Q.: We are working on a project where we placed a waffle-pan slab. Recently, we cored through the slab to install piping and conduit. The slab thickness above the pans was specified to be 4 inches. After analyzing the cores, we discovered that the slab is as thin as 3 inches in some sections and as thick as 4 1/2 inches in others. What is the tolerance for this type of slab, and is this slab acceptable? If not, what should I do in lieu of ripping out the slab?

A.: American Concrete Institute Committee 117 report, "Standard Specifications for Tolerances for Concrete Construction and Materials," offers tolerances for cast-in-place concrete construction. For cross-sectional dimensions of members such as columns, beams, piers, walls (thickness only), and slabs (thickness only) the tolerances are as follows:

  • 12 inches or less +3/8 inch -1/4 inch
  • More than 12 inches but not over 3 feet +1/2 inch -3/8 inch
  • Over 3 feet +1 inch -3/4 inch

For a 4-inch-thick slab, this would limit the thickness to between 3 3/4 and 4 3/8 inches, so this particular slab does not meet the required thickness after applying the tolerance.

Before planning on removing the slab, present this problem to the structural engineer. Possibly the 4-inch thickness was chosen because the designer wanted to keep nominal dimensions for ease of construction. The 3-inch-thick slab areas may be adequate to handle the necessary loads. The dimensions of the joists and the placement of reinforcement within them are often the critical elements in a waffle slab. The engineer may want to check if a reduction of the load limit is acceptable to the owner. If it is, it would be more economical than ripping out the slab. In any case, the engineer should check the capacity of the floor in areas subject to large point loadings, such as those from heavy equipment or bookshelves.