How Concrete Screw Anchors Work: XBolt Screw Anchor Performance 101

Key things to remember

The essentials from this document in plain words. Check the full document before you rely on a figure.

• ✓Screw anchors suit medium duty fixings that need fast installation, close edge distance, close spacing and an anchor that can be removed. They tap their own thread in the drilled hole and lock mechanically to the concrete.
• ✓Ease of driving depends mostly on the thread pitch and profile and on the steel's strength and hardness: a wider pitch drives faster, a taller thread takes more effort, and stronger concrete takes more energy to tap.
• ✓The thread has to be hard enough that abrasive concrete does not wear it away, but not so hard that it chips off as it is driven.
• ✓Load performance is set by whichever failure mode governs: steel in tension or shear, concrete cone, pullout, concrete edge, pryout or bending. At the embedments screw anchors are used at, the concrete usually fails before the steel, even at class 8.8.
• ✓A narrow thread pitch leaves less concrete between threads and, with a poor thread profile, can sharply reduce pullout strength.
• ✓The XBolt described is case hardened C10B21 carbon steel with an ultimate steel strength of 1,200 N/mm², mechanically galvanised zinc yellow, with a serrated flange head and a tapered tip.

Who it helps: Builders, engineers and staff who want to understand screw anchor performance before choosing one.