Recycled Crushed Concrete 2

[bearjew]

Hi all,

This is my first post so go easy. I am involved with a project currently in which we are placing a large truck dock at a facility which lies in a flood plain. The soil conditions here are poor...lots of soft spots and generally weak soil. I can provide more info if needed. The adjacent bldg. sits on driven piles. We got the job before we knew about the poor soil (don't ask) and thus are now having to throw more concrete at our pad in order to compensate for the bad subgrade modulus. Fortunately, after some on site testing, we got word from the geotech that we can comfortably use a subgrade modulus of 100pci and be in the clear.

The contractor is wanting to use recyclded crushed concrete (approx. 3" nominal size) below the standard layer of crushed run stone that we normally spec. They reason that it will help with the soft spots. Of course the crushed run stone will still be placed and compacted. I do not see any problems with this, but I am young and wanting to understand what the implications of this could be.

Thanks

 

[BUGGAR]

I can't speak for the 3" size of concrete, but I experienced a crushed concrete fill that would not give a reliable reading on a nuclear density meter. Just a note to keep you from going insane if this ever happens to you.

 

[bearjew]

I have heard of this happening with crushed concrete as well BUGGAR. We would be placing 6" of the crushed run stone overtop of the recycled crushed concrete. If they only sink the nuclear gauge to the depth of the crushed stone, they may be able to get a reliable reading. I have used these gauges in the past, but never testing material that is overtop of an 'untestable' material such as the case here.

 

[fattdad]

the subgrade modulus will only affect the design of the slab and is influence by the soil properties in the upper 2 or 3 ft. The presence of soft soils, potentially at 5 or 10 ft (i.e., there's some reason the adjacent building is on piles and it's not because of low subgrade modulus) will require more analyses about settlement - immediate settlement, consolidation and secondary compression. Subgrade modulus does not help with these other aspects of load-deformation response.

I have worked for decades with structural engineers and this topic is a source of confusion. Subgrade modulus does not help a geotechnical engineer evaluate settlement. It's a structural contrivance (a good one) for the benefit of structural design.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Ron]

bearjew....my understanding is that the crushed concrete is being used to further stabilize the subgrade prior to placing the crusher run subbase for the concrete pavement slab. There is nothing wrong with this procedure and it clearly aids in the constructability of the crusher run subbase. Density testing can be done with a sand cone if density is a requirement; however, for such a constructability aid, proofrolling is likely all that's required. You can then run nuclear density tests in the crusher run material just as you've described.

 

[bearjew]

bearjew....my understanding is that the crushed concrete is being used to further stabilize the subgrade prior to placing the crusher run subbase for the concrete pavement slab. There is nothing wrong with this procedure and it clearly aids in the constructability of the crusher run subbase. Density testing can be done with a sand cone if density is a requirement; however, for such a constructability aid, proofrolling is likely all that's required. You can then run nuclear density tests in the crusher run material just as you've described.

Exactly what I needed to hear. Thanks for the advice Ron.

From a learning standpoint, how exactly does the RCC help to stabilize the subgrade? Intuitively it seems like it would consolidate soft spots which could then just be filled in with the crushed stone to bring it back up to grade, but I'm not sure if this is the correct thinking.

 

[BUGGAR]

"You can then run nuclear density tests in the crusher run material just as you've described." Not my experience. But let us know your experience.

 

[ATSE]

...subgrade modulus does not help a geotechnical engineer evaluate settlement. It's a structural contrivance (a good one) for the benefit of structural design.

I hope my structural brethren are listening.