Low Concrete Strength and Air Content
Low Concrete Strength and Air Content
(OP)
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
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






RE: Low Concrete Strength and Air Content
Dik
RE: Low Concrete Strength and Air Content
RE: Low Concrete Strength and Air Content
Check JAE's FAQ
RE: Low Concrete Strength and Air Content
RE: Low Concrete Strength and Air Content
Air is markedly higher... might indicate some other mix problem.
RE: Low Concrete Strength and Air Content
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.
RE: Low Concrete Strength and Air Content
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