Achieving an (almost) crack free elevated slab

[lexeng18]

Hi everyone,

I’m working on a project involving a conventional steel office building with a 2nd-floor concrete slab over a metal deck. The project requires the elevated slab to be as crack-free as possible, and budget constraints are not a major concern. I’ve been researching extensively and would appreciate feedback on my approach and any additional insights.

Here’s what I’ve been considering:

Shrinkage Compensating Concrete​

1. I’m exploring the use of shrinkage-compensating concrete (Type K cement) for the slab on grade but haven’t found much information about its use for slabs on metal deck. I suppose slabs on metal deck are highly restrained by the bond between the decking and the concrete, making it impossible to guarantee that the reinforcement will be absorbing the expansion forces (and therefore ineffective in compensating for the later shrinkage). Is this understanding correct?
2. Would using a non-composite (form) deck with a bond breaker on the decking surface help? This might allow the slab reinforcement to restrain the expansion better. This is more of a thought experiment than anything, I likely will not be that experimental on this project.

General Best Practices for Minimizing Cracks​

• Metal Deck Stiffness: Use a very stiff metal deck to reduce variation in slab thickness between secondary beams and areas above them.
• Composite Action Design: Design beams and floor for composite action with shear studs. Would full vs. partial composite action make a significant difference?
• Beam Deflections: Target beam deflections of L/400 or less, focusing on limiting differential deflection rather than overall amounts.
• Reinforcement:
  • Use top and bottom conventional rebar mats with tightened spacing.
  • Specify chairs for placement to maintain proper positioning.
  • Design reinforcement for full negative moment, assuming continuity over beams.
• Steel Fibers: Add steel fibers to the mix alongside conventional rebar. I’ll consult with a fiber supplier for dosage recommendations while considering workability.
• Clear Cover: Specify 1.5-inch clear cover from the top surface.

Concrete Mix and Placement​

• Use 1-inch nominal aggregate size (max allowed with 1.5-inch clear cover).
• Specify a maximum water/cement ratio of 0.45. Is this appropriate, or should it be adjusted?
• Apply a curing compound. I’m seeking recommendations for effective products here.
• Address environmental conditions during placement, such as wind speed and air temperature.

Questions​

1. Are my concerns about Type K cement on metal deck valid?
2. Are there better approaches or additional factors I should consider?

Thank you in advance for your insights and expertise!

 

[haynewp]

Where did you find 1.5" of cover, from ACI? If it's T&S reinforcing the SDI used to recommend it being closer to the top surface, 3/4" to 1" cover.

 

[human909]

Crack free? Well surely you would be wanting to look at keeping it in compression. Eg post tensioning.

I think you need to explain the reasons why cracking is a problem. Visual, corrosion, seal etc...

 

[ANE91]

It’s gonna crack, as I’m sure you know. If you really wanted to prevent cracks in service, then you’d need to swap the structural system to something like a series of topped Class U prestressed concrete beams.

If your goal is simply to limit crack widths, then, again, I’d recommend prestressed concrete. Specifically, a post-tensioned slab is great at this.

For concrete over metal deck, I’m going to assume that the goal here is to minimize unsightly cracks for a polished top finish. Dial in on the concrete mix and curing methods. Minimize paste content by substituting for SCMs. Minimize water content with HRWs. Cure with steam or wet burlap. As,min at the top surface won’t cut it; check out what ACI 350 has to say about As,req’d for minimizing cracks.

 

[CJLCivilStruct]

With traditional reinforced concrete water retaining structures, we typically specify a limestone aggregate (low thermal expansion co-efficient) and 50% GGBS cement replacement (lower heat generation during hydration) as it is better for early age thermal cracking. May be a consideration for this scenario?

Although, as others have mentioned pre or post stressed seems to be the more obvious choice to achieve the requirements.

 

[jhnblgr]

My experience is composite metal decking rarely cracks unless the placement of the concrete was done improperly or the structure wasn’t designed for the dead load and deflect during placement. The composite metal decking really doesn’t experience the shrinkage like slabs on grade likely due to the continuous bottom reinforcing and the ribs. I don’t think type k cement would have much impact due to this. Mistakes made is insufficient coverage and stone size when reinforcement or radiant tubing is added. Mistakes made in structural design to make it stiff enough to not deflect when concrete is being placed or finished. Steel joists should be braced during the concrete placement to prevent any deflection. Avoiding any 90 degree bends in the floor and replace with rounded corners. Don’t allow heavy ride on power trowels. Use heavy gauge wire mesh, the light stuff is easily bent and wavy. Run rebar over girders or beams. Use heavy gauge decking and review the construction spans.

 

[HTURKAK]

AFAIK , composite slabs are designed as a simply supported reinforced concrete slab which the steel deck acting as sagging reinforcement . Typical mesh used for temperature control will not provide enough negative reinforcement for beam spacing of 3- 6 mt. Imo , the best practice to provide a nominal hogging reinforcement at supports although the slab is designed as a simple span . Theory of RC is that, cracking of the slab in the negative regions is normally expected and we may discuss to limit the crack widths .
My suggestion would be , design the composite deck with at least min.temp reinf , provide hogging reinf. at supports and consider 20 mm thick. polymer modified topping screed .
















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