Precast topping reinforcement 1

[CTSeng]

Have had lots of complaints lately about specifying a minimal welded wire fabric for a 2" hollow core slab topping with contractors preferring no reinf. or fiber mesh. The hollow core manufacturers catalog states that the topping reinforcement is to comply with ACI318 as determined by the EOR.

The topping is primarily for the composite slab action and a decent finished floor surface. It’s not really needed for the diaphragm and is in a non-seismic region so it could be considered plain (unreinforced) concrete. Anyone else putting reinforcment in toppings?

 

[Lion06]

I would always put reinforcement in topping for crack control. I can't imagine you would ever need it for composite action because it doesn't cross the horizontal shear plane and, as a result, won't offer any composite value.

 

[cvg]

if just for crack control, why can't fibers be used?

 

[Lion06]

Someone started a thread asking that question some time ago, and I didn't see (or don't remember) the outcome.

 

[UcfSE]

Fibers are not a replacement for wwf.

 

[JLNJ]

If the slab cannot slide (bonded to the relatively rough substrate) how would the cracks form?

I don't like the WWF due to the high degree of difficulty for placement. Maybe fibers would help reduce the plastic shrinkage but I'm not sure they're worth the effort either.

 

[Lion06]

JLNJ-
The same way that S-O-G's crack is the same way the topping could crack. A SOG is just as bonded to the stone beneath it, but because of the drag on that stone, it cracks. If it wasn't bonded at all, then it wouldn't crack (like pre-cast).
The restraining forces not allowing it to shrink is what causes the cracking

 

[archeng59]

In my opinion, the topping slab is the diaphragm and should be reinforced to transfer the horizontal shear forces into the lateral load resisting system and to control the inevitable cracking.

 

[civilperson]

Use stiff sheets of WWF not rolls for easier placement, Use six inch maximum spacing, i.e. 6 x 6, W10 x W10. Equivalent to #4 @ 12" both ways.

 

[Gordy2]

I only use WWF if the topping is 4" thick (nominal). Any thinner and it's too difficult to place. Otherwise I only use fiber. Yes the topping slab is the diaphragm, use a bar centered in the slab at the perimeter.

As far as it being for composite action, when you consider the camber in the slab how much topping do you really have? Personally I feel that composite topping is a bad idea, there are too many variables - the biggest one being the topping thickness.

I highly recommend using at least a 3" topping. I used to get complaints constantly about "too much camber in the plank". I have not had one complaint since going to 3" topping.

 

[civilperson]

Two inch topping can be exactly placed if the the alignment of the cambered hollowcore are pulled into uniformity. Hanging weights attached to the individual planks can align the surface. The WWF can be placed on #4 bars for spacers or chairs with 1 1/2" average cover. The addition of structural topping increases the load capacity for very little cost.

 

[Gordy2]

Civilperson you must be specifying 2" minimum topping? Otherwise you will not get 2" due to camber in the plank (not necessarily differences in camber). I'm assuming topping is poured level.

 

[civilperson]

No, 2" topping is a uniform thickness. The camber is usually designed to be equal to dead load deflection so the topping becomes "level".

 

[Gordy2]

Camber in precast plank is a result of the manufacturing process, prestressing force causes the camber. It is not designed in.

Read the PDF pages 33-39 sections 2.4 & 2.5

https://www.pci.org/view_file.cfm?file=MNL-126-981.PDF

,

http://www.kerkstra.com/hcload/Load_Table_10_inch.pdf

shows estimated camber, notice how more heavily reinforced plank has more camber.

I understand the theory with composite topping but when you factor in the tolerance for levelness of the bearing surface, camber in the plank and differential camber in the plank, which all effect the thickness of the topping, I'm quite sure you don't get what you think. You can spec 2" uniform thickness but the floor may not end up level.
Also, proper surface preparation is extremely important. The precast mfr. in my area (Kerkstra) no longer has load tables for plank with composite topping for theses reasons.

 

[McSEpllc]

I talked to a precast engineer about this a year back. He recommended strongly against using fibermesh in the topping. He said it usually creates large cracks at plank joints every 40-ft or so that it separates the planks. WWF apparently doesn't do that.

One thing I learned the hard way is not to expect the camber listed in the PCI Manual. I have 12'-5" clear-spanning 6" planks with a 1" camber!

I consider the planks without the topping slab as a diaphragm as long as the key way between planks is not cracked.

Eric McDonald, PE
McDonald Structural Engineering, PLLC

 

[csd72]

Sorry to say but I disagree with you eric, how can the planks act as a diaphragm if they can potentially move in relation to each other?

The only way to endure it acts as a diaphragm is to put in some WWF.

CTSeng,
builders are always complaining, just do what you think is right. What is the point of saving $10/sqft on a $100+/sqft floor if it is only half as good?

It will be you that is sued if they dont like the cracks, not the builder.

 

[UcfSE]

With the keys ways grouted they can act as a unit for diaphragm action.

 

[csd72]

UcfSE,

I would question the effectiveness of those keys ways if the topping concrete cracks due to shrinkage.

 

[csd72]

I stand corrected, refer attached web link:

http://www.spancrete.com/pdfs/ResearchNotes/researchNotes1011.pdf