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lightweight concrete topping on structural slab

lightweight concrete topping on structural slab

lightweight concrete topping on structural slab

For a new multistory concrete building, I am placing a 4" thick lightweight concrete topping on top of the normal weight structural slab at various locations (the required "depressed" area accounts for 70-80% of the floor plate, hence the entire structural floor is set at 4" down and the other 20-30% is brought back up to finish floor line by topping).

The flooring sitting on this topping will be terrazzo, ceramic tiles, and carpeting.

I am attempting to do the following:

1. The 4" lightweight topping will be reinforced with welded wire mesh to control shrinkage crack because there will be no control joints.
2. Specify the largest practical maximum size aggregate and/or the highest practical coarse aggregate content to minimize drying shrinkage.
3. Dowels will be located along the edge and at the corner of the topping area to reduce potential curling, or separation from the structural slab below.
4. Specify low strength, about 2500 psi, and low slump lightweight concrete topping.

I don't find many references or guidelines for such construction in ACI or PCA publications, and I appreciate it if anyone has any comments or suggestions on the approach listed above. Also, I am debating to see if there is any need to either roughen the structural slab or apply bonding grout before placing the topping as the structural slab below is still relatively green when this occurs. Thank you.

RE: lightweight concrete topping on structural slab

What density concrete are you trying to achieve?

You might be able to reference some Solite aggregate references written by the guru of lightweight aggregate, Thomas Holm. He was specialist in structural lightweight concrete, but also had a vat knowledge of other uses of lightweight aggregate.


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

RE: lightweight concrete topping on structural slab

What is the design of the structural slab (post tensioned or normally reinforced) and what is the time difference between placing the structural slab and the topping.

I would not provide a bonding agent or a scrabbled surfaced because bonding the topping to the structural slab will only introduce an additional restraint to the topping which will promote cracking.

Don't higher grade concretes have less shrinkage? What is the reason for specifying 2500psi?  

RE: lightweight concrete topping on structural slab

Thank you two for the responses.

I'm looking for a density of 110 pcf (=/-)


It's reinforced concrete slab (not prestressed one). It's difficult to estimate the time difference as it is up to  contractor's logistics. But I would guess somewhere between a few weeks to couple months.
The only reason I'm thinking of 2500 psi is to reduce the cement content so it will be a bit more permeable to allow the water at the bottom of the slab to exit more quickly to reduce shrinkage and curling. (not sure if I'm thinking right though)
I like your idea of not bonding the two to reduce the cracking. However, I need to dowel the topping down along the edge to prevent possible curling, which means restraint -- major conflict of interest here, right?


RE: lightweight concrete topping on structural slab

The better lightweight aggregates (lightweight fines and coarse) may provide a lower cement content and consequently less shrinkage. Since you do not have a requirement for extremely low density, some natural normal weight aggregates may be used in lieu of lightweight fines to meet the 110 pcf.

I would talk to a local supplier to see what the options are for materials and mix designs, since the distance from the aggregate sources and distance from the R/M plant can be a big cost factor.

If this mix is to be pumped, it may be required to pre-soak or saturate the lightweight aggregates to prevent them from absorbing too much mixing water at the wrong time. This depends on the type of aggregate used. Some of the lightweight manufacturers have very good, experienced technical representatives and cover large areas because of multiple plants.


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

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