Precast Topping 1

[a3a]

Our building sturcture has some precast-prestressed hollowcore floor slabs. On top of the hollowcore slabs, a 2 in topping slab is placed to finish it off. The mix design of the topping is the same as the interior slabs on grade (3/4" aggregate w/ min. strenght of 3500psi, but 28-day breaks have been 4000+ typically).

I guess the designers wanted this, but I have never been around this type of work.

Is this common?
Isn't a 2" slab more prone to cracking (from both deflection and shrinkage)? The only reinforcement will be welded wire mesh (which I assume will end up at the bottom).

 

[KarlT]

Topping can be called up for many reasons. One reason is to provide a level floor surface due to the camber of the slabs. This is usually only called up as a nominal skim coat.

If the precaster is asking for 2" of "Composite" topping, then they are counting on it in the design of the slabs. Composite topping typically increases the strength a fair bit (Increase of "d"), but you have to take great care that the contractor does a proper job in placing the topping. If the topping debonds from the slabs due to poor placement, the topping will only act as dead load and the original slabs may then become overstressed depending on the design assumptions the precaster made.

When the topping cures, you'll most likely see small cracks at the joints - the mesh is for crack control reinforcing. As long as it's properly bonded to the slabs below, shrinkage cracks shouldn't be a problem. If you see very wide shrinkage cracks then that is probably a good indication that the topping has debonded.

 

[a3a]

KarlT,

Thanks for the reply. I see what you mean about composite action and bonding. However, what type of bonding are you speaking of? The precast hollowcore slabs are flat and smooth. Granted the hollowcore will be clean, the only bonding present is from friction (i.e., there are no shear studs or any other methods of mechanical bonding).

Is there enough friction between the hollowcore and topping slab to form a composite section? Or are you saying the bond from the friction is enough prevent only major shrinkage cracks (but still not have enough bond to act as a composite section).

The precaster didn't specifically mention to us (the CM) that it was a "composite" topping slab.

 

[KarlT]

a3a

The PCI Hollowcore Design Manual (see

http://www.pci.org)is

an excellent resource for the design and fabrication of hollowcore slabs.

In terms of the composite topping, the horizontal shear stresses are typically checked (Based on max. horiz shear stress at support, NOT the average from zero moment to max moment) and compared to an upper limit of 80 psi. (See section 17.5.2.1 of ACI 318-95)

Testing done by several precast plants has indicated that the normal finished surface of hollowcore will develop the above shear stresses if the hollowcore has been thoroughly cleaned before the topping is placed. Unfortunately (or fortunately depending on how you look at it!) for the precast manufacturer, they are at the mercy of the contractor and engineer of record because they typically don't design the topping mixture, or specify the surface preparation requirements.

Locally, the slabs we have used in the past are extruded with a zero slump mixture (you can walk on the slab after the machine has passed by). However the concrete is still plastic enough to roughen up the slabs with a stiff bristle broom and increase the bonding to the slabs.

Just be careful that if the contractor applies a bonding agent, or rubs a sand cement grout into the slabs (as per ACI) that they don't allow it to set prior to topping placement or else it will act as a debonding agent.