Lightweight concrete construction

[Orange44]

This is in reference to a medical building where the construction is 3.5" lightweight concrete (f'c=4000 psi) on 2" metal deck.

It has recently come to our attention that some manufacturers of rubber flooring will not warrant their product if it is installed over concrete that is less than 115 pcf (regardless of moisture content). This has our client worried, that they will not be able to install their desired flooring because of the type of concrete that is specified for our project.


The intent of this warranty language is probably in reference to poor mixes, underlayment such as Gyp-crete, or fillers; not properly mixed lightweight concrete. Because the warranty language states 115 pcf as a cut-off, though, we’ve been informed that it has been used to excuse flooring deficiencies on any lightweight mix that is poured at anything less than specified, even if it’s 114 pcf (which is within tolerance for lightweight concrete).


Is this an issue which has been brought to your attention for any other projects? If so, is there a remedy, such as particular specification language, that is acceptable to concrete manufacturers as well as flooring manufacturers?

Thanks for any help.

 

[concretemasonry]

Just write a spec for 116 to 120 pcf range of concrete density. Any reasonably capable concrete supplier should be able to hit that. A spec writer may have to "bend" the boiler plate and think a bit. For that minor density difference in the property spec for a 3.5" slab there should be minimal, if any, construction cost and you will be able to give the owner a floor guarantee.

Technically, it is not lightweight, but it could meet your requirements from a dead load standpoint. The lightweight aggregate supplier could provide assistance on any differences in fire ratings, which are also categorized.

The categories are just broad ranges and have some convenient lines drawn to differentiate the ACI/ASTM categories by definition.

Specific densities can be easily specified. What you are after is the "hard numbers" like weight and strength and getting a product that will meet the floor manufacturers requirement to provide a guarantee. - Are there any other property requirements in addition to the 15 pcf "escape clause"?

 

[271828]

Be careful coming up with a concrete that's not "lightweight" because of fire ratings. I can't remember what unit weights they use as "light weight," but D916 among others require more concrete for "normal weight."

Also, we've seen problems with composite slabs and sheet vinyl floors regardless of concrete weight. Very tiny cracks (some you can't even see) over beams at columns cause "ant trails" that can been seen. Some of the contractors we worked with came up with elaborate processes for getting rid of these little cracks--something like wait 'til they appear, bead blast (?) them and then seal them before putting down the sheet vinyl.

 

[concretemasonry]

If you are concerned with fire rating, lean on a lightweight aggregate supplier through a major local concrete supplier.

If you chose to go with the arbitrary definitions of lighweight concrete, you are not wrong, but there may be a better engineering apprach like specifying a density and strength.

The only difference between lighweight concrete and normal weight concrete is the amount of lightweight aggregate that is used and the gradations and type of the different raw materials. Cement contents are just adjusted to meet the strength requirement. It is really a gradual progression and not a step function.

A good supplier of lightweight aggregates and a good ready-mix supplier can give you the documentation you need if you wish to meet the owners requirements. If not, take the prescriptive appraoch and go to normal concrete and add to the thicknesses and structural system.

 

[271828]

concretemasonry, I agree completely. I only pointed out this stuff because bldg officials get really hung up on the nit picky details when it comes to fire ratings.

D916 for example, might say 110-115 pcf. To catch 116 pcf is an easy "checkbox item" for somebody to find.