Most retail and commercial structures have floors which are simply concrete poured on grade. The earth is part of the form work and its levelness, compaction and grading affect the thickness of poured concrete as does the presence or absence of stones, debris and like materials. If the underlying earth is not perfectly level, variances in the concrete thickness occur throughout the slab.

Owners historically demand slabs be of at least specified concrete thickness. Concrete pouring contractors, and general contractors, generally pour to specified thicknesses, but concrete shrinks during the curing period. Accordingly, slabs generally cure to a thickness less than poured and often in a non-uniform manner across the area of the slab. Most specifications for retail or commercial concrete slabs specify 4 to 6 inches of thickness.

Owners and contractors have historically disputed whether a poured slab meets a specification because of its thickness. An article once reported that the thickness of one specified 6 inch slab varied from 2 ¾ inches to 8 inches, but the “normal” range should have been between 4 ½ inches to 7 ½ inches of thickness with an “average” concrete thickness of 5 ¼ to 5 ½ inches. Most concrete slabs, as poured, do not conform to specified thickness. That fact can, and does, result in litigation or arbitration.

In a recent case, the following specification was at issue:

Slab Design Criteria: Min. slab thickness shall be 4 inches, based on a minimum 3,000 PSF soil bearing pressure, 100 PSF uniform floor design load with 400 psf fixture point loading. Engineer of record shall verify slab thickness based on site specific conditions, modify the design criteria thickness as required for site specific conditions and report any deviation from the design criteria to Owner’s PM.

Additionally, the Concrete Mix specification required that the mix be poured to achieve 3,000 psi when cured after a twenty-eight day cure period.

Once the slab was poured it was tested by coring and found thin. It averaged 3 ½ inches in thickness, with one core sample actually below the minimum ACI tolerance level. The Owner claimed the slab failed to meet the 4 inch specification and constituted defective construction. The contractor and subcontractor contended the slab exceeded the required specification because higher strength concrete actually poured exceeded the specified load bearing requirements of 3,000 psi. The concrete poured exceeded 3,700 psi when cured. In terms of load bearing capacity, the slab “as poured” was structurally stronger than the slab “as specified.”

Concrete strengthens with the passage of time. Consequently, if there is going to be a concrete slab failure, it generally occurs earlier rather than later. The purpose of a concrete slab is to support required loads determined in advance by architects and engineers. Specifications require a minimum thickness of slabs, but they also require that the mixture as poured be capable of sustaining loads of a certain number of pounds per square inch when cured. The load bearing capacity of a slab is the result of a combination of several factors, not just slab thickness. The two most significant factors are the concrete strength and the slab thickness. Specifications for slabs on grade are often treated as design specifications because they provide the contractor with the exact thickness of the slab desired. Tolerances also need to be considered.

Structural engineers and architects have debated tolerances for thickness of concrete slabs poured on grade for years. The American Concrete Institute’s ACI-117 has gone through several iterations since the late 1930’s. The latest ACI version is ACI-117-10. Published articles report that few slabs on grade actually meet thickness specifications. ACI-117-10 Commentary states “No structure is exactly level, plumb, straight and true,” and for such reason tolerances are provided for.

The American Concrete Institute initially provided tolerances for thickness which were both positive and negative. Ultimately, ACI eliminated its tolerance for a greater thickness, and provided a tolerance from specified thickness of a negative 3/8 inch with no one sample less than a negative 3/4 inch irrespective of specified thickness. ACI-117.10 ¶ 4.4(a).

The specification in the recent case cited above was found to be a performance specification. The specification did not simply require a 4 inch slab, but read as the whole, required a slab to support intended loads for the useful life of the structure. While the slab at issue was not as thick as specified, its compressive strength and load bearing capacity was proven by the contractor to exceed design requirements. At the time of decision, no failure(s) had occurred, and the slab had been in use for more than two years without cracking, spalling or flaw.

Whether a specification is a design or performance specification is a question of law, the resolution of which can be impacted by facts. The debate over slab thickness and tolerances will continue, but the purpose of a slab will not; to bear required loads for the useful life of a structure.