Show Us the Data

“The operator should not use the vibrator to cause concrete to flow from one location to another, because such a practice usually causes segregation.”

We could find no published evidence or data showing that flowing concrete laterally usually causes segregation. In fact, the ACI Committee E-703 publication Cast-in-Place Walls⁴ states the following:

It is necessary to place the concrete from one side of a blockout so that it flows under and completely fills the area under the blockout. If concrete is placed from both sides, a void or honeycomb is likely to occur beneath the blockout.

Moving concrete laterally is also needed at the boundary element in shear walls (see photo). The boundary elements are so congested with reinforcing steel that it’s not possible to insert a pump hose or a vibrator from the top of the element. Instead, the vibrator operator is part of the placing crew. The crew member directing the discharge from the pump hose in the shear wall portion adjacent to the boundary element works with the vibrator operator, who vibrates the concrete so it flows laterally into the element. The vibrator operator observes the lateral flow to ensure the concrete completely fills the element.

If segregation occurs when concrete is moved laterally, it’s most likely to be a problem with the mixture proportions. Segregation causes are discussed in ACI 212.3R-10, “Report on Chemical Admixtures for Concrete.” That document notes, in referring to flowing concrete, that segregation and bleeding can be reduced by increasing the fine-to-coarse aggregate ratio, improving the aggregate grading, or adding other fine material. Entrained air and viscosity modifying admixtures also help to minimize segregation.

What about empirical evidence?

Some ACI documents use empirically based design formulas. The deflection formulas in Chapter 9 of ACI 318 are an example of this approach. Is empirical evidence considered to be as flawed as anecdotal evidence? Not at all. Anecdotal evidence is often used to deduce a conclusion that doesn’t follow from it, usually by generalizing from an insufficient amount of evidence. Empirical evidence is measured, unbiased and replicable.⁵ The formulas in ACI 318 meet these requirements. They may not predict deflections with great accuracy, but they are based on a lot of measured, experimental data. In short, they have the numbers to back them up.

References

1. Suprenant, B.A. and Malisch, W.R., Tolerances for Cast-in-Place Concrete Buildings, American Society of Concrete Contractors, St. Louis, Mo., 2009, pp. 61-62.

2. Novella, Steven, “The Role of Anecdotes in Science-Based Medicine,” Science-Based Medicine, Jan 30, 2008. www.sciencebasedmedicine.org/index.php/the-role-of-anecdotes-in-science-based-medicine/

3. Suprenant, B.A. and Malisch, W.R., “How Clean Must Rebar Be?” Concrete Construction, June 1998, pp.517-523.

4. ACI Committee E-703 Concrete Craftsman Series 2, Cast-in-Place Walls, American Concrete Institute, Farmington Hills, Mich. 2000, p. 40.

5. Parr, Ben, “Ben Parr’s Entrepreneurial Musingshttp://benparr.com/2010/02/anecdotal-evidence/

Note: ACI documents cited can be purchased at www.concrete.org.

Ward R. Malisch, PE, PhD, FACI, technical director for the American Society of Concrete Contractors, can be reached at wmalisch@ascconline.org. Bruce A. Suprenant, PE, PhD, FACI, concrete consultant, can be reached at bsuprenant@bsuprenant.com.

This column is sponsored by the American Society of Concrete Contractors, but the views expressed are solely those of the authors.