Direct Answer
Post-installed anchors (epoxy anchors, mechanical wedge anchors, or undercut anchors) are used when anchor bolts were omitted from the concrete pour, mislocated, or when new structural connections need to be made to existing concrete. They are always less preferable than cast-in-place anchors — their capacity is typically lower and their installation requires core drilling and careful execution to achieve certified capacity.
Full Explanation
Post-installed anchor types: (1) Adhesive (epoxy) anchors — a threaded rod set in an epoxy-filled drilled hole; capacity depends on hole cleanliness and epoxy injection quality. ICC-ES listed anchors (Hilti HIT, Simpson SET) provide verified design values. (2) Mechanical expansion anchors (wedge anchors) — expand against the hole wall; adequate for lighter loads, not recommended for seismic or sustained tension. (3) Undercut anchors — mechanically interlock with a mechanically formed undercut in the concrete; highest post-installed capacity. Post-installed anchors in seismic applications must meet ACI 318 Chapter 17 requirements for ductile or brittle behavior.
What This Means for Your Shop
When anchor bolts are mislocated and require post-installed supplemental anchors, coordinate with the engineer immediately. The engineer must confirm that the new anchor group (original + post-installed) achieves the required capacity. Document the mislocated condition with photos and measurements before drilling.
Common Mistakes
Installing epoxy anchors without following the manufacturer's installation requirements — specifically, hole cleaning (two brush passes, two blow-out passes, vacuum) is required for full capacity. Skipping cleaning reduces epoxy anchor capacity by up to 30%.
Recommended Process
For all post-installed anchor work, use an ICC-ES listed anchor system with manufacturer's installation instructions. Follow the hole cleaning procedure exactly. Allow full cure time per the manufacturer (temperature-dependent) before applying load. Document installation with photos of each step.