High slump and air entrainment in exterior concrete 3

[whir]

I have a job where I specified a 4000 psi concrete for a truck dock slab with slump limits from 5"-7" and 4.5% air entrainment. The testing lab technician is telling me that some of the concrete was placed with a 9 3/4" slump and some was placed with 9% air entrainment. If no superplasticizers were approved to be used, the high slump concrete could have cracking and durability problems. The high air entrainment could cause cracking problems as well after freeze/thaw cycles. Does anyone have any experience with these problems? I am on the fence about having them remove the concrete.

 

[emmgjld]

The high slump will result is some strength loss, Not a good thing on a truck dock.
The high slump will also probably result in excessive cracking, allowing possibly allowing deep entry of deicing compounds and water, probably resulting in spalling and crack widening over time.

The high air content will contribute to the lower strength.
Depending on the finish placed on the concrete, The high air content may result in severe delamination of the slab surface.

Non of these possibilities would amuse me and I cannot imagine the owner/user being happy with them.

 

[civilperson]

Slump is out of date for a criteria to reject concrete. The w/c ratio is paramount. If the cylinders or cores achieve required compressive strength, then the concrete is OK. High air will deduct from strength, not frost protection.

 

[JAE]

If the cylinders or cores achieve required compressive strength, then the concrete is OK.

I would question this blanket statement. Here's why:

1. The cylinders taken from the concrete could have been taken after the addition of water at the site or before. I would find out from the technician (if they remember) and see what went down. If water was after the cylinders, then the slump test, the cylinder breaks would show test results for concrete with a different water/cement ratio - which is an important element as civilperson points out. Thus, your cylinder breaks may or may not be representative of the in-place concrete.

2. If the high slump is a reflection of a high water/cement ratio, then even with cylinder tests coming out OK, you might still have excessive shrinkage over time due to the high amount of water...thus shrinkage cracks.

3. The high air content might cause finishing problems by creating blisters in the top surface layers of the slab. When troweling occurs, the dragging force and the vibration of the equipment compels the air to rise to the surface at the same time that the equipment is denisfying the top layers, prohibiting the air from getting out. Thus you get small delaminated pockets of air. These can be discovered by chain dragging the surface, or simply tapping on it in areas where blister cracks appear....but blisters can occur without the blister cracks.

Again..the concrete cylinder tests aren't the only measure of acceptance.

 

[hokie66]

Yes, I would agree with JAE. Although cylinder strengths have always been an acceptance criteria for concrete, in a situation such as described here, the cylinder strengths are probably a very poor indicator of the long term performance of the truck dock slab.

 

[Rjeffery]

Additionally, the owner was cheated our of 4.5% of his concrete... (it was replaced by air)

The best way to test something is to squeeze it, slowly, until it breaks!

 

[dougantholz]

The contractor did not install what you specified. I would tell the contractor they own the liability for that slab. First in person (phone is okay) then in writing. The item is non-conforming. Now the contractor has two choices: 1) do nothing and hope no problem comes up in the future; 2) rip it out now and save the liability in the future. The owner should participate in this and be the final arbiter.
When notifying the contractor in writing make sure you use language such as "indemnify against all future liability" and have them sign a copy and return a copy to you for your file.
If you head down this road you are likely to find the contractor will voluntarily rip out the slab when they see what could be hanging around their neck in the future.

 

[hokie66]

I don't like the idea of giving the contractor the option of signing an indemnity form. He may sign it and skip town.

 

[cvg]

if the contractor was required to provide a warranty, he is probably bonded. If the contractor skips town, he would have to forfeit the bond back to the bank...

 

[civilperson]

A minimum wage technician reporting a high slump could be in error, (bumping the board while measuring). The high air reading can come from mis-calibration of the meter or leaving voids at the top of the sample bucket. Cores can establish in place compressive strengths and micro examination of the cores can estimate the air entrained. Starting a claim war with the contractor over a single testing report is not wise.

 

[Ron]

The way the concrete was placed is certainly not within the specified limits and, on that basis, is rejectable. The high slump is indicative of poor control of the concrete at placement and leads to lower compressive strength and compromised durability. This application demands both strength and durability. Just because the test specimens might have met the strength requirement does not mean that the concrete will be durable for the application.

Someone in the thread said that as long as the compressive strength is ok, then the concrete is ok...NOT TRUE. As JAE mentioned, it is only one of several criteria of evaluation.

Keep in mind that the test specimens are intended to reflect the mix design...not the in-place concrete. The mix was obviously adulterated and you will likely see a compromise to the durability of the concrete, most likely in higher shrinkage (cracks/joints open more than expected), lower resistance to abrasion (critical for forklift traffic/warehousing operations), increased potential for dusting and spalling.

Accepting an indemnification is just temporarily passifying a potential long term problem. Do a little more testing (cores, joint measurements, and petrography)and if those results still show a potential for problems, reject the concrete.

 

[hokie66]

I would just reject the slab and put the onus on the contractor if he wants to try to prove it is satisfactory. I suppose it would depend on whether you have faith in the technician or not, but this report is so bad that I can't imagine that he has it all wrong.

 

[dik]

whir:
The original slump specified is higher than I spec, usually 4" max. Unless there has been superplasticiser added the supplied concrete is unacceptable. With the spec'd concrete, I'd expect shrinkage cracks, in particular if you have 'dock pits'.

For exterior exposure for freeze-thaw conditions the air should be about 5% to 8% max. Even the upper limit has a bit of an effect on strength.

Generally, unless you have shear issues, for small amounts of reinforcing or chemical resistance, the concrete strength has little bearing on the behaviour of the concrete in the structure.

Dik