sand with bags of Portland cement vs 12" of cement stabilized sand

[oengineer]

A contractor is asking to place sand with bags of Portland cement in lieu of 12" cement stabilized sand under the base of a wet well slab.

Does sand with bags of Portland cement offer better (or equal) bedding support underneath a foundation vs using 12" of cement stabilized sand?

Comments/suggestions are appreciated.

 

[phamENG]

I would assume not. It's like filling a fence hole with a bag of dry quickcrete instead of mixing it properly and placing the concrete. The end product is woefully inferior. I haven't spec'd cement stabilized sand before, but I understand there is a science to it and there should be a mix design with required amounts of water mixed in. Usually needs some minimum compression strength - 100psi or so?

 

[oengineer]

I would assume not. It's like filling a fence hole with a bag of dry quickcrete instead of mixing it properly and placing the concrete. The end product is woefully inferior. I haven't spec'd cement stabilized sand before, but I understand there is a science to it and there should be a mix design with required amounts of water mixed in. Usually needs some minimum compression strength - 100psi or so?

Thank you for your response.

I personally am not familiar with the use of "sand with bags of Portland cement" underneath a foundation.

 

[phamENG]

Sounds like a contractor trying to hack it and pinch a few pennies. I'd tell him no.

 

[AskTooMuch]

Cement stabilized sand isnt 100 psi, that's flowable fill. Cement stabilized sand is much harder. If I remember right it's how many bags/yd is the spec for it when you buy from supplier, not psi. We don't use it under foundation because it requires compaction which is additonal work unlike flowable fill. We use cement stab sand on ditch slopes to stabilize it obv flow fill cant be used as it will just flow to the ditch.

Go call suppliers and ask them about cement stabilized sand and ask how they mix it and you can have a feel if this can be done in the field by construction guys.

Why not use flow fill, it wont require compaction.

 

[Tomfh]

If it’s placed dry, no, it’s not as good as a wet mix of sand and cement.

 

[phamENG]

AskTooMuch - I agree that 100psi is a common strength spec for flowable fill, but that doesn't mean it isn't viable for cement stabilized sand. Here's a spec from a municipality in Texas. 100psi is stated in section 1.04...

 

[AskTooMuch]

You don't order from a soil/sand supplier and tell them you want 100 psi cement stabilized sand. They wouldn't know what it means. You tell them how many bags of cement per cu yd of sand. We just buy from sand/soil suppliers.
We buy flowable fill from concrete batch plant that has people capable of computing psi like in concrete. Just my experience though, maybe different for others.

 

[Ron]

Cement stabilized sand is a form of soil-cement. It will typically have a compressive strength of 150-300 psi. Haphazardly mixing portland cement with sand without an appropriate design mix will yield inconsistent results. (as phamENG noted).

Soil cement mixtures are also susceptible to a variety of issues that affect strength. If the soil is organic, it can affect the setting. If the soil is of a certain variety, it can absorb calcium from the cement and reduce the strength. Both issues can be accommodated by proper testing and mix designs.

Flowable fill is a different material altogether. It is delivered like ready-mix concrete and has a compressive strength of between 500-1000 psi.

Much of this will depend on where the water table is. A thick layer of graded aggregate (No. 57 stone or similar) might be a better option to consider.