DGA as SubGrade Material 1

[asiem]

I am structural engineer working on a new supermarket project. The client is particularly concerned about curling of the concrete slab on grade. Originally, we had specified a 3" sand layer below the 5" fiber-reinforced slab. Now, the client wants to change to a 4" slab on DGA material. Also, after discussion with the geotechnical engineer, the client and the design team have decided to eliminate the vapor barrier. I am not familiar with DGA, so my questions are: What is it? Are there any drawbacks? Is it appropriate for this application? Any helpful information would be greatly appreciated.

As an alternative, they are thinking of placing the slab on a stone subgrade. Are there any advantages to using DGA vs. stone?

 

[Focht3]

By DGA, do you mean, "decomposed granite aggregate"?

Where is your site?

I wouldn't eliminate the vapor barrier, particularly if this site is in southern California...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[asiem]

DGA stands for Dense Graded Aggregate. My understanding is that this material consists of range of particles ranging from about 1" down to dust-sized. That's really all I know about DGA.

The site is in Central New Jersey.

As far as the elimination of the vapor barrier goes, the owner, the contractor, and the geotech discussed this at length before deciding to do away with it.

 

[cvg]

what you describe sounds like road base material and may be a good material for placing under a structure. Will the larger stones puncture the plastic?

 

[LHA]

Here in PA, DOT classification has a stone it calls "2A", which fits your description (I know in NJ you can use PennDOT agg. spec, and contractors and engineers will know what you mean), but can range up to 2". As cvg says, this is the preferred roadway subbase, but it will puncture plastic.

I would bet DGA is NJDOT's equivalent of PennDOT 2A...ask around.

 

[SirAl]

A well graded gravel base product is structurally superior to a sand base of equal thickness without question. Whether a switch from 3" of sand to 3" of crush gravel (if that is what is proposed) can justify a 1" reduction in the thickness of your concrete slab thickness remains a question mark. What are the subgrade conditions (i.e. soil conditions below the granular base)? The use of stone/tile rock will offer immediate drainage. We will also use a tile rock in areas of wet or weak subgrade where excavation and drying or replacement are not practical.

 

[Focht3]

Sorry for the delay in this post. I've been quite busy -

The decision has been made. So what are the possible consequences?
[ol][li]Moisture migration through the slab.[/li][li]Effluorescence forming along and adjacent to cracks in the slab.[/li][li]Accelerated rebar corrosion.[/li][li]Debonding of glues holding down sheet vinyl or VCT.[/li][li]Debonding of tile grout.[/li][li]Rotting carpet backs.[/li][li]Mold and mildew.[/li][li]Odors from mold and mildew.[/li][/ol]



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[jheidt2543]

I would NOT eliminate the vapor barrier. There has been a LOT of debate over the last 10-15 years about vapor barriers and if they are one of the causes curling or not and I believe the having a vapor barrier has won out in both the PCA and ACI.

I believe a vapor barrier is essential in the reduction of moisture transmission from the subgrade through the concrete slab, particularly if any finishes are to be installed on the slab.

I would leave the vapor barrier in place, pour the slab as stiff as possible, make sure to thoughly wet cure the slab for at least seven (7) days and then use a high solids cure-n-seal agent that will be compatible with the floor finishes. Call the local reps from W.R. Grace, Sonoborn-Chemrex, and Master Builders and discuss it with them. Do a test pour of about 20'x20' in a maintenence area of the building utilizing the recommended finishing, curing and cure-n-seal products to verify the methods.

 

[Laser28]

Are you using a true vapor barrier or is this just 6 mil poly? Poly reduces vapor transmission about the extent a well-cured 3000 psi concrete mix will. In a situation such as this, where there will be floor coverings, materials stored in contact with the floor, a proper asphalt impregnated water barrier should be spec'd.

As well, a quick check indicates that a reduction in thickness of 1" is not justified by the change in base material.

Also, dense graded aggregate is just that, dense graded, and does not prevent water from coming in contact with the u/s of the slab. In fact, the coefficient of drainage is lower than sand, which would provide relatively the same structural support.

Leave it at 5", cure well, use any base you want, but use a proper water barrier. Reduce the chance of curling by reducing joint spacing. Yes - I read the part of the post that said the floor was fiber-reinforced. You will still get cracking. Trust me...