Replacement of Portion of Roof Slab With Simply Supported Slab

A large wastewater pumping station has a reinforced concrete slab at ground level that serves as the roof for the debris screening room below. (The steel screens intercept solids in the wastewater influent stream before they can get to the suction of the pumps and create clogs or cause damage.) The slab has a severly corroded portion due to exposure to corrosive gasses from the wastewater as well as chemicals draining to the slab openings from the ground level. Cores have been taken and indicate cracking and delamination in the interior of the slab in addition to the corrosion at the surfaces. At the perimeter of what is known to be deficient concrete, are massive concrete beams and walls that appear to be relatively sound.

The intent is to remove the corroded portion of the slab by sawcutting the bad sections while cutting a notch into the beams and walls below that will allow the slab to bear on the existing concrete and function as a simply supported slab. We are pretty comfortable with the design we have generated and are not anticipating major problems. I am wondering if anyone has suggestions or comments about our approach to this problem. The portion of the slab to be removed is roughly 17 ft. X 24 ft. X 12 inches thick.

 

[ishvaaag]

As long as the beams are still structurally competent after notching I don't see why the scheme would need to be changed. Choose the most chemically resilient concrete, impervious, compact, and maybe seal bot top and bottom with silicon spray and mastic, except some expert says it won't work; if you have one of these make him made recommendations for the concrete and surface protection.

 

[paddingtongreen]

My first thought was to wonder how much of the slab also figured as part of the beam but if the numbers work and the beam is okay after notching it should be good.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[TXStructural]

It sounds like the slab would have been designed as continuous, so there would be concern about strength or serviceability of the remaining, adjacent slab sections. You will be removing a lot of weight, and eliminating the continuity provided by the existing slab. Increased deflection may result in cracking to the underside of adjacent slabs, increasing risk of further deterioration. I would also consider torsion on the beams from remaining structure and the new, discontinuous beam.

The size of the notch required will remove significant section, so try to verify the existing beam reinforcing.

Assuming that the section you are proposing to remove is one full bay, dimensions indicate that it might be one-way or two-way. Do you know how this was designed?