Waer Content

[BigH]

What is the definition of water content, if:

a. You are an agronomist?
b. You are a geotechnical/civil engineer?
c. You are a roller compacted concrete mix design specialist?

 

[BigH]

No takers, eh? . . .

 

[GeoPaveTraffic]

I seem to remember that agronomist's use the volume of water divided by the volume of solids, but I may be remembering incorectly.

For b, it is the weight of water divided by the weight of the solids.

Lastly, I would expect that for c, it would be the weight of water divided by the weight of cement in the mix, i.e. the same way that the water/cement ratio is calculated for concrete.

 

[boffintech]

For B, if you have 100 lbs of soil at 25% moisture the water content is 20 lbs. When the water content is expressed as a percentage, it is (Mw/Ms)x100.

Mw = mass water
Ms = mass solids

 

[cvg]

for c you defined the "water/cement ratio". However, the water "content" is the weight of water in lbs per cy of concrete.

 

[jdonville]

I get the odd request now and again from environmental types on remediation jobs to perform volumetric water contents, in-situ void ratio determinations, etc. It's funny how almost all of them have no clue about the importance of obtaining a relatively undisturbed sample in order to make these determinations accurately...

Just an observation FWIW.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

 

[Grouser]

As I recall:
A. is the weight of the water [÷] the total sample weight, max value = 1;

B. is as above mass of water [÷] mass of soil, max value = [∞]

C. I don't know but cvg's answer seems right to me.

Cheers

 

[bmike]

Having recently been involved with discussions with one of the pioneers of TDR soil water content measurement, who is a soil physicist semi-retired from Agriculture Canada, I can pretty much guarantee that agronomists use volumetric water content, and calculate it from volume of water divided by bulk volume.

In response to jdonville's post, I was also amazed to learn that when the volumetric water content is determined with electromagnetic methods (TDR,GPR, and FDR), and the equipment is calibrated to the particle minerals present in the soil or aggregate, the result is independent of the bulk density or void ratio, and it is unnecessary to have undisturbed samples, etc. It is only when the result has to be referenced to gravimetric values that the problems jdonville points out arise.

This kind of begs an important question or reflection, why do we (geotechnical and civil engineers) think in terms of gravimetric water content, on a dry basis? Is this simply a result of the ease with which this can be determined? Since the mechanical and hydraulic behaviour are more closely related to the degree of saturation than the gravimetric water content maybe we should be looking outside of our box?

The geotechnical/civil convention has been accurately described by others, although I am finding more younger geotechnical engineers with backgrounds in unsaturated soil mechanics or evironmental areas thinking in terms of degree of saturation or volumetric water content.

I have no idea what the roller compacted concrete people use.

As an additional point of confusion, combustion engineers consider the water content of a fuel (e.g., hog fuels) on a gravimetric basis, based on wet total weight.

Talk about a need for some inter-silo translation.

 

[oldestguy]

BigH you should list item d.

d. Water content for foundry sand.

I recall using the Speedy Moisture meter (from the foundry industry) and having to change reading to match our usual terms. I believe it was: water divided by total weight x 100.

Raise your hand if you used that thing. It was handy.

 

[BigH]

The reason I asked this was to see if any RCC types would respond. Of course, for civil engineers, it is on the dry basis - i.e., wt water divided by wt dry soil. Agronomists use the wt water divided by the wet wet of soil. I have seen RCC specialists do it both ways and it makes life very difficult when sizing up RCC articles that talk of the water content of the RCC - as they seldom, if ever, really identify which they have used.
I prefer using dry weight - that is what I have always done. Focht3 several years ago discussed that Proctor himself used wet weight - not dry weight and was throughly PO'd that it was conventionally changed (true or not ???) For some formations (e.g., roadway pavements), it wouldn't make much difference as the optimums are in the order of only 5%; but for sandier/siltier layers, it obviously would.
The speedy moisture gives you the "%" moisture - but in actuality you need to calibrate your particular device with lab dried samples - much the way that I have calibrated my nuke's water contents in the past (I found out that the actual moisture content of oven dried samples compared to our nukes (1980s - early) was 70% or so.

 

[oldestguy]

BigH

Right on for Speedy calibration.

Here is a question. It has been some time since I have seen the ASTM specs for nukes. As I recall ASTM recommends job calibration.

However, I wonder how many people ever calibrate their nukes for the on-site soil.

I did a grad level reserch project in 1954-6 using some of the first experimental nuclear moisture meters. Can you imagine taking a lab scaler out in the field and using a generator for AC?. It was physics dept. development.

This was a "down the hole" type, with neutrons as the "stuff" doing the sensing.

Anyhow, even then it was evident that it had to be re-calibrated for different sites, not calibrated against some lab standard alone.

I just don't trust them without direct on the job calibration for both moisture and density..

 

[BigH]

I started on the nukes when I had to use charts to determine the "weight of water" and "weight of wet soil" instead of the black-box calculations today. Most contractors would accept the nuke without question if it was good. As soon as a bad result came, they would 'demand' in their polite way the use of the Washington Densometer, sand cone or rubber balloon.

 

[oldestguy]

BigH

Yup, a common problem. Nuke shows specs not met.
In my past experience, usually it was another engineer in town that "rejected" the job (his favorite word). Of course no calibration of the device. Also, maybe only one Proctor to boot. Contractor was stopped or delayed, so he turns for help.
Instead, we looked at the job and our crude preliminary jabs with hand probe showed it might be OK and then sand cone proved it OK. It might also take a few more Proctors. This policy of not wasting the contractor's money and keeping the job moving came about due to us looking at our basic policies. The main item there was we provide a service, not a head ache. Reputations like that, in time, kept the jobs coming in. Ya can't be too nice to the contractor, but be reasonable and you get more done.