Modeling software predicts 24% of the structure is more than 1/4-in. too thin.
Crystal Ball modeling software Modeling software predicts 24% of the structure is more than 1/4-in. too thin.

ACI 117 includes tolerances for fabrication, placement, cover, thickness, and elevation. Certain combinations of these result in conflicts and remediation that increases project costs. The challenge facing the design/construction team is to identify and remove conflicts prior to construction.

One example of these internal conflicts is found in floor construction. ACI 117-10 contains a tolerance for the elevation of the top surface of formed suspended slabs, before removal of supporting shores (±3/4 inch, Section 4.4.1). There also is a tolerance on the elevation of formed surfaces before shore removal (±3/4 inch, Section 4.4.2) and on the thickness of the suspended slab between surface and forms (-1/4 inch, Section 4.5.3).

A brief examination of the form and surface tolerances might lead one to conclude that they are compatible, because both are the same. The reality, though, is the combination of these two tolerances can result in a slab with a significant area that is more than 1/4 inch too thin.

If the forms and slabs are installed independently, the elevation of the formwork and that of the slab also will vary independently. Modeling software can forecast the resulting structures’ thickness if the mean elevation separation between the formwork and the top surface of the slab is exact and the form and slab surface are allowed to vary within tolerances specified by ACI 117. The result, as can be seen from Figure 1, is about 24% of the structure and can be expected to be more than 1/4 inch too thin under those circumstances. In this example, the total structure depth has been set at 24 inches. The percentage of structure falling below the thickness of 23 3/4 inches is slightly less than 24% for the 500,000 trial runs.

Who’s responsible?

Responsibility for tolerance conformance depends on how the work is contracted. If one subcontractor is responsible for the forms, reinforcing, and concrete placement (turnkey), then one entity is clearly responsible. If the work is split among subcontractors, then responsibility is not clear-cut. Often the GC provides benchmarks for subcontractors. An independent forming contractor might install the forms using elevations provided by the GC. A separate concrete contractor might install the concrete work under a place-and-finish contract using elevations also provided by the GC. The forming contractor is clearly responsible for conforming to the form elevation tolerance and the concrete contractor is responsible for conforming to the slab elevation tolerance. Who would be responsible for the slab thickness? Often, the GC will push responsibility to the subcontractors when questions arise about thickness.

Available strategies

As a general rule, the slab surface can be installed more accurately than formwork. The smartest strategy might require improved accuracy be relinquished to provide necessary structural thickness. If the project is turnkey, then one strategy is to place the formwork carefully, gage up off the forms to establish concrete elevation, and finish the surface to specified finish tolerances. When the work is split, the following options should be considered:

  • Exclude conformance to slab thickness tolerances from the subcontractor.
  • Add 1/4 inch to the specified separation between form and concrete surface elevation benchmarks. The additional separation should reduce the area of deficient slab thickness from 24% to about 10%.
  • Gage up off the formwork to establish concrete elevation. Normally, the place-and-finish subcontractor will not have the money or time in his bid to cover an independent survey of the formwork before concrete placement, so this approach presumes that the forms have been installed within the necessary elevation envelope.

Eldon Tipping, PE, FACI, is president at Structural Services Inc., Richardson, Texas.