Concrete around the turn of the 20th century 2

[Robvh1984]

I am analyzing the retaining wall of a water reservoir in Baltimore County, Maryland that was built in the 1920's and I have no other information regarding the concrete other than it is designated on an old drawing as "Class A" for the wall/footing and "Class B" for the Containment bottom slab.

The only concrete standards I could think of at that time were ASTM, and the early workings of ACI. AASHTO uses letters to designate classes of concrete so I thought maybe they were a carryover from those early mix designs.

-Robert Miller, E.I.T.
KPA Structural Engineers

 

[kieran1]

Could you get a core sample and do some testing?

Kieran

 

[Robvh1984]

Kieran,

I wish it was in the budget right now. However, if the wall analysis failed with the most reasonably accurate estimate of f'c, we would request for cores to be completed before the additional loads were imposed on the wall.

A previous analysis was completed by a firm, which used f'c = 2500 psi. I have a sneaky suspicion that the design compression strength was higher, not to mention that the curing over 90 years has probably increased quite significantly.

-Robert Miller, E.I.T.
KPA Structural Engineers

 

[Ron]

Concrete of that era could have been anywhere from 2500 to 5000 psi. Mix control quality was probably a bit better than you might expect; however, the materials quality assessment was not quite as good as you would like. In many cases those were offsetting considerations, resulting in fairly good quality and strength concrete.

While portland cement concrete does gain strength with age, that strength gain will plateau after the first year or so. One thing that makes concretes of that era gain strength for a longer period of time was the coarseness of the cement grinding. Even though that has occurred over time, I would not consider much beyond about 110 percent of expected design strength, simply because of the variability.

Concrete can lose strength with age, mostly due to environmental factors. Consistent wetting and drying is one action that can cause this.

I can't understand why your company has gone into an evaluation of an aged structure without due consideration for evaluation of the materials, including coring, petrographic examination and strength testing. These items should have been in the budget.

 

[Robvh1984]

Ron, I agree. I am not sure why that is the case, but I am just an E.I.T. I cant tell u why that is the case. Any references for classes of concrete?

-Robert Miller, E.I.T.
KPA Structural Engineers

 

[Ron]

Robert,
I have some older references in my library at the office. I'll check on Monday.

Ron

 

[Ron]

Robert,
Checked a few references I have, the oldest back to 1929, but no mention of "Class" of concrete. In my experience, that terminology often comes from state Departments of Transportation and as you noted, AASHTO, though in that time frame, it was AASHO, not AASHTO. Further, the Bureau of Reclamation drove a lot of standards and terminology in that era.

I would suggest two paper search approaches....

1. Check with the Portland Cement Association (PCA) in Skokie, Illinois. Their reference librarian/archivist might have something.

2. Check with the Maryland DOT to see if their early specifications used that terminology. Also, it is possible that Baltimore had its own building code at that time. Check that.

Good luck...these things are often difficult to chase down, but always a learning experience!

Ron

 

[SLTA]

I found a reference to Class 1 and Class 2, from a turn-of-the-century book called "Architects' and Builders' Pocket-Book" by Kidder. Class 1 is "Foundations constructed in situations where the natural soil is sufficiently firm to bear the weight of the intended structure." Class 2 is "Foundations in situations where an artificial bearing-stratum must be formed, in consequence of the softness or looseness of the soil."

I also found the attached PDF of concrete info, from about 1900. There is a reference to class, and it may help with starting design figures.

cheers,
Linnea

 

[Robvh1984]

Thanks! This is helpful.

-Robert Miller, E.I.T.
KPA Structural Engineers

 

[RHTPE]

I recently did some falsework design for an arched bridge rehab in the Boston MA area. The original bridge arches and walls were constructed around the same era. AASHTO recommends the assumption that concrete of that era has a compressive strength of 2,500 PSI. This was inadequate for the anchors I was using, so I requested that the contractor have a testing lab obtain and test a few cores from the walls that the anchors were going into. The test results indicated an average strength of 5,010 PSI with little variation. The point being that the concrete could have a surprisingly good compressive strength that only testing will determine.

Ralph
Structures Consulting
Northeast USA

 

[Robvh1984]

RHTPE,

I agree with that AASHTO recommendation and I know that the concrete has significantly more strength than 2500psi, but we cant core drill so there is not a more precise way to quantify it. I think the thought process (from my superior) is that if the concrete fails in analysis with the best possible assumptions, we can go to gathering core samples. However, if it passes analysis with 2.5ksi, we have a conservative solution.

I just like to have all my I's dotted and T's crossed before I assume anything. You know that old idiom...

-Robert Miller, E.I.T.
KPA Structural Engineers

 

[nuche1973]

In the midwest I've heard some old-timers refer to Class A and Class B in regards of finishes. Class A would be minimal fins and bugholes. Class B was rougher. Not sure that helps.

There are days when I wake up feeling like the dumbest man on the planet, then there are days when I confirm it.

 

[Robvh1984]

nuche1973, I doubt it because these classes were called out by leaders on a cross sectional drawing of a concrete wall, so I doubt that it was referencing a finishing technique.

In the end, I'm leaning towards AASHTO (or AASHO as it was formed in 1914) or the local jurisdiction of the structure. A phone call to the local jurisdiction might have answered it..

-Robert Miller, E.I.T.
KPA Structural Engineers

 

[msquared48]

I googled "Class A concrete" and came up with a lot, one of which is this article:

http://www.hpcbridgeviews.com/i55/Article3.asp

You might check further...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

Mike,
Common sense, practicality and experience are not allowed...stop it!

 

[msquared48]

Proof that I, too, have moments of unbounded brilliance.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[bridgebuster]

2,500 is a reasonable assumption and you have AASHTO to hang your hat on but things aren’t always what we assume. For example, one of my colleagues is working on a bridge rehab – a bascule bridge constructed circa 1908 –he was given some core results taken from the walls of the bascule pit that yielded an f’c of 1,750 psi, considerable less than what he assumed.

In your case, did the firm that performed the previous analysis provide any basis for 2500? On the plus side the reservoir is still standing.

 

[Robvh1984]

bridgebuster:

I am betting that the environmental factors were a bit harsher on that concrete for your colleagues bascule bridge. Unfortunately for our client, the previous firm's analysis was awful, along with their short report describing their findings. One of many things, they assumed deformed bar at 36 ksi, which was not even manufactured until the 30's.

However, from some of my cross referencing and from some help from some people on eng-tips, 2500 psi seemed to fall in line with old AASHO & ASTM standards for dams/retaining walls with moderate exposure.

-Robert Miller, E.I.T.
KPA Structural Engineers

 

[racookpe1978]

When did "we" (you guys who actually design with concrete that is!) actually start getting reliable, consistent concrete in the field.

I understand the Panama Canal locks were one of the first large modern reinforced concrete structures, and they were 1910-1916 time frames. This problem is only a few years after that.

 

[Robvh1984]

racookpe1978: I couldn't speak to that, specifically. Somebody with greater historical knowledge is needed. I would venture to say that by the 30's there was enough statistical data to have generally reliable working stress empirical formulas. IF the field guys cast the concrete such as the specifications spelled out, the variability should have been within the ACI, ASTM, AASHTO, etc. standards of the time.

Also, as we all know, concrete is inherently relatively unpredictable, so until recently (say 30 years or so), admixtures have allowed greater performance and predictable behavior, especially with the greater implementation of post-tensioned nuclear containment buildings in the late 70's/early 80's.

So...my best guess is the time period when post-tensioning of nuclear structures began.

-Robert Miller, E.I.T.
KPA Structural Engineers