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Normal Weight Concrete vs. Light Weight Concrete Composite Constructio

krus1972 (Structural) (OP)
10 Jul 08 18:10
We are designing slabs on metal deck for a 2nd floor office building. It will be composite construction - i.e. steel beams with shear studs on top.

Does it matter if normal weight concrete is used vs. light weight as long as the required concrete strength is achieved?

Other then the dead weightt and added costs I am trying to understand the pros & cons between using the different weights for the composite construction design.

Any help would be appreciated.
jike (Structural)
10 Jul 08 18:41
The choice to go normalweight or lightweight often has to do with firerating.

For instance, a 2 hour fire rating would require 4.5" NW or 3.25" LW over metal deck. Another possibility might be 2.5" NW over metal deck plus spray on fireproofing. Which one is more cost effective would determine the way I would go.

You should be familiar with the costs in your area to make the proper selection.

If you have no required fire rating, I would choose normal weight because of the large cost premium to use lightweight. As an aside, finishers do not usually like lightweight.
msucog (Civil/Environmental)
10 Jul 08 19:46
jike is dead on. also, i've seen where the concrete placed was 10+ lbs too heavy and everyone had to jump through hoops to prove everything was fine because the fear that fire rating diminished. after we were able to provide test results for the dry unit weight that were only marginally heavy, the engineer and architect were okay with the concrete placed.
lkjh345 (Structural)
11 Jul 08 7:43
Jike & msucog are right about the fire rating. We often find this is THE determining factor.

Couple other things to keep in mind:

- you need more composite studs for lightweight concrete than normal weight. See AISC Chapter I (Green Book).

- all other things being equal, normal weight concrete is often cheaper than lightweight.

- if you know who the contractor will be, talk to them first. Many (most) contractor/subs will preferr to place normal weight concrete as placing and finishing normalweight is less man power intensive. Contractors often bring in extra finishers if the pour is lightweight concrete in comparision if the pour was done in normalweight.  
Dinosaur (Structural)
11 Jul 08 8:36
Do not forget that the modulas of elasticity is lower for LW concrete so your "n" value will change.
strguy11 (Structural)
13 Jul 08 20:09
Be sure to check vibration.  Normal weight may help you if you need help in controlling vibrations.   

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