Pea gravel for interior floor slab support
Pea gravel for interior floor slab support
(OP)
Does anyone of any suggestions or information regarding the use of pea gravel beneath interior floor slabs? Recently, a local builder used pea gravel up to 6 inches in thickness above the vapor retarder and under a post-tensioned slab. The builder used the increased pea gravel thickness to help re-establish grade due to winter construction conditions. The local builder believes the pea gravel to be "self-compacting" which is not correct.
Thanks for your help.
Thanks for your help.





RE: Pea gravel for interior floor slab support
RE: Pea gravel for interior floor slab support
With respect to the gravel being "self-compacting"; while you are correct that it is not self-compacting it is almost impossible to compact pea gravel. If you run a roller over it, you may get the roller stuck because of the way the rock will continue to move around beneath the roller. You may be able to run a plate compactor over the rock, but the upper 1/2 will not look or behave any different and the benefit to the lower portion of the rock may not be much better.
Hope this helped a little.
RE: Pea gravel for interior floor slab support
I've seen a lot of sand bases in which, after the slab was placed and after it was removed (for various reasons) we discovered that during the concrete placement, the construction worker's footsteps could be observed and the slab had a very non-uniform thickness...even where an intended 5" slab had its sand pumped up from footsteps to create 2" thick areas.
So my concern would be to ensure that the base was stable under foot traffic and wouldn't behave like a Caribbean
beach. Any thoughts on this?
RE: Pea gravel for interior floor slab support
RE: Pea gravel for interior floor slab support
On the day of the pour you pretty much have to use boards/bridges resting on the formwork to keep from disturbing the pea gravel.
I said it could be done with good results, not that it was easy. :)
RE: Pea gravel for interior floor slab support
RE: Pea gravel for interior floor slab support
Sheldon Shaw PE
Soils Alaska
Fairbanks
RE: Pea gravel for interior floor slab support
Also, a thought regarding capillary break material under the floor slab. The point is to stop free water from migrating by capillary action to the bottom of the concrete or even the vapor barrier. The material must be gap-graded to limit capillary action. A clean sand or sand and gravel might be free-draining but could still allow capillary water action. I have this discussion with contractors all the time and also remind our plan review people to watch for this. We like to do a simple lab test with the proposed material in a plastic container/jug with small holes in the bottom. Set the container into a shallow pan with about a 1/2" to 1" of dyed water and observe how high the water is pulled up the side of the container.
RE: Pea gravel for interior floor slab support
RE: Pea gravel for interior floor slab support
RE: Pea gravel for interior floor slab support
thanks
RE: Pea gravel for interior floor slab support
RE: Pea gravel for interior floor slab support
RE: Pea gravel for interior floor slab support
If this site will ever experience water seepage requiring a drain system, water will flow easily though this pea gravel, due to the "large" void "diameters". Should the soil from whch the water comes or that under the pea gravel not be well graded granular material, such as fine sand, then you easily can have sand migrating out of the area under the slab through the drain system. Geotech fabric around the drains may stop this migration.
However, if there is any "holiday" in the filter, be prepared, in time, for big voids under the slab, footings settled, etc.
The cure for the situation is to use a well graded granular filter material instead of the single sized stuff. The US Army Corps of Engineers years ago found that the best filter material is ASTM-C-33 Fine agregate, (concrete sand). Cost is same as pea gravel and it really does the job.
It is very difficult to goof up a drainage job if concrete sand is the main drainage medium. With pea gravel, I've seen many a goofed up job.