Pea gravel for interior floor slab support 4

[RosevilleGE]

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.

 

[LCruiser]

Then how would one "compact" pea gravel?

 

[GeoPaveTraffic]

In the mid west (USA) where I practice crushed limestone is normally used beneath floor slabs. I have however specified that pea gravel be used in a couple situations. Six inches of pea gravel, I assume that the max particle size is 0.5" or so, should be fine support for most floor slabs. I have used it beneath a waffle type slab on very expansive weak shales and/or hard clays. The ribs of the slab bear on the pea gravel with a very low bearing capacity factor of safety. If (when) the soil swells a bearing capacity failure occurs beneath the ribs and some of the gravel flows into the waffle area of the slab and as a result the floor does not heave.

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.

 

[JAE]

GeoPaveTraffic, one question for you - my fear in using pea gravel, or even some sands for that matter, is that with material like this that has uniform gradation, I always sense that there is a difficulty in compacting the material in a way in which it "nests" into itself and remains stable under foot traffic, etc.

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?

 

[cvg]

a better material to use would be a well graded aggregate base course. it is readily available, compacts well and provides good drainage - and doesn't exhibit any of the problems mentioned with pea gravel or sand

 

[GeoPaveTraffic]

JAE, your point about movement is very accurate. It is the single biggest problem we have seen constructing these foundations. The best solution is to structure the pours to minimize the amount of traffic, to dress the areas of traffic with comealongs/lutes/rakes following disturbance, to use bridges/boards, and to place plywood in heavy traffic areas.

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.

 

[bushel3]

Just to add to what Geopave was saying, my company often allows CA-7 (Illinois DOT spec) under floor slabs. That is a 3/4 inch clean angular crushed limestone. You can not compact this. We typicaly specify that the contractor tamp (roll over w/ bobcat or tamp w/ bucket of backhoe) the stone during placement to achieve particle interlock. Since the stone is angular and not round much less movement/pumping occurs under foot traffic.

 

[srshaw]

INteresting.. I often spec pea gravel under tanks. In this circumstance it is nearly self-compacting. But as noted, foot traffic would cause some surficial displacement of the material. I prefer to use a compacted and well watered sandy gravel base beneath concrete slabs.

Sheldon Shaw PE
Soils Alaska
Fairbanks

 

[Geodude1]

Pea gravel is still commonly used under floor slabs and tight places. Floor contractors don't like working with it because it displaces so easily under foot and while placing concrete. Many contractors in the Puget Sound region have opted for clean, crushed rock such as 5/8". The only product I know of that is "self compacting" is Controlled Density Fill (flowable fill pumped from a concrete truck). I used to hear the contractor tell me that pea gravel goes in automatically at 95% compaction. For relatively thin fills its probably not an issue. We have been recommending that contractors at least vibrate the surface of thicker sections of pea gravel (hoepac or sled) to help settle the material, especially around manhole structures where equipment access is restrictive. No need to conduct in-place density tests on the pea gravel. I had one site that had 5' of pea gravel that I could T-probe 4' to the handle. As recommended, they used a conrete stinger and the next day I could only probe 6".

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.

 

[kozera]

Has anyone every seen an engineering definition of "pea gravel" which includes a gradation requirement?

 

[GeoPaveTraffic]

Pea Gravel refers to the rounded to sub-rounded nature of the stone, not the gradation. You could have 6- to 12-inch pea gravel or minus 3/8-inch pea gravel.

 

[kozera]

Thanksw for the reply. This is also my contention. When pea gravel is specified I have not seen it with a size other than say something like 3/4" pea gravel. However, when one has to size subdrainage pipe openings the D85 of the material is required and thus my question regarding a gradation. Are there tolerances +/- when a size of pea gravel is specified?
thanks

 

[CarlB]

I've always understood pea gravel to be about the size of peas, or about 1/4". The ACPA site (

http://www.pavement.com/PavTech/Tech/Glossary/P.html)

has a definition of "Pea Gravel"- Screened gravel the particle sizes of which range between 3/16 and 3/8 inch in diameter.

 

[LCruiser]

My experience with it is in shotcrete - 3/8" max.

 

[oldestguy]

Here is another thought.
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.