Lightweight Aggregate, LWA, Typical CBR Values? 3

[smp123069]

Where can I find typical CBR values for Lightweight aggregate? We would like to use a LWA for a road subbase but can not find documentation of the CBR values for any of the products that we have evaluated.

 

[Ron]

LWA is generally too expensive to use as roadway fill or base material. Consequently, you are not likely to find a good source for such data, except if one of the suppliers of the aggregate has some data.

If you are trying to cut down the overburden weight on some soft underlying soils, it would be better to do that in the subgrade, not the base. That can be done in a variety of ways, including using cellular concrete and polystyrene in extreme cases.

Cellular concrete alone is a good choice for a relatively high modulus material that will significantly reduce the weight of overburden (typical soil unit weight - about 100 pcf...typical cellular concrete unit weight - about 40 psf); keeping in mind that it isn't cheap. But then, if you have adverse subsurface soil conditions, a successful solution of any sort won't be particularly cheap.

 

[SlideRuleEra]

One place to look for information is the "Expanded Shale, Clay and Slate Institute" at this link

http://www.escsi.org/

Try the "Geotechnical" and the "Literature & Technical Documents" areas of the website.

http://www.SlideRuleEra.net

 

[smp123069]

Thank-you for your responses. I have been to escsi.org and have found that resource to be helpful. I am waiting to hear back from them on one of the publications that they don't have posted on their web site.

Ron, I didn't give you the full story. The road that we are providing a section for his on top of a parking structure. We are using the LWA for the subbase of variable thickness and then using base and surface asphalt on top of that. We are attempting to reduce the load on the garage slab that has already been built without taking into account the deadload of the pavement section above it.


SMP

 

[Ron]

SMP..thanks for the clarification. Be careful with that pavement section you described. I am assuming since you need a subbase, base, and surface layer that you have significant traffic. Considering that, your layer stresses will concentrate (because of your high modulus materials from the parking deck upward) and you will likely see elevated lateral stress levels at the bottom of the asphalt layer under traffic loading.

You might want to consider a hybrid of the cellular concrete for this application. You could add some lightweight aggregate to the cellular concrete and still achieve a low unit weight and relatively high strength. You could then cover that with a relatively thin layer of polymer concrete, effectively cutting out your subbase layer.

 

[BigH]

Ron is right in that the use of a material like Elastizel is probably the way to go - it is used to for insulation purposes. It weighs in at about 1/5th that of normal concrete. If you have the roof of a parking garage, I am not really sure why you want to go to subbase, base, and asphalt? Seems to me that if you have a grade to raise, this will be adding quite a dead load to the roof. I see the use of Elastizel as likely the only viable solution to raising grade on an existing roof and minimizing the load - you can place a thin concrete slab on top and provide a chip seal - as the Elastizel is not likely durable enough to take traffic loadings on itself. But, you might like to contact Elastizell directly (two "l"s).

 

[smp123069]

Thank-you Ron and BigH. The reason that we have a subbase, Base and Surface asphalt is that the County requires this pavement section. We are required to have 2" surface asphalt and 5" of base asphalt. The asphalt is required by Code to be on a subbase having a CBR of 30 or higher. We had intended to use 21A aggregate for the subbase on top of the garage slab but the loading on the garage slab has become an issue.

I'm sorry but I don't have a very good understanding of lateral stresses at the bottom of the asphalt layer. It is our plan to use the Stalite product under the asphalt layers to serve as our subbase with a CBR of 30 or higher. The Stalite will be a variable depth to a maximum of about 18". I'm hoping that this section will be sufficient. I have a phone call into Stalite to confirm this. I'll mention the lateral stresses to their engineer to make sure that it will not be an issue for us.

I reviewed the Elastizell product and I can see what the advantages would be. What is the typical thickness for the concrete carrying slab? I think that since we don't need much fill (18") that we may be okay with the Stalite alone.

Thank-you both for your responses. They have been very helpful!
SMP

 

[TDAA]

I would smack the county around. That is typical min. for a road on soils (depending on the area). You are on a structure, and one you can drive on!

 

[smp123069]

We have evaluted the LWA and agree that we can not pave on it due to it's uniformly gradation. The engineer at Stalite recommend 4" of open graded aggregate (the VDOT 21A stone) on top of the Stalite so that we can pave on this. We are still having a problem with this section (with the 4" of 21A Stone) being too heavy for the garage slab underneath that has already been built.

The County may allow us to reduce the asphalt layers of 2" surface and 5" base course, but it would require a lengthy review process that we are hoping to avoid.

One idea that we are tossing around is laying some type of geomat on the LWA to contain it so that we may pave on it. This would eliminate the additional 4" of stone that is only being put in place to contain the LWA. Does this sound like a solution that would address the issue of elevated lateral stresses? Does anyone have a product in mind so that I can pursue this solution through one of their technical representatives?