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.

 

[ajk1]

- Looks like you have indeed done your homework. Yes those are all the references that I would have recommended. The Chrest book is excellent, but it describes American practice which is much less stringent than Canadian as prescribed in CSA S413. If you have been talking to Conforce, my understanding is that they are an excellent precaster and should be very knowledgeable about parking garages. I am from Toronto, so I have no direct experience with them.

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.

 

[dik]

...and prior to adding the topping the surface of the DT's should be saturated surface dry/damp.

Dik

 

[hope9010]

Right on!

 

[KootK]

That helps a TON AJK. Thanks for taking the time to share all that with me. My tender drawings go out on Monday. I wish I could fly you out here for a concept review somehow. You've earned a beer or four.

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.

 

[ajk1]

Thank you for the very kind words. Good luck with your project.

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?

 

[hokie66]

Durability of the structure is not the only concern about leakage. I know of post-tensioned, cast in place carparking structures here in Australia where the issue has been leakage onto the automobiles below. And of course, where parking is over commercial or residential space, that is arguably worse.

 

[ajk1]

CSA S413 requires a waterproofing membrane over all cast-in-place post tensioned systems with unbonded tendons. For cast-in-place concrete, only bonded tendon systems allow omission of the waterproofing membrane if the designer so chooses. I would hope no one is so unwise as to build a garage over occupied space, irrespective of the structural system, without a waterproofing membrane. If any engineer thinks that post-tensioning can prevent all leakage, he is unwise, to put it gently.

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.

 

[Ingenuity]

For cast-in-place concrete, only bonded tendon systems allow omission of the waterproofing membrane if the designer so chooses. I would hope no one is so unwise as to build a garage over occupied space, irrespective of the structural system, without a waterproofing membrane. If any engineer thinks that post-tensioning can prevent all leakage, he is unwise, to put it gently.

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.

 

[Ingenuity]

I should more correctly state that "shrinkage-limited" cement not technically shrinkage-compensated cement is used.

25% initial stressing at 24 hours is also used, along with high-early strength concrete.

 

[ajk1]

How much corrosive deicing salts do they use in the winter in Australia? That cover would be totally inadequate here!

Interseting that in Australia you have succeeded in making it watertight without a membrane, with no failures.

 

[Ingenuity]

ajk1,

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.

 

[hokie66]

I agree with ajk1 that concrete roofs, floors or whatever, over occupied spaces should have a membrane. But when it is done carefully, with enough crack control reinforcement, and with attention to detail as to restraint, they work most of the time without membranes. We don't use unbonded PT in Australia.

 

[ajk1]

I wonder if in Australia you have the seasonal temperature swings we have in most of Canada and the northern U.S. i.e. in Toronto from minus 20 C to plus 35 C, and greater in places like Calgary. That produces significant stresses that low shrinkage concrete does not address. Perhaps that is why you can do some things in Australia that we cannot safely do in most of Canada. When I was in Sydney 6 years ago I some things with masonry construction that absolutely astounded me...it would never work here because of the freeze thaw cycles.

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?

 

[hokie66]

You are right...Australian climate conditions are much less severe than in Canada. Most of the population here is in cities close to the coast, and we complain about the cold when it gets down to 10C (50F), although we have had a few cool 4C mornings lately in Brisbane. In some of the inland centres such as Canberra, it goes below freezing, but nothing like your winters.

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.

 

[dik]

hokie and ajk:

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

 

[hokie66]

But dik, notwithstanding the inadequacy of strength at Elliot Lake, the system sounded very similar to what is being discussed here...precast, bonded topping, and reliance on sealant joints. The joints leaked profusely, and in this case the supporting structure for the precast was structural steel, which failed due to corrosion. Correct?

 

[dik]

Yup... but, the original system would not likely have worked... the HC was overspanned, and the engineer had no concept of the strength of the system selected by the architect. The topping was just poured on top without regard to shrinkage, etc. 200' or so or topping without any control joints... to crack at random with no consideration of a waterproof membrane... a recipe for failure... failure occured by corrosion of the structural steel that was missed by numerous top quality consultants many of which I have provided consulting services to, resulting in a fatal collapse...

Dik

 

[hokie66]

Maybe they were "top quality consultants", but in this case their work was not of top quality.

 

[dik]

agreed, but there is no indication what the scope of their work was...

Dik

 

[hokie66]

I think that is a copout. We all know that many architects are incompetent, but engineers should accept being held to a higher standard of accountability. If an engineer cannot adequately define and complete a task, he should not accept the assignment.