Over the course of the last few years, members of The Concrete Sawing & Drilling Association (CSDA) have observed failures when using 0.5-in. (1.3 cm) drop-in anchors and many assumed the problem was with the anchors.
The majority of failures have occurred in the internally threaded region of the anchor, causing half of the anchor to remain in the concrete and the other half to be pulled out. This mode of failure poses a serious hazard to operators, as the drill rig could suddenly fall on them. Anchor failure also could result in the rotation of the entire core drill around the drill bit that also could cause serious injury to an operator.
In order to evaluate the reasons behind drop-in anchor failures, the Concrete Sawing and Drilling Association sponsored a senior engineering project at the University of Toledo College of Engineering. Skip Aston, chair of CSDA Safety Committee, had worked with the University of Toledo Engineering Department in the past and believed a project of this nature would benefit both the engineering students and members of the Concrete Sawing and Drilling Association.
The objective of the project was to evaluate the performance of 0.5-in. drop-in anchors in a core drilling application. The project was divided into four phases: an analysis of stress in the anchor; a comparison of strength in five popular brands of drop-in anchors; a determination of the effects of installation parameters on anchor strength; and development of design recommendations to the core drill set-up as well as the development of an additional method to reduce anchor loading.
Based on their results, the students defined a summary of important installation parameters when using 0.5-in. drop-in anchors. Those parameters include, anchors should be installed perpendicular to the surface; anchor countersink depth should be limited to 0.5 in.; and expansion plugs should be fully set and the threaded rod should be fully engaged with the anchor.
Other important guidelines are that leveling screws should be adjusted before the base plate is installed on the threaded rod and the installation torque on the threaded rod should be limited to 19 ft-lbs.
For more information, visit www-mime.eng.utoledo.edu/design_clinic/design_expo/Fall07Pages/2007-04-03/index.html.