Standard vs. Modified Proctor for Crushed Concrete

[Chewi00]

I have two questions relating to testing crushed concrete as a backfill material. I recieved some results an environmental engineer had performed and noticed some problems.

1) The crushed concrete contains more than 30% greater than 3/4" (52%) and therefore, can I consider the standard proctor results with correction to be accurate? I suspect not as stated by the ASTM.

2) Is there any rule of thumb when performing a proctor on crushed concrete to be used as backfill (i.e. Standard vs. Modified)? For most granular soils, I usually specify modified and was wondering if there was any special reason the lab would have performed a standard proctor on the crushed concrete.

Thanks in Advance
Adam

 

[msucog]

1: i don't have astm in front of me, but if the correct method (C, D or whatever) was used, it is what it is. you should keep in mind sampling of the material and uniformity of the material. if a piles been pushed around into a stock, the coarser material may fall out so depending on where you sample, the numbers will change.

2: i personally would only spec a high modified % if the material is graded and a "controlled" material...(i.e. GAB material from a quarry). another reason is that (if i remember correctly) based on the gradation, it sounds like it might be a modified D which means using the big mold with 56 blows in 5 layers with the heavy hammer...OUCH!

i would consider evaluating this like a true "rock fill" and throw out a performance spec...proofrolls, periodic tests, very thin lifts, specified gradation, big equipment, etc....but that's just me.

 

[rockiologist]

1. Did they do a bulk specific gravity correction on the +3/4" fraction? If they did not you will get lower compaction and higher moisture reuslts. How much fines are in the specimen? If you have to little fines I do not care how much you pond the sample you will not get a good curve. Try running relative denisty on the sample.

I would caution you when running a modified tests with concrete or any soft rock. You will end up crushing the rock and possibly throwing off your results. I hope this helps.

 

[Chewi00]

They did the bulk density correction (D4718). The lab performed a standard proctor and put a note saying that because the amount of material greater than 3/4" was higher than 30%, the corected proctor values may be inaccurate. What can I do with this? Is it worthless?

Secondly, Did they use a standard proctor because of the crushing factor you mentioned? If so, im assuming it was better to use a standard over a modified on this material?

 

[Chewi00]

As a follow up, this material is mostly graded between 2" and No. 4 with only 3 or 4% passing the number 4. Would any compaction be required, it seems this is compacted as placed and only rolling for smoothing would be required.

 

[msucog]

sounds a little large to be doing compaction testing. i would treat it like a rock fill and use a performance specification. make the contractor put it in thin with big equipment tracking in the material and running large compaction equipment over the top. evaluate the fill placement by noting lift thickness, amount of effort to place the material and proofrolling the top. in a "structural" area, i would limit the lift size to 1 foot and require something big like a D8 tracking in the material with repeated passes. proofroll each lift with the loaded haul trucks to look for instability. it may sound like overkill, but there's not specific test that would be performed to back up that the material is well compacted. i would also specify that the material should be well-graded down to fines. i would also insist on full time observation by a testing firm. contractors often insist to me that aggregate is "self compacting"...those funny contractors...what the heck are ya going to do with 'em?

 

[Chewi00]

sorry for the ignorance here...just a couple questions on your post.

Is the thin lifts and heavy compacting because of bridging or voids that may be created during placement with an angular gravel? Wouldnt the rock-fill be compact upon placement if its mostly gravel with no fines to fill the voids? I just cant think of what heavy compaction would achieve with this type of gravel material...although im relatively inexpierienced?

 

[Chewi00]

oh and why would we require it graded to fines...to fill in the voids? Whats the downside to having a gravel with hardly any sand or fines?

Thanks Again.

 

[msucog]

yes, compacting will reduce void spaces. think of what you gravel as being smaller...say the size of sand...compaction effort is obviously required for sand. it's the same concept. and graded down to fines does fill in the voids. i'd allow some rock fills be placed in lifts up to about 4' thick..for true rock fills in non-structural areas. i usually run across rock mixed with soil which is not the same thing. for an aggregate fill of significant size and no fines, i've seen washing of adjacent soils in to the layer which created additional problems. it also creates a cavity for water to collect. from my experience with crushed concrete, there are usually enough of the finer material to take care of having a well graded material. if you're really having a hard time getting enough finer material, use soil and keep the lifts thin and alternating. i would suggest using caution with this if the soil is wet since you'll end up with a water bed and looking rather silly. however, if you have enough soil in the lifts, you could run density tests on those layers.