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mcollins16 (Specifier/Regulator)
10 Jan 07 8:45
I'm developing a multifamily project that has 1 1/2" of lightweight concrete on the exterior balconies of the apartments.  Is 4000 psi lightweight just as good if not better than 2500 psi lightweight with fibermesh?
bridgebuster (Civil)
10 Jan 07 9:04
Are you asking about a 1 1/2" topping?
mcollins16 (Specifier/Regulator)
10 Jan 07 9:49
Here's the situation:  Our plans call out a 1 1/2" 2500 psi lightweight with fibermesh topping for the balconies.  Our General Contractor poured the first 20 balconies with a 1 1/2" 4000 psi lightweight topping with no fibermesh.  Should I be concerned with this or is it comparable?
DaveAtkins (Structural)
10 Jan 07 10:14
Fibermesh is for crack control.  The PCA recommends that for unreinforced slabs (on grade), without reinforcement, control joints should be cut in at intervals of 24--36 times the slab thickness.  In your case, control joints are needed at intervals of 3' to 4'-6" oc.  If a balcony is bigger than 4'-6" in either direction, I would ask for a control joint.  Otherwise a crack is likely to form.

If these balconies were poured last week, or before last week, the cracks may already have formed.

DaveAtkins

COEngineeer (Structural)
10 Jan 07 12:33
I usually do not specify 1 1/2" topping unless it is going to be covered with tile, veneer, or wood flooring.  I dont think 1 1/2" of concrete should be a finish floor.  Thats just what I do on all of my projects.  If it is going to be concrete floor, I put at least 3".
Taro (Structural)
10 Jan 07 13:06
Who specified the original 2500 psi mix and what was the basis of design for this mix (strength, crack control, durability, etc.)?  You need to know this before you can determine whether the substitution is acceptable.

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