Using recycled concrete and asphalt

[ohengineer]

I am currently working on a project where the client wants to use recycled concrete from previously demoed floor slabs and foundations as fill material and recycled asphalt as a base course.

For recycled concrete, they know to remove all metal and construction debris. They say their crusher can crush the pieces into 6 inch pieces. My thoughts are that this would be fine provided there were enough fines in the material and treat it like a shot rock fill.

For the recycled asphalt, the asphalt is about 2 to 3 inches thick with fatigue cracking everywhere (very poor condition) Boring logs indicate two or less inches of crushed stone base. Our client want to mill the asphalt and use it as the base. My concern is contamination of the underlying soil subgrade into the milled asphalt. In addition several people at our firm have had negative experiences using milled asphalt as a base material.

Would appreciate any comments, concerns, or literature that could help. Thanks

 

[msucog]

1st of all, i suggest you try to find out if the undersoil is in poor condition. if it's more or less good, then maybe consider pulverizing to a depth an inch or two in to the soil to take care of any potential surficial soft spots. prior to pulverizing, spread out the crushed concrete on top a few inches thick. pulverize it all together to maximum thickness of 8 inches or so. maybe even add a little cement or lime depending on the overall gradation, composition and stability of the materials. lay it back down just like a soil cement or cement/lime stabilized base. depending on the pavement use and anticipated loading, that may be good enough but a few inches of GAB might be especially helpful. if this is a privately owned pavement area, then they might be willing to accept a few cracks. if it's more critical, they should do more upfront anaylsis and testing. in order to go through a bona fide soil cement type design, the max partial size would likely need to be smaller. it might be cheaper overall to have the contractor reduce the concrete to save 2-3 inches depending on what the reclamation contractor might charge to pulverize 6 inch material with the asphalt. at least with the smaller size, you could probably put a soil cement strength with a design in the event someone needs paper on it. GAB and asphalt makes a fine soil cement without a whole lot of cement. add a lot of soil and it throws the cement on up due to the fines, mica content and clay fraction. again, this is for a true soil cement. if you're looking to just make it "hard", then the cement might be cut in half. even if you throw say 2-3 inches of fine grained soil in to the mix, the asphalt, GAB, and concrete should do very well with a minimal amount of cement.

try the PCA site. they have some free publications and very inexpensive publications that have great info in them on the subject.

http://www.cement.org/

also check ASTM and your DOT sites for additional info.

 

[dirtsqueezer]

I assume this is for some sort of roadway. 6" stone is a big material. I'd be worring about the fines migrating to the concrete base and creating voids. The other thing with recycled concrete is that it isn't very angular, at least not as much as, say, basalt. It won't hold as much of an edge, and may even settle if you're talking thick sections. It also can't be tested for compaction.

I've used both recycled concrete and asphalt for roadways. Recycled asphalt can be tricky to test for compaction. A nuclear densiometer tests for dry density and moisture. The problem with the asphalt is that there is hydrogen in the oil, which counts for moisture, which makes your dry densities low, making your compaction measurements read as low (even though they're not).

I've seen recycled concrete used in the 1 1/2" size and it tested well. I wouldn't use that ashpalt/crushed/soil for base/top course unless you had to. Maybe for fill in case the recycled concrete runs out. Crushed rock is always the best.

 

[msucog]

dirtsqueezer brings up a good point...sometimes, the testing has to be based on a performance criteria such as general observations, proofrolls, etc. if the material is the more desireable minus 2" size, then sand cone density tests should be satisfactory (nukes don't work here when mica involved). more importantly, the observed procedures and performance of the materials under the typically large compaction equipment should be satisfactory for the application.

 

[ohengineer]

The concrete is just going to be used as a mass fill (probably 6 to 8 feet thick in a low lying area in the parking lot. My thoughts are that if it is treated like a shot rock fill and chocked off at the surface it should perform well. I do not have experience using recycled concrete and its performance. From what I have read, it can be used as fill for roadbeds and backfill behind MSE walls.

For the asphalt, there is currently existing asphalt in place, they would like to just mill up the existing 2 to 3 inches of asphalt and immediately compact it behind the milling process and consider that the base material without any additional base. Once the asphalt is milled, then they would put 3 to 4 inches of asphalt on top. My calculations for 100,000 ESAL's indicates approx. 4 inches of asphalt and 6 inches of crushed stone base.

My opinion is that the recycled asphalt will not work without the addition of more crushed stone and if the milled asphalt becomes "contaminated" with soil then structural capacity becomes less. From talking with some more experienced engineers, they seem to have had negative experiences with using recycled asphalt as base material.

Any comments still appreciated. Thanks

 

[boffintech]

"For recycled concrete, they know to remove all metal and construction debris."

If you are after structural fill that's where it goes wrong. It's the knowing AND the doing that matters. Actually performing the task of debris removal is time consuming and labor intensive. The owner should be advised of the nearly constant inspection/observation/oversight that will be required to verify this.

 

[msucog]

all the jobs i've had, we recommended that they still overlay the reclaimed material with a few inches of base course material to keep surface cracking down. i would probably object to just milling and laying it back down as the base course layer. i think that your structural number would be drastically reduced for the reclaimed asphalt versus fresh asphalt or even base course material. if it were me, i'd get the concrete down to a more preferable size, and do the whole thing as a bona fide soil cement design process. your structural number should be down in the range of GAB or a little better for that layer. and i'd still go with some thickness of base course material, but that may not be desireable from the cost standpoint. it might be more cost effective to just find a placement area onsite for all the concrete and leave it bigger, then deal with the asphalt independently to avoid additional crushing costs.

it is not cheap to reclaim material as i describe so your project may not be big enough to be cost effective. i suggest you avoid a "billy bob" type reclaiming contractor since they could easily screw up the entire process for a whole lot of money.

go to this site to see if it helps. the basic asphalt recycling manual is very good.

http://www.arra.org/

good luck

 

[fattdad]

If the material is 6 in size (and finer), I'd still recommend a typical lift thickness of 18 inches. I would consider some performance-level placement criteria that includes some consideration on moisture content and number of passes.

f-d

¡papá gordo ain’t no madre flaca!

 

[jtatlow]

I HAVE USED PULVERIZED AC AS A BASE MATERIAL BEFORE. THE GRINDER SHOULD PULVERIZE THE MATERIAL TO AN AVERAGE OF 3/4 INCH. YOU MAY HAVE SOME CHUNKS, AND IT WOULD BE A GOOD IDEA TO USE AN 815 FOR COMPACTION BEFORE THE SMOOTH DRUM ROLLER.