Technical Questions

ACI Committees, Membership, and Staff have answered common questions on a variety of concrete related topics.

Tolerances for concrete slabs supporting steel tanks

Q. My company will be placing a concrete slab foundation designed to directly support the base of a steel tank. Does ACI have any documents that discuss tolerances for elevation of such slabs after placement?


A. Concrete slabs supporting steel tanks are not covered by the ACI specification for tolerances in concrete construction (ACI 117-10). We also could not find this application in any other ACI document discussing foundations or slabs-on-ground. However, the information you are looking for is provided in standards from the American Water Works Association (AWWA).

For example, Provision 12.6.2 of ANSI/AWWA D100-11 discusses tolerances for concrete foundations: “The tolerance on poured concrete before grouting shall be ±1 in. (±25 mm).” In addition, per Section 12.6, “Unless otherwise specified, the foundation shall be graded to slope uniformly upward to the center of the tank with a minimum slope of 1-in. (25-mm) vertical to 10-ft (3.0-m) horizontal”; and “Unless otherwise specified, an oiled sand cushion shall be used under the tank bottom.” Also, for tanks supported on concrete slabs (Provision 12.6.1): “A sand cushion not less than 1-in. (25-mm) thick shall be provided between the flat bottom and the concrete slab foundation. In lieu of a sand cushion, the bottom may be supported on 1/2-in. (13-mm) thick cane fiber joint filler meeting the requirements of ASTM D1751.

The shell of mechanically anchored tanks shall be supported by grout.” For requirements for bolted steel tanks for water storage, refer to ANSI/AWWA D103-09.

Although the relevant statements are in mandatory language, it’s important to note that the AWWA standards are not specifications, as they “describe minimum requirements and do not contain all the engineering and administrative information normally contained in specifications.” If the contract documents do not explicitly state the required tolerances, you should ask for clarification from the owner or general contractor.


References:  AWWA D100-11; ASTM D1751

Topics in Concrete:  Tank, Concrete; Slab, Concrete

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