Compacting Crushed Concrete

[Mattyhooks]

Crushed concrete fill:
-max dry density of material < 3/4 inch 117 PCF at 13.5% moisture the standard proctor
-oversize material 20-30% > 3/4 inch

Compaction method:
-probe sub grade and compact if necessary
-compact #57 stone to 6 inches above pipe
- 9 inch lifts of crushed concrete fill
- 6+ passes with 4 ton vibratory roller

I am hoping someone can help me figure out my conundrum on the compaction of crushed concrete. We have been backfilling a utility trench all week and I had a thought regarding the oversize in the fill. I ran the proctor test myself and perform field one-points, that range from 11-13% moisture, to verify moisture and density prior to compaction. I am getting consistent nuke readings of 11-13% moisture and a wet density around 135 PCF, with little to no change after 6 passes.

I agree with correcting the max dry density for oversize material, however I disagree with a corrected optimum moisture. Since the moisture contents of the material > 3/4 inches is lab tested 3-5%, the corrected weighted average optimum moisture should be around 10.5-11.5% moisture. I disagree with this statement considering the concrete can absorb moisture and not deform like other soil materials. We have encountered better compaction with moisture values near 13% versus 11% both density wise and visually after equal number of passes. Additionally, the specific gravity of the oversize material is so variable, it is difficult to accurately determine corrected maximum dry density value.

What are your thoughts on correcting for oversize and confirming compaction?

 

[bb29510]

my crushed concrete runs about 126 lbs per cu ft with a moisture of about 8%. one thing you need to consider is, if they are over compaction to an modified. yes you testing to a standard but a modified on a crushed concrete will run about mid 130's Our biggest problem is not the oversize but no fines. we have to add sand to meet gradation

master ICC inspector, AWS CWI

 

[Ron]

bb29510 is correct. By all indications, you are compacting past your standard Proctor value....somewhere in the range of 102 to 105 percent compaction. You indicated little or no change after 6 passes, also indicates you have achieved all the compaction you are likely to get....it is now just a numbers game.

I'm not sure why you are using a standard Proctor comparison on crushed concrete. It has few fines and generally should be compared to a modified Proctor. If so, your MDD will be higher.

As for your moisture content, generally a Speedy moisture value in crushed concrete is more accurate than a nuclear moisture test.