Honestly, if you just screw a bench down with normal bolts, a thief with a wrench can have it gone in five minutes. The best method I’ve used is to anchor it with concrete wedge anchors combined with tamper-resistant hardware — and if you want it truly permanent, add epoxy.
Start by positioning the bench exactly where you want it and mark the mounting holes through the feet or frame. Use a hammer drill with a carbide-tipped masonry bit that matches the diameter of your anchors, usually 3/8 inch or 1/2 inch for a typical bench. Drill each hole about 1/2 inch deeper than the anchor length so dust and debris don’t stop the anchor from seating. Clean the holes with a vacuum, blow-out bulb, or compressed air; skipping this step is the main reason anchors fail to grip.
For most outdoor benches, 3/8-inch or 1/2-inch wedge anchors are the best balance of strength and ease. Slide the anchor through the bench foot into the hole, hammer it down lightly, then tighten the nut to expand the wedge inside the concrete. If the bench feet are thick wood or metal, choose longer anchors so enough thread is embedded. Where the mounting holes are slotted or oversized, add a heavy stainless or galvanized washer under the nut.
To make it theft-resistant rather than just bolted, replace standard hex nuts with security nuts or breakaway shear nuts. If you must use standard hex-head hardware, tighten it fully, then fill the head recess with epoxy or metal patch, or use a center punch to distort the threads above the nut so a wrench can’t easily back it off. In public or commercial areas, use stainless steel or hot-dipped galvanized hardware so rust doesn’t weaken the anchor.
If the bench doesn’t have pre-drilled feet, you can install anchors in the concrete first, set the bench over the studs, and fasten it down. Concrete screws like Tapcons are faster, but they resist prying a little less than wedge anchors. In high-theft spots, I combine methods: epoxy-set threaded rod plus a tamper-proof nut and fender washer. It feels like overkill, but that bench won’t disappear without power tools and serious noise.