Q.: In the article "Shear Collars Save a Parking Garage" (Concrete Construction, October 1988), the authors refer to a chain drag delamination survey. This method seems to be much faster than the traditional method of tapping with a hammer to detect delaminations. Do you have specific details on the method?

A.: Gary Klein of Wiss, Janney, Elstner replies. The method simply involves dragging lengths of chain across the top of a concrete surface. A distinctly hollow, drum-like sound is heard when delaminations are encountered. Chain drag surveys are particularly effective in locating shallow delaminations on uncovered decks. The method can't be used to locate delaminations in the concrete below bituminous wearing surfaces, although debonding between wearing surface and underlying concrete can generally be detected by a chain drag survey.

A hammer or steel rod can be used to conduct a similar survey on vertical and overhead surfaces, although it's generally not practical to survey the entire underside of a bridge deck or parking deck.

Automated acoustical sounding devices also are available. We have found, though, that the chain drag survey is more cost efficient and reliable. In all cases, however, you should verify that a delamination is present by core drilling.

The locations of delaminations are typically marked on the deck surface and plotted on a plan view sketch of the bridge deck or parking structure. The percentage of delaminated area can then be determined. Quantities of repair materials used substantially exceed quantities predicted on the basis of delamination area measurements. That's because some incipient delaminations aren't detected by the chain drag survey. Also, some sound concrete has to be removed at the perimeters of the delaminated areas.

For more information about delamination surveys, see the article by Bill Perenchio entitled "The Condition Survey" in Concrete International, January 1989, page 59. For a copy of the article, contact the American Concrete Institute, P.O. Box 19150, Detroit, Michigan 48219.