Recycled Concrete Road Base Compaction 1

[jjesik]

HAs anyone had experience using this stuff. It is aggregate Road Base CDOT Class 6, if anyone is familiar with CDOT. But the compaction keeps coming up in the low 90's with a modified proctor, moisture looks good.

The contractors seem to be laying it in there and compacting it with a vibratory roller, I'm thinking they may need a larger one.

I'm just curios if there is some reason with recycled concrete that it wouldn't meet compaction. It basically looks like aggregate with concrete powder more or less. I didn't notice and visible chunks of concrete in the stuff.

 

[GeoPaveTraffic]

Have you checked to see if the modified Proctor laboratory test resulted in crushing of the material? It is possible that the higher energy of the modified Proctor lab test has crushed a significant portion of the material that is not occuring in the field. This can shift the maximum density thereby making compaction very difficult to obtain.

 

[eric1037]

In my experience with recycled concrete, it is possible for some asphalt pieces to get mixed in. This will throw off the nuke guage (assuming that's what you are using) due to the hyrdocarbons.

 

[jjesik]

Thanks for the info guys, I'll check and see about the modified test actually cruching the material, that does make sense. There could possibly be recycled asphalt in the material I suppose, but I haven't seen any of it.

 

[Ron]

For materials contain cement, it is important to take the density test immediately after compaction. When taken later after some hydration has occurred, the density values tend to go down. I've seen this in soil-cement, crushed recycled concrete, and cement-treated base materials. This is particularly true when a nuclear density gage is used. Try a sand cone correlation just to check it.

 

[Scienceguy57]

This is kind of a late response, but I ran into that problem in the last two weeks in Colorado as well. I assume that’s where you’re at since you mention CDOT. What was your proctor like for the material? The material I was testing was crushed concrete, kind of reddish brown. I did the mod. proctor on it myself and came up with 124.7 p.c.f. at 10.7 percent moisture. I don’t think the mod. hammer could have pulverized any of the material because the material seemed to be stout. Haven’t noticed any asphalt either. The contractor would get about 88% off the bat then would struggle to get up to even 92%. I think the problem was on my job was that the contractor put in an 18" lift, then hit it with a vibrating sheep’s foot rather than a smooth drum, wheel rolled it with the other tandem trucks dumping the material and with a loader, then finally put water on it after it was compacted. After 4 retests they finally got one final spot to pass on the first lift. It will be interesting to see if they get the final lift to pass more easily this week since it's "only" a 12" thick lift.
Ron, shouldn't the hydration process be totally finished after the concrete was initially poured, or can some cement be left over if enough water isn't used?

 

[GeoPaveTraffic]

Scienceguy57,

I expect that the problem you saw had a lot more to do with the thickness of the lift and using he wrong kind of compactor. I would be willing to bet that the sheeps foot roller DID cause at least some of the concrete to crush, thereby creating more fines and making it harder for the contractor to get compaction.

I would recommend using a vibratory smooth drum roller along with wheel rolling to compact this type of material with loose lift thicknesses of ~9 inches.

 

[jjesik]

Basically what we ended up doing is laying down 4" lifts, had the semi's run over it and a new vibratory smooth steel drummed roller go over it eight times. Total thickness was 8 inches. The base course looked like it was aggregate with concrete fines in it already, so I wasn't to worried about over crushing the aggregate, unless we ended up crushing the actual rock, but that would be very hard to do unless we had bad rock in the first place and the LA abrasion came back really good.

The new roller really shook the base course and that did the trick, we ended up with compactions from 95.5 to 98 percent of the modified.

This looks like really good material to me and actually prefer it over regular base course. It actually seems to harden up a bit on the surface after it gets wet and is more stable, especially if you have traffic on it than just rock base course. I guess if I learned anything from this, make sure that your base course is properly processed at the plant and broke down enough so that there aren't any larger concrete pieces in it (pieces you can see) because if you need to wack it hard it'll probably crush and change your material properties.

 

[jjesik]

Our proctor was 119.1 pcf at 13.9%, which seems a little low to me for base course. We had another done at the pit with 122.7pcf and a moisture of 11.3%. It does seem like the recycled concrete base course does require more compactive effort than regular base course.

 

[rockiologist]

Are you allowed to burn a moisture off in the field right after you take the nuke test? That seems to work for us whenever we encounter recycled concrete material for subbase. Using the Dry Density from a lab compaction curve the field technician will use an older style nuclear gauge and get a wet density. He will then take a small amount of subbase directly below where he took a nuke test making sure he dug down to the depth of the lift. He will then burn off a moisture in the field. Hope this helps.

 

[Ron]

scienceguy57....there is almost always unhydrated cement left in concrete. In almost all of the recycled, crushed concrete I have seen over the years, latent hydration occurs after compaction.

 

[Scienceguy57]

That's neat. I have noticed that before when the surface of crushed concrete hardens up. Wasn't sure if there was actually unused cement left over causing that. Yeah I thought that was the problem in my case, they put in way too big of a lift and then used the wrong type of compactor on it. May have helped if I had sampled for the proctor from where they compacted already rather than from the stockpile if they were really crushing the aggregate.