Topping Bond Strength on Precast Double tees

[ajk1]

What minimum bond stregth should I specify for a new parking garage, for the cast-in-place concrete topping that will be poured on top of the new precast prestressed double tee units that will have a deliberately roughened surface? I believe that CSA A23.1 gives a minimum bond strength for toppings of 1.0 MPa (145 psi) but I was wondering if anyone has experience with this, what bond strength is generally obtained when the topping is placed on deliberately roughened tees, and whether I can specify something > 1.0 MPa (perhaps 1.5 MPa) without encountering push-back from the contractor. A greater bond than 1.0 MPa might be desirable, given that this is a roof level open air parking subject to the full summer to winter temperature variation of minus 20 deg. C to plus 35 deg. C in this area.

Also, is there any surface prep required beyond the deliberately roughened surface and a saturated surface dry condition before placing the concrete?

Is any bonding agent used, such as a rich cement slurry scrubbed into the surface of the roughened tee? I think not, but I am not sure.

 

[Ron]

Ajk1... Instead of specifying a bond strength why not specify tensile bond test and require the failure to be in the substrate not at the bond interface. This assure you that the bond strength is at least as high as the tensile strength of the precast

 

[ajk1]

Ron - that would force it into a bond strength of about 3 MPa which is much greater than the CSA A23.1 requirement of 0.9 MPa minimum. I would be concerned that to set such a high acceptance criterion would require a bonding agent, perhaps an epoxy bonding agent (even then we may not consistently achieve it), and we would get push-back against this from the contractor. I am wondering what the industry practice is for bond strength of topping on precast tees for parking structures, or is it not checked on each project?

 

[Ron]

Ajk1....rarely checked in my area

 

[BAretired]

It's never checked in my area either. A roughened surface is really all that is typically used. Any debris lying on the surface of the tees should be blown off before casting topping.

BA

 

[ajk1]

ok, thank you both.

 

[hokie66]

I don't even know how you would measure it, as I doubt it is ever attempted. I would avoid bonding agents, and they can easily turn into debonding agents if the timing is not exact.

My bigger concern would be waterproofing. How do you intend to make this type of deck watertight?

 

[ajk1]

to hokie66 - there is a Canadian Standards Association (CSA) Standard A23.2 that covers how to do the bond test. Basically drill a 3" diamter core thru the topping and a bit into the substrate, then epoxy a plate to the concrete; special testing device that pulls up, is screwed onto the plate' The device has a gaiuge that measures the pull force. It is a very common test here and all of the Canadian testing agencies that I have worked with know how to do it. I have had hundreds of these tests done over the last 30 years. I think ASTM may have a simialr test.

The precast form of parking structure construction with cast-in-place topping is popular in North America, but it is very specialized and the precaster has to know what he is doing and have the experience.

It is watertight by virtue of the uncracked prestressed double tee units, and the caulked joints in the topping which are located at the edges of the tees. These caulked joints do require some maintenance over the years, but in general (with some exceptions) they have perfromed reasonably well. I take it from your comments that it is not used in Australia, where I take it is your territory.

 

[hokie66]

No, these type of parking garages are not used in Australia, at least to my knowledge. The prestressed precast industry here is not very prevalent in buildings, only in bridges. Not sure, but I believe there would be building code provisions which would prohibit dependence on caulked joints in roof structures.

 

[Ron]

ajk1...similar to the ACI test. I've done them probably a dozen times in 30+ years.

 

[BAretired]

What happens if the topping concrete meets or exceeds f'c but fails to meet specified bond strength? You ask the contractor to remove and replace the topping?



BA

 

[ajk1]

hokie66 - yes the caulked joints are the weak link in the system. If they develop a leak that is not repaired, then major damage can be done to the structure including the tees, which are for practical purposes almost impossible to repair. It is not my favourite way to design a garage, but it is permitted by CSA S413 Parking Structures, and with the proper maintenance they have generally performed satisfactorily. If it is a foolish owner who does not maintain it properly, and does not realize that it is cost effective to do leakage repairs as soon as possible without delay, then it will have a limited life. I have heard in recent years some engineers may want to put a traffic topping waterproofing membrane on the concrete topping, but then the cost advanatge over cast in place disappears. Many of the shopping centre garages around Toronto have this precast system (without a membrane) but when I looked after the structural design of the 9300 car parking structure at the Toronto airport new terminal 1 a number of years ago, I recommended against this system and it was built as cast-in-place (as are many other large airport garages).

BAretired - same as for any other aspect of construction that does not meet spec. As you know, usual procedure is to do the calculations to see if spec can be relaxed. If it cannot be relaxed, then yes the topping would have to be removed or perhaps an extended warranty could be sought so long as the structure had the required strength and there was no life safety issue. However, my understanding is that it is no problem to meet the required bond strength and it would fail only if really bad practices were used on site, in which case the contractor deserves to pay for its removal. Some bond strength tests should be done as soon as possible after the first area of topping is placed so that any general problem will be detected as soon as possible. I don't see the point of CSA specifying a minimum bond strength, if no testing is done on site to verify that it is being achieved.

 

[Agent666]

In New Zealand you are allowed to add reinforcement in the Double Tee flanges to increase the longitudinal shear capacity at the precast/in situ interface. Typically this consists of spirals of 6mm plain bars about 100mm diameter, length to suit your needs. They are usually placed over the ribs. I'm assuming your code specifies the minimum bond stress to ensure there isn't a longitudinal shear failure and the topping delaminates.

 

[ajk1]

Agent666 - I understand what you are saying, but generally it is not necessary to add interface rebar for parking garage spans and loads (2.4 kPa). In most of Canada, that would be counterproductive because it would put more rebar in the cast-in-place topping where it would be eventaully subject to corrosion from the de-icing salts tracked into the garage in winter, which are absorbed into the pore structure of the concrete. I am guessing that is not an issue in most of New Zealand...so thanks for identifying where you are.

 

[hokie66]

ajk1,
After the Elliott Lake collapse, I wonder if the Canadian authorities might be having second thoughts about relying on maintaining sealant joints as a means of waterproofing.

 

[ajk1]

The double tee garage with caulked joints and no membrane is used throughout North America, not just in Canada, and there are currently no plans to revise the parking garage standard. Double tee garages have a reasonably good track record thoughout North America. I don't know if they are used in Europe. So best not to jump to conclusions before knowing the facts.

A few facts:

- Each parking garage system has its pros and cons, and each type has had its horror story. Eliott Lake is only the most recent horror story.
- Elliott lake was NOT a precast double tee system.
- Eliott Lake was a precast hollow core system - something I would never use.
- It had been leaking for 30 years with no repairs.

Do you know of any garage, irrespective of type of construction that would not have been subject to collapse if neglected for 30 years in a corrosive chloride deicing environment? I don't, and I have specialized in parking garages for 30 years.

Don't get me wrong, I have always argued that a cast-in-place system with a good waterproofing membrane system protection is a superior system, even though there are costs to installing and maintaining the membrane.

In fact I had a battle with the architect for a 9300 car garage a number of years ago...the architect insisted that it had to be a double tee garage with caulked joints. I was against it for the reason that in a garage that size they would be repairing the joints almost continuously...although they had another precast double tee garage at the same site that was about 20 years old and reportedly had been doing well. The owner of the garage wisely decided to listen to me rather than the architect so I won that battle (the architect never forgave me) and today there is a beautiful 9300 car cast-in-place garage there.

 

[KootK]

This is pretty timely for me as I'm also working on a one story parking structure in Canada. Thanks for blazing the trail ajk! I think that we want two important things from our topping:

1) Durability. I've read that the topping should be at least 4-5 ksi to match the likely durability characteristics of the double TEE's. Obviously, resistance to delamination is big too, hence the pull-off tests.

2) Longitudinal shear capacity (Agent's point). Do pull-off tests correlate to longitudinal shear capacities somehow? They must. I'd like to know more about that if anyone is well versed on the issue.

In my region, it now common to use pre-topped double Tees. This seems like a much better way to go from both a strength and a durability standpoint. Without any reinforcing in the CIP topped system, I'm surprised that the Tee flanges area able to resist code specified point loads. Your effective d wouldn't be more than an inch or two. In my parking structure research, I came across a company that markets FRP reinforcing for the flange toppings. Clever.

I'm using hollow core plank with topping for some areas of my structure. How come you avoid that system?





The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[ajk1]

Kootk - where in Canada is the garage to be? what is it to serve - a shopping mall, or an office building or a hospital or other?

- Do you live in Canada? If you don't you will need to get CSA Standard S413 "Parking Structures" immediately. If you live in Canada, you will liklely already have it. Read it cover to cover. Not only is S413 mandatory in every province of Canada, it has a wealth of very practical infomation in its annexes, which you would be hard pressed to find in any other single document.

- I did not say the flange has no reinforcemnt. It does have mesh. However unless you are an engineer working for a precaster, you need not concern yourself about the design of the tee and the topping. There are precasters who have very specialized knowledge about precast double tee garages and they have developed specialized details over the last 30 years. As well they know how to design and reinforce for concentrated loads, etc. The usual practice os for the consulting engineer to design the foundations, and write a performance spec for the precats and the precaster's engineer deigns all the precats elements and cast-in-place topping and any required reinforcing in the topping.

- As for the pre-topped unit, I have not used it, do not use it, and never will, for a lot of reasons. In the Toronto area, I don't know of any so-called pre-topped tee (I call them untopped tees).

- I also would not use hollow core. If anyone does design it, my recommendation would be to put a reinforced waterproofing membrane on top and overlaid with 2.5" minimum (perhaps 3") of compacted asphalt wearing surface. There are numbe rof waterproofign detials that you must show on your drawings.

- If this is the first garage that you have ever designed in Canada, I would recommend that you hook up with a structural engineer experienced in parking garage design in Canada. The Canadian regulations are significantly more stringent than American.

- A lot of people who have neve designed a parking structure think that there is nothing much to them. Actually they are amongst the more complex buildings and require experience and specialized knowledge to design properly.

 

[KootK]

@ajk:

- I practice in Canada presently.
- the structure will serve a medical clinic in Alberta.
- this is indeed my first stand alone outdoor parking structure.

I agree, parking structures are pretty specialized. It's been fun to learn about them. Going to the mall and the airport are entirely different experiences now. Instead of seeing dead people, I see durability details...

I've digested S413, the Chrest book, PCI's offerings, product literature from a few local precasters, and all the random goodness available on google. I should be able to conjure up a parking structure through sheer application of will now. I've also been in close contact with LaFarge's precast group and a precast specialist that works for my firm out east.

However, it sounds as though you might be the guy that I need to talk to. Can you tell me why you're so skeptical of pre-topped double tees and hollow core?

Pretopped tees are purportedly more durable, attractive, stronger, and more economical than site topped tees. They even can be made with a great mechanically raked traction surface. I reviewed several pretopped parking structures in the field. They look great. As far as I can see the only disadvantages are diaphragm strength and more potential water leakage paths. The diaphragms strength is easily resolved with connection hardware and water leakage can be addressed through sealant maintenance. If the precasters themselves are to be believed, pretopped tees are "the future".

As for the site topped hollow core, how is that fundamentally any different from site topped tees from a waterproofing standpoint?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[KootK]

This was the article that got me worried about the concentrated loads: Link.

I've also read that it's common practice to consider any flange point load as shared with the flange of the neighbouring tee. While I'm sure that's reasonable, it seems a bit liberal to me. Not all tee flanges even have neighbours.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.