Slump and Air Content tests yielding different results at beginning and middle third. 1

[acer49]

Link

Going back to this thread and I had an issue on site with the Contractor, Concrete Testing Agent, and Concrete Supplier/QC person while on site.

Our specification states we need a MINIMUM of (1) test for slump and air content per ASTM guidelines which state to perform these test in the middle third of the pour. Usually this isn't a problem at job sites because they test prior to the pour for acceptance and then test again during the middle third for our records. Results usually stay the same or don't vary by a large magnitude.

I was met with a lot of push back from the contractor, testing agent, and supplier for this particular job. All of them said it was strict and never have seen it tested in the middle in their 20-30 years of experience. They refused to test the first truck at the middle and all hell broke loose. Their reasoning was that they had a previous job in which the concrete had to be replaced. I'm the EOR as well and had to call the contractors supervisor to get the subsequent trucks tested in the middle.

Our specification requires air content to be 6% plus or minus 1%. The second and third trucks met spec prior to pour but the air dropped to 3.5% when tested at the middle third. What course of action should I take or taken in the field? Is our specification too strict? What is y'alls experience with the ASTM guidelines versus acceptance?

 

[Ron]

@acer49....you are seeing something most of us have seen many times. Keep in mind that a contractor who says he's been doing this for 20-30 is one of those contractors who has 1 year of experience, 30 times....not 30 years of experience.

Your specification is not out of line. Also any contractor who says that he has not had concrete testing in the middle third of a load has never had concrete tested properly. Stick to your guns. The standards are there for a reason. As the EOR YOU DECIDE...not the contractor, not the ready mix supplier, not the testing lab.

Not one thing wrong with having a spec and sticking to it. They should have read it when they put in their bid...all of them. Don't back down.

 

[acer49]

@Ron so if the middle third of the load is not meeting specification what course of action would you recommend to take when the concrete is already in the hole? Would you tear out concrete for low air or is that unreasonable? My guess is that it is up for the owner to decide ultimately?

The concrete supplier is threatening to pull of the job because they find our specifications to be too strict with the testing. The only problem is the job is in a remote area.

 

[MTNClimber]

Test results can and usually will vary throughout the load. Which is why I think it’s better to just test the middle third as the first and last thirds are not representative. 6% plus or minus 1 is pretty standard so I don’t think you’re too strict.

3.5% of entrained air measured on fresh concrete doesn’t necessarily mean that the concrete isn’t satisfactory for freeze-thaw durability. Sometimes those values will be actually higher when entrained air is measured on hardened concrete. See this FHWA report on marginal entrained air.

This is one of those situations where you’ll have to use your judgement on how imperative freeze-thaw durability is of your concrete. Will there be waterproofing or insulation on the exterior of the concrete? Is the concrete poured for a foundation wall, a slab, or a footing? Is the project in an area where freeze-thaw is frequent or occasional? Freeze-thaw durability will be more important in some of those situations than the others.

 

[Ron]

@acer49...MTNClimber is correct. Since the concrete did not meet the specification requirement, make the contractor provide a petrographic examination of the concrete to determine the actual air content. If he refused to do so, reject the concrete and have them remove it. If they pull off the job, recommend no payment for any of the placed concrete. Check their contract. It probably says that they must comply with the specifications.

 

[fel3]

Acer49....

Whether or not 3.5% air is OK from a technical standpoint, the 6%±1% requirement is part of the contract between the owner and contractor. The contractor signed the contract, so he is responsible to meet the contract requirements. It sounds like this particular requirement is reasonable, common, and achievable, so I wouldn't give any weight to the concrete supplier's whining, which is almost certainly about profit, not difficulty.*

If you unilaterally decided to allow 3.5% air, you would be allowing the contractor to deviate from the contract without owner approval. I have never had such authority and you probably don't either.** From a strictly legal perspective, accepting 3.5% air requires owner approval and a change order. I would also demand a credit for the difference in cost between 6%±1% air and 3.4% air.

What to do when the concrete is already in the hole?
-- You and the owner could accept 3.5% air and go through the change order procedure I described above.
-- You could require the contractor to tear it out and try again. I've never had to do this with concrete, but I've had to do it with other things, including piping, tank coatings, asphalt pavement, etc.***

Fred


* I recently helped a young engineer in another office with a specification for a pre-cast concrete vault for a storm drainage project. I prepared the spec from a master specification that I have used for more than two decades (updated along the way, of course). This spec has put more than 100 pre-cast vaults in the ground, with nary a complaint from anyone. Well, said young engineer told me about a month ago that the contractor had complained several times about "that stupid spec, yada, yada, yada." Well, whaddayouknow, said young engineer called me a week ago to tell me that she had just received a submittal for the vault I spec'ed, including drawings and calculations, and it looked like the submittal met the spec. Amazing!
** That's not to say that many of us (myself included) haven't accepted minor deviations without going through a formal change order, but we really shouldn't do that. If the owner is going to get what he contracted and paid for, we must hold the contractor to the contract.
*** Closest to your situation: I once had a small concrete block masonry building for a pumping station torn down twice and rebuilt. In this case, the order came from the City's very experienced inspector not me. However, both times he called me first to confirm before giving the order....I was two hours away, so a spur-of-the-moment site visit to confer was not practical. For the first build, the contractor grouted all cells, but the structural engineer had called for grouting only cells with rebar. Solid grouting resulted in much larger lateral seismic forces than the structure was designed to handle, so my structural recommended rebuilding, I concurred, and so told the City's inspector. The inspector had already figured this out. For the second build, the masonry courses were not level and the horizontal grout lines varied between 1/4 inch and more than 1 inch. Fortunately, the third time was the charm.


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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[MTNClimber]

As much as I agree with the contractural side of things, I would make sure that stopping the progress of the project is necessary. Include the owner in the conversation if you think you can get away with concrete with less entrained air as delays for concrete removal may cost them time and money in the long run.

When I was an entry level engineer I did a fair amount of concrete testing. Reality is, concrete pours were a pain in the ass half of the time. I’ve turned around or been involved with the decision to turn around so many trucks. It always involves a HUGE argument. That’s why it’s important to do things per ASTM standard specified in the contract. I would always have a copy of the standards on me to show the contractor or concrete rep when things went south. I understand how it’s important in some cases to stick to your guns so contractors don’t think they can walk all over you. But when everything has been placed, it’s probably a good time to re-evaluate what is necessary.

 

[wohoodiii]

Assuming the initial test was at the back (or front) of the mixer and that the test from the middle of the load was not at the end of the pump line, other causes could be as simple as the slump decreased and made the air content lower. The mixer could have been stopped for a while and the concrete sampled without remixing. If the beginning of the load was tested and accepted for placement and in the middle of the load it was low accept it and continue to check. Maybe it wasn't the concrete but a bad sample (especially from the end of the pump line if not sampled correctly). I've had projects where every load was initially tested and on the cylinder load was re-tested in the middle. In my experience the difference is minimal. Follow the code. The ready mix producer needs to figure out why the air is decreasing in the middle of the load. Not your problem but work with the contractor and ready mix supplier.