Light Weight Concrete Mix for Metal Deck

[Lot2Learn]

I am trying to specify light weight concrete for a conc on mtl deck design. Vulcraft has used fc=3000 psi and conc. density of 110pcf. 3” deck with 6.5” total depth with conc.

I have three questions:

1) Is 4”+- 1” slump ok?
2) What should be the nominal max aggregate size: 1”, ¾”, ½” or 3/8”. I am thinking ½”.
3) Max air content?

 

[concretemasonry]

It may be difficult to get 110 pcf with relatively small maximum sized aggregate for several reasons:

1. The smaller diameter of MOST lightweight aggregates have a higher density than the larger particles that would not be present. The uncrushed manufactured lightweight aggregates are far more costly than the normal aggregates where the aggregate is cruched to get a specific gradation.

2. With smaller particles, there is more surface area and the mix matrix and sand have a higher density.

As with all aggregate, there is a geographic effect on the materials and costs thereof, but lightweight concrete also depends on the supplier of the aggregate to the ready-mix plant and the history based on previous mix designs. If you are pumping, this adds and additional factor.



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[a2mfk]

Do you really need lightweight, you are specifying a pretty common slab?

 

[Lot2Learn]

@a2mfk: This is a single span floor infill 8'x11' aprox, and the existing beams don't have enough capacity.

 

[TXStructural]

Consider letting the ready mix supplier design your mix, based on the strength and placement requirements. Most suppliers have standard mixes which they can make consistently and that have a history of successful placement with correct strength. You should specify only the minimum requirements (especially since you are unfamiliar with mix design.) ACI 318 and IBC 2006 requires the designer to specify only compressive strength of the concrete, unless there are durability requirements for exposure. 318 5.2, et seq. talk about proportioning of mixtures, if you decide to design your own mix.

Slump determines how easily it can be pumped, placed, and finished.
ACI 301 requires suppliers to provide a slump of 4 inches unless otherwise specified by the designer (or 2-4 inches before water-reducing admixtures are added.)

Volume of cement and water-cement ratio (w/cm) will determine the amount of shrinkage and subsequent cracking. Lightweight aggregates will absorb more water, so specifying w/cm is counterproductive for lightweight mixes. Consider allowing a water-reducing admixture to improve workability.

Larger aggregates generally reduce the amount of cement required, and as long as it can be pumped, mixes with larger aggregates will often have lower shrinkage and perform better.

For floors, there is minimum cement content based on aggregate size (ACI 301-10 4.2.2.1) which place restrictions on the supplier/contractor.