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Understanding the requirements to ensure proper performance
Post-installed adhesive anchors, also known as epoxy anchors, chemical anchors, bonded anchors, and adhesive-bonded anchors, have been successfully used to make connections to concrete structures for many years. With the failure of adhesive anchors in the Boston I-90 Tunnel Project, commonly called the Big Dig, the use of these types of anchors has been called into question. This article reiterates the proper installation and associated inspections necessary for adhesive anchors to achieve the desired performance.
WHAT ARE ADHESIVE ANCHORS?
Adhesive anchors in concrete or masonry derive their resistance to applied tension load by adhesion or bond. The adhesive, available in both cartridge and capsule configurations, consists of two essential components: a resin and a hardener.
When cartridges are used, the two components are contained in separate parallel tubes connected to a manifold that proportions the materials in the proper ratio and mixes them together. The cartridge tool forces the materials out of the tubes, through the manifold and mixing nozzle, and into the drilled hole. The mixing nozzle ensures that the components are well mixed so the adhesive resin is activated by the hardener.
When an adhesive capsule is used, the resin and hardener are kept separate but are contained within a single glass or foil capsule. After inserting the entire capsule into the drilled hole, the anchor element, usually a threaded rod, is inserted into the predrilled hole with a rotational motion using a rotary drill. The rotary motion of the anchor breaks the capsule, causing the resin and hardener to mix and initiating the chemical reaction that hardens the adhesive.
Adhesive anchors are available in a variety of chemistries, each with its own specific characteristics and capacities. The adhesive materials include epoxies with many different formulations, acrylates, vinyl esters, polyesters, and hybrid mortars. The specifier, installer, and end user should become familiar with the requirements of each specific application to ensure the selected anchor and adhesive materials are appropriate for the given application.
SELECTION OF ADHESIVE ANCHORS
Selection of the appropriate adhesive anchor system requires an understanding of the loads to be resisted. Not only the direction of the load (tension, shear, or a combination of tension and shear), but also the duration (sustained or...