Concrete Under Strength 5

[SperlingPE]

I have a building (6 story CIP) that has a floor that did not meet strength requirements. The contractor took it upon himself to have cores drilled at various locations around the floor. The core strengths were all over the place. One very low, but the majority made strength or were within the 500 psi limit. I asked that more core samples be taken in the area of the very low sample. My thought is that the average of these cylinders will satisfy code requriements. My questions are: What can be done to strengthen the floor system (one way pan joists)? Can I re-rate the live load capacity based on the core sample breaks?

The cylinder breaks for columns on this floor also came back low.

I am trying to ascertain what the cause of the low strengths is. The concrete supplier, testing agency, contractor, owner, and flat work sub are all involved. I am not looking forward to this possible mess.

 

[JAE]

I would recommend that you closely follow ACI 318, Section 5.6.5. You are getting ahead of the "process" by starting to talk about reducing load capacities or strengthening the floor.

The steps I'd take:

1. Review what cylinder tests (2 cylinders = 1 test) have come up below the required strength per 5.6.3.3 (this is where your 500 psi comes in). If you are within the 500psi limit (5.6.3.3(b)) but are below the required f'c (5.6.3.3(a)) then what you are required to do is take steps to increase the concrete strength. This is outlined in the commentary of 5.6.3.4. You do NOT have to take cores, or do anything else...just take steps so that future concrete comes up better.

2. IF you DO come in below the 500 psi limit, you have to take steps to assure the capacity of the structure. (see 5.6.5.1).

3. These steps would first be to take the NEW, LOWER f'c that comes out of the original cylinder tests and go back to your calculations and recheck the area of the affected structure. This may surprise you as f'c doesn't have a huge impact on flexure, but may have on shear...check the shear of the joists especially. If the various parts of the floor design come up OK based on your original required Live Loads, etc. then you are OK - no other steps are necessary.

4. IF the lower f'c causes problems in your design capacity, then you go to the cores. The coring takes place in threes - the average of three cores must be > 85% of f'c and no single core can be less than 75% f'c. (see 5.6.5.4). If the cores meet the above, then you are OK, no other steps are required.

5. IF the cores don't meet the required levels, then I would take the next step of either more cores or perform a load test (ACI Chapter 20) or strengthen the floor with supplemental structural members - but ONLY strengthen for those areas deemed understrength per step 3 above.

All of the above is laid out in 5.6.5 of the ACI Code.

 

[SperlingPE]

Steps 1 to 3 were followed and I wrote the original post in the midst of step 4. The remaining floors that were poured above this one were meeting f'c requirements. I am waiting on results of the core tests for the problematic area of the floor in question.
Also, concrete cylinder tests for the columns on this floor came back below strength. I am currently checking how this affects the remainder of the structure.

 

[BigH]

It would help in these threads to actually give the strengths of the cylinders/cores, etc. rather than justl saying they are low. Just a suggestion.

JAE is quite correct in going about your consideration of the data in a rational step by step way - don't try to get ahead, don't try to make decisions without all the information - and don't get paranoid about things at first.

I must reiterate earlier comments I and many others have made in other threads - the cylinder strengths tested (cast and cured according to specified standard) are not really any indication of the actual in situ strength of the concrete. Curing is differnt; conditions are different, etc. It is, in my view, for the purpose of showing that you have consistet quality concrete.

Secondly, as per AASHTO, you may neglect a 28-day single cylinder strength if there is evidence that the cylinder has been damaged/wasn't properly cast, etc. Then, the test result (usually the average of the two) would be based on the single remaining cylinder strength. This is why it is so important when you have low strneght test result of a cylinder (or cube if you use those) to take the time and effort to observe the break, the quality of the concrete, etc - to see if there is something "in error" that has caused a low cylinder (cube) strength.