In place density of 3/4 Crushed stone 2

[massbridge]

We have compacted 3/4" crushed stone for a leveling course prior to concrete placement. The compaction was tested using a nuclear density machine. He used the "back scan" option. Doesn't that give you a direct reading of the in place density rather than a value to compare to the lab density or(Proctor)?

 

[Ron]

Nuclear density determination on gravel with no fines is not particularly accurate whether you use backscatter or direct transmission.

For determination of in-place density, use a direct volume replacement method such as the rubber ballon method (hard to find these days. You can also use the sandcone method, but you'll have to isolate the voids in the gravel by using Saran wrap to line the hole. Make sure the Saran wrap is loose so that the sand conforms to the particle surfaces.

 

[TigerGuy]

If the 3/4" crushed stone has sufficient fines in it, a proctor curve can be obtained. We see it used extensively on projects.

Ron is correct that nuclear density testing on clean rock is inaccurate at best.

The choice between backscatter and direct transmission can be governed by the layer thickness. Most meters on backscatter only test the upper 4 inches.

 

[Ron]

Since this is being used as a leveling course under a concrete placement, do you really need to know the density? I get the impression this is a coarse aggregate leveling course, not a graded aggregate subbase for the concrete. As tigerguy notes, if there are sufficient fines in the material, a moisture-density relationship can be done in the lab, to which you would compare the field density for compaction assessment. If that is not the case, the density of the clean aggregate is somewhat irrelevant. Just compact it with several overlapping passes of a compactor to achieve a modicum of uniformity, without regard to the actual in-place density.

 

[massbridge]

The contractor took a sample and came up with a dry rodded unit weight. The inspector used the Nuclear Density gauge (on back scatter) to come up with a density reading. The spec calls for the stone to be compacted to 100% of the rodded value. I don't believe that you can compare a back scattered compaction value with the rodded density. I thought that a back scatter value is a direct reading of the (some what accurate) density in place. Does that sound right to you?

 

[Ron]

Well, density is density, so theoretically you can compare the two. However, as we've stated, the nuclear density gage for this application is not very accurate.

Keep in mind also that the dry rodded unit weight is done in a confined "bucket". Loosely placed gravel has little or no confinement and no cohesion, so you're not really comparing apples to apples.

 

[massbridge]

Ron,
I attached a photo of the 3/4" stone which was placed and compacted. There is approximately 2 feet of this stone. It is placed directly on rock. There is a 15' x 34' x 3' deep 4000 psi reinforced concrete footing over this. I am very comfortable with the integrety of the structure. Would you concur?

 

[TigerGuy]

It doesn't look like there is sufficient lateral support for the rock, if the footing has a significant load. How much settlement can the structure withstand?

I have monitored placement of clean gravel on numerous projects. Our standard requirements are maximum lift thickness of 6-8 inches and a minimum of 3 passes with a vibratory compactor. Generally, it's for backfilling building foundations under sidewalks or lawn areas.

 

[Ron]

Whoa, massbridge...let's take these one at a time! The 3/4" material appears to be well placed and assuming it was compacted in shallow lifts, it is probably OK. The thickness is a bit more than I would typically use. If the rock below is that uneven, I would look at removing some of the rock. I don't particularly like thick sections of gravel for two reasons: (1) under heavy foundation loads, the elastic strain can be high and (2) the high voids tend to hold a lot of water.

Structural integrity goes to a lot more than just a leveling course of gravel, so I won't go there.

 

[massbridge]

Thanks everybody!