Precast Double T-Beam Cracking Question

[LStrang]

I am working on concrete repairs in the parkade of an existing building. There is a horizontal crack in one of the precast double T-beam webs staring from the steel embed shoe (see attached photos for reference). The crack is 50 mil at the largest and then tapers to nothing approximately 30” away. This cracking occurs on both sides of one of the webs in a double T-beam at the end where it is being supported on a beam ledge.

Is this an acceptable shear crack that is being controlled by the reinforcing steel or is this something more serious? I don’t have a strong knowledge of cracking in precast elements and would appreciate people’s thoughts on this. Thanks!

 

[Mark_J]

It looks like the steel bearing plate is heavily corroded.

 

[jhnblgr]

Cracks in prestressed concrete is a major concern. Over 0.004” is considered insignificant but over that is concerning.

 

[ANE91]

Need a lot more info. @jhnblgr is right to be concerned.

How do the adjacent beams look? Is that ponded water coming from above? Do you have at least a rough idea of the layout of the prestressing? Does the prestressing at least fan out at beam ends and cross the crack? Have you checked span tables for this cross-section?

I am inclined to agree with @Mark_J, but I don’t see rust stains on the concrete surfaces. Can OP confirm if bearing pads are indeed steel?

Once you figure out the cause, an FRP wrap might be viable as a solution. Or it could simply be a matter of getting rid of the water and patching the concrete. Lots to think about.

 

[JohnRwals]

This crack itself is local issue (connection design) rather than global issue.
Unless you can access to bearing plate design, it's difficult to evaluate analytically.

 

[LStrang]

Need a lot more info. @jhnblgr is right to be concerned.

How do the adjacent beams look? Is that ponded water coming from above? Do you have at least a rough idea of the layout of the prestressing? Does the prestressing at least fan out at beam ends and cross the crack? Have you checked span tables for this cross-section?

I am inclined to agree with @Mark_J, but I don’t see rust stains on the concrete surfaces. Can OP confirm if bearing pads are indeed steel?

Once you figure out the cause, an FRP wrap might be viable as a solution. Or it could simply be a matter of getting rid of the water and patching the concrete. Lots to think about.

The adjacent double T-beams either have either no cracking at this location or hairline cracks. Cracks of this size have only been observed at two webs (on two different double T sections)

The double T beams have a concrete topping on top (creating the parkade slab surface above), but there was previously no waterproofing so water has been entering this level of the parkade and sitting on the beam ledges. New traffic coating waterproofing is being put in place above as part of this project, so there should be no further water ingress at these locations.

The building was constructed in the 1960’s and unfortunately we do not have any structural drawings/plans at our disposal.

The bearing pads are steel. There is a cast in steel embed plate in the beam ledge and then a cast in steel plate on the underside of the end of the double T beam. These steel supports have been exposed to water as described above. There are not rust stains on the surface of the concrete ledge in the original photo because the concrete has recently been repaired there (I have attached a photo to this reply from before the repair where you can see rust staining).

The web is slightly tapered, I’m not sure if this impacts using an FRP wrap as a solution.

Let me know if there is any other info that would help. Thanks! (Also I'm a new member so my posts/replies require moderator approval before they show up (which I totally understand), but please bear with any delayed response times)

 

[JAE]

I would be concerned.
Any shear-type cracking is worrisome - if there's any failure it's abrupt with little warning.

Is this the only member with cracks like this?

 

[HTURKAK]

The crack implies a problem for the tension steel ( Z2) at the following figure. ( Copy and paste from BetoKALENDER Precast Concrete Structures )



Can you post the details of reinforcement and loading history ?

 

[ANE91]

The adjacent double T-beams either have either no cracking at this location or hairline cracks. Cracks of this size have only been observed at two webs (on two different double T sections)

The double T beams have a concrete topping on top (creating the parkade slab surface above), but there was previously no waterproofing so water has been entering this level of the parkade and sitting on the beam ledges. New traffic coating waterproofing is being put in place above as part of this project, so there should be no further water ingress at these locations.

The building was constructed in the 1960’s and unfortunately we do not have any structural drawings/plans at our disposal.

The bearing pads are steel. There is a cast in steel embed plate in the beam ledge and then a cast in steel plate on the underside of the end of the double T beam. These steel supports have been exposed to water as described above. There are not rust stains on the surface of the concrete ledge in the original photo because the concrete has recently been repaired there (I have attached a photo to this reply from before the repair where you can see rust staining).

The web is slightly tapered, I’m not sure if this impacts using an FRP wrap as a solution.

Let me know if there is any other info that would help. Thanks! (Also I'm a new member so my posts/replies require moderator approval before they show up (which I totally understand), but please bear with any delayed response times)

Great info. Ok, I’m convinced that at least some of the cracking is corrosion-related, but I’m not convinced that you don’t have a shear problem. Two cracked webs in 60 years of service is not bad. At that time horizon, I might accept hairline flexural cracks, but I wouldn’t ignore these. As others have said, the failure would be abrupt. My guess is that stirrups cross the crack but strands don’t. You have a bigger problem if slack strands are crossing the crack; could always do a screwdriver penetration test to qualitatively check.

Unfortunately, it’ll be extremely difficult to analytically quantify the shear capacity; it’s directly related and inextricably tied to how much prestress is still in the beam. This should be intuitive — the more you “squeeze” the beam axially, the harder it is to shear apart. Plus you’re in the D-zone, complicating matters further. Since you also don’t have drawings, you’re stuck with doing tests. Core samples for f’c. GPR for reinforcement sizes/layouts. Instrument and load-test for prestress, thought you might be able to lowball an estimate around 140ksi, instead.

Taper is normal and won’t mess with FRP. Bonded steel plates could also work. Call around and see what replacing it costs…your fees for all the above could easily break $50k.

 

[BAretired]

I think you have to assume there is a shear problem with the web, and possibly with the ledge, and reinforce for that condition.

 

[Mark_J]

I think you have to assume there is a shear problem with the web, and possibly with the ledge, and reinforce for that condition.

It looks like a fairly large failure at the support. Shear, support, bearing plates. 1/2" steel and connectors seem like the path forward

 

[LStrang]

Thank you for the reply. Considering your reply and the others above I am going to treat this as a serious shear issue, and look into repair options including FRP, steel plates, or adding a new support below the double T. The double T beam is being shored in the meantime.

 

[Greenalleycat]

This looks pretty bad and I'd be nervous
Here's a paper that you may find of some use that identifies different failures of precast double tees

https://cdn.ymaws.com/concretenz.org.nz/resource/resmgr/docs/conf/2018/s2_p3.pdf

 

[NedGan76]

IMHO, these cracks indicate structural shear issue and they are too large to be neglected.
Seems as if more vertical stirrups are required at the end of the higher section.

 

[JohnRwals]

Many tests released in the PCI Journal shows diagonal cracks.
But, this case shows almost horizontal crack.
Isn't it separation failure (hanger reinforcement required) between nib (upper portion) and dap (bottom area)?