How Do Concrete Anchors Work? | Load Transfer Explained

Concrete anchors transfer structural loads from a fastener into the concrete through mechanical interlock, expansion, or adhesive bond.

Whether you’re hanging a handrail or securing heavy equipment, the principle behind a concrete anchor is the same: it creates a connection that spreads tension and shear forces into the concrete substrate instead of letting them concentrate on a single point. The way a concrete anchor works depends almost entirely on which type you choose and how well you install it.

Cast-In vs. Post-Installed Anchors

All concrete anchors fall into one of two families, and the distinction comes down to timing. Cast-in anchors are placed in the form before the concrete is poured, becoming part of the slab itself. Post-installed anchors are drilled into hardened concrete and then expanded, bonded, or screwed into place, per Caltrans’ bridge design guidance.

For most DIY and renovation work, you’re dealing with post-installed anchors. These grip the concrete in one of three ways:

• Mechanical interlock — the anchor physically keys into irregularities in the hole
• Expansion/wedging — the anchor’s sleeve or wedge spreads against the sides of the drilled hole
• Adhesive bond — a chemical compound glues the anchor into the hole

How Wedge Anchors Create Their Grip

Wedge anchors are the most common mechanical post-installed type, and understanding them explains how concrete anchors work in general. When you tighten the nut, it pulls the anchor upward while an expansion clip at the bottom forces a wedge outward against the walls of the drilled hole. That outward pressure creates the mechanical interlock that resists both tension (pulling out) and shear (lateral force).

Wedge anchors have a few identifying details worth knowing.

The type of concrete matters too. Anchors must match whether the concrete is cracked or uncracked, and whether it’s rated for the exposure conditions you’re working in.

Adhesive Anchors and the Installation Rules

Adhesive anchors work differently — instead of mechanical wedging, a chemical compound bonds the anchor to the hole walls. That makes them ideal for thin or cracked concrete where expansion forces could cause damage. But their performance depends almost entirely on installation quality.

Michigan DOT’s field manual is blunt about this: drilled-in anchors should be installed by qualified contractors following the manufacturer’s printed instructions. For grouted anchors specifically, the bonding surfaces must be cleaned, the hole wire-brushed, and the hole saturated with potable water before grouting — then free surface water removed right before the grout goes in.

Skip the cleaning step and you compromise the bond. Adhesive suitability also has to match the specific condition: dry, water-filled, submerged, or overhead installations each have their own requirements.

The Load Path and Concrete Condition Decide Everything

The anchor doesn’t do the work alone — it’s part of a load path running through the concrete. Your selection must account for the concrete’s condition and the intended load. Using the wrong anchor type for cracked concrete, moisture exposure, or overhead tension can invalidate performance or code compliance entirely. Treat structural anchoring as an engineered connection, not a hardware decision; MDOT directs structural anchoring questions to Bridge Field Services.

The standard installation sequence is simple in theory: drill the specified hole, clean it thoroughly, place the anchor, and for adhesive systems support the anchor correctly during curing. The failures almost always trace back to one of two mistakes — inadequate hole prep or ignoring the manufacturer’s instructions. Best anchors for a concrete wall compares the top-rated options if you’re choosing hardware for your own project.

PCI’s certification addendum reinforces the same point for post-installed anchors: qualified installers and manufacturer instructions aren’t optional extras, they’re the difference between a connection that holds and one that fails. A concrete anchor doesn’t fail silently — when it goes, it takes whatever it was holding with it.