Concrete sampling frequency before steel erection

November 19, 2002

Paul Bartleson

Safety Director

Kraemer Brothers, LLC

[Recipient address omitted]

Re: steel erection concrete testing requirements; number of samples required; §1926.752(a)(1)

Dear Mr. Bartleson:

By letter of January 7, 2002, you requested clarification of the concrete testing requirements in §1926.752(a)(1) of the steel erection standard. We responded by telephone on February 8, 2002; we understand that you subsequently requested a written response. We have paraphrased your questions and included an illustrative example as follows.

Question 1(a): Section 1926.752(a)(1) sets strength requirements for concrete in footings, piers and walls that support steel erection loads. In assessing concrete strength, how many samples must be taken?

Answer: Section 1926.752(a)(1) requires that:

Before authorizing the commencement of steel erection, the controlling contractor shall ensure that the steel erector is provided with the following written notification: the concrete in the footings, piers and walls and the mortar in masonry piers and walls has attained, on the basis of an appropriate ASTM standard test method of field-cured samples, either 75 percent of the intended minimum compressive design strength or sufficient strength to support the loads imposed during steel erection.

As you are aware, the standard does not specify the number of test cylinders that must be taken to assess concrete strength. However, since the standard does require the concrete to attain a specified strength, the number of samples taken must be sufficient for the tester to make a reasonably accurate assessment. In determining the number of samples, the tester will have to consider conditions and factors typically used by professionals in making these judgments. These include the volume of concrete being used, number of batches, whether different classes of concrete are used, and amount of surface area (for slabs and walls).

Reliance may be placed on established engineering practices, local building codes, or industry consensus standards (such as the American Concrete Institute's "Building Code Requirements for Structural Concrete" (ACI 318-02) and accompanying "Commentary" (ACI 318R-02)).[1] Reliance may also be placed on testing instructions set out in project specifications that have been signed by a registered professional engineer.

Question 1(b): Scenario: A wall is being poured that will support a line of columns. The 60 cubic yards of concrete will be used for every 5 bearing (anchorage) points. The 60 cubic yards yield less than 5,000 square feet of surface area for the wall. The truck loads are from the same batch. One test is conducted for every 60 yards of concrete. Is this a sufficient number of samples?

Answer: If you use the ACI 318-02 and its Commentary ACI 318R-02, which, as explained in the answer to Question 1(a) is acceptable to rely on, this would be a sufficient frequency of sampling, and would be in compliance with the §1926.752(a)(1) requirement.

Question 1(c): Scenario: Three concrete footings are poured from one concrete truck; that concrete is from one batch. Is one test sample sufficient to cover all three footings, or do we have to conduct a separate sample for each footing?

Answer: Under the ACI code referred to above, which would call for one sample per 150 cubic yards in this situation, one sample would be sufficient to cover all three footings; there would be no need to have a separate sample for each footing.

Question 1(d): Scenario: A slab is poured, which takes 50 cubic yards. Is one sample sufficient for that slab?

Answer: The answer in this case depends on the surface area of the slab and whether the concrete comes from a single batch.[2] As long as the surface area is less than 5,000 square feet, under the ACI code referred to above, one sample would be sufficient.

If you need additional information, please do not hesitate to contact us by fax at: U.S. Department of Labor, OSHA, Directorate of Construction, [Office of Construction Standards and Guidance], fax # 202-693-1689. You can also contact us by mail at the above office, Room N3468, 200 Constitution Avenue, N.W., Washington, D.C. 20210, although there will be a delay in our receiving correspondence by mail.

Sincerely,

Russell B. Swanson, Director

Directorate of Construction

[1] Note, though, that section 5.6.2.3 of the ACI code states that "when total quantity of a given class of concrete is less than 50 yd3, strength tests are not required when evidence of satisfactory strength is submitted to and approved by the building official." Since the steel erection standard requires the concrete strength to be assessed on the basis of field-cured samples, in no instance may the assessment be made with no samples. [ back to text ]

[2] Section 5.6.2.2 states: "On a given project, if total volume of concrete is such that frequency of testing required by 5.6.2.1 would provide less than five strength tests for a given class of concrete, tests shall be made from at least five randomly selected batches or from each batch if fewer than five batches are used." [ back to text ]