed undisturbed. However, there are many ruts, a 2 or 3 dewatering holes 2 feet deep x 2' dia., and ridges of disturbed soil that occured during forming and pouring of the footers.

I told him that code requires compaction of disturbed soil (IRC 506.2) prior to pouring the slab. He stated that they will just fill the holes and rake out the surface before placing the 4" of stone and then pouring the slab.

My concern is cracking of the slab over any areas that are not sufficiently supported. He indicated that basement slabs always crack. I commented that I am sure they do if this is the way they typically build them.

Can someone tell me if I am off-base with my concern, or if my interpretation of 506.2 is wrong?

Thanks

 

[jheidt2543]

arbitrader:

The project manager is right about one thing, most basement floors do crack, but most basement floors are not installed correctly.

You don't mention what type of soils you are dealing with, but my suggestions would be:

1. You mention dewatering, I'm assuming that is for surface water (rain) and it won't be a continuing problem. If you are dealing with groundwater, that will require special drainage and waterproofing provisions.

2. Compaction is NECESSARY! If it were mine, I would require compaction of the subgrade and then compaction of the drainage course too. It is not that time consuming or expensive to run a plate compactor over the area.

3. Insist on a vapor barrier between the drainage course and the concrete slab of at least 6 mil poly, 10 mil is better. There are also specialty products for more intense moisture problems. Lap the joints at least 12" and up the walls behind the expansion joint too (you can cut the excess off at floor line after the floor is poured.

4. A 5 or 5.5 bag mix design concrete is fine, the key is to pour it as dry as possible. No water should be added at the site! The wetter the mix, the more suseptable the slab is to cracking.

5. Have the floor control joints saw-cut to 1/4 the slab depth as soon as possible after pouring, certainly less than 24 hours. The Soft-Cut method is the best, but not many home builders are using it.

6. Use a semi-rigid joint filler (caulk) at the slab/wall joints and control joints.

 

[GeoPaveTraffic]

I would not be too concerned about compacting the subgrade, there is very little load on a basement slab. However, the base rock should be compacted.

Most basement slabs crack because the concrete is way to wet, has too much water in it, when it is placed. I would get a copy of the mix design from the batch plant and insist that the slump is not greater than the mix design when the concrete is placed. Also, you will need to make sure that the concrete finishers do not add water to the surface of teh concrete during finishing.

As for control joints, cutting with in 24 hours is best and depending on weather and the concrete mix, cutting may be needed within 8 hours.

However, when all is said and done, the slab is still likely to get some cracks. Basement floors have so many corners and penetrations, that it is unlikly to avoid all cracks, just to minimize the ones you do get.

 

[arbitrader]

Soil is primarily sandy with a some clay. Not plastic at all. Looks like good structural fill but it is native. Site has always been farmlands.

I would be happy to just have them run a plate compactor over the stone drainage base. Yes they are using 6mil poly barrier.

I'm not sure I even saw expansion joint against walls of homes already built. Plans don't show a detail.

The only leverage I have is if things aren't to code. Can someone confirm that compaction is definitely required per IRC 506.2.1?

 

[techmaximus]

Instead of fighting with this guy why not rent a plate compactor, and get a couple of day laborers with a couple of rakes and do it yourself.

Techmaximus

 

[arbitrader]

I asked if I could do this and they said due to insurance concerns I am strictly prohibited from working on my site.

However, problem is resolved! I wrote faxed a letter to the company citing the code and telling them that they must compact or I will ask code official for his interpretation. They immediately called back and agreed to use a plate compactor.

Thanks for all comments!