Low strength concrete in suspended slab 3

[Hemifun]

I have a small building with CIP concrete walls and a CIP suspended slab. The building is roughly 27'X22' in plan size. The contract called for 3750 psi concrete and the slab was designed with f'c of 3500 psi. The cylinders broke low so we had cores taken. The cores broke at an average of about 1900 psi(56 days). I checked the calcs. and structurally the slab is ok even with the low breaks. I know most will say I need to have the slab removed and replaced but there are dowels all around the perimeter which tie the slab to the walls and replacement would be difficult. What are some cons besides durability for leaving the slab in place? The entrained air is 7%. Any suggestions??

 

[msquared48]

In the IBC, depending on your seismic zone, there is a certain minimum of f'c that must be used.

Was the 3500 psi value reflective of this requirement? If so, this item may not be "negotiable" if you know what I mean.

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

 

[Ron]

For structural concrete, the code MINIMUM is 2500 psi. You have not achieved this. To get 1900 psi at 56 days on a 3750 psi mix is deplorable. Why would you even consider leaving it in place? You have structural and contractual implications that must be covered...and you're considering trying to work it out. Grow a pair and reject it. The dumba$$ contractor should have thought of the implications of removal if rejected before he adulterated the concrete. If you let him get by with it, he'll do it again on the next job. Time to get him squared away. You have everything on your side to reject it...just do it.

 

[rapt]

I wouldn't call that concrete and use stronger for paving slabs around my house. For a suspended concrete slab, never. Agree with Ron!

 

[JAE]

If you have very low breaks, there is the possibility that the variability in strength throughout the slab is very high.

This means that you need to see the core strength results as an average, with the possibility, if not the probability, that you could have some areas with higher, and some with much lower strength than 1900 psi.

Be careful.

 

[msquared48]

Maybe I was a bit too diplomatic here... Putting things another way here, if you are the EOR for the project and the contractor screwed up, why would you remove his tit from the ringer and insert yours? That would be foolish, and very painful for you. He made the mistake and he needs to pay for it.

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

 

[rscassar]

I wouldn't consider accepting this concrete, strength is just one thing that must be considered and if it does pass strength, it would most likely fail for service and durability.

A low break like this could have been pick up with 7-day breaks. I would request the contractor to submit the mix design to see if you can pin point why the strength is so low but under no circumstances would I consider accepting it.

 

[hokie66]

Agree with all of above, and Ron said it best.

 

[a2mfk]

Ouch Mike, that is some kind of imagery, but point well taken. Some demo contractor is about to get a big job.

 

[beton1]

I think that the documents, ie. concrete delivery tickets are needed as a start, for one. The question is : was the contractor negligent by ordering an inferior quality, or tampering, or was the concrete ready-mix supplier responsible? The 7% air for a interior suspended slab is a question . In this day and age, for concrete to be that poor (compressive strength) is beyond me. The specifications call for something that is routinely and readily achievable, yet it was not done.

 

[Hemifun]

Thank you gentlemen for your input. I made my initial post from home and didn't have the actual core values in front of me. I now have the values in front of me and would like to clarify. The average of the uncorrected core breaks is about 2700 psi(56 days). The average of the corrected breaks, per ACI 214.4R Chapter 8, is about 2900 psi. The 1900 psi value is the equivalent f'c value calculated by the "tolerance factor" method found in Section 8.4.1. The mix design looked good. Slump and air also were OK. The lab tech who broke the cylinders and the cores did make an observation that some of the breaks were more of a "crumbling" than a typical break. He also noted that on one of the cores the upper half was very noticeably lighter in color than the bottom half.

 

[Ron]

I'm confused! Your average core strength is 2700 psi, but corrected it is 2900 psi? Usually when core strength correction factors are applied, they reduce the strength, not increase it.

Your technician made a valuable observation...friability and different coloring.

You say the mix design "looks good"....by what account? It doesn't appear that you received anything like what was supposed to be in the mix design. Why? As someone else pointed out, you have look at more than just strength, particularly when the indications of high variability are as strong as in this case.

To allow acceptability under ACI 318, your average core strength, assuming you took an adequate sampling, would have to reach 0.85(3750), or 3190 psi. You're not there. Even if you try to rationalize that your true f'c is 3500 psi, then you're still not there.

All indications are that you have a problem. Determining who is responsible for the problem could involve several players. Have you reviewed the delivery tickets? Was there water added at the site? Have you had petrography done to determine the air content of the hardened concrete and other void characteristics? Petrography can also tell you if the mix was retempered or if it had an inordinate amount of bleed water. Was there a batching problem? When was the last time the batch plant was inspected? Are the scales certified? Are the dosing valves/site glasses good?

Lots of things to check in order to determine responsibility. Still think you probably have bad concrete. Your liability..have fun. Good luck.

 

[a2mfk]

Ron, I think he met the design itself looked to be adequate for his structural design assumptions, not that the actual mix was good

Ron- what are your initial thoughts on concrete that "crumbles" rather than fractures, if you had to take a guess? I have seen stucco that was having major problems do that, I am wondering if it is a cement, water or contamination issue?

 

[Ron]

a2mfk...no guess necessary. It is related to poor bond between the coarse aggregate and cement paste...usually one of two causes for that...contaminated aggregate or retempering.

When that occurs in stucco, it is usually a result of an over-sanded mix or very dry conditions, both atmospherically and substrate.

As for the concrete dilemma...you're right...could probably rationalize it, but variability makes it "iffy".

I will be testifying in a trial next week on a building that the owner wanted demolished because the rebar placement was suspect. Yes, the rebar was a bit out of place vertically on a suspended slab, but the results were not that bad. I recommended a load test and pointed out to the building official that a load test is required by code if the analytical results are inconclusive. Building official ordered a load test...owner refused and is demolishing the building. We'll see!!

 

[a2mfk]

Ron- that sounds like a very interesting one, if you don't mind posting when it sorts itself out, as much as you are legally allowed to say

 

[Hemifun]

Ron- If you look at the eqn. for equiv. in-place strength(eqn. 8-1) in ACI 214.4R, there are 4 correction factors which must be applied to the core breaks. The most influential of these is the Fd factor which accounts for damage due to drilling which is a factor of 1.06 which accounts, in part, for the increase in strength. When I said the mix design looked OK, I meant just that. The submitted mix design looked acceptable. I think you hit the nail on the head re: why the cores crumbled rather than fractured. There is a noticeable lack of crushed aggregate in the mix. The large aggregate appears to be smooth, rounded river or bank gravel.

As I originally stated, strength wise the slab is still adequate. I also agree that the slab should be removed and replaced although it will be difficult because it is doweled to the walls. However, I know I will receive a lot of opposition not only from the contractor but possibly the owner as well and they will say "whats the problem, it is still strong enough". I guess I'm looking for more justification to reject the concrete in view of the fact that it still has adequate strength.

Thank you everyone for your comments. This is a great board not just for advice but also for tossing around different opinions and ideas.Thanks!

 

[dik]

any chance it froze?

Dik

 

[Hemifun]

Dik- I dont think so. The slab was in a heated enclosure.

 

[Ron]

Hemifun...thanks. I wasn't sure where you were coming up with higher average values. Can't replicate, since we don't know your raw data, but quickly ran through a couple of scenarios to get a range from 2760 to 2860.

You are using an older version of ACI 214.4R. In the 2010 version, your equations are in Chapter 9. Everything in there refers to "existing" buildings, not new construction. Chapter 8 of the newer report, directs you to go back to ACI 318 to evaluate new construction (which I believe is your condition) and directly evaluate based on that criteria. For that, if your core strengths do not reach 85% of the mix design strength, it doesn't meet code requirements. The only correction that is applied to the cores for that analysis is L/d. There's your justification to remove.

 

[a2mfk]

Could bond strength with the rebar, which is critical for composite action, not also be an issue with a suspect mix? I would think so...

None of this is your fault so I would not lose much sleep over it, this is on the contractor and the plant to make right. I saw a Discovery channel special on some bridge in middle America where one of those giant tower piers on a large river bridge had bad concrete, in the realm of hundreds of truckloads. It got rejected and the plant's insurance was on the hook for millions. They had to jackhammer it out of a heavily reinforced tower-column that was hundreds of feet tall.

So your situation is not nearly as nightmarish!