Low Concrete Strength and Air Content 1

[GotSoiled]

Recently a subcontractor of ours poured weather exposed concrete with a design strength of 5000psi and air content to be in conformance with Air limits as specified in ACI 301. 1 of the 4 pours came up super low with a corresponding air content of 9.0%. The 9% air concrete was noted in the field and was allowed to be poured (that's another story). Here are the 28 day break results and corresponding AC readings for the first 3 pours and 14 day for pour4:

Pour1- 5720psi, 6.8% Air
Pour2- 6435psi, 5.7% Air
Pour3- 2690psi, 9.0% Air
Pour4- 4715psi, 7.5% Air (14day)

Questions:
1.)Does pour 3 have any chance at gaining 85% f'c (4259)? (I think I know the answer to that one)

2.)If the results of the coring are within 85% f'c is this acceptable or is it purely up to the engineer of record to determine acceptance?

3.)Are there any other methods of testing in-situ concrete which I am missing?

I have dealt with this issue as an engineer now I'm on the contractor side (the dark side) and ripping this stuff out while possible, will be extremely costly

Thank you for any insight

 

[dik]

JAE had an excellent description of what to do, either in a FAQ or one of his postings. If you can't find it, I can dig up a copy... stole it (with permission) for my own use...

Dik

 

[JAE]

faq507-1575

 

[Ron]

Answer to 1)...no
Check JAE's FAQ

 

[BigH]

Would there have been any chance that the cylinders were 'damaged' during transport to the lab, a rookie taking the cylinders and not sampling and casting them correctly, or other issues that could be attributable to flawed cylinders - and similarly to the air content? It is most likely that pour #3 will not make the grade but one can't be wholly sure about the concrete in situ. Was this part of a mass pour - or beam/column placement? How closely spaced were the samples taken - part of the same placement or taken at separate placements? How "critical" is the member in question? Was fly ash involved? If so, what is the percentage and what percentage increase could you expect between 28 days and, say, 90 days? On a job I was on recently, we had a 30% increase (I know it wouldn't be enough) for 25% fly ash between 28 and 90 days - for a 50% fly ash substitution, we had an increase of about 50%. I would think that cores would be your best bet on determining the strength and then applying as JAE stated the rules of the game for acceptance.

 

[dik]

BigH:
Air is markedly higher... might indicate some other mix problem.

 

[concretemasonry]

How many samples were taken for each of the pours?

If it was a limited number, it could have just one batch and not representative of the entire pour.

Damaged cylinders due to handling or poor preparation are very possible, but the unusually high air content indicates at least one bad batch in the pour.

The high air content could be due to bad field testing procedures.

As an ex-concrete inspector (in the dark ages) in both plants and in the field, we ran parallel samples to discover the range of results that could be expected while still essentially following the standard practices and found it was easy to skew the strength results by 15% (up or down). Because this was for the DOT and we had our own lab, this luxury was possible. Much is due to the techniques of making cylinders, but not enough to justify the difference between the specified strength the actual test results provided.

Low compressive strengths due to handling/transportation is more common than it should be, but typically, technicians are seasonal and underpaid. - I know since I was one for 4 summers as part of a 5 year engineering curriculum.

There is a good history of comparisons between field cylinders and core samples, especially if there are any field note on the location of the cylinders and the area cored (especially depth if it is mass concrete).

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[GotSoiled]

It was a small pour. We used 2 trucks, 7 yards each because of the total amount being just over 1 truck's capacity. The structure in question is 1 of 4 pile caps for a moorings constructed as a pier retrofit. I assume designed to handle mostly lateral impact loads from docked ships (again I'm the contractor and it's not my job to speculate on design purpose or adequacy).

I'm not sure what percent if any fly ash was used. I have attached the mix design.

As an ex concrete inspector myself I agree that sometimes the inspectors are not the best. However, the testing agency we used was instructed to use ACI field grade inspectors as per the specification and the guys on site thought he was competent enough. I think the air content is the main problem. We are trying to set up a meeting with the engineer of record to lay it on the table I can only hope he is as knowledgeable as some of you guys and works with us if possible. I will have to set back and let the engineer decide the next steps (coring, windsor probe, etc.)

I appreciate everybody's input