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98 or 95 percent Mod. Proctor density under slab?

98 or 95 percent Mod. Proctor density under slab?

98 or 95 percent Mod. Proctor density under slab?

Our clients, an architect and a structural engineer, are complaining about our recommendation for a 98 percent Modified Proctor dry density in the last foot underneath the floor slab-on-grade for an addition to a high school.  It is a 150 psf slab, with no extraordinary criteria.  The soils are loose to firm sands to 20 feet.  The 98 percent requirement is mostly to get contractors to do their work right.  It appears to be fairly easy to get 98 percent if the subgrade is properly compacted.  The client's complaint is that 98 percent instead of 95 percent will cost significantly more money.  I think this is baloney.  In loose to firm sands, 98 percent compaction within a foot of the surface in an area easily accessible to a medium to large compactor, which they'll need anyway, is easily achievable with little to no extra effort.  There doesn't seem to be much difference between 95 and 98, but the contractors I have dealt with put a little extra effort into making the slab subgrade evenly and properly compacted if they think they need 98 percent.

Is a 98 percent Modified Proctor dry density specification under slab reasonable, or am I being overconservative?

RE: 98 or 95 percent Mod. Proctor density under slab?

Are you compacting an engineered fill such as well graded crushed stone? . . . or are you compacting the natural soil only?  What is the gradation of the sand - is it a medium to fine sand?  a fine sand? or a well graded sand with coarse sizes too?

RE: 98 or 95 percent Mod. Proctor density under slab?

Compaction is of natural soil.  Natural soil is fine sand.  I didn't run a gradation, but sample observation indicates about 99% between #10 and #200, poorly graded.  Nothing in 12 small samples larger than #10 sieve.  This kind of natural soil is what we would use for engineered fill (not crushed stone) for most purposes in this area.

RE: 98 or 95 percent Mod. Proctor density under slab?


As far as I can remember, I've never specified 98 percent modified.  That said for material such as what you have described, I would not think it that difficult to obtain.  Do I think you are being over conservative, probably, but I don't know the type of contractors or clients that you typically deal with.

The only two problems that I have seen from specifying too high a density is crushing of the material and increased cost.

RE: 98 or 95 percent Mod. Proctor density under slab?

jgailla....98 percent under slabs exceeds the typical requirement for slabs on grade.  95 percent is common.  Compactive effort is not linear.  It is fairly easy to get from 90 to 95 percent in clean sands, but 95 to 98 percent actually does require more effort and more attention to optimizing conditions.

Look at your Proctor Curve.  Plot the moisture ranges in which 95 percent compaction should be achievable then compare the moisture ranges in which 98 percent compaction is achievable.  You will see from this the need to have more optimized conditions to achieve the 98 percent compaction.

Another issue is that this opens your client up to more costs and claims of delay if the compaction is difficult to achieve.  If the contractor bids assuming 95 percent (not an unreasonable assumption, even if he did assume) and the requirement then becomes 98 percent, he will have at least some validity in his claim for more effort, more equipment, or delays or all of the above.

Stick to requirements that others in your practice area would deem reasonable and 95 percent of the Modified Proctor under building slabs is reasonable.  Save the 98 percent for the pavement.

RE: 98 or 95 percent Mod. Proctor density under slab?

We don't normally even use the Modified in the midwest here - usually the standard, which opens up a whole 'nuther can of worms.  But the 98% modified - for a building slab - wow -

The kind of loading in a high school slab is just not all that intense...the 150 psf not-withstanding.  For a slab taking heavy forklift wheel loads with an exposed concrete surface, well then, maybe a consideration for a high level of compaction is warranted.  But for a school - I would go with 95% Modified as a max.

But keep the contractor to that specified - as you imply, you need to hold them tight to the spec to get the quality you want.  Perhaps you should focus on the testing frequency and accountability rather than just upping the compaction level.

RE: 98 or 95 percent Mod. Proctor density under slab?

    There are a few problems I see.  First, the effect of such heavy compaction is really dependent on the kind of compactor you use.  Are you wanting to densify just the upper zone so you can place the concrete?  Or are you trying to reach looser zones at a greater depth?  Getting 98% (or 95%) MDD in the upper 300mm with looser zones below - wondering why - I'd probably just proofroll it to a uniform density and leave it at that.  
    The next problem and the most troublesome, as I see it, is how are you going to protect the fine sand when you place the concrete?  It will dry out after compaction and displace with every footstep!  On our project they insist on using 150mm of sand on top of clay soil with a 150mm lean concrete for a a mudmat.  And, when they are pouring the concrete, the sand is moving everywhere and they basically get a mix of the sand and the lean concrete.  To place steel without any mudmat - troublesome.  I would suggest that you consider a layer of crushed stone on the sand, say 100mm thick or so, placed in such a fashion as to prevent the sand from being disturbed.  Then you can used the crushed stone for placing the steel.  Given this scenario, what difference would 95 or 98% mean?  I think that JAE has it right - go with 95% MDD modified and this would be enough.

RE: 98 or 95 percent Mod. Proctor density under slab?

For sands in south florida, 95% density for the modified proctor will get you about a qc value of 35 for a cone pentrometer and at least an N=8 to 9 value for spt test. The modified proctor is standard in sands here. The compaction of 95% is good enough for a slab.

A big consideration is that you mention that it is an addition. That means the compaction equipment must be performed with the vibratory mode off in order not to damage the adjacent structure to the addition. The lift thickness must be reduced in this case, or it will take them a longer (how much?) time rolling the 1 foot thickness to get the required density. It does take longer in the case of getting 98%, thats for sure, and if small plate compactors are used, it takes sometimes much longer.

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