Checking bad concrete mix

[khinz]

How do you check for bad mix when the concrete is still solid say inside a column or beam (meaning not the honeycomb section)? and what is the compression strength for a bad mix.. say the original is 4000 psi.. would it become 2000 psi or just 500 psi? how can you use a hammer to test a bad mix?

So a bad mix lacks cement paste to the aggregates but still enough to make it solid? is this it? how do you know how many cement paste lacks since everything is still solid.

This is important to know because replacing the concrete is very expensive with very specialized equipments.

 

[Ron]

There are many forms of "bad" concrete.....low strength, compromised durability, excessive voids, lack or air entrainment, too much air entrainment....the list goes on.

To determine if you have "bad" concrete, the most conclusive way to check is to remove a core from the questionable concrete and subject the core(s) to various tests, including petrographic examination.

Other methods to check the concrete are indirect and can only provide indications, not conclusory evidence.

 

[khinz]

Ron, In another thread (Column Discoloration) you wrote "In your photo, you appear to have elongated coarse aggregate particles with preferential, stratified alignment. Further, these stratifications appear to have little paste between them, indicating accumulated, segregated aggregate. This is different than "honeycomb" which is evidenced by sparse coarse aggregate and little paste."

I'm not sure what you are referring to. Are you referring to the following in the picture (left inside blue circle)? If not, which part is this "elongated..."? On the right is the comparison with surface of concrete removed (inside the same blue circle). The manager said it's solid inside. Is it not possible it's only the outer surface what is the problem caused perhaps by the aggregates not attaching to the surface especially as it is blocked by stirrups? How do you discount this possibility? because this is their arguments and I can't have the column replaced without giving good explanations of why the inside mix are soft. Also since concrete is composed of water, sand, cement, aggregates, admixtures. Are you saying one of these is missing inside? But the concrete if hammer appear so hard. Can't it just be good concrete?

(detail pics)

http://img27.imageshack.us/img27/4688/surfaceg.jpg

 

[Ron]

Look below the blue line in the left photo...see the "flaky" looking area? That's it.

 

[patbrry]

If you're pouring buckets down the col formwork, you might get that effect.

The rebar acts like a sieve, catching and slowing the large aggregates. So you get a thin layer of weak cement landing, and then a layer of gravel on top. Next bucket a similar thing happens, and you end up with thin layers of cement missing gravel = weak and gravel missing cement = weak.

It wasn't vibrated properly either. The area to the right of the blue circle is where the vibrator was resting against the formwork. It was either running at low speed, or wasn't there for long. It was then pulled up quickly (too quickly) to near the top of the blue circle (leaving behind that smeared effect (and probably an area with low amounts of aggregate). At that height the vibration was done properly.

 

[khinz]

patbrry, thanks.. do you think it just occurs in the sides of the columns or at the interior of the column too? have you encountered such before or seen other contractors do.. what did you or do they do?