Old Concrete Mix Spec 1

[msquared48]

Got a set of old drawings, probably in the very early 1900's, where the concrete is spec'd as a 1:2:4 mix with 7# of Hydro*** (unreadable) per sack of cement. Slump is specific as not to exceed 4". I need to confirm that the ratio is most likely the cement, sand, and aggregate, in that order, plus water (Hydro***) to define the mix.

This seems like a 2000 psi mix to me, conservatively, and I can see nothing to indicate safely assuming more.

Any thoughts on the possibility of 2500 or 3000 for the concrete? This gets into the capacity to sustain a new load pattern for an existing concrete beam.

Mike McCann
MMC Engineering

 

[Jerehmy]

Is there any way to determine a capacity that the concrete had to have based on events it may have seen during its service such as snow loads or wind loads?

 

[SlideRuleEra]

1 part cement: 2 parts sand: 4 parts aggregate
From: Trautwine's "The Civil Engineer's Reference Book" 21st Edition (1937), Page 1255.
This was a general purpose mix often used for floors (girders, beams & slabs). Also for stairways and roofs.

Based on lab tests performed in 1899, your estimate of 2000 psi is a reasonable minimum for concrete mixed in the field using materials from that time. The testing was on cubes, instead of modern cylinder shape, but the 2000 psi assumption makes allowances for that variation. Here is a summary of those test results:

http://chestofbooks.com/architectur...essive-Strength-of-Concrete.html#.Uh0MLEV33Tp

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[jayrod12]

If the concrete is that old wouldn't 2000psi be overly conservative?

Concrete continues to gain strength over it's lifetime (albeit after the first month or so the growth rate drops significantly.

Can you take a core from a non-essential area and get it tested?

 

[msquared48]

Thanks. I have Trautwine too. I'll have to check that out. Thanks for the reminder.

It's not in good condition now, so I keep my copy at home.

Mike McCann
MMC Engineering

 

[Ron]

jayrod...the curve for concrete strength gain flattens tremendously at about a year. After that, even though it can continue to gain strength under the right conditions, concrete in dry interior conditions usually has about 97+ percent of its ultimate strength at the one-year or so mark.

 

[jayrod12]

Ron,

That's why I noted that it drops significantly (I was tempted to say the growth becomes negligible but changed the wording).

And even still, 2000psi? I can mix concrete that strong by hand in a wheelbarrow without measuring anything. If it were me and I needed to know what strength the in-place concrete was I would be hoping to take some cores and test it.

Especially in this case where he needs the number to be higher. 2000psi is really low however I understand in those days it was probably standard.

 

[SlideRuleEra]

Jayrod12, your suggestion to test is reasonable and the results very well could show higher strength - 2500 psi would not surprise me. 3000 psi... maybe.

Your analogy to mixing concrete by hand in a wheelbarrow is very close to how it would have been done c. 1900:
1 shovelful of cement, then
2 shovelfuls of sand, then
4 shovelfuls of aggregate... all into a wheelbarrow.
Repeat this process, add water (amount more or less based on the experience of the person doing the mixing), mix (with the shovel), and dump wheelbarrow into the forms.
Perhaps, tap the forms or rod the concrete inside the forms to provide "vibration"... maybe not.

Of course the materials at that time would be much more variable than today. Possibly local sand and aggregate (whether good or bad - by today's standards). Cement shipped in by rail, but what quality?

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[msquared48]

Coring and testing is not really an option here, or necessary, in my opinion. I'll try and see what the 2K figure gives me and move on here. Thanks all.

Mike McCann
MMC Engineering

 

[msquared48]

Well, when I ran the numbers, the beams are no where near what they were rated for (125 psf live load on 8" slab), by a factor of 4, so either I missing something here, or something is just not correct. A higher concrete strength alone of 4 ksi does not make up for the deficiency.

A7 steel assumed with two 1" bent and two 1" straight bars for the positive steel. One end pinned and one end fixed, so wl^2/8 still applies for positive steel.

Looks like I am just going to have to over frame as close to the existing columns and shear heads as I can.

Mike McCann
MMC Engineering

 

[msquared48]

Quite frankly, based on having done this many time, I would not expect that kind of a deficiency from a 1922 drawing.

Mike McCann
MMC Engineering

 

[BigH]

SRE - how do you get the 2000 psi making the allowance for cubes/cylinders? I took a quick glance and the article didn't state that the test results were converted for cylinders. Normally, we have used 80% - so that 2000 psi cube strength would be only 1600 psi cylinder strength. This is consistent with numbers by Shetty.

 

[SlideRuleEra]

BigH - I gave the link as a quick summary of the 1899 tests and you are right it does not make a comparison of tests on cubes vs. cylinders. A more detailed account of the 1899 tests (I give a link to that in a minute) does confirm that the tests were performed on 12" cubes and that the cubes were moist cured (more or less by burying them in wet ground until tests were performed).

Note that the average test results, in 1899, for 1:2:4 concrete are:

2399 PSI @ One Month
2896 PSI @ Three Months
3826 PSI @ Six Months

Based on these values I am confident that 12" CUBES with noted compressive strengths would have tested at a 2000 PSI, or better, IF the same samples had been formed and tested as modern 6" x 12" cylinders with normal laboratory curing. A faulty assumption on my part? Maybe, you be the judge.

For a more detailed account of the 1899 tests see the 1912 book, "A Treatise on Concrete, Plain and Reinforced" by Taylor & Thompson; see pages 365 - 367. Here is the link:

http://books.google.com/books?id=lA...own Arsenal for Mr. George A. Kimball&f=false

Unrelated to your question I do want to point out that the original poster was later able to pin down the date of the drawings to 1922 instead of the earlier assumption of "the very early 1900's", which I assumed to be c. 1900.
Concrete from 1922 is likely to be MUCH better than 1900 concrete, even for the same mix (1:2:4 in this case). Much improvement in understanding of water / cement ratio. Maybe delivery of better material by truck. Possibly the use of a mechanized concrete mixer, instead of wheel barrows, hand mixing, etc.
IMHO, much more likely that 1:2:4 concrete (made in 1922) would test at 2500 PSI.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net