Historic Concrete Strength 1

[cgstrucg]

Hello,

I have a project in which I want to check the strength of existing slab and reinforce if it is failing with new loads. Project is a concrete slab in NYC, North America. As that existing slab was made in 1930's I have no way of knowing what was it's strength back then. Does anyone have any idea regarding this? I know the range is between 2-4 ksi but what was the most commonly used strength and what type of concrete was used in NYC region in 1930's.

Thanks.

 

[PEinc]

Core a hole and send the core out for compression testing. However, the compressive strength of the reinforced concrete may not be as critical as the amount, size, and spacing of the reinforcing steel. Check your design for its sensitivity to compressive strength. The steel is doing most (all?) of the work in resisting tension. Compression may not be your problem.

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[CANPRO]

I agree testing is the way to go here. But in testing, consider the sample size...I don't think they had the same QA/QC process for concrete in the 1930's so the results could vary significantly throughout the building.

 

[cgstrucg]

Issue is we can't send it for testing at this phase of the project. I am looking for some literature from UBC or any other manufacturer which states some range of concrete used during that period and type of concrete also. Thanks

 

[PaulG93]

Could you use a Schmidt Hammer? They are extremely easy to use and will give you in situ strength within 500 psi, if properly calibrated. 500 psi may seem like a lot, but a 100% increase in f'c will only increase moment capacity by 10% or so. In about half an hour you could get the strength at 10 or so locations along the slab.

 

[dik]

and send out cores for petrographic analysis. Schmidt hammer not too reliable. Strength could be as low as 2000 psi or as high as 6000 psi... no wild assed guess.

Dik

 

[WARose]

From that era, I am use to seeing about 2-4 ksi. (Typically 3.) I use to retrofit a lot of process buildings from the 1930's, and the drawings called for 3 ksi.

But I agree with the others: a 28 day strength from 80+ years ago is meaningless. Some testing at some point (for it and the rebar) is in order.....I know you say that is down the road.....but I'd be careful how much strength I assume. This issue may be a good item to cover in the scope/contract documents (under "assumptions").

 

[cgstrucg]

Yes WARose and thanks a lot everyone for the response. I also saw a lot of 3ksi concrete in other projects dwg from that period and I am going to mention it that we assumed 3ksi in design criteria. I was just thinking if anyone knows of any journal/book where I can get it confirmed.

 

[dik]

WARose:

and occasionally a high one... the McGregor Armouries in Winnipeg had structured slab strengths of nearly 6ksi... and, it was constructed about 1914...

Dik

 

[JAE]

It is hard to actually get below 3,000 psi for most concrete mixes over the years.
So the original concrete, assuming it is in good condition now, is most likely at least 3,000 psi.

Agree 100% with PEinc - f'c doesn't matter for flexure. But it does significantly affect shear strength of concrete beams and joists.
And these, typically, are the problem children of most old structures asked to take additional load.
Shear design, and the prevalence of shear reinforcement, was somewhat rare back in the day.



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[bridgebuster]

A core won't lie, anyway here's an excerpt from the 1922 NYC Building Code (2,000 psi minimum):

If you have a drawing with the mix design the attached may be of help. There's a table that compares mix designs vs. strength.

 

[JLNJ]

I think it unlikely that you will find anything which gives you absolute certainty.

It sticks in my mind that columns were often specified as 1:1:2 and beams and joists as 1:2:4. I used 3000 psi for the beams and 4000 psi for the columns.

You might find some clues here: Books By George Hool.

Page 318 of the first reference has New York Building Code requirements. The Chicago Building Code starting on page 321 actually has strength references for different mixes (2000 psi for 1:2:4 and 2900 psi for 1:1:2 mix).

(I used to have this book and it walked off. Grrrr.)

 

[Tmoose]

New York in the 1930s. I wonder if the dealings and doings of Tammany Hall and the like might have influenced the quality of the concrete delivered to job sights in any way. Certainly they influenced the price.

 

[RickyTickyTavi]

There is an ACI 318 predecessor code for 1927, Serial Designation A-1A-27T, Page 650 has indications of F'c as low as 1500 psi. ACI501-36T, is the equivalent ACI318-36. Has the lowest F'c at 2000 psi. By 1941 the code was officially ACI318-41.

That being said, I generally agree with WARose, I normally see 2-3.5ksi. Dont think I've ever seen 1500 psi specified for a structural concrete.

For yield strength, this historical CSRI rebar tables are useful if you cant test.

 

[dik]

BB: Handy booklet to have...

Dik

 

[sandman21]

I have had a building with 1800 psi built around your time-frame, only way to know is to take cores.

 

[JAE]

All those 1500 - 2000 psi strengths, remember, are way back in time. Concrete does continue to increase strength over time indefinitely....albeit in much smaller rates as time goes by.



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[bridgebuster]

dik, I found it a few years ago on a website that has 1000's of historic catalogs.

Link

 

[structSU10]

I had a building that had cores taken recently, same vintage, and we got breaks in the 1500-1900 psi range in all cores.

 

[sandman21]

My 1800 psi number comes from core breaks as well