Suspended Floor Slabs

[rmayo (Randy Mayo, P.E.)]

For newer home inspectors the subject of suspended concrete floor slabs is somewhat complicated, but can be broken down into a few basic types: (I am still tweaking this article)

• Solid concrete slabs options with conventional reinforcing steel, i.e., bottom of slab is visible.
  • Clear span concrete slab between foundation walls in both directions.
    One-way slab – Where the length (L) divided by the width (W) is greater than or equal to two. The main reinforcing steel is placed in the short direction.
    Two-way slab – Where the length (L) divided by the width (W) is less than 2. Main reinforcing steel is distributed in both directions.
  • Concrete slab supported on one or more steel beams anchored into the foundation walls. Longer steel beams may have intermediate steel columns for added beam support.
  • Concrete slab supported on one or more steel or concrete columns (no visible beams).

PROBLEMS

 * Cracks that leak (poor waterproofing)
 * Rusted/expanding reinforcing steel
 * Negative bending cracks over intermediate support beams
 * Punching shear where support columns punch through slab

• Concrete slabs poured on top of corrugated metal decking, with or without visible support beams.
  • Composite vs Non-Composite design.

Composite steel forms typically are embossed which helps bond the concrete to the forms. The composite steel form acts as the main reinforcement for the slab. Only smaller temperature/shrinkage steel is place below the concrete surface.

Non-Composite steel forms are typically smooth and do not bond to the concrete. The concrete needs to be reinforced for bending. The steel forms do not provide any strength in the slab design

PROBLEMS

 * Cracks that leak rusting the steel decking (poor waterproofing)
 * Rusted/expanding reinforcing steel
 * Negative bending cracks over intermediate support beams

• Clear span precast hollow concrete panels.
  • With or without concrete overlay

PROBLEMS

 * Reflective cracking if the joints are not grouted or joined properly.
 * Leaks due to poor waterproofing maintenance.
 * Debonding of slab overlay

[mcyr (Marcel Cyr, CMI)]

Worked with all of those Randy and the smooth form decks usually looked like this in different gauges.

The one your showing looks like B-deck used on the roof except it is shown upside down.

[chall20 (Chris Hall)]

Really interesting to read. I had this today and the area was very wet at crawlspace. Steel beams were rusted much more then I usually see. Did not appear to be surface layer and actual pieces were missing. I am planning on referring to an engineer. Is there a general rule as to how much rust is significant? Or is any non surface rust and deterioration an issue?

[bcawhern1 (Brian Cawhern, CMI)]

This will provide some helpful guidelines.

https://inspectapedia.com/structure/Steel-Column-Rust-Damage.php

Interestingly, a patina can form on some (such as a man-hole cover) which is not harmful. Not something I typically see on load bearing components.

Stages of Rust

[rmayo (Randy Mayo, P.E.)]

The section loss looks significant, but where the section loss is located is critical. Loss in the flanges is critical in the area of maximum moment (middle 1/3 on simple span beams), shear is critical at the bearings at the supports. The process involves removing the rust so the current amount of section loss can be calculated to determine the new Moment of Inertia which is used to calculate the beam stress.

[chall20 (Chris Hall)]

Thanks to both. This was the first time I had seen rust that was not surface layer. I always check them real close.

[mcyr (Marcel Cyr, CMI)]

@bhubbard3

if these forms are significantly rusting and falling apart, is there much concern

No, the form deck is exactly that, a form to hold the wet concrete inbetween the beam supports. it comes in 20, 22, or 24 gauge. Once the concrete is set the form deck doesn’t do anything.
That rust on the support beam is a concern as Randy mentioned.