DT Flange Connection Corrosion and Repair

[therrero]

Long time lurker, first time poster and I have a question for the group when it comes to an existing precast parking garage located near Denver, Colorado.
We were recently contacted to put together an engineer estimate for some repair work to fix a large number of completely corroded flange connector shear plates. The garage is slab on grade lower floor and precast double tee upper floor. It appears that the upper floor has some major sealing issues and water (CaCl2 during the winter months) is working its way below the topping and making its way to the shear connectors - the majority of them are completely corroded. The contractor wants to leave the existing shear connection in place and add an additional C10 channel centered between the existing connection with adhesive anchors embedded in the two inch concrete flange to replace the shear capacity and provide continuity between flanges for vehicle loading. After this fix, they will reseal the upper deck. Supposedly they have done this many times and had no issue with it. This detail seems pretty questionably to me. The structure will see moderate wind and seismic loads.
I have attached some photos as well as a sketch of what the contractor is proposing. In my opinion, it would be best to chip and remove the concrete flange and add in new shear connections, but I know the client would never go for it. This structure needs to be kept accessible as much as possible. Does anyone know of a reliable fix (or design standard) for this kind of situation? We have reviewed ACI 362.2 as well as PCI Parking Garage design and they really never address typical repair and replacement details. I appreciate the comments in advance. Thanks!

 

[dik]

You have a messy and costly (labour intensive) repair problem.

Can you talk to the precast supplier and get some assistance from him? Loads, design of DT's, suggested repair?

You mention CaCl2 in the winter months. Is this a de-icer? or was this used as an accelerator during concrete placement in the winter months?

When was the parkade constructed? Should the original consultant known about corrosion problems?

Your use of a C10 is a bit of a query. Is this just to tie the panels together and offer a small amount of shear resistance? Could you use a C6 on the flat with the web against the concrete?

Can you cut out the offending inserts and cast a small concrete shear key to resist longitudinal shear? The inserts will continue to corrode and cause problems.

Dik

 

[therrero]

dik, thanks for the response! Q and A below.

Q: Can you talk to the precast supplier and get some assistance from him? Loads, design of DT's, suggested repair?

A: We are just putting an estimate together at the moment, so we don't have any further information other than the pictures and measurements we took. I am sure we could get this information, but I like to have some details in my pocket before we commit to any project.

Q: You mention CaCl2 in the winter months. Is this a de-icer? or was this used as an accelerator during concrete placement in the winter months?

A: Just being general, they use a de-icer during the winter months on the top deck to keep it clear of ice and snow.

Q: When was the parkade constructed? Should the original consultant known about corrosion problems?

A: Not sure of the date of original construction - based on the area it was most likely late 90s , but the last time it was sealed was 3 years ago and obviously they didn't do a good job at all. From our observation, it appears that there was never any caulking or joint pack between the flanges, so water is free to move around below or above the topping slab.

Q: Your use of a C10 is a bit of a query. Is this just to tie the panels together and offer a small amount of shear resistance? Could you use a C6 on the flat with the web against the concrete?

A: If we were to prove the contractor's detail would be adequate (we would definitely not want to reinvent the wheel so we would like to see something similar used in this application - a contractor saying "we do this all the time" is like me selling a bridge in Brooklyn), we would provide a detail similar to what you mentioned - web side face up to the bottom of the concrete flange. Our thoughts are that the anchors embedded into 2 inches of concrete in this manner will provide minimal shear in the transverse direction, but for transferring vehicle load flange to flange it would be basically worthless.

Q: Can you cut out the offending inserts and cast a small concrete shear key to resist longitudinal shear? The inserts will continue to corrode and cause problems.

A: It's a thought, the clients main objective is to not have the parking area on the top floor compromised at all and saw cutting through the flange would definitely be a deal breaker - a wheel poking through the topping would not be ideal .

 

[dik]

It would be best that you contact the precast supplier... their input can have a significant impact on your work and fees.

If constructed in the late 1990's, then the designer should have been aware of many of the corrosion issues.

Can you hydroblast the offending concrete away from the anchorages... coat the anchorages with zinc rich paint and use a Sika patch.

What about the other concrete and reinforcing in the DT flanges? or the main rfg in the DT's. I assume the rfg is draped and bonded strand.

Have you done any other tests for degree of chloride invasion or carbonation? I'm not sure that half cell is applicable for the individual DT's. It would be good to know if you are putting a bandaid on a deceased patient...

Dik

 

[therrero]

dik, thanks again!

We provided an engineering estimate with about 100 caveats (sorta exaggerating ).

You are definitely correct in needing to contact the precast supplier prior to any significant detailing. We really just want to find out why these plates are corroding so badly in such a relatively short amount of time. I am almost convinced they used carbon steel with no cathodic protection at all. I swung by the site this morning to take a less rushed look and every single plate has moderate to significant corrosion (even at the high points of the deck).

We are thinking of two options:

1 - The detail we discussed above.
2 - Sawcut the general area, removal of the existing embeds, provide new adhesive dowels into the concrete flange (horizontal dowels), adding a new shear plate between the joint of the flanges, and welding the dowels to the shear plate.

If the existing embeds are left in place they would still continue to corrode (albeit at a slower pace even if a bullet proof watertight system is installed above) due to atmospheric exposure so these would also need to be protected, else spalling continues and the cars below would catch the concrete (not something the client would probably want). So we were thinking along the lines of your comment - the existing plates and spalled concrete can be hydroblasted, the existing embeds can be painted with a zinc rich paint (or for belt and suspenders we could use Sika FerroGard-903 as well) and the concrete can be patched with a Sikaquick product.

The stems of the DTs look fine and away from the embeds the flanges look pristine. This appears to be a case of localized corrosion.

Prior to actually performing the work, we would have the contractor sawcut and remove a few of the most heavily corroded embeds and get a handle on how far the corrosion has penetrated in these areas. It might just come down to a full face lift (removal of the majority of the flange and replacement). A half cell test could come in handy if we discover after the first embed removal that the concrete is just crumbling away from the reinforcement.

Any other thoughts? You have been quite a help and we truly appreciate it!

 

[dik]

Cathodic protection of the individual anchors is not a practical solution. The fasteners are located at places where the topping is likely to crack and promote the ingress of salts that will help cause corrosion. They should have been HDG at very least with the welded area cleaned of zinc and a zinc rich primer applied. There are several ways of dealing with this at the time of construction and the original designer should have considered corrosion issues. Parkade corrosion has been reasonably well known since 1975 or thereabouts.

Dik