How Does Post-Installed Anchor Design Work in Concrete?

Post-installed anchors are designed using manufacturer test data published in evaluation reports: adhesive anchors develop load through bond stress along the drilled hole, expansion anchors through friction from the wedged sleeve, and screw anchors through thread interlock with the concrete. Design checks the anchor's tested tensile and shear capacities against the same concrete failure modes as cast-in-place anchors — breakout, pullout, pryout — using the product's qualified values and installation conditions. The mindset shift from cast-in-place design: you're not calculating capacity from material properties, you're applying tested values within their qualified limits. The evaluation report is part of the design documents, not a product brochure.

Adhesive anchors — a threaded rod or rebar bonded into a drilled hole with epoxy, vinylester, or hybrid mortar — are the heavy lifters: highest capacities, qualified for cracked concrete and seismic use, but sensitive to installation and needing cure time. Their design centers on bond strength, which varies with concrete condition, temperature, and whether the hole was drilled with a hammer drill or a diamond core bit. Expansion anchors — wedge, sleeve, and undercut types — develop load mechanically and are ready immediately, which makes them the choice for speed and for lighter loads. Screw anchors cut their own threads into the concrete, install fast with an impact wrench, and are removable, which earns them a place in temporary bracing and adjustable connections. Undercut anchors, which lock into an enlarged hole bottom, give near-cast-in-place performance for the most demanding retrofits. Each family has its own acceptance criteria and its own set of qualified products — the design starts by matching the product's qualifications to the job conditions.

Post-installed anchor failures are installation failures far more often than design failures. The capacities on paper assume the hole was drilled, cleaned, and filled the way the testing assumed — and field conditions love to deviate. I treat the installation procedure as a design parameter, not a contractor means-and-methods footnote.

• Name the basis-of-design product and require engineered review of any substitution
• Verify the product is qualified for the actual conditions: cracked concrete, seismic, temperature, wet holes
• Enforce the hole preparation procedure — cleaning steps are what the test values were based on
• Require special inspection for structural adhesive anchors; it's a code requirement, not optional
• Check edge distances and spacing against the report, not just the plans — existing concrete has real constraints