Low strength concrete in suspended slab
Low strength concrete in suspended slab
(OP)
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??






RE: Low strength concrete in suspended slab
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
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
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.
RE: Low strength concrete in suspended slab
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Low strength concrete in suspended slab
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.
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
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.
RE: Low strength concrete in suspended slab
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?
RE: Low strength concrete in suspended slab
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!!
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
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!
RE: Low strength concrete in suspended slab
Dik
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
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.
RE: Low strength concrete in suspended slab
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! ;)
RE: Low strength concrete in suspended slab
a2mfk- Yes, bond is affected by concrete strength. And you're right, in the grand scheme of things my 27'X22' slab is relatively minor. lol
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
A lot of great information from others on the concrete mix. I would like to address your concern about removing the slab and affecting the dowels coming out from the walls. I have been involved in many total slab replacement projects in parking structures.
Contractors will usually opt for the following procedure:
- shore walls/columns where necessary to replace any lateral support provided by the slab to be removed.
- install formwork on the underside of the slab
- sawcut slab into squares for removal, keeping sawcuts a minimum of 3 feet away from walls/columns.
- jackhammer concrete in the 3 foot zone beside walls/columns, thus preserving any existing dowels. Use small jackhammers for final chipping to avoid bruised concrete at the bond line.
- level formwork installed previously
- install new steel, lapped onto existing dowels and pour new slab.
A contractor experienced in total slab replacement/rehabilitation would have no problem with this type of removal.
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
RE: Low strength concrete in suspended slab
I don't know if I'm in the "tear it out" camp, but even if they pay you handsomely to re-analyze it there's a lot to think about before accepting responsibility.
RE: Low strength concrete in suspended slab
This FAQ is a brief summary of the steps in Chapter 5:
FAQ507-1575: How to Evaluate Concrete Tests per ACI 318