CRACKING IN POST TENSIONED BEAMS

[tbone73]

Fellow structural engineers, do you know of a good reference for guidance in classifying crack types for several types of construction? I recently made a site visit for a cast in place post tensioned parking garage that we designed. The only cracking I noticed were a few beams have 1 to 3 vertical hairline cracks a couple of feet away from the beam column intersection. The crack is very small in width, you would be hard pressed to fit a hair into the crack. I have attached a picture of one such location. My initial thought is that it is a crack related to temperature expansion. Your thoughts, as always is appreciated. Thank you.

 

[Teguci]

Maybe shrinkage cracks prior to stressing, but I don't think temperature strain will cause cracking in a tensioned slab. If the column is fighting the volume change in a P/T slab, the column will lose.

The cracks appear vertical on the left and then sloped like a shear crack pattern towards the right.

Any other input you can provide? How many days between pour and tension?

 

[hokie66]

Restraint shrinkage cracking is more common in the thinner element, in this case the slab. Without knowing more about the structure, I would guess that these are shear cracks.

 

[rapt]

The beams look to be fairly deep. Was any side face reinforceemnt provided?

 

[rowingengineer]

can we get a beam loading diagram?

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[hokie66]

Are there top bars in the beams near the columns? The cracks seem to have propagated from the top, at least they look like they go to the slab soffit, and stop in the compression zone near the bottom. All as expected with flexural cracking. As the tendons drape, there is a zone above the tendons which is in tension, requiring reinforcement to control the cracking.

 

[tbone73]

Teguci, 2-3 days between pouring and stressing. Early concrete strength monitoring was implemented to allow the contractor to stress as soon as f'c reached 3000-3500 psi.

The beams are 36 inches total depth. There are top, bottom, and each face bars continuous end to end with both the top and bottom bars hooked into the column and fully developed. The stirrup spacing at the column face is 2 legs of #4 bars, 5 @ 2", 5 @ 6".

The loading is uniformly distributed dead and live loads, but to this point the deck should only have been subjected to full dead load and construction live load, not full live load.

The cracks appear to run from near the bottom of the slab to the bottom of the beam.

The construction is 1 and 2 62 ft span beams. 7 inch slab, 16 inch wide and 36 inch total beam depth.

I hope this provides for a better picture.

 

[BAretired]

Perhaps the concrete is trying to shorten because of the prestressing force but this is resisted by the continuous reinforcement, causing cracks in the face of the concrete.

BA

 

[hokie66]

That is too long to wait prior to stressing, so may indicate that at least some of the cracks started as drying shrinkage cracks. Can you see cracks on the top surface of the slab? Is your photo taken at the end support or the middle support? If at the end, how big and tall is the column, and is there something else restraining the beam? Are the tendons bonded?

 

[tbone73]

hokie66, 2-3 days has been pretty standard in my experience. No I couldn't see cracks in the top of the slab. The majority of the cracks take place at the end support. The first crack is a couple of feet away from the column face (end support) and the next crack is about 12 inches from the first one, and typically at one beam end only. The columns vary in height. At the exterior columns range from 11'-4" to 12'-8". Many of the interior ramp columns have beams framing into them at different heights, so that the columns are shorter. Typical column size is 24" x 30" (30" in the direction of the frame). Nothing restraining the beam that I know of, and the tendons are unbonded.

 

[Teguci]

A couple of ideas:
1. the drape of the tendon at the continuous end is putting the bottom of the beam in tension allowing the shrinkage cracks to open up.
2. Cracks occurred prior to tensioning and aren't being closed.

In the photo, is the column to the right?

In any case, the reinforcement seems appropriate and cracking on the underside of a slab shouldn't be a concern regarding maintenance. The cracks should close even further once live load is placed on the slab.

 

[tbone73]

I have attached a simple sketch of the two span conditions.

 

[Teguci]

OK, my option 1 is out. Option 2 is still a possibility except that normal PT stressing would close those cracks up so tight you wouldn't see them except when wet. So why, with 400+psi stressing, are there still visible cracks?

I am assuming that the prestressing at this end of the beam is in the centroid of the shape (discontinuous end). Is it possible that the PT duct is eccentric to the centerline of the beam? Do the cracks appear on both sides of the beam or just one face? Short of that, I am at a loss.

 

[BAretired]

If cracks occurred prior to stressing, the conventional reinforcement will resist volume change in the concrete. How much conventional reinforcement do you have?

BA

 

[tbone73]

Teguci, to my recollection the cracks do appear on both sides of the beams. The centroid of the tendons was located at the centroid of the beams at the ends vertically. One end detail had one tendon in the slab on one side of the beam and two on the other side. This slight horizontally eccentric detail was not typical, but happened at only a few locations.

BAretired, typically 4 in[sup]2[/sup] top reinforcing continuous and fully developed into the columns, and 3 in[sup]2[/sup] bottom reinforcing midspan with 2 in[sup]2[/sup] at each end fully developed into the columns. Usually minimum reinforcing controlled (A[sub]s[/sub] = 0.004 A[sub]ct[/sub]).

 

[VTPE]

Is your slab tensioned too? Primary stressing perpendicular to beams with T&S stressing parallel to beams? The beam may be trying to shorten but be restrained by the slab so the system is relieving the stresses through cracks.

 

[tbone73]

VTPE, it is exactly as you stated, the slab is post tensioned in the direction perpendicular to beam post tensioning, and some T&S stressing parallel to the beams.