Topping Bond Strength on Precast Double tees
Topping Bond Strength on Precast Double tees
(OP)
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.
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.






RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
BA
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
My bigger concern would be waterproofing. How do you intend to make this type of deck watertight?
RE: Topping Bond Strength on Precast Double tees
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.
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
BA
RE: Topping Bond Strength on Precast Double tees
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.
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
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.
RE: Topping Bond Strength on Precast Double tees
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.
RE: Topping Bond Strength on Precast Double tees
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.
RE: Topping Bond Strength on Precast Double tees
- 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.
RE: Topping Bond Strength on Precast Double tees
- 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.
RE: Topping Bond Strength on Precast Double tees
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.
RE: Topping Bond Strength on Precast Double tees
Untopped (Pretopped) Tees
- I don't like untopped (or if you buy into their PR, "pretopped"). If however, you have been to see a couple of them, and you have found that they are performing well over say a 20 year period, with no signs of previous extensive concrete repairs, and you also think they look great, then you would be justified in going for it. Places to look for deterioration are i) the ledge of the inverted T beam that supports the double tees, ii) column corbels, supporting stairs well walls and the like, iii)the soffit of the stems of the tees, and iv) expansion joints. All these locations are subject to reinforcement corrosion due to leakage and concrete spalling. Item i)can be particlarly common and difficult to repair properly. Item iii) is virtually impossible to repair and has required demolition of at least one 30 year old garage in Toronto within the last coupel of years.
The following are my reasons for not using it.
a) they have thicker slab (I believe it is 100 mm slab) and so cannot be warped as much to provide the required drainage slope;
b) the adjacent units cannot provide a perfectly aligned top surgace the way a cast-in-place topping can, and so in my opinion do not look as good, and perhaps you might feel it a bit when drive over it;
c) they are only rarely used in the Toronto area which is my main area of practice; I know of only one that is untopped and that was built a long time ago. I went to see it about 20 years ago and did not find that it looked as good as c.i.p topping garages;
d) they may be more prone to leakage, which is the Achilles heal of the precast systems;
e) they are heavier than untopped tees so fewer can be transported by truck and transporation cost goes up (but that does not affect durability, but is just something to keep in mind).
I have never ever heard of any problems with concentrated loads on double tees, so I think the experienced parkign garage precasters know how to design for that. Perhaps some of the design is based on load testing. The precast and topping is generally designed by the precaster, but I would insist on a precaster who has proven long term (at least 20 years) experience on parking structure design. You have that in the Calgary area which is at least as sophisticated if not more so than Toronto. There used to be someone (I cannot recall his name; he is no longer with Conforce) from Conforce who was on the CSA S413 technical Committee with me.
As for hollow core, I would not use it because:
a) it is only 1.2 m wide so there are two to three times as many joints to leak as with the double tee system!;
b) doubtful that uniform bearing can be achieved on the suppoorting beam over the full 1.2 m width of the precast unit; would that cause rocking of the panel?;
c) if water gets into the cores and freezes, what then?
d) they may be relatively flexible (compared to double tees) and I have heard that this may cause some breakdown of the joints between units;
I know of only one garage (for a medical centre) of hollow core in Toronto, perhaps 10 years old, but there could be more, I don't know. It has a waterproofing membrane and several inches of asphalt over it. It still leaks but is in generally decent condition except for signs of minor scaling at the soffit adjacent to all supports...later I will see if I can find some pictures and send to you. Also one location where the leakage has caused severe corrosion of supporting steel. If I did ever use hollow core, I would insist on a rugged waterproofing system over it, with the membrane reinforced over all joints.
Have you been to see any hollow core slab garages and how they are performing?
By the way, there is a new edition of S413 that is to be published I believe later this year but there are no really major changes, except we have deleted the statement that said only alternate joints in hollow core system need be tooled and sealed!.
I have to run to an appointment so I have not had time to proof read the above...likely lots of typos. Sorry. Hope the above helps a bit.
RE: Topping Bond Strength on Precast Double tees
Dik
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
For my project, the hollow core is being used for a ramp structure rather than the main parkade. Still, the same concerns should apply. In particular, I think that water in the hollow core is a serious concern. I'll detail the planks with end grouted cores to try to mitigate that.
I gather that your primary objection to pretopped tees boils down to this: Pretopped = more sealant joints = more water leakage = more durability issues. As I understand it, the system hasn't been prevalent until pretty recently in Calgary. All of the structures that I toured were built within the last five years and so I can't personally vouch for the long term performance.
I did my tour of a year old parkade on a rainy day as it were. While taking a picture of a vector connector from beneath the roof deck, a big droplet of salty water fell down from the ceiling right into my eyeball. It was quite unpleasant and altogether consistent with your appraisal of things.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Topping Bond Strength on Precast Double tees
As far as pretopped being the wave of the future, I have a hard time believing that because they are nothing new...been around for at least 30 years I would say, and they certainly have not caught on in this area. Perhaps the costs and economies of the different systems are differnt in various parts of the country, or maybe the different precast manufaturers have different preferences.
How many stalls will this garage have, and how many stories?
Is it all above grade?
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
CSA S413 states in the first paragarph that one must protect against both corrosion AND leakage.
If you are interested in garage design for durability, CSA S413 is the best thing to read up on.
RE: Topping Bond Strength on Precast Double tees
CODE --> ajk1
In Australia, it is common practice to have bonded PT roof level framing systems (incl. parking decks) over occupied spaces with decades of success in terms of durability and serviceability, withOUT waterproof membranes.
Local construction practices avoid construction joints, opting for large pour areas. High strength concrete using shrinkage compensating cement are common.
Design and detailing includes min P/A of 2 MPa (300 psi), continuous top reinf in both directions, 35mm+ top cover, and details that minimize restraint.
RE: Topping Bond Strength on Precast Double tees
25% initial stressing at 24 hours is also used, along with high-early strength concrete.
RE: Topping Bond Strength on Precast Double tees
Interseting that in Australia you have succeeded in making it watertight without a membrane, with no failures.
RE: Topping Bond Strength on Precast Double tees
I used to work in Toronto (2 years in late 80', early 90's) with a consulting structural engineering firm, and your weather and environment is like a real-world testing lab. Did they ever pull down the Gardner Expressway!
No de-icing salt used in Australia.
Our chlorides come in the form of "coastal chlorides" and our codes stipulate the exposure classification based upon proximity to the body of salt water, and min cover is determined from the grade of concrete used, with a min grade specified for each exposure classification.
There are some failures. The main objection to waterproof membrane is that they are tough to maintain and they cost a lot install and replace.
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
It also depends what is in the occupied space - if it expensive electrical equipment, or say clothing store, etc., leakage could do a lot of costly damage. Have you ever been to Canada during our winter - your summer?
RE: Topping Bond Strength on Precast Double tees
I know about cold winters, having been born and raised in Virginia, but have spent no winters farther north than New Jersey. But Blacksburg, VA can be cold. -15F, with 40MPH wind, caused classes to be cancelled one day in 1963.
RE: Topping Bond Strength on Precast Double tees
I've done a lot of research on Elliot Lake collapse... and the problem was the system used. The HC was overspanned and the topping was never used properly... It was an architectural error that an engineer 'walked' into. He was correct in stipulating the loads... only the system chosen could never have worked and there were problems almost from day one.
As far as using HC slabs... there's a parkade in Winnipeg that I did about 35 years or more ago that is still up and running... using 12" HC with a topping... and proper jointing system.
Dik
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
Dik
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
Dik
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
Dik
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
Trow was/is a high quality group (now exp consultants). Not sure of their exposure in this one.
RE: Topping Bond Strength on Precast Double tees
BA
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
RE: Topping Bond Strength on Precast Double tees
Dik