Emergency Concrete Construction Joint

[ATSE]

During the concrete pour for a wastewater lift station and emergency basin, the concrete pump had a mechanical failure
about the time 1 cubic yard of concrete was placed.

3 hours passed by. All the ready mix trucks were sent away.

Then a new concrete pump showed up, and a new ready mix truck, and they started work again. Pouring over and vibrating thru the semi-plastic concrete from 3-hours earlier.

Before the "new" work began, I advised the contractor to abort the pour, and remove the concrete before proceeding by removing a lower form panel. He said he was taking the risk (not sure what that means in this circumstance).

The structure is a big underground box, with 28 inch thick x 30 ft tall walls. With waterstops at CJs.

Mix design is standard for wastewater: 5,000 psi, 20% fly ash, with air entrainment and plasticizing admixtures (but no retarder). w/cm = 0.40.

Structure will be water-tested.

From the structural engineer's perspective, anything meaningful to do now, after-the fact? Besides visually inspecting the walls after the forms come off, not sure how to confirm the structural integrity (especially shear strength and bond strength for rebar development).

 

[a2mfk]

Others will ask about the reinforcing specifics.

Wondering if this will not end up being a good situation for carbon fiber placed across the cold joint...

 

[cvg]

he says he is taking the risk, but really the owner is stuck with it. Did he place a waterstop at the joint? Did he place dowels? Did he do anything at all to prepare the cold joint? I would recommend cores taken at the cold joint location and to the lab for analysis. CTL is a good company to do this. Put the contractor on notice that the concrete pour may be rejected.

http://www.concrete.org/FAQ/afmviewfaq.asp?faqid=588

 

[hokie66]

What kind of concrete remains "semi-plastic" for three hours?

 

[Brad805]

It seems to me the risk is the contractors if he proceeded against your recommendation. In this case I think by "risk" he meant "cost/time to demolish".

I don't think there is much you can do right now. If the tank fails the leak test, then you will need to add some waterproofing or grind out the joint and implement a joint repair. Kryton and Xpex have details and products that can deal with non-structural cracks. If you want, you can implement a cold joint repair before testing. It is a lot easier for him to grind out the joint when the concrete is green. If that does not work, then I would think about the reinforcing idea suggested by a2.

Brad

 

[Ron]

What was the air temperature at the time of placement and what was the concrete temperature at the time of placement?

Instead of "advising" the contractor to abort the placement, if you are the structural engineer of record, your comment should have been..."This concrete is rejected. Take it out." The get the owner's rep to support that.

Contractors will almost always "take the risk". They count on being able to bulldoze their way through controversy when it finally hits, if it does.

When the forms are pulled if you can tell where the first concrete placement occurred, you probably have a bond/consolidation issue. Reject it.

 

[ATSE]

Thanks for the replies.

hokie66,
Regarding time and set, the outside temperature was between 34 and 36 degrees F most of the morning. I wasn't at the jobsite, but even on cold days, I've never seen fresh concrete stay workable for more than 2 hours.

Ron,
You sounded like my dad for a second, but you're probably right. I didn't think that quickly.

Coming from the contractor side: Let's say that after the forms are removed, the joint looks good, and the tank passes the water test with minor or no leaking. The contractor will say, "See, I told you, the concrete is tight. No problems."

I'm not sure what criteria there is to reject hardened concrete if there isn't clear visible defects. Not that I'm eager to reject; I'm just hesitant to accept.

Destructive testing may tell me quite a bit, but the operative word is destructive. What non-destructive or minimally destructive tests are most useful for analyzing the concrete around this joint, and what is the criteria for acceptance? Any guidance from ACI or ICRI publications?

 

[Ron]

If you have access to both sides of the wall, you can use pulse velocity to check for changes in the concrete. You can also use radiography, but for a 30-inch thick wall, the shot would be long and would likely have to be done with Cobalt 60 as a radiation source....only a few labs in the country have that capability. I have done radiographic testing on walls up to about 16 inches thick using Iridium 192, but the shot time was over an hour. That's a long time to have unshielded radiation out on a construction site.

You might also consider impact-echo testing of the concrete. This can be done from one side.

 

[hokie66]

ATSE,
The reason for my question was your statement that they poured over and vibrated through the semi-plastic concrete from 3 hours earlier. If they could vibrate through the earlier concrete, it is not a cold joint. Since you didn't see it, I doubt that is an accurate description of what happened in the field. If it did happen that way, the concrete set must have been retarded.

 

[HSIII]

ATSE,

We did hqave a situation where the concrete did not harden after two to three hours, but that was due to a severe retarder (site was on an island, access was not instant, etc, etc). My guess would agree with hokie, there was some form of retarder in the concrete mix, although the batch ticket may not say so. If you weren't on the site, but a third-party inspector was, did the inspector call you while all of these problems happened, or after everything was all said and done and you received word about it through their report? If that's the case, then shame on your inspection company, as they should have informed you right away. Watch for visible joint lines, honeycombing, cracking, etc to show external damage, which may be an indicator of internal damage. Personally, I think that impact echo or pulse velocity are your two best ways to determine internal damage regardless if external damage is evident (the contractor could parge the wall after form removal when nobody is looking).