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Existing concrete compressive strength - Core samples

Existing concrete compressive strength - Core samples

Existing concrete compressive strength - Core samples

Hi All,

I have analyzed and existing concrete pan joist roof structure (1958)for snow drift loads that the minimum compressive strength of the concrete specified on the drawings is 3,000 psi. The joists are adequate in flexure, but are about 15% overstressed in shear. I have already taken advantage of the 1.1 increase in shear capacity per ACI section 9.8. The joists are adequate in shear utilizing 4,000 psi compressive strength. The only reinforcement scheme I think is feasible is carbon fiber reinforcement.

I am considering proposing to the owner and architect the option of taking some cores for testing to determine if the concrete has 4,000 psi compressive strength and possibly eliminate the need for any reinforcement. It is a hospital, so it may not be very easy to obtain the cores, and it is difficult to find a location where it can be done. The roof is being replaced so it could be coordinated to do the cores from the top side of the roof, but I am not sure how I feel about coring through a beam, but it can be done. The roof deck between the joist is 2" thick and will not work for testing per ASTM C42. The minimum diameter is 3.70 in., so the desired length or depth of the core is 4" to 8", with reduction factors for a core l/d less than 2. I am looking into taking 2-3 cores and also some form of non-destructive testing that can use the cores to correlate the results as added verification.

What I am struggling with is if I can take the average of the core compressive strength tests or if those will have to be reduced based on the limited amount of test similar to cast cylinder testing for new construction. I think there is a good probability that the existing concrete fc is 4,000 psi+, but if it needs to test at 5,200 psi to say it is good for 4,000 psi that may be a stretch and not worth exploring.

I would appreciate any recommendations based on a similar investigation. Thanks!

RE: Existing concrete compressive strength - Core samples


Depending on the test results, you may have to do some increased sampling to get your required strength to pan out:

RE: Existing concrete compressive strength - Core samples

Unless shear is an issue... for small amounts of reinforcing, concrete strength is of little use... if the construction was done a bit back, most deflection is already in place. You have to be prepared to accept whatever results you get from testing... it could be ugly...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates


RE: Existing concrete compressive strength - Core samples

You can't use the average strength in the shear strength formula, you would have to use the lower characteristic (maybe 'specified' is the ACI term) strength like Ingenuity said.

The shear strength formulas, safety factors etc. all are calibrated based on the use of the lower characteristic strength and not the average.

For concrete of that age, you would have to expect a fairly large coefficient of variation for the strength of the cores, around 15-20%. From similar experience of coring 70 year old concrete, we have previously found COV ~ 17-18%.

According to my rough calculations, if you need 4000 psi characteristic strength for shear, you would need your cores to show an average of about 6000 psi (after applying correction factors) (assuming 3 cores). With 10 cores, that drops to about 5600 psi.

RE: Existing concrete compressive strength - Core samples

Thank you all for your input. Shear strength is the concern, as the joist are adequate in flexure. Unfortunately it is only ONE location where it is deficient in shear (by 15%). To me it does not make sense to have one location strengthened with carbon fiber due to the cost of mobilization, ect. for such a small amount of work. I am not quite sure how you would reinforce the ends of the joist otherwise. The shear is an issue at the first interior span along a 36 in wide beam x 12 in deep. The pan joist are 10-1/2" deep.

RE: Existing concrete compressive strength - Core samples

I would not do cores. I would contact a carbon fiber manufacturer and get their input on how to reinforce the members for shear, using carbon fiber. The likely hood of you getting the concrete strength required is not good. Now the client has spent money for cores and you are still back to carbon fiber. Skip the cores and do carbon fiber.

RE: Existing concrete compressive strength - Core samples

How is reroofing changing the snowdrift loads? Or are you adding some equipment or something to it where there is a greater potential for snowdrifts? Make sure you aren't applying modern snow loads to a historic structure unless you are required to.

Regardless of the reason for the overstress, I would personally not core based on the information provided. If it really is only one location on a joist that would need to be strengthened for shear, that's like a $10,000 repair all in for a specialty concrete repair / FRP repair contractor. When wrapped into a reroofing project, it's not gonna move the needle a whole lot and won't be very invasive. You will be in a similar ballpark for the coring option. You will need to retain a coring subcontractor, some NDE subcontractor to locate and avoid reinforcing, send the cores to a testing lab, and then reduce the data with ACI 562 or equivalent. All of that will end up being a decent amount of money, maybe $10,000 - $20,000. Not even mentioning the headaches of coring at a hospital, that will be a huge PITA. Even with all of that work, you could end up just needing to FRP this spot anyway.

I would personally discuss this with the owner and the architect to get there buy in. If they feel strongly about coring, fine, but at least you can then say you warned them about the cost, logistics, and possibility the number still won't work out.

Delegating FRP design is pretty easy if you don't feel comfortable detailing everything. The big FRP manufacturer's will provide support to sell their products.

RE: Existing concrete compressive strength - Core samples

Quote (StructEngBrah)

How is reroofing changing the snowdrift loads? Or are you adding some equipment or something to it where there is a greater potential for snowdrifts? Make sure you aren't applying modern snow loads to a historic structure unless you are required to.

I don't think the re-roofing itself is changing the snow drift load, but obviously there is some new work that's either causing the snow drift or adding some load combined with the current drift (OP can clarify).

Also, I don't recall many instances where you are allowed to ignore current design loads and/ or design procedures just because it's an existing structure. If the new roof work requires analysis, OP has to use the current code's snow load & drift loading.

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