Cast-in-Place topping on precast prestressed 9 m span hollow core slabs

[ajk1]

What is generally done about the camber in precast prestressed hollow core slabs, with respect to how it affects the topping thickness? We have a 50 mm specified topping but after placing the topping there is only 20 mm left for the topping at midspan due to the precast camber, if we are to have a flat horizontal floor. It is an office building. There was reportedly little reduction in the camber after the topping was placed. Is that normal? Seems that we will need a special concrete mix for the topping if we are to abide by the code requirement that coarse aggregate not exceed 1/3 of the thickness (20/3 = 7 mm maximum coarse aggregate size). Or is it normally accepted that there will be some camber remaining?

 

[ajk1]

This has been posted several days and yet no answer. I would expect it is a common issue. Can anyone answer it?

Thanks.

 

[JedClampett]

I always feel kind of bad responding to these type questions, because there's people that know lot more about it than I do. I only dabble with hollowcore. Plus you never told us your span or the depth of your hollowcore.
But to your question. Not only is there a camber left in hollowcore after topping, it gets worse due to creep (yes; the hollowcore goes up). The weight of the topping is not enough to negate all the tension on the strands.
I've never heard of adjusting the mix design due to reduced thickness. I'm guessing the finishers push the large pieces around until they fit in the pour and the finish look pretty.

 

[ajk1]

Thanks jedClampett, both for your response and for you refreshing modesty. Both are very much appreciated. You are a gentleman. The span is 9 m and I am not sure of the hollow core thickness (I am assisting another engineer in the office on this and can't find out missing info till Monday) but I think it is 200 mm. The concrete topping mix would usually have 20 mm aggregate I would think, although for 50 mm topping the maximum aggregate size should really be 16 mm. The smallest aggregate generally available in the Toronto area is 14 mm. When the topping thickness is 20 mm at midspan, perhaps they can make a 14 mm aggregate fit. I can call a floor finisher on Monday and ask this question. Seems to me that if we want to use 20 mm aggregate, we should specify a topping thickness of 3 x 20 + an allowance of 30 mm for camber = 90 mm, but this seems like rather a lot. For 14 mm aggregate, the specified topping thickness = 3x14+30 = 72 mm. Even this I suspect is thicker than normally used, and concrete made with smaller aggregate has a higher water demand and therefore more shrinkage. I should call a hollow core manufacturer and get his comment, but if you have any further thoughts please do send them.

 

[BAretired]

The span is given in the title to the thread, namely 9m.

Standard practice is to provide the specified topping thickness, in this case 50mm or 2" at the highest point, i.e. where camber is greatest. The thickness of topping at the supports will be greater than 2" by the magnitude of camber.

BA

 

[ajk1]

Thanks BA retired. I knew I could count on you. I wondered where you were! Much appreciated. Of course that means that the added weight of the topping in those parts of the span where it is thicker than 50 mm must be accounted for in the loading for the precast and all other supporting members. I guess engineers who design this type of system must know that.

 

[doka1]

I just completed a 6 story topping project where the topping assisted with the diaphragm of the building. Our project was 23,000 sf but had one group of planks that had a far greater camber than other planks On The same level. We used 1.5" inches above this set, ended up with about 3 to 3.5" on the edges. The concrete was 3/8" stone with a heavy dosage of structural fibers. Of course there were bars around the perimeter, 2 #4

We didnt find that the camber would go down much during the pour. Also, in speaking the precaster and seeing differences from floor to floor, planks with the same span would have different camber. The precaster a lot has to do with when it is cast and how quickly it is transported.

Our issue was tying in a level floor with hard points like stair wells that were precast as well that did not receiver a topping. After the first slab the precast installer realized hard points like stair cases would have to be held 3/4 inch higher than theoretical. ( 2" topping, min thickness was reduced to 1.5", therefor camber was around 1.25"). Raising the grade for each floor by 3/4" did impact the headers in the prefabricated panels. Fabricator did have enough play in headers, but could have been a nightmare

 

[BAretired]

Some differential camber can be expected, but camber can be controlled at the plant before casting by adding top strands to partially compensate for the upward lift of the main prestressing. But camber will grow with time, so planks which have been stored for a long time will have more camber than those fresh out of the plant.

BA

 

[JLNJ]

Camber is tough to control and predict. It seems to rear its ugly head on many projects. The best way to combay this is to allow for enough topping so that, even when the camber exceeds expectations, you still have adequate material over top of the precast.

I normally spec a max camber, but this is tough to enforce. The precast guys will tell you that the camber is simply a function of the prestress force and there is not much they can do about it. I'm sure they have more control than they let on, but I'm also sure that there is quite a bit of unpredictability in the casting, curing, shipping, and setting process.

I don't believe the PCA is much help. While it has strict and enforceable requirements for most of the geometric properties, camber is left for you to negotiate with the precaster. I have generally found precasters to be a squirrely bunch to deal with. All you can do is set out the expectations ahead of time. Keeping the spans and depths reasonable so that you don't have a super-heavily reinforced section (which will have more camber).

With your reduced topping thickness do you have reduced section capacity? I have given up on counting on the composite section and just plan on non-composite action. You also can force the design for a reduced thickness (due to camber) on the precaster. Let him decide if or how much of the topping to count on as composite. And while he's at it, make him specify the topping and the surface prep as well. Some want to scarify the top of the precast, but others swear that the as-cast condition will fully develop the composite action. I say leave all that up to the precaster.

Now, if you are down to 20mm (and you don't need the composite action) you might need to provide a thin setting leveling mortar to fill the space. There are all kinds of possibilities for that.

Even when I'm on the lookout for issues and try to mitigate things ahead of time, camber seems to jump up and bite me on planks, tees, and stadia!

 

[ajk1]

Thank you Doka for your informed and helpful comments; I can see that hard points are an additional consideration.

Thank you BAretired for the clever idea of adding top strands to compoensate for the camber. Have you ever done this? I suppose there would be a fair amiount of added cost.
I wonder if they could reduce the camber by reducing the prestress and adding bottom rebar?

Thank you JLNJ. You have identified a number of interesting issues and approaches. Sounds like you have had issues with precasters and your comments ring true. Your advice to calculate strength based on no strength contribution from the topping seems wise and good advice to follow.

 

[BAretired]

ajk1,

Years ago, I prepared load tables for planks and joists for a precast supplier in my location. Top strands with a stipulated force were provided to control camber. Slabs were extruded in a very long casting bed with all strands stretched horizontally over the full length of bed and anchored at each end.

After extruding the slabs and allowing time to cure, planks were cut to length. Conventional reinforcement was not used because at the time of extruding, the length of individual planks was not known to the operator, so he would not know where to splice bars.



BA

 

[ajk1]

To BARetired - very interesting; very good point; I had not considered the bar splice points.