QA/QC testing for soil-cement 3

[fattdad]

Dear Forum:

With limited experience in soil-cement testing I'm wondering about the following:

The Proctor: Do you air dry, mix in the cement, pound the sample and then allow hydration, prior to determining the moisture and density for that sample conditon?

The field density test: Do you apply the cement, mix and then test the density? Or, do you apply the cement, mix, wait a day or so (i.e., for hydration) and then test the density.

I can read the ASTMs and will do exactly that, I'm curious about practical experience and you all's experience (well and to learn a few things too).

Thanks!

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

i wouldn't wait to test...true soil cement is rather hard at 24 hours. pulverize, moisture condition, add cement, moisture, mix, moisture, mix, compact and walk out of it. test say 30-60+ mins after walking out of it.

keep in mind that the density result is not the most important thing with soil cement. who cares that it's 97% instead of 98% if everything goes fine in the field? field observations are much more important. out of everything, the preliminary work (soil-cement design and field evaluation of exisitng conditions) is the most critical step to a good product.

 

[fattdad]

thanks msucog! Any comments on the Proctor preparation? I have read the ASTM, just any other comments?

f-d

¡papá gordo ain’t no madre flaca!

 

[Ron]

f-d...the proctor test is done before the cement hydration takes place.

Density tests should also be take prior to hydration. If you take the density test afterward, say a day after compaction, the tests will sometimes show a lower density. That's a bit erroneous, but it's because the moisture in the S-C gets bound up in hydration, giving erroneous moisture content readings, particularly with a nuke gage.

Field specimens for unconfined compressive strength must be molded prior to hydration.

 

[msucog]

to be completely honest, i don't remember about the proctor. i do remember that the topic is rather convoluted when you get down to the nuts and bolts of it. basically (and rather oddly enough now that i think of it), with soil cement, i focus on everything but density testing other than to make to make someone feel all warm and fuzzy about the end product. basically, if the soil underlying the layer to receive soil-cementing is firm and the appropriate cement applied to the correct depth, mixed properly, moisture conditioned/reconditioned, and compacted until the big sheepsfoot roller walks out of it, then i don't reallly care about the compaction. it should be a performance spec as far as i'm concerned. i do measure cement application rate too. i'm also there for quite a bit of it and have my field guys there for the rest. i've caught problems by standing there which i wouldn't have caught until after the fact had i not been standing there.

i can't stress enough, test gradation, depths of existing conditions and do the design before hand. as you likely know, the gradation is one of the big factors that controls the design. the correlations is pretty amazing if you have enough data sets (you can use gradation to estimate cement required after you've developed the curves and my curves fall within about 2% cement for the soils/materials i touch). use RAP blended whereever possible to cut the cement required. there are other factors as well. optimization of all things considered is the name of the game. too much cement and you'll have problems...too little and you'll have problems. get it in the right range and it'll work like a charm. oh, and i guess i should throw out there that having a qualified/good contractor involved is half the battle.

references

http://arra.org/

http://www.cement.org

http://www.millergroup.ca/

 

[Ron]

At the risk of being a smart a$$, one of the reasons for QA/QC testing is to feel "warm and fuzzy about the end product". While I agree that full time observation of the process by a qualified technician or engineer goes a lot further toward achieving a good product than a few test results, you still have to prove that the intent has been achieved by something other than a divining rod.

The mechanical properties of the soil-cement that are achieved in the field are necessary to validate the design assumptions. For instance, if the desired strength of the soil-cement is 250 psi in 7 days, then you need to show that it is achievable and has been achieved. While the gradation of the parent material helps to define the design cement content, the other parameters of compaction and unconfined compressive strength need to be checked as well. For instance, a low density will generally lead to lower compressive strength, thus a lower resilient modulus, thus a lower fatigue life.

Many premature soil-cement failures take the form of poor durability at the asphalt/soil-cement interface where the lateral strains are high in the asphalt layer. This reduced durability is related to a lower strength of the soil-cement.

To quote the Portland Cement Association..."The purpose of field inspection and control of soil-cement during construction— cement content, moisture content, mixing, compaction, and curing—is to ensure that the results set out in the plans and specifications are obtained and if problems do arise they can be handled immediately."

 

[msucog]

and at the risk of sounding like a smart arse, what difference does it make if the density is 97% vs 98%? what are you going to do, go back and recompact the next day??? and the "warm and fuzzy feeling" comes from my PE stamp that the work is acceptable. if the construction process is fully documented and the end result works, i could really care less about what the density it as long as the compactor walks out of it.

here's a point: in the field, if you perform density testing after the area has been constructed, is that really in the same field as the proctor testing as you describe where the moisture is taken before hydration? this is the convoluted arguement i mention in my previous post. bottom line, if it's moving (ie. unstable) after the area has been constructed, then your neck is on the line and you should be kicked since you probably tried to build it on top of unstable soils (deeper than the 8-12" layer on top) or your design didn't encompass the range of soils likely to be encountered. the warm and fuzzy feeling the owner gets comes when they're laying down asphalt within a few days after the area is initially torn up...nobody cares that the density is a percent low (according the test). let's not even get in to the "variability of testing" argument either. throw in the concept of cement hydration in to the mix and we could argue for days/weeks.

and i just to be clear, i'm not so sure we disagree as much as it seems...i'm more proving my points for those reading the thread that may be about to take that leap of faith in to soil-cement stabilization/design. field inspection and control is crucial...density testing is not so much.

good luck f-d.

 

[Ron]

msucog...you're right...we're not as far apart on these issues as our discussion might imply.

Doing density testing is to set the construction procedure, much in the same manner as one does with asphalt construction. After all, you wouldn't attempt to recompact asphalt the next day either.

Doing initial density testing is a means of establishing a compaction procedure. It can tell you very quickly how many passes of the available compactor will get you the result you want. The soil conditions to which you allude are amenable to compaction with a sheepsfoot roller; whereas in other areas, the soils may be compacted without a sheepsfoot roller so "walking out" of the soil is not an observable parameter. I've worked on soil-cement projects where there wasn't a sheepsfoot roller within a hundred miles of the site (and didn't need to be). Further, you are implying the use of a mix-in-place process for soil-cement, where again, in other areas pugmill mixing is more common.

Checking compaction of soil-cement has to be done quickly while the mixture is still a "soil-like" material. Once you've missed that opportunity, density testing is worthless. I've seen laboratories try to do density testing in soil-cement after the cement has hydrated. They almost always come up with an apparently low density. It isn't necessarily low, but the test procedures applicable to soil density do not apply to cement treated material once hydration has occurred. As I mentioned previously, the moisture content relative to the Proctor is almost impossible to properly discern and is irrelevant at that point anyway.

To your point about using your P.E. as the measure of the validity of the soil-cement quality is reasonable with two exceptions....one is that not all P.E.'s are created equal with respect to experience and capability in field quality assurance/control of soil-cement. I'm sure someone else who is also licensed but nearly as diligent as you appear to be with soil-cement might sign off that it is fine, only to have premature failures with no objective measure of material properties. The second is that in the event of an issue, either during or after construction, again you have no objective measure of the characteristics of the soil-cement that one could use for analytical evaluation.

Yes..I have my wooden dowel that I use for "sounding" the soil-cement. But I also like to have some initial compaction testing to validate the design characteristics of the soil-cement so that I can get a reasonable measure of its structural efficacy.

 

[msucog]

good points. and a PE is nothing more than an overblown piece of paper without professional execution...

broom stick and my trusty pokey stick #2 (aka probe rod). #1 pokey stick was worn down to a speared point and a foot shorter than it started...i like to poke!

 

[Ron]

msucog....sorry, I left out a word in a sentence above...

" but nearly as diligent as you" should read

"but [highlight]not[/highlight] nearly as diligent as you"

 

[cvg]

generally, approval of soil cement is based on compressive strength. density is one indicator of "potential" strength. Cylinders, broken in the lab are the actual test for strength. However, since the cylinders can't even be broken to test for strength until they have cured for some amount of time, real time quality control and assurance is absolutely necessary to avoid any issues later with poor quality. So, a qualified engineer and inspector should be looking at everything on a frequent basis including:

- mix design, especially any modifications to it
- batch plant records, especially any adjustments in the water, cement, fly ash, aggregate quantities
- quality control of the aggregate stockpiles
- of course compaction, but especially sticking to the method approved during the test strip unless adjustments are recommended
- time limits including mixing, placement and compaction times
- proper curing and treatment of cold joints
- temperature and adequate measures during hot or cold or rain conditions
- compressive strength 1-day, 3-day, 7-day running averages

 

[fattdad]

Well, I'm just back from a few college tours with my rising senior (1,500 miles). Thanks so much for the interesting information! I have really learned a lot.

For context this is a pad-ready site for a large distribution center. We provided QA (i.e., not QC) for the contract buyer. (The contract seller provided the QC services.) The pad is well compacted and the native soils are compact - both are silty sand with minor variation. The contractor placed into his work soil cement protection of the building pad to limit construction disturbance during winter work (i.e., foundations, steel and pre-cast wall panels). The contractor has the "design" and we've just been asked to do the QC for the owner (we're the Special Inspector). It's a change of service, so I'm learning what I can in these few days.

Work begins tomorrow or the next day. We'll be doing Proctors and density testing soon. I'm fully confident in our laboratory and field staff. Just want to "see" the data correctly.

You've helped me, so thanks!

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

the pca website has a great manual for soil cememt. measure spread rates...either place several small bug sample bags on ground pinned by flags or lay down a cy tarp. good luck