Map Cracking on Precast Double Tee Flange

[KimWT]

Hi!

You can see some severe cracks in the flange.
These two photos show how bad they are looking.
These are from typical double tees of two parking garages;
one is pre-topped and the other is field-topped.

I wonder what caused this problem.

I am looking forward to precast engineers' opinion.

Thanks!

 

[TehMightyEngineer]

ASR?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[JAE]

Looks like ASR

(some images of ASR

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[TehMightyEngineer]

The only other thing I could think it would be is a poor mix causing high surface shrinkage, but I'd expect that to show up on the top surface and not a formed surface.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[KimWT]

ASR?

This sometimes happens, not often.
It happens only in the flange, not stems.

Why?

 

[BridgeSmith]

ASR = alkali-silica reactivity. It's activated by moisture.

 

[TehMightyEngineer]

https://www.understanding-cement.com/alkali-silica.html

Does the precaster have records from the aggregate supplier for tests of ASR reactivity?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[JAE]

It happens only in the flange, not stems.

Why?

Perhaps more moisture in the flanges than in the stems?

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[KimWT]

Aggregates do not contain any special component causing ASR.
If they do, ASR should happen everywhere and every component.
This problem happens only in double tees.
Slabs, walls and spandrels are clean.

 

[BridgeSmith]

If it's not ASR, it may be an issue with the curing process for those components, such as inadequate temperature or moisture control in the casting beds for the double tees.

However, if the flanges of the double tees see more moisture than the stems or other precast sections, the effects of the ASR may show up there before other places.

 

[KimWT]

This is precast concrete; one double tee is only one day old...
Can ASR happen within 24 hours after concrete is poured?

 

[BridgeSmith]

Yeah, you wouldn't see ASR in 'young' concrete. It's a process that usually takes years or decades to show. Assuming there's actually cracking of substantial width, and it's not just an irregular drying pattern, it would have to be due to something wrong in the concrete mix or improper curing. Presumably these sections are prestressed, so it's natural that the stems would not have cracks, since that is the part of the section that's in compression, while the flange probably carries some tension, at least until it's loaded.

 

[Compositepro]

The cracks could be knit lines where concrete flow fronts meet after falling though the rebar.

 

[jayrod12]

The cracks could be knit lines where concrete flow fronts meet after falling though the rebar.

That's an interesting theory, I actually kind of like it. I've never heard of them being called that before.

 

[TehMightyEngineer]

This is precast concrete; one double tee is only one day old...

Yep, not ASR. My best guess is plastic shrinkage cracking though I generally wouldn't expect that on the formed surface. My reasoning for this is between the double-tee stems you have the typical square-shaped map cracking caused by restraint from the stems and shrinkage cracking of the long flange slab. Then on the flanges on the outside of the stems you only have cracking perpendicular to the long direction of the double tee as the stem no longer is restraining the flange during curing.

I'd suggest you adjust your mix to retard the initial curing period and see if these cracks don't show up.

The cracks could be knit lines where concrete flow fronts meet after falling though the rebar.

In my experience I don't believe this crack pattern would be caused by that; I've seen that happen and it looks different than the cracking shown here. You tend to get lines of bug holes that follow the rebar and you don't get the square-shaped random pattern of map cracking shown in OP's 2nd picture.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)